ICE University of Baghdad

INTERNAL COMBUSTION ENGINES

(ICE)

COURSE PLAN

1. Principles of SI and CI engine operation, 2-stroke engines, 4-stroke engines

2. Ideal standard cycles, thermal efficiencies, comparison, deviations

3. Classification of engine fuels

4. Characteristics of engine fuels, knock resistance, ignition tendency, combustion

chemistry (air excess ratio, calorific value, adiabatic flame temperature,
dissociation)

5. Real engine strokes, induction stroke, volumetric efficiency

6. Compression stroke, combustion in SI engines and influencing parameters

7. Abnormal combustion, parameters influencing knock and early ignition

8. Combustion in CI engines, parameters influencing ignition delay

9. Expansion and exhaust strokes, exhaust emissions

10. Mixture preparation in SI engines

11. Carburetor fundamentals, fuel injection, control of A/F ratio

12. Mixture preparation in CI engines, injection pumps, injectors

13. Fuel injection systems in Diesel engines, Atomization, combustion chamber types
in Diesel engines

14. Engine characteristics and performance.

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PRINCIPLES OF IC ENGINE OPERATION

• INTRODUCTION

• OPERATION PRINCIPLES

• CLASSIFICATION OF ENGINES

• FOUR-STROKE AND TWO-STROKE ENGINES

• SI ENGINES, CI ENGINES
Heat pump
Heat engine

INTRODUCTION

Engine

An engine is a device which transforms one form of energy into another form. Normally,
most of the engines convert thermal energy into mechanical work and therefore they are
called “heat engines”.

Heat engine

Heat engine is a device which transforms the chemical energy of a fuel into thermal
energy and utilizes this thermal energy to perform useful work.

Heat engines can be broadly classified into two categories:

1- Internal Combustion Engines (ICE)

2- External Combustion Engines (ECE)

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External combustion engines are those in which combustion takes place outside the
engine whereas in internal combustion engines combustion takes place within the engine.
For example, in a steam engine or a steam turbine, the heat generated due to the
combustion of the fuel is employed to generate high pressure steam which is used as the
working fluid in a reciprocating engine or a turbine. In case of gasoline or diesel engines,
the products of combustion generated by the combustion of fuel and air within the
cylinder form the working fluid.

Engines whether internal combustion or external combustion are of two types:

1- Rotary engines
2- Reciprocating engines

Advantages of ICE engines over ECE

1- Absence of heat exchangers in the passage of the working fluid (boilers and
condensers in steam turbine plant).
2- All its components work at an average temperature which is much below the
maximum temperature of the working fluid in the cycle.
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3- Higher thermal efficiency can be obtained with moderate maximum working pressure
of the fluid in the cycle, and therefore the weight to power ratio is less than that of the
fluid in the turbine plant.
4- It has been possible to develop reciprocating internal combustion engines of very
small power output (even fraction of a kilowatt) with reasonable thermal efficiency
and cost.

Disadvantages of reciprocating engines

1- The problem of vibration caused by the reciprocating components.

2- It is not possible to use a variety of fuels in this engines, only liquid or gaseous fuels
of given specification can be efficiently used.

Internal Combustion Engines reciprocating engines

Internal Combustion Engines (IC-engines) produce mechanical power from the chemical
energy contained in the fuel, as a result of the combustion process occurring inside the
engine.

IC engine converts chemical energy of the fuel into mechanical energy, usually made
available on a rotating output shaft. Chemical energy of the fuel is first converted to
thermal energy by means of combustion or oxidation with air inside the engine, raising
the T and p of the gases within the combustion chamber.

The high-pressure gas then expands and by mechanical mechanisms rotates the
crankshaft, which is the output of the engine. Crankshaft is connected to a
transmission/power-train to transmit the rotating mechanical energy to drive a vehicle.

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There are two types of engines Spark Ignition (SI) engines – Otto or gasoline engines and
Compression Ignition (CI) engines – Diesel engines.

EARLY HISTORY

Atmospheric engines

Earliest IC engines of the 17th and 18th centuries are classified as atmospheric engines.
These are large engines with a single cylinder which is open on one end. Combustion is
initiated at the open cylinder and immediately after combustion; cylinder would be full of
hot gases at atmospheric pressure. The cylinder end is closed at this time and trapped
gases are allowed to cool. As the gases are cooled, vacuum is created within the cylinder
causing pressure differential across the piston (atmospheric pressure on one side and
vacuum on the other side). So piston moves due to this pressure difference doing work.

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Huygens (1673) developed piston mechanism

Hautefeuille (1676) first concept of internal combustion engine

Papin (1695) first to use steam in piston mechanism

“Modern” engines using same principles of operation as present

engines – previously no compression cycle

Lenoir (1860) driving the piston by the expansion of burning

products -first practical engine, 0.5 HP later 4.5
kW engines with mech. efficiency up to 5%

Rochas (1862) four-stroke concept was proposed

Otto – Langen (1867) produced various engine

improved efficiency to 11%

Otto (1876) Four-stroke engine prototype built, 8 HP and

patented

Clark (1878) Two-stroke engine was developed

Diesel (1892) Single cylinder, compression ignition engine

Daimler/Maybach Incorporated IC engine in automobile

(1882)

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BASIC ENGINE COMPONENTS AND NOMENCLATURE

Even though reciprocating internal combustion engines look quite simple, they are highly
complex machines. There are hundreds of components which have to perform their
functions satisfactorily to produce output power.

Engine components

A cross section of a single cylinder spark-ignition engine with overhead valves

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The major components of the engine and their functions are:

1- Cylinder block: The cylinder block is the main supporting structure for the various
components. The cylinders of a multicylinder engine are cast as a single unit, called
cylinder block.

2- Cylinder: It is a cylindrical vessel or space in which the piston makes a reciprocating

motion. The cylinder is supported in the cylinder block.

3- Piston: It is cylindrical component fitted into the cylinder forming the moving
boundary of the combustion system. It fits perfectly (tightly) into the cylinder
providing a gas-tight space with the piston rings and the lubricant. It forms the first
link in transmitting the gas forces to the output shaft.

4- Combustion chamber: the space enclosed in the upper part of the cylinder, by the
cylinder head and the piston top during the combustion process, is called the
combustion chamber. The combustion of fuel and the consequent release of thermal
energy results in the building up of the pressure in this part of the cylinder.

5- Inlet manifold: It is the pipe which connects the intake system to the inlet valve of the
engine and through which air or air-fuel mixture is drawn into the cylinder.

6- Exhaust manifold: It is the pipe which connects the exhaust system to the exhaust
valve of the engine and through which the products of combustion escape into the
atmosphere.

7- Inlet and exhaust valves: Valves are commonly mushroom shaped poppet type. They
provided either on the cylinder head or the side of the cylinder for regulating the
charge coming into the cylinder (inlet valve) and for discharging the products of
combustion (exhaust valve) from the cylinder.
8- Spark plug: It is a component to initiate the combustion process in spark-Ignition (SI)
engines and usually located on the cylinder head.

9- Connecting rod: It interconnects the piston and the crankshaft and transmits the gas
forces from the piston to the crankshaft. The two ends of the connecting rod are called
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as small end and the big end. Small end is connected to the piston by gudgeon pin and
the big end is connected to the crankshaft by crankpin.

10- Piston rings: It fitted into the slots around the piston, provide a tight seal between
the piston and the cylinder wall thus preventing leakage of combustion gases.

11- Gudgeon pin: It forms the link between the small end of the connecting rod and the
piston.

12- Camshaft: The camshaft and its associated parts control the opening and closing of
the two valves. This shaft also provides the drive to the ignition system. The
camshaft is driven by the crankshaft through timing gears.

13- Fly wheel: In order to achieve a uniform torque an inertia mass in the form of a
wheel is attached to the output shaft.

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Nomenclature

1. Cylinder bore (d): the nominal inner diameter of the working cylinder and is
designated by the letter d and is usually expressed in millimeter (mm).

2. Piston area (A): the area of a circle of a diameter equal to the cylinder bore is called
the piston area and is designated by the letter A.

3. Top Dead Center (TDC): the extreme position of the piston at the top of the cylinder.
In the case of the horizontal engines this is known as the outer dead center (ODC).

4. Bottom Dead Center (BDC): the extreme position of the piston at the bottom of the
cylinder. In horizontal engine this is known as the Inner Dead Center (IDC).

5. Stroke (L): the distance between TDC and BDC is called the stroke length and is
equal to double the crank radius, is usually expressed in millimeter (mm).
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6. Stroke to bore ratio: L/d ratio is an important parameter in classifying the size of the
engine.

low RPM /long stroke engine/ heavy vehicales

If d < L, it is called under-square engine. If d = L, it is called square engine. If d > L,
it is called over-square engine. An over-square engine can operate at higher speeds
because of larger bore and shorter stroke. high RPM /short stroke engine / high speed
vehicales

7. Swept volume or displaced volume (Vs): the volume swept by the working piston
when travelling from one dead center to the other. It is expressed in terms of cubic
centimeter (cc) and given by

π 2
Vs  A  L  d L
4

8. Cubic capacity or engine capacity (CC): the displacement volume or Swept volume
of a cylinder multiplied by number of cylinders in an engine will give the cubic
capacity or the engine capacity (CC). For example, if there are K cylinders in an
engine, then

Cubic capacity (CC) = Vs X K

9. Clearance volume (Vc): the nominal volume of the combustion chamber above the
piston when it is at the top dead center. It is designated as Vc and expressed in cubic
centimeter (cc).

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 Compression ratio (r): it is the ratio of the total volume V of the cylinder to the
clearance volume Vc, and is denoted by (r),

V Vc  Vs V
r   1 s
Vc Vc Vc

where V = Vc + Vs

THE WORKING PRINCIPLE OF ENGINES

If an engine is to work successfully then it has to follow a cycle of operations in a

sequential manner. The sequence is quit rigid and cannot be changed.

In the following sections the working principle of both SI and CI engines is described.
Even though both engines have much in common there are certain fundamental
differences.

The credit of inventing the spark-ignition engine goes to Nicolaus A. Otto (1876) whereas
compression-ignition engine was invented by Rudolf Diesel (1892). Therefore, they are
often referred to as Otto engine and Diesel engine.

gasoline engine

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Four-stroke spark-ignition engine

In a four-stroke engine, the cycle of operation is completed in four strokes of the piston or
two revolutions of the crankshaft. During the four strokes, there are five events to be
completed, viz., suction, compression, combustion, expansion and exhaust. Each stroke
consists of 180o of crankshaft rotation and hence a four-stroke cycle is completed through
720o of crank rotation. The cycle of operation for an ideal four-stroke SI engine consists
of the following four strokes:

i. Suction or intake stroke.

ii. Compression stroke
iii. Expansion or power stroke.
iv. Exhaust stroke.

The details of various processes of a four-stroke spark ignition engine with overhead
valves are shown in the figure below

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When the engine completes all the five events under ideal cycle mode, the p-V diagram
will be as shown below

p-V diagram

1. Suction or intake stroke

0→1 (p-V diagram), the inlet valve is open and the exhaust valve is closed.
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Starts with the movement of the piston from TDC to BDC, while drawing fresh charge
(air + fuel mixture) into the cylinder through the open inlet valve.
O
To increase the mass inducted, inlet valve opens for a period of 220 – 260 CA.

2. Compression stroke

The charge taken into the cylinder during the suction stroke is compressed by the return
stroke of the piston 1→2 (p-V diagram), during this stroke both valves are closed. The
mixture which fills the entire cylinder volume is compressed into clearance volume. At
the end of the compression stroke the mixture is ignited with the help of a spark plug
located on the cylinder head.

In ideal gas it is assumed that burning gas takes place instantaneously when the piston is
at the top dead center and hence the burning process can be approximated as heat
addition at constant volume. During the burning process the chemical energy of the fuel

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is converted into heat energy producing a temperature rise of about 2000 oC 2→3 (p-V
diagram). The pressure at the end of the combustion process is considerably increased
due to the heat release from the fuel.

3. Expansion or power stroke

The high pressure of the burnt gases forces the piston towards the BDC 3→4 (p-V
diagram). Both the valves are in closed position. Of the four-strokes only during this
stroke power is produced. Both pressure and temperature decrease during expansion.

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4. Exhaust stroke

At the end of the expansion stroke the exhaust valve opens and the inlet valve remains
closed. The pressure falls to atmospheric level a part of the burnt gases escape 4→1 (p-V
diagram).
The burned gases exit the cylinder through the open exhaust valve, due to the pressure
difference at first and then swept by the piston movement from BDC to TDC 5→0 (p-V
diagram).
Exhaust valve closes when the piston reaches TDC at the end of the exhaust stroke and
some residual gases trapped in the clearance volume remain in the cylinder.

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These residual gases mix with the fresh charge coming in during the following cycle,
forming its working fluid.

