SCHOOL OF MECHANICAL ENGINEERING

DEPARTMENT OF MECHATRONICS

UNIT – 3 – Applied Thermodynamics – SMRB1303

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UNIT 3 GAS POWER CYCLES AND INTERNAL COMBUSTION ENGINES

Air standard cycles - Otto, Diesel and Dual cycles. Derivation of expression for air standard
efficiency and mean effective pressure .IC Engines- Introduction-Classification, Comparison
between four stroke and two stroke, petrol & diesel engines-Performance testing on internal
combustion engines, Performance curves..

DEFINITION OF A CYCLE
A cycle is defined as a repeated series of operations occurring in a certain order. It
may be repeated by repeating the processes in the same order. The cycle may be of imaginary
perfect engine or actual engine. The former is called ideal cycle and the latter actual cycle. In
ideal cycle all accidental heat losses are prevented and the working substance is assumed to
behave like a perfect working substance.
AIR STANDARD EFFICIENCY
To compare the effects of different cycles, it is of paramount importance that the
effect of the calorific value of the fuel is altogether eliminated and this can be achieved by
considering air (which is assumed to behave as a perfect gas) as the working substance in the
engine cylinder. The efficiency of engine using air as the working medium is known as an
“Air standard efficiency”. This efficiency is often called ideal efficiency. The actual
efficiency of a cycle is always less than the air-standard efficiency of that cycle under ideal
conditions. This is taken into account by introducing a new term “Relative efficiency” which
is defined as the ratio of Actual thermal efficiency to Air standard efficiency.
The analysis of all air standard cycles is based upon the following assumptions:
1. The gas in the engine cylinder is a perfect gas i.e., it obeys the gas laws and has constant specific

heats.
2. The physical constants of the gas in the cylinder are the same as those of air at moderate

temperatures i.e., the molecular weight of cylinder gas is 29.Cp = 1.005 kJ/kg-K, Cv = 0.718
kJ/kg-K.
3. The compression and expansion processes are adiabatic and they take place without

internal friction, i.e., these processes are isentropic.

4. No chemical reaction takes place in the cylinder. Heat is supplied or rejected by bringing a

hot bodyor a cold body in contact with cylinder at appropriate points during the process.
5. The cycle is considered closed with the same ‘air’ always remaining in the cylinder to repeat the

cycle.

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CONSTANT VOLUME OR OTTO CYCLE

This cycle is so named as it was conceived by ‘Otto’. On this cycle, petrol, gas and
many types of oil engines work. It is the standard of comparison for internal combustion
engines.
Figs. 1 (a) and (b) shows the theoretical p-V diagram and T-s diagrams of this cycle respectively.
 The point 1 represents that cylinder is full of air with volume V1, pressure P1 and
absolute temperature T1.
 Line 1-2 represents the adiabatic compression of air due to which P1, V1 and T1
change to P2, V2 and T2 respectively.
 Line 2-3 shows the supply of heat to the air at constant volume so that P2 and T2
change to P3 and T3 (V3 being the same as V2).
 Line 3-4 represents the adiabatic expansion of the air. During expansion P3, V3 and T
3 change to a final value of P4, V 4 or V1 and T4, respectively.
 Line 4-1 shows the rejection of heat by air at constant volume till original state
(point 1) reaches. Consider 1 kg of air (working substance):

Fig. 1

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This expression is known as the air standard efficiency of the Otto cycle. It is clear from the
above expression that efficiency increases with the increase in the value of r, which means we
can have maximum efficiency by increasing r to a considerable extent, but due to practical
difficulties its value is limited to about 8. The net work done per kg in the Otto cycle can also

be expressed in terms of p, v. If p is expressed in bar i.e., 105 N/m2, then work done

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MEP may be thought of as the average pressure acting on a piston during different portions
of its cycle.It is the ratio of the work done to stoke volume of the cycle

CONSTANT PRESSURE OR DIESEL CYCLE

This cycle was introduced by Dr. R. Diesel in 1897. It differs from Otto cycle in that
heat is supplied at constant pressure instead of at constant volume. Fig.2 shows the p-v and
T-s diagrams of this cycle respectively.
This cycle comprises of the following operations:

(i) 1-2 ...Adiabatic compression.

(ii) 2-3... Addition of heat at constant pressure.

(iii) 3-4... Adiabatic expansion.

(iv) 4-1 ... Rejection of heat at constant volume.

