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Page 1

THERMAL ENGINEERING
UNIT I

GAS POWER CYCLES
Otto, Diesel, Dual, Brayton cycles, Calculation of mean effective pressure and air
standard efficiency, Actual and theoretical PV diagram of four stroke engines, Actual and
theoretical PV diagram of two stroke engines

CONCEPT OF AIR STANDARD CYCLES

An internal combustion (I.C.) engine works on open cycle but it is desirable to
device closed cycle that would approximate the actual open cycle. In an actual engine, the
working fluid changes from air and fuel to products of combustion during the cycle.
However nitrogen being the main constituent of air which does not undergo any chemical
reaction in the combustion chamber, therefore, the working fluid closely resembles with
air at all times. The closed cycles with air as working substance which closely resembles
with actual open cycles is called an air standard cycle. In practice, it is impossible to
repeat the real cycles prescribed by the theoretical cycles for the following reasons:

(a) Internal and external irreversibility like fluid and mechanical friction, combustion etc.
(b) Unintended heat losses.
(c) Different properties of working medium.
(d) Variation in specific heat of gases with temperature.

However, the air standard cycles are useful and become an important tool to
provide a simple method for achieving a quantitative comparison of cycle performance.

ASSUMPTIONS OF AIR STANDARD CYCLE

Following are the assumptions made in the analysis of air standard cycles:
1. The working medium in the cylinder is air.
2. The air behaves like an ideal gas i.e. it obeys the gas laws and its specific heat is

constant at all temperatures. Cp = 1.005 kJ /kg K, Cv = 0.718 kJ /kg K, γ = 1.4
3. The charging and discharging operations are omitted. therefore, .a constant mass

of the medium is carried through the entire cycle.
4. All the processes are internally reversible. There are no unintended heat losses

and friction is neglected.
5. The heat energy added to the working medium is transferred from a heat

reservoir. There is also a provision for transfer of heat from the working medium
to a heat reservoir sink so that the cycle is completed

TERMS USED

1. CLEARANCE VOLUME VC
The volume contained in the cylinder above the piston from its top dead centre
position is called clearance volume

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VALVE TIMING DIAGRAM FOR A FOUR-STROKE CYCLE DIESEL ENGINE
In the valve timing diagram as shown we see that the inlet valve opens before the

piston reaches TDC; or in other words while the piston is still moving up before the
beginning of the suction stroke. Now the piston reaches the TDC and the suction stroke
starts. The piston reaches the BDC and then starts moving up. The inlet valve closes,
when the crank has moved a little beyond the BDC.

This is done as the incoming air continues to flow into the cylinder although the piston is
moving upwards from BDC. Now the air is compressed with both valves closed. Fuel
valve opens a little before the piston reaches the TDC. Now the fuel is injected in the
form of very fine spray, into the engine cylinder, which gets ignited due to high
temperature of the compressed air. The fuel valve closes after the piston has come down a
little from the TDC. This is done as the required quantity of fuel is injected into the
engine cylinder. The burnt gases (under high pressure and temperature) push the piston
downwards, and the expansion or working stroke takes place. Now the exhaust valve
opens before the piston again reaches BDC and the burnt gases start leaving the engine
cylinder. Now the piston reaches BDC and then starts moving up thus performing the
exhaust stroke. The inlet valve opens before the piston reaches TDC to start suction
stroke. This is done as the fresh air helps in pushing out the burnt gases. Now the piston
again reaches TDC, and the suction starts. The exhaust valve closes when the crank has
moved a little beyond the TDC. This is done as the burnt gases continue to leave the
engine cylinder although the piston is moving downwards.

COMPARISON OF PETROL AND DIESEL ENGINES

Following points are important for the comparison of petrol engines and diesel engines:

Petrol Engines Diesel Engines
• A petrol engine draws a mixture of

petrol and air during suction stroke.
• The carburetor is employed to mix

• A diesel engine draws only air
during suction stroke

• The injector or atomizer is

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air and petrol in the required
proportion and to supply it to the
engine during suction stroke

• Pressure at the end of compression
is about 10 bar

• The charge (i.e. petrol and air
mixture) is ignited with the help of
spark plug

• The combustion of fuel takes place
approximately at constant volume.
In other words, it works on Otto
cycle

• A petrol engine has compression
ratio approximately from 6 to 10.

• The starting' is easy due to low
compression ratio.

• As the compression ratio is low, the
petrol engines are lighter and
cheaper.

• The running cost of a petrol engine
is high because of the higher cost of
petrol.

• The maintenance cost is less.
• The thermal efficiency is up to

about 26%.
• Overheating trouble is more due to

low thermal efficiency.
• These are high speed engines.
• The petrol engines arc generally

employed in light duty vehicles
such as scooters, motorcycles, cars.

These are also used in aero planes

employed to inject the fuel at the
end of compression 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 (about 600"C at a
pressure of about 35bar) is
sufficiently high to ignite the fuel.

• The combustion of fuel takes place
approximately at constant pressure.
In other words. It works on Diesel
cycle.

• A diesel engine has compression
ratio approximately from 15 to 25.

• The starting is little difficult due. to
high compression ratio.

• As the compression ratio is high.
the diesel engine;; are heavier and
costlier.

• The running cost of diesel engine is
low because of the lower cost of
diesel.

• The maintenance cost is more.
• The thermal efficiency is up to

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 etc.

.
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

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