Cranfield University

Piston Engines Module Lecture Notes

Contents
The main lecture notes are supplemented by additional material presented during lectures and
provided in electronic form.

1. Piston Engines Introduction 1

2. Petrol Engine Fuel Injection Systems 30

3. Fuel and Combustion 42

4. Exhaust Emissions 48

5. Turbocharging 56

6. Appendix 1 - Burning LPG 65

7. Wankel Engine 71

8. Appendix 2 – Trunk piston and radial engines 77

Piston Engines Introduction

Air Standard Cycles

The basic air standard cycle assumes that air is the working fluid, and that all processes are completely
reversible. For the Otto and diesel cycles, we assume a closed system that works on the same quantity of air
throughout the cycle. Combustion of a hydrocarbon or other combustible fuel is modelled as a heat addition;
since the quantity of air is many times larger that the fuel, this is a good qualitative model that requires no
knowledge of the actual combustion. Heat is usually rejected in these devices by the exhaust of combustion
products; this is replaced with a heat rejection process. Two concepts are the compression ratio and the
mean effective pressure.

The Air Standard Otto Cycle

The Otto cycle is the idealized model representing a standard spark-ignition, four stroke engine. The
processes are:
i. a reversible and adiabatic compression (1 to 2).
ii. a constant volume heat addition, representing combustion at the end of the piston stroke (2 to 3).
iii. a reversible and adiabatic expansion, representing the power stroke of the piston (3 to 4).
iv. a constant volume heat rejection, representing the exhaust process (4 to 1).

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The Air Standard Diesel Cycle

The diesel cycle is the idealized model representing a compression-ignition, four stroke engine. The
processes are:
i. a reversible and adiabatic compression (1 to 2).
ii. a constant pressure heat addition, representing combustion at the end of the piston stroke (2 to 3).
iii. a reversible and adiabatic expansion, representing the power stroke of the piston (3 to 4).
iv. a constant volume heat rejection, representing the exhaust process (4 to 1).

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Dual Combustion Cycle
A practical representation of the diesel cycle has the heat transfer partly at constant volume and partly at
constant pressure.
i. a reversible and adiabatic compression (1 to 2).
ii. a constant volume heat addition, representing combustion at the end of the piston stroke (2 to 3).
iii. a constant pressure heat addition, representing combustion at the end of the piston stroke (3 to 4).
iii. a reversible and adiabatic expansion, representing the power stroke of the piston (4 to 5).
iv. a constant volume heat rejection, representing the exhaust process (5 to 1).

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Internal Combustion Engine
The petrol engine is a spark ignition engine and requires the fuel charge in the cylinder to be ignited by a
high temperature spark. The timing of the spark is critical. The compression ratio of the petrol engine is
relatively low in order to prevent ignition of the fuel charge due to the temperature of the compression; this
produces “knocking” which is the effect of pre-ignition of the fuel. The fuel and air charge are introduced
into the cylinder by means of a carburettor or a fuel injection system, (the fuel injector introduces a
measured quantity of petrol into the air flowing to the cylinders). This means that the air/fuel mixture is
compressed in the cylinder.

A diesel engine is a compression ignition engine and this means that the air charge in the cylinder must be
compressed to a high enough temperature to ensure that the fuel will ignite spontaneously when it is injected
into the cylinder. The compression ratio is chosen to achieve the desired air temperature in the cylinder at
the end of compression.

Gas engines operate in a similar way to the petrol engine in that the air and gas are mixed before the charge
enters the cylinder and so the mixture of fuel and air are compressed.

The governing equations

P 1V 1 P2V 2

T1 T2 ---------- eqn 1

and P1 (V1)γ = P2 (V2)γ ----------- eqn 2

 1
V1
may be combined to give T2 = T1 X V2 ----------- eqn 3

where P1 is the initial cylinder pressure

V1 is the initial cylinder volume
T1 is the initial air temperature
P2 is the final cylinder pressure
V2 is the final cylinder volume
T2 is the final air temperature
γ is the index of air compression

It can be seen from equation 3 that the final temperature T 2 depends upon the initial temperature of the air
in the cylinder as well as on the compression ratio V1/V2. Any reduction in initial air temperature will
reduce the final air temperature but any leakage of air from the cylinder will also reduce the final
temperature, which could influence fuel ignition.

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Turbocharging and Supercharging. A two-stroke cycle engine requires scavenge air to be supplied
at a pressure above that of the atmosphere in order to ensure that exhaust gas is removed from the cylinder
and a fresh air charge is available for the next cycle. This air can be supplied by a number of systems
including engine driven rotary blowers, engine driven reciprocating pumps or electrically driven rotary
blowers, however, since the 1950s it has been the practice to employ rotary compressors driven by single
stage gas turbines which derive their energy from the engine exhaust gas. Use of such gas has many
advantages including an improvement in operating efficiency and an increase in power available at the
crankshaft as no power is removed to drive the scavenge pumps or blowers. Supercharging is a means of
increasing engine power output by increasing the air supply which enables more fuel to be burned. A
supercharger can be any device which increases the pressure of the combustion air supply above that which
is normally required. A supercharger can be a turbocharger but it can also be any of the other devices which
increase air pressure. A turbocharger can be a supercharger but only if it supplies air to the engine at a
pressure above that which is needed just for effective scavenging. Supercharging may be applied to petrol
engines, gas engines and diesel engines, the effect being the same.

The effect of supercharging can be seen from the equation,

pv = mRT ---------------- eqn 4.

where p = cylinder pressure (N/m2)

v = cylinder volume (m3)
R = Gas constant (J/kg K)
T = air temperature (K) {Celsius Temperature + 273)

m = pv/RT ---------------- eqn 5.

The cylinder volume when the scavenge ports are covered is constant, and if the air temperature is kept
constant then the mass of air varies directly with the pressure. Doubling the pressure will double the mass of
air in the cylinder and, theoretically the fuel mass burned may be doubled allowing for an increase in power
developed. A critical factor in this is the air temperature as too high an air temperature will cause cylinder
problems and possibly combustion problems. Intercoolers are fitted between the supercharger unit and the
cylinders in order to reduce the air temperature.

Engine Parameters. The size of an engine depends upon a number of factors and an important feature is
the power to be developed. Naturally the number of cylinders will govern the power of the engine but the
bore of the cylinder and the length of the stroke will determine the power developed in the cylinder.

The standard equation governing a two-stroke cycle engine cylinder power is:

Power (Watts) = pm x A x L x n ----------eqn 6.

Where pm = Mean cylinder pressure (N/m2)

A = Piston area (m2)
L = Length of piston stroke (m)
n = Revolutions per second (1/s)

Note: Sometimes the acronym PLAN is used in order to aid memory of the cylinder power calculation.

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The cylinder bore governs the value of `A` and this can vary from 980mm for large engines down to 260mm
for the smallest engines. During the final decade of the 20 th century brake mean effective pressures have
increased from about 18.0bar to 19.0bar and at the same time maximum cylinder pressures have risen from
about 135bar to 150bar.

Engine Cycles

Medium speed engines may operate on the four stroke cycle or the two stroke cycle. Although the former is
favoured by most medium speed marine engine builders a number of two stroke medium speed engines are
still in service. The fundamental structure of both types is similar and the combustion process is the same,
the basic difference is in the gas exchange process. All crosshead type marine propulsion (and shore power
plant) engines operate on the two stroke cycle.

Four Stroke Cycle

The four stroke cycle engine has four distinct piston strokes or two revolutions of the crankshaft for each
power output stroke.

Air inlet Exhaust Fuel

Valve Valve Injector

Suction Compression Power Exhaust

The Four Stroke Cycle

a. During the suction stroke the piston moves down and air is drawn into the cylinder through the air
inlet valve(s).
b. During the compression stroke all valves are closed and the piston moves up.
c. Near the top of the stroke the fuel injector opens and sprays a quantity of fuel into the cylinder. This
ignites quickly if the temperature is correct, producing a rapid rise in pressure which causes the
piston to move down the cylinder. Power is transferred to the crankshaft via the connecting rod.
d. During the exhaust stroke the piston is moved upwards by the crankshaft and the products of
combustion – the exhaust gases – are forced out of the cylinder through the exhaust valve(s).

It can be seen that power is produced only during one of the piston strokes during the four stroke cycle.

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 Two Stroke Cycle
Two Stroke Cycle
The two stroke cycle has one power stroke of the piston for each crankshaft revolution. There are no distinct
exhaust and air suction strokes and so provision must be made for the removal of the exhaust gas from the
cylinder before it is recharged with fresh combustion air. This process is known as „scavenging‟ and
involves the incoming charge air forcing out the exhaust gas through ports cut in the cylinder liner or valves
in the cylinder cover. An essential feature of the two stroke cycle engine is that combustion air must be
provided at a pressure above that of the atmosphere to ensure that exhaust gas will be forced from the
cylinder by the incoming scavenged air. Such air can be supplied by crankshaft driven pumps or chain
driven rotary blowers, but most engines employ exhaust gas driven turbochargers. An air trunking, generally
known as the scavenge air manifold, runs the length of the engine and surrounds the scavenge air ports. Air
is supplied to the cylinders through ports cut in the lower part of the liner. This means that the pistons of two
stroke cycle engines must be provided with long skirts to ensure that the ports remain covered when the
piston is at the top of its stroke. If this was not so there would be a loss of air into the crankcase and, in the
case of an engine with exhaust ports, blowback from the exhaust into the crankcase and scavenge manifold
with its associated problems, including the risk of explosion. Such two stroke cycle engine pistons require
sets of sealing rings at the lower part of the skirt.

Port Scavenging
For a port controlled two
stroke cycle engine there
are exhaust and scavenge
air ports cut in opposite
sides of the lower part of
the cylinder liner. As the
piston moves downwards
during the power stroke it
will uncover the exhaust
ports and the cylinder
pressure will fall to a
value below that of the
scavenge air pressure by
the time the piston
uncovers the scavenge
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ports. The piston crown is profiled to encourage the incoming air to flow upwards and so effectively remove
all exhaust gas from the cylinder, which is known as cross flow scavenging. On the upward or compression
stroke of the piston the scavenge ports are covered before the exhaust ports and some air is lost from the
cylinder. However, the designer will have allowed for this and there will still be sufficient air in the cylinder
for efficient combustion provided the scavenge air supply pressure is at the design value. The area of
scavenge and exhaust ports is crucial to effective engine operation and it is essential that these ports be kept
clear of carbon deposits.

Uniflow Scavenging
In some cases one or more exhaust valves in the cylinder cover are used to control cylinder exhaust, the
opening and closing of such a valve being regulated by a camshaft. The exhaust valve will open before the
piston uncovers the scavenge air ports in the lower part of the liner, allowing the cylinder pressure to fall
below the scavenge air pressure. When the scavenge ports are uncovered, air flows upwards and forces the
remaining exhaust gas from the cylinder. This is known as uniflow scavenging.

Medium Speed Engine Classification

The classification of the marine medium speed engine is broad, encompassing a wide range of operating
speeds, two operating principles and propulsion and electrical power generation applications. The broad
classification means that it is difficult to define and no book can give comprehensive coverage, at least
within a reasonable sized volume. This book is intended only as a guide, and reference should be made to
operational manuals for individual engines before any action is taken.

Definition of Medium Speed

A medium speed engine can be considered as one operating within the normal speed range of 300 rev/min –
1000 rev/min. For propulsion purposes the shaft speed must generally be reduced by gearing or through the
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adoption of a diesel-electric drive, to obtain the most efficient propeller speed. When employed as a prime
mover for electrical generation purposes the engine operating speed is chosen, in conjunction with the
design of the electrical equipment, to give the desired power supply frequency. Aboard ship a frequency of
60Hz is usual, therefore the engine speed and electrical equipment arrangement must be capable of
producing this.

Trunk Piston Arrangements

Although over the years many different designs of medium speed engine have evolved the most common
type at the time of writing is the trunk piston arrangement. This form provides a low engine height, favoured
by shipowners due to the headroom saving compared with the tall crosshead engine. Trunk engines have
cylinder liners which open directly into the crankcase so that products of combustion and unburnt fuel pass
directly from the cylinder into the crankcase causing contamination of the lubricating oil; however, given
that no more satisfactory arrangement exists, the trunk piston arrangement is the best option available. A
connecting rod provides the drive from the piston directly to the crankshaft and the angularity of the
connecting rod means that there is a side thrust from the piston to the cylinder liner. The magnitude and
direction depend on the force on the piston and the direction in which the piston travels, up or down the
cylinder. This side thrust can increase cylinder wear.

Typical In-Line and Vee-Type Medium Speed Engine Arrangements

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Diesel Engine Types
For marine purposes two types of engine are in common use namely the crosshead type and the trunk
piston type. The crosshead engine has its combustion cylinders separate from the crankcase in which the
crankshaft turns; in order to achieve this a crosshead is provided and this has a bearing which allows for
reciprocating action of the piston to be translated into semi-rotary motion at the connecting rod. A
diaphragm plate separates the engine cylinders from the crankcase and a gland allows the piston rod to
pass through that plate. The crosshead also has guide shoes which run in guide bars, the purpose of
these being to take the side-thrust exerted by the angular position of the connecting rod. In the trunk
piston engine this side thrust is taken by the piston which presses against the cylinder wall and thus
there is a chance of increase in cylinder and piston ring wear.
Exhaus
Turbo- t Valve
charger Cylinder
Exhaust
Gas to Cover
Funnel

Air
Cooler

Diaphragm
Scavenge
Trunking
Diaphragm
Gland Crosshead

Guide
Bar
A-Frame

Connecting
Crankshaf Rod
t
Crank

Bedplate

Crosshead Engine Arrangement

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Trunk piston cylinders connect, at their lower ends, with the engine crankcase space and so used
cylinder lubricating oil, combustion products (e.g. carbon) and unburnt fuel can enter the crankcase and
contaminate the crankcase lubricating oil. The advantage of the trunk piston type compared with the
crosshead engine it that it is of relatively low height and so occupies less headroom.

Cylinder Trunk Piston Engine

Arrangement
Piston

Gudgeon Pin
{Top End}

Connecting Rod
Lower
Cylinder

{Trunk}
Crankshaft Bottom
End
Bearing
Crank
V-type trunk piston engines are also available and these allow for more cylinders to be connected to the
crankshaft without too great an increase in the space occupied; the penalty for such an arrangement is an
increase in complication of the engine parts. Trunk piston engines tend to operate at higher speeds than
crosshead engines and so they are usually connected to the propeller by means of a reduction gearbox
and a controllable pitch propeller which allows for change in ship speed and direction without changing
the speed, or rotational direction of the engine. Crosshead engines are generally connected directly to
the propeller, the engine speed and rotational direction being changed in order to manoeuvre the ship.

Engine Operation
The basic operation of the four and two stroke cycle engines have already been mentioned but it should
be emphasised that effective removal of exhaust gas from the cylinder and replacement by a new charge
of clean air is essential to effective power production, it does not matter which operating cycle is
employed to achieve that end. Means are available to determine the quality of exhaust removal and air
charge replacement, this being the use of indicator diagrams or electronic means on more modern
engines.

The process common to all engine cycles is that of fuel combustion and it is during the
combustion/power stroke of the piston that most problems occur. These problems may be due to the
fuel itself, the injection process or deficiencies in the fresh air charge and indicator diagrams may also
allow the engineer to determine the nature and cause of combustion troubles. There are five basic
requirements of good cylinder combustion for any engine;

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1. Air/Fuel Ratio; there must be the correct amount of air available in the cylinder to
completely burn the charge of fuel supplied. Usually engines are supplied with excess
air in order to ensure that complete combustion of the fuel can take place.

2. Atomisation; in order to obtain rapid and complete combustion the fuel is broken into
very fine droplets during injection into the cylinder {this is known as atomisation and is
obtained by forcing high pressure fuel through very small holes in the fuel injector
which is positioned in the cylinder cover}. Very fine droplets have a large surface area
which allows for rapid oxidation (combustion) of the carbon and hydrogen in the fuel. If
droplets are too large they pass through the combustion air charge and impinge upon the
cylinder liner or piston causing burning of these items and/or resulting in unburnt fuel
running down the liner wall into the scavenge space (crosshead engines) or into the
crankcase (trunk piston engines) where it will contaminate the crankcase lubricating oil.

Large Fuel Droplets Small Fuel Droplets

{Slow burning due to small fuel surface {Rapid combustion due to large surface
area} area}

Fuel Atomisation

3. Mixing; oil droplets and fuel must mix in order to achieve rapid combustion. Correctly
operating fuel injectors ensure that fuel droplets penetrate into the cylinder but if the
droplets are of incorrect size they may not penetrate adequately (if too small) or may
touch the liner cylinder wall and piston (too large); by giving the incoming scavenge air
a swirling motion effective mixing is promoted.

4. Fuel Injection Timing; fuel must be injected at the correct time to ensure that it starts
burning just as the piston passes the top of its stroke and begins moving downwards
during the power stroke. If injection takes place too soon very high cylinder pressures
will result and these cause overload on engine bearings and possible damage. If injection
is too late the fuel may still be burning when exhaust commences resulting in a loss of
power. Correct timing depends upon the ignition quality of the fuel and that differs with
different fuels; it important that the engineer knows what the effect of ignition quality is
on the combustion of any fuel he burns and is able to make adjustments of the injection
timing in order to compensate for changes in that quality. The taking of indicator
diagrams allow the engineer to assess whether the correct timing is being achieved.

5. Compression Temperature; the Diesel engine is a compression ignition engine which

means that ignition of the fuel is caused by the high temperature of the air charge in the
cylinder. If that temperature is too low the fuel may not ignite or will ignite late thus
12
causing power production problems and after burning of the fuel {the burning of the fuel after exhaust
commences}. Low compression temperature can result from faulty piston rings which allow air leakage
and so do not enable the correct compression ratio and temperature to be achieved.
Maintaining of fuel pumps, fuel injectors and other parts of the fuel system in good order is essential to
effective combustion and power production. Most fuels burned in marine Diesel engines are of the
residual type which means that they have higher viscosity than Diesel oils used for car and lorry
engines. Residual fuels must be heated in order to reduce the viscosity and allow the oil to be atomised
correctly; heating also promotes rapid ignition when the fuel is injected into the cylinder. Heating
temperature must be correctly controlled as too low a temperature results in poor ignition whilst too
high a temperature can cause the "gassing up" of fuel pumps {fuel vapour forms in the pump cylinder
during its suction stroke resulting in difficulties injecting the fuel}. Worn or defective fuel pumps doe
not achieve the correct fuel pressure due to leakage and fuel injection timing is also incorrect. Worn fuel
injector spray holes may result in larger fuel droplets than intended causing impingement of these
droplets on the liner walls and in poor combustion; leakage past the injector spindle will also result in
low injection pressure and faulty atomisation. It is the engineers responsibility to ensure that the fuel
injection system is maintained in good order.

Fuel Oil
Oils are hydrocarbons and often contain impurities which can influence combustion and engine
maintenance. Any oil is a product of the refinery process but there are many different grades which can
be obtained depending upon the stage of the refinery process at which the oil is extracted. Distillates, as
the name implies, are obtained by distilling the vapours extracted at certain levels in the pipe still at the
refinery; hot oil is introduced to pipe stills, and vapour extracted at different levels, the level governing
the temperature and hence the product obtained. At relatively low temperature very volatile products
such as petroleum are produced whilst at higher temperatures kerosene and gas oil are obtained. Diesel
oil, the product burned in Diesel engines such as are found in lorries and buses, is very much like gas oil
and may be considered as a distillate. However, there are "heavier" oils which are also classed under the
name of Diesel oils and these are not of such high quality as gas oils. Distillates contain very low levels
of impurities because they are essentially condensed vapours and unless the impurities can pass over
with the vapour then the distillate will be impurity free; distillate oils are expensive.

The refinery process begins with an atmospheric pipe still where oil is heated under atmospheric
pressure and vapours extracted; because the residual oil from this will still be high in the good quality
products such as petroleum and kerosene the residual is subject to a further distillation but this time the
pipe still is maintained under partial vacuum. World demand for petroleum and kerosene (aviation fuel)
has increased considerably and petroleum companies have sought more ways of obtaining there
valuable products from the residual oil leaving pipe still, two such processes are catalytic cracking and
viscosity breaking. In the catalytic cracker the hot residual oil is passed through a bed containing a
catalyst which generally comprises very fine particles of aluminium and/or silica. The process allows
more of the valuable light products to be extracted from the residual oil and hence the residue leaving
the catalytic cracker will be a very viscous fluid with high levels of impurities; in this state it is of very
little direct use as a general fuel and resembles tar which is used for road making. Heating can,
however, reduce its viscosity and it may be able to be burned in some boilers or engines. The vis`
breaker is a device which reduces the viscosity of high viscosity oils such the residues from catalytic
crackers and other refinery processes. The basic process involves subjecting the residual oil to high
temperature and pressure for a period of time and this breaks the bonds of the long molecular chains
giving shorter molecular chains which thus result in the oil having a lower viscosity. Viscosity is
directly linked to the length of the molecular chain. By this means residual oils of reasonable viscosity
13
may be produced and these are offered for sale for use as boiler fuel or as fuel for large Diesel engines
which are found aboard ship or in power stations ashore.