Point 1 represents that the cylinder is full of air. Let P1, V1 and T1 be the corresponding pressure,

volume and absolute temperature. The piston then compresses the air adiabatically (i.e., pV r =
constant) till the values become P2, V2 and T2 respectively (at the end of the stroke) at point 2.
Heat is then added from a hot body at a constant pressure. During this addition of heat let volume
increases from V2 to V3 and temperature T2 to T3, corresponding to point 3. This point (3) is
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called the point of cut-off. The air then expands adiabatically to the conditions P4, V4 and T4
respectively corresponding to point 4. Finally, the air rejects the heat to the cold body at constant
volume till the point 1 where it returns to its original state

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It may be observed that eqn. for efficiency of diesel cycle is different from that of the Otto
cycle only in bracketed factor. This factor is always greater than unity, because r > 1. Hence
for a given compression ratio, the Otto cycle is more efficient.

DUAL COMBUSTION CYCLE

This cycle (also called the limited pressure cycle or mixed cycle) is a combination of Otto and
Diesel cycles, in a way, that heat is added partly at constant volume and partly at constant
pressure ; the advantage of which is that more time is available to fuel (which is injected into
the engine cylinder before the end of compression stroke) for combustion. Because of lagging
characteristics of fuel this cycle is invariably used for diesel and hot spot ignition engines.
The dual combustion cycle (Fig 3) consists of the following operations :
(i) 1-2—Adiabatic compression

(ii) 2-3—Addition of heat at constant volume

(iii) 3-4—Addition of heat at constant pressure

(iv) 4-5—Adiabatic expansion

(v) 5-1—Rejection of heat at constant volume.

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Fig.3
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COMPARISON OF OTTO, DIESEL AND DUAL COMBUSTION CYCLES
Following are the important variable factors which are used as a basis for comparison of the
cycles:
 Compression ratio.
 Maximum pressure
 Heat supplied
 Heat rejected
 Net work
Some of the above mentioned variables are fixed when the performance of Otto, Diesel
and dual combustion cycles is to be compared.

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Efficiency Versus Compression Ratio
Fig shows the comparison for the air standard efficiencies of the Otto, Diesel and Dual
combustion cycles at various compression ratios and with given cut-off ratio for the Diesel
and Dual combustion cycles. It is evident from the Fig that the air standard efficiencies
increase with the increase in the compression ratio. For a given compression ratio Otto cycle
is the most efficient while the Diesel cycle is the least efficient.
Note. The maximum compression ratio for the petrol engine is limited by detonation. In their
respective ratio ranges, the Diesel cycle is more efficient than the Otto cycle.
For the Same Compression Ratio and the Same Heat Input
A comparison of the cycles (Otto, Diesel and Dual) on the p-v and T-s diagrams for the same
compression ratio and heat supplied is shown in the Fig.4

Fig.4

Fig.5

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 Since all the cycles reject their heat at the same specific volume, process line from state 4 to
1, the quantity of heat rejected from each cycle is represented by the appropriate area under
the line 4 to 1 on the T-s diagram. As is evident from the eqn. the cycle which has the least
heat rejected will have the highest efficiency. Thus, Otto cycle is the most efficient and
Diesel cycle is the least efficient of the three cycles.

otto >dual > diesel

For Constant Maximum Pressure and Heat Supplied

Fig. 6 shows the Otto and Diesel cycles on p-v and T-s diagrams for constant maximum
pressure and heat input respectively.

Fig.6

For the maximum pressure the points 3 and 3′ must lie on a constant pressure line.

On T-s diagram the heat rejected from the Diesel cycle is represented by the area under the
line 4 to 1 and this area is less than the Otto cycle area under the curve 4′ to 1 ; hence the
Diesel cycle is more efficient than the Otto cycle for the condition of maximum pressure and
heat supplied

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1. An engine working on the Otto cycle is supplied with air at 0.1 MPa, 350C. The
compression ratio is 8. Heat supplied is 2100 kJ/kg. Calculate the maximum pressure
and temperature of the cycle, the cycle efficiency, and the mean effective pressure.
(For air, cp = 1.005kJ/kgK, cv = 0.718 kJ/kgK, and R = 0.287 kJ/kgK)

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2. An engine of 250 mm bore and 375 mm stroke works on constant volume cycle. The
clearance volume is 0.00263 m3 . The initial pressure and temperature are 1 bar and
500C. If the maximum pressure is 25 bar, determine (i) the air standard efficiency of
the cycle and (ii) the mean effective pressure.
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3. The compression ratio of an air standard diesel cycle is 16. The temperature and
pressure at the beginning of isentropic compression are 150C and 0.1 MPa
respectively. During the constant pressure process, the heat is added until the
temperature reaches 14800C. Determine (i) the cutoff ratio (ii) the heat supplied per kg
of air, (iii) the cycle efficiency, and (iv) the m.e.p

