DIESEL ENGINE THEORY

Carburetor on a gasoline engine.

Fundamental Theory
 Its purpose is to mix gasoline with air
What is a Diesel Engine? for ignition. Carburetors are made in
different forms with various control
 A diesel engine is an engine which devices to control the quality of air
uses a low – grade oil for fuel and and the quantity of gasoline mixed the
ignites this fuel by the heat of air mixture is compressed in the upper
compression. part of the cylinder by the upward
movement of the piston.
Where did the Diesel Engine get its name?
What causes air to heat when compressed?
 From Dr. Rudolf Diesel, an inventor,
who obtained his patent on the  The mechanical force pressing against
principle of the Diesel engine in 1893? the air particles, or molecules,
squeezes then into a smaller space.
Where can a Diesel Engine are used? This pressure causes them to move or
vibrate very rapidly, thus the friction
 Almost any place where power is of the molecules creates heat.
needed whether on land or water.
How do Diesel Engines differ in the burning
What are the main difference between a of the fuel?
Diesel Engine and a gasoline engine?
 In the gasoline engine, the spark
 The Diesel Engine may use a low – ignites the mixture causing an
grade fuel oil which ignited in the explosion and an immediate rise in
cylinder by the heat of air pressure.
compression. The gasoline engine  In the Diesel Engine, the lighter
requires high – grade gasoline for fuel bodies of the fuel oil begin burning as
which is ignited by an electric spark soon as they enter the heated air and
after the gasoline has been mixed with the particles gradually become fully
air in a carburetor, injected into the ignited, thereby creating a gradual
cylinder, and the mixture compressed. increase in pressure in the cylinder of
the engine.
How does a carburetor in a gasoline engine
vaporize the fuel? Why is the Diesel Engine known as a
constant – pressure engine?
 By blowing a stream of air over or
through the fuel, sending it out in the  Because the fuel oil ignites as soon as
form of a mist or a mist gas. This the first particles enter the cylinder
makes it easier to ignite than if it were and it continues to ignite. Burn, and
in the liquid state. expand during the entire admission
period of the fuel oil.
What is a four – stroke – cycle engine?  The created pressure moves the piston
and. As more pressure is created, the
 A four – stroke – cycle engine is one piston continues moving and giving
which for strokes of the piston are more space to the expanded gases. The
required to complete the necessary result is an almost constant pressure
series of events required to produce on the piston.
one power stroke.
What is a two – stroke – cycle engine? Why is the gasoline engine known as a
constant – volume engine?
 A two – stroke cycle engine is one in
which two strokes of the piston are  Because a volume of gasoline and air
required to complete the necessary mixture enters the cylinder and is
series of events required to produce compressed, then ignited by a spark.
one power stroke.
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 This creates an explosion and an
instantaneous high pressure which Firing Order
decreases quickly as the piston moves
downward and gives more space to the  Firing order refers to the order in
burned gases. which each of the cylinders in a multi-
cylinder engine fires (power stroke).
Diesel Engine Fundamentals:  For example, a four cylinder engine's
Operational Terminology firing order could be 1-4-3-2.
 This means that the number 1 cylinder
Bore and Stroke fires, then the number 4 cylinder fires,
then the number 3 cylinder fires, and
 Bore and stroke are terms used to so on.
define the size of an engine.  Engines are designed so that the
 BORE refers to the diameter of the power strokes are as uniform as
engine's cylinder, and possible, that is, as the crankshaft
 STROKE refers to the distance the rotates a certain number of degrees,
piston travels from the top of the one of the cylinders will go through a
cylinder to the bottom. power stroke.
 The highest point of travel by the  This reduces vibration and allows the
piston is called top dead center power generated by the engine to be
(TDC), and the lowest point of travel applied to the load in a smoother
is called bottom dead center (BDC). fashion than if they were all to fire at
 There are 180o of travel between TDC once or in odd multiples.
and BDC, or one stroke.
Compression Ratio and Clearance Volume
Engine Displacement
 Clearance volume is the volume
 Engine displacement is one of the remaining in the cylinder when the
terms used to compare one engine to piston is at TDC.
another.  Because of the irregular shape of
 Displacement refers to the total the combustion chamber (volume in
volume displaced by all the pistons the head) the clearance volume is
during one stroke. calculated empirically by filling the
 The displacement is usually given in chamber with a measured amount of
cubic inches or liters. fluid while the piston is at TDC.
 To calculate the displacement of an  This volume is then added to the
engine, the volume of one cylinder displacement volume in the cylinder to
must be determined (volume of a obtain the cylinders total volume.
cylinder = (Πr2)h; where h = the  An engine's compression ratio is
stroke). determined by taking the volume of
 The volume of one cylinder is the cylinder with piston at TDC
multiplied by the number of cylinders (highest point of travel) and dividing
to obtain the total engine the volume of the cylinder when the
displacement. piston is at BDC (lowest point of
travel).
Degree of Crankshaft Rotation  This can be calculated by using the
following formula:
 All events that occur in an engine are  Compression Ratio =
related to the location of the piston. displacement volume + clearance
 Because the piston is connected to the volume divided by clearance volume
crankshaft, any location of the piston or V1/V2
corresponds directly to a specific
number of degrees of crankshaft
rotation.
 Location of the crank can then be
stated as XX degrees before or XX
degrees after top or bottom dead
center.
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 The Four-Stroke Cycle

Intake

 The intake valve is open, the exhaust

 The compression ratio is a measure of
valve is closed, and the piston moves
how much the engine compresses the
down, bringing fresh air into the
gasses in the engine's cylinder.
cylinder.
 In a gasoline engine the compression
 As the piston passes TDC and begins
ratio (which controls the compression
to travel down the cylinder bore, the
temperature) is limited by the air-fuel
movement of the piston creates a
mixture entering the cylinders.
suction and continues to draw fresh air
 The lower ignition temperature of
into the cylinder.
gasoline will cause it to ignite (burn)
at a compression ratio of less than
Compression
10:1.
 The average car has a 7:1  Both intake and exhaust valves are
compression ratio. closed, and the air is compressed by
 In a diesel engine, compression the upward movement of the piston.
ratios ranging from 14:1 to as high as  The volume of air is reduce thus
24:1 are commonly used. increasing the temperature and
 The higher compression ratios are pressure use to ignite the fuel.
possible because only air is
compressed, and then the fuel is
injected.
 This is one of the factors that allows
the diesel engine to be so efficient.

THE BASIC DIESEL CYCLE

To convert the chemical energy of the fuel into

useful mechanical energy all internal
combustion engines must go through four
events:

 intake, Fuel Injection

 compression,
 Injection,  Before the piston reaches TDC, the
 power, and fuel has been injected and is starting
 exhaust. to burn.
 Fuel injection starts at 28° BTDC and
ends at 3° ATDC; therefore, fuel is
injected for a duration of 31°.

