unit 5, EME uptu first year

Internal Combustion Engines
Unit – V
( EME)

Topics covered
I.C. Engines
 Introduction of I.C. Engines
 Classification of I.C. Engines
 Constructional details of I.C. Engine
 Comparison of Two-stroke and Four-stroke Cycle Engine
 Working of two stroke and four stroke diesel
and petrol engines
 Comparison of Petrol and Diesel Engine

Thermodynamics
• Thermodynamics definition
• It is a Greek word which means flow of
heat in physical and chemical reactions.
• Thermodynamics is a branch of science
which deals with study of different forms of
energy and their interconversions. It deals
with energy changes in physical and
chemical processes.

Importance of thermodynamics
• Useful to predict whether any chemical
reaction can occur under specified
conditions
• Used to predict the extent of chemical
reaction before equilibrium is reached
• Used to derive important laws like Law of
equilibrium

System, surrounding and universe
System
• May be defined as the part of universe selected for thermodynamic
consideration i.e. to study the effect of temperature, pressure etc.
• It may also be defined as specified part of universe in which energy
changes are taking place
Surrounding
• The remaining portion of universe excluding the system is called
Surrounding
Universe = System + Surrounding
• The System And Surrounding can be separated by real and
imaginary boundary

Types of System
Open System: Here mass and energy can be exchanged
with surroundings eg. If some water is kept in open
vessel or hot tea in open cup

Closed System
• In a closed system, there is only the exchange of energy
with surroundings, no exchange of mass takes place
• For example, if water is placed in closed metallic vessel
or hot tea placed in closed tea pot.

Isolated System
• There is neither exchange of mass nor energy with
surrounding.
• Eg. Water placed in a vessel which is closed as well as
insulated or tea placed in a thermos flask

Laws of thermodynamics
• Zeroth law of thermodynamics
• First law of thermodynamics
• Second law of thermodynamics

Zeroth law of thermodynamics
• Temperature is a property of a system
which determines the degree of hotness.
Obviously, it is a relative term.
• eg: A hot cup of coffee is at a higher
temperature than a block of ice. On the
other hand, ice is hotter than liquid
hydrogen.

• Two systems are said to be equal in
temperature, when there is no change in
their respective observable properties
when they are brought together. In other
words, “when two systems are at the
same temperature they are in thermal
equilibrium”(They will not exchange heat).

• When two bodies A and B are separately in
thermal equilibrium with a third body, they in turn
are in equilibrium with each other.
• Let us say TA,TBand TCare the temperatures of
A,B and C respectively.
• A and C are in thermal equilibrium. Ta=Tc
• B and C are in thermal equilibrium. Tb=Tc
• Consequence of ‘0’th law
• A and B will also be in thermal equilibrium TA=TB

First law of thermodynamics
• Energy can neither be created nor
destroyed although it may be converted
from one form to other
• The total energy of the universe remains
constant although it may undergo
transformation from one form to other
• The energy of an isolated system remains
constant

• 1st
Law of thermodynamics
∆U= q + W
• Where
∆U = change in internal energy
q = heat energy
W = work done

Internal Energy
• Every system is associated with a definite amount of
energy, which is called its internal energy. It is donated
by E or U
• It depends upon the various factors such as
temperature, pressure and chemical nature of the
substance
Change in Internal Energy
• The change in internal energy in a chemical reaction is
the difference in the internal energies of the products
and the reactants
• ∆E= E(products)-E(reactants)
• = Ep-Er

Enthalpy(H)
• Enthalpy or heat content of a system may
be defined as the sum of the internal
energy and the product of its pressure and
volume
H =E + PV
Where E is the internal energy
P is pressure
V is the volume of the system

Change In Enthalpy
Change In Enthalpy
• It is the difference in the enthalpies of the
products and the reactants
∆H = H(products)- H(reactants)
= H p- Hr

Property, process, path
• Property- It is some characteristic of the
system to which some physically
meaningful numbers can be assigned
without knowing the history behind it.
• These are macroscopic in nature.
• Invariably the properties must enable us to
identify the system.