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INTERNAL COMBUSTION ENGINES
INTRODUCTION
As the name implies or suggests, the internal combustion engines (briefly written as IC
engines) are those engines in which the combustion of fuel takes place inside the engine cylinder.
These are petrol, diesel, and gas engines.
CLASSIFICATION OF IC ENGINES
The internal combustion engines may be classified in many ways, but the following are
important from the subject point of view
1. According to the type of fuel used
(a) Petrol engines. (b) Diesel engines or oil engines, and (c) Gas engines.
2. According to the method of igniting the fuel
(a) Spark ignition engines (briefly written as S.1. engines), (b) Compression ignition
engines (briefly written as C.I. engines), and (c) Hot spot 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 (also known as constant volume cycle) engines, (b) Diesel cycle (also known
as constant pressure cycle) engines, and (c) Dual combustion cycle (also known as semi-
diesel cycle) engines.
5. According to the speed of the engine
(a) Slow speed engines, (b) Medium speed engines, (c) High speed engines.
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 6. According to the cooling system
(a) Air-cooled engines. (b) Water-cooled engines. (c) Evaporative cooling engines.
7. According to the method of fuel injection
(a) Carburetor engines, (b) Air injection engines, (c) Airless or solid injection
engines.
8. According to the number of cylinders
(a) Single cylinder engines (b) Multi-cylinder engines.
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, (j) Opposite-cylinder engines, (g) Opposite-
piston engines

MAIN COMPONENTS OF IC ENGINES

Fig.7
As a matter of fact, an IC engine consists of hundreds of different parts, which are important
for its proper working. The description of all these parts is beyond the scope. However, the main
components, which are important from academic point of view, are shown and are discussed below:
1. Cylinder. It is one of the most important part of the engine, in which the piston moves

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to and fro in order to develop power. Generally, the engine cylinder has to withstand a high
pressure (more than 50 bar) and temperature (more than 2000°C). Thus the materials for an 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 aluminium alloys. In case of multiple cylinder engines, the cylinders are cast
in one block known as cylinder block.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
can 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 acts 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 contains nozzle (i.e. fuel valve) for injecting the fuel into the cylinder. The cylinder
head is, usually, cast as one piece and bolted to one end of the cylinder. Generally, 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 exeI1ed by the burning of charge to the connecting rod. The pistons are generally made of
aluminium alloys which are light in weight. They have good heat conducting property and also
greater strength at higher temperatures.
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 to 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.

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6. Crankshaft: It is considered as the backbone of an l.c. engine whose function is to
convert 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. That part of the crank, to
which bigger end of the connecting rod is fitted, is called crank pin.It has been experienced that too
many main bearings create difficulty of correct alignment. 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 the power stroke, which is returned during
other strokes.
FOUR STROKE CYCLE PETROL ENGINE
It is also known as Otto cycle. 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 carburetor known as charge) are described below:
1. Suction or charging stroke: In this stroke, the inlet valve opens and charge is sucked
into the cylinder as the piston moves downward from top dead centre (T.D.C.). It continues till the
piston reaches its bottom dead centre (B.D. C.) as shown in (a).
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 B.D. C. to TD. C. As a result of compression, the
pressure and temperature of the charge increases considerably (the actual values depend upon the
compression ratio). This completes one revolution of the crank shaft. The compression stroke is
shown in (b).

Fig.8
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3. Expansion or working stroke Shortly before the piston reaches T.D.C. (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, 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. It may be noted that during this working stroke, as shown in (c), both the
valves are closed and piston moves from T.D.C. to B.D.C
4. Exhaust stroke: In this stroke, the exhaust valve is open as piston moves from B.D.C. to T.D.C.
This movement of the piston pushes out the products of combustion, from the engine cylinder and
are exhausted through the exhaust valve into the atmosphere, as shown in (d). 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 p\ace 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 or charging stroke: In this stroke, the inlet valve opens and pure air is sucked
into the cylinder as the piston moves downwards from the top dead centre (TDC). It continues till
the piston reaches its bottom dead centre (BDC) as shown (a).
2. Compression stroke: In this stroke, both the valves are closed and the air is compressed as the
piston moves upwards from BDC to TDC. As a result of compression, pressure and temperature of
the air increases considerably (the actual value depends upon the compression ratio). This
completes one revolution of the crank shaft. The compression stroke is shown in (b).

Fig.9
3. Expansion or working stroke: Shortly before the piston reaches the TDC (during the
compression stroke), fuel oil is injected in the form of very fine spray into the engine cylinder,
through the nozzle, known as fuel injection valve. At this moment temperature of the compressed

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air is sufficiently high to ignite the fuel. It suddenly increases the pressure and temperature of the
products of combustion. The fuel oil is continuously injected for a fraction of the revolution. 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. It may be noted that
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.
This 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.