Distillates

Atmospheri
c Distillates
Crude Distillation Distillates
Oil
Visbreaker
Vacuum
Distillation
Straight Run
Residual Fuel
Distillates

Fluid Catalytic
Residual Fuel Cracker (FCC)

Residual Fuel Oil

Primary & Secondary Refinery Processes

The problem with residual oils is that they contain higher levels of impurities than did the original crude
oil simply because the lighter elements have been removed but the impurities have not. A further
problem resulting from the use of a vis` breaker is that the viscosity of the residual oil has been reduced
thereby giving the impression that it is a low viscosity oil but it does still contain the same levels of
impurities of the more viscous oil from which it originated. In former years it was possible to order fuel
simply by viscosity and be assured that the levels of impurity were related to viscosity, with modern
fuels that is no longer possible. Ideally fuels should be tested in order to determine the properties and so
make adjustments to the engine so that it may burn the fuel with minimum problem. Boilers are not as
sensitive as internal combustion engines to oil quality but they are susceptible to problems due to certain
impurities which cause slag deposits on tubes or corrosion in the uptakes.

Viscosity influences the ability of the oil to flow and as any system requires the oil to be moved from
storage to the point of use it is essential that some means of changing the viscosity is available. Aboard
ship fuel is stored in bunker tanks, which may also be double bottom tanks, and these are fitted with
heating coil so that the temperature of the oil may be raised when pumping is to take place. Treatment
of the oil in centrifugal separators also requires the oil to be heated as does injection into the engine as
the spray pattern from the injector and the size of fuel droplet produced is governed by viscosity,
together with other factors. In order to control the amount of heating it is necessary to know the initial
viscosity of the oil as overheating can result in problems such as gassing-up in fuel pumps and the
production of volatile vapours in tanks which can present a hazard.

14
Fuel Injection
In order that fuel will burn quickly when injected into an engine cylinder, or boiler furnace, it is broken
into very fine droplet (atomisation) which present a large surface area per unit mass and so will heat up
very quickly. Combustion is an oxidation process whereby oxygen combines with hydrogen to produce
H2O and with carbon to produce CO2, both processes being accompanied by the liberation of heat. Oils
are hydrocarbon chains and in order for the combustion process to take place hydrogen and carbon
atoms must be separated and this is achieved by means of heat which breaks the bonds in the molecular
chains. In order to obtain rapid combustion, which is essential in an internal combustion engine so that
the process may be complete before exhaust commences, atomisation of the oil is required so that the
molecular chains heat rapidly. With a very viscous oil the atomisation process in the fuel injector, or
boiler burner spray, is poor and so very fine droplets are not produced; droplet size is influenced by fuel
pressure and the size of hole in the atomiser spray but viscosity influences the pressure which can be
achieved in the fuel line by a particular pump and the drag effects on the oil flowing through that line.
Heating of the oil enables the optimum viscosity to be produced and it also raises the temperature of the
droplets so that on mixing with the hot air in the cylinder very little additional heat is needed in order to
bring about a breaking of molecular chains.

Plunger at the Commencement of End of fuel delivery; Plunger at the top

bottom of its fuel delivery; plunger lower edge of plunger of its stroke; no fuel
stroke just covers spill port helix uncovers spill port being delivered

Commencement of End of fuel delivery with Plunger rotated so that

fuel delivery; plunger increased effective pump grooves are in line with spill
rotated to give greater stroke and more fuel ports; no fuel delivery during
fuel delivery delivered any part of the pump stroke
15
 Injector Opening
Detection
Transducer
Lifting
Pressure
Adjusting
Screw

Oil Supply Pipe

Connection

Setting
Spring

Pressure Oil
Supply Injector
passage Body

Nozzle
Holder

Needle
Valve

Nozzle
Tip
Spray Holes

Typical Diesel Engine Fuel Injector

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Fuel injection systems for diesel engines are complex, the fuel injection pump, high pressure pipe
and injectors being closely matched in order to provide optimum fuel injection. All parts are critical
to performance and a slight deterioration in any can have adverse effects upon cylinder combustion
and the operating life of other components in the system. Although items such as pumps and
injectors will be considered separately it must be remembered that the fuel injection system is a
matched system and not just a collection of parts.

Fuel Injection Pumps

Most diesel engines operate on the jerk pump fuel injection system in which each cylinder has its
own fuel pump supplying high pressure fuel to a fuel injector fitted in the cylinder head; in some
cases there may be more than one injector but the majority of engines have one injector per cylinder.
The fuel pump is actuated by means of a cam fitted to the camshaft, timing of injection being
controlled by the cam. Fuel pumps are of the helical control type and the high pressure pipe carrying
fuel to the injector is sheathed in order to prevent fuel leakage in the event of HP pipe failure;
maximum pressures in the HP pipe can be as high as 2000 bar. Engines may operate on distillate
fuel, diesel oil or gas oil, but many highly rated engines employed for propulsion purposes burn
residual fuel; in some cases blended fuel is used in order to reduce the viscosity. It should be noted
that blending is carried out in order to reduce the viscosity of the fuel but levels of damaging
impurities such as sulphur and vanadium will not be reduced by the same percentage as the viscosity
is reduced. The burning of residual, or blended, fuel requires a heater in the fuel supply line together
with lagging on pipes in order to ensure that the fuel remains at the desired viscosity in order to
achieve the correct atomisation at the injectors. Some form of fuel treatment, e.g. centrifugal
separation, is required prior to the supply of fuel to the engine.

Although there are differences between individual designs all jerk type fuel pumps employing
helical control operate on fundamentally the same principle. A plunger reciprocates in the barrel of
the pump, clearances being very fine on order to prevent leakage. Delivery of the fuel takes place
when the pressure lifts the discharge valve fitted in the pump cover. Upwards movement of the
plunger is achieved by the cam acting through the follower and fuel pump tappet. The tappet is
spring loaded in order to provide for a downwards movement of the plunger in order to give a
suction stroke. The actual stroke of the fuel pump plunger is the total lift of the plunger and is equal
to the distance between the cam base circle and the cam peak, however, fuel may not be delivered
for the entire duration of the plunger stroke. The portion of the stroke during which fuel is delivered
is called the effective stroke and it comes to an end when spill takes place. Spill is the connection
between the high pressure part of the pump cylinder with the spill port and results in rapid drop in
fuel delivery pressure and consequent closing of the fuel injector needle valve. The rotational
position of the plunger regulates the point in the stroke at which spill takes place, the plunger being
rotated by means of a rack and pinion mechanism which is actuated by the fuel control.

Helical control plungers allow the quantity of fuel being delivered to be varied hence the power
generated in the cylinder may be varied. Movement of the control rack rotates the pinion which
forms part of a sleeve which engages with a rectangular section near the foot of the fuel pump
plunger. At the top of the plunger is a helical groove cut in the side, there being a vertical groove at
the mating ends of the helical groove. In most cases there are actually two sets of grooves
diametrically opposite each other, this providing for balance when spill occurs. When the spill port
is uncovered there is a sudden drop in oil pressure acting on the side of the plunger in the annular
space below the helix and this forces the plunger against the pump barrel. By providing two identical
sets of grooves diametrically opposite each other the pressure forces are balanced and this minimises
wear. When the vertical groove is in line with the spill port no oil pressure is produced when the
17
plunger moves upwards as oil can simply flow to spill; this is the no load position of the pump.
Rotating the plunger so that it is covered by the side of the plunger above the helix means that fuel
can be delivered. Delivery commences as soon as the top of the plunger covers the spill port;
pressurised oil will flow down the vertical groove and occupy the annular space below the helix but
it will remain pressurised as it cannot escape. Delivery of fuel continues until the helix uncovers the
spill port allowing the pressure oil in the annular space and in the space above the plunger top to
flow to spill. Immediately this happens delivery of oil to the injector ceases although the plunger will
still be moving upwards. The length of the effective stroke, and hence the amount of fuel delivered,
is changed by rotating the plunger thus allowing the helix to uncover the spill port earlier in the
plunger stroke for less fuel or later in the stroke for more fuel.

With a flat topped plunger delivery of fuel commences at the same point in the plunger stroke, and
consequently at the same camshaft and crankshaft angular position, for all. This is satisfactory in
most case but there are times when changing the timing of fuel injection commencement has
advantages. Engine operating efficiency is greatest when peak pressure and ignition temperature are
maintained at their maximum permissible value; the load on gudgeon pin and large end bearings
limit the value of peak pressure and a high maximum temperature can result in high levels of NO X
(Oxides of Nitrogen emissions). Advancing the injection timing at slightly reduced loads (between
about 90% and 100% of MCR {Maximum Continuous Rating}) enables peak pressure to maintained
virtually constant thus keeping operating efficiency at a high level. This is achieved by raising the
control edge of the pump plunger in these regions so that the spill port is covered earlier in the
plunger stroke and hence injection commences earlier. At very low loads there might be insufficient
energy in the exhaust gas to drive the turbocharger effectively thus resulting in a reduction in air
supply and consequent combustion problems. Retarding the injection timing at very low loads
reduces cylinder efficiency but allows sufficient energy to be available in the exhaust gas to enable
the turbocharger to provide the correct quantity of air without the need for auxiliary blowers.
Lowering the control edge on the plunger top delays the covering of the spill port and hence delays
or retards the commencement of fuel injection.

Fuel Injectors
Fuel injectors are basically spring loaded non-return valves which when they open allow high
pressure oil to flow through spray holes in the nozzle tip. For efficient cylinder performance the fuel
must burn quickly and this means that fuel droplets of about 10-15m diameter must be produced
during injection. Droplet size is a function of spray hole diameter and fuel pressure, which
influences the velocity of the oil passing through the holes. Very fine droplets burn quickly as they
have a large surface area per unit mass thus the oxygen can readily reach the hydrocarbon molecules,
which can only be done at the surface of the fuel droplet. However, fine droplets do not penetrate
readily into the compressed air in the combustion chamber thus they do not mix with the available
oxygen and so combustion can be impaired. Large droplets will penetrate but burning is slow
because of the relatively small surface area per unit mass of fuel; penetration can be too good and
unburnt fuel droplets may impinge on the liner and piston crown surfaces. Large droplets can also
result in afterburning which can damage exhaust valves. Optimum size is, therefore, a compromise
but the size can tend towards fine droplets if the air in the combustion chamber is encouraged to
swirl thus encouraging mixing between fuel and air. Swirl is promoted by the shape of the piston
crown in four-stroke engines and by the piston crown shape and port design in two-stroke engines.

Most engines employ a centrally fitted injector and the nozzle tip will be provided with a number of
spray holes. The disposition of the holes allows for production of a droplet spray pattern which suits
the shape of the combustion chamber. Droplets should not impinge on the piston crown, cylinder
18
head or the liner as this not only reduces performance but it may cause local burning of these parts
and unburnt fuel will be scraped off the liner into the crankcase where it will contaminate the
lubricating oil. In addition a defective fuel injector may result in poor engine emissions. During
service nozzle spray holes tend to erode and this enlargement results in a change in droplet size and
possibly in spray pattern. Injectors need to be removed from the cylinder at regular intervals, e.g.
MAN-B&W L58/64 engines after 2-3,000 hours. Hole size is checked by inserting a Go/No-Go
gauge and if the No-Go gauge will fit the holes are too large and the nozzle must be replaced. This
method is fine if holes erode in a truly circular manner but in many cases they do not and care is
needed in judging the size of a spray hole which has worn in a non-circular manner.

High pressure fuel enters the body of the injector via a filter unit and then flows down a hole drilled
in the body to the needle valve unit. A vent valve is fitted in order to allow for the removal of air
from the system. At the nozzle tip the high pressure acts on the annular face of the needle valve
which is immediately below the guided portion of the valve. When fuel pressure has increased
sufficiently to overcome the spring loading on the needle valve assembly the needle valve will lift
and high pressure fuel can flow through the nozzle holes. As the needle lifts the lower tip of the
valve is exposed to high pressure fuel and the valve immediately lifts to its fully open position,
which allows full fuel flow to the spray holes. Full lift is very small, only a few millimetres, as the
seat diameter of the valve is also only small. Lifting pressure of a valve is set by an adjusting screw
which acts on the setting spring, the lifting pressure being in the region of 200 bar for most engines.
At normal loads pressure in the fuel pipe will rise much higher than the lifting pressure and it is this
maximum pressure which governs the fuel droplet size; maximum pressures of 1500 - 2000 bar may
be encountered in highly rated engine fuel pipes. Overheating of the fuel at the nozzle tip can result
in pre-ignition of the fuel, with consequent burning of the nozzle tip or the formation of carbon
trumpets which destroy the spray pattern. In order to restrict the temperature at the tip of the nozzle
cooling of the injector is employed. In many cases this is achieved by simply placing the injector in a
water cooled pocket in the head but some engines, particularly those operating on residual fuel, are
fitted with injectors which are cooled by circulating lubricating oil through drilled passageways in
the body and nozzle tip.

Fuel does not ignite immediately it is injected, there is an ignition delay period during which the
combustion process is being initiated. It takes some time for a flame to appear and for there to be any
pressure increase in the cylinder. The duration of this ignition delay depends upon the ignition
quality of the fuel being burned. Ignition quality relates to the Calculated Carbon Aromacity Index
(CCAI) of the fuel and the higher the CCAI the longer will be the ignition delay. During the delay
period fuel is still being injected and as soon as the flame appears all fuel in the cylinder will burn at
once resulting in a very high peak pressure which could damage bearing surfaces. This is a problem
for high powered engines, with high rates of fuel injection, which operate on residual fuels. Later
injection of the fuel is not a real option as the ignition delay period would still be the same and the
effect would be to reduce power and possibly lead to afterburning. A two stage or pilot injection
system can avoid the problems of high injection rates with poor ignition quality fuels and the
Wartsila Vasa 46 engine is provided with a pilot injection system employing a single fuel pump.
Each cylinder has two injectors, the main injector being placed in the centre of the cylinder head and
the pilot injector to one side. The pilot injector is designed to give the required droplet size and spray
pattern at a lower fuel pressure than the main injector and is also set to lift at a lower pressure. When
the fuel pump pressure rises the pilot injector will lift first, at the correct crank angle timing to give
effective combustion. At the ignition point of the fuel there is only a small quantity of fuel in the
cylinder and this will burn but the cylinder pressure rise will be limited. The main injector is set to
open close to piston top dead centre (TDC) and when this happens the main fuel charge is injected;
19
this will burn immediately because of the pilot flame provided by the pilot injection. Cylinder
pressure, and temperature, rise will be reasonable but the full charge of fuel can be injected without
the risk of afterburning or high loads on bearings. The added advantage of the system is that peak
temperature is lower than with single stage injection and this has the effect of reducing the formation
of NOX

Injection Lag and Ignition Delay

20
The Common Rail Fuel Injection System
A problem with large crosshead type diesel engines is the camshaft drive system which is heavy,
consumes engine power and is expensive. A solution to the problem is to remove the camshaft
completely and operate the exhaust valves by means of an external system and fit a common rail fuel
system which does not rely upon camshaft driven pumps. Basically there is a fuel pump system
which supplies high pressure fuel to a common rail and individual cylinders are supplied with fuel
from that rail via injection control units which regulate the fuel quantity and injection timing.

21
 Diesel Engines
Lubrication
A lubricant serves to reduce friction and carry away heat from the components rubbing together (in
some cases heat from other sources may also be removed from the locality). In terms of the Diesel
engine there are two separate areas of lubrication, namely the crankcase and the cylinders, although the
same lubricant may serve both locations. In general engine terms lubricants used are mineral oils, ie
they are based upon crude oil stock and so are hydrocarbons (synthetic oils may be used in specialist
applications but this does not generally extend to Diesel engines). Particular properties may be imparted
to a lubricant by careful choice of additives but the basic stock is still hydrocarbon bases.

Crankcase Lubrication
In a crankcase there is a need to lubricate a number of bearings and also to carry away heat from these
bearings but in some engines the crankcase lubricant is also the piston coolant whilst in others (trunk
piston engines) the crankcase lubricant is also the cylinder lubricant. In terms of friction reduction there
are no particular aspects which present problems as mineral oils of the correct viscosity provide for a
low coefficient of friction at the bearing surfaces; the important factor is the correct viscosity at the
working temperature. Capability of withstanding loads is important, particularly at top-end bearings;
loading introduces stress in the oil film but the stress can be minimised by providing a large bearing
area (see notes on crossheads). Most mineral oils, with or without additives, can act as effective
crankcase lubricants but the use of additives provides for properties which may be desirable. The
quantity of oil supplied, usually under pressure to all bearings, is an important factor as it is essential
that there is always a film of oil at the bearing surfaces.

As crankcase oil is a hydrocarbon means that oxidation is a problem; oxidation is the conversion of the
basic Hydrogen and carbon elements into their oxidises forms of H2O and CO2 but the process is very
slow compared with rapid oxidation which occurs during combustion. During this slow oxidation
process sticky deposits are produced and it is these present problems; the rate of oxidation increases
with temperature and if the oil is heated oxidised deposits will form quickly. These deposits block oil
passageways leading to poor lubrication of bearings and they can also restrict the flow of oil through
filters, etc. Oxidised oil deposits are not as good at lubricating as the oil from which they were formed
and they cannot support high loads thus oxidation of oil should be avoided. Where oil acts as a piston
coolant, or cylinder lubricant, temperature increase is likely and hence the rate of oxidation will
increase. Anti-oxidants are usually employed as additives in most oils in order to reduce the rate at
which oxidation takes place. Where the oil acts as a piston coolant high temperature also results in
carbonizing of the oil and this causes additional deposits to form of cooling surfaces (eg within the
piston cooling space) resulting in overheating.

Note: most modern crankcase oils are referred to as High Duty (HD) oils as they contain levels of
additives which enhance performance potential.
Crankcase Oil Contamination may come from a number of sources and these often depend upon the
type of engine and features incorporated in the engine.

Water contamination may be fresh , salt or even bilge water. Fresh water comes from the jacket system,
salt water usually from leakage at coolers and bilge water from leaking tank-top plates. Bilge water may
be fresh or salt or a combination of both.

22
Fresh water brings water treatment chemicals to the crankcase and these may have a harmful effect on
the oil and its additives but the same is also true for salt water due to the sodium chloride. All water
tends to increase the risk of corrosion within the crankcase particularly if sulphur is present (from
leaking fuel). Water also forms an emulsion with the oil and emulsion is not as good at supporting loads
at bearings as the good quality oil used in crankcases. Centrifuging may remove water but certain of the
additives in the oil are water soluble and centrifuging also removes these thereby reducing additive
protection. The only safeguard is to prevent water contamination in the first place.

Fuel contamination may be by heavy oil or diesel oil depending upon the fuel being burned. Both types
of fuel reduce the flash point and increase the level of volatile vapours in the crankcase. Sulphur in the
fuel can cause corrosion. Heavy fuel oil increases the crankcase oil`s viscosity whilst diesel oil can lead
to a reduction. In trunk piston engines unburnt fuel is scraped off the liner into the crankcase but in
crosshead engines the diaphragm prevents this from happening. However, a defective diaphragm gland
can result in some contamination but if the cam box lub` oil system is part of the crankcase system
leaking fuel pump seals result in contamination of the cam box oil which then finds its way into the
crankcase. For this reason most modern engines have separate cam lub` oil supply systems.

Carbon contamination is mainly a problem with trunk piston engines as the products of combustion on
the liner walls are scraped into the crankcase. Carbon will obstruct lub` oil flow through pipes and
passageways and may even fill oil grooves in bearings resulting in complete failure. Straight mineral
oils can hold about 1% carbon in suspension but the use of detergents and dispersants keeps surfaces
clean and allows up to 5% carbon particles to be held in suspension.

Bacteria, yeasts and fungi will attack oil under certain circumstances although the aerobic bacterial
attack of lub` oil is the major problem area: aerobic attack requires air to be present but anaerobic attack
takes place with certain bacteria when there is no oxygen presence. In an aerobic bacterial attack of lub`
oil (or fuel oil for that matter) there is a break down of the oil resulting in a loss of its lubricating and
load bearing capability. Water needs to be present for any bacterial attack of this form and the action of
the bacteria is to form tight emulsion which cannot break down readily. An increase in acidity occurs
resulting in corrosion of bright metal parts and the discolouration and blistering of paint work in the
crankcase. Highly corrosive honey coloured films form on working parts. Dead bacteria block filters
and restrict oil flow through pipes and passageways in bearings. Bacterial attack results in a foul smell,
like rotten eggs, in the crankcase and so a severe attack can be easily recognised but it is important to
ensure that the infestation is detected before it becomes severe. {It should be noted that such attack can
take place in any place where oil is present is water can enter that oil.}. Use of bacterial detection
devices (such as Easi-cult) indicate the presence of bacteria in the oil. For a severe attack the only
solution is complete replacement of the oil charge after the entire oil system has been cleaned and
sterilised, usually by circulating a flushing oil containing a strong biocide. it is essential that all parts of
the system are sterilised as a small trace of contaminated old oil will result in contamination of the fresh
oil charge. For a mild attack the oil can be pumped to the renovating tank and heated for 24 hours at
about 85OC before being returned to the crankcase via the centrifuge. (Heating to this temperature may
have an effect on the oil, particularly oxidation and so the oxygen level (air space) in the renovating
tank should be minimised by pressing up the tank.) As water is an essential for any bacterial growth the
best way to avoid such problems is to eliminate water; means to prevent water contamination should
always be in the mind of the engine designer and the engine operator; as already mentioned centrifuging
is not the complete solution as this can remove water soluble additives in the oil. Biocide oil additives
are available and these will destroy bacteria but such additives are not always compatible with other
desired oil additives.

23
Cylinder Lubrication
The requirement in an engine cylinder is to minimise wear and also to carry away products of
combustion and wear particles. Additives are used to minimise wear (molybdenum disulphide) but
when running in an engine a straight mineral oil should be used in order to get to the running condition
as quickly as possible: running-in is a wear process and the use of anti-wear additives slows that
process; the quantity of cylinder oil should be increased during running-in to ensure that wear particles
and products of combustion are carried away.