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 The Two-Stroke Cycle

Intake

 air under slight pressure, is blown into

and fills the cylinder through the side
port.
 Scavenging, the piston continues
downward and uncovers the
scavenging and charging – air port at
the side, through which air, under
Power pressure, enters and forces out
whatever remaining gases there maybe
 Both intake and exhaust valves are in the cylinder, leaving clean air for
closed and combustion occurs with a the next compression stroke.
resultant increase in pressure.
 After the piston passes TDC, heat is
rapidly released by the ignition of the
fuel, causing a rise in cylinder
pressure. Combustion temperatures
are around 2336°F.
 This rise in pressure forces the piston
downward and increases the force on
the crankshaft for the power stroke

Exhaust

 when the piston is near the end if its

stroke, the exhaust valve in the head
opens and permits the exhaust gases to
escape.
 The exhaust gasses start to escape into
the exhaust manifold.
Exhaust

 The exhaust valve is open, the intake

valve is closed, and the upward
movement if the piston forces the
products of combustion from the
engine.
 the exhaust gasses start to flow out the
exhaust valve due to cylinder pressure
and into the exhaust manifold.

Compression

 the piston is starting upward to

compress the air to produce heat to
ignite the fuel oil.

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 Comparison of Power between a 2-stroke
and 4-stroke engine.

 In a two stroke diesel engine, only

about 38% of the generated
power is harnessed to do work, about
30% is wasted in the form of heat
rejected to the cooling system,
and about 32% in the form of heat is
rejected out the exhaust.
 In comparison, the four-stroke
diesel engine has a thermal
distribution of 42% converted to
Injection, the fuel valve opens and fuel oil is useful work, 28% heat rejected to the
injected into the hot air where it ignites and cooling system, and 30% heat rejected
start burning. out the exhaust.

 The first minute droplets of fuel enter Cross-hatch Finish

the combustion chamber and are  A "cross-hatch" pattern is used to
quickly vaporized. retain oil or grease to ensure proper
lubrication and ring seal of pistons in
cylinders.
 A smooth glazed cylinder wall can
cause piston ring and cylinder
scuffing.
 The "cross-hatch" pattern is used on
brake rotors, and flywheels.
 Other similar processes are lapping
and superfinishing.

Honing tool

Power

 The power stroke starts after the

piston passes TDC, the power stroke
which continues until the piston
reaches 91° ATDC, at which point
the exhaust valves start to open and a
new cycle begins.

Cross hatch pattern

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Engine Construction The bedplate

 The bedplate is the foundation of the

engine.
 Everything is built from it, so it must
have sufficient strength to support all
of the engine’s fixed and moving
parts.

Cross Section of a V-type Four Stroke Diesel

Engine

 is attached by its lower flanges to the

ship’s structure.
 The bedplate has a machined top
surface and it supports the crankshaft
and bearings directly.

Crankcase and Oil Pan

Major Components of a Diesel Engine The crankcase is usually located on the bottom
of the cylinder block.
The Cylinder Block
 The crankcase is defined as the area
The cylinder block is generally a single unit
around the crankshaft and crankshaft
made from cast iron.
bearings.
 provides the structure and rigid frame  This area encloses the rotating
for the engine's cylinders, water crankshaft and crankshaft counter
coolant and oil passages. weights and directs returning oil into
 Provides support for the crankshaft the oil pan.
and camshaft bearings.
The oil pan is located at the bottom of the
crankcase.

 The oil pan collects and stores the

engine's supply of lubricating oil.
 Large diesel engines may have the oil
pan divided into several separate pans.
 In box – type construction, the oil is
often contained within the lower part
of the bedplate, an area called the
crankcase.
 In the marine – type construction,
the lubricating oil is stored in a

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 separate area because the bedplate is  This type of liner has direct water
very shallow. contact with both the engine water
 The engine frame is held in place by jacket and the liner.
the through bolts, which also secure  Therefore, the heat flows from the
the engine block (not shown), to the cylinder to the liner to the water
frame. jacket.
 The wet liner is typically found on
Cylinder Sleeve or Bore engines with bores greater than eight
to ten inches.
Two types of cylinders.
3. Integral liner
1. In one type, each cylinder is simply
machined or bored into the block casting,  It has water jacketing, but in this case
the water jacketing is included in the
making the block and cylinders an
liner.
integral part.  There is no water jacket in the engine
2. In the second type, a machined steel block per se.
sleeve is pressed into the block casting to  the heat flow is from the cylinder to
form the cylinder. the liner to the liner water jacket.
 With either method, the cylinder  Usually found in opposed piston type
sleeve or bore provides the engine engine
with the cylindrical structure needed
to confine the combustion gasses and The Sleeve or Bore
to act as a guide for the engine's
pistons.
 There are two types of sleeves: wet
and dry.
 A dry sleeve is surrounded by the
metal of the block and does not come
in direct contact with the engine's
coolant (water).

LINERS

1. Dry type liners

• use a counter-bored cylinder with the

liner installed in the bore of the
cylinder.
• There is a tight seal between the liner
and cylinder wall.
• Heat from the cylinder flows to the
liner through the cylinder wall and  A wet sleeve comes in direct contact
into the water jacket. with the engine's coolant.
 The Figure provides an example of
• Use in small diesel engine
a wet sleeve.
 The volume enclosed by the sleeve
2. Wet type liners
or bore is called the combustion
 also use counter-bored cylinder blocks chamber and is the space where the
and there are gaskets on the top and on fuel is burned.
the bottom of the liner.
 The top gasket prevents water from Liner/sleeve
leaking out of the water jacket.
1. Dry liners
 The bottom gaskets are O rings around
the liner to prevent water from leaking
 liner is inserted inside the bore cooling
into the crankcase.
is in the blade
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2. Wet liner  The skirt of the piston must be able to
withstand the side thrust forces
 cooling water is in direct contact with developed by the angularity of the
both the jacket and the liner connecting rod movement.
 The upper part of the piston is slightly
3. Integral liner tapered to allow for expansion due to
high combustion temperatures.
 water jacketing is inside the liner;  To prevent the combustion gasses
used in large engines and in opposed from bypassing the piston and to
piston keep friction to a minimum, each
piston has several metal rings around
 In either type of cylinder, sleeved it.
or bored, the diameter of the cylinder
is called the bore of the engine.
 Most diesel engines are multi-cylinder
engines and typically have their
cylinders arranged in one of two ways.
 In an in-line engine, as the name
indicates, all the cylinders are in a
row.
 In a "V" type engine the cylinders
are arranged in two rows of cylinders
set at an angle to each other that align
to a common crankshaft.
 Each group of cylinders making up
one side of the "V" is referred to as a
bank of cylinders.
Function of rings:
Piston and rings
 the seal between the piston and the
cylinder wall;

 act to reduce friction by minimizing

the contact area between the piston
and the cylinder wall.

 The piston transforms the energy of

the expanding gasses into mechanical
energy.
 The piston rides in the cylinder liner
or sleeve
 Pistons are commonly made of
aluminum or cast iron alloys. The rings are usually made of cast iron and
 The trunk piston is used in engines
coated with chrome or molybdenum.
where the connecting rod goes directly
from the piston to the crankshaft. The top ring (s)

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  acts primarily as the pressure seal.  The ring is place on the liner with
least wear, thus the ring would be in
 The upper rings on the piston are the liner as if it is in the piston
compression rings, which seal the
combustion space during the  The minimum and maximum
compression process and power stroke clearance is specified by manufacturer
to prevent leakage of combustion
gasses to pass between liner and
piston.