TWO-STROKE CYCLE PETROL ENGINE

A two-stroke cycle petrol engine was devised by Duglad Clerk in I RHO. In this cycle, the
suction, compression, expansion and exhaust takes place during two strokes of the piston. It means
that there is one working stroke after every revolution of the crank shaft. 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 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 (a).
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
(b). In this stage, the inlet port opens and fresh fuel-air mixture enters into the crank case.
3. Expansion stage: Shortly before this piston reaches the 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, practically, remains constant. Due to
rise in the pressure, the piston is pushed downwards with a great force as shown in (c). 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.

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 Fig.10
1. 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 (d). This completes the cycle and the engine cylinder is ready to suck the
charge again
TWO STROKE AND FOUR STROKE CYCLE ENGINE

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 more precisely in inward stroke), expansion and exhaust processes in
the second stroke (or more precisely 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. It will be
interesting to know that from the thermodynamic point of view, there is no difference between two-
stroke and four-stroke cycle engines. The difference is purely mechanical.
Advantages and Disadvantage of Two-stroke 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. Theoretically, a two-stroke cycle engine should
develop twice the power as that of a four-stroke cycle engine. But in actual practice, a two-
stroke cycle engine develops 1.7 to 1.8 times greater value for slow speed engines the power
developed by four-stroke cycle engine of the same dimensions and speed. This is due to
lower compression ratio and effective stroke being less than the theoretical stroke.
2. For the same power developed, a two-stroke cycle engine is lighter, less bulky and occupies
less floor area. Thus it makes, a two-stroke cycle engine suitable for marine engines and
other light vehicles.
3. As the number of working strokes in a two-stroke cycle engine are twice than the

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 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.
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,
because a two-stroke cycle engine has less compression ratio than that of a four-stroke
cycle engine.
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 some time. In spite of careful design, 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 is twice as those of a
four-stroke cycle engine. Thus the capacity of the cooling system must be higher. Beyond a
certain limit, the cooling capacity offers a considerable difficulty. Moreover, 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 create noise, because of short time available
for their exhaust.

COMPARISON OF PETROL AND DIESEL ENGINES (SI and CI Engines)

Following points are important for the comparison of petrol engines and diesel engines:
Table.1
Petrol Engines Diesel Engines
 A petrol engine draws a mixture of  A diesel engine draws only air
petrol and air during suction stroke. during suction stroke
 The carburetor is employed to mix  The injector or atomizer is
air and petrol in the required employed to inject the fuel at the
proportion and to supply it to the end of compression stroke.

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 engine during suction stroke  Pressure at the end of compression
 Pressure at the end of compression is about 35 bar.
is about 10 bar  The fuel is injected in the form of
 The charge (i.e. petrol and air fine spray. The temperature of the
mixture) is ignited with the help of compressed air (about 600"C at a
spark plug pressure of about 35bar) is
 The combustion of fuel takes place sufficiently high to ignite the fuel.
approximately at constant volume.  The combustion of fuel takes place
In other words, it works on Otto approximately at constant pressure.
cycle In other words. It works on Diesel
 A petrol engine has compression cycle.
ratio approximately from 6 to 10.  A diesel engine has compression
 The starting' is easy due to low ratio approximately from 15 to 25.
compression ratio.  The starting is little difficult due. to
 As the compression ratio is low, the high compression ratio.
petrol engines are lighter and  As the compression ratio is high.
cheaper. the diesel engine;; are heavier and
 The running cost of a petrol engine costlier.
is high because of the higher cost of  The running cost of diesel engine is
petrol. low because of the lower cost of
 The maintenance cost is less. diesel.
 The thermal efficiency is up to  The maintenance cost is more.
about 26%.  The thermal efficiency is up to
 Overheating trouble is more due to about 40%
low thermal efficiency.  Overheating trouble is less due to
 These are high speed engines. high thermal efficiency
 The petrol engines arc generally  These are relatively low speed
employed in light duty vehicles engines.
such as scooters, motorcycles, cars.  The diesel engines are generally
These are also used in aero planes employed in heavy duty vehicles
like buses. trucks, and earth moving
machines etc.

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TEXT / REFERENCE BOOKS
1. Nag P.K., “Engineering Thermodynamics”, Tata McGraw Hill Education, 2009.
2. Yunus A. Cengel, Michael A. Boles, “Thermodynamics: An Engineering Approach”,
McGraw Hill Education, 2014.
3. Rajput R.K., “Engineering Thermodynamics”, Laxmi Publications, 2010.
4. Khurmi R.S., Gupta J.K, “Thermal Engineering”, S Chand, 2006.
5. P.L. Ballaney,”Thermal Engineering”, Khanna Publisher, 2005.

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