Types of properties
Intensive Properties
• They do not depend on the size or extent of the system
or quantity of matter present in it.
• They are dependent on the nature of substance present
in it.
• Example: pressure, temperature, density, surface
tension
Extensive Properties
• Depend on the Quantity or extent of matter present in
the system
• Examples: volume, energy, heat capacity, entropy

State of System
• They are the condition of system
which is described in terms of
certain measurable properties
such as temperature(T),
pressure(P), volume(V) etc. of the
system.

State Variables
• The properties of system such as
temperature (T), pressure (P),
volume (V) when changed, the
system also changes. These
properties are called state
variables

State function
• It is defined as the property whose value depends only
upon the state of the system and is independent of the
path by which state has been reached
• For example: a person standing on the roof of the
building has a fixed value of potential energy and the
potential of person does not depend whether he has
reached there by stairs or lift. Ex. Potential energy,
pressure, volume, temperature, internal energy etc.

Process: Any change that a system undergoes
from one equilibrium state to another.
Process
• When state of system changes then
process is said to occur the first and last
state of process are called initial and final
state respectively

Types of processes
1. Isothermal Process:
• It is defined as the process in which
temperature of system remains constant.
• Heat can flow from system to surrounding
and vice versa in order to keep the
temperature constant.
PV= CONSTANT

Isochoric Process:
• Volume of system remains constant
during the process.

Isobaric process
• A process is one which the pressure is
kept constant.

Cyclic Process
• Here the system undergoes series of
changes and finally returns to its initial
state.

Reversible Process
• Such a process is carried out
infintesimally slowly so that all
changes occuring in the direct
process can be reversed and the
system and the surrounding remain in
state of equilibrium
• It is an ideal process and cannot be
realized in actual process

Irreversible process
• Change is brought about rapidly
and the system does not attain
equilibrium
• The force which drives the
reactants towards products is
greater than opposing force which
is to carry reverse process

Path
• Path: The series of states through which a
system passes during a process.

Second law of thermodynamics
• Second law: Thermal reservoir
Statement 1: It is impossible to
construct a device which
operating in a cycle will produce
no effect other than raising of a
weight and exchange of heat with
a single reservoir.

• Note the two underlined words. II Law
applies only for a cycle -not for a process!!
(We already know that during an
isothermal process the system can
exchange heat with a single reservoir and
yet deliver work)
• !!There is nothing like a 100% efficient
heat engine!!

Kelvin Planck statement
• Kelvin-Planck’s statement is based on the fact that the
efficiency of the heat engine cycle is never 100%. This
means that in the heat engine cycle some heat is always
rejected to the low temperature reservoir. The heat
engine cycle always operates between two heat
reservoirs and produces work
• The Kelvin - Planck statement states that "It is
impossible for a heat engine operating in a cyclic
manner to exchange heat with only one reservoir".
In other words, this law implies that it is impossible to
build a 100% efficient heat engine.

Clausius’ statement
• Clausius’ statement-It is impossible to construct a
device which operates in a cycle and produces no effect
other than the transfer of heat from a cooler body to a
hotter body.
• Clausius statement states “it is impossible for a self
acting machine working in a cyclic process without
any external force, to transfer heat from a body at a
lower temperature to a body at a higher temperature.
The Kelvin Planck statement is used for heat engines
and Clausius statement is for refrigerators.

Introduction of I.C. Engine
Heat Engines - A machine or device which derives heat from the
combustion of fuel and converts part of this energy into
mechanical work is called a heat engine. Heat engines may be
classified into two main classes as follows:
1. External combustion engines
2. Internal combustion engines.
1. External Combustion Engines - In this case, combustion of
fuel takes place outside the cylinder as in the case of steam
engines where the heat of combustion is employed to generate
steam which is used to move a piston in a cylinder. Other
examples of external combustion engines are hot air engines,
steam turbine and closed cycle gas turbine.

Introduction of I.C. Engine (contd..)
2. Internal Combustion Engines - In this case, combustion of
fuel with oxygen of the air occurs within the cylinder of the
engine. The internal combustion engines group includes
engines employing mixtures of combustible gases and air,
known as gas engines, those using lighter liquid fuel or spirit
known as petrol engines and those using heavier liquid fuels,
known as oil, compression ignition or diesel engines.
The important applications of I.C. engines are: (i) Road vehicles,
locomotives, ships and aircraft, (ii) Portable standby units for
power generation in case of scarcity of electric power, (iii)
Extensively used in farm tractors, lawn movers, concrete mixing
devices and motor boats.