 The upper ring also serve to transmit

heat from the piston to cylinder liner
wall.

The intermediate ring (s)

 acts as a wiper ring to remove and

control the amount of oil film on the
cylinder walls.

The bottom ring (s)

 is an oil ring and ensures that a supply

of lubricating oil is evenly deposited
on the cylinder walls.
Too Little Gap (insufficient)
 The oil rings are the lowest rings on
the upper part of the piston.  End will butt due to thermal
expansion.
 There also may be an oil ring on the  it will tend to expand tightly against
lower part of the skirt of the piston. the liner causing too much friction and
possibility of piston seizure
 The oil rings’ function is to remove
excess oil from the cylinder wall and Too much Gap (excessive)
scrape it back into the crankcase.
 Possibility of blow by
Oil drain hole  Loss of compression
 Exhaust gasses get into the crankcase
 There are small drain holes through  clearance be less than the minimum;
the piston behind the oil ring, which  a possibility exists that the ends will
allow the oil to drain back to the butt due to thermal expansion of the
crankcase. ring on the hot engine
 There would be no direct path for the
oil flow back to the crankcase if these Butting end ring may cause:
holes were not provided.
 Whenever pistons are pulled and the  to expand tightly against the liner,
rings cleaned, it is important that drain significantly increasing friction and
holes are cleaned and opened. the probability of piston seizure.

 the clearance be greater than the

Piston ring clearance maximum specifications,
 combustion gases will blow – by the
Ring end clearance or gap
rings and exhaust gases will enter the
crankcase.

193
  This will be accompanied by a loss of  this ring groove wear is more common
compression. on high speed engine (4 stroke)

Ring End Clearance or Gap  it increases as the engine speed rises.

 The ring is place on the liner with Back clearance

smallest wear, the would be in the
liner as if it is in the piston  The ring thickness must be less than
 The minimum and maximum the groove depth, the outer face is
clearance is specified by manufacturer slightly inside the ring groove to have
 Should the clearance be less than the a back clearance
minimum, a possibility exists that the
ends will butt due to thermal Without back clearance;
expansion of the ring on the hot
engine.  the ring might take the side thrust
 Butting ring ends would cause the ring instead of the piston trunk or skirt
to expand tightly against the liner, resulting in high friction or piston
significantly increasing friction and seizure.
the probability of piston seizure.
 Should the clearance be greater than Variety of compression and oil rings
the maximum specifications,
combustion gases will blow – by the  Compression rings are normally;
rings and exhaust gases will enter the butt or angle cut (45 deg is common)
crankcase. This will be accompanied
by a loss of compression.  step cut rings
to minimize blow -by
Side Clearance
 bronze insert
 Is the distance from the top of the ring allow rapid wear-in and conformance
to the bottom of the ring land above of the ring to the cylinder
the ring
 It is measured by the use of feeler  quick seating
gauge wear and conform quickly
 Manufacturers specifies a minimum
and maximum clearance

Too little

 The combustion gasses will NOT be

able to get behind the ring
 the ring will not be pushed against the
liner wall for sealing for this purpose,
 It is not only the tension which
maintain the seal but gas pressure as
well
 The combustion gas might act on the
outer or contacting surface and push
the ring inside creating – blow by

The chrome – faced ring increases ring life as

chrome wears slowly.
Clearance is to great
 Chrome rings cannot be used in
 excess ring groove wear will occur as
chromed cylinder liner is not porous
result of ring snapping to the top of
and will not hold the oil film
the ring groove at the end of exhaust
necessary for piston lubrication.
stroke.

194
  Chrome rings are used on new cast  The bottom bearing cap is split to
iron liners where both the liner and allow connection to the crankshaft.
rings are initially circular.
2. Marine type – connecting rods have the
Piston pin three pieces:

 A steel pin that is passed through the a. the rod

Piston, it is used as a base upon which
to fasten the upper end of the b. the upper bearing cap
Connecting rod.
 It is round and may be hollow. c. the lower bearing cap
 Also called wrist pin or gudgeon pin.
 The marine – type rod is connected to
the piston pin as is the automotive
type.
 In marine – type construction there is
bearing box that can be broken into
two separate pieces.
 The connecting rod is attached to the
top of the bearing box and joins the
connecting rod with the crankshafts.
particularly on the upper half of the
connecting rod bearing shell and on
the lower half of the main bearing
shells.
Three types of piston pins
 This tends to lower the compression
ratio, allowing a greater clearance
1. Full – floating pistons pins volume at top dead center.
 To compensate for this, shims are
 are those in which the piston pins and
installed so the volume at top dead
the connecting rod can rotate center remains constant. Thus, the
independently. design compression ratio can be
maintained.
2. Semi – floating piston pins

 have the connecting rod rigidly

fastened to the wrist pins where the
connecting rod may move but the
wrist pin; this allow no bearing
material in the rod.

3. Fixed wrist pins

 where the connecting rod may move

but the wrist pin is held rigidly.

 Fixed pins are used where oil cooling  The connecting rod connects the
of the piston is required and oil piston to the crankshaft.
passages go through the wrist pin to  The rods are made from drop-forged,
the piston. heat-treated steel to provide the
required strength.
CONNECTING RODS  Each end of the rod is bored, with the
smaller top bore connecting to the
1. Automotive type – is built in two pieces: piston pin (wrist pin) in the piston
 The large bore end of the rod is split in
 the rod and the bearing cap. half and bolted to allow the rod to be
attached to the crankshaft.
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  Some diesel engine connecting rods  The crankshaft has large weights,
are drilled down the center to allow oil called counter weights, that balance
to travel up from the crankshaft and the weight of the connecting rods.
into the piston pin and piston for  These weights ensure an even
lubrication. (balance) force during the rotation of
 A variation found in V-type engines the moving parts.
that affects the connecting rods is to
position the cylinders in the left and
right banks directly opposite each
other instead of staggered (most
common configuration).
 This arrangement requires that the
connecting rods of two opposing
cylinders share the same main journal
bearing on the crankshaft.
 To allow this configuration, one of the
connecting rods must be split or
forked around the other.

Crankshaft
 In considering the moving parts of the
 The crankshaft transforms the linear engine, we will start at the bottom,
motion of the pistons into a rotational with the crankshaft. Notice the main
motion that is transmitted to the load. journal (the horizontal part of the
 Crankshafts are made of forged steel. crankshaft), two crank webs, the main
 The forged crankshaft is machined to
bearing, and the crank pin journal.
produce the crankshaft bearing and
connecting rod bearing surfaces. The crank pin journal is also called the
 The rod bearings are eccentric, or connecting rod is fastened to the
offset, from the center of the crankshaft.
crankshaft.
 This offset converts the reciprocating The crankshaft’s construction may be of
(up and down) motion of the piston two types:
into the rotary motion of the
crankshaft. 1. one piece, found in most engines,
 The amount of offset determines the 2. or with a shrink fitting holding the webs to
stroke (distance the piston travels) of the main journals, (found in Sulzer
the engine (discussed later). engines.)
 The crankshaft does not ride directly
on the cast iron block crankshaft  The term crank throw covers the two
supports, but rides on special bearing webs plus the crank pin journal.
material .  The length of the crank throw is
 The connecting rods also have measured from the centerline of the
bearings inserted between the main journals to the centerline of the
crankshaft and the connecting rods. connecting rod journal.
 The bearing material is a soft alloy of  Thus, the vertical distance the web
metals that provides a replaceable travels is equal to half the stroke of the
wear surface and prevents galling engine;
between two similar metals (i.e.,  there must be sufficient space on the
crankshaft and connecting rod). bottom of the engine, above the
 Each bearing is split into halves to bedplate, to permit the webs to rotate.
allow assembly of the engine.
 The crankshaft is drilled with oil
passages that allow the engine to feed
oil to each of the crankshaft bearings
and connection rod bearings and up
into the connecting rod itself.