Normal cylinder liner water is due to:

1. Corrosion due to acid products of combustion
2. Friction due to the rubbing together of rings and liner surfaces.
3. Abrasion caused by solid particles between rings and liner.

Under normal circumstances liner wear is maximum towards the top of the stroke due to the conditions
of temperature and pressure which exist; the oil film is thinned due to the temperature and high
pressures behind the rings force these hard into contact with the liner. Hot corrosion can also take place.
The piston is moving slowly at the top of its stroke and so an oil wedge does not form between rings
and liner. Wear reduces towards the middle of the stroke as the piston speed increases allowing an oil
wedge to form and conditions of pressure and temperature are less difficult. Towards the bottom of the
stroke wear may increase again, particularly with two-stroke engines and with long stroke engines.
Incoming scavenge air can scour the oil film from the lower part of the liner if the velocity is high and if
it contains water droplets (use of scavenge air water separators minimises this problem - see turbo-
charger notes). The piston is also moving slowly at the bottom of the stroke and an oil wedge cannot
form. Some engine builders provide additional quill points at the bottom of the liner. Abnormal wear
takes place if there is insufficient oil or if the oil does not provide the correct neutralising effect.

Clover leafing takes place when there is insufficient neutralising effect from the oil due to incorrect
Total Base Number (TBN) or incorrect quantity. Corrosive wear takes place in the positions between
and above the quills, the patterns being of clover leaf shape. When wear is excessive rings are not
supported at these points and gas can act on the front faces of the rings leading to collapse; this produces
blow past which removes the oil film from the liner and excessive wear and loss of power then takes
place. The solution is to ensure that oil of the correct neutralising effect and correct quantity is being
delivered. A liner suffering from severe clover-leafing must be replaced.
Cylinder
Cylinder Liner Lubricator Quill
Corrosive Wear Patches on
Liner Wall

Clover Leaf Wear Patches

Piston Ring

Combustion gases act on face of

piston ring in wear regions
leading to ring collapse, gas blow
past, loss power, removal of oil
fim and excessive ring & liner
Cylinder Lubrication Quills wear

24
Scuffing or Microseizure occurs when high spots on rings and liners come into contact due to
insufficient oil film. This results in local seizure at these high spots which causes local heating and a
changes in structure at these points. Because the piston is moving the seized sections shear as soon as
they form and scuff or seizure marks occur along the axis of the liner. The surfaces of rings and liner at
these locations becomes hard and glassy which prevents the oil from "keying" onto the surface; oil runs
off the surface and further wear and local overheating takes place due to incorrect lubrication. As rings
turn in their grooves the scuffing moves circumferentially around the liner and eventually covers the
entire surface.

Piston Ring

Liner Areas of Local

Welding

The Action of Scuffing or

Micro-seizure

Scuff or Micro-seizure Marks on

Liner Wall
In most cases the liner and rings must be replaced but it is essential that the problems with cylinder
lubrication which have cause the scuffing are corrected otherwise it will occur again. in the case of mild
scuffing the rings should be replaced and cylinder lubrication corrected; the scuffed area of the liner
should be roughened with a coarse grinding stone in order to break down the scuffed surface and
provide a key for the oil to attach itself to the liner.

Rings & Liners

Piston rings and liners are made from cast iron due to the ease with which this material can be cast and
the self lubricating properties it has due to the free graphite flakes which exist. The background matrix
of the grey cast iron is essentially pearlite plus ferrite which is relatively weak; the graphite flakes
further weaken the structure. However, by the use of suitable processes it is possible to increase the
tensile strength of cast iron; alloying elements, particularly magnesium, modifies the structure and
causes the graphite to form into spheroids rather than flakes. The matrix forms as pearlite which is
25
effectively steel. Although the spheroids of carbon weaken the matrix to some extent the material is
much stronger than grey cast iron but it does not have the same self lubricating properties. Spheroidal
graphite cast iron (sometimes referred to as nodular cast iron) is generally used for cylinder liners of
modern engines which require high strength in order to withstand high combustion pressures. usually
liner and piston ring rubbing surfaces have had the same degree of hardness in order to ensure that the
wear rates are equal but the rings wear faster than the liners due to the fact that they do more rubbing
per unit surface area (rings are more easily replaced). For modern long stroke engines this situation
would result in frequent ring replacement and it is now the case that rings are made slightly harder on
their rubbing surfaces than the liners; this reduces ring wear rate although it slightly increases liner wear
rate, however, long stroke liners have less contact with the rings per unit area and per unit time. Rings
are cased hardened on the rubbing surface, generally using a Nitriding technique. This method avoids
cracking and distortion which can occur with other methods of case hardening. In order to Nitride cast
iron it must contain 1.5% each of chromium and aluminium. In some cases cylinder liners are also case
hardened by Nitriding.

Some piston rings have a profiled rubbing surface, the idea of this is to encourage the ring to wear in the
form which allows the generation of an oil wedge. Putting an chamfer on the edge of rings achieves this
in many cases. Certain engine builders have used copper and carbon coatings on ring surfaces as a
means to assist running-in; the value of such coatings is disputed by many and the fact that it is not
common practice indicates that plain rings, with or without profiled surfaces, are satisfactory for most
engines. A good ring will bed itself to the liner provided that cylinder lubrication and operating
conditions are within reasonable limits.

Ring Clearances
It is essential that ring butt clearance be within certain limits; the butt clearance is needed to allow for
thermal expansion of the ring so that is it is insufficient the butts will come together and the ring will
jam in the liner. If the clearance is excessive it will promote ring blowpast when wear has taken place.
Axial groove clearance is needed to ensure that the ring is free within its groove. Excessive clearance
means that the ring can deform or bend within the groove causing breakage; insufficient clearance
means that the ring may jam in the groove when deposits build-up causing a scraping action against the
liner or ring blowpast. In order to allow for reconditioning of ring grooves to bring the axial clearance
back to its original value with new rings most engine builders provide groove inserts at the bottom of
the groove. These are welded into the groove and may be replaced as required during reconditioning of
the piston.

Butt
Clearanc
e

Axial Gas Cushion

Clearanc Piston Space
e

Piston Radial
Ring Clearanc
e
Groove
Insert

26
Radial ring clearance in the groove is needed in order to ensure that the ring does not touch the bottom
of the groove and so prevent it from sticking out from the piston face; axial clearance also provides the
space into which gas may pass in order to provide the cushion and gas force on the back of the ring
which forces it into contact with the liner.

Rings must be of good quality material which retain their spring even when subject to high temperature;
this spring ensures the seal between ring and liner which is then assisted by gas pressure behind the
ring. Rings must not crack in service due to thermal stress.

Timed Cylinder Lubrication

It is important that there is an oil film over the entire rubbing surface of the liner and that this film is of
sufficient thickness to provide the necessary oil wedge and neutralise acid products of combustion. The
scraping action of piston rings removes this film and so it must be constantly replaced; with trunk piston
engines splash lubrication is employed and this requires that scraper rings be fitted near the bottom of
the piston skirt in order to regulate the film on the liner and distribute this over the liner surface. If these
rings are not maintained in good condition defective lubrication will result. With crosshead engines,
which generally have longer strokes than trunk piston engines, oil is supplied directly tot he cylinders
through quills. These are essentially non-return valves positioned at certain points around the liner and
at particular point(s) down the liner. The actual location of the quills is critical to performance of the
cylinder lubrication system. It is also important to inject cylinder oil at the correct time which is
generally between the upper two piston rings when the piston is moving upwards, the position being at
about mid-stroke; the rings spread the cylinder oil over the liner surface. Unfortunately the piston is
moving at its quickest at this point and so it is very difficult to get very small quantities of oil through
the quills in such a short period of time. Positioning the quills slightly higher in the stroke means that
the piston is moving more slowly when it passes the quills and so there is more time available to hit the
gap between the upper two rings; there is a greater need for oil in the upper part of the liner and so
positioning the quills in this section is also very useful. If the oil is injected before the piston reaches the
quill point all of the oil is scraped upwards by the top ring and is burned; if the oil is injected after the
piston has passed it is all scraped off the liner on the next downwards stroke of the piston. Trying to
inject such small quantities of oil very precisely is extremely difficult and most engine builders accept
that precise timing of cylinder lubrication is an ideal but is not possible in practice. Reasonably accurate
timing is possible with pumps which are controlled very much like fuel injection pumps but the long
pipe run between pumps and quills results in a delay in the injection which means that absolutely
precise timing is not possible. (Temperature variation along the pipes also influences injection of the
cylinder oil.) In order to ensure that sufficient oil is spread over the liner most engine builders inject
more oil than is theoretically necessary to provide the oil film; spreader grooves in the liner surface
allow for a reservoir of oil and spread the oil circumferentially and axially.

A good cylinder oil should have the following properties:

1. Alkaline in order to neutralise the acid products of combustion.

2. Of sufficient viscosity so that it spreads over the liner surface at the working temperature in
order to form a tough film which resists the scraping action of the piston rings.
3. It must seal the piston rings against the liner surface.
4. Produce a soft deposit when it burns.
5. Keep surfaces clean by holding deposits in suspension.

27
Lubricating Oil Additives.
Additives provide the required properties and make a straight mineral oil into an oil which suits any
particular purpose. Careful choice of additives is needed both in terms of type and quantity. Some of the
main additives are:

1. Anti-oxidants; Use in most oils in order to reduce the rate at which oxidation takes place. Of
particular use in oils subject to high temperatures such as trunk piston engine oils and oils used
as piston coolants.
2. Detergents and Dispersants; Detergents keep surfaces clean whilst dispersants hold deposits in
suspension and so prevent them from adhering to surfaces. Such additives are particularly useful
in cylinder oils and crankcase oils for trunk piston engines.
3. Extreme Pressure Agents; These are chemical which maintain the strength of the oil film under
conditions of high temperature and/or pressure. They are useful for crankcase oils of highly
rated trunk piston engines and for cylinder oils.
4. Viscosity Index Improvers; Used to minimise change in viscosity with change in temperature.
They effectively make the oil a multi-grade and may be used where the oil is likely to be subject
to change in temperature.

Other additives are often defined by their names and they include; anti-wear, anti-corrosion, anti-
bacteria, anti-foaming, etc.

28
Automobile Compression Ignition Engines and Fuel Injection
Spark ignition engines are often called gasoline engines and compression ignition engines are often
called diesel engines. The basic difference between the two is the means by which the fuel and air
are mixed and burned. In a gasoline engine, fuel and air are mixed external to the cylinder volume;
during intake a fuel-air mixture is inducted through the intake valve into the cylinder. In a diesel
engine, fuel and air are mixed internally; during intake only air is inducted into the cylinder.

If the compression ratio of a gasoline engine is too high, then some of the mixture will autoignite
and burn so quickly that it will rattle the engine parts; the attendant noise is called knock.
Autoignition occurs because compression of the fuel-air mixture raises its temperature too high.
Since knock is undesirable, the engine designer must limit the compression ratio so that it does not
occur. As we shall see, this is one factor that limits the efficiency of gasoline engines.

Diesel engines are able to operate at higher compression ratios than gasoline engines because the
fuel is mixed with air at the time combustion is to commence. The compression ratio is deliberately
selected to be high enough so that the gases near the end of the compression stroke are hot enough so
that the fuel autoignites very soon after injection starts. The remaining fuel to be injected can then
burn no faster than it is injected. The period between the start of injection and autoignition is called
the ignition delay. Its duration depends upon the engine design and the fuel type. The designer
strives for a small ignition delay, for once the mixture autoignites all the fuel already injected burns
very quickly. If too much fuel burns in this phase instead of at a rate limited by the fuel injection, the
engine will knock intolerably. Diesel fuel should possess the ability to autoignite easily, whereas
gasoline should resist autoignition. Clearly, gasoline would make a poor diesel fuel and vice versa.

Example of indirect injection or divided chamber diesel engine are shown below. In this type of
engine the combustion chamber is two volumes connected by a passage. During compression,
gases will be forced to enter the spherical swirl chamber through a tangential passage. This creates
a rotating or swirling flow in the chamber and assists mixing of the fuel and air during injection.
Since the swirl is high, less demand is put on the injector to produce a finely atomized spray.
During combustion the flow will reverse itself and rich combustion products will flow back into
the cylinder or main chamber. Recesses are in the piston surface to direct that flow and mix it with
air to complete combustion. Notice, too, that this engine has a glow plug. The swirling air motion
also enhances convective heat transfer. One result is that in a cold engine the gases compressed
into the swirl chamber lose so much heat that ignition may be difficult or impossible to achieve.
The glow plug is a resistance heater turned on at this time to aid in starting. After the engine has
fired a few times, the walls of the swirl chamber become hot, thereby reducing the heat loss during
compression. The glow plug is thus used only for starting. Divided-chamber engines tend to be
used where the engine is expected to perform over a wide range of speeds and loads such as in an
automobile. Whenthe operating range of the engine is less broad such as in trucks, ships,
locomotives, or electric power generation, open chamber engines predominate. Figures below show
different combustion chambers to illustrate that a great deal of flexibility exists in the design of
diesel engines. That each engine manufacturer has worked to optimize the design for a particular
application and that each manufacturer has produced an engine with unique characteristics
illustrates that the optimum design is highly dependent upon the specific application.

29
 Volkswagen Diesel Engine with Indirect Fuel Injection via a Divided Chamber

Oldsmobile Swirl Chamber Daimler Benz Precombustion Chamber

30
Petrol Engine Fuel Injection Systems
For most of the existence of the internal combustion engine, the carburettor has been the device that supplied
fuel to the engine. On many small petrol driven machines it still is but as the automobile evolved, the
carburettor became more and more complicated trying to handle all of the operating requirements. For instance,
to handle some of these tasks, carburettors had five different circuits:

Main circuit - Provides just enough fuel for fuel-efficient cruising

Idle circuit - Provides just enough fuel to keep the engine idling
Accelerator pump - Provides an extra burst of fuel when the accelerator pedal is first depressed, reducing
hesitation before the engine speeds up
Power enrichment circuit - Provides extra fuel when the car is going up a hill or towing a trailer
Choke - Provides extra fuel when the engine is cold so that it will start

In order to meet stricter emissions requirements, catalytic converters were introduced. Very careful control of
the air-to-fuel ratio was required for the catalytic converter to be effective. Oxygen sensors monitor the amount
of oxygen in the exhaust, and the engine control unit (ECU) uses this information to adjust the air-to-fuel ratio
in real-time. This is called closed loop control - it was not possible to achieve this control with carburettors.
There was a brief period of electrically controlled carburettors before fuel injection systems took over, but these
electrical carburettors were even more complicated than the purely mechanical ones.

Induction Line Fuel Injection

Single Point Continuous Flow Petrol Injection (Bosch) Mechanical Multi-point petrol Injection System

At first, carburettors were replaced with throttle body fuel injection systems (also known as single point or
central fuel injection systems) that incorporated electrically controlled fuel-injector valves into the throttle body.
These were almost a bolt-in replacement for the carburettor, so the automobile makers didn't have to make any
drastic changes to their engine designs. Gradually, as new engines were designed, throttle body fuel injection
was replaced by multi-port fuel injection (also known as port, multi-point or sequential fuel injection). These
systems have a fuel injector for each cylinder, usually located so that they spray right at the intake valve. These
systems provide more accurate fuel metering and quicker response. The accelerator pedal in the car is connected
to the throttle valve which regulates how much air enters the engine; the fuel pedal is really the air pedal. When
you depress the accelerator pedal, the throttle valve opens up more, letting in more air. The engine control unit
(ECU, the computer that controls all of the electronic components on your engine) "sees" the throttle valve open
and increases the fuel rate in anticipation of more air entering the engine. It is important to increase the fuel rate
as soon as the throttle valve opens; otherwise, when the accelerator pedal is first pressed, there may be a
hesitation as some air reaches the cylinders without enough fuel in it. Sensors monitor the mass of air entering

31
the engine, as well as the amount of oxygen in the exhaust. The ECU uses this information to fine-tune the fuel
delivery so that the air-to-fuel ratio is just right.

The Injector

A fuel injector is nothing more than

an electronically controlled valve. It
is supplied with pressurized fuel by
the fuel and it is capable of opening
and closing many times per second.

When the injector is energized, an

electromagnet moves a plunger that
opens the valve, allowing the
pressurized fuel to squirt out
through a tiny nozzle. The nozzle is
designed to atomize the fuel - to
make as fine a mist as possible so
that it can vaporize easily. The amount of fuel supplied to the engine is determined by the amount of time the
fuel injector stays open. This is called the pulse width, and it is controlled by the ECU.

Fuel injectors mounted in the intake manifold of the engine

The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe
called the fuel rail supplies pressurized fuel to all of the injectors. In order to provide the right amount of fuel,
the engine control unit is equipped with a number of sensors.

Engine Sensors
In order to provide the correct amount of fuel for every operating condition, the engine control unit (ECU) has
to monitor a large number of input sensors. These include the following:

Mass airflow sensor - Tells the ECU the mass of air entering the engine
Oxygen sensor(s) - Monitors the amount of oxygen in the exhaust so the ECU can determine how rich or lean
the fuel mixture is and make adjustments accordingly
Throttle position sensor - Monitors the throttle valve position (which determines how much air goes into the
engine) so the ECU can respond quickly to changes, increasing or decreasing the fuel rate as necessary
Coolant temperature sensor - Allows the ECU to determine when the engine has reached its proper operating
temperature
Voltage sensor - Monitors the system voltage in the car so the ECU can raise the idle speed if voltage is
dropping (which would indicate a high electrical load)
Manifold absolute pressure sensor - Monitors the pressure of the air in the intake manifold.
The amount of air being drawn into the engine is a good indication of how much power it is producing; and the
more air that goes into the engine, the lower the manifold pressure, so this reading is used to gauge how much
power is being produced.
Engine speed sensor - Monitors engine speed, which is one of the factors used to calculate the pulse width

There are two main types of control for multi-port systems: The fuel injectors can all open at the same time, or
each one can open just before the intake valve for its cylinder opens (this is called sequential multi-port fuel
injection). The advantage of sequential fuel injection is that if the driver makes a sudden change, the system can
respond more quickly because from the time the change is made, it only has to wait only until the next intake
valve opens, instead of for the next complete revolution of the engine.

32
Engine Controls and Performance Chips
The algorithms that control the engine are quite complicated. The software has to allow the car to satisfy
emissions requirements for 100,000 miles, meet fuel economy requirements and protect engines against abuse.
And there are dozens of other requirements to meet as well. The engine control unit uses a formula and a large
number of lookup tables to determine the pulse width for given operating conditions. The equation will be a
series of many factors multiplied by each other. Many of these factors will come from lookup tables. Below is a
simplified calculation of the fuel injector pulse width. In this example, the equation will only have three factors,
whereas a real control system might have a hundred or more.

Pulse width = (Base pulse width) x (Factor A) x (Factor B)

In order to calculate the pulse width, the ECU first looks up the base pulse width in a lookup table. Base pulse
width is a function of engine speed (RPM) and load (which can be calculated from manifold absolute pressure).
Say the engine speed is 2,000 RPM and load is 4. We find the number at the intersection of 2,000 and 4, which
is 8 milliseconds.
RPM Load
1 2 3 4 5
1,000 1 2 3 4 5
2,000 2 4 6 8 10
3,000 3 6 9 12 15

4,000 4 8 12 16 20

In the next examples, A and B are parameters that come from sensors. Let's say that A is coolant temperature
and B is oxygen level. If coolant temperature equals 100 and oxygen level equals 3, the lookup tables tell us that
Factor A = 0.8 and Factor B = 1.0.
A Factor A B Factor B
0 1.2 0 1.0
25 1.1 1 1.0
50 1.0 2 1.0
75 0.9 3 1.0
100 0.8 4 0.75

So, since we know that base pulse width is a function of load and RPM, and that pulse width = (base pulse
width) x (factor A) x (factor B), the overall pulse width in our example equals:
8 x 0.8 x 1.0 = 6.4 milliseconds.
From this example, it can be seen how the control system makes adjustments. With parameter B as the level of
oxygen in the exhaust, the lookup table for B is the point at which there is (according to engine designers) too
much oxygen in the exhaust; and accordingly, the ECU cuts back on the fuel. Real control systems may have
more than 100 parameters, each with its own lookup table. Some of the parameters even change over time in
order to compensate for changes in the performance of engine components like the catalytic converter. And
depending on the engine speed, the ECU may have to do these calculations over a hundred times per second.

33
Performance Chips
Performance chips are made by aftermarket companies, and are used to boost engine power. There is a chip in
the ECU that holds all of the lookup tables; the performance chip replaces this chip. The tables in the
performance chip will contain values that result in higher fuel rates during certain driving conditions. For
instance, they may supply more fuel at full throttle at every engine speed. They may also change the spark
timing (there are lookup tables for that, too). Since the performance-chip makers are not as concerned with
issues like reliability, mileage and emissions controls as the carmakers are, they use more aggressive settings in
the fuel maps of their performance chips.

Direct Petrol Injection

As the name suggests, Direct Petrol Injection uses injectors that add fuel directly to the combustion chamber.
Like diesel engines, the air/fuel mixing occurs inside the combustion chamber, rather than in the inlet ports.
Taking this approach gives far greater control over the combustion process, allowing for a variety of
combustion operating modes, including those having ultra-lean air/fuel ratios.

System Mechanicals
The diagram on the left shows the
layout of the Bosch direct injection
system. Direct injection systems
differ from conventional port
injection in several ways.