Each cylinder of a four stroke engine completes the above four operation in two engine
revolutions, one revolution of the crankshaft occurs during the suction and compression
strokes and the second revolution during the power and exhaust strokes. Thus, for one
complete cycle there is only one power stroke while the crankshaft turns by two
revolutions. For getting higher output from the engine the heat release (process 2→3 in p-
V diagram) should be as high as possible and the heat rejection (process 3→4 in p-V
diagram) should be as small as possible. So, one should be careful in drawing the ideal p-
V diagram.
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Four-stroke compression-ignition engine

The four-stroke CI engine is similar to the four-stroke SI engine but it operates at a much
higher compression ratio. The compression ratio of an IS SI engine is between 6 and 10

while for a CI engine it is from 16 to 20.

In the IC engine during suction stroke, air instead of a fuel-air mixture is inducted. Due to
the high compression ratio employed, the temperature at the end of the compression
stroke is sufficiently high to self-ignite the fuel which is injected into the combustion
chamber. In CI engines, a high pressure fuel pump and an injector are provided to inject
the fuel into the combustion chamber. The carburetor and ignition system necessary in the
SI engine are not required in the CI engine.

The ideal sequence of operations for the four-stroke CI engine as shown in figure below
is as follows:

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1. Suction stroke

Air alone is induced during the suction stroke. During this stroke intake valve is open and
exhaust valve is closed.

2. Compression stroke

Air inducted during the suction stroke is compressed into the clearance volume. Both
valves remain closed during this stroke.

3. Expansion stroke

Fuel injection starts nearly at the end of the compression stroke. The rate of injection is
such that combustion maintains the pressure constant in spite of the piston movement on
its expansion stroke increasing the volume. Heat is assumed to have been added at
constant pressure. After the injection of fuel is completed (i.e. after cut-off) the products
of combustion expand. Both the valves remain closed during the expansion stroke.

4. Exhaust stroke

The piston travelling from BDC to TDC pushes out the products of combustion. The
exhaust valve is open and the intake valve is closed during this stroke.

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The ideal p-V diagram is shown below

Due to higher pressures in the cycle of operations the CI engine has to be more sturdy
than a SI engine for the same output. This results in a CI engine being heavier than the SI
engine. However, it has a higher thermal efficiency on account of the high compression
ratio (of about 18 as against about 8 in Si engines) used.

Comparison of SI and CI engines

In four-stroke engines, there is one power stroke for every two revolutions of the
crankshaft. There are two non-productive strokes of exhaust and suction which are
necessary for flushing the products of combustion from the cylinder and filling it with the
fresh charge. If this purpose could be served by an alternative arrangement, without the
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movement of the piston, it is possible to obtain a power stroke for every revolution of the
crankshaft increasing the output of the engine. However, in both SI and CI engines
operating on four-stroke cycle, power can be obtained only in every two revolution of the
crankshaft.

Since both SI and Ci engines have much in common, it is worthwhile to compare them
based on important parameters like basic cycle of operation, fuel induction, compression
ratio etc. The detailed comparison is given below

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Two-stroke engine

As already mentioned, if the two unproductive strokes, viz., the suction and exhaust could
be served by an alternative arrangement, especially without the movement of the piston
then there will be a power stroke for each revolution of the crankshaft. In such an
arrangement, theoretically the power output of the engine can be doubled for the same
speed compared to a four-stroke engine. Based on this concept, Dugald Clark (1878)
invented the two-stroke engine.

In two-stroke engines, the cycle is completed in one revolution of the crankshaft. The
main difference between two-stroke and four-stroke engines is in the method of filling the
fresh charge and removing the burnt gases from the cylinder. In the four-stroke engine
these operations are performed by the engine piston during the suction and exhaust
strokes respectively. In a two-stroke engine, the filling process is accomplished by the
charge compressed in crankcase or by blower. The induction of the compressed charge
moves out the product of combustion through exhaust ports. Therefore, no piston strokes
are required for these two operations. Two strokes are sufficient to complete the cycle,
one for compressing the fresh charge and the other for expansion or power stroke.

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The above figure shows one of the simplest two-stroke engines, viz., the crankcase
scavenged engine. The figure below shows the ideal indicator diagram of such an engine.
The air or charge is indicated into the crankcase through the spring loaded inlet valve
when the pressure in the crankcase is reduced due to upward motion of the piston during
compression stroke. After the compression and ignition, expansion takes place in the
usual way.

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During the expansion stroke the charge in the crankcase is compressed. Near the end of
the expansion stroke, the piston uncovers the exhaust ports and the cylinder pressure
drops to atmospheric pressure as the combustion products leave the cylinder. Further
movement of the piston uncovers the transfer ports, permitting the slightly compressed
charge in the crankcase to enter the engine cylinder. The top of the piston has usually a
projection to deflect the fresh charge towards the top of the cylinder before flowing to the
exhaust ports. This serves the double purpose of scavenging the upper part of the cylinder
of the combustion products and preventing the fresh charge from flowing directly to the
exhaust ports.

The same objective can be achieved without piston deflector by proper shaping of the
transfer port. During the upward motion of the piston from BDC the transfer ports close

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first and then the exhaust close when compression of the charge begins and the cycle is
repeated.

Comparison of four-stroke and two-stroke engines

The two-stroke engine was developed to obtain a greater output from the same size of the
engine. The engine mechanism also eliminates the valve arrangement making it
mechanically simpler. Almost all two-stroke engines have no conventional valves but
only ports (some have an exhaust valve). This simplicity of the two-stroke engine makes
it cheaper to produce and easy to maintain. Theoretically a two-stroke engine develops
twice the power of a comparable four-stroke engine because of one power stroke every
revolution (compare to one power stroke every two revolutions of a four-stroke engine).
This makes the two-stroke engine more compact than a comparable four-stroke engine. In
actual practice power output is not exactly doubled but increased by only about 30%
because of

1- Reduced effective expansion stroke

2- Increased heating caused by increased number of power strokes which limits the
maximum speed.

The other advantages of the two-stroke engine are more uniform torque on crankshaft and
comparatively less exhaust gas dilution. However, when applied to the spark-ignition
engine the two-stroke cycle has certain disadvantages which have restricted its
application to only small engines suitable for motor cycles, scooters, lawn mowers,
outboard engine etc. In the SI engine, the incoming charge consists of fuel and air. During
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scavenging, as both inlet and exhaust ports are open simultaneously for some time, there
is a possibility that some of the fresh charge containing fuel escapes with the exhaust.
This results in high fuel consumption and lower thermal efficiency. The other drawback
of two-stroke engine is the lack of flexibility, viz., the capacity to operate with the same
efficiency at all speeds. At part throttle operating condition, the amount of fresh mixture
entering the cylinder is not enough to clear all the exhaust gases and a part of it remains
in the cylinder to contaminate the charge. This results in irregular operation of the engine.

The two-stroke diesel engine does not suffer from these defects. There is no loss of fuel
with exhaust gases as the intake charge in diesel engine is only air. The two-stroke diesel
engine is used quit widely. Many of the high output diesel engines work on this cycle. A
disadvantage common o all two-stroke engines, gasoline as well as diesel, is the greater
cooling and lubricating oil requirements due to one power stroke in each revolution of the
crankshaft. Consumption of lubricating oil is high in two-stroke engines due to higher
temperature. A detailed comparison of two-stroke and four-stroke engines is given below

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Actual engines

Actual engines differ from the ideal engines because of various constraints in their
operation. The indicator diagram also differs considerably from the ideal indicator
diagrams. Actual indicator diagram of a two-stroke and a four-stroke SI engines are
shown in figures a & b below respectively. The various processes are indicated in the
respective figures.

CLASSIFICATION OF ENGINES

Internal combustion engines are usually classified on the basis of the thermodynamic
cycle operation, type of fuel used, method of charging the cylinder, type of ignition, type
of cooling and the cylinder arrangement etc.

Details are given in the following figure:

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1- Cycle of operation

According to the cycle of operation, IC engines are basically classified into two
categories

i. Constant volume heat addition cycle engine or Otto cycle engine. It is also called a
Spark-Ignition engines, SI engine or Gasoline engine.

ii. Constant pressure heat addition cycle engine or Diesel cycle engine. It is also
called a compression-ignition engine, CI engine or Diesel engine.

2- Type of fuel used

Based on the type of fuel used engines are classified as

I. Engines using volatile liquid fuel like gasoline, alcohol, kerosene, benzene etc.
The fuel is generally mixed with air to form a homogeneous charge in a carburettor
outside the cylinder and drawn into the cylinder in its suction stroke. The charge is
ignited near the end of the compression stroke by an externally applied spark and
therefore these engines are called spark-ignition engines.

II. Engines using gaseous fuels like natural gas, Liquefied Petroleum Gas (LPG), blast
furnace gas and biogas.
The gas is mixed with air and the mixture is introduced into the cylinder during the
suction process. Working of this type of engine is similar to that of the engines
using volatile fuels (SI engine).

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III. Engine using solid fuels like charcoal, powdered coal etc.
Solid fuels are generally converted into gaseous fuels outside the engine in a
separate gas producer and the engine works as a gas engine.

IV. Engines using viscous (low volatility at normal atmospheric temperatures) liquid
fuels like heavy and light diesel oils.
The fuel is generally introduced into the cylinder in the form of minute droplets by
a fuel injection system near the end of the compression process. Combustion of the
fuel takes place due to its coming into contact with the high temperature
compressed air in the cylinder. Therefore, these engines are called compression-
ignition engines.

V. Engines using two fuels (dual-fuel engines)

A gaseous fuel or a highly volatile liquid fuel is supplied along with air during the
suction stroke or during the initial part of the compression through a gas valve in
the cylinder head and the other fuel (a viscous liquid fuel) is injected into the
combustion space near the end of the compression stroke (dual-fuel engines).

3- Method of charging

According to the method of charging, the engines are classified as

i. Naturally aspirated engines: Admission of air or fuel-air mixture at near

atmospheric pressure.

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ii. Forced induction engines: admission of air or fuel-air mixture under pressure, i.e.,
above atmospheric pressure.
If the power for the induction is provided mechanically by means of a belt, gear,
shaft, or chain connected to the engine's crankshaft it is called a supercharged
engine. When power is provided by a turbine powered by exhaust gas, it is known
as a turbocharged engine.

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Supercharged Engine

Four-Stroke Engine with Turbocharger

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Four-Stroke Engine with Turbocharger

4- Type of ignition

Spark-ignition engines requires an external source of energy for the initiation of spark and
thereby the combustion process. A high voltage spark is made to jump across the spark
plug electrodes. In order to produce the required high voltage there are two types of
ignition system which are normally used. They are:

1- Battery ignition system

2- Magneto ignition system

They drive their name based on whether a battery or a magneto is used as the primary
source of energy for producing the spark.
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In the case of CI engines there is no need for an external means to produce the ignition.
Because of high compression ratio employed, the resulting temperature at the end of the
compression process is high enough to self-ignite the fuel when injected. However, the
fuel should be atomized into very fine particles. For this purpose a fuel injection system is
normally used.

5- Type of cooling

Cooling is very essential for the satisfactory running of an engine. There are two types of
cooling systems in use and accordingly, the engines is classified as

1- Air-cooled engine

2- Water-cooled engine

6- Cylinder arrangements

Another common method of classifying reciprocating engines is by the cylinder

arrangement. The cylinder arrangement is only applicable to multicylinder engines.

A number of cylinder arrangements popular with designers are described below

1. In-line engine: the in-line engine is an engine with one cylinder bank, i.e. all cylinders
are arranged linearly, and transmit power to a single crankshaft. This type is quite
common with automobile engines. Four and six cylinders in-line engines are popular
in automotive applications.

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2. ‘V’ Engine: in this engine there are two banks of cylinders (i.e., two in line engines)
inclined at an angle to each other and with one crankshaft. Most of the high powered
automobiles use the 8 cylinder ‘V’ engine, four in-line on each side of the ‘V’.
Engines with more than six cylinders generally employ this configuration.

3. Opposed cylinder engine: this engine has two cylinder banks located in the same
plane on opposite sides of the crankshaft. It can be visualized as two ‘in-line’
arrangements 180 degrees apart. It is inherently a well balanced engine and has the
advantages of a single crankshaft. This design is used in small aircrafts.

4. Opposed piston engine: when a single cylinder houses two pistons, each of which
driving a separate crankshaft, it is called an opposed piston engine. The movement of
the pistons is synchronized by coupling the two crankshafts. Opposed piston
arrangement, like opposed cylinder arrangement, is inherently well balanced. Further,
it has the advantage of requiring no cylinder head. By its inherent features, this engine
usually functions on the principle of two-stroke engines.

5. Radial engine: Radial engine is one where more than two cylinders in each row are
equally spaced around the crankshaft. The radial arrangement of cylinders is most
commonly used in conventional air-cooled aircraft engines, where 3, 5, 7 or 9
cylinders may be used in one bank and two to four banks of cylinders may be used.
The odd number of cylinders is employed from the point of view of balancing. Pistons
of all the cylinders are coupled to the same crankshaft.

6. ‘X’ type engine: this design is a variation of ‘V’ type. It has four banks of cylinders
attached to a single crankshaft.
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7. ‘H’ type engine: the ‘H’ type is essentially two ‘Opposed cylinder’ type utilizing two
separate but interconnected crankshafts.