196
  Diesel engines have two methods of
admitting and exhausting gasses from
the cylinder.
 They can use either ports or valves
or a combination of both.
 Ports are slots in the cylinder walls
located in the lower 1/3 of the
bore. for examples of intake ports, and
note their relative location with
respect to the rest of the engine.
 When the piston travels below the
level of the ports, the ports are
"opened" and fresh air or exhaust
gasses are able to enter or leave,
Flywheel depending on the type of port.
 The flywheel is located on one end  The ports are then "closed" when the
of the crankshaft and serves three piston travels back above the level of
purposes. the ports.
 First,
through its inertia, it reduces Valves
vibration by smoothing out the
power stroke as each cylinder fires.  are mechanically opened and closed to
 Second, admit or exhaust the gasses as needed.
it is the mounting surface used to bolt  The valves are located in the head
the engine up to its load. casting of the engine.
 Third,  The point at which the valve seals
on some diesels, the flywheel has gear against the head is called the valve
teeth around its perimeter that allow seat.
the starting motors to engage and  Most medium-sized diesels have
crank the diesel. either intake ports or exhaust valves or
both intake and exhaust valves

Cylinder Heads and Valves

 A diesel engine's cylinder heads

perform several functions.
 First,
they provide the top seal for the
cylinder bore or sleeve.
 Second,
they provide the structure holding
exhaust valves (and intake valves
where applicable), the fuel injector,
and necessary linkages.

A diesel engine's heads are manufactured in

one of two ways.

 In one method, each cylinder has its  Valve are timed while on the base
own head casting, which is bolted to circle valve lash or tapped clearance
the block. This method is used
primarily on the larger diesel engines. Too Small
 In the second method, which is used
on smaller engines, the engine's head  Valve will open too early and close
is cast as one piece (multi-cylinder late
head).  Worst if valve will never close or seat
 Too long opening

197
  The pushrods and rocker arms transfer
the reciprocating motion generated by
the camshaft lobes to the valves and
injectors, opening and closing them as
needed.
 The valves are maintained closed by
springs.
 As the valve is opened by the
camshaft, it compresses the valve
spring.
 The energy stored in the valve spring
is then used to close the valve as the
camshaft lobe rotates out from under
the follower.
 Because an engine experiences fairly
large changes in temperature (e.g.,
Too Large ambient to a normal running
temperature of about 190°F), its
 Valve will open late and close early components must be designed to allow
 Too little opening time for thermal expansion.
The slope in the cams will determine the rate /  Therefore, the valves, valve
pushrods, and rocker arms must have
speed of valve opening and closing
some method of allowing for the
expansion.
Timing Gears, Camshaft, and Valve
 This is accomplished by the use of
Mechanism valve lash.
 Valve lash is the term given to the
"slop" or "give" in the valve train
before the cam actually starts to open
the valve.
 The camshaft is driven by the engine's
crankshaft through a series of gears
called idler gears and timing gears.
 The gears allow the rotation of the
camshaft to correspond or be in time
with, the rotation of the crankshaft
and thereby allows the valve opening,
valve closing, and injection of fuel to
be timed to occur at precise intervals
in the piston's travel.
 In order for a diesel engine to operate,
 To increase the flexibility in timing
all of its components must perform
the valve opening, valve closing, and
their functions at very precise intervals
injection of fuel, and to increase
in relation to the motion of the
power or to reduce cost, an engine
piston.
may have one or more camshafts.
 To accomplish this, a component
Typically, in a medium to large V-
called a camshaft is used.
type engine, each bank will have one
 A camshaft is a long bar with egg-
or more camshafts per head. In the
shaped eccentric lobes, one lobe for
larger engines, the intake valves,
each valve and fuel injector. Each lobe
exhaust valves, and fuel injectors may
has a follower.
share a common camshaft or have
 As the camshaft is rotated, the
independent camshafts. Depending on
follower is forced up and down as
the type and make of the engine, the
it follows the profile of the cam lobe.
location of the camshaft or shafts
 The followers are connected to the
varies.
engine's valves and fuel injectors
 The camshaft(s) in an in-line engine is
through various types of linkages
usually found either in the head of the
called pushrods and rocker arms.
198
 engine or in the top of the block What are the advantages of a pre-
running down one side of the cylinder combustion chamber?
bank.
 Figure 10 provides an example of an  The fuel oil injected into a pre-
engine with the camshaft located on combustion chamber is more
the side of the engine. Figure 3 thoroughly atomized and also partly
provides an example of an overhead ignited in advance of reaching the
cam arrangement as on a V-type cylinder.
engine. On small or mid-sized V-type  This produces a more uniform
engines, the camshaft is usually cylinder pressure and a smoother
located in the block at the center of the running engine.
"V" between the two banks of  The pre-combustion chamber in figure
cylinders. In larger or multi- (a) uses a pintle nozzle.
camshafted V- type engines, the  this type of chamber contains only
camshafts are usually located in the 35% to 45% of the clearance volume.
heads.  Ignition starts in the pre-combustion
chamber, and the flame and fuel
expand rapidly into the main
Combustion Chamber combustion chamber where air cell.
 In the air cell there is just air; no
 The combustion process is the key to combustion process occurs there.
engine operation .  The pintle nozzle opens between
 Turbulence greatly aids in the 1,200 to 2,000 psi and the fuel sprays
combustion process. in a fine hollow cone pattern rather
 Combustion chambers are designed to than mixing dispersion and
promote turbulence. penetration.
 a pintle nozzle is required in a
Turbulence turbulence chamber, rather than the
multi-orifice type used in main
 Violent mixing of air and fuel mainly chamber combustion
produced by the tangential cut of
intake parts to swirl the inlet air.
 The shape of combustion chamber is
another way to increase turbulence.

What is a pre-combustion chamber on a

Diesel engine?

 A small chamber directly connected

with the cylinder through a restricted
channel into which the fuel oil is
injected before it reaches the main
power cylinder.
 Many of the old Diesel engines were
designed with pre-combustion
chambers.
 Some of the later designs use an intake
– air pre-heater to aid in starting at
cool temperatures. How does a pre-combustion chamber cause
 Pre-heater equipment usually consists better atomization and more uniform
of a hand priming pump, to pump fuel
pressure?
oil into the intake manifold, and a
glow plug which is electrically heated
 It creates a better turbulence a better
by the battery.
mixing of the fuel oil and air through
 Fuel oil is burned in the intake
the whirling motion gives to the air
manifold. This heats the intake air.
and fuel and forces the fuel mixture

199
 through the narrow neck between  It increases the power which may be
chamber and the cylinder. required at interval and give more
power where space does not permit
What is meant by turbulence as applied to larger engine.
Diesel engine operation?
How does supercharging increase the power
 It means the disturbance or agitation of the engine?
of the sprayed fuel oil and the air
within the combustion chamber or  By the increase in amount of air which
cylinder. provides the fuel oil with more oxygen
for combustion of a larger charge of
What methods are used to provide air fuel.
turbulence?