The fuel supply system uses two fuel

pumps – a conventional electrical
fuel pressure supply pump (in the
past dubbed a high pressure pump
but now referred to in this system as
a low pressure pump) and a
mechanically-driven high pressure
injection pump. The low pressure
pump works at pressures of 3.0 5.0
Bar (0.3 – 0.5 MPa) while the high
pressure pumps boost this very
substantially to 50-120 Bar (5 – 12 MPa). The high pressure fuel is stored in the fuel rail that feeds the injectors.
The fuel rail is large enough to minimised pressure fluctuations as each injector opens. The pressure of the fuel
in the injector supply rail is controlled by an electronically-controlled bypass valve that can divert fuel from the
high pressure pump outlet back to its inlet. The fuel bypass valve is varied in flow by being pulse-width
modulated by the Electronic Control Unit (ECU). A fuel pressure sensor is used to monitor fuel rail pressure.

34
 The diagram on the left shows a cross-sectional view of an injector. Compared with
a conventional port fuel injection system, the fuel injectors must be capable of
working with high fuel pressures and also injecting a large amount of fuel in a short
period of time. The reason for the much reduced time in which the injection can be
completed is due to the fact that all the injection must sometimes occur in a portion
of the induction stroke. Conventional port fuel injectors have two complete rotations
of the crankshaft in which to inject the fuel charge – at an engine speed of 6000
rpm, this corresponds to 20 milliseconds. However, in some modes, direct fuel
injectors have only 5 milliseconds in which to inject the full-load fuel. The fuel
requirements at idle can drop the opening time to just 0.4 milliseconds. Direct
injection fuel droplets are on average only one-fifth the droplet size of traditional
injectors and one-third the diameter of a human hair. The very lean air/fuel ratios at
which direct injection systems can operate results in the production of large
quantities of oxides of nitrogen (NOx). As a result, direct injected cars require both
a primary catalytic converter fitted close to the engine, and also a main catalytic
converter - incorporating a NOx accumulator - that is fitted further downstream.

Combustion Modes
The really radical nature of direct fuel injection can be seen when the different combustion modes are
examined. There are at least six different ways in which combustion can take place.

Stratified Charge Mode

At low torque output up to about 3000 rpm the engine is operated in Stratified Charge Mode. In this mode the
injector adds the fuel during the compression stroke, just before the spark plug fires. In the period between the
injection finishing and the sparkplug firing, the airflow movement within the combustion chamber transports
the air/fuel mixture into the vicinity of the sparkplug. This results in a portion of relatively rich air/fuel mixture
surrounding the sparkplug electrode while the rest of the combustion chamber is relatively lean. The gas filling
the rest of the chamber often comprises recirculated exhaust gases which results in a reduced combustion
temperature and so decreased NOx emissions.

In Bosch direct injection systems, the air/fuel ratio within the whole combustion chamber can be as lean as 22:1
– 44:1. Mitsubishi states that total combustion chamber air/fuel ratios of 35 – 55:1 can be used. This can be
compared with a conventional port fuel injected engine that seldom uses an air/fuel ratio leaner than 14.7:1.

Homogenous Mode
Homogenous Mode is used at high torque outputs and at high engine speeds. Injection starts on the intake stroke
so there is sufficient time for the air/fuel mixture to be distributed throughout the combustion chamber. In this
mode Bosch systems use an air/fuel ratio of 14.7:1 (the same as with port fuel injection at light loads), while
Mitsubishi use air/fuel ratios from 13 – 24:1.

Homogenous Lean-Burn Mode

In the transition between Stratified and Homogenous Modes the engine can be run with a homogenously lean
air/fuel ratio.

Homogenous Stratified Charge Mode

Initially, this mode doesn’t seem to make sense – how can the combustion process be both homogenous and
stratified? However, what occurs is not one but two injection cycles.

The initial injection occurs during the intake stroke, giving plenty of time for the fuel to mix with the air
throughout the combustion chamber. Then, during the compression stroke, a second amount of fuel is injected.
35
This leads to the creation of a rich zone around the sparkplug. The rich zone easily ignites, which in turn ignites
the leaner air/fuel ratio within the remainder of the combustion chamber. Of the total fuel addition,
approximately 75 per cent occurs during the first injection and 25 per cent in the second. The Homogenous
Stratified Charge Mode is used during the transition from Stratified Charge to Homogenous Modes.
In addition there are at least two more modes – Homogenous
Anti-Knock and Stratified Charge Cat-Heating. The first is used
at full throttle and the second to rapidly heat the catalytic
converter to operating temperature. A final mode – mentioned
in only some of the literature – is Rich Homogenous Mode,
which is used to regenerate the NOx cat. (The NOx cat deposits
oxides of nitrogen in the form of NHO3 nitrates. When the cat
is regenerated, the nitrate, together with carbon monoxide, is
reduced in the exhaust to nitrogen and oxygen.)

The diagram above shows the two primary combustion modes – stratified charge and homogenous modes.

Electronic Control Systems

As was indicated earlier, the injectors must be opened against very high fuel pressures. So that this can happen,
a peak/hold strategy is employed where the opening current is very high and the ‘hold’ current much reduced. A
dedicated triggering module is used to control the injectors, with a booster capacitor providing 50 – 90 volts to
initially open the injector.

The sensing of how much gas is in the cylinder is more complex on a direct injected engine than a conventional
port injected engine. This is because at times recirculated exhaust gas forms a major component of the total
cylinder charge. As a result, two cylinder charge sensors are used. These comprise a conventional hot-film mass
airflow sensor (ie similar to a hot-wire airflow meter) and a manifold pressure sensor (MAP sensor). The flow
through the airflow meter is used as an input into the calculation of the pressure within the intake manifold and
this is then compared with the actual intake manifold pressure measured by the MAP sensor. The difference
between the two indicates the mass flow of the recirculated exhaust gas.

As with many conventional engine management systems, direct injection requires the use of an electronically-
controlled throttle. However, unlike conventional systems where the actual throttle opening more or less follows
the driver’s accelerator pedal torque request, in the case of direct injected engines, for much of the time the
throttle is fully open - engine torque output is instead regulated by varying the fuel delivery, just like a diesel.

At a certain point, which corresponds on an engine-specific basis to engine speed and the amount of torque
required, the engine changes to Homogenous Mode. With the change in modes, the throttle valve opening
becomes related to the driver’s torque request and the air/fuel ratio holds a constant stoichiometric air/fuel ratio
(that is, 14.7:1 or Lambda = 1) across the rest of the engine load range.

The system incorporates an operating-mode co-ordinator which maps operating mode against engine speed and
torque request. Before the selected combustion mode starts to occur, control functions for exhaust-gas
recirculation, fuel tank ventilation, charge-movement flap (ie port tumble valves or variable length intake
manifold), and electronic throttle settings are initiated as required. The system waits for acknowledgement that
these actions have been carried out before altering fuel injection and ignition timing.

The advantage of having the electronic throttle valve fully open at low loads is a large reduction in pumping
losses – the engine is no longer trying to breathe through the restriction of the nearly-closed throttle. However,
the downside of this approach is that the partial vacuum that is normally available for the servo-assisted brake
booster is no longer available. To overcome this problem, a vacuum switch or pressure sensor monitors brake
booster vacuum, and if it is necessary, the combustion mode is altered so that vacuum again becomes available.
36
In addition to the reduction in pumping losses occurring as a result of the throttle being wide open at low loads,
during Stratified Charge Mode thermodynamic efficiencies are also increased. This is because the rich cloud of
combustible air/fuel mixture around the sparkplug is thermally insulated by the layer of air and recirculated
exhaust gas that surrounds it. Together with the much leaner air/fuel ratios than can be used in a conventional
port injected engine, the result is a fuel efficiency improvement that can be up to 40 per cent at idle. Mitsubishi
state that at 35 km/h their direct injected engines use 35 per cent less fuel than a comparably sized conventional
engine and that in the Japanese 10-15 Urban Driving Cycle (albeit a slow speed cycle), the direct injected
engine uses less fuel than even a comparable diesel engine. During homogenous mode operation, both the use
of an air/fuel ratio that is never richer than 14.7:1 and the higher compression ratios normally associated with
direct injection engines result in a fuel saving of about 5 per cent.

Petrol Engine Exhaust Emissions

Exhaust emissions are strongly influenced by the air to fuel ratio of the mixture.
Defining lambda coefficient:

The air/fuel mixture may be classified as:

λ < 1: mixture is fuel rich; air is less than the
quantity required for a stoichiometric ratio
λ = 1: stoichiometric ratio of air/fuel
λ > 1: mixture fuel weak; air is greater than for a
stoichiometric ratio (lean mix).

The optimum air fuel ratio must be achieved to

reduce emissions, as the different behaviors of CO,
HC and NOX emissions vary according to the λ
value. The figure above shows how exhaust
emissions vary with the λ value. CO and HC
emission curves rise sharply in proximity of rich
mixtures; they reach a minimum for λ=1.05-1.1;
they tend to become higher for lean mixtures. NOX
Variation of CO, HC and NOx Against λ emissions have a different behavior, as they remain
high around the stoichiometric ratio, going down
37
both for rich and lean mixtures.

The shapes of these curves indicate the complexities of emission control; moreover, several difference can be
found between petrol and diesel engine. Petrol engines normally operate close to stoichiometric, or slightly fuel-
rich, to ensure smooth and reliable operation. With diesel engines the fuel is injected into the cylinder just
before combustion starts, so it is more sensible to fuel distribution; for this reason, it is generally preferable to
work with an excess of air to ensure the complete burn of the fuel.

Exhaust Emissions
Ideal combustion with stoichiometric mixtures results in theoretical exhaust emissions; under these conditions,
exhausts should contain only water vapor (H2O) and carbon dioxide (CO2), according to the following chemical
reaction;

Actual combustion takes place under different conditions from theory as it is impossible to have pure
hydrocarbon fuel droplets, due the presence of other constituents (like sulphur) that influence the fuel quality.
Pollutants are generally combustion sub-products, dangerous for environment and also for human life. The main
results are:
 Increase of toxic gases percentage per each life form;
 Production of carbon dioxide affecting the natural cycle of plants re-production;
 Change of the balance between energy absorbed and radiated from the earth (greenhouse);
 Damage of the plant and animal life with the discharge of harmful substances;
 Contamination of aquifers through chemical reagents

The motor vehicles are the major sources of air pollution in urban high traffic areas, especially with regard to
the layers of air closest to the ground. The majors responsible of environmental footprint can be considered
carbon monoxide (CO), nitrogen oxides monoxide (NO) and dioxide (N02), generally referred to as (NOX), a
variety of hydrocarbons from combustion or partial decomposition (called HC). Exhaust gases contain several
pollutants, such as solid particles carried in suspension by the exhaust gas (collectively called particulates),
oxidation products of fuel impurities (sulphur), carbon dioxide, etc,.

Carbon Monoxide (CO)

Carbon monoxide is the product of a combustion having low air to fuel ratio, this means that there isn’t enough
oxygen in the cylinder thus leading to an incomplete combustion of fuel. CO emissions are higher in gasoline
engines with respect to diesel ones, mainly due to the oxygen amount content in the mixture, higher in diesel
engines. Excess air allows complete combustion with oxidation of CO in CO2. CO is an odourless and tasteless
toxic gas which can cause damages to sight and muscular system and also impede oxygen transport in the blood
and this can cause asphyxiation. A concentration in excess, also minimally, can be fatal.

Nitrogen Oxides (NOX)

Nitrogen oxides, produced during combustion, are mainly nitric oxides (NO) and nitrogen dioxide (NO 2), with a
less relevant presence of nitrous oxide (N2O). NOX are produced by high temperature processes where air
containing nitrogen is burned. The zone of NOX formation is the burned gas zone behind the flame. The
production of NOX is higher in diesel engines than petrol gasoline engines; this is due to high air excess and
higher maximum temperature of combustion. Nitrogen oxides can cause irritation at eyes, nose and throat.
Moreover NOX form secondary pollutants reacting in atmosphere, dangerous for health, such as acid rain and
photochemical smog.

38
Unburnt Hydrocarbons
Hydrocarbons emissions in exhaust gas are a mixture of lubricating oil and hydrocarbons caused by incomplete
combustion particles in combination with new hydrocarbon compounds generated by chemical combinations
during the same combustion processes; alkynes, alkenes, alkanes and aromatics are the principal components.
Hydrocarbons emissions depend upon fuel composition and for this reason gasoline engines emit hydrocarbons
of lower molecular weight with respect to diesel ones. Hydrocarbons emissions follow the same behavior of
CO; they are influenced by the concentration of air in the combustion. Hydrocarbons irritate the mucous
membranes and also have narcotic effect. Hydrocarbons are involved in secondary pollutants reacting with
NOX.

Oxides of Sulphur
Sulphur dioxides (SO2) are due to sulphur contained in fuels. Like nitrous oxides, SO2 contributes to the
creation of acid rains reacting in atmosphere. However, this pollutant is limited by the use of low-sulphur
content fuels.

Particulates
Particulate is a peculiarity of diesel engine emissions mainly due to incomplete combustion of diesel fuel.
molecules), and dehydrogenation (to form molecules of a lower hydrogen/carbon ratio).

Secondary Pollutants
All pollutants that react in atmosphere after their emission can be classified as secondary pollutants. Ozone (O3)
and photochemical smog are the products of the reaction on atmosphere of NO X and hydrocarbons (especially
alkanes) at the presence of sunlight. Excess of ozone causes problem to respiratory apparatus, premature ageing
and also cause slows plant growth in nature. Acid rains are due to the reaction between acid gases produced as
a result of combustion and the water in atmosphere. These reactions lead to sulphurous, sulphuric and nitric
acids production. The precipitation of these substances can cause damage to building, death of forests and
acidification of waterways. Numerous pollutants also participate to global warming process, such as carbon
dioxide emissions. They contribute to this process with larger classes of other gases involved, called greenhouse
gases, that include tropospheric ozone O3, NOX and chlorofluorocarbons.

Petrol Engine Emission Control Catalyst

Catalysts are substances with the
Catalytic Converter ability to accelerate certain
chemical reactions in exhaust
gases. The aim of the catalytic
converter in a petrol engine
system is to reduce emissions of
the three types of pollutants in
exhaust gases: carbon monoxides
(CO), hydrocarbons and nitrogen
oxides (NOX).

In order to reduce NOX emissions

two types of catalyst are used:
NOX absorber catalysts and SCR
process; Selective Catalytic
Reduction units are applicable to
diesel engines.

The NOX absorber catalysts

employ two subsequent phases to
39
reduce this kind of emissions. First phase is the loading-phase: NO reacts with oxygen on the active oxidation
catalyst sites to form NO2. Chemical reactions increase with catalysts elements such as alkaline earths (Barium
(Ba), Calcium (Ca), Strontium (Sr), and Magnesium (Mg) or alkaline metals (potassium (K), Sodium (Na),
Lithium (Li), Cesium (Cs)) and other metals like Lanthanum (La) and Yttrium (Y) a storage of NO X arises.
Where the catalyst is Barium (Ba) the reactions are as follows:

The second stage involves the use of catalysts absorber to form inorganic nitrate. In such stage, called removal
and conversion, is instable and interacts with CO, as reducing agent, to form N 2,the reaction being
as follows;

One of the disadvantages of this method is the reactivity with any sulphur compounds present, with formation
of BaSO4.

Three-way Catalytic Converter

The three-way catalytic converter is considered one of the most effective form of emission control for petrol
engine. It is part of a bigger system of exhaust control and works in conjunction with the control system
associated with both petrol direct-injection engines and manifold-injection engines.

As the three-way catalytic converter

requires to work with approximately
stoichiometric air/fuel mixtures, it is
generally connected with an electronic
system able to switch data with the oxygen
sensor, so as to ensure an air/fuel mixture
at a value of λ=1.

The three-way catalytic converter is able to

simultaneously reduce three kind of
pollutants: carbon monoxide (CO),
hydrocarbons (HC) and nitrous oxides
(NOX). The optimal range of temperature,
both for high exhaust conversion and long
life service of the converter is set around
400 – 800°C; lower values (under 300°C)
or higher values (excess of 1000°C), make
the conversion ineffective. When the
engine starts from cold, the catalytic
converter does not start immediately, but it
remains ineffective for a few minutes until
its temperature reaches 300°C: this is
called light-off time. Local heating may be
applied to reduce the inactivity time.

40
The converter is made up with a steel casing, a substrate of ceramic or metallic monoliths, a washcoat substrate
coating (Al2O3), able to increase the effective surface of the catalytic converter by a factor of 7000. The core of
the reaction is assisted by the action of the active noble-metal catalysts, like platinum and palladium (to
accelerate the oxidation of carbon monoxide and hydrocarbons) and rhodium (nitrous oxides abatement).

Pollution Emission Regulations for Automobiles

The increment of pollutants favored by car emissions led governments to create specific rules in order to
safeguard the environment. European standards on emissions of pollutants are restrictions on emissions from
cars sold in the EU Member States. Standards are identified with the Euro-symbol followed by a number. The
European Community regulates common requirements for emissions by the introduction of several standards
that manufacturers are obliged to respect, upgrading previous production with new restrictions.
Since 1991, the European Union has issued a series of directives aimed at reducing environmental pollution
produced by light vehicles, precisely The Regulations covers vehicles with a reference mass not exceeding 2
610 kg. Based on these regulations, vehicles have been identified during the years in some categories associated
with the Euro symbol plus a series of numbers, which are growing from 0, related to the most polluting vehicles
matriculated before December 1992, until the current standard, indicated as 5, and the future enforcements,
respectively Euro 6.

In order to limit the negative impact of road vehicles on the environment and health, regulations covers a wide
range of pollutant emissions: carbon monoxide (CO), non-methane hydrocarbons and total hydrocarbons,
nitrogen oxides (NOX) and particulates (PM). It covers the tailpipe emissions, evaporative emissions and
crankcase emissions. The quantities are measured considering the mass quantity reversed in the environment (g
or mg) for each kilometer driven.

In this category are included all the gasoline vehicles without catalytic converters or
vehicles not eco-diesel. It is referred to cars registered before 31/12/1992, after which
Euro 0
the approval became mandatory class Euro 1. Since highly polluting, in many cities
they cannot move even apart from the blocks traffic (except for some exceptions such
as vintage cars).
The legislation take effects since 1993 and forced to equip new vehicles with catalytic
Euro 1
converters and use the fuel injection in gasoline engines.
Legislation is in force since 1997 and has imposed changes to reduce emissions
Euro 2
differentiated between gasoline and diesel.
This 3 legislation is in force since 2001 and has imposed the adoption of a system
Euro 3
called EOBD, to control the pollution control system.
The legislation is in force since 2006 and imposes even more stringent. Although in
Euro 4 some cases was already present on diesel Euro 3, the filter starts to spread on the Euro
4.
Cars registered after 1/1/2009, have to require the adoption of the diesel particulate
Euro 5
filter and also reduce the emission level of petrol cars.
The Euro 6 standard will come into force on 1 September 2014 for the approval of
Euro 6
vehicles, and from 1 January 2015 for the registration and sale of new types of cars.

41
Petrol Car Emission Limits

Chart above show the maximum concentration (in g/km) of different kinds of pollutants for each limit imposed
by the EURO regulations. For Euro 1 and Euro 2 NOX and HC emissions are taken together, while for the later
legislation, they have been divided into two different groups.

Over the years, emission maximum limits have been reduced to follow the international restrictions, with the
tendency of an overall pollution reduction began with the common agreement by all the industrial nations.

42
Fuel and Combustion Combustion
Nature of Fuels

Fuels for diesel engines are obtained from crude petroleum which, when distilled, yields gasoline,
kerosene, gas oil, diesel oil, fuel oil, lubricating oil stocks and waxes. All these oils are composed of
mixtures of hydrocarbons, each of which has a formula of the general form Cx Hy where both x and
y can range from unity to several hundreds. The lighter fractions in which x and y are small are
gases, for example Methane, the chief constituent of natural gas, has the formula CH,. The range
extends to heavy fractions where x and y are large and the molecular structure is complex.
Hydrocarbons are classified into groups having similar molecular structure and members of the
same group exhibit largely similar properties and patterns of behaviour during combustion.
The paraffins are one important group and include methane, ethane, propane and butane.

The paraffins occur naturally in petroleum but some of the other groups are produced by refinery
processes breaking down complex molecules into simpler ones. Such a process is "cracking" in
which the hydrocarbon is subjected to heat or a catalyst and which results in a greater yield of fuels
that are light or have special properties than would result from simple distillation. Most gasoline is
produced by cracking. Certain hydrocarbon groups occur solely or mainly in breakdown products, a
typical series being the Olefines having the formula C nH2 with n = 2 as the first number of the series.

The various fuels derived by distillation and other means consist of mixtures of hydrocarbons and
are described as lighter or heavier in relation to their neighbours without any well defined
boundaries. In fact there is considerable overlap between the divisions. A light fuel is more volatile
than a heavy one and has a lower viscosity and a lower specific gravity. The heaviest fuels are the
residuals which remain after the other grades have been removed by distillation. A heavy viscous
residual fuel may be thinned by mixing with it a small quantity of light fuel, the result being a
blended fuel of intermediate viscosity. The combustion properties of such blended fuels are usually
the same as those of the residual fuels from which they originate, the chief reason for the blending
being to reduce the viscosity to make them easier to handle. Crude petroleum is found in many parts
of the world and the proportions of the constituent hydrocarbons vary according to the source.

Although composed predominantly of carbon and hydrogen, liquid fuels may contain sulphur : up
to about 1 % in light diesel oils and up to 3% or 4% in heavy residual fuels. In combustion sulphur
oxides are formed which combine with moisture to form corrosive acids detrimental to the engine.
The energy from the combustion of sulphur is negligible and it is preferable for a fuel to contain as
little sulphur as possible.