8. ‘U’ type engine: the ‘U’ type is a variation of opposed piston arrangement.

9. Delta type engine: the delta type is essentially a combination of three opposed piston
engine with three crankshafts interlinked to one another.

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In general, automobile engines and general purpose engines utilize the ‘in-line’ and ‘V’
type configuration or arrangement. The ‘radial’ engine was used widely in medium and
large aircrafts till it was replaced by the gas turbine. Small aircrafts continue to use either
the ‘opposed cylinder’ type or ‘in-line’ or ‘V’ type engines. The ‘H’ and ‘X’ types do not
presently find wide applications, except in some diesel installations. A variation of the
‘X’ type is referred to as the ‘pancake’ engine.

APPLICATION OF IC ENGINES

The most important application of IC engines is in transport on land, sea and air. Other
applications include industrial power plants and as prime movers for electric generators.
Table below gives in a nutshell, the application of both IC and EC engines.

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1. Two-stroke gasoline engines

Small two-stroke gasoline engines are used where simplicity and low cost of the prime
mover are the main considerations. In such applications a little higher fuel
consumption is acceptable. The smallest engines are used in mopeds (50 cc engine)
and lawn mowers. Scooters and motor cycles, the commonly use two wheeler
transport, have generally 100-150 cc, two-stroke gasoline engines developing a
maximum brake power of about 5 kW at 5500 rpm. High powered motor cycles have
generally 250 cc two stroke gasoline engines developing a maximum brake power of
about 10 kW at 5000 rpm. Two-stroke gasoline engines may also be used in very
small electric generating sets, pumping sets, and outboard motor boats. However, their
specific fuel consumption is higher due to the loss of fuel-air charge in the process of
scavenging and because of high speed of operation for which such small engines are
designed.

2. Two-stroke diesel engines

Very high power diesel engines used for ship propulsion are commonly two-stroke
diesel engines. The brake power on a single crankshaft can be up to 37000 kW.
Nordberg, 12 cylinder 800 mm bore and 1550 mm stroke, two-stroke diesel engine
develops 2000 kW at 120 rpm. This speed allows the engine to be directly coupled to
the propeller of a ship without the necessity of gear reducers.

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3. Four-stoke gasoline engines

The most important application of small four-stroke gasoline is in automobiles. A

typical automobile is powered by a four-stroke four cylinder engine developing an
output in the range of 30-60 kW at a speed of about 4500 rpm. American automobile
engines are much bigger and have 6 or 8 cylinder engines with a power output up to
185 kW. However, the oil crisis and air pollution from automobile engines have
reversed this trend towards smaller capacity cars.

Four-stroke gasoline engines were also used for buses and trucks. They were generally
4000 cc, 6 cylinder engines with maximum brake power of about 90 kW. However, in
this application gasoline engines have been practically replaced by diesel engines. The
four-stroke gasoline engines have also been used in big motor cycles with side cars.
Another application of four-stroke gasoline engine is in small pumping sets and
mobile electric generating sets.

Small aircraft generally use radial four-stroke gasoline engines. Engines having
maximum power output from 400 kW to 4000 kW have been used in aircraft. An
example is the Bristol Contours 57, 18 cylinder two rows, sleeve valve, air-cooled
radial engine developing a maximum brake power of about 2100 kW.

4. Four-stroke diesel engines

The four-stroke diesel engine is one of the most efficient and versatile prime movers.
It is manufactured in sizes from 50 mm to more than 1000 mm of cylinder diameter
and with engine speeds ranging from 100 to 4500 rpm while delivering outputs from 1
to 35000 kW.

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Small diesel engines are used in pump sets, construction machinery, air compressors,
drilling rigs and many miscellaneous. Tractors for agricultural application use about
40 to 100 kW diesel engines. Generally, the diesel engines with higher outputs than
about 100 kW are supercharged. Earth moving machines use supercharged diesel
engines in the output range of 200 to 400 kW. Locomotive applications require
outputs of 600 to 4000 kW. Marine applications, from fishing vessels to ocean going
ships use diesel engines from 100 to 35000 kW. Diesel engines are used both for
mobile and stationary electric generating plants of varying capacities. Compared to
gasoline engines, diesel engines are more efficient and therefore manufactures have
come out with diesel engines in personal transportation. However, the vibrations from
the engine and the unpleasant odor in the exhaust are the main drawbacks.

THE FIRST LAW ANALYSIS OF ENGINE CYCLE

Before a detailed thermodynamic analysis of the engine cycle is done, it is desirable to

have a general picture of the energy flow or energy balance of the system so that one
becomes familiar with the various performance parameters. Figure below shows the
energy flow through the reciprocating engine

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And figure below shows its block diagram as an open system.

According to the first law of thermodynamics, energy can be neither be created nor
destroyed. It can only be converted from one form to another.

Therefore, there must be an energy balance of input and output to a system. In the
reciprocating internal combustion engine the fuel is fed into the combustion chamber
where it burns in air converting chemical energy of fuel into heat. The liberated heat
energy cannot be totally utilized for driving the piston as there are losses through the
engine exhaust, to the coolant and due to radiation. The heat energy which is converted to
power at this stage is called the indicated power, Pi and it is utilized to drive the piston.
The energy represented by the gas forces on the piston passes through the connecting rod
to the crankshaft. In this transmission there are energy losses due to bearing friction,
pumping losses etc. In addition, a part of the energy available is utilized in driving the
auxiliary devices like feed pump, valve mechanisms, Ignition systems etc. The sum of all
these losses, expressed in units of power is termed as frictional power, Pf.

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The remaining energy is the useful mechanical energy and is termed as the brake power,
Pb. In energy balance, generally, frictional power is not shown separately because
ultimately this energy is accounted in exhaust, cooling water, radiation, etc.

Pi  Pb  Pf

ENGINE PERFORMANCE PARAMETERS

The engine performance is indicated by the term efficiency, . Five important engine
efficiencies and other related engine performance parameters are given below:

i. Indicated thermal efficiency (ith)

ii. Brake thermal efficiency (bth)
iii. Mechanical efficiency (m)
iv. Volumetric efficiency (v)
v. Relative efficiency or Efficiency ratio (rel)
vi. Mean effective pressure (pme)
vii. Mean piston speed (Vm)
viii. Specific power output (Ps)
ix. Specific fuel consumption (sfc)
x. Inlet-valve Mach Index (Z)
xi. Fuel-air or air-fuel ratio (A/F)
xii. Calorific value of the fuel or Specific heating value (The fuel power) (shv)

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Figure below shows the diagrammatic representation of energy distribution in an IC

engine.

Energy losses in exhaust,

coolant, radiation etc.
Energy (E) in fuel (kW)

Energy losses in friction,

pumbing, etc
Pi (kW)

Pb (kW)

The most important engine performance parameters are power, torque. Brake torque is
normally measured with a dynamometer – engine is mounted on a test bed and the shaft is
connected to the dynamometer rotor. The rotor is coupled electromagnetically,
hydraulically or by mechanical friction to a stator, which is supported in low friction
bearings. Torque exerted on the stator with the rotor turning is measured by balancing the
stator with weights, springs or pneumatic means.

The torque exerted by the engine is,

T  F b

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Indicated power

Indicated power (Pi) is the theoretical maximum output power of the engine. The
indicated power is the total power available from the expanding of the gases in the
cylinders negating any friction, heat loss or entropy within the system.

Brake power

Brake power (Pb) is the power output of the drive shaft of an engine without the power
loss caused by gears, transmission, etc. It's called also pure power, useful power, true
power or wheel power as well as other terms.

The power delivered by engine is, product of torque and angular speed,

2
Pb  NT
60
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where N is the number of crankshaft revolution per minute (r.p.m)

Indicated Thermal Efficiency (ith)

Indicated thermal efficiency is the ratio of energy in the indicated power (Pi) to the input
fuel energy in appropriate units. Input fuel energy is the fuel energy (specific energy
content) multiplied by fuel mass flow rate

Pi
 ith 
m f  shv

where m f is mass flow rate of the fuel (kg/s) and shv is Calorific value of the fuel
(kJ/kg).

Brake Thermal Efficiency (bth)

Brake thermal efficiency is the ratio of energy in the brake power (Pb) to the input fuel
energy in appropriate units.

Pb
 bth 
m f  shv

Mechanical Efficiency (m)

Mechanical efficiency is the ratio of brake power (delivered power) to the indicated
power (power provided to the piston).

Pb Pb
m  
Pi Pb  Pf
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It can also be defined as the ratio of the brake thermal efficiency to the indicated thermal
efficiency.

Volumetric Efficiency (v)

The intake system, the air filter, carburetor, and throttle plate, intake manifold, intake
port, intake valve, restricts the amount of air which an engine of given displacement can
induct. The parameter used to measure the effectiveness of an engine's induction process
is the volumetric efficiency (v).

Volumetric efficiency is only used with four stroke cycle engines which have a distinct
induction process. It is one of the very important parameters which decide the
performance of four-stroke engines. Four-stroke engines have distinct suction stroke and
therefore the volumetric efficiency indicates the breathing ability of the engine. It is to be
noted that the utilization of the air is what going to determine the power output of the
engine. Hence, an engine must be able to take in as much air as possible.

Volumetric efficiency is defined as the volume flow rate of air into the intake system
divided by the rate at which the volume is displaced by the piston.

ma
v 
 airVs N 2

where m a is mass flow rate of the inlet air (kg/s) and air is the inlet air density.

It is to be noted that irrespective of the engine whether SI or CI, volumetric rate of air
flow is what to be taken into account and not the mixture flow. The inlet density may
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either be taken as atmosphere air density (in such case volumetric efficiency measures the
pumping performance of the entire inlet system) or may be taken as the air density in the
inlet manifold (in such case volumetric efficiency measures the pumping performance of
the inlet port and valve only).

Typical maximum values of volumetric efficiency for naturally aspirated engines are in
the range 80 to 90 percent. The volumetric efficiency for diesels is somewhat higher than
for SI engines. Gas engines have much lower volumetric efficiency since gaseous fuel
displaces air and therefore the breathing capacity of the engine is reduced.

Relative Efficiency or Efficiency Ratio (rel)

Relative efficiency or Efficiency ratio (rel) is the ratio of thermal efficiency of an actual
cycle to that of the ideal cycle. The efficiency ratio is a very useful criterion which
indicates the degree of development of the engine.

Actual thermal efficiency

 rel 
Air - standard efficiency

Mean Effective Pressure (pme)

A more useful relative engine performance measure is obtained by dividing the work per
cycle by the cylinder volume displaced per cycle. The parameter so obtained has units of
force per unit area and is called the mean effective pressure (pme).

Mean effective pressure is the average pressure inside the cylinders of an internal
combustion engine based on the calculated or measured power output. It increased as
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manifold pressure increases. For any particular engine, operating at a given speed and
power output, there will be a specific indicated mean effective pressure, pime and a
corresponding break mean effective pressure, pbme. They are derived from the indicated
and brake power respectively.

Since, the indicated power is related to the indicated work per cycle by

Wc , i N
Pi 
nR

where nR is the number of crank revolutions for each power stroke per cylinder. For four-
stroke cycles, nR equals 2; for two-stroke cycles, nR equals 1. So,

Pi n R
pime 
Vs N

Pb n R
pbme 
Vs N

Another way of specifying the indicated mean effective pressure, pime is from the
knowledge of engine indicator diagram (p-v diagram). In this case, specific indicated
mean effective pressure, pime may be defined as

Area of the indicator diagram

pime 
Length of the indicator diagram

where the length of the indicator diagram is given by the difference between the total
volume and the clearance volume.
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Mean Piston Speed (Vm)

Mean piston speed is the distance traveled by the piston per unit of time,

2 LN
Vm  m/ s
60

It is an important parameter in engine applications and it is often a more appropriate than

crank rotational speed for correlating engine behavior as function of speed.

Resistance to gas flow into the engine or stresses due to the inertia of the moving parts
limit the maximum value of mean piston speed (Vm) to within 8 to 15 m/s. Automobile
engines operate at the higher end and large marine diesel engines at the lower end of this
range of piston speeds.

Specific Power Output (Ps)

Specific power output (Ps) of an engine is defined as the power output (brake power) per
unit piston area and is a measured of the engine designer’s success in using the available
piston area regardless of cylinder size. The specific power can be shown to be
proportional to the product of the mean effective pressure and mean piston speed.

Pb
Ps 
A

 constant  pbme  Vm

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As can be seen the specific power output consists of two elements, viz., the force
available to work and the speed with which it is working. Thus, for the same piston
displacement and brake mean effective pressure (pbme), an engine running at a higher
speed will give a higher specific output. It is clear that the output of an engine can be
increased by increasing either the speed or the brake mean effective pressure (pbme).
Increasing the speed involves increase in the mechanical stresses of various engine
components. For increasing the brake mean effective pressure (pbme) better heat release
from the fuel is required and this will involve more thermal load on engine cylinder.