 Designs of spray nozzles, piston Diesel Engine Support Systems

heads, and precombustion chambers
which tend to create more violent  A diesel engine requires five
agitation. supporting systems in order to operate:
cooling, lubrication, fuel injection, air
How does turbulence aid combustion? intake, and exhaust. Depending on the
size, power, and application of the
 By causing a more thorough mixing of diesel, these systems vary in size and
the hot compressed air with the complexity.
injected fuel oil. Thereby bringing
about more complete combustion and Engine Cooling
more even cylinder pressure.
 Nearly all diesel engines rely on a
Explain the process that air and fuel oil liquid cooling system to transfer
undergo when a cylinder is equipped with a waste heat out of the block and
internals.
pre-combustion chamber.

 The piston, compressing the air, forces

it through the narrow neck which give
it a whirling motion around the
chamber.
 At the same time friction produced by
the air as it is forced through a narrow
neck gives higher temperature.
 This ignites particles of the fuel oil
entering the chamber which in turn
creates a pressure and sends mixture
forcibly into the cylinder spreading
itself to all part thus creating more
uniform combustion and pressure
 Design of pistons and cylinder heads
has been altered to increase whirling
motion and give improved turbulence.

What is mean by supercharging? The cooling system consists of a major

components:
 Increasing the total amount of
charging air in the working cylinder of  Water pump,
the engine.  radiator or heat exchanger
 water jacket (which consists of
What is purpose of supercharging or coolant passages in the block and
increasing the supply of charging air? heads)
 and a thermostat.
200
Engine Lubrication

 An internal combustion engine would

not run for even a few minutes if the
moving parts were allowed to make
metal-to-metal contact.
 The heat generated due to the
tremendous amounts of friction would
melt the metals, leading to the
destruction of the engine.
 To prevent this, all moving parts ride
on a thin film of oil that is pumped
between all the moving parts of the
engine.

The oil serves two purposes.

Fuel System
1. One purpose is to lubricate the bearing
surfaces.  All diesel engines require a method
2. The other purpose is to cool the bearings to store and deliver fuel to the
by absorbing the friction- generated heat. engine.
The flow of oil to the moving parts is  Because diesel engines rely on
accomplished by the engine's internal injectors which are precision
lubricating system. components with extremely tight
tolerances and very small injection
 The filters clean the oil and remove holes, the fuel delivered to the
any metal that the oil has picked up engine must be extremely clean and
due to wear. free of contaminants.
 The cleaned oil then flows up into
the engine's oil galleries.
 A pressure relief valve(s) maintains
oil pressure in the galleries and returns
oil to the oil pan upon high pressure.
 The oil galleries distribute the oil to all
the bearing surfaces in the engine.
 Once the oil has cooled and lubricated
the bearing surfaces, it flows out of
the bearing and gravity-flows back
into the oil pan.
 In medium to large diesel engines, the
oil is also cooled before being
distributed into the block.
 This is accomplished by either an
internal or external oil cooler.  This is usually accomplished
 The lubrication system also supplies through a series of in-line filters.
oil to the engine's governor, which is  Commonly, the fuel will be
discussed later in this module. filtered once outside the engine and
then the fuel will pass through at least
one more filter internal to the engine,
usually located in the fuel line at
each fuel injector.
 In a diesel engine, the fuel system is
much more complex than the fuel
system on a simple gasoline engine
because the fuel serves two purposes.

201
  One purpose is obviously to supply various combustion chambers
the fuel to run the engine; discussed in this chapter.
 The other is to act as a coolant to  The pintle shape is slightly expanded
and tapered which causes throttling of
the injectors.
the fuel leaving the tip.
 To meet this second purpose, diesel  The fuel spray pattern is conical, and
fuel is kept continuously flowing solid stream of atomized fuel is
through the engine's fuel system at a directed on the inside, this causes a
flow rate much higher than penetration across the entire chamber
required to simply run the engine,  A throttling pintle nozzle is located
 The excess fuel is routed back to the directly across the energy cells and a
fuel pump or the fuel storage tank throttling pintle is used.
depending on the application.  The fuel is injected across the top of
the main chamber where combustion
FUEL INJECTION NOZZLES starts, but the solid stream is able to
penetrate into the energy cell, and
 A different type of fuel injection
combustion also occurs there.
nozzle, a pintle nozzle (shown in
 The fuel is injected across the top of
figure 5.3), is required in a turbulence
the main chamber where combustion
chamber, rather than the multi-orifice
starts, but the solid stream is able to
type used in main chamber
penetrate into the energy cell, and
combustion. The multi-orifice nozzle
combustion also occurs there.
(also called a differential hydraulic
injector and illustrated in figure 5.4)
1. Injector tip
requires a fuel oil pressure of 2,500
TO 3,500 psi to open, while pintle 2. Injector seat
nozzle requires a much lower opening 3. Needle and guide assembly
pressure. 4. Holding nut
 The pintle nozzle opens between 5. Injector body
1,200 to 2,000 psi and the fuel sprays 6. Push rod
in a fine hollow cone pattern rather 7. Spring
than: mixing dispersion and
8. Spring shims
penetration.
 The multiorifice nozzle does not 9. External lift adjustment
provide the needed fuel penetration in 10. Drain tube
certain instances. These results in only 11. Fuel line connection
air supply near the nozzle being used 12. Fuel line
for combustion, rather than the air
supply throughout the combustion
chamber.
 In the turbulence chamber it is
desirable that the fuel penetrate across
the entire chamber, but it should not
impinge the wall of the turbulence
chamber.
 A modification to the pintle nozzle
called a throttling pintle injector is
shown in figure 5.5. The pintle share
is slightly expanded and tapered which
causes throttling of the fuel leaving
the tip. The fuel spray pattern is
conical as the pintle nozzle and, in
addition, a solid stream of atomized
fuel is directed on the inside. This
causes a penetration across the entire
chamber. There are several instances
where this type of nozzle is used in the AIR INTAKE

202
Where do the cylinders get air for dirt in the air is removed by the oil in
compression? the filter.
 The air then flows through a
 From the atmosphere, Air is drawn in screen-type material to ensure any
by the suction, or intake, stroke of the entrained oil is removed from the air.
piston.
 In the four – stroke – cycle engine. A Oil Bath Air filter
supercharger, or a blower, may be
used to provide an adequate supply of
scavenging air for the cylinders.

What is meant by turbulence as applied to

Diesel engine operation?

 It means the disturbance or agitation

of the sprayed fuel oil and the air
within the combustion chamber or
cylinder.

What methods are used to provide air

turbulence?