Classification of I.C. Engines
The internal combustion engines may be classified in the
following ways:
1. According to the type of fuel used
a) Petrol engines, b) Diesel engines, and c) Gas engines.
2. According to the method of igniting the fuel
a) Spark ignition engines, and b) Compression ignition
engines.
3. According to the number of strokes per cycle
a) Four stroke cycle engines, and b) Two stroke cycle
engines.
4. According to the cycle of operation
a) Otto cycle engines, b) Diesel cycle engines, and c) Dual
cycle engines.

Classification of I.C. Engines (contd..)
5. According to the speed of the engine
a) Slow speed engines, b) Medium speed engines, and
c) High speed engines.
6. According to the cooling system
a) Air-cooled engines, and b) Water-cooled engines.
7. According to the method of fuel injection
a) Carburettor engines, and b) Air injection engines.
8. According to the number of cylinders
a) Single cylinder engines, and b) Multi-cylinder engines.

Classification of I.C. Engines (contd..)
9. According to the arrangement of cylinders
a) Vertical engines, b) Horizontal engines, c) Radial engines,
d) In-line multi-cylinder engines, e) V-type multi-cylinder
engines,
f) Opposite-cylinder engines, and g) Opposite-piston engines.
10. According to the valve mechanism
a) Overhead valve engines, and b) Side valve engines.
11. According to the method of governing
a) Hit and miss governed engines, b) Quantitatively
governed engines, and Qualitatively governed engines.

Basic Idea of I.C. Engines
The basic idea of internal combustion engine is shown in Fig.
(Basic idea of I.C. engine). The cylinder which is closed at one
end is filled with a mixture of fuel and air. As the crankshaft
turns it pushes cylinder. The piston is forced up and
compresses the mixture in the top of the cylinder. The mixture is
set alight and, as it burns, it creates a gas pressure on the
piston, forcing it down the cylinder.
This motion is shown by arrow ‘1’. The piston pushes on the rod
which pushes on the crank. The crank is given rotary (turning)
motion as shown by the arrow ‘2’. The flywheel fitted on the end
of the crankshaft stroes energy and keeps the crank turning
steadily.

Fig. Basic idea of I.C. engine

Constructional details of I.C. Engines
A cross-section of an air-cooled I.C. engine with principal parts
is shown in Fig. (Air-cooled I.C. engine).
A. Parts common to both Petrol and Diesel engine:
1. Cylinder, 2. Cylinder head, 3. Piston,
4. Piston rings, 5. Gudgeon pin, 6. Connecting rod,
7. Crankshaft, 8. Crank, 9. Engine bearing,
10. Crank case. 11. Flywheel, 12. Governor,
13. Valves and valve operating mechanism.
B. Parts for Petrol engines only:
1. Spark plug, 2. Carburettor, 3. Fuel pump.
C. Parts for Diesel engine only :
1. Fuel pump, 2. Injector.

Constructional details of I.C. Engines (contd..)
The details of the I.C. Engine parts are:
1. Cylinder - It is one of the most important part of the engine, in
which the piston moves to and fro in order to develop power.
The engine cylinder has to withstand a high pressure (more than
50 bar) and temperature (more than 2000 deg C). Thus the
material for the engine cylinder should be such that it can retain
sufficient strength at such a high pressure and temperature. For
ordinary engines, the cylinder is made of ordinary cast iron. But
for heavy duty engines, it is made of steel alloys or aluminum
alloys.
Sometimes, a liner or sleeve is inserted into the cylinder, which
can be replaced when worn out. As the material required for
liner is comparatively small, it cab be made of alloy cast iron
having long life and sufficient resistance to rapid wear and tear
to the fast moving reciprocating parts.

2. Cylinder head - It is fitted on one end of the cylinder, and act
as a cover to close the cylinder bore. Generally, the cylinder
head contains inlet and exit valves for admitting fresh charge
and exhausting the burnt gases. In petrol engines, the cylinder
head also contains a spark plug for igniting the fuel-air mixture,
towards the end of compression stroke. But in diesel engines,
the cylinder head contain nozzles, (i.e. fuel valve) for injecting
the fuel into the cylinder.
The cylinder head is cast as one piece and bolted to one end of
the cylinder. The cylinder block and cylinder head are made
from the same material. A copper or asbestos gasket is provided
between the engine cylinder and cylinder head to make an air-
tight joint.