Specific Fuel Consumption (sfc)

The specific fuel consumption (sfc) is a formula that is used to measure the fuel efficiency
of a basic engine or a shaft rotating engine. This formula in general, is the specific fuel
consumption in kilograms of fuel divided by the brake power in kilowatt-hour. This ratio
is used to determine the amount of power provided by an engine when compared to the
consumption of fuel required to generate that power. It measures how efficiently an
engine is using the fuel supplied to produce work. It is an important parameter that
reflects how good the engines performance is. It is inversely proportional to the thermal
efficiency of the engine. In general,

m f
sfc 
Power

Brake specific fuel consumption and indicated specific fuel consumption, abbreviated as
sfcb and sfci, are the specific fuel consumption on the basis of brake power (Pb) and
indicated power (Pi) respectively.

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Inlet-Valve Mach Index (Z)

In a reciprocating engine the flow of the intake charge takes place through the intake
valve opening which is varying during the induction operation. Also, the maximum gas
velocity through this area is limited by the local sonic velocity. Thus gas velocity is
finally chosen by the following equation,

A
u Vm
Ci Ai

where

u: gas velocity through the inlet valve at smallest flow area

A: piston area
Ai: nominal intake valve opening area
Ci: inlet valve flow coefficient

and

u A Vm
Z 
 Ai Ci

2
 d  Vm
Z   
 Di  Ci

where

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d: cylinder diameter
D: inlet valve diameter
: inlet sonic velocity
Ci: inlet valve flow coefficient
Z: inlet valve Mach index.

Large numbers of experiments have been conducted on single cylinder Cooperative Fuel
Research (CFR) engine with gaseous mixtures and short induction pipe lengths, at fixed
valve timing and fixed compression ratio, but varying inlet valve diameter and lift. The
results are quite revealing as regards the relationship of volumetric efficiency versus
Mach index are concerned. From the figure below, it could be seen that the maximum
volumetric efficiency is obtained for an inlet Mach number of 0.55. Therefore, engine
designers must take care of this factor to get the maximum volumetric efficiency for their
engines.

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Fuel-Air or Air-Fuel Ratio (A/F)

The relative proportions of the fuel and air in the engine are very important from the
standpoint of combustion and the efficiency of the engine. This is expressed either as a
ratio of the fuel to that of the air or vice versa.

In the SI engine, the fuel-air ratio practically remains a constant over a wide range of
operation. In CI engines at a given speed the air flow does not vary with load; it is the
fuel flow that varies directly with load. Therefore, the term fuel-air ratio is generally used
instead of air-fuel ratio.

A mixture that contains just enough air for complete combustion of all the fuel in the
mixture is called a chemically correct or stoichiometric fuel-air ratio. A mixture having
more fuel than that in a chemically correct mixture is termed as rich mixture and a
mixture that contains less fuel (or excess air) is called a lean mixture. The ratio of actual
fuel-air to stoichiometric fuel-air ratio is called equivalence ratio and denoted by .

Actual fuel - air ratio


Stoichiometric fuel - air ratio

Accordingly, means stoichiometric (chemically correct) mixture, < 1 means lean

mixture and > 1 means rich mixture.

Calorific Value of the Fuel or Specific Heating Value (The Fuel Power)
(Shv)

Specific heating value (shv) or specific energy content and sometimes called the heat of

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combustion or calorific value of the fuel is an important property of fuels. This property
helps scientists and engineers determine the usefulness of a fuel. Specific heating value
(shv) is the amount of heat produced by the burning of 1 gram of a substance, and is
measured in joules per gram (J/g, kJ/kg or kcal/kg).

Gaseous fuels specific heating value is given in terms of energy content per unit volume
(kJ/liter or kJ/m3, kcal/m3).

Heating value of the combustible air-fuel mixture is a significant factor for engine
performance.

You can determine the specific heating value (shv) of a fuel by burning an amount of the
fuel and capturing the heat released in a known mass of water in a calorimeter. If you
measure the initial and final temperatures, the energy released can be calculated using the
equation

H = ∆t m Cp

where H = heat energy absorbed (in J), ∆t = change in temperature (in °C), m = mass (in
g), and Cp = specific heat capacity for water (4.18 J/g °C).

Dividing the resulting energy value by grams of fuel burned gives the specific heating
value (shv) (in J/g).

When the products of the combustion are cooled to 25 oc practically all the water vapour
resulting from the combustion process is condensed. The specific heating value (shv) so
obtained is called the higher calorific (HHV) or gross calorific value of the fuel. The
lower or net calorific value (LHV) is the heat released when water vap/our in the products
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of the combustion is not condensed and remains in the vapour form.

The formula for calculating the fuel power is the amount of fuel multiplied by the specific
heating value (thermal energy of fuel),

Fuel Power  m f  shv

Design and Performance Data

Engine ratings usually indicate the highest power at which manufacturers expect their
products to give satisfactory economy, reliability, and durability under service conditions.
Maximum torque, and the speed at which it is achieved, is also usually given. Since both
of these quantities depend on displaced volume, for comparative analysis between
engines of different displacements in a given engine category normalized performance
parameters are more useful.

Typical design and performance data for SI and CI engines used in different applications
are summarized in the table below. The four-stroke cycle dominates except in the
smallest and largest engines. The larger engines are turbocharged or supercharged. The
maximum rated engine speed decreases as engine size increases, maintaining the
maximum mean piston speed in the range of about 8 to 15 m/s. The maximum brake
mean effective pressure for turbocharged and supercharged engines is higher than for
naturally aspirated engines. Because the maximum fuel-air ratio for SI engines is higher
than for CI engines, their naturally aspirated maximum brake mean effective pressure
levels are higher. As the engine size increases, brake specific fuel consumption decreases
and fuel conversion efficiency increases due to the reduced heat losses and friction. For
the large CI engines, brake thermal efficiencies of about 40 % and indicated thermal

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efficiencies of about 50 % can be obtained in modern engines.

Example (1):

The cubic capacity of a four-stroke over-square spark-ignition engine is 245 cc. The over-
square ratio is 1.1. The clearance volume is 27.2 cc. Calculate the bore, stroke and
compression ratio of the engine.

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Solution:


Cubic capacity (CC)  Vs  K  d 2L
4

d
 1.1
L

d 3  343.138 , Bore, d = 7 cm

7
Storke, L   6.36 cm
1.1

Vc  Vs 27.2  245
Compression ratio, r    10
Vc 27.2

Example (2):

The mechanical efficiency of a single-cylinder four-stroke engine is 80 %. The frictional

power is estimated to be 25 kW. Calculate the brake power (Pb) and indicated power (Pi)
developed by the engine.

Solution:

Pb Pb
m  
Pi Pb  Pf

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Pb
0.8 
Pb  25

Pb  0.8Pb  20

20
Pb   100 kW
0.2

Pi  Pb  Pf  100  25  125 kW

Example (3):

A 42.5 kW engine has a mechanical efficiency of 85 %. Find the indicated power and
frictional power. If the frictional power is assumed to be constant with load, what will be
the mechanical efficiency at 60 % of the load?

Solution:

Pb 42.5
Indicated power, Pi    50 kW
m 0.85

Frictional power, Pf  Pi  Pb  50  42.5  7.5 kW

Brake power at 60 % load  42.5  0.6  25.5 kW

Pb 25.5
Mechanical efficiency,  m    0.773  77.3%
Pb  Pf 25.5  7.5

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Example (4):

Find out the speed at which a four-cylinder engine using natural gas can develop a brake
power of 50 kW working under following conditions. Air-gas ratio 9:1, calorific value of
the fuel = 34 MJ/m3. Compression ratio 10:1, volumetric efficiency = 70%, indicated
thermal efficiency = 35%, the mechanical efficiency = 80% and the total volume of the
engine is 2 liters.

Solution:

2000
Total volume / cylinder, V   500 cc
4

V 500
Clearance volume/ cylinder, Vc    50 cc
r 10

Swept volume / cylinder, Vs  500  50  450 cc

Volume of air taken in / cycle   v  Vs

 0.7  450  315 cc

315
Volume of gas taken in / cycle   35 cc
9

Input energy / cylinder  35  106  34  103  1.19 kJ

Pb 50
Indicated power, Pi    62.5 kW
m 0.8

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Pi
 ith 
Fuel Power

Pi 62.5
Fuel Power    178.571 kW
 ith 0.35

178.571
Input energy/cylinder  s   44.642 kJ
4

Input energy/cylinder  s
Now, input energy/power stroke 
N 2  60

44.642
1.19 
N 2  60

N  4500 rpm

Example (5):

A four-stroke, four-cylinder diesel engine running at 2000 rpm develops 60 kW. Brake
thermal efficiency is 30% and calorific value of fuel is 42 MJ/kg. Engine has a bore of
120 mm and stroke of 100 mm. Take a = 1.15 kg/m3, air-fuel ratio = 15:1 and m = 0.8.
Calculate (i) fuel consumption (ii) air consumption (iii) indicated thermal efficiency (iv)
volumetric efficiency (v) brake mean effective pressure and (vi) mean piston speed.

Solution:

. Pb 60
Fuel consumption, m f    4.76  10 3 kg/s
bth  shv 0.3  42000

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.
mf 4.76 10 3
Air consumption, Va  A / F   3 3
15  62.09  10 m /s
a 1.15

Pb 60
Indicated power, Pi    75 kW
m 0.8

Pig 75
ith    0.37515  37.51%
m f  shv 4.67 10 3  42000

π 2
Vs  d  L  1.1309  10 3 m 3
4

62.09  10 3
Air flow rate / cylinder   15.52  10 3 m3 /s
4

V 15.52  10 3
v  a   0.823 =82.3%
Vd N 3 2000
1.1309  10 
60

Pb n R
pbme 
Vd N

60  10 3
  31.83  10 5 N/m 2
2000
1.1309  10 3 
2  60

pbme
pbme / cylinder   7.96 bar
4

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2lN
Vm  m/ s
60

2  0.1  2000
Vm   6.67 m/s
60

Example (6):

A single-cylinder, four-stroke hydrogen fuelled spark-ignition engine delivers a brake

power of 20 kW at 6000 rpm. The air-gas ratio is 8:1 and the calorific value of the fuel is
11000 kJ/m3. The compression ratio is 8:1. If volumetric efficiency is 70%, indicated
thermal efficiency is 33% and the mechanical efficiency is 80%, calculate the cubic
capacity of the engine.

Solution:

Pb 20
Pi    25 kW
m 0.8
Pi
 ith 
Fuel power

25
Fuel power   75.76 kJ/s
0.33

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N
Number of power strokes / s 
2  60

6000
  50 stroke/s
120

75.76
Input energy / power stroke   1.52 kJ
50

Input energy / power stroke

Volume of H 2 taken in 
shv

1.52
  0.00013818 m 3  138.18 cc
11000

Volume of air taken in   A / F  138.18  1105.44 cc

Volume of air taken in

Swept volume, Vs 
v

1105.44
  1579.2 cc
0.7

Cubic capacity, CC = Vs  K

 1579.2  1  1579.2 cc

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FIRST LAW OF THERMODYNAMICS

Whenever a system undergoes a cyclic change, the algebraic sum of work transfer is
proportional to the algebraic sum of heat transfer as work and heat are mutually
convertible from one form into the other.

For a closed system, a change in the energy content is the algebraic difference between
the heat supply, Q, and the work done, W, during any change in the state. Mathematically,

dE = Q – W

The energy E may include many types of energies like kinetic energy, potential energy,
electrical energy, surface tension etc., but from the thermodynamic point of view these
energies are ignored and the energy due to rise in temperature alone is considered. It is
denoted by U and the first law written as:

dU = Q – W

or during a process 1-2, dU can be denoted as

2
U 2  U 1   Q  W 
1

PROCESS

A change in the condition or state of a substance is called a process. The process may
consist of heating, flow from one place to another, expansion etc. In general, a process
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may be divided into non-flow and flow processes.

Non-flow Processes

If there is no flow of material into or out of a system during a process, it is called a non-
flow process. This is the simplest kind of process, and much can be learned about it by
applying the principle of conservation of energy. Non-flow processes are divided to,

1. Constant volume or isochoric process

2. Constant pressure or isobaric process

3. Constant temperature or isothermal process

4. Reversible adiabatic or isentropic process

5. Reversible polytropic process

IDEAL AIR STANDARD CYCLES

Introduction

The operation cycle of an internal combustion engine can be broken down into a
sequence of separate processes viz., intake, compression, combustion, expansion and
exhaust. The internal combustion engine does not operate on a thermodynamic cycle as it
involves an open system i.e., the working fluid enters the system at one of conditions and
leaves at another. However, it is often possible to analyze the open cycle as though it

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were a closed one by imagining one or more processes that would bring the working fluid
at the exit conditions back to the condition of the starting point.

The accurate analysis of internal combustion engine processes is very complicated. In

order to understand them it is advantageous to analyze the performance of an idealized
closed cycle that closely approximates the real cycle. One such approach is the air-
standard cycle

Assumption

1. The working medium is assumed to be a perfect gas and follows the relation pV =

mRT or p = RT.