 Designs of spray nozzles, piton heads,  In a dry filter system, paper, cloth, or a
and precombustion chambers which metal screen material is used to catch
tend to create more violent agitation. and trap dirt before it enters the
engine (similar to the type used in
How does turbulence aid combustion? automobile engines). In addition to
cleaning the air, the intake system is
 By causing a more thorough mixing of usually designed to intake fresh
the hot compressed air with the air from as far away from the
injected fuel oil. Thereby bringing engine as practicable, usually just
about more complete combustion and outside of the engine's building or
more even cylinder pressure. enclosure. This provides the engine
with a supply of air that has not
Air Intake System been heated by the engine's own
waste heat. The reason for ensuring
 Because a diesel engine requires close that an engine's air supply is as cool as
tolerances to achieve its compression possible is that cool air is more dense
ratio, and because most diesel engines than hot air.
are either turbocharged or  This means that, per unit volume, cool
supercharged, the air entering the air has more oxygen than hot air.
engine must be clean, free of Thus, cool air provides more oxygen
debris, and as cool as possible. per cylinder charge than less dense,
 Also, to improve a turbocharged or hot air. More oxygen means a more
supercharged engine's efficiency, the efficient fuel burn and more power.
compressed air must be cooled after After being filtered, the air is
being compressed. routed by the intake system into
 The air intake system is designed to the engine's intake manifold or air
perform these tasks. box.
 Air intake systems vary greatly  The manifold or air box is the
from vendor to vendor but are component that directs the fresh air to
usually one of two types, wet or each of the engine's intake valves
dry. or ports.
 In a wet filter intake system, the air is  If the engine is turbocharged or
sucked or bubbled through a housing supercharged, the fresh air will be
that holds a bath of oil such that the compressed with a blower and
possibly cooled before entering the
203
 intake manifold or air box. The intake  The blower is driven through gears
system also serves to reduce the air directly from the engines crankshaft.
flow noise.  The most common type of blower uses
two rotating rotors to compress the
The compressed air serves two functions. air.
 Supercharging is more commonly
1. First, it increases the engine's found on two-stroke engines where
available power by increasing the the higher pressures that a
supercharger is capable of generating
maximum amount of air (oxygen) that is
are needed.
forced into each cylinder. This allows
more fuel to be injected and more power Advantages of Supercharging
to be produced by the engine.
2. The second function is to increase  increase the mass of air in the
intake pressure. This improves the cylinder
scavenging of the exhaust gasses out of  increase the break mean effective
the cylinder. pressure
 increase the break horsepower
 Turbo charging is commonly found on  decrease break specific fuel
high power four-stroke engines. consumption
 It can also be used on two-stroke  Super charging can increase BHP by
engines where the increase in intake 50% to 75% over the same non –
pressure generated by the turbocharged engine.
turbocharger is required to force the
fresh air charge into the cylinder and Turbo charging
help force the exhaust gasses out of
the cylinder to enable the engine to  Turbo charging an engine occurs
run. when the engine's own exhaust gasses
are forced through a turbine
Supercharging (impeller), which rotates and is
connected to a second impeller located
 Supercharging an engine performs the in the fresh air intake system.
same function as turbo charging an  The impeller in the fresh air intake
engine. system compresses the fresh air.
 The difference is the source of power
used to drive the device that Turbo Charger
compresses the incoming fresh air.
 Allows greater mass of air into the  Uses energy of the exhaust gases to
drive a turbine and have this turbine
combustion chamber thus greater mass
drive a compressor (blower)
fuel can be burned, providing greater  Operates in conjunction with positive
release, increasing the power displacement method of scavenging
potential. air in an engine
 In a supercharged engine, the air is  Turbo charger is driven by exhaust
commonly compressed in a device gases of engine rated speed, thus
called a BLOWER. increasing the “engine load” will also
increase the speed of T/C
Blower  Thus T/C is load dependent
 At low load, the T/C is un-operational
 The diesel engine's blower is part of because the amount of exhaust gas is
the air intake system and serves to not enough to bring the turbines into
compress the incoming fresh air for operational speed.
delivery to the cylinders for  If an engine is turbo charged there will
combustion. be more amount of fuel burned, then
 The blower can be part of either a the total heat removal responsibility of
turbocharged or supercharged air the cooling system is also increased.
intake system.  For example: power is increased 30% ,
30% more cooling would be required.
204
  Valve over-lap is the way for The engine speed will not increase with load,
supercharged engine to bypass some it may decrease a little.
of this cooling responsibility
 Extended overlap period allows Horsepower
cooling of the exhaust V/V, piston
head and cylinder walls  Power is the amount of work done per
 Supercharging causes higher overage unit time or the rate of doing work.
pressures to act on the bearings  For a diesel engine, power is rated in
(higher bearing sustained pressure). units of horsepower.
 This requires increase in oil flow to
maintain proper bearing temperature. Indicated horsepower is the power
 Increases the air supply with load transmitted to the pistons by the gas in the
 As the engine is loaded, more exhaust
cylinders and is mathematically calculated.
gas is generated providing more
energy to be extracted in the turbine, Brake horsepower refers to the amount of
hence, more air can be compressed.
usable power delivered by the engine to the
Scavenging crankshaft.

 Forcing the products of combustion  Indicated horsepower can be as much

out of the cylinder with fresh air as 15% higher than brake horsepower.
charge  The difference is due to internal
 Valve overlap period also serves to engine friction, combustion
cool valves inefficiencies, and parasitic losses, for
 Overlap increase purging period of example, oil pump, blower, water
exhaust gases from cylinder pump, etc.
 The ratio of an engine's brake
Types of Scavenging horsepower and its indicated
horsepower is called the mechanical
1. Direct or Cross Flow scavenging – the efficiency of the engine.
intake and exhaust parts are opposite one  The mechanical efficiency of a four-
cycle diesel is about 82 to 90 percent.
another (the inlet air is deflected by the
 This is slightly lower than the
piston shape) efficiency of the two-cycle diesel
engine.
2. Loop scavenging – both inlet port and  The lower mechanical efficiency is
exhaust ports are on the same side ( piston due to the additional friction losses
shape directs the air flow from intake o the and power needed to drive the piston
exhaust port) through the extra 2 strokes.
 Engines are rated not only in
3. Uniflow – there are intake ports and horsepower but also by the torque they
exhaust valve (intake parts are located produce.
around the periphery and exhaust valve on
Torque is a measure of the engine's ability
top; air flow is in uniform direction
to apply the power it is generating.
Roots - Type Blower – used on small engines Torque is commonly given in units of lb-ft.
to force exhaust gases of the cylinder (there
are driving gears on one end of the blower and
DIESEL ENGINE SPEED, FUEL
either a lobe type compressor or positive
CONTROLS, AND PROTECTION
displacement pump to the other end); gear
driven Engine Control
Root – Type blower speed = engine speed  The control of a diesel engine is
accomplished through several
Positive displacement overload with by components: the camshaft, the fuel
burning more fuel, there will not be more air injector, and the governor.

205
  The camshaft provides the timing chamber by two funnel-shaped ports
needed to properly inject the fuel, the in the plunger bushing.
fuel injector provides the component  The motion of the injector rocker arm
that meters and injects the fuel, and (not shown) is transmitted to the
the governor regulates the amount of plunger by the injector follower
fuel that the injector is to inject. which bears against the follower
 Together, these three major spring.
components ensure that the engine
runs at the desired speed.

