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

SurroundingUniverse = 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 energyq = heat energyW = 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 + PVWhere 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 propertiesIntensive 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

Process

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

Isothermal Process:

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

TWO STROKE ENGINE

Four stroke 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.

Design of I.C . ENGINE

Fig. Air-cooled I.C. engine

Parts of I.C. Engine

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.

Constructional details of I.C. Engines (contd..)

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.

Constructional details of I.C. Engines (contd..)

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.

Constructional details of I.C. Engines (contd..)

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.

Constructional details of I.C. Engines (contd..)

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

CARNOT CYCLE

CARNOT CYCLE

P- V and T-S diagram for carnot cycle

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.

• Process (3-4): Reversible adiabatic 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

• Process (1-2):           Reversible adiabatic compression

• Process (2-3):           Constant pressure heat supply

• Process (3-4):           Reversible adiabatic 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.

Comparison of Two-stroke and Four-stroke 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.

Comparison of Two-stroke and Four-stroke 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

Two stroke 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

Four stroke 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 Engines1. 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.