3. Piston – It is considered as the heart of an I.C. engine, whose
main function is to transmit the force exerted by the burning of
charge to the connecting rod. The piston are generally made of
aluminium alloys which are light in weight. They have good heat
conducting property and also greater strength at higher
temperature.
4. Piston rings – These are circular rings and made of special
steel alloys which retain elastic properties even at high
temperatures. The piston rings are housed in the circumferential
grooves provided on the outer surface of the piston. Generally,
there are two sets of rings mounted for the piston. The function
of the upper rings is to provide air tight seal to prevent leakage
of the burnt gases into the lower portion. Similarly, the function
of the lower rings is to provide effective seal to prevent leakage
of the oil into the engine cylinder.

5. Connecting rod – It is a link between the piston and
crankshaft, whose main function is to transmit force from the
piston to the crankshaft. Moreover, it converts reciprocating
motion of the piston into circular motion of the crankshaft, in the
working stroke. The upper (i.e. smaller) end of the connecting
rod is fitted to the piston and the lower (i.e. bigger) end of the
crank.
The special steel alloys or aluminium alloys are used for the
manufacture of connecting rods. A special care is required for
the design and manufacture of connecting rod, as it is subjected
to alternatively compressive and tensile stresses as well as
bending stresses.

6. Crankshaft – It is considered as the backbone of an I.C.
engine whose function is to covert the reciprocating motion of
the piston into the rotary motion with the help of connecting rod.
This shaft contains one or more eccentric portions called
cranks. This part of the crank, to which bigger end of the
connecting rod is fitted, is called crank pin. Special steel alloys
are used for the manufacture of crankshaft. A special care is
required for the design and manufacture of crankshaft
7. Crank case – It is a cast iron case, which holds the cylinder
and crankshaft of an I.C. engine. It also serves as a sump for the
lubricating oil. The lower portion of the crank case is known as
bed plate, which is fixed with the help of bolts.
8. Flywheel – It is a big wheel, mounted on the crankshaft,
whose function is to maintain its speed constant. It is done by
storing excess energy during power stroke, which, is returned
during other stroke.

Terms relating to I.C. Engines
The various terms relating to I.C. engines are elaborated in Fig.
1. Bore – The inside diameter of the cylinder is called bore.
2. Stroke – As the piston reciprocates inside the engine cylinder,
it has got limiting upper and lower positions beyond which it
cannot move and reversal of motion takes place at these limiting
positions. The linear distance along the cylinder axis between
two limiting positions, is called stroke.
3. Top Dead Centre (T.D.C.) – The top most position towards
cover end side of the cylinder is called “top dead centre”. In
case of horizontal engines, this is known as inner dead centre.
4. Bottom Dead Centre – The lowest position of the piston
towards the crank end side of the cylinder is called “bottom
dead centre”. In case of horizontal engines it is called outer
dead centre.

Basic thermodynamic cycles
•CARNOT CYCLE
•OTTO CYCLE
•DIESEL CYCLE

P- V and T-S diagram for carnot
cycle

CARNOT CYCLE
• The Carnot cycle is the most efficient cycle
possible. It consists of four basic reversible
processes meaning that the cycle as a whole is
also reversible. The four reversible processes
are:
• 1. Isothermal expansion
• 2. Adiabatic expansion
• 3. Isothermal compression
• 4. Adiabatic compression

• The Carnot cycle comprises two ideal
reversible isothermal and two reversible
adiabatic processes in a heat engine. The
Carnot theorem and second law of
thermodynamics are based on the Carnot
cycle, which shows the maximum
efficiency that can be achieved by the
engine.

OTTO CYCLE
• It is theoretical cycle for analysis of
behavior of reciprocating Spark-Ignition
engines. It is also termed as constant
volume cycle. The cycle consists of two
reversible isochoric (V = C) processes and
two reversible adiabatic (PVy
= C)
processes. The sequence of processes is
as follows

OTTO CYCLE
• Process (1-2): Reversible adiabatic
compression.
• Process (2-3): Constant volume heat
addition.
expansion.
• Process (4-1): Constant volume heat
rejection.