Chemistry of Combustion
In complete combustion a fixed amount of fuel combines with a fixed amount of oxygen and
liberates a definite amount of energy as heat in the process. The overall process can be illustrated
using the basic chemistry of combustion of the elementary fuels, carbon and hydrogen, according
to their proportions in the fuel and regardless of their combination into hydrocarbons. Using the
usual chemical symbols and equations:

C + 02 → CO2 + 393.8 MJ/kmgl

2H2 + 02 → 2H20 + 261.06 MJ/kmol (liquid at 25°C)

The combined proportions by weight are obtained by considering the molecular weights
H2 = 2; C = 12; 02 = 32
thus 12 parts by weight of carbon combine with 32 parts oxygen to form 44 parts of CO2 with a
release of 393.8 MJ/Kmol.

43
C + 02 → CO2 + 393.8
12 + 32 → 44
and also
2H2 + 02 → 2H20 + 261.06
4 + 32 → 36
Suppose a. fuel to consist of 87% carbon and 13% hydrogen by weight. Then of 1 kilogramme of
this fuel 0 87 kilogramme would be carbon, the combustion of which would liberate
0.87 x 393.8/12 = 28.55 MJ.

The remaining 0.13 kilogrammes would be hydrogen and its combustion would liberate
0.13 x 261.06/2 = 16.96 MJ.

The gross calorific value of the fuel would thus be

28.55 + 16.96 = 45.51 MJ/kg.
The latent heat of steam
.
at 25oC is
.
583 cal/g. The mass of steam formed by .the combustion
. .
of 0 .13
kg of hydrogen = 0 13 x 36/4 = 1 17 kg and the latent heat of this steam = 1 17 x 2 441 = 2 85. The
net calorific value of this fuel would thus be
45.51 - 2.85 = 42.65 MJ/kg (18 240 BTU/lb).
The mass of oxygen required to burn 1 kilogramme of this fuel can be found as follows:
C + 02 → C02
12 + 32 → 44

0.87 +0.87 x 32/12 → 0.87 x 44/12

0.87 + 2.32 → 3.19

2H2 + 02 → 2H20

4 +32 → 36

0.13 + 0.13 x 32/4 → 0.13 x 36/2

0.13 + 0.975 → 1.17

Oxygen per kg of fuel = 2.32 + 0.975 = 3.295 kg.

This oxygen is provided as part of the air. Air is a mixture of oxygen and nitrogen with a small
proportion of carbon dioxide and rare gases which are usually considered as part of the nitrogen
content. The proportions by volume are 20-9% oxygen and 79-1% nitrogen and by weight 23-2%
oxygen and 76.8% nitrogen.

The air required per kg of fuel is therefore

3.295/0.232 = 14.15 kg.

44
Air/fuel Ratio
The exact proportion of air to fuel for complete combustion is known as the theoretical or
Stoichiometrical mixture and is expressed by weight for liquid fuels. For most diesel fuels it lies
between 14 and 14.5 to 1. Mixtures with less air than the theoretical are known as "rich" and those
with excess air are known as "lean" or "weak".
The diesel combustion process requires considerable excess air, air/ fuel ratios of 30 to 1 and more
are not uncommon. The carbon in the fuel burns completely to form carbon dioxide, COZ. There is
no partial combustion of carbon to form carbon monoxide, CO, as in the gasoline engine. Analysis
of the exhaust gases may be carried out, the results being based on volumes at room temperature as
is usual with gas analysis. The H2O in the exhaust gas will have condensed so that its volume is
negligible and the constituents will be CO2, 02 and N2, the first two being measured by absorption
and the last one usually being obtained by difference.

The proportions of these gases that are to be expected may be derived from a consideration of the
chemical equations together with Avagadro's hypothesis which states that equal volumes of all
gases at the same temperature and pressure contain the same number of molecules. Thus
C + 02 → CO2
1 volume + 1 volume → 1 volume
2H2 + 02 → 2H20
2 volumes + 1 volume → 2 volumes (steam)
Taking as an example the .fuel already used, of one hundred volumes of air used in combustion of a
stoichiometric mixture 20 9 volumes are oxygen.

2.32/3295 x 20.9 = 14.8 volumes of this oxygen combine with the carbon in the fuel.

Of the 100 volumes of air 79.1 volumes are nitrogen. As the theoretically correct mixture strength
has been assumed there will be no free oxygen and as already explained the H2O will have
condensed to a negligible volume of water, therefore the exhaust gas will occupy
14.8 + 79.1 = 93.9 volumes

The CO2 is therefore 14.8/93.9 = 15.7% of the exhaust gas and the N2 is 79-1/93-9 = 84.3% of the
exhaust gas. Now consider X% excess air to be supplied, that is the air to fuel ratio

= (1 + X /100) x theoretical.

The
.
air supplied = (100 + X) volumes and in the exhaust .
gas there will be 14.8 volumes of CO2,
79 1 volumes of N 2 associated with the burnt oxygen, (0 791 x) volumes of N2 associated with the
excess air and (0.209 x) volumes of 02. The exhaust gas will occupy a total of (93 .9 + X) volumes
and the fraction of CO2 will be given by

Y = 14.8/(93.9 + X)

thus if Y, the percentage of CO2 in the exhaust gas is obtained by analysis the excess air for this

particular fuel may be calculated from the following equation

X = (14.8/Y) – 93.9

45
The table below shows the constituent proportions and the theoretical air required for three grades
of fuel.

Light Diesel Oil Marine Diesel Oil Heavy Fuel Oil

. . .
Specific gravity 15 5°C 0 870 0 940 0.960
Analysis:
%C 87.1 86.1 86.1
%H 12.7 12.2 11.9
1.7
%S 0.2 1.4
0.3
% Ash - -

Viscosity:
Centistokes at 500C 2.8 15 200
Seconds Red. No.1 at 100°F 32 90 1800
Theoretical air/fuel ratio 14.41 14.16 14.1
Calorific Value
gross MJ/kg 45.22 43.96 43.54
net MJ/kg 42.32 41.45 41.03

Range of inflammability
For every hydrocarbon and for each fuel, being a mixture of hydrocarbons, there are limiting air
fuel ratios in both rich and lean regions, beyond which combustion is not self propagating. The
range between these limits is known as the range of inflammability. The limits vary very widely for
different hydrocarbons and are affected by temperature and pressure.
When fuel is injected into a diesel engine cylinder during the combustion process it mingles with
the air to form local mixtures that vary in strength, temperature and pressure throughout the
combustion chamber space and throughout the combustion process time. An exact knowledge of the
limits of inflammability is of little practical significance but their existence helps to account for the
completeness or otherwise of combustion under various circumstances.

Fuel Octane Number

If a petrol-air mixture is compressed sufficiently it will ignite spontaneously. This means that there
is a limit to the compression ratio if controlled combustion is to be obtained from spark ignition.
However, before this limit is reached for the whole charge, spontaneous ignition can occur in the
unburnt charge after combustion has commenced normally. The unburnt gas, compressed by the
advancing flame front, is raised in temperature and may reach the point of self-ignition. This
produces an uncontrolled combustion and its occurrence may be heard as a knocking sound. A
critical condition can be reached which is called detonation, or heavy knock. The advancing flame
front is suddenly accelerated by the occurrence of a high-pressure wave and the flame front and
shock wave traverse the cylinder together. The detonation wave suffers successive reflections, and a
high-frequency noise is created. This combustion phenomena is usually as knock. One of the results
of knock is that local hot spots can be created which remain at a sufficiently high temperature to
ignite the next charge before the spark occurs, known as pre-ignition, and this can help to promote
further knocking. The result is a noisy, overheated, and inefficient engine, and eventual mechanical
failure. There is a delay period between the occurrence of the spark and a noticeable rise in the

46
pressure curve from that of normal compression. This is a time delay which is independent of
engine speed so that as the engine speed is increased the spark point must occur earlier in the cycle
to obtain the best position of the peak pressure. This ignition advance can be accomplished
manually, but can also be controlled automatically by a timing advance mechanism in the
distributor which is sensitive to engine speed; an additional control is obtained at small throttle
openings by a pressure connection from the distributor to the induction manifold; the vacuum
advance mechanism. Modern petrol engines have electronic ignition systems which can be
controlled independently and as necessary.

The compression ratio which can be utilized depends on the fuel to be used and a scale has been
developed against which the knock tendency of a fuel can be rated. The rating is given as an
octane number. The fuel under test is compared with a mixture of iso-octane (high rating) and
normal heptane (low rating), by volume. The octane number of the fuel is the percentage of octane
in the reference mixture which knocks under the same conditions as the fuel.. High-octane fuels
(up to 100) can be produced by refining techniques, but it is done more cheaply and more
frequently by the use of anti-knock additives, such as tetraethyl lead. (An addition of 1.1 cc of
tetraethyl lead to 1 litre of 80-octane petrol increases the octane number to 90.) Levels of lead
now in the atmosphere due to the spark ignition engine are harmful to health and the use of
unleaded fuel is now applicable. Engine manufacturers have adjusted engine designs so that
unleaded fuel can be used without causing knocking. Lead also destroys the catalyst in the
catalytic converter and so is no longer used.

Cetane Number and Ignition Quality

The effect of compression ratio in the compression ignition engine is simpler than in the spark
ignition engine. For combustion to occur at the temperature produced by the compression of the air
a compression ratio of 12:1 is required. The efficiency of the cycle increases with higher values of
compression ratio and the limit is a mechanical one imposed by the high pressures developed in the
cylinder, a factor which adversely affects the power-weight ratio. The normal range of compression
ratios is 13:1 to 17:1, but may be anything up to 25:1. The combustible mixture in the spark ignition
engine is formed before compression, but with the compression ignition engine this mixture has to
be formed after compression and after injection begins. This leads to delay periods in the
compression ignition engine which are greater than those in the spark ignition engine; (The fuel
droplets injected have to evaporate and mix with oxygen to give a combustible mixture). The delay
period forms the first phase of the combustion process, and is dependent on the nature of the fuel.
The second phase consists of the spread of flame from the initial nucleus to the main body of the
charge. There is a rapid increase in pressure during this phase and the rate of pressure rise depends
to some extent on the availability of oxygen to the fuel spray, which in turn depends on the
turbulence in the cylinder. The main factor, however, is that of the delay period. A long delay
period means more combustible mixture has had time to form, and so more charge will be involved
in the initial combustion. As the speed increases the rate of pressure rise in this phase also increases.
This is because the delay period is a function of time if surrounding conditions remain constant, and
at the higher engine speeds more mixture will be formed in the delay period. The initial rapid
combustion can give rise to rough running and a characteristic noise called diesel knock. During the
third phase of combustion the fuel burns as it is injected into the cylinder, and this phase gives more
controlled combustion than that of phase two. One of the main factors in a controlled combustion is
the swirl which is induced by the design of the combustion chamber. It has been stated that the
delay period depends on the nature of the fuel, and a fuel with a short delay period, or high
ignitability, is required. The ignitability of a fuel oil is indicated by its cetane number, and the
procedure for obtaining it is similar to that for obtaining the octane number of petrol.

47
Exhaust Emissions
Engine emissions depend upon the fuel being burned but even if hydrogen was being burned in air there will
be harmful oxides of nitrogen (NOx) produced because of the high temperature combination of nitrogen and
oxygen in the combustion air. If fuel is burned with stoichiometric air then there should be no free oxygen
available for the formation of NOx but that is very difficult to produce and diesel engines generally are
supplied with excess air in order to ensure that all of the fuel will be burned. The diesel engine has a
problem in that it is compression ignition and fuel only mixes with the air in the cylinder so excess air is
supplied to ensure that there is always sufficient oxygen available near the fuel injector. Where fuel and air
are mixed outside of the cylinder as in the petrol engine it is easier to control the mixture and so it is
possible to get close to stoichiometric mixtures.

Emissions from a petrol engine are generally reduced by the use of a catalytic converter located in the
exhaust pipe. In a diesel engine systems such as direct water injection are used to lower the peak cylinder
temperature and so reduce the formation of NOx but this reduces operating efficiency. Setective catalytic
reduction may also be used but this requires the injection of ammonia or urea into the exhaust stream.

Pollutants Produced by a Car Engine

In order to reduce emissions, modern car engines carefully control the amount of fuel they burn. They try to
keep the air-to-fuel ratio very close to the stoichiometric point, which is the calculated ideal ratio of air to
fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For gasoline,
the stoichiometric ratio is about 14.7:1, meaning that for each pound of gasoline, 14.7 pounds of air will be
burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving.

Sometimes the mixture can be lean (an air-to-fuel ratio higher than 14.7), and other times the mixture can be
rich (an air-to-fuel ratio lower than 14.7).

The main emissions of a car engine are:

Nitrogen gas (N2) - Air is 78-percent nitrogen gas, and most of this passes right through the car
engine.
Carbon dioxide (CO2) - This is one product of combustion. The carbon in the fuel bonds with the
oxygen in the air.
Water vapour (H2O) - This is another product of combustion. The hydrogen in the fuel bonds with
the oxygen in the air.

These emissions are mostly benign (although carbon dioxide emissions are believed to contribute to global
warming). But because the combustion process is never perfect, some smaller amounts of more harmful
emissions are also produced in car engines:

Carbon monoxide (CO) - a poisonous gas that is colourless and odourless

Hydrocarbons or volatile organic compounds (VOCs) - produced mostly from unburned fuel
that evaporates

Sunlight breaks these down to form oxidants, which react with oxides of nitrogen to cause ground
level ozone (O3), a major component of smog.
Nitrogen oxides (NO and NO2, together called NOx) - contributes to smog and acid rain, and

48
 also causes irritation to human mucus membranes

These are the three main regulated emissions, and also the ones that catalytic converters are designed to
reduce.

Action of a Catalytic Converter

Most modern cars are equipped with three-way catalytic converters. "Three-way" refers to the three
regulated emissions it helps to reduce -- carbon monoxide, VOCs and NOx molecules. The converter uses
two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types consist of a
ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to
create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also
minimizing the amount of catalyst required (they are very expensive).

There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads.
Most cars today use a honeycomb structure.

The Reduction Catalyst

The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help
reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the
nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen
atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N2 + O2 or 2NO2 => N2 + 2O2

The Oxidization Catalyst

The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned
hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst.
This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas.
For example:

2CO + O2 => 2CO2

The Control System

The third stage is a control system that monitors the exhaust stream, and uses this information to control the
fuel injection system. There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is
closer to the engine than the converter is. This sensor tells the engine computer how much oxygen is in the
exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting
the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running
at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to
allow the oxidization catalyst to burn the unburned hydrocarbons and CO.

Alternative Ways to Reduce Pollution

The catalytic converter does a great job at reducing the pollution, but it can still be improved substantially.
One of its biggest shortcomings is that it only works at a fairly high temperature. When you start your car
cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust.

One simple solution to this problem is to move the catalytic converter closer to the engine. This means that
49
hotter exhaust gases reach the converter and it heats up faster, but this may also reduce the life of the
converter by exposing it to extremely high temperatures. Most carmakers position the converter under the
front passenger seat, far enough from the engine to keep the temperature down to levels that will not harm
it.

Preheating the catalytic converter is a good way to reduce emissions. The easiest way to preheat the
converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars
don't provide enough energy or power to heat the catalytic converter fast enough. Most people would not
wait several minutes for the catalytic converter to heat up before starting their car. Hybrid cars that have
big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly.

Tailpipe Emission Standards (USA)

“Tailpipe” emission standards specify the maximum amount of pollutants allowed in exhaust gases
discharged from a diesel engine. The tailpipe emission standards were initiated in California in 1959 to
control CO and HC emissions from gasoline engines. Today, emissions from internal combustion engines
are regulated in tens of countries throughout the world. The regulated diesel emissions include:

Diesel particulate matter (PM), measured by gravimetric methods. Sometimes diesel smoke opacity
measured by optical methods is also regulated. Nitrogen oxides (NOx), composed of nitric oxide (NO) and
nitrogen dioxide (NO2). Other oxides of nitrogen which may be present in exhaust gases, such
as N2O, are not regulated.

Hydrocarbons (HC), regulated either as total hydrocarbon emissions (THC) or as non-methane

hydrocarbons (NMHC). One combined limit for HC + NOx is sometimes used instead of two separate
limits.

Carbon monoxide (CO).

Emissions are measured over an engine or vehicle test cycle which is an important part of every emission
standard. Regulatory test procedures are necessary to verify and ensure compliance with the various
standards. These test cycles are supposed to create repeatable emission measurement conditions and, at the
same time, simulate a real driving condition of a given application. Analytical methods that are used to
measure particular emissions are also regulated by the standard.

Emission cycles are a sequence of speed and load conditions performed on an engine or chassis
dynamometer. Emissions measured on vehicle (chassis) dynamometers are usually expressed in grams of
pollutant per unit of travelled distance, e.g., g/km or g/mi. Emissions measured according to an engine
dynamometer test cycle are expressed in grams of pollutant per unit of mechanical energy delivered by the
engine, typically g/kWh or g/bhp/hr. Depending on the character of speed and load changes, cycles can be
divided into steady state cycles and transient cycles. Steady state cycles are a sequence of constant engine
speed and load modes. Emissions are analysed for each test mode. Then the overall emission result is
calculated as a (weighted) average from all test modes. In a transient cycle the vehicle (engine) follows a
prescribed driving pattern which includes accelerations, decelerations, changes of speed and load, etc. The
final test results can be obtained either by analysis of exhaust gas samples collected to plastic bags over the
duration of the cycle or by electronic integration of a fast response, continuous emission measurement.

Regulatory authorities in different countries have not been unanimous in adopting emission test procedures
and many types of cycles are in use. Since exhaust emissions depend on the engine speed and load
conditions, specific engine emissions which were measured on different test cycles may not be comparable
50
even if they are expressed or recalculated into the same units of measure. This should be kept in mind
whenever comparing emission standards from different countries. Tailpipe emission standards are usually
implemented by government ministries responsible for the protection of environment, such as the EPA
(Environmental Protection Agency) in the USA. The duty to comply with these standards is on the
equipment (engine) manufacturer. Typically all equipment have to be emission certified before it is released
to the market.

Occupational Health and Safety Standards

Applications of diesel engines in confined spaces are regulated through occupational health and safety
ambient air quality standards rather than (or in addition to) the tailpipe regulations. The ambient air quality
standards specify maximum concentrations of air contaminants called Permissible Exposure Limits (PEL)
which are allowed in the workplace.

Gases found in diesel emissions including carbon monoxide, nitric oxide, nitrogen dioxide, sulphur dioxide
and many other compounds have their PELs set by occupational health and safety authorities. Diesel
particulate matter has been also listed by a growing number of occupational health and safety standards as a
toxic air contaminant.

These regulations are set and enforced by occupational health and safety authorities such as OSHA
(Occupational Safety and Health Administration) in the USA. The duty to comply is on the end-user
(warehouse operator, mine operator, etc.) who has to make sure that the emission control measures which
have been employed are adequate to the type and number of polluting equipment. Engine or equipment
manufacturers do not have any direct obligations in regard to the
occupational health and safety air quality standards.

Large Diesel Engine Emissions

The Problem
The burning of fuel creates exhaust gases which vary with the nature of the fuel and the quality of its
combustion. Fuels as burnt in boilers and internal combustion engines essentially consist of hydrogen and
carbon but impurities can also be present the most common of which is sulphur; other impurities include
sodium, vanadium and aluminium but these tend to cause the formation of deposits within the engine or
boiler rather than damaging emissions. Problems are similar for internal combustion engines and boilers but
much depends upon the type and quality of the fuel, coal having certain problems which oil fuel does not.
Gaseous fuels will also produce emissions which may be harmful but they do not result in solid particulates
entering the atmosphere. I.C. engines require cylinders to be lubricated in order to minimise wear and some
of this oil will burn in the cylinder; how this influences the quantity and type of emissions depends upon the
nature of additives in the cylinder oil and because these are neither constant nor in many cases known they
will be ignored in this instance.

Carbon in a fuel should burn completely to form carbon dioxide (CO2), if it does not then there is a loss of
energy and carbon monoxide (CO) is formed. Both are damaging gases but little can be done about CO if
fossil fuels are to be burned. CO2 is a greenhouse gas which means that it can built up in the atmosphere and
prevent the escape of heat from the earth thus resulting in overheating of the planet. Some effect of this 2
nature is required in order to allow life to develop on the planet but if the balance is disturbed by excessive
CO2 in the upper atmosphere then overheating may result; the mean temperature of the planet only has to
rise by a few degrees in order to cause melting of ice caps which will result in a rise in ocean levels with
consequent flooding of low lying ground. A rise in the general temperature will also cause changes in
weather patterns and increased risk of storms. Because even the most efficient combustion of fuels
containing carbon result in the formation of CO2 the only real solution is to reduce the combustion of fossil
fuels.
51
Carbon monoxide is toxic and should be avoided by good combustion; CO 2 is not toxic but will not support
life and so excessive quantities in confined spaces can be damaging.

Oxides of Nitrogen (NOx) are produced when nitrogen is heated to high temperature in the presence of
oxygen. This situation occurs during fuel combustion as there is nitrogen in the air supplied for combustion
and excess air, hence excess oxygen, is supplied to ensure complete combustion takes place. There are
various oxides of nitrogen, Nitric oxide, nitrous oxide, etc. but they are all damaging and are grouped under
the general heading of NOx. This substance is an irritant and damages lung tissue resulting in respiratory
ailments. It also contributes to acid rain. Although nitrogen and oxygen are present during the combustion
process the formation of NOx depends upon the maximum temperature reached and so the combustion
process itself directly influences the formation of NOx.