2. The working medium has constant specific heats throughout the cycle.

3. The physical constants i.e., Cp, Cv,  and M of working medium are the same as those

of air at standard atmospheric conditions. For example in SI units,

Cp = 1.005 kJ/kg K M = 29 kg/kmol

Cv = 0.717 kJ/kg K 

4. Neglect heat transfer to and from cylinder walls.

5. Replace combustion process by a heat addition process that occurs at constant volume

(in Otto cycle) or at constant pressure (in Diesel cycle).

6. Do not consider gas exchange process.

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Otto cycle

The Otto cycle is used as a basis of comparison for SI engine. The cycle consists of four
processes,
1 – 2 isentropic compression from V1 to V2.

2 – 3 addition of heat Q23 at constant volume.

3 – 4 isentropic expansion to the original volume.

4 – 1 rejection of heat Q41 at constant volume.

 Work done during the cycle, 1-2-3-4 is,

Wcycle   pdV   Tds

Constant volume heat input to the cycle per unit mass of working fluid

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T3
Q23   cv dT  cv T3  T2 
T2

Constant volume heat extraction from the cycle per unit mass

T1
Q41    cv dT  cv T1  T4   cv T4  T1 
T4

1st law of thermodynamics,

dE = Q – W
dE = 0

Thermal efficiency,

W work done
t Otto  
Q23 heat input

Q23  Q41 Q
t Otto   1  41
Q23 Q23

T4  T1
t Otto  1 
T3  T2

Initial pressure p1 and temperature T1, using p1V1  p2V2 for an adiabatic compression
and pV  mRT from ideal gas law

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
V 
p 2  p1  1 
 V2 

 1
V  cp
T2  T1  1  where  
 V2  cv

V 
Define compression ratio as r   1 
 V2 
T2  T1r  1

 From 2 → 3, constant volume heat addition,

p2V2 = mRT2

p3 V3 = mRT3

V2 = V3

p3 p2

T3 T2

p3
T3  T2
p2

p 
Let pressure ratio as    3 
 p2 

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T3  T1r  1

 From 3 → 4, adiabatic expansion,

p4V4  p3V3

p4V4V4 1  p3V3V3 1

From ideal gas law p4V4 = mRT4 p3V3 = mRT3

V4 V1
 r
V3 V2

T4V4 1  T3V3 1

T3
T4 
r  1

T4  T1

Thermal efficiency,

Thermal efficiency of Otto cycle is given by,

T4  T1
t Otto  1 
T3  T2

Placing the temperatures T2, T3 and T4 in terms of T1

76 Dr. Munther Abdullah Department of Mechanical Engineering-2018
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1
 t Otto  1 
r  1

Note that the thermal efficiency of Otto cycle is a function of compression ratio r and the
ratio of specific heats . As  is assumed to be constant for any working fluid, the
efficiency is increased by increasing the compression ratio. Further, the efficiency is
independent of heat supplied and pressure ratio. The use of gases with higher  values
would increase efficiency of Otto cycle. See the figure above.

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DIESEL CYCLE

 Work done during the cycle, 1-2-3-4 is,

Wcycle   pdV   Tds

Constant pressure heat input to the cycle per unit mass of working fluid

T3
Q23   c p dT  c p T3  T2 
T2

Constant volume heat extraction from the cycle per unit mass

T1
Q41    cv dT  cv T1  T4   cv T4  T1 
T4

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1st law of thermodynamics,

dE = Q – W
dE = 0

Thermal efficiency,

W work done
t Diesel  
Q23 heat input

Q23  Q41 Q
t  Diesel   1  41
Q23 Q23

T4  T1
 t  Diesel  1 
 T3  T2 

T2  T1r  1

V3
T3  T2  T1r  1
V2

T3  1
T4   1  T1 
r

Defining “cut-off ratio” or “load ratio”

V3

V2

Thermal efficiency of Diesel cycle is given by,

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T4  T1
 t  Diesel  1 
 T3  T2 

Placing the temperatures T2, T3 and T4 in terms of T1

1    1 
 t  Diesel  1   
r  1     1

It may be noted that the efficiency of the Diesel cycle is different from that of the Otto
cycle only in the bracketed factor. This factor is always greater than unity. Hence for a
given compression ratio, the Otto cycle is more efficient. In practice, the operating
compression ratios of diesel engines are much higher compared to spark-ignition engines
working on Otto cycle. Due to the higher compression ratios used in diesel engines the
efficiency of a diesel engine is more than that of the gasoline engine.

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DUAL CYCLE

 Work done during the cycle, 1-2-3-4-5 is,

Wcycle   pdV   Tds

Constant volume heat input followed by constant pressure heat input to the cycle per unit
mass of working fluid

Q23  cv T3  T2   c p T4  T3 

Constant volume heat extraction from the cycle per unit mass

T1
Q51    cv dT  cv T1  T5   cv T5  T1 
T5

Thermal efficiency,

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cv T5  T1 
 t  Dual  1 
cv T3  T2   c p T4  T3 

T2  T1r  1

T3  T1r  1

T4  T1r  1

T5  T1  

Thermal efficiency of Dual cycle is given by,

1    1
 t  Dual  1 
r  1   1     1

Putting,

 = 1 Otto cycle thermal efficiency is obtained

 = 1 Diesel cycle thermal efficiency is obtained

Otto cycle

1
 t Otto  1 
r  1

Diesel cycle

  1
1
 t  Diesel  1   1
r    1

Dual cycle

1    1
 t  Dual  1 
r  1   1     1
82 Dr. Munther Abdullah Department of Mechanical Engineering-2018
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COMPARISON OF IDEAL CYCLES

For  > 1 and > 1

  1
term is greater than 1
   1

Therefor t-otto >  t-diesel for a constant value of compression ratio

Also  t-otto >  t-dual >  t-diesel

Efficiency of Dual cycle lies between Otto and Diesel cycles according to the value of 

In real engines, SI engines have a compression ratio between 6:1 to 10:1, this value is
limited due to engine knock.

CI engines have compression ratio higher than 14:1 to provide temperature and pressure
required for self-ignition of the fuel.
Compression ratio of 16:1 to 18:1 is sufficient for efficiency, but used for improving
ignition quality.
High compression ratio increases thermal and mechanical stresses

Example (1):

An engine working on Otto cycle has the following conditions: pressure at the beginning
of compression is 1 bar and pressure at the end of compression is 11 bar. Calculate the
compression ratio and air-standard efficiency of the engine. Assume  = 1.4.

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Solution:

1
  1
V  p2    
r  1     11 .4
1  5.54
V2  p1 

1
Air  standard efficiency  1 
r  1

0.4
 1 
 1    0.496
 5.54 

 49.6%

Example (2):

In an engine working on ideal Otto cycle the temperature at the beginning and end of
compression are 50 oC and 373 oC. Find the compression ratio and the air-standard
efficiency of the engine.

Solution:

1
V1  T1   1
r  
V2  T2 

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1
 646  0.4
   5.66
 323 

1 T1
Otto  1   1
 1
r T2

323
 1  0.5  50%
646

Example (3):

A Diesel engine has a compression ratio of 20 and cut-off takes place at 5% of the stroke.
Find the air-standard efficiency. Assume  = 1.4 .

Solution:

85 Dr. Munther Abdullah Department of Mechanical Engineering-2018

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V1
r  20
V2

V1  20V 2

Vs  V1  V2  20V2  V2  19V2

V3  0.05Vs  V2  0.05 19V2  V2  1.95V2

V3 1.95V2
   1.95
V2 V2

1   1
  1
r  -1    1

1  1.951.4  1 
 1  0.4     0.649  64.9%
20 1.4  1.95  1 

Example (4):

Determine the ideal efficiency of the diesel engine having a cylinder with bore 250 mm,
stroke 375 mm and a clearance volume of 1500 cc, with fuel cut-off occurring at 5% of
the stroke. Assume  = 1.4 for air.

Solution:

 
Vs  d 2L   252  37.5  18407.8 cc
4 4
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Vs 18407.8
r  1  1  13.27
Vc 1500

1   1
  1
r  -1    1

V3

V2

Cut  off volume  V3  V2  0.05Vs

Vs
r 1   12.27
Vc

V3  V2  0.05 12.27Vc

V2  Vc

V3  1.6135Vc

V3
  1.6135
V2

1 1.61351.4  1
  1 
13.27 0.4 1.4  1.6135  1

 0.6052  60.52%

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IC-ENGINE FUELS

In IC engines, the chemical energy contained in the fuel is converted into mechanical
power by burning (oxidizing) the fuel inside the combustion chamber of the engine. As a
result of the chemical reactions which occur inside the cylinder, heat is released. The
fuel-air mixture (the working fluid before combustion) must stay in the cylinder for a
sufficient time so that the chemical reactions can be completed.

Classification of Engine Fuels

Fuels suitable for fast chemical reaction have to be used in IC engines,

1. Hydrocarbons in liquid form

2. Alcohols (methanol, ethanol)
3. LPG (propane and butane)
4. Natural gas (methane)
5. Hydrogen

Liquid hydrocarbons CnHm

Engine fuels are mainly mixtures of hydrocarbons, with bonds between carbon atoms and
between hydrogen and carbon atoms.
During combustion, these bonds are broken and new bonds are formed with oxygen
atoms, accompanied by the release of chemical energy. Principal products are carbon
dioxide and water vapour. Fuels also contain small amounts of O2 , N2 , S , H2O

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I. Alkanes

Alkanes or Paraffins can in general be represented by CnH2n+2, all the carbon bonds are
single bonds. They are “saturated” with high number of H atoms, high heat content and
3
low density (620 – 770 kg/m )
The carbon atoms can be arranged as a straight chain or as branched chain compounds.

1. Straight chain group (Normal paraffins)

Shorter the chain, stronger the bond not suitable for SI engines, high tendency for auto
ignition according to the value of “n” in the formula, they are in gaseous (1 to 4), liquid
(5 to 15) or solid state (>16).

The simplest possible alkane is methane, CH4 and Ethane, C2H6. Another example is
Hexane

2. Branched chain compounds (Isoparaffins)

When four or more C atoms are in a chain molecule it is possible to form isomers, they
have the same chemical formula but different structures, which often lead to very
different chemical properties.

Example: Iso-octane C8 H18

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Branched chain compounds (isoparaffin)

iso-octane 2. 2 .4 trimethyl pentane tri methyl 3 (CH3)

3. Naphthenes

Naphthenes also called cycloparaffins CnH2n. Saturated hydrocarbons which are arranged
in a circle have stable structure and low tendency to auto ignite compared to alkanes
(normal paraffins).

Naphthenes can be used both in SI-engines and CI-engines. Low heat content and high
3
density (740 – 790 kg / m )

II. Alkenes

Also called olefins, mono-olefins CnH2n or dio-olefins CnH2n−2, have the same C-to-H ratio
and the same general formula as Naphthenes, their behavior and characteristics are
entirely different.

They are straight or branch chain compounds with one or more double bond. The position
of the double bond is indicated by the number of first C atom to which it is attached, ie,

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CH2=CH.CH2.CH2.CH3 called pentene-1

CH3.CH=CH3 called butene-2

Olefinic compounds are easily oxidized, have poor oxidation stability. It can be used in
SI-engines, obtained by cracking of large molecules low heat content and density in the
3
range 620 – 820 kg/m

Hexen (mono-olefin)

Butadien (dio-olefin)

III. Aromatic

Aromatic hydrocarbons CnH2n−6 are so called because of their aromatic odor. They are
based on a six-membered ring having three conjugated double bonds. Aromatic rings can
be fused together to give Polynuclear Aromatics, PAN, also called Polycyclic Aromatic
Hydrocarbons, PAH, simplest member is benzene (C6H6).

Aromatic hydrocarbons can be used in SI-engines, to increase the resistance to knock, not
suitable for CI-engines due to low cetane number.
3
Low heat content and high density in the range 800 – 850 kg/m

Benzene C6H6

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Alcohols (monohydric alcohols)

These include methanol (methyl alcohol), ethanol (ethyl alcohol), propanol (propyl
alcohol), butanol (butyl alcohol) as compounds.

The OH group which replaces one of the H atoms in an alkane, gives these compounds
their characteristic properties.
Specific heating value is lower than gasoline (42 – 43 MJ/kg) methanol (19.7 MJ/kg) and
ethanol (26.8 MJ/kg) for air-fuel mixture s.h.v. is comparable with gasoline (MJ/kg-
mixture at stoichiometric mixtures), other alcohol groups such as dihydric and trihydric
alcohols are not used as a fuel in IC engines.

i. Methanol CH3OH

Methanol also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits. It
acquired the name "wood alcohol" because it was once produced chiefly as a byproduct
of the destructive distillation of wood. Modern methanol is produced in a catalytic
industrial process directly from carbon monoxide, carbon dioxide, and hydrogen.
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Methanol is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with
a distinctive odor very similar to that of ethanol (drinking alcohol). However, unlike
ethanol, methanol is highly toxic and unfit for consumption. At room temperature, it is a
polar liquid, and is used as an antifreeze, solvent, fuel, and as a denaturant for ethanol. It
is also used for producing biodiesel via transesterification reaction.