Otto cycle with P -V and T-S
diagram

Diesel cycle
• It is theoretical cycle for
Compression-Ignition engines or
diesel engines analysis. The cycle
consists of two reversible adiabatic,
one constant volume and one
constant pressure process. The
operations are as follows:

DIESEL CYCLE
compression
• Process (2-3): Constant pressure
heat supply
expansion
• Process (4-1): Constant volume
heat rejection

Diesel cycle with P -V and T-S
diagram

Fig. Terms relating I.C. engines

Terms relating to I.C. Engines (contd..)
5. Clearance volume – The volume contained in the cylinder
above the top of the piston, when the piston is at top dead
centre, is called the clearance volume.
6. Swept volume – The volume swept through by the piston in
moving between top dead centre and bottom dead centre, is
called swept volume or piston displacement. Thus, when piston
is at bottom dead centre,
Total volume = swept volume + clearance volume.

Sequence of Operation
The sequence of operation in a cycle are as follows:
1. Suction stroke – In this stroke, the fuel vapour in correct
proportion, is applied to the engine cylinder.
2. Compression stroke –. In this stroke, the fuel vapour is
compressed in the engine cylinder.
3. Expansion stroke – In this stroke, the fuel vapour is fired just
before the compression is complete. It results in the sudden rise
of pressure, due to expansion of the combustion products in the
engine cylinder. This sudden rise of pressure pushes the piston
with a great force, and rotates the crankshaft. The crankshaft, in
turn, drives the machine connected to it.
4. Exhaust stroke – In this stroke, the burnt gases (or
combustion products) are exhausted from the engine cylinder,
so as to make space available for the fresh fuel vapour.

Two-stroke and Four-stroke Cycle
In a two-stroke engine, the working cycle is completed in two
strokes of the piston or one revolution of the crankshaft. This is
achieved by carrying out the suction and compression
processes in one stroke (or in inward stroke), expansion and
exhaust process in thr second stroke (or in outward stroke).
In a four-stroke engine, the working cycle is completed in four
strokes of the piston or two revolutions of the crankshaft. This
is achieved by carrying out suction, compression, expansion
and exhaust processes in each stroke.

Comparison of Two-stroke and Four-stroke
Cycle Engine
Following are the advantages and disadvantages of two-stroke
cycle engines over four-stroke cycle engines:
Advantages
1. A two stroke cycle engine gives twice the number of power
strokes than the four stroke cycle engine at the same engine
speed.
2. For the same power developed, a two-stroke cycle engine is
lighter, less bulky and occupies less floor area.
3. As the number of working strokes in a two-stroke cycle
engine are twice than the four-stroke cycle engine, so the
turning moment of a two-stroke cycle engine is more uniform.
Thus it makes a two-stroke cycle engine to have a lighter
flywheel and foundations. This also leads to a higher
mechanical efficiency of a two-stroke cycle engine.

Cycle Engine (contd..)
4. The initial cost of a two-stroke cycle engine is considerably
less than a four-stroke cycle engine.
5. The mechanism of a two-stroke cycle engine is much simpler
than a four-stroke cycle engine.
6. The two-stroke cycle engines are much easier to start.
Disadvantages
1. Thermal efficiency of a two-stroke cycle engine is less than
that a four-stroke cycle engine.

Cycle Engine (contd..)
2. Overall efficiency of a two-stroke cycle engine is also less
than that of a four-stroke cycle engine because in a two-stroke
cycle, inlet and exhaust ports remain open simultaneously for
sometime. A small quantity of charge is lost from the engine
cylinder.
3. In case of a two-stroke cycle engine, the number of power
strokes are twice as those of a four-stroke cycle engine. Thus
the capacity of the cooling system must be higher. There is a
greater wear and tear in a two-stroke cycle engine.
4. The consumption of lubricating oil is large in a two-stroke
cycle engine because of high operating temperature.
5. The exhaust gases in a two-stroke cycle engine creates noise,
because of short time available for their exhaust.