Sulphur Dioxide (SOx) is produced when sulphur burns and when combined with H2O, water vapour,
formed by the burning of hydrogen, Sulphur Trioxide (H2SO3) is produced which results in acid rain and
causes respiratory ailments in excessive concentrations. The main way of minimising this problem is to
switch to fuels which contain little or no sulphur but that is not always easy.
Under the action of ultra violet light unburnt hydrocarbons will break down and produce a photo-chemical
smog which is both damaging to health and restricts the passage of sunlight. (Los Angeles is subject to such
smog almost perpetually due to the high use of automobiles and the plentiful sunshine). Hydrocarbons also
irritate the eyes.

Other undesirable emission include Hydrogen Chloride (HCl) but the level of this depends upon impurities
in the fuel. Particulates can give rise to bronchitis, asthma and pulmonary emphysema.

Oil Fired Internal Combustion Engines

The burning of residual fuels as in marine practice is more problematic than the burning of diesel oil or gas
oil; such distillate fuels generally contain very low levels of impurities which result in an absence of certain
pollutants such as SO2. Oil fuels of all types do, however, result in the formation of certain emissions and so
the problems involving residual fuels will be considered.

Unburnt Hydrocarbons Emission of these into the atmosphere not only results in pollution but also
reduces operating efficiency of the engine thus it is in the interest of the operator to reduce levels of unburnt
hydrocarbons (HC) to a minimum. Also though fuel oil is the main source lubricating oil also contributes to
the emission of unburnt hydrocarbons. Although catalysts are used for reduction of unburnt hydrocarbons in
automobile engine exhaust these are not economically practical for large engines and the best solution is
control of the engine`s combustion process. This involved adjustment in timing and in the air supply; lean
burn engines have a higher air:fuel ratio than is normally used and this assists in regulating the hydrocarbon
emissions. Careful control of engine performance is important in keeping levels reasonable but if more
stringent regulations concerning HC are introduced Selective Catalytic Reduction (SCR) by means of such
converters will be the only way of dealing with the problem.

CO This is also a function of efficient engine combustion as complete conversion of carbon into carbon
dioxide will eliminate the problem. Sufficient time for complete combustion and an adequate air supply are
required. Engine maintenance and monitoring will minimise the development of conditions which result in
inefficient combustion.

CO2 Formation of this is a function of burning carbon based fuels and it cannot be reduced unless the
amount of carbon in the fuel is reduced.

SOx Combustion control cannot reduce levels of SOx in the exhaust gas and the only way to do so is by
burning fuels having a lower sulphur content. Such fuels are available but at very high cost.

52
Smoke/Particulates These are a function of combustion and the nature of the fuel used. Changes in
combustion conditions employed to reduce other emissions tend to reduce combustion efficiency which in
turn increases smoke and particulate formation. Increasing fuel injection pressure and promoting better
mixing of air and fuel in the combustion chamber result in improved, and more complete, combustion which
leads to a lower level of smoke and particulates. Lubricating oils contribute to particulate emissions and
control of cylinder lubrication is the only way of minimising such problems.

Post treatment of engine exhaust can reduce levels of emissions and this is done in some cases where engine
exhaust is used for inert gas supply to cargo tanks. Scrubbers and absorbtion units together with catalytic
converters are of benefit but they are costly and restrict the flow of exhaust from the engine thus producing
inefficient operation. Such devices are classed as secondary measures, the primary measures are those
concerned with cylinder operation.

Oxides of Nitrogen (NOX) These are formed during combustion and result from the chemical
combination of oxygen and nitrogen at high temperature. As both of these gases are in the atmosphere their
formation is unavoidable. Two-stroke cycle engines are optimised for high efficiency and this means high
maximum cylinder temperatures which in turn result in high levels of NO X formation. NOX contributes to
the formation of “Smog” and acid rain and they also cause damage to lung and other delicate tissue.
Reduction may be by engine based, or primary, methods or by secondary methods which is effectively the
fitting of a Selective Catalytic Reduction (SCR) unit. Primary methods involve reducing maximum cylinder
temperature by delaying fuel injection or through the use of emulsified fuels or water injection. These
systems tend to increase fuel consumption and hence CO2 emissions. SCR requires the fitting of a catalytic
unit in which the NOX is reduced to harmless components through reaction with a chemical injected into the
exhaust gas flow.

Control of NOX emissions may be by primary methods (engine based) or by secondary methods (acting on
the exhaust gas), or a combination of the two. NOX emissions occur even with the highest quality fuel
because they are formed by high temperature combination of oxygen and nitrogen, both of which exist in
the atmosphere. Lowering the maximum cylinder temperature can reduce the rate at which NO X form and
this is the aim of most primary methods. Delaying cylinder fuel injection, injection of water or exhaust has
recirculation all lower the maximum cylinder temperature and so reduce the formation of NO X but the
reduced peak temperature also results in a reduction in engine efficiency and an increase in specific fuel
consumption; this also increases the amount of carbon dioxide produced (and oxides of sulphur depending
upon the sulphur quantity in the fuel). This is known as the Diesel Dilemma and it basically means that you
cannot have a reduction in all engine emissions from purely primary means.

Primary means of NOX reduction are as below and these have been used mainly for medium speed engines.

Exhaust Gas Recirculation (EGR) is a method of reducing NOX formation where some of the exhaust gas
is drawn back into the engine thus reducing the percentage of oxygen in the “air” charge drawn into the
cylinders. This means that there is less oxygen available for combustion and the formation of NO X but
because the fuel has first call on the oxygen it is employed for combustion leaving less available for the
formation of NOX. There is a full charge of “air” in the cylinder which is heated but combustion is slower
because of the reduced oxygen content hence the maximum cylinder temperature is lower and less NO X
forms. Care must be taken to ensure that the amount of exhaust gas recirculated is kept under control
otherwise cylinder combustion will suffer.

The injection of water into the cylinder, known as fumigation, has the effect of reducing the maximum
cylinder temperature because some of the energy released during combustion of the fuel must be employed
in evaporating the water and even dissociating the steam. With a lower maximum temperature less NO X is

53
generated but energy from the expanding steam and possible subsequent combining of the oxygen and
hydrogen from dissociation allow some of the energy imparted to the water to be reclaimed. The quantity of
water injected must be carefully regulated otherwise there can be adverse effects on performance. The use
of emulsified fuel has a similar effect to water injection.

Control systems for EGR and water injection must be monitored in order to ensure that the systems only
allow the correct quantity of exhaust gas or water into the cylinder. If control is not exact damage to the
engine may result and the NOX emission level will not be reduced.

Other emissions include smoke and particulates which result from poor combustion, ash in the fuel, cylinder
oil and deposits coming loose from the combustion chamber or exhaust gas system.

STRATIFIED CHARGE ENGINES

Although there remains a great deal of flexibility in the design of diesel engines the same cannot be said
of the spark ignition. This is primarily because the fuel-air mixture is more or less homogeneous at the
time of spark. Engine manufacturers have long recognized this and thus have explored ways to produce a
heterogeneous mixture at the time of spark. Thus stratified charge engines have been developed. But they
only became cost effective in certain applications because of changing constraints on engine design
imposed to control emissions.

Like diesel engines, stratified charge engines have two basic configurations: open chamber and divided
chamber. The Honda engine shown below is typical. The Honda engine has three valves. A rich premixed
fuel-air charge is inducted into the prechamber and a lean premixed fuel-air charge is inducted into the main
chamber. This flow and subsequent compression sets up a distribution in the fuel-air ratio varying from rich
at the spark plug to lean at the main chamber valves, as the diagrams indicate. There are more degrees of
freedom in the design of this engine than in a conventional spark ignited engine. These include: ratio of
prechamber to clearance volume, cross-sectional area of passage throat, ratio of prechamber to main
chamber fuel-air ratio, third valve size and timing and passage angle.
These extra degrees of freedom allow lower emissions and a lower octane requirement (i.e., gasoline less
resistant to autoignition is required) with no sacrifice in indicated thermal efficiency.
The extra degrees of freedom afforded in the stratified charge engine show relative to conventional
spark ignition engines include:
 fuel injection timing
 fuel nozzle orientation and number of holes
 distance between injector and spark plugs
 ratio of cup volume to clearance volume
 fuel pressure
Again, these extra degrees of freedom allow construction of a better engine. In fact, dependent upon the
fuel-injection timing, the engine can operate anywhere from more or less homogeneous combustion to
combustion as heterogeneous as in a diesel engine.

If, for example, fuel is injected early in the intake stroke, there is plenty of time for the fuel to evaporate and
mix thoroughly with the air prior to spark ignition. But if the fuel is injected later when it is intended to
burn, then no more time for mixing is available than in a diesel engine. With late injection the engine is a
multifuel engine since autoignition is not relied upon to start the combustion (ignition delay is fixed by the
54
time required for fuel to traverse from the injector to the spark plug) and combustion can be limited by the
rate of fuel injection. If a high quality diesel fuel is burned in the late injection stratified charge engine such
that ignition delay is less than the time required for fuel to reach the spark plug, no problem is created since
the fuel will autoignite and subsequently burn at a rate limited by the fuel injection. If a poor quality diesel
fuel is burned, such as gasoline or, worse yet, methanol, still no problem arises since the spark limits the
ignition delay, thereby preventing a large amount of fuel from burning rapidly once autoignition occurs, as
would be the case in a diesel engine.

Stratified Charge Engine Arrangement

In fact, a very appealing piston engine vehicle can be constructed. Powered by a direct injection stratified
charge engine fueled with methanol, it would have the advantages of a diesel engine, high compression ratio
and no air throttling, but not the disadvantage of a smoke limited bmep because there is no smoke limit due
to the presence of oxygen already in the fuel (CH3OH). However, one must recognize that consumers try to
maximize kilometers traveled per dollar rather than per liter. Although it is very fuel efficient, this car
would not sell if the manufacturing of the fuel were inefficient and therefore expensive. To date the
tradeoffs involved have not favored such construction.

55
 Turbo-Charging
In order to burn fuel air must be supplied to the cylinder and with a two stroke engine air under pressure is needed in
order to remove the exhaust gas when scavenging. The higher the air pressure the greater the quantity of air in the
cylinder and hence the greater the amount of fuel which can be burnt. That allows for increased power development.
This is pressure charging.

Gas Inlet
Gas Outlet

Turbocharger Arrangement

Pressure charging can be obtained by a number of means including scavenge pumps, chain driven rotary blowers and
exhaust gas turbine driven blowers. Theoretically gas in the cylinder can be expanded further allowing more power to
be developed and that power used to drive scavenge pumps or chain driven blowers. In practice it is more efficient to
use exhaust gas to drive turbo-chargers because further expansion in a cylinder requires an increased stroke. Increased
stoke poses problems with crankshaft construction, cylinder lubrication, effective scavenging and engine height.
Expansion of low pressure gas in the cylinder is not really effective because energy recovery from such low pressures is
minimal in a reciprocating machine due to the fact that frictional losses are involved.

Low pressure expansion, and compression, is more efficiently achieved in a rotary machine hence the use of turbo-
chargers. Increasing the pressure increases the air density, hence pressure charging. However, if the temperature
increases by the same amount then there is no actual change in density. Cooling of the air down to the lower acceptable
temperature allows maximum density to be achieved. In practice there is a limit to cooling as too low a temperature can
lead to thermal shock within the cylinders with certain engines. Cooling below the dew temperature leads to the
formation of water droplets and if these enter the cylinder they rub the oil film from the liner causing increased wear. If
air temperature is too high then maximum power might not be available because of reduced air mass and because high
air temperature leads to high exhaust temperature and there is always a limit on maximum exhaust temperature.

Cooling of the air to an optimum temperature provides the best option but if that means cooling below the dew
temperature then there are problems. The ideal solution is to fit a water separator in the air trunking between
cooler and scavenge space in order to remove any water which might form. The simplest form is the grin type
separator which consists of a number of blades placed at an angle to the air flow. Air hitting these blades is simply
deflected but water droplets have a higher density and hence a higher inertia and so cannot

56
change direction easily. They run along the face of the blades to the gutters down which they fall to the
condensate trough. From here they are removed from the scavenge space via a trap. Two rooms of blades
restore axial air flow and can remove up to 85% of water droplets. The grid type separator requires no
actuating energy (it is static), occupies very little space and offers only a small resistance to air flow.
This arrangement allows maximum air density to be achieved and also avoids problems related to water entering
the cylinder.

Two sets of
Blades

Water
Air Flow

Grid Type Water

Separator

The scavenge and exhaust period on an engine may be divided up into three separate periods;
1. Blowdown: Exhaust only is open and cylinder pressure falls to or below the scavenge pressure.
2. Scavenge: Incoming air forces out remaining exhaust gas. Types of scavenging systems need to be taken into
account in overall design. Uniflow scavenging is the most efficient especially for long stroke engines but it does
require the use of exhaust valve or opposed piston design.
3. Post Scavenge: Exhaust only is open and some air is lost from the cylinder. Not ideal but has to be accepted from a
constructional point of view if there is to be a blowdown period. Use of rotary exhaust valve (Sulzer RD),
advancing top piston or timing exhaust valve can reduce post scavenge.

57
Blower Systems
There are two basic blower systems, the Pulse system and the Constant pressure system. Variations of these exist and
sometimes the Pulse system has been referred to as a constant volume system.

The Pulse system was the first type introduced and this makes use of the high pressure energy in the exhaust gas when
exhaust commences. With early arrangements exhaust was often commenced earlier in the stroke in order to provide
sufficient energy to drive the turbo-charger. The high pressure pulse energy falls by time scavenge commences and then
remains at about the same pressure until exhaust closes. Turbines must, therefore, be designed to deal with wide ranges
of gas pressures and are not very efficient because of this. An exhaust period lasts for about 180 degrees of crank
movement and so three periods can be conveniently arranged without exhausts from cylinders interfering with each
other. If two cylinders were open to exhaust at the same time then the pulse from one might be directed back into the
exhaust line of another during its post scavenge period (when scavenge has closed and exhaust is still open) thus
leading to possible operational defects due to poor scavenging and combustion. For engines having multiples of three
cylinders it is convenient to arrange a pulse system but for other than multiples of three cylinders problems can occur.

This simple answer is to connect blowers with up to three cylinders such as a five cylinder engine with two blowers one
fed by three cylinders and the other by two. That is practical and possible but it requires different sized blowers in
terms of impellers, blade discs and nozzle rings to be provided. A more acceptable situation is to provide a balanced
system. This has each blower being supplied with the same amount of gas allowing them to be of the same size. Such
an arrangement needs split entry blowers where gas pulses do not mix until after the blower or divided gas flows with
gas from any particular cylinder being split between two blowers so that all blowers receive the same amount of gas.

Balanced Pulse System

{Split entry turbocharger}

58
Constant pressure systems have all cylinders supplying exhaust gas to a single large volume exhaust manifold which
then supplies the turbocharger(s). The system is more efficient because exhaust can be commenced later in the stroke as
pulse energy is not required. This gives a saving in expansive cylinder working. Turbines can be designed to operate
under steady gas conditions rather than the wide range of gas conditions met in a pulse system thus they are more
efficient. At brake mean effective pressures above about seven bar the constant pressure system is better than the pulse
system as it can supply more scavenge air and is more efficient. Theoretically only one turbocharger needs to be
supplied with any number of cylinders for a constant pressure system but in practice two or more are often fitted for the
following reasons :

1. Mechanical breakdown,
2. Mounting problems for a large and very heavy blower,
3. Cylinders furthest away from the blower would receive air at reduced pressure thus leading to cylinder power
imbalance.

Constant Pressure
Turbocharging

Because of the wide range of gas conditions involved pulse system blades are of thicker cross section and have well
rounded noses in order that the gas may enter the blades without shock due to the variation in gas velocities. Constant
pressure blades meet with lower maximum pressures so they may be thinner in cross section. In addition the gas flow
is steady over the working cycle and so blades may have sharper noses because relative gas velocity remains almost
constant.

Bearings
Most main engine turbochargers are water cooled in order to keep temperatures reasonable. Some uncooled blowers
are available but they tend to be of the smaller types. In fact they are cooled by air circulation and so design allows
temperatures to remain relatively low and within the capabilities of the construction material. As no cooling jacket is
required the bearings may be placed between blade disc and impeller allowing for better rotor support but in most
blowers bearings have to be positioned at the ends of the shaft because of the cooling water jacket. Bearings must
always be in the coolest position and easily accessible because they require more frequent attention than other items.

Bearings must allow the shaft to rotate and they must be of sufficient area to give maximum support. Plain white metal
sleeve bearings have an indefinite life provided that they are lubricated correctly but they need oil to be supplied under
pressure and that requires a pump system. In order to provide safeguard in the event of pump failure an external oil
supply system is used with header tanks giving a backup supply to allow time for another pump to be started or the
blower to be brought to rest with the engine stopped. Pumps usually take the oil from the bearing sumps and pass it to
the header tank, coolers keeping the temperature with set limits. With the engine at rest natural draught can set the rotor
turning and so it must either be locked or the oil supply maintained in order to prevent damage to the bearings.

59
 Oil Radial & Axial Deflector Damping Springs

Turbocharger Ball Bearing Arrangement

Ball or roller races only require boundary lubrication and that may be supplied by means of a shaft driven gear pump
from an integral sump. Whenever the shaft turns oil is supplied to the bearings, provided that there is some oil in the
sump. A water jacket surrounding the sump keeps that oil relatively cool. Ball races have a definite life of about 15,000
hours in order to avoid failure due to fatigue or brinelling, thus regular replacement is needed and so bearings must be
reasonably accessible. In order to dampen out transmission of vibration from casing to rotor through the ball races radial
and axial damping springs are provided. These consist of leaf type spring elements wrapped around the bearing and
fitted at the bearing ends.

An axial thrust exists due to the gas force on the turbine blades and this must be balanced. (The impeller is of the radial
type and so does have a pressure drop across its faces thus no axial force is produced by the impeller). A thrust bearing
is required to balance the axial thrust from the turbine and this can be provided by the ball races if they are large enough
or by miniature michell type bearings. Although the gas thrust only acts in one direction a double faced thrust is
provided. This prevents contact by rotating part with the casings whenever the ship pitches and so keeps the rotor firmly
located in position axially.
Labyrinth seals are provided at each end of the rotor and between turbine and impeller in order to prevent the passage of
gas and avoid oil laden air being drawn through the bearing into the eye of the impeller. Oil seals in the form of thrower
plates are also fitted at bearings to prevent the passage of oil along the shaft. Generous oil quantities are supplied to
bearings in order to allow for cooling as well as lubrication.

Bled Air

Pi>P1>P2>P3>P4>P5>P6>P7>P8>

Labyrinth Seal Arrangement

60
Labyrinth seals consist basically of a number of projections from the casing almost touching the rotor. Any gas, or air,
passing through the narrow gap between a projection and the rotor will expand into the large volume chamber contained
between adjacent projections and so will be subject to a pressure reduction. This means that chambers of the labyrinth
will be maintained at successively lower pressure and that prevents leakage down the rotor as at some point the pressure
will be the same as atmospheric. Bled air under pressure is fed to a point near the middle of the labyrinth and this
expands both ways and ensure a very effective seal. Care must be taken to ensure that deposits do not build up and block
the labyrinth chambers as that will cause defective sealing and may lead to a binding effect on the shaft. At the end of
the rotor on the gas side the labyrinth prevents gas escape into the bearing whilst on the air side it avoids air being
drawn in through the bearing and hence drawing oil with it. Between impeller and blade disc a labyrinth is provided to
allow a controlled air leakage from aid side to gas side. That effects sealing and the air tends to cool the shaft
minimising heat transmission along the rotor from gas to air sides.

Construction
A turbocharger may be considered as two separate sections, gas side and air side. The gas side is made from cast iron,
is in two parts and generally water cooled. The turbine inlet casing carries the nozzle blade shroud ring and forms the
bearing housing. The turbine outlet casing forms the main part of the blower which includes the mountings. In addition
it forms a shroud for the shaft and contains bled air passageways for supplying air to labyrinth seals.

Compressor casing is also in two parts but is made from aluminium alloy. The inlet casing may be arranged to draw
air from the engine room or from deck but in either case it provides connection for the air filter and is lined with noise
insulating material. Compressor outlet casing is of a special volute shape which converts velocity energy into pressure
energy; for larger blowers diffuser blades may also be fitted within the casing to assist in this energy transformation.
Outlet casings also provide the bearing housing.

Rotors must be capable of maintaining strength at high temperature and so the material used is usually a chromium
steel. The rotor for a small blower may be a single piece forging but for large blowers it may consist of separate
sections of shaft and blade disc, bolted connections being provided at the blade disc. Blades must also be capable of
maintaining strength at high temperatures but should also have some degree of corrosion resistance. Stainless steel is
frequently used for blade construction. Blades are mounted axially in the disc using inverted fir-tree roots or similar,
e.g. "T" piece or cylindrical roots. Locking strips are provided to prevent axial movement of the blades in the disc due
to the axial gas force acting, such locking strip being like tab washers or rivets. Blades are not force fits into the disc but
the fitting is relatively "loose". This means that there is no stress on the root because of its fitting. Thermal expansion
and centrifugal stress are applied when the blower operates and if there was further stress from force fitting the
possibility of yielding would exist. To avoid this the "loose" fitting is used.