Methanol is produced naturally in the anaerobic metabolism of many varieties of bacteria,

and is commonly present in small amounts in the environment. As a result, there is a
small fraction of methanol vapor in the atmosphere. Over the course of several days,
atmospheric methanol is oxidized with the help of sunlight to carbon dioxide and water.

Methanol burns in oxygen, including open air, forming carbon dioxide and water:

2 CH3OH + 3 O2 → 2 CO2 + 4 H2O

Methanol ingested in large quantities is metabolized to formic acid or formate salts,

which is poisonous to the central nervous system, and may cause blindness, coma, and
death. Because of these toxic properties, methanol is frequently used as a denaturant
additive for ethanol manufactured for industrial uses.

Methanol near and long-term potential, it has high octane quality (130 RON, 95 MON), it
can be used in low-concentration (5-15 %) in gasoline to increase octane number of the
mixture. Problems; poor solubility in gasoline, toxicity, low energy content (about half of
gasoline), high latent heat of vaporization and oxygen content contribute to poor
driveability, incompatibility with some metals.

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ii. Ethanol C2H5OH

Ethanol produced from biomass, it has high octane number so it can be used in low–
concentrations in gasoline.

Ethanol is an alcohol fuel made from the sugars found in grains, such as; Corn, Sorghum
and Barley. Other sources of sugars to produce ethanol include: Potato skins, Rice, Sugar
cane, Sugar beets, Yard clippings, Bark and Switchgrass. Most of the ethanol used in the
United States today is distilled from corn. Scientists are working on cheaper ways to
make ethanol by using all parts of plants and trees rather than just the grain.

About 99% of the ethanol produced in the United States is used to make "E10" or
"gasohol," a mixture of 10% ethanol and 90% gasoline. Any gasoline powered engine can
use E10, but only specially made vehicles can run on E85, a fuel that is 85% ethanol and
15% gasoline.

Biodiesel

Biodiesel is a fuel made from vegetable oils, fats, or greases such as recycled restaurant
grease. It refers to a vegetable oil - or animal fat-based diesel fuel consisting of long-
chain alkyl (methyl, ethyl, or propyl) esters. Biodiesel is typically made by chemically
reacting lipids (e.g., vegetable oil, animal fat with an alcohol producing fatty acid esters.

Biodiesel is meant to be used in standard diesel engines and is thus distinct from the
vegetable and waste oils used to fuel converted diesel engines. Biodiesel can be used
alone, or blended with petrodiesel in any proportions. Biodiesel blends can also be used
as heating oil.

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It is the first and only Environmental Protection Agency (EPA) designated Advanced
Biofuel in commercial-scale production across the country (USA) and the first to reach 1
billion gallons of annual production (1 US gal = 3.78541 Liter). Meeting strict technical
fuel quality and engine performance specifications, it can be used in existing diesel
engines without modification and is covered by all major engine manufacturers’
warranties.

Biodiesel’s environmental benefits have recognized by classifying it as an Advanced

Biofuel, making biodiesel the only commercial-scale U.S. fuel produced nationwide.
According to the EPA, biodiesel reduces greenhouse gas emissions by at least 57 percent
and up to 86 percent when compared to petroleum diesel – making it one of the most
practical and cost-effective ways to immediately address climate change. In addition,
biodiesel sharply reduces major tailpipe pollutants from petroleum diesel, particularly
from older diesel vehicles. This is important because the EPA has consistently cited
diesel exhaust – primarily from older trucks, buses and other vehicles – as one of the
nation's most dangerous pollutants.

Blends of biodiesel and conventional hydrocarbon-based diesel are products most

commonly distributed for use in the retail diesel fuel marketplace. Much of the world uses
a system known as the "B" factor to state the amount of biodiesel in any fuel mix:

 100% biodiesel is referred to as B100

 20% biodiesel, 80% petrodiesel is labeled B20
 5% biodiesel, 95% petrodiesel is labeled B5
 2% biodiesel, 98% petrodiesel is labeled B2

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Blends of 20% biodiesel and lower can be used in diesel equipment with no, or only
minor modifications, although certain manufacturers do not extend warranty coverage if
equipment is damaged by these blends. Biodiesel can also be used in its pure form
(B100), but may require certain engine modifications to avoid maintenance and
performance problems.

Biodiesel is made through a chemical process called transesterification whereby the

glycerin is separated from the fat or vegetable oil. The process leaves behind two
products; methyl esters (the chemical name for biodiesel) and glycerin (a valuable
byproduct usually sold to be used in soaps and other products).

GASEOUS FUELS

Due to storage and transportation problems, they are not widely used. They reduce
volumetric efficiency and power output of engine (5 – 10 %). They have low tendency to
knock and low emissions.

Liquefied petroleum gas

LPG (liquefied petroleum gas) consists of propane and butane provides good mixture
with air, cleaner combustion has excellent cold weather performance, low sulphur
content, high octane number (propane 111 RON and 100 MON), lower density and lower
heat content of LPG versus gasoline (23.5 MJ/liter for propane and 32 MJ/liter for
gasoline) results a loss in fuel economy, but better combustion efficiency can reduce this
loss.

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LPG is prepared by refining petroleum or "wet" natural gas, and is almost entirely derived
from fossil fuel sources, being manufactured during the refining of petroleum (crude oil),
or extracted from petroleum or natural gas streams as they emerge from the ground. It
was first produced in 1910 by Dr. Walter Snelling, and the first commercial products
appeared in 1912. It currently provides about 3% of all energy consumed, and burns
relatively cleanly with no soot. As it is a gas, it does not pose ground or water
pollution hazards, but it can cause air pollution. Varieties of LPG bought and sold include
mixes that are primarily propane (C3H8), primarily butane (C4H10) and, most commonly,
mixes including both propane and butane. A powerful odorant, Ethanethiol, is added so
that leaks can be detected easily

LPG has a lower energy density than either petrol or fuel-oil, so the equivalent fuel
consumption is higher. Many governments impose less tax on LPG than on petrol or fuel-
oil, which helps offset the greater consumption of LPG than of petrol or fuel-oil. Actually
not all automobile engines are suitable for use with LPG as a fuel. LPG provides less
upper cylinder lubrication than petrol or diesel, so LPG-fueled engines are more prone to
valve wear if they are not suitably modified. Many modern common rail diesel engines
respond well to LPG use as a supplementary fuel.

LPG is heavier than air, thus will flow along floors and tend to settle in low spots, such as
basements. There are two main dangers from this. The first is a possible explosion if the
mixture of LPG and air is within the explosive limits and there is an ignition source. The
second is suffocation due to LPG displacing air, causing a decrease in oxygen
concentration.

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Natural gas

It is a fossil fuel formed when layers of buried plants and animals are exposed to intense
heat and pressure over thousands of years. The energy that the plants originally obtained
from the sun is stored in the form of carbon in natural gas. Natural gas is a nonrenewable
resource because it cannot be replenished on a human time frame. It may be found above
oil deposits, or may be collected from landfills or wastewater treatment plants where it is
known as biogas. Natural gas is a hydrocarbon gas mixture consisting primarily
of methane, but commonly includes varying amounts of other higher alkanes and even a
lesser percentage of carbon dioxide, nitrogen, and hydrogen sulfide. Natural gas is an
energy source often used for heating, cooking, and electricity generation. It is also used as
fuel for vehicles and as a chemical feedstock in the manufacture of plastics and other
commercially important organic chemicals.

Before natural gas can be used as a fuel, it must undergo processing to remove impurities,
including water, to meet the specifications of marketable natural gas.
Natural gas is often informally referred to simply as gas, especially when compared to
other energy sources such as oil or coal. However, it is not to be confused with gasoline,
especially in North America, where the term gasoline is often shortened in colloquial
usage to gas.

CNG (Compressed natural gas)

CNG (Compressed natural gas) (Methane stored at high pressure) can be used in place
of gasoline (petrol), Diesel fuel and propane/LPG. CNG combustion produces fewer
undesirable gases than the fuels mentioned above. It is safer than other fuels in the event
of a spill, because natural gas is lighter than air and disperses quickly when released.

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CNG is made by compressing natural gas (which is mainly composed of methane, CH4),
to less than 1 percent of the volume it occupies at standard atmospheric pressure. It is
stored and distributed in hard containers at a pressure of 200–248 bar, usually
in cylindrical or spherical shapes.

CNG is used in traditional gasoline/internal combustion engine automobiles that have

been modified or in vehicles which were manufactured for CNG use, either alone
('dedicated'), with a segregated gasoline system to extend range (dual fuel) or in
conjunction with another fuel such as diesel (bi-fuel)

CNG does not contain any lead; thereby eliminating fouling of spark plugs so it have
lower maintenance costs than other hydrocarbon-fuel-powered vehicles. Because of the
higher octane levels (around 130 RON) of natural gas, the engine runs quieter resulting to
minimize engine nose increased life of lubricating oils, as CNG does not contaminate and
dilute the crankcase oil. CNG is less likely to ignite on hot surfaces, since it has a high
auto-ignition temperature (540 °C) and less pollution and more efficiency: CNG emits
significantly fewer pollutants. CNG cylinders take up a lot of storage space and generally
have to be placed in the boot of the car. Also the bodies of the cylinders have to be made
of good grade steel capable of handling the roughs and toughs of travelling.

Hydrogen

Hydrogen-burning internal combustion engines trace their roots back to some of the very
earliest developments in internal combustion engine development. Initially, gaseous fuels
like hydrogen were preferred to liquid fuels like gasoline because they were considered
safer to work with, due to the low pressures used for the gaseous fuels and the quick
dissipation of the gases in the event of a leak. In 1807 Issac de Rivas built the first

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hydrogen internal combustion engine, and although the design had serious flaws, it was a
more than 50 years ahead of the development of gasoline internal combustion engines
(Taylor 1985). Technological advances in gasoline engines, such as the development of
the carburetor (which allowed air and gasoline to be consistently mixed), eventually led
to other fuels being largely passed over in favor of gasoline. Until recently, hydrogen has
been relegated to niche uses, such as in experimental vehicles or in the space program.

Much like hydrogen fuel cell vehicles, hydrogen ICE vehicles present a considerable
promise: the chance to improve energy security and reduce carbon dioxide emissions by
weaning the light duty vehicle sector off of gasoline. There are significant barriers to the
adoption of hydrogen ICE vehicles, involving both technological improvements so it is
competitive with gasoline-based alternatives as well as implementing a hydrogen fueling
infrastructure. The most critical differences are the power produced by the engine, the
fuel economy, the fuel tank size, and the state of development of the technology. If the
fuel cell technology is developed to its potential. This is particularly true because the
higher fuel economy allows for a smaller fuel tank size for the same range, and fuel tank
size is almost certain to be a key limitation for hydrogen vehicles. However, the issue of
power may prove to be a thorn in the side of fuel cell vehicles, particularly for vehicles
that need the capacity to perform at high loads, since adding more fuel cell stacks can add
significantly to cost of the vehicle. Buses and trucks clearly fall into this category, and
light duty vehicles such as light trucks and sport-utility vehicles may also fall into it,
depending on the eventual cost of fuel cells.

Manufacture of Engine Fuels (oil refinery)

An oil refinery or petroleum refinery is an industrial process plant where crude oils
processed and refined into more useful petroleum products, such as gasoline, diesel fuel,

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asphalt base, heating oil, kerosene, and liquefied petroleum gas.

Oil refineries are typically large sprawling industrial complexes with extensive piping
running throughout, carrying streams of fluids between large chemical processing units.
In many ways, oil refineries use much of the technology of, and can be thought of as
types of chemical plants. The crude oil feedstock has typically been processed by an oil
production plant. There is usually an oil depot (tank farm) at or near an oil refinery for
storage of bulk liquid products.

Crude oils separated into fractions by fractional distillation. The fractions at the top of the
fractionating column have lower boiling points than the fractions at the bottom. The
heavy bottom fractions are often cracked into lighter, more useful products. All of the
fractions are processed further in other refining units.

In general, oil refining is going through three stages, namely:

1. Separation: separating different materials by heat, components with a high boiling

point remain at the bottom of the tower and components with low boiling point rise to
the top of the tower and pulled out.

2. Conversion: make some chemical processes to convert some of the components

resulting from the tower to desirable products such as polymers (plastics and
elastomers)

3. Treatment: purification of petroleum products from impurities and prepared them for
consumption and also the gases are extracted for use in the rest of the production
processes, such as the production of hydrogen gas from heavy Naphtha for use in
hydrocracking units, where they are to take advantage of the last drop of crude oil.
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Oil can be used in a variety of ways because it contains hydrocarbons of varying

molecular, masses, forms and lengths such as paraffins, aromatics, naphthenes, alkenes,
dienes, and alkynes.
While the molecules in crude oil include different atoms such as sulfur and nitrogen, the
hydrocarbons are the most common form of molecules, which are molecules of varying
lengths and complexity made of hydrogen and carbon atoms, and a small number of
oxygen atoms. The differences in the structure of these molecules account for their
varying physical and chemical properties, and it is this variety that makes crude oil useful
in a broad range of applications

Petroleum products are usually grouped into three categories:

1. Light distillates (LPG, gasoline, naphtha).

2. Middle distillates (kerosene, diesel).
3. Heavy distillates and residuum (heavy fuel oil, lubricating oils, wax, asphalt).
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This classification is based on the way crude oil is distilled and separated into fractions
(called distillates and residuum). Generally, distillation of crude oil produces 30%
naphtha, 20-40 % diesel fuel, 20 % heavy fuels, 10-20 % heavy oils.