Two-stroke Cycle Petrol Engine
In this cycle, the suction, compression, expansion and exhaust
takes place during two strokes of the piston. There is one
working stroke after every revolution of the crankshaft. A two
stroke engine has ports instead of valves. All the four stages of
a two stroke petrol engine are described below:
1. Suction stage – In this stage, the piston, while going down
towards bottom dead centre (BDC), uncovers both the transfer
port and the exhaust port. The fresh fuel-air mixture flows into
the engine cylinder from the crank case, as shown in Fig.
2. Compression stage – In this stage, the piston, while moving
up, first covers the transfer port and then exhaust port. After
that the fuel is compressed as the piston moves upwards as
shown in Fig. In this stage, the inlet port opens and fresh fuel-air
mixture enters into the crank case.

Fig. Two-stroke cycle petrol engine

Fig. p-V diagram for a two-stroke cycle engine

Two-stroke Cycle Petrol Engine (contd..)
3. Expansion stage – Shortly before this piston reaches the top
dead centre (TDC) during compression stroke, the charge is
ignited with the help of a spark plug. It suddenly increases the
pressure and temperature of the products of combustion but the
volume remains constant. Due to rise in the pressure, the piston
is pushed downwards with a great force as shown in Fig. The
hot burnt gases expand due to high speed of the piston. During
this expansion, some of the heat energy produced is
transformed into mechanical work.
4. Exhaust stage - In this stage, the exhaust port is opened as
the piston moves downwards. The products of combustion,
from the engine cylinder are exhausted through the exhaust port
into the atmosphere, as shown in Fig. This completes the cycle
and the engine cylinder is ready to suck the charge again.

Two-stroke Cycle Diesel Engine
In a two-stroke Diesel cycle engine all the operations are the
same as in the spark ignition (Otto cycle) engine with the
differences; firstly in this case, only air is admitted into cylinder
instead of air-fuel mixture and secondly fuel injector is fitted to
supply the fuel instead of a sparking plug. All the four stages of
a two-stroke cycle diesel engine are described below:
1. Suction stage – In this stage, the piston while going down
towards Bottom Dead Centre (BDC) uncovers the transfer port
and the exhaust port. The fresh air flows into the engine cylinder
from the crank case.
2. Compression stage – In this stage, the piston while moving
up, first covers the transfer port and then exhaust port. After
that the air is compressed as the piston moves upwards. In this
stage, the inlet port opens and the fresh air enters into the crank
case.

Two-stroke Cycle Diesel Engine (contd..)
3. Expansion stage – Shortly before the piston reaches the Top
Dead Centre (TDC) during compression stroke, the fuel oil is
injected in the form of very fine spray into the engine cylinder
through fuel injection valve. At this moment, temperature of the
compressed air is sufficiently high to ignite the fuel. It suddenly
increases the pressure and temperature of the products of
combustion. The fuel oil is assumed to be burnt at constant
pressure. Due to increased pressure, the piston is pushed with a
great force. The burnt gases expand due to high speed of the
piston. During the expansion, some of the heat energy produced
is transformed into mechanical work.
4. Exhaust stage – In this stage, the exhaust port is opened and
the piston moves downwards. The products of combustion from
the engine cylinder are exhausted through the exhaust port into
the atmosphere. This completes the cycle, and the engine
cylinder is ready to suck the air again.

Four-stroke Cycle Petrol Engine
It requires four strokes of the piston to complete one cycle of
operation in the engine cylinder. The four strokes of a petrol
engine sucking fuel-air mixture (petrol mixed with proportionate
quantity of air in the carburettor known as charge) are described
below:
1. Suction stroke – In this stroke, the inlet valve opens and
charge is sucked into the cylinder as the piston moves
downward from TDC. It continues till the piston reaches its BDC
as shown in Fig.
2. Compression stroke – In this stroke, both the inlet and
exhaust valves are closed and the charge is compressed as the
piston moves upwards from BDC to TDC. As a result of
compression, the pressure and temperature of the charge
increases considerably. This completes one revolution of the
crank shaft. The compression stroke is shown in Fig.

Fig. Four-stroke Otto cycle engine

Fig. Theoretical p-V diagram of a four-stroke Otto cycle engine

Four-stroke Cycle Petrol Engine (contd..)
3. Expansion stroke – Shortly before the piston reaches TDC,
the charge is ignited with the help of a spark plug. It suddenly
increases the pressure and temperature of the products of
combustion but the volume, practically remains constant. Due to
the rise in pressure, the piston is pushed down with a great
force. The hot burnt gases expand due to high speed of the
piston. During this expansion, some of the heat energy
produced is transformed into mechanical work. During this
working stroke, as shown in Fig., both the valves are closed and
piston moves from TDC to BDC.
4. Exhaust stroke – In this stroke, the exhaust valve is open as
piston moves from BDC to TDC. This movement of the piston
pushes out the product of combustion, from the engine cylinder
and exhausted through the exhaust valve into the atmosphere,
as shown in Fig. This completes the cycle, and the engine
cylinder is ready to suck the charge again.