Loose fitting also helps to dampen out blade vibration which is caused by gas pulses from the nozzle blades especially
if these are partially blocked. For larger blades lacing wire is the means of damping out vibration. A frictional damping
capacity exists between the blade through which the wire passes and the wire itself. Wire is fitted about 1/3rd of the
way down from the blade tip and may group several blades together or pass completely around the entire disc.
Crimping of the wire prevents its rotation. Damping due to friction and stiffening up because of connection of a
number of blades avoid vibration. The main problem with lacing wire, usually of wrought iron, is that it breaks and
sections fall out resulting in an unbalanced rotor. Balance of the rotor is essential in order to avoid vibration and blade
damage due to impact, corrosion, erosion and deposit build-up may all cause problems.

61
 62
Arrangement
For the air side this usually consists of injecting a limited quantity of water into the eye of the impeller, the water
droplets then wipe the oily film from the surface but often deposit this on the cooler from where it must also be
removed. If heavy deposits do form on the impeller and volute casing the risk of surging increases. On the gas side
deposits may be very heavy and corrosive but regular cleaning is required in order to keep them within limits so as to
avoid vibration, fall off in performance and possible surging. The usual in service cleaning method for blower gas side
employs water but it is also possible to make use of ground rice or walnut shells. Whichever method is used care must
be exercised.

In service water washing required the blower speed to be reduced to half or below in order to avoid impact damage
by the water droplets.

Casing drain is opened and water injected into the gas flow before the blower. The flow rate of water is controlled by
means of an orifice in the line and in some cases air operated atomisers are used. The basic principle is that fine droplets
of water impact with the blades producing a shock blasting effect to remove deposits. How long the water is applied
depends upon the nature and amount of deposit but in service cleaning should not be used with heavily fouled turbines
as the risk exists of only partial deposit removal which can give rise to vibration damage. Rotor vibration can effect all
parts but bearings are particularly susceptible. A check on the condition of water flowing through the casing drain will
give a reasonable idea as to the amount of deposit still being removed; clean water indicates a relatively clean blower.
With the water shut off blower speed should be increased to normal gradually over about a half hour period. This avoids
temperature problems and minimises the risk of excessive vibration if deposits have only been partially removed. Dry
cleaning using ground rice or nut shells is similar but speed restrictions do not necessarily apply.

Turbine Water Washing Arrangement

Blower surging takes place if the air mass delivered by the blower falls off at a faster rate than the air pressure of
delivery. With all blowers it is possible to produce a graph showing this effect. Surging gives an unpleasant noise which
causes the engineer to do something about the problem as he is made immediately aware of the condition. The initial
action in order to stop surging is to slow the engine until it ceases, or reduce load in the case of a constant speed engine.
Blower efficiency is highest closer to the surge line and so if a high efficiency is demanded there is little leeway against
surging. In practice the fitting of blowers is a compromise between a reasonable blower efficiency and an acceptable
degree of safeguard against surging. Surging can occur for a wide variety of reasons many of which are not obvious but
the more obvious ones include, cylinder power imbalance, faulty injectors, deposits on blades or impeller, damage to
blades, nozzles or impeller, etc.,

63
 Air
Delivery
Pressure

Turbocharger Compressor Characteristic Graph

{Surge Graph}

In order to achieve higher scavenge air pressures two stage turbocharging is used with an intercooler fitted between the
stages. Such a system, although more complex and expensive than single stage turbocharging is more efficient and as
modern engines aim at maximum efficiency the arrangement is acceptable despite the increased complexity. The use
of an intercooler between the stages improves efficiency and keeps the air temperature relatively low thus increasing
air density and minimising cylinder temperature problems. A number of different arrangements are possible and the
one shown is typical.

Exhaust Exhaust Air

Gas Gas 2nd Stage
Outlet Inlet Outlet

Two Stage Turbocharger

64
LPG Burning in Automobile Engines
Liquefied Petroleum Gas (LPG) - hydrocarbon gases under low pressure
Most people call liquefied petroleum gas (LPG) "propane." That is because LPG is mostly made up of
propane. Actually, LPG is made of a mixture of propane and other similar types of hydrocarbon gases.
Different batches of LPG have slightly different amounts of the different kinds of hydrocarbon molecules.
These hydrocarbons are gases at room temperature, but turn to liquid when they are compressed. LPG is
stored in special tanks that keep it under pressure, so it stays a liquid. The pressure of these tanks is usually
about 200 pounds per square inch. LPG comes from natural gas wellhead processing or from petroleum
refining. LPG occurs naturally in crude oil and natural gas production fields and is also produced in the oil
refining process.

There are LPG cars commercially available to the public and you can also get a current car converted to take
LPG if it is not too old.

The LPG used in vehicles is the same as that used in gas barbecues and camper appliances. LPG is also used
in many homes in the country, where there are no natural gas pipelines. These homes use LPG for heating,
cooking, hot water and other energy needs. (Calor Gas in the UK.) LPG fuelled engines can pollute less than
petrol and diesel engines. LPG usually costs less than petrol for the same amount of energy. In the
Netherlands over 10% of the motor fuel used is LPG.

The components of an Automobile LPG-system

The basic components of a LPG-system are:
A LPG storage tank
A tank valve to shut off the tank when the engine stops
A vaporiser / regulator to vaporise the liquid and regulate the amount of LPG to the engine
A main flow adjuster to adjust the LPG-system to the engine
A mixer to mix the LPG with the air which enters the engine
A petrol shut off system

LPG tanks
The LPG tanks can be classified by their shape :
• Cylindrical tanks: the most common straight round tanks (cheap)
• The duo-tank: flattened tank cylindrical to reduce height
• Twin-tank: 2 cylindrical tanks are welded together
• Ring-tank: doughnut-shape tank with a hole in the middle which can be installed in the spare wheel
well of the boot
• U.F.O.-tank: a round pill-shape tank which can be installed in the spare wheel well of the boot

The LPG tank incorporates safety valves :

• Filling-hose connection with a 80% shut off
• Overpressure relief valve which opens at ±30 bar
• A shut off to close the tank by hand or electronically (solenoid)
• Fuel gauge which gives you an indication of the amount of LPG in the tank
• The valves must be covered by an air-tight box with an atmospheric vent to avoid LPG entering the
car.

Those safety devices can be gathered in one valve. This valve is called a multi-valve and can only be
installed in a single-hole-tank. The multi-hole-tank is the most common tank. In a multi-hole-tank are
separate valves installed. The installation of the tank is much easier than a single hole-tank because there is
more space between the components. Filling up the tank can be done much faster because of the larger
connection between the tank and the filler connection (specially in winter).

Gaseous Fuels 1
LPG shut off solenoid.
Under the bonnet is a solenoid installed close to the vaporiser. The length of the LPG-drain-pipe must be
less than 30 cm. The function of this solenoid is to close the system when the engine stops or when the
engine is running on petrol. On modern cars this is done electronic by a relay connected to the ignition
system.

Vaporiser / regulator.
The vaporiser/ regulator, mostly called vaporiser, has two major functions. The first is to heat up the liquid
LPG so the vaporisers wont freeze when the LPG becomes a vapour. Normally this is done by making a
connection to the car's water cooling circuit. Some engines have an air cooling system. In that case the liquid
LPG has to be preheated before it goes into the vaporiser. This can be done with a special heat exchanger
installed in the exhaust system . The second is to regulate the amount of LPG that goes to the engine. Just
like a carburettor it tries to keep the mixture of LPG and air at the optimum proportion. Therefor the
vaporiser must deliver a stable LPG pressure to the engine. A problem is the changing of the pressure in the
tank. The pressure depends on the outside temperature and the amount of LPG left in the tank (4bar up to
30bar). The manufactures of vaporisers solved this problem by lowering the pressure in several stages.

The vaporisers can be classified in the way they are built :

Two-stage vaporisers. The two stage vaporisers are the most common. They lower the pressure in two
stages. The first stage lowers the tank pressure from ± 10 bar to ± 2 bar. The second lowers the operating
pressure to ± 0,7 bar so the mixer can cope with it. The advantages are: they are small and can be cheap.
Disadvantages are: they have a limited range (0,5L up to 1,5L or 1,0L up to 2,5L or 2,0L up to 4.0L engine
size) {L is litre or 1000cc}.
Two-stage-tandem- vaporisers. The two-stage-tandem- vaporisers are functioning in almost the same way as
the two stage vaporisers. The biggest difference is the second stage. The second stage is regulated in two.
One accurate second stage regulates small amounts and a second big stage delivers high quantity's of LPG.
The small stage is operating when small quantities of LPG are required by the engine. Because of the small
internal size of the valve the LPG release pressure is very stable. The big stage will be opened by the small
stage as soon as the required amount of LPG is reaching above the capacity of the small stage. The big stage
is capable of dosing large quantities of LPG. Advantages: for small and big engines (0.5L up to 7L).
Disadvantages: the large size, expensive.

Three-stage- vaporisers. This type of vaporiser lowers the tank pressure in three stages after each other.
Because of the three stages the vaporiser is extremely stable. Advantages: they can cope with large engines
(4L up to 8L) Disadvantages: the large size, difficulties with delivering low quantities of LPG, expensive.
All vaporisers shut off LPG flow as soon as the engine stops. This is done in two ways: - by vacuum - by
electronics

By vacuum. If the closing of the vaporiser can be done by vacuum there is a connection made from the
vaporiser to the air intake manifold. As soon as the engine runs the vacuum in the air intake opens a valve in
the vaporiser. The vaporiser will now deliver LPG to the engine. When the engine stops and the vacuum
drops the valve will shut down the LPG flow to the engine. Starting on LPG is impossible with those
vaporisers, unless there is a solenoid installed on the vaporiser which can be opened manually for pre-
injecting LPG. A button must be installed in the cabin to operate the vaporiser solenoid.

By electronic circuit. If the vaporiser closes electronic there is a connection made from the vaporiser to an
electronic control relay. This relay gets a signal from the ignition system when the engine is running and
opens the vaporiser solenoid. As soon as the engine stops the relay closes the solenoid on the vaporiser and
the LPG flow will stop. To start the engine you have to switch the ignition on. At that moment a relay
activates the vaporiser solenoid for some seconds. A small amount of LPG will be injected in the air intake
manifold before the engine is started. When the temperature is around 0 degrees or lower it might be
recommendable to enrich the mixture by switching on the ignition twice before starting the engine. This
procedure gives some additional LPG to the engine (choke).

Gaseous Fuels 2
Main LPG adjuster.
The main LPG adjuster is installed between the vaporiser and the mixer. ItÕs function is to adjust the LPG
system to the engine. The LPG system must create a LPG/air mixture of a constant ratio. If the mixture is a
little too rich the engine uses more LPG, the temperature in the combustion chamber stays lower and the
engine delivers its maximum power. This lowering of the temperature is desirable for older engines. If the
mixture is too lean the performance of the engine drops dramatically, it is using more LPG as necessary, the
pollution is rising and the temperature in the combustion chamber might rise dramatically. The high
temperature might damage the engine. There are 3 different types of main LPG adjusters:

• A fixed main flow adjuster; The fixed main flow adjuster can't interact with the engine. The result
is a mixture which is too rich or too lean. Adjusting the main flow adjusters is easy. The
manufacturer gives a Co value (mostly between 05% and 1.0%) at a particular amount of revolutions
(mostly 3000 rpm). This type of main flow adjuster ignores the performance delivered by the engine.
Advantages: cheap, trouble free and easy adjustable.
• A vacuum controlled main flow adjuster; The vacuum controlled main flow adjusters have two
settings. The first setting must be adjusted in accordance with the specifications given by the
manufacturer of vaporisers (Co usually between 0.5% and 1.0% mostly at 3000 rpm). Before you can
start adjusting you have to disconnect the vacuum hose and blank it off. The second setting has to be
done at the same rpm. but the vacuum hose must be re-connected. Make the mixture as lean as
possible. Therefore you have to look at the HC-scale and close the second setting until the HC
reaches the lowest level (on LPG this will be ±150 HC). The result will be a very economic LPG
consumption at partial load and the engine is still capable of giving maximum power at full load. If
the system is installed in an older engine the vacuum controlled main flow adjuster is capable to
lower the combustion temperature by enriching the mixture even more than strictly necessary.
Advantages: cheap, easy adjustable, might give a fuel consumption drop between 5% tot 15%
compared with a fixed main flow adjuster and less chance of getting a back-fire. Disadvantages:
none
• An electronic controlled main flow adjuster; The electronic controlled main flow adjusters
(stappen motors) are operating by computer. The computer constantly gets information about the
combustion from the lambda probe which is installed in the exhaust system. The electronic
controlled main flow adjusters are capable to reach good economics and (when required) maximum
power. The advantages are: self learning, fuel consumption drops between 10% tot 20% compared
with a fixed main flow adjuster. The disadvantages are: pre adjusting must be done by an installer by
computer.

LPG mixer
The LPG-mixer (also called mixer) as two functions.
• Giving a vacuum signal to the vaporiser; The vacuum signal must be as constant as possible
compared with the amount of air that is going trough the venturie of the mixer. To achieve this the
mixer must be carefully designed. One of the most important components is the venturi. The LPG/air
exit of the venturi must have an ± 8 degrees angle to achieve a constant ratio of LPG with air. One of
the problems of this mixer will be his length ( ± 30 CM.), which makes it almost impossible to
install. Theoretically this mixer won't produce any swirl, so that the gases wonÕt mix with etch
other.
• Mixing up the LPG and air; To get a fast and almost complete carbonation the LPG and air must not
only be in the right proportion but also be proper mixed. Therefor it's necessary that the gases LPG /
air) swirl. To combine those two demands is almost impossible. Therefor a mixer will always be a
compromise. The manufacturer of mixers has to work out the best mixer for every single car. The
result mostly is that the mixer gives a right mixture in the partial load and a lean mixture at full load.
Not only the shape of the mixer is important but also the size of the venturi. The smaller the
diameter, the higher the vacuum signal to the vaporiser, the more accurate the LPG flow. The
disadvantage is that the engine gets less mixture because of the small diameter. This means it looks
as if your engine only can drive half throttle. Especially cars with injection or big carburettors will
Gaseous Fuels 3
 have a high power loss. (up to 20%). Therefore we recommend that :
- The venturies must be at least 75% of the superficial measure of the carburettors venturie or
butterfly valve if its an EFI engine.
- The venturies superficial measure must be 7.5MM2 and if possible 10 MM2 for every HP. If you
have a 3.5 Lr 200Hp engine you need a mixer with a superficial measure of
200X7.7MM=1500MM2. You need a mixer with a venturie diameter of at least 43 MM. If you have
a 3.5 Lr 200Hp engine you the need a mixer with a superficial measure of 200X10MM=2000MM2.
You need a mixer with a venturie diameter of 50 MM for the best performance. The manufacture or
importer can advise you whit a chose of the mixer.

Simulator
Modern cars are filled up with electronics. Those electronics provide the driver with information when one
of the measurements are out of tolerance. When a LPG system is installed the motor management system
detects that the fuel injection has stopped, the lambda signal has changed etc. etc. The modern LPG system
has to cope with those problems. According to the car there will be installed: - injection simulator - lambda
simulator - auto petrol start Injection simulation.
The injection simulator has 3 functions:
• shut down the petrol injection by blocking off the signals to the injectors
• tricking out the motor management system by giving a similar signal as the injectors to the motor
arrangement system
• avoiding the chances of getting a back-fire by delaying the shutting down of the petrol injection by
several seconds after activating the LPG system Lambda simulator.

If an engine has a constant learning motor management system a lambda simulator has to be installed This
constant learning motor management system will try to reduce the amount of fuel constantly until it gets a
low read out of the lambda probe. Because the LPG computer is regulating the mixture the lambda will give
a lambda "1" read out. The motor management system will lower the injected amount of petrol and still get a
lambda "1". This will result in lowering and lowering the amount of petrol until the system is completely
closed. Sometimes this will result in activating an alarm. At the moment the engine has to run on petrol
again the motor management system is completely out of its proper working range. This means that the car
can't run on petrol any more and needs some time to reprogram itself. Sometimes starting is impossible
because the engine has to start on petrol before switching over to LPG. To start the engine the motor
management system has to be reset by the dealer.

Back-fire safety protection

There are two ways of back-fire protection.
• Active back-fire protection; Active back-fire protection is a way to minimise the chances of getting a
back-fire. This can be done by: - starting automatically on petrol - giving LPG and petrol for several
seconds when switching over from petrol to LPG The chances of getting a back-fire are the highest
during the starting procedure. This risk can be minimised: - Start on petrol and switch after a little
while over to LPG. This can be done by an electronic switch with a time delay, with a vacuum
switch which measures the under pressure in the air intake manifold, a ref. counter or a combination
of the three. - Starting on petrol and LPG. After deactivating the starter engine of the petrol injection
will be shut off.
• Passive back-fire protection; Passive back-fire protection is a way to minimise the damage of a back-
fire. This can be done by: - Installing blow-back valves which will open by overpressure in the air
intake system. The overpressure can escape and afterwards the valve closes itself. Plastic air filter
boxes can't cope with high over pressure and will split open when a back-fire occurs. - Installing an
air-flap opener. The airflap will be opened during the time the engine is running on LPG. If a back-
fire happens the pressure can pass without damaging the airflap. They can be operated by LPG
pressure from the vaporiser or by the under pressure in the air intake manifold. Once the airflap is
damaged it has to be replaced. The replacement is very expensive.

Gaseous Fuels 4
Liquefied Natural Gas (LNG) - natural gas that is very, very cold

Natural gas can be made into three forms. One kind is the low-pressure form you use to cook or heat your
home. It comes from the underground pipe from the gas company. Another form is compressed natural gas
(CNG). This form is compressed into high-pressure fuel cylinders to power a car or truck. It comes from
special CNG fuel stations. The third form is liquefied natural gas (LNG). LNG is made by refrigerating
natural gas to condense it into a liquid. The liquid form is much more dense than natural gas or CNG. It has
much more energy for the amount of space it takes up. So, much more energy can be stored in the same
amount of space on a car or truck. That means LNG is good for large trucks that need to go a long distance
before they stop for more fuel.

Liquefied natural gas is made by refrigerating natural gas to minus 260 degrees Fahrenheit (260 degrees
below zero!) to condense it into a liquid. This is called liquefaction. The liquefaction process removes most
of the water vapor, butane, propane, and other trace gases, that are usually included in ordinary natural gas.
The resulting LNG is usually more than 98 percent pure methane. As this was first written (in 1999),
Caterpillar, Cummins, Detroit Diesel, Mack and Navistar sell heavy-duty natural gas engines that can
operate on LNG.

Liquefied Natural Gas is called LNG. It has a higher energy density than any other fuel available today.
LNG also has a higher octane rating, which allows vehicles to run more efficiently. Natural Gas is not a
new intervention, but has been around for over 100 years. LNG is a likely fuel for cars in the near future.

Natural gas is a, natural mixture of gaseous found issuing from the ground or obtained from specially
driven wells. The composition of natural gas varies in different localities. Its chief component, , usually
makes up from 80% to 95%, and the balance is composed of varying amounts of , , , and other
hydrocarbon compounds. Some of the hydrocarbons found in also occur as vapours in natural gas; by
liquefying these hydrocarbons, gasoline can be obtained.
Although commonly associated with deposits it also occurs separately in sand, sandstone, and limestone
deposits. Some geologists theorize that natural gas is a byproduct of decaying vegetable matter in
underground strata, while others think it may be primordial gases that rise up from the mantle. Because of its
flammability and high calorific value, natural gas is used extensively as an illuminant and a fuel.

Natural gas was known to the ancients but was considered by them to be a supernatural phenomenon
because, noticed only when ignited, it appeared as a mysterious fire bursting from the ground. One of the
earliest attempts to harness it for economic use occurred in the early 19th cent. in Fredonia, N.Y. Toward the
latter part of the 19th cent., large industrial cities began to make use of natural gas, and extensive pipeline
systems have been constructed to transport gas.

Liquefied natural gas,. or LNG, is natural gas that has been pressurized and cooled so as to liquefy it for
convenience in shipping and storage. The boiling point of natural gas is extremely low, and only in the
1970s did cryogenic technology (see -temperature physics) advance enough to make the production and
transport of LNG commercially feasible.

Gaseous Fuels 5
The Carburettor
The goal of a carburettor is to mix just the right amount of gasoline with air so that the engine runs properly.
If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially
damages the engine. If there is too much fuel mixed with the air, the engine "runs rich" and either will not
run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. The
carburettor is in charge of getting the mixture just right.

On new cars, fuel injection is becoming nearly universal because it provides better fuel efficiency and lower
emissions. But nearly all older cars, and all small equipment like lawn mowers and chain saws, use
carburettors because they are simple and inexpensive.

• A carburettor is essentially a tube.

• There is an adjustable plate across the tube called the throttle plate that controls how much air can flow
through the tube.
• At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is
created. In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel.

The carburettor is operating "normally" at full throttle. In this case the throttle plate is parallel to the length
of the tube, allowing maximum air to flow through the carburettor. The air flow creates a nice vacuum in the
venturi and this vacuum draws in a metered amount of fuel through the jet

When the engine is idling, the throttle plate is nearly closed (the position of the throttle plate in the photos is
the idle position). There is not really enough air flowing through the venturi to create a vacuum. However,
on the back side of the throttle plate there is a lot of vacuum (because the throttle plate is restricting the
airflow). If a tiny hole is drilled into the side of the carburettor's tube just behind the throttle plate, fuel can
be drawn into the tube by the throttle vacuum. This tiny hole is called the idle jet and it controls the amount
of fuel that flows through the idle jet.

Both the Hi and Lo screws are simply needle valves. By turning them you allow more or less fuel to flow
past the needle. When you adjust them you are directly controlling how much fuel flows through the idle jet
and the main jet.