Finally, the major products that are obtained from the refining of crude oil are,

1. Liquefied petroleum gas (LPG)

2. Gasoline (also known as petrol)
3. Naphtha
4. Kerosene and related jet aircraft fuels
5. Diesel fuel
6. Fuel oils
7. Lubricating oils
8. Paraffin wax
9. Asphalt and tar
Oil refineries also produce various intermediate products such as hydrogen and light
hydrocarbons. These are not usually transported but instead are blended or processed
further on-site. Chemical plants are thus often adjacent to oil refineries.

Naphtha

normally refers to a number of flammable liquid mixtures of hydrocarbons, i.e. a

component of natural gas condensate or a distillation product from petroleum, coal, tar, or
peat boiling in a certain range and containing certain hydrocarbons. It is a broad term
covering among the lightest and most volatile fractions of the liquid hydrocarbons in
petroleum. Naphtha is a colorless to reddish-brown volatile aromatic liquid, very similar
to gasoline.

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In petroleum engineering, full range naphtha is defined as the fraction of hydrocarbons in

petroleum boiling between 30 °C and 200 °C. It consists of a complex mixture of
hydrocarbon molecules generally having between 5 and 12 carbon atoms. It typically
constitutes 15–30% of crude oil, by weight. Light naphtha is the fraction boiling between
30 °C and 90 °C and consists of molecules with 5–6 carbon atoms. Heavy naphtha boils
between 90 °C and 200 °C and consists of molecules with 6–12 carbons.

Naphtha is used primarily as feedstock for producing gasoline (via the catalytic
reforming process). It is also used in the bitumen mining industry as a diluent, the
petrochemical industry for producing olefins in steam crackers, and the chemical industry
for solvent (cleaning) applications. Common products made with it include lighter fluid,
fuel for camp stoves, and some cleaning solvents. Light Naphtha is also used directly as a
blending component in the production of gasoline.

Catalytic reforming

Catalytic reforming is a chemical process used to convert petroleum refinery naphtha’s,

typically having low octane ratings, into high-octane liquid products called reformates
which are components of high-octane gasoline (also known as high-octane petrol).

Basically, the process re-arranges or re-structures the hydrocarbon molecules in the

naphtha feedstock’s as well as breaking some of the molecules into smaller molecules.
The overall effect is that the product reformate contains hydrocarbons with more complex
molecular shapes having higher octane values than the hydrocarbons in the naphtha
feedstock. In so doing, the process separates hydrogen atoms from the hydrocarbon
molecules and produces very significant amounts of byproduct hydrogen gas for use in a
number of the other processes involved in a modern petroleum refinery. Other byproducts
are small amounts of methane, ethane, propane, and butanes.
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Gasoline

Gasoline, also spelled gasolene, also called gas or petrol, mixture of volatile, flammable
liquid hydrocarbons derived from petroleum and used as fuel for internal-combustion
engines. It is also used as a solvent for oils and fats. Originally a by-product of the
petroleum industry (kerosene being the principal product), gasoline became the preferred
automobile fuel because of its high energy of combustion and capacity to mix readily
with air in a carburetor. Gasoline is a complex mixture of hundreds of different
hydrocarbons.

It consists mostly of aliphatic hydrocarbons obtained by the fractional distillation of

petroleum, enhanced with isooctane or the aromatic hydrocarbons, toluene and benzene to
increase its octane rating. Small quantities of various additives are common, for the
purposes of tuning engine performance or reducing harmful exhaust emissions. Some
mixtures also contain significant quantities of ethanol as a partial alternative fuel.

Under normal ambient conditions, its material state is liquid and not a true gas as opposed
to liquefied petroleum gas or "natural gas".

Gasoline is produced in oil refineries. Material that is separated from crude oil via
distillation, called straight-run gasoline, does not meet the required specifications for
modern engines (in particular octane rating), but will form part of the blend. The bulk of a
typical gasoline consists of hydrocarbons with between four and 12 carbon atoms per
molecule.

Overall, a typical gasoline is predominantly a mixture of paraffins (alkanes), cycloalkanes

(naphthenes), and olefins (alkenes). The actual ratio depends on:

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1. The oil refinery that makes the gasoline, as not all refineries have the same set of
processing units.

2. Crude oil feed used by the refinery.

3. The grade of gasoline, in particular, the octane rating.

Currently, many countries set limits on gasoline aromatics in general, benzene in

particular, and olefin (alkene) content. Such regulations led to increasing preference for
high-octane pure paraffin (alkane) components, such as alkylate, and is forcing refineries
to add processing units to reduce benzene content.

Gasoline for reasonable approximation can be considered as single component

hydrocarbon fuel, such as:

1. Gasoline with chemical formula C8H15 and molecular weight = 111

2. Iso-Octane with chemical formula C8H18 and molecular weight = 114

Fuel Composition

 C and H: carbon content of aromatics is around 89 %, and of paraffins and naphthenes

is around 86 %>

 Benzene: max allowable concentration is specified because it is highly toxic material,

the level is 5 %.

 Sulphur content: HC fuels contain free sulphur, hydrogen sulfide and other sulphur
compounds which are objectionable because it is a corrosive element that can corrode

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fuel lines, carburetor and injection pump. It will unite with oxygen to form sulphur
dioxide, which in presence of water at low T, forms sulfurous acid. It has low ignition
T, promote knock in SI engines, limited to approx. 250 Parts per million (ppm).

 Gum deposits: gasoline with unsaturated HCs forms gum in the engine, paraffin,
naphthene and aromatic HCs also form some gum. It causes operating difficulties,
sticking valves and piston rings, deposits in the manifold etc.

 Water: both dissolved and free water can be present in gasoline, free water is
undesirable because it can freeze and cause problems. Dissolved water is usually
unavoidable during manufacture.
 Lead: for leaded and unleaded gasoline mixture, lead content specified. Lead causes
pollution and destroys catalytic converters in the exhaust system.

 Manganese: used for antiknock in gasoline Methylcyclopentadienyl manganese

tricarbonyl (MMT), max amount is specified, 0.00025 to 0.03 g Mn/L

 Oxygenates: oxygenated compounds such as alcohols are used in gasoline to improve

octane rating.

FUEL SPECIFICATIONS

The major fuel specifications are:

 Relative density (specific gravity)

 Specific heating value

 Flash point

 Viscosity

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 Surface tension

 Freezing point

Relative density
Relative density, or specific gravity (Sg), is the ratio of the density (mass of a unit
volume) of a substance to the density of a given reference material. Specific gravity
usually means relative density with respect to water. The term "relative density" is often
preferred in modern scientific usage.

If the reference material is water it’s assumed to be at 4 °C (or, more precisely, 3.98 °C,
which is the temperature at which water reaches its maximum density). In SI units, the
density of water is (approximately) 1000 kg/m3 , which makes relative density
calculations particularly convenient: the density of the object only needs to be divided by
1000.


Sg 
 water

American Petroleum Institute also defines degrees API or API gravity as, a measure of
how heavy or light petroleum liquid is compared to water. If its API gravity is greater
than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. API
gravity is thus an inverse measure of the relative density of a petroleum liquid and the
density of water, but it is used to compare the relative densities of petroleum liquids. For
example, if one petroleum liquid floats on another and is therefore less dense, it has a
greater API gravity. Although mathematically, API gravity has no units, it is nevertheless
referred to as being in "degrees". API gravity is gradated in degrees on a hydrometer
instrument. The API scale was designed so that most values would fall between 10 and 70
API gravity degrees.
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141.5
Sg 
API  131.5

For gasoline, the relative density is around 0.72 to 0.78, which is equivalent to an API
range of 65 to 50, ρ = 700 −800 [kg/m3 ]. For unleaded gasoline, this value is higher due
to the aromatics. For diesel fuel, ρ = 830 − 950 [kg/m3].

Specific heating value

Specific heating value or specific energy content and sometimes called the heat of
combustion is an important property of fuels. This property helps scientists and engineers
determine the usefulness of a fuel. Energy content is the amount of heat produced by the
burning of 1 gram of a substance, and is measured in joules per gram (J/g, kJ/kg or
kcal/kg).

Gaseous fuels specific heating value is given in terms of energy content per unit volume
(kJ/liter or kJ/m3, kcal/m3).

Heating value of the combustible air-fuel mixture is a significant factor for engine
performance.

You can determine energy content of a fuel by burning an amount of the fuel and
capturing the heat released in a known mass of water in a calorimeter. If you measure the
initial and final temperatures, the energy released can be calculated using the equation

H = ∆t m Cp

Where H = heat energy absorbed (in J), ∆t = change in temperature (in °C), m = mass (in
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g), and Cp = specific heat capacity for water (4.18 J/g °C). Dividing the resulting energy
value by grams of fuel burned gives the energy content (in J/g).
(More details in p.59)
Flash point

The flash point is defined as the lowest temperature at which the vapor formed above a
pool of the liquid ignites in existence of ignition source in air at a pressure of 1
atmosphere.
The flash point is often used as a descriptive characteristic of liquid fuel. Liquids with a
flash point less than 37.8 °C are considered flammable, while liquids with a flash point
above those temperatures are considered combustible.

For gasoline, it is 25 °C, diesel fuel 35 °C and heavy diesel 65 °C.

Viscosity

It is an important parameter for CI engines, also influences fuel metering orifices since Re
is an inverse function of fuel viscosity, lower the viscosity, smaller the diameter of the
droplets in the spray. Below certain limits, low viscosity increases the leaks in the fuel
system. It is a strong function of T, must be given at certain T values.

Surface tension

Surface tension is a parameter which effects the formation of fuel droplets in sprays,
increasing the surface tension will reduce mass flow and air-fuel ratio in gasoline engines,
lower the value, smaller the droplet diameter.

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Freezing Point

The precipitation of paraffin crystals in winter can lead to clogged filters. It can be
prevented by either removing paraffins from the fuel or adding flow improvers
(additives). Anti-freezing properties are determined by its filterability. For gasoline
freezing point is –65 °C and for diesel fuel –10 °C.

Volatility

It is the tendency of a substance to vaporize. Volatility is directly related to a

substance's vapor pressure. At a given temperature, a substance with higher vapor
pressure vaporizes more readily than a substance with a lower vapor pressure.

The volatility of gasoline affects ease of starting, length of warm-up period, and engine
performance during normal operation. The rate of vaporization increases as the
temperature increases and as pressure decreases. The volatility of gasoline must be
regulated carefully so that it is volatile enough to provide acceptable cold weather
starting, yet not be so volatile that it is subject to vapor lock during normal operation.
Refiners introduce additives to gasoline to control volatility according to regional
climates and seasons,

a. Starting Ability

To provide satisfactory cold weather performance and starting, the choke system
causes a very rich mixture to be delivered to the engine. Gasoline that is not volatile
enough will cause excessive amounts of raw non-vaporized fuel to be introduced to
the combustion chambers. Because non-vaporized fuel does not burn, it is wasted.
This reduces fuel economy and causes a condition known as crankcase dilution.

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b. Crankcase Dilution

Crankcase dilution occurs when the fuel that is not vaporized leaks past the piston
rings and seeps into the crankcase. The non-vaporized fuel then dilutes the engine
oil, reducing its lubricating qualities. A certain amount of crankcase dilution occurs in
all engines during warm-up. It is not considered harmful in normal quantities because
it vaporizes out of the oil as the engine warms-up. The vapors are then purged by the
crankcase ventilation system.

c. Vapor Lock.

Vapor lock is one of the difficulties experienced in hot weather when using highly
volatile fuels. When fuel has a tendency to vaporize at normal atmospheric
temperature, it may under higher temperature form so much vapor in the fuel line that
the action of the fuel pump will cause a pulsation of the fuel vapor rather than normal
fuel flow. Heats insulating materials or baffles are often placed between the exhaust
pipe and fuel line to help avoid vapor lock. Hot weather grades of gasoline are
blended from lower volatility fuels to lessen the tendency toward vapor lock.

Antiknock Quality

Knocking

Knocking, in an internal-combustion engine, sharp sounds caused by premature

combustion of part of the compressed air-fuel mixture in the cylinder. In a properly
functioning engine, the charge burns with the flame front progressing smoothly from the
point of ignition across the combustion chamber. However, at high compression ratios,
depending on the composition of the fuel, some of the charge may spontaneously ignite
ahead of the flame front and burn in an uncontrolled manner, producing intense high-
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frequency pressure waves. These pressure waves force parts of the engine to vibrate,
which produces an audible knock.