Four-stroke Cycle Diesel Engine
It is also known as compression ignition engine because the
ignition takes place due to the heat produced in the engine
cylinder at the end of compression stroke. The four strokes of a
diesel engine sucking pure air are described below:
1. Suction stroke – In this stroke, the inlet valve opens and pure
air is sucked into the cylinder as the piston moves downwards
from the TDC. It continues till the piston reaches its BDC as
shown in Fig.
2. Compression stroke – In this stroke, both the valves are
closed and the air is compressed as the piston move upwards
from BDC to TDC. As a result of compression, pressure and
temperature of the air increases considerably. This completes
one revolution of the crank shaft. The compression stroke is
shown in Fig.

Fig. Four-stroke Diesel cycle engine

Fig. Theoretical p-V diagram of a four-stroke Diesel cycle

Four-stroke Cycle Diesel Engine (contd..)
3. Expansion stroke – Shortly before the piston reaches the
TDC, the fuel oil is injected in the form of very fine spray into the
engine cylinder, through fuel injection valve. At this moment,
temperature of the compressed air is sufficiently high to ignite
the fuel. It suddenly increases the pressure and temperature of
the products of combustion. The fuel oil is assumed to be burnt
at constant pressure. Due to increased pressure, the piston is
pushed down with a great force. The hot burnt gases expand
due to high speed of the piston. During this expansion, some of
the heat energy is transformed into mechanical work. During
this working stroke, both the valves are closed and the piston
moves from TDC to BDC.
4. Exhaust stroke – In this stroke, the exhaust valve is open as
the piston moves from BDC to TDC. The movement of the piston
pushes out the products of combustion from the engine cylinder
through the exhaust valve into the atmosphere. This completes
the cycle and the engine cylinder is ready to suck the fresh air
again.

Comparison of Petrol and Diesel Engines
Petrol Engines
1. A petrol engine draws a mixture
of petrol and air during suction
stroke.
2. The carburettor is employed to
mix air and petrol in the required
proportion and to supply it to the
engine during suction stroke.
3. Pressure at the end of
compression is about 10 bar.
4. The charge (i.e. petrol and air
mixture) is ignited with the help
of spark plug.
Diesel Engines
A diesel engine draws only air
during suction stroke.
The injector or atomiser is
employed to inject the fuel at the
end of combustion stroke.
Pressure at the end of
compression is about 35 bar.
The fuel is injected in the form of
fine spray. The temperature of the
compressed air is sufficiently high
to ignite the fuel.

Comparison of Petrol and Diesel Engines (contd..)
5. The combustion of the fuel takes
place at constant volume. It
works on Otto cycle.
6. A petrol engine has compression
ratio from 6 to 10.
7. The starting is easy due to low
compression ratio.
8. As the compression ratio is low,
the petrol engines are lighter and
cheaper.
9. The running cost of a petrol
engine is high because of the
higher cost of petrol.
The combustion of the fuel takes place
at constant pressure. It works on
Diesel cycle.
A diesel engine has compression
ratio from 15 to 25.
The starting is difficult due to high
compression ratio.
As the compression ratio is high,
the diesel engines are heavier and
costlier.
The running cost of diesel engine is
low because of the lower cost of
diesel.

Comparison of Petrol and Diesel Engines (contd..)
10. The maintenance cost is less.
11. The thermal efficiency is about
26%.
12. Overheating trouble is more due
to low thermal efficiency.
13. These are high speed engines.
14. The petrol engines are generally
employed in light duty vehicle
such as scooters, motorcycles
and cars. These are also used in
aeroplanes.
The maintenance cost is more.
The thermal efficiency is about
40%.
Overheating trouble is less due to
high thermal efficiency.
These are relatively low speed
engines.
The diesel engines are generally
employed in heavy duty vehicles
like buses, trucks, and earth
moving machines.

unit 5, EME uptu first year

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