When the engine is cold and you try to start it with the pull cord, the engine is running at an extremely low
RPM. It is also cold, so it needs a very rich mixture to start. This is where the choke plate comes in. When
activated, the choke plate completely covers the venturi. If the throttle is wide open and the venturi is
covered, the engine's vacuum draws a lot of fuel through the main jet and the idle jet (since the end of the
carburettor's tube is

Gaseous Fuels 6
THE WANKEL ENGINE

Like a conventional piston engine, the Wankel engine is an internal combustion engine and operates on a
four-stroke cycle. Also, it runs on gasoline and the spark is generated by a conventional distributor-coil
ignition system. However, the similarities end there.

In a Wankel engine, the cylinders are replaced by chambers, and the pistons are replaced by rotors. The
chambers are not circular, but have a curved circumference that is identified as an epitrochoid. An
epitrochoid is the curve described by a given point on a circle as the circle rolls around the periphery of
another circle which is twice the radius of the generating circle.

The rotor is three-cornered, with curved sides. All three corners are in permanent contact with the
epitrochoidal surface as the rotor moves around the chamber. This motion is both orbital and rotational, as
the rotor is mounted off centre. The crankshaft of a piston engine is replaced by a rotor shaft, and crank
throws are replaced by eccentrics. Each rotor is carried on an eccentric. Any number of rotors is possible,
but most engines have one or two rotors. The valves of the piston engine are replaced by ports in the Wankel
engine housing. They are covered and uncovered by the path of the rotor.

One of the important differences between the Wankel rotary engine and the piston engine is in the
operational cycle. In the piston engine, all the events take place at the top end of the cylinder (intake,
compression, expansion, and exhaust). The events are spaced out in time only. The Wankel engine is the
opposite. The events occur at the same time but at different places around the rotor-housing surface.

The intake phase takes place next to the intake port and overlaps with the area used for compression.
Expansion takes place opposite the ports, and the exhaust phase takes place in the area preceding the exhaust
port, overlapping with the latter part of the expansion phase. All three rotor faces are engaged in one of the
four phases at all times

Cutaway view of a two rotor Wankel rotary engine

The path of the rotor in the Wankel engine. Note the

constantly varying shape of the combustion chamber and the
two spark plugs per cylinder.

1
Rotary Engines
In the Wankel a triangular rotor incorporating a central ring gear is driven around a fixed pinion within an
oblong chamber

The fuel/air mixture is drawn in the intake port during this

phase of the rotation.

The mixture is compressed here

The mixture burns here, driving the rotor around.

And the exhaust is expelled here.

The rotory motion is transferred to the drive shaft via an eccentric wheel (illustrated in blue) that rides in a
matching bearing in the rotor. The drive shaft rotates once during every power stroke instead of twice as in
the Otto cycle. The Wankel promised higher power output with fewer moving parts than the Otto cycle
engine, however technical difficulties have apparently interfered with widespread adoption.

2
Rotary Engines
The Basics
Like a piston engine, the rotary engine uses the pressure created when a combination of air and fuel is
burned. In a piston engine, that pressure is contained in the cylinders and forces pistons to move back and
forth. The connecting rods and crankshaft convert the reciprocating motion of the pistons into rotational
motion that can be used to power a car.

In a rotary engine, the pressure of combustion is contained in a chamber formed by part of the housing and
sealed in by one face of the triangular rotor, which is what the engine uses instead of pistons.

The rotor and housing of a rotary engine from a Mazda RX-7: These parts replace the
pistons, cylinders, valves, connecting rods and camshafts found in piston engines.

The rotor follows a path that looks like something you'd create with a spirograph. This path keeps each of
the three peaks of the rotor in contact with the housing, creating three separate volumes of gas. As the rotor
moves around the chamber, each of the three volumes of gas alternately expands and contracts. It is this
expansion and contraction that draws air and fuel into the engine, compresses it and makes useful power as
the gases expand, and then expels the exhaust.

The Parts
A rotary engine has an ignition system and a fuel delivery system that are similar to the ones on piston
engines. If you've never seen the inside of a rotary engine, be prepared for a surprise, because you won't
recognize much.

Rotor
The rotor has three convex faces, each of which acts like a piston. Each face of the rotor has a pocket in it,
which increases the displacement of the engine, allowing more space for air/fuel mixture.

3
Rotary Engines
 The Rotor

At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber. There
are also metal rings on each side of the rotor that seal to the sides of the combustion chamber. The rotor has
a set of internal gear teeth cut into the centre of one side. These teeth mate with a gear that is fixed to the
housing. This gear mating determines the path and direction the rotor takes through the housing.

Housing
The housing is roughly oval in shape (it's actually an epitrochoid). The shape of the combustion chamber is
designed so that the three tips of the rotor will always stay in contact with the wall of the chamber, forming
three sealed volumes of gas.

Each part of the housing is dedicated to one part of the combustion process. The four sections are:

• Intake
• Compression
• Combustion
• Exhaust

The intake and exhaust ports are located in the housing. There are no valves in these ports. The exhaust port
connects directly to the exhaust, and the intake port connects directly to the throttle.

Output Shaft
The output shaft has round lobes mounted eccentrically, meaning that they are offset from the centreline of
the shaft. Each rotor fits over one of these lobes. The lobe acts sort of like the crankshaft in a piston engine.
As the rotor follows its path around the housing, it pushes on the lobes. Since the lobes are mounted
eccentric to the output shaft, the force that the rotor applies to the lobes creates torque in the shaft, causing it
to rotate.

4
Rotary Engines
 The Housing and shaft
showing the position of the
Exhaust ports, cooling
passageways and Spark Plugs

The two end covers contain the seals and bearings for the output shaft. They also seal in the two sections of
housing that contain the rotors. The inside surfaces of these pieces are very smooth, which helps the seals on
the rotor do their job. An intake port is located on each of these end pieces.

The next layer in from the outside is the oval-shaped rotor housing, which contains the exhaust ports. This is
the part of the housing that contains the rotor.

In the centre of each rotor is a large internal gear that rides around a smaller gear that is fixed to the housing
of the engine. This is what determines the orbit of the rotor. The rotor also rides on the large circular lobe on
the output shaft.

The part of the rotor housing that holds the rotors (Note the exhaust port location.)

5
Rotary Engines
Producing Power

Rotary engines use the four-stroke combustion cycle, which is the same cycle that four-stroke piston engines
use. But in a rotary engine, this is accomplished in a completely different way.

The heart of a rotary engine is the rotor. This is roughly the equivalent of the pistons in a piston engine. The
rotor is mounted on a large circular lobe on the output shaft. This lobe is offset from the centreline of the
shaft and acts like the crank handle on a winch, giving the rotor the leverage it needs to turn the output shaft.
As the rotor orbits inside the housing, it pushes the lobe around in tight circles, turning three times for every
one revolution of the rotor.

As the rotor moves through the housing, the three chambers created by the rotor change size. This size
change produces a pumping action. Let's go through each of the four stokes of the engine looking at one face
of the rotor.

Intake
The intake phase of the cycle starts when the tip of the rotor passes the intake port. At the moment when the
intake port is exposed to the chamber, the volume of that chamber is close to its minimum. As the rotor
moves past the intake port, the volume of the chamber expands, drawing air/fuel mixture into the chamber.

When the peak of the rotor passes the intake port, that chamber is sealed off and compression begins.

Compression
As the rotor continues its motion around the housing, the volume of the chamber gets smaller and the
air/fuel mixture gets compressed. By the time the face of the rotor has made it around to the spark plugs, the
volume of the chamber is again close to its minimum. This is when combustion starts.

Combustion
Most rotary engines have two spark plugs. The combustion chamber is long, so the flame would spread too
slowly if there were only one plug. When the spark plugs ignite the air/fuel mixture, pressure quickly builds,
forcing the rotor to move.

The pressure of combustion forces the rotor to move in the direction that makes the chamber grow in
volume. The combustion gases continue to expand, moving the rotor and creating power, until the peak of
the rotor passes the exhaust port.

Exhaust
Once the peak of the rotor passes the exhaust port, the high-pressure combustion gases are free to flow out
the exhaust. As the rotor continues to move, the chamber starts to contract, forcing the remaining exhaust
out of the port. By the time the volume of the chamber is nearing its minimum, the peak of the rotor passes
the intake port and the whole cycle starts again.

The neat thing about the rotary engine is that each of the three faces of the rotor is always working on one
part of the cycle -- in one complete revolution of the rotor, there will be three combustion stokes. But
remember, the output shaft spins three times for every complete revolution of the rotor, which means that
there is one combustion stroke for each revolution of the output shaft.

Key Differences
There are several defining characteristics that differentiate a rotary engine from a typical piston engine.

6
Rotary Engines
MAK 16 cylinder turbocharged V-type engine
Two banks of cylinders each with its own turbocharger which can be seen at the end of the engine. The charge air
coolers are arranged together below the turbochargers.

Engine driven lubricating oil and cooling water pumps are located below the charge air cooler

13
Radial Engine

Radial engines typically have anywhere from three to nine cylinders. The radial engine has the same sort of
pistons, valves and spark plugs as any other four-stroke engine. The main difference is in the crankshaft.
Instead of the long shaft as fitted in a multi-cylinder car engine, all of the piston connecting rods connect to
a hub. One connecting rod is fixed, and it is generally known as the master rod. The others are articulating
rods and they attach to the hub/fixed rod by means of pins that allow them to rotate as the crankshaft and the
pistons move.

Pistons move up and down in the cylinders and the valves operate to allow air/fuel to enter the cylinders and
exhaust gas to leave.

Radial engines have several advantages for aircraft in that their power/weight ratio is high and as they have a
relatively low operating speed they may be used to drive a propeller directly. Because all of the pistons are
in the same plane, they all get even cooling and normally can be air-cooled. This saves the weight of water-
cooling.

8
Rotary Engines
Colour Page
CRANFIELD UNIVERSITY

Diesel Engines
for
Power and Heat

Engines
 Diesel Engine Types
• Two stroke operating cycle or four stroke
operating cycle
• Crosshead engine (2-stroke) or trunk piston
type (4-stroke)
• In-line or Vee-type trunk piston engines
• Crosshead engines 350mm bore to 1000mm
bore; 2,000kW to 80,000kW depending upon
number of cylinders
• Trunk piston medium speed engines 150mm
bore to 520mm bore; 750kW to 15,000kW
depending upon number of cylinders
Crosshead Diesel Engine
Medium Speed
Trunk Piston Diesel Engine

In-line Engine Vee-type Engine

Electronically Controlled Camshaftless Crosshead Engine
Sulzer RT-flex Engine 98mm bore
Electro-Hydraulic Valve and Injection System
Section Through a Trunk-Piston
Vee-type Engine
Diesel Engine Advantages

• High thermal efficiency meaning lower fuel consumption

per unit power (up to 50% thermal efficiency)

• Wide range of powers available from engines of different

sizes giving good range of [power outputs

• Spares readily available

• Maintenance is relatively straight forward

• Engines can operate on fuels of very low quality (and

hence cost)
Diesel Engine Disadvantages

• High lubricating oil consumption compared with other

prime movers

• Requires more frequent maintenance than other prime

movers

• Noise and vibration must be overcome

• Low power to weight ration compared with gas turbine

 Fuels for Diesel Engines

• Distillate fuel oil (diesel oil)

• Heavy fuel oil (residual oil)
• Bio-fuel oil (vegetable oil)
• Gaseous fuel oil (LNG/methane,
marsh gas, etc)
Diesel Engine Solid Fuel Injection
(Atomisation)
• High pressure fuel injection requires high
pressure fuel pumps and timed injection
• Atomisation requires one or more fuel injectors
in the cylinder cover
• Diesel engine is a compression ignition engine
so high temperature air compression is needed
• The correct mass of air is required for the fuel
injected; normally excess air is supplied to
ensure correct combustion
 Four- Stroke Diesel Cycle
(Power and exhaust strokes not shown)
 Gas Burning in a Diesel Engine

• Gas and fuel oil may be burned together but

control can be difficult
• Natural gas has a low ignition quality and so
must be ignited by a flame or spark. Dual fuel
engines use flame from liquid fuel to ignite the
gas
• Dual fuel engines may burn gas or liquid fuel
• Gas mixed with air and the mixture compressed
• Different systems for two and four stroke
engines
Diesel Engine Operation on Dual Fuel
(Power and exhaust strokes not shown)
 Gas Engines

• Gas engines operate on gas alone

• Require a secure supply of fuel

• Ignition of gas is by means of a spark

Diesel Engine Operation on Gas
(Power and exhaust strokes not shown)
 Burning of Gas in a Diesel Engine

• Gas is compressed and mixed with the

combustion air, the mixture being compressed in
the cylinder
• In a four stroke engine the gas is injected into
the combustion air stream during the piston
suction stroke
• In a two stroke engine the gas is compressed to
a higher pressure and injected into the cylinder
during the piston compression stroke
 Diesel Engine Emissions

• CO2 [Caused by burning fossil fuels and cause global

warming]

• NOx (Oxides of Nitrogen) [Caused by high

temperature combustion in air (oxygen and nitrogen)
– results in acid rain, damage to lung tissue etc.]

• SOx (Oxides of sulphur) [Caused by burning fuels

containing sulphur]

• Unburnt hydrocarbons [Caused by incomplete

combustion of fuel oil and by cylinder lubricating oil]

• Particulates
 Emission Control
• CO2 – Burn less fossil fuel by more efficient combustion
and energy recovery

• NOx – Engine based systems such as water injection to

reduce peak temperature or secondary systems such as
selective catalytic reduction

• Sox – Use low sulphur fuels

• Unburnt hydrocarbons – better control of combustion and

cylinder oil use

• Particulates – cleaner combustion

 Engine Support Systems

• Fuel preparation unit – cleaning and heating

• Cooling water system – for cooling cylinders and
turbochargers
• Lubrication systems – for lubrication of engine
parts including the cylinders
• Air supply system – air induction arrangements
with filters
• Exhaust gas removal system – to ensure safe
and efficient removal of exhaust gas
• Engine mountings, etc
Colour Page
CRANFIELD UNIVERSITY

Diesel Engines
for
Power and Heat

Diesel Engine
Installation and Operation
Diesel Plant Requirements (Systems)
• Engine cooling water system

• Cooling system for the cooling water; radiator, cooling

tower, sea water

• Lubricating oil supply, storage and cooling system

• Fuel treatment plant (storage tanks and

separator/filtration system)

• Operating personnel

• Road access/ access for fuel supply

Diesel Plant Requirements (Installation)

• Space; land area and height

• Pipework for coolants and oil

• Holding down arrangements to keep engines in place

• Access ladders, etc

• Workshop and maintenance areas

• Cranes and lifting equipment

Diesel Plant Requirements (Maintenance)

• Skilled personnel availability (in house or service

contracts)

• Tools and spare gear storage and requisition systems

• Record keeping

• Safety systems
Diesel Plant Requirements (Legislation)

• Emission control

• Water pollution if using water cooling of engine coolant

• Air pollution if using radiators or cooling towers

• Noise pollution (silencers on the engine exhausts)

Diesel Plant Requirements (Operation)
• Use a number of engines of different powers so the running engines
are operating most efficiently

• Fuel supply must be reliable

• Monitor operation electronically with automatic power management

system to start and stop engines as required

• Manual intervention must always be available to safeguard the

system

• Must be able to meet load requirements by operating engines over a

wide load range
Colour Page
CRANFIELD UNIVERSITY

Diesel Engines
for
Power and Heat

Crosshead Diesel Engines

 Crosshead Diesel Engine
• Bore about 300mm to 1100mm
• Stroke to bore ratio 2.0:1 to about 3.6:1
• Marine engines direct propeller drive
• Marine Engines are reversible
• Started by compressed air
• Slow speed operation 90rpm – 200rpm
Colour Page
CRANFIELD UNIVERSITY

Diesel Engines
for
Power and Heat

Medium Speed Diesel Engines

Medium Speed Trunk Piston Engines

Bore up to 400mm

Stroke to bore ratio about 2.5:1

Speed up to about 700rpm

In-line or vee type

Colour Page
CRANFIELD UNIVERSITY

Diesel Engines
for
Power and Heat

Energy Recovery Systems

 Diesel Engine Heat Balance

Output Power 50%

Exhaust gas 25%
Scavenge air cooling 14%
Jacket water cooling 6%
LO cooling 4%
Radiation 2%
 Engine Operation

Engines should be operated close to their maximum operating

efficiency as this produces the lowest emission levels

Engines should be operated at about 80% of Maximum

Continuous Rating (MCR) and this offers the lowest Specific
Fuel Consumption (SFC)

Specific Fuel Consumption is the fuel used per unit power per
hour.

Best SPC is about 172g/kw.h

Specific Fuel Consumption
Engine Operation
Multiple engines of different powers offer greatest operating
range potential
eg. Six engines, three of 12MW and three of 16MW power
This allows output range of:

12MW (1 x 12MW)
16MW (1 x 16MW)
24MW (2 x 12MW)
28MW (1 x 12MW + 1 x 16MW)
32MW (2 x 16MW)
Etc.
 Energy Saving

• Energy recovery is only worthwhile if that recovery

costs less than the saving made
• Savings must be made from the 50% of thermal energy
which is thrown away
• Thermal Efficiency of a Modern Diesel Engine is about
50%; there is little further scope for improvement
without high cost
 Waste Heat Recovery Potential

• Large plants give higher energy recovery (even

small plants offer savings such as the heater on
a car)

• Waste heat should be available continuously

otherwise backup plant is required

• Recovery costs must be as low as possible

 Waste Heat Recovery Potential
(Exhaust Gas)

• Exhaust gas is at a relatively high temperature

• Turbocharging (Provide increased charge air

quantity)

• Exhaust gas boiler; provides steam for power

generation and heating – steam pressure about 9.5
bar
 Steam Waste Heat Recovery

• Applicable to larger plants (above about 15MW

depending upon location)

• Ideally plant should be running 24 hours per day, 7

days per week

• Depends upon the initial cost of the installation and

the running; retrofits are generally more expensive

• Depends upon space available

 Waste Heat Turbogenerator
Steam pressure 8.5 – 9.5 bar
Reheat steam at 160oC and 3.0 – 3.5 bar
Up to 7.0MW depending upon steam availability
 Marine Total Heat Recovery Plant

HP and LP evaporation and steam reheat

Feed water heating from jacket cooling water and scavenge air cooling
Power generation turbine using engine exhaust gas
 Marine Total Heat Recovery Plant

• Marine diesel engine 75,658kW service output

• Engine installation MCR 89,000kW

(120,000bhp)

• Potential power recovery with Total Heat

Recovery Plant 7,590kW

• Efficiency gain approximately 12%

 Total Heat Recovery Plant
Increase in overall efficiency
Slight increase in specific fuel consumption
Increase power in electrical generation
 Cost Saving from Heat Recovery

• Fuel cost; HFO $250 per tonne, MDO cost $500 per
tonne
• 7,590 kW (free energy from waste heat)
• Based on sfc of 170g/kWh annual cost saving is
$2,825,000 when burning HFO or $5,650,000 when
burning MDO
• Maintenance costs to be considered
• LO saving costs to be considered
• Using Net present Value calculations and interest rate
of 6% payback in 4 to 5 years
 General Waste Heat Recovery

• District heating hot water systems

• Cooling systems using chiller

• Fresh water generation from sea water

Hot Water Heating System
Chiller Unit From Waste Heat
Lithium-Bromide Absorbtion Chiller
 Barajas Airport, Madrid

• Six Wärtsilä 18V32DF dual-fuel engines

• Generator sets must continue running without

interruption even if there is a total failure in the heat
recovery system

• Back-up coolers which will remove all heat not captured

by the heat recovery system

• Six single-stage absorption chillers; powered by the

120°C heat recovery circuit
 Barajas Airport, Madrid
• Power at generator terminals 33.0 MW
• Electrical efficiency 42.4%
• Gross thermal power from heat recovery 24.6
MW
• Total thermal power (heat recovery + gas fired
boilers) 30.9 MWth
• Heat recovery circuit water 120/80°C
• Total efficiency of cogeneration 74%
• Absorption chiller capacity 18.0 MW(cooling)
• Total chiller capacity (absorption + compressor
chiller) 37.4 MW(cooling)
• Normal fuel - gas: Back-up fuel - light fuel oil
• Note: Thermal power and chilling do not run
together
 Low Temperature Evaporator
Using Engine Cooling Water as the Heat Source

Air Ejector

Sea Water
Cooling

Condensate
Pump
Probe
Demister

Non-return
Valve
Heating Pump
{Jacket Cooling Brine Starter
Water} Outlet

Feed Indicator
Brine
Inlet

Flow Filter
Controller

Salinometer
Flash Evaporator
• Multiple stages, 5 to 8, from a single heating
• May get 250 to 300 tonnes of water per day
• Each stage maintained at a successively lower pressure
• Heat source is cooling water from the engine which may be
supplemented by steam
Flash Evaporator
Further Information
Wartsila Sulzer Diesel Engines
http://www.wartsila.com

MAN B&W Diesel Engines

http://www.manbw.com

Caterpillar Diesels
http://www.cat.com
Colour Page
Absorption Chiller
System
 Water as a Refrigerant
• Water absorbs heat when it evaporates and this
cools the surrounding region
• Water changes to steam which raises the system
pressure
• Must maintain the pressure low to enabled
continued evaporation
• Require a system to remove the water vapour in
order to maintain a low pressure
Dry Chemical Moisture Absorber
Common salt and other materials will absorb
moisture but it soon becomes saturated and the
system will stall
 Basic Lithium Bromide Arrangement
• Heat source may be low grade
• Heat source may be engine cooling water
Lithium Bromide System
Lithium Bromide Chiller