Fuel Injectors

 Each cylinder has a fuel injector

designed to meter and inject fuel into
the cylinder at the proper instant.
 To accomplish this function, the
injectors are actuated by the engine's
camshaft.
 The camshaft provides the timing and
pumping action used by the injector to
inject the fuel.
 The injectors meter the amount of fuel
injected into the cylinder on each
stroke.
 The amount of fuel to be injected by
each injector is set by a mechanical
linkage called the fuel rack.
 The fuel rack position is controlled by
the engine's governor.

How do the cylinders get fuel oil for ignition

to create the power stroke? Governors

 By forcing the fuel oil into them either Speed sensitive device used to control or limit
by injection pumps or by high fuel oil the engine speed
pressure. The fuel oil enters the
cylinder through fuel injectors which Two type of Governors
spray the fuel into the cylinders. 1. Direct Acting – (mechanical governors) –
 The governor determines the amount linked directly to the fuel pump racks so
of fuel required to maintain the that the action of flyweights on the
desired engine speed and adjusts the governor will cause the fuel pump rack to
amount to be injected by adjusting the move.
position of the fuel rack. 2. Indirect acting – (relay or hydraulic
 Each injector operates in the following governors) – consist of 3 main parts: a.
manner. As illustrated in Figure fuel
speed sensitive action, b. a power section,
under pressure enters the injector
through the injector's filter cap and c. compensating section
filter element.
Function of Governor
 From the filter element the fuel travels
down into the supply chamber (that
1. Regulation
area between the plunger bushing and
the spill deflector). a. Constant speed governors – (maintains
 The plunger operates up and down in
constant speed regardless of load ) a
the bushing, the bore of which is open
to the fuel supply in the supply for diesel generators

206
 b. Variable speed regulating governors Dead Band – narrow band of speed variation
(maintain set output speed at any which the governor makes no connections of
given load) *for M/E the energy medium (fuel); refers to the
sensitivity of governors
2. Limit Speed
Hunting – a rhythmic variation of speed
 regulates both maximum speed and
idling speed; all other speed are Isochronous – constant speed, specific the
normally set. (for small vessel; truck same avenue speed regardless of load; zero
and automotive engines) speed droop
 Prevent the engine from over speeding
when load is suddenly taken off the Overspeed – a speed greater than the selected
engine and from stalling when the normal speed or greater than the highest speed
throttle is in the lowest possible attained for a specified load selected.
position. Parallel Operation – two or more prime
3. Engine Over Speed Protection – have two movers (diesel) are electrically and driving a
types, manual and automatic reset common shaft and load

 Either used with regulating governors Smoke Limiters, Load Limiters, Maximum
to provide protection in case he latter Fuel Stops- device used to limit the maximum
should fail amount of fuel delivered to the engine
 The automatic type does not stop the
engine, the governor resets itself. Maximum Speed Stop – a device used to
 Both types will “sound an alarm” limit the spring force on a regulating – type
indicating malfunction has occurred. governor.
 Both types act by stopping the flow of
free to the engine Hydraulic Governors: 3 Main Parts:

Definitions: 1. Speed Sensitive Section – senses the

engine speed and tries to maintain a
Speed Drop – a progressive drop in speed as constant engine speed
load is picked up by the prime mover from no 2. Power Section – actuates the fuel
load to full load with out manually changing racks controlling fuel flow to the engine
the speed setting. 3. Compensating Section –
compensates the reactions time between
Expressed in rpm or percentage
the engine and the governor
% speed droop = no load rpm – full load rpm x 100%
Full load rpm
Engine Time Lag

Speed droop rpm = no load rpm – full load rpm Time from the moment the load or/
speed changes of the engine occur until the
Sensitivity – the smallest speed change that engine is again running at the correct load or
will induce the governor to after the flow of speed.
energy medium make corrective movement.
5 Factors Affecting Time Log
Stability – ability of the governor to correct a
speed disturbance with a minimum of false 1. Delay of the governor in sensing the
motion change in load/speed
2. Delay caused by the inertia of the
Compensation – mechanical and/or hydraulic governors part (from flyweights sense the
action that prevents overcorrection of the action to the moment governor moves)
energy medium (fuel) supply