The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in
the piston's stroke. Knock occurs when the peak of the combustion process no longer
occurs at the optimum moment for the four-stroke cycle. The shock wave creates the
characteristic metallic "pinging" sound, and cylinder pressure increases dramatically.
Effects of engine knocking range from inconsequential to completely destructive.

Knocking can cause overheating of the spark-plug points (see picture below), erosion of
the combustion chamber surface, rough, inefficient operation and sometimes this can
cause holes in engine components and small indents on the piston face as well as very
rapid wear and stress on all engine components. It can be avoided by adjusting certain
variables of engine design and operation, such as compression ratio and burning time; but
the most common method is to burn gasoline of higher octane number.

Knocking should not be confused with pre-ignition. They are two separate events;
however, pre-ignition is usually followed by knocking.

Pre-ignition (or preignition) in a spark-ignition engine is a technically different

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phenomenon from engine knocking, and describes the event wherein the air/fuel mixture
in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition
source other than the spark, such as hot spots in the combustion chamber, a spark plug
that runs too hot for the application, or carbonaceous deposits in the combustion chamber
heated to incandescence by previous engine combustion events.

Pre-ignition causes massive stresses on the rotational components of the engine, as full
ignition occurs before peak cylinder pressure and far before TDC. The premature ignition
attempts to push the piston backwards against the rotational inertia and can often lead to
broken connecting rods (see picture below), wrist pins and other component failure. In the
case of highly supercharged or high compression multi-cylinder engines, pre-ignition can
quickly melt or burn pistons (see picture below) since the power generated by other still
functioning pistons will force the overheated ones along no matter how early the mix pre-
ignites. Many engines have suffered such failure where improper fuel delivery is present.

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The phenomenon is also referred to as 'after-run', or 'run-on' or sometimes dieseling,

when it causes the engine to carry on running after the ignition is shut off. This effect is
more readily achieved on carbureted gasoline engines, because the fuel supply to
the carburetor is typically regulated by a passive mechanical float valve and fuel delivery
can feasibly continue until fuel line pressure has been relieved, provided the fuel can be
somehow drawn past the throttle plate. The occurrence is rare in modern engines with
throttle-body or electronic fuel injection, because the injectors will not be permitted to
continue delivering fuel after the engine is shut off, and any occurrence may indicate the
presence of a leaking (failed) injector.

Pre-ignition is often caused by excessive temperatures in the combustion chamber from

high intake temperature, high compressions, high levels of boost (which cause high intake
air temperatures), physical hot spots due to piston imperfections or damage, or low-grade
gasoline (lower octane gasoline is easier to ignite.) Prevention of pre-ignition is achieved
largely through proper selection of parts, operating parameters (boost, fuel used, etc.) and
engine building. Pre-ignition is usually an indication of a larger scale problem with the
engine. Pre-ignition can also be avoided by using items such as an intercooler to lower
intake temperatures or methods to increase the effective octane rating of the air/fuel
mixture.

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OCTANE NUMBER

Octane rating or octane number (ON) is a standard measure of the performance of a fuel.
The higher the octane number, the more compression the fuel can withstand before
detonating. In broad terms, fuels with a higher octane rating are used in high-compression
engines that generally have higher performance. In contrast, fuels with lower octane
numbers (but higher cetane numbers) are ideal for diesel engines. Use of gasoline with
lower octane numbers may lead to the problem of pre-ignition and or engine knocking.

Octane rating does not relate to the energy content of the fuel. It is only a measure of the
fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled
manner. Where octane is raised by blending in ethanol, energy content per volume is
reduced.
Octane is a hydrocarbon and an alkane with the chemical formula C 8H18, and the
condensed structural formula CH3(CH2)6CH3. Octane has many structural isomers that
differ by the amount and location of branching in the carbon chain. One of these isomers,
2,2,4-trimethylpentane (isooctane) is used as one of the standard values in the octane
rating scale.

The octane number of a fuel is measured in a test engine, and is defined by comparison
with the mixture of 2,2,4-trimethylpentane (iso-octane) and heptane also called n-heptane
which would have the same anti-knocking capacity as the fuel under test: the percentage,
by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel. For
example, fuel with the same knocking characteristics as a mixture of 90% iso-octane and
10% heptane would have an octane rating of 90. This does not mean that the fuel contains
just iso-octane and heptane in these proportions, but that it has the same detonation
resistance properties. Because some fuels are more knock-resistant than iso-octane, the
definition has been extended to allow for octane numbers higher than 100.
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n-Heptane is the straight-chain alkane with the chemical formula H3C(CH2)5CH3 or

C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane
fuel is the zero point of the octane rating scale (the 100 point is a 100% iso-octane)

Measurement methods

Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane Number
(RON). RON is determined by running the fuel in a test engine with a variable
compression ratio under controlled conditions, and comparing the results with those for
mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)

There is another type of octane rating, called Motor Octane Number (MON), which is a
better measure of how the fuel behaves when under load, as it is determined at 900 rpm
engine speed, instead of the 600 rpm for RON. MON testing uses a similar test engine to
that used in RON testing, but with a preheated fuel mixture, higher engine speed, and
variable ignition timing to further stress the fuel's knock resistance. Depending on the
composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower
than the RON, however there is no direct link between RON and MON. normally; fuel
specifications require both a minimum RON and a minimum MON.

Anti-Knock Index (AKI)

In most countries, including Australia and all of those in Europe, the "headline" octane

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rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some
other countries, the headline number is the average of the RON and the MON, called the
Anti-Knock Index (AKI, and often written on pumps as (R+M)/2). It may also sometimes
be called the Pump Octane Number (PON).

Difference between RON and AKI

Because of the 8 to 10 point difference noted above, the octane rating shown in Canada
and the United States is 4 to 5 points lower than the rating shown elsewhere in the world
for the same fuel. This difference is known as the fuel's sensitivity, and is not typically
published for those countries that use the Anti-Knock Index labelling system.

Observed Road Octane Number (RdON)

Another type of octane rating, called Observed Road Octane Number (RdON), is derived
from testing gasoline in real world multi-cylinder engines, normally at wide open throttle.
It was developed in the 1920s and is still reliable today. The original testing was done in
cars on the road but as technology developed the testing was moved to chassis
dynamometers with environmental controls to improve consistency.

Antiknock agents

Modern gasoline contains some chemical additives designed to improve fuel quality.
These are used to raise octane number, control surface ignition, reduce spark plug
fouling, resist gum formation, prevent rust, reduce carburetor icing, remove carburetor or
injection system deposits, minimize deposits in intake system, prevent valve sticking.

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Octane number can be increased by antiknock agents at less expense than modifying
hydrocarbons composition by refinery process. An antiknock agent is a gasoline
additive used to reduce engine knocking and increase the fuel's octane rating by raising
the temperature and pressure at which ignition occurs.

Early research into this effect was led by A.H. Gibson and Harry Ricardo in England and
Thomas Midgley, Jr. and Thomas Boyd in the United States.

The discovery that lead additives modified this behavior led to the widespread adoption
of the practice in the 1920s and therefore more powerful higher compression engines. The
most popular additive was tetraethyllead. However, with the discovery of the
environmental and health damage caused by the lead, and the incompatibility of lead with
catalytic converters found on virtually all US automobiles since 1975, this practice began
to wane in the 1980s. Most countries are phasing out leaded fuel although different
additives still contain lead compounds. Other additives include aromatic hydrocarbons,
ethers and alcohol (usually ethanol or methanol).

The typical antiknock agents in use are:

1. Tetraethyllead (Still in use as a high octane additive)

2. Methylcyclopentadienyl manganese tricarbonyl (MMT)
3. Ferrocene
4. Toluene
5. Isooctane

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Tetraethyllead

In the United States, where lead was blended with gasoline (primarily to boost octane
levels) since the early 1920s, addition of about 0.8 g lead per liter provides a gain of
about 10 octane number in gasoline. Starting to phase out leaded gasoline was first
implemented in 1973. From January 1, 1996, the Clean Air Act banned the sale of leaded
fuel for use in on-road vehicles. Possession and use of leaded gasoline in a regular on-
road vehicle now carries a maximum $10,000 fine in the United States. However, fuel
containing lead may continue to be sold for off-road uses, including aircraft, racing cars,
farm equipment, and marine engines. The ban on leaded gasoline led to thousands of tons
of lead not being released in the air by automobiles. Similar bans in other countries have
resulted in lowering levels of lead in people's bloodstreams.

A side effect of the lead additives was protection of the valve seats from erosion. Many
classic cars' engines have needed modification to use lead-free fuels since leaded fuels
became unavailable.

In some parts of South America, Asia and the Middle East, leaded gasoline is still in use.
Leaded gasoline was phased out in sub-Saharan Africa with effect from 1 January 2006.
A growing number of countries have drawn up plans to ban leaded gasoline in the near
future.

Methylcyclopentadienyl manganese tricarbonyl (MMT)

Methylcyclopentadienyl manganese tricarbonyl (MMT) has been used for many years in
Canada and recently in Australia to boost octane. It also helps old cars designed for
leaded fuel run on unleaded fuel without need for additives to prevent valve problems.

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United State Federal sources state that MMT is suspected to be a powerful neurotoxin and
respiratory toxin, and a large Canadian study concluded that MMT impairs the
effectiveness of automobile emission controls and increases pollution from motor
vehicles.

Ferrocene

Ferrocene is the organometallic compound. Ferrocene and its derivatives are antiknock
agents used in the fuel for petrol engines; they are safer than tetraethyl lead, previously
used. There are petrol additive solutions which contain Ferrocene which can be added to
unleaded petrol to enable it to be used in vintage cars which were designed to run on
leaded petrol.

The iron containing deposits formed from ferrocene can form a conductive coating on the
spark plug surfaces.

Toluene

Toluene is a clear, water-insoluble liquid with the typical smell of paint thinners. It is an
aromatic hydrocarbon that is widely used as an industrial and as a solvent. Like other
solvents, toluene is also used as an inhalant drug for its intoxicating properties.

Toluene can be used as an octane booster in gasoline fuels used in internal combustion
engines. Toluene at 86% by volume fueled all the turbo Formula 1 teams in the 1980s,
first pioneered by the Honda team. The remaining 14% was a "filler" of n-heptane, to
reduce the octane to meet Formula 1 fuel restrictions.

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Toluene at 100% can be used as a fuel for both two-stroke and four-stroke engines;
however, due to the density of the fuel and other factors, the fuel does not vaporize easily
unless preheated to 70 degrees Celsius (Honda accomplished this in their Formula 1 cars
by routing the fuel lines through the muffler system to heat the fuel).

Toluene also poses similar problems as alcohol fuels, as it eats through standard rubber
fuel lines and has no lubricating properties as standard gasoline does, which can break
down fuel pumps and cause upper cylinder bore wear.

Isooctane

2,2,4-Trimethylpentane, also known as isooctane, is an octane isomer which defines the

100 point on the octane rating scale (the zero point is n-Heptane). It is an important
component of gasoline. Isooctane is produced on a massive scale in the petroleum
industry, usually as a mixture with related hydrocarbons.

CETANE NUMBER

Cetane number or CN is a measurement of the combustion quality of diesel fuel during

compression ignition. It is a measure of a fuel's ignition delay, the time period between
the start of injection and the first identifiable pressure increase during combustion of the
fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay
periods than lower cetane fuels.

In short, the higher the cetane number the more easily the fuel will combust in diesel
engine. The characteristic diesel "knock" occurs when the first portion of fuel that has
been injected into the cylinder suddenly ignites after an initial delay. Minimizing this
delay, results in less unburned fuel in the cylinder at the beginning and less intense knock.
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Therefore higher-cetane fuel usually causes an engine to run more smoothly and quietly.
This does not necessarily translate into greater efficiency, although it may in certain
engines.

Running on low cetane number will produce cold start problems, peak cylinder p,
combustion noise, hydrocarbons emissions will be increased, more fuel will be injected
before ignition and less time for combustion.

Higher cetane number results in a sooner ignition. Extremely high cetane number may
ignite before adequate Fuel-Air mixing can take place leading to higher emissions. Power
output can be reduced if burning starts too early.

Cetane number is measured by comparing the “ignition delay time” of the sample fuel
with a mixture of cetane (C16H34) and alphamethyl naptane (C10H7 CH3). The cetane
percentage in the mixture gives the CN of the sample fuel.

Cetane number of the reference fuel cetane (C16H34) is arbitraryly set at 100, and of
alphamethyl naptane (C10H7 CH3) at 0.

Accurate measurements of the cetane number is rather difficult, as it requires burning the
fuel in a rare diesel engine called a Cooperative Fuel Research (CFR) engine, under
standard test conditions. The operator of the CFR engine uses a hand-wheel to increase
the compression ratio (and therefore the peak pressure within the cylinder) of the engine
until the time between fuel injection and ignition is 2.407ms. The resulting cetane number
is then calculated by determining which mixture of cetane and alphamethyl naptane will
result in the same ignition delay.

Cetane number is in the range of, 50 - 60 for high speed diesel engines and 25 - 45 for
low speed diesel engines. Normal Diesel fuel cetane number is 40 – 55.
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