207
3. Time from when the governors causes  The large hydraulic piston is linked to
the fuel p/p settings to be damage until the the fuel rack and its motion resets the
fuel pump is changed. fuel rack for increased/decreased fuel.
4. Time that it takes corrected fuel
Simplified Operation of the Governor
change, once the pump is reset, to entering
the engine cylinder  With the engine operating, oil from
5. Time required for the correct fuel the engine lubrication system is
charge to be converted into the correct supplied to the governor pump gears,
engine load / speed. as illustrated in Figure 29.
 The pump gears raise the oil pressure
GOVERNOR to a value determined by the spring
relief valve.
Operation of a Governor  The oil pressure is maintained in the
annular space between the undercut
 The following is an explanation of the portion of the pilot valve plunger and
operation of a constant speed, the bore in the pilot valve bushing.
hydraulically compensated governor  For any given speed setting, the spring
using the Woodward brand governor speeder exerts a force that is opposed
as an example. by the centrifugal force of the
 The principles involved are common revolving flyweights.
in any mechanical and hydraulic  When the two forces are equal, the
governor. control land on the pilot valve plunger
 The Woodward speed governor covers the lower ports in the pilot
operates the diesel engine fuel racks to valve bushing.
ensure a constant engine speed is
maintained at any load. Detailed Operation of the
 The governor is a mechanical- GovernorCutaway of a Woodward Governor
hydraulic type governor and receives
its supply of oil from the engine  Under these conditions, equal oil
lubricating system. pressures are maintained on both
 This means that a loss of lube oil sides of the buffer piston and tension
pressure will cut off the supply of oil on the two buffer springs is equal.
to the governor and cause the  Also, the oil pressure is equal on both
governor to shut down the engine. sides of the receiving compensating
 This provides the engine with a built- land of the pilot valve plunger due to
in shutdown device to protect the oil passing through the compensating
engine in the event of loss of needle valve.
lubricating oil pressure.  Thus, the hydraulic system is in
balance, and the engine speed remains
Simplified Operation of the Governor constant.
 When the engine load increases, the
 The governor controls the fuel rack engine starts to slow down in speed.
position through a combined action of  The reduction in engine speed will be
the hydraulic piston and a set of sensed by the governor flyweights.
mechanical flyweights, which are The flyweights are forced inward (by
driven by the engine blower shaft. the spring), thus lowering the pilot
 The position of the flyweights is valve plunger (again, due to the
determined by the speed of the engine. downward spring force).
 As the engine speeds up or down, the  Oil under pressure will be admitted
weights move in or out. under the servo-motor piston (topside
 The movement of the flyweights, of the buffer piston) causing it to rise.
due to a change in engine speed, This upward motion of the servo-
moves a small piston (pilot valve) in motor piston will be transmitted
the governor's hydraulic system. through the terminal lever to the fuel
 This motion adjusts flow of hydraulic racks, thus increasing the amount of
fluid to a large hydraulic piston fuel injected into the engine.
(servo-motor piston).
208
  The oil that forces the servo-motor to cut off fuel to the engine and
piston upward also forces the buffer alarm at a certain preset rpm.
piston upward because the oil  This is usually accomplished by
pressure on each side of the piston isolating the governor from its oil
is unequal. This upward motion of supply, causing it to travel to the
the piston compresses the upper no-fuel position, or it can override
buffer spring and relieves the the governor and directly trip the fuel
pressure on the lower buffer spring. rack to the no-fuel position.
 The oil cavities above and below
the buffer piston are common to Water jacket
the receiving compensating land on
the pilot valve plunger.  Water-cooled engines can overheat
 Because the higher pressure is below if the cooling water system fails to
the compensating land, the pilot valve remove waste heat.
plunger is forced upward, recentering  Removal of the waste heat prevents
the flyweights and causing the control the engine from seizing due to
land of the pilot valve to close off the excessive expansion of the
regulating port. components under a high
 Thus, the upward movement of the temperature condition.
servo-motor piston stops when it has  The cooling water jacket is commonly
moved far enough to make the where the sensor for the cooling water
necessary fuel correction. system is located.
 Oil passing through the compensating  The water jacket temperature sensors
needle valve slowly equalizes the provide early warning of abnormal
pressures above and below the engine temperature, usually an alarm
buffer piston, thus allowing the function only.
buffer piston to return to the center  The setpoint is set such that if
position, which in turn equalizes the condition is corrected in a
the pressure above and below timely manner, significant engine
the receiving compensating land. damage will be avoided.
 The pilot valve plunger then moves to  But continued engine operation at
its central position and the engine the alarm temperature or higher
speed returns to its original setting temperatures will lead to engine
because there is no longer any damage.
excessive outward force on the
flyweights. Exhaust, exhaust temperatures
 The action of the flyweights and
the hydraulic feedback mechanism  In a diesel engine are very important
produces stable engine operation by and can temperatures - provide a vast
permitting the governor to move amount of information regarding
instantaneously in response to the load the operation of the engine.
change and to make the necessary  High exhaust temperature can
fuel adjustment to maintain the indicate an overloading of the
initial engine speed. engine or possible poor performance
due to inadequate scavenging (the
Overspeed device cooling effect) in the engine.
 Extended operation with high
 Because a diesel is not self-speed- exhaust temperatures can result in
limiting, a failure in the governor, damage to the exhaust valves,
injection system, or sudden loss piston, and cylinders.
of load could cause the diesel to  The exhaust temperature usually
overspeed. provides only an alarm function.
 An overspeed condition is extremely
dangerous because engine failure is Low lube oil pressure
usually catastrophic and can possibly
cause the engine to fly apart.  Low oil pressure or loss of oil pressure
 An overspeed device, usually some can destroy an engine in short order.
type of mechanical flyweight, will act Therefore, most medium to larger
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 engines will stop upon low or loss of  The engine will then accelerate to idle
oil pressure. speed. When the starter motor is
 Loss of oil pressure can result in the overdriven by the running motor it
engine seizing due to lack of
will disengage the flywheel.
lubrication. Engines with
mechanical-hydraulic governors will  Because a diesel engine relies on
also stop due to the lack of oil to the compression heat to ignite the fuel, a
governor. cold engine can rob enough heat from
 The oil pressure sensor usually stops the gasses that the compressed air falls
the engine. The oil pressure sensors below the ignition temperature of the
on larger engines usually have two fuel.
low pressure setpoints.  To help overcome this condition,
 One setpoint provides early warning some engines (usually small to
of abnormal oil pressure, an alarm medium sized engines) have
function only. The second setpoint can glowplugs.
be set to shutdown the engine before  Glowplugs are located in the cylinder
permanent damage is done. head of the combustion chamber and
use electricity to heat up the electrode
at the top of the glowplug.
High crankcase pressure
 The heat added by the glowplug is
sufficient to help ignite the fuel in the
 High crankcase - pressure is usually
cold engine.
caused by excessive blow-by (gas
 Once the engine is running, the
pressure in the cylinder blowing by
glowplugs are turned off and the
the piston rings and into the
heat of combustion is sufficient to
crankcase).
heat the block and keep the engine
 The high pressure condition indicates
running.
the engine is in poor condition. The
 Larger engines usually heat the block
high crankcase pressure is usually
and/or have powerful starting motors
used only as an alarm function.
that are able to spin the engine long
enough to allow the compression heat
Starting Circuits
to fire the engine. Some large engines
use air start manifolds that inject
 Diesel engines have as many
compressed air into the cylinders
different types of starting circuits
which rotates the engine during the
as there are types, sizes, and
start sequence.
manufacturers of diesel engines.
 Commonly, they can be started by air
motors, electric motors, hydraulic Engine Performance
motors, and manually.
 The start circuit can be a simple  Indicated power developed by actual
manual start pushbutton, or a complex engines of those factors by which the
auto-start circuit.
differ from ideal engines.
 But in almost all cases the following
events must occur for the starting
engine to start. Factors that affecting Performance
 The start signal is sent to the starting
motor. The air, electric, or hydraulic 1. Valve resistance
motor, will engage the engine's 2. Residual Gases – lower the amount of
flywheel. fresh change
 The starting motor will crank the
 dilute the fresh charge, increase the
engine. The starting motor will spin
amount of inert gas
the engine at a high enough rpm to
3. Heat Loss Conditions – heat exchange
allow the engine's compression to
between the gases and the walls or parts
ignite the fuel and start the engine
running of the engine coming in contact with the
gases

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Factors Affecting Heat Loss:  Bypass the cooler to increase the
temperature
1. Duration of combustion of the charge  By-passing occurs for density start-up
air and on bleed basis during actual
2. Temperature of combustion (depends engine running as the engine outlet
on fuels, ratio of compression and engine temp. varies.
load)  Ideal outlet temp. 1600 – 1800F / 72 –
3. Speed of the engine 820C.
4. Shape of combustion space
5. Size of the cylinder Cooling Water Pumps
6. Timing of the ignition (spark
ignition) / fuel injection (in diesel engine)  Forces of the fresh water through the
7. Heat flow from walls engine
8. Charge dilution  FW P/p’s are driven directly by the
9. Charge efficiency average for high and medium speed
10. Atmospheric condition engine
 The pump should be kept running
Cooling and Lubrication System when the temp. diff. across the
engine is zero.
Two main function;
Piston Cooling
1. To remove 30 – 35% of the heat from
the system  Water removes more heat efficiency
2. to cool the engine’s lube oil  Specific heat of water is about twice
that of the line lube oil/ only half
Two general types of Cooling System amount of the water must be
circulated to remove the same
1. Open system – sea water is in direct amount of the heat.
contact with sea water
 Max. outlet temp. 1200F = 490C
 Salt will precipitate on the engine
block
2. Closed system – uses fresh water
 Water circulates starting at the
bottom where the engine is coolest.
 Desirable temperature diff. = 100 –
200F
 (to prevent built up of thermal
stress)
 Expansion tank capacity 50 – 10% of
the total cooling H2O volume
 To compensate for density variation
in the water due to increase or
decrease in temperature.

3 Way Thermostatic Valve

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