Educational purpose

1. MODULE – 2
AIR STANDARD CYCLES & WORKING
OF IC ENGINES

2. AIR STANDARD CYCLES
• A cycle is defined as a repeated series of operations occurring in a certain
order.
• The efficiency of engine using air as the working medium is known as an
“Air standard efficiency”. This efficiency is oftenly called ideal efficiency.

3. Assumptions made in Air standard cycles
• 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 body or 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.

4. THE CARNOT CYCLE
• This cycle has the highest possible efficiency and consists of four simple operations namely,
(a) Isothermal expansion
(b) Adiabatic expansion
(c) Isothermal compression
(d) Adiabatic compression.
• The condition of the Carnot cycle may be imagined to occur in the following way :
One kg of a air is enclosed in the cylinder which (except at the end) is made of perfect
non conducting material. A source of heat ‘H’ is supposed to provide unlimited quantity of heat,
non conducting cover ‘C’ and a sump ‘S’ which is of infinite capacity so that its temperature
remains unchanged irrespective of the fact how much heat is supplied to it. The temperature of
source H is T1 and the same is of the working substance. The working substance while rejecting
heat to sump ‘S’ has the temperature. T2 i.e., the same as that of sump S. Following are the four
stages of the Carnot cycle. Refer Fig.

5. • Stage (1). Line 1-2 [Fig. (a)] represents the isothermal expansion which takes place at
temperature T1 when source of heat H is applied to the end of cylinder. Heat supplied
in this case is given by RT1 loge r and where r is the ratio of expansion.
• Stage (2). Line 2-3 represents the application of non-conducting cover to the end of
the cylinder. This is followed by the adiabatic expansion and the temperature falls
from T1 to T2.
• Stage (3). Line 3-4 represents the isothermal compression which takes place when
sump ‘S’ is applied to the end of cylinder. Heat is rejected during this operation whose
value is given by RT2 loge r where r is the ratio of compression.
• Stage (4). Line 4-1 represents repeated application of non-conducting cover and
adiabatic compression due to which temperature increases from T2 to T1.
• It may be noted that ratio of expansion during isotherm 1-2 and ratio of compression
during isotherm 3-4 must be equal to get a closed cycle.
– Fig. (b) represents the Carnot cycle on T-s coordinates.
– Now according to law of conservation of energy,

7. 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. (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 T3 change to a final value of p4, V4 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) :

10. 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. (a and b) 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., pVγ = 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 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.

13. 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 for combustion. Because of lagging characteristics of fuel this cycle is
invariably used for diesel and hot spot ignition engines.
• The dual combustion cycle 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.
(*** No Derivation for exams.)

16. Joule cycle
Brayton cycle is a constant pressure cycle for a perfect gas. It is also called
Joule cycle. An ideal gas turbine plant would perform the processes that make up a
Brayton cycle. The cycle is represented on p-v and T-s diagrams in Figs. (b) and (c).
The various operations are as follows :
• Operation 1-2. The air is compressed isentropically from the lower pressure p1 to
the upper pressure p2, the temperature rising from T1 to T2. No heat flow occurs.
• Operation 2-3. Heat flows into the system increasing the volume from V2 to V3 and
temperature from T2 to T3 whilst the pressure remains constant at p2. Heat received
= m Cp (T3 – T2).
• Operation 3-4. The air is expanded isentropically from p2 to p1, the temperature
falling from T3 to T4. No heat flow occurs.
• Operation 4-1. Heat is rejected from the system as the volume decreases from V4 to
V1 and the temperature from T4 to T1 whilst the pressure remains constant at p1.
Heat rejected = m Cp (T4 – T1).

18. INTERNAL COMBUSTION ENGINES
Heat Engines
Any type of engine or machine which derives heat energy from the combustion of fuel or any
other source and converts this energy into mechanical work is termed as a heat engine.
Heat engines may be classified into two main classes as follows :
1. External Combustion Engine.
2. Internal Combustion Engine.
1. External Combustion Engines (E.C. Engines)
In this case, combustion of fuel takes place outside the cylinder as in 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. These engines are generally needed for driving locomotives,
ships, generation of electric power etc.
2. Internal Combustion Engines (I.C. Engines)
In this case combustion of the 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.

19. Different parts of I.C. Engines
A cross-section of an air-cooled I.C. engines with principal parts is shown in Fig.
A. Parts common to both petrol and diesel engines
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 plugs 2. Carburettor 3. Fuel pump.
C. Parts for Diesel engine only
1. Fuel pump. 2. Injector.

20. Spark Ignition (S.I.) Engines
These engines may work on either four stroke cycle or two stroke cycle.
FOUR STROKE PETROL ENGINE :
Fig. illustrates the various strokes/series of operations which take place in a four stroke petrol
(Otto cycle) engine.
Suction stroke. During suction stroke a mixture of air and fuel (petrol) is sucked through the inlet
valve (I.V.). The exhaust valve remains closed during this operation.
Compression stroke. During compression stroke, both the valves remain closed, and the pressure
and temperature of the mixture increase. Near the end of compression stroke, the fuel is ignited by
means of an electric spark in the spark plug, causing combustion of fuel at the instant of ignition.
Working stroke. Next is the working (also called power or expansion) stroke. During this stroke,
both the valves remain closed. Near the end of the expansion stroke, only the exhaust valve opens
and the pressure in the cylinder at this stage forces most of the gases to leave the cylinder.
Exhaust stroke. Next follows the exhaust stroke, when all the remaining gases are driven away
from the cylinder, while the inlet valve remains closed and the piston returns to the top dead
centre. The cycle is then repeated.

22. • Two stroke petrol engine :
• A cycle which could be completed in two strokes of piston rather than four strokes as is the case
with the four stroke cycle engines. The engines using this cycle were called two stroke cycle
engines. In this engine suction and exhaust strokes are eliminated. Here instead of valves, ports
are used. The exhaust gases are driven out from engine cylinder by the fresh change of fuel
entering the cylinder nearly at the end of the working stroke.
• Fig. shows a two stroke petrol engine .The cylinder L is connected to a closed crank chamber
C.C. During the upward stroke of the piston M, the gases in L are compressed and at the same
time fresh air and fuel (petrol) mixture enters the crank chamber through the valve V. When the
piston moves downwards, V closes and the mixture in the crank chamber is compressed.
• Refer Fig. (i) the piston is moving upwards and is compressing an explosive change which has
previously been supplied to L. Ignition takes place at the end of the stroke. The piston then
travels downwards due to expansion of the gases [Fig. (ii)] and near the end of this stroke the
piston uncovers the exhaust port (E.P.) and the burnt exhaust gases escape through this port [Fig.
(iii)]. The transfer port (T.P.) then is uncovered immediately, and the compressed charge from the
crank chamber flows into the cylinder and is deflected upwards by the hump provided on the
head of the piston. It may be noted that the incoming air petrol mixture helps the removal of
gases from the engine-cylinder ; if, in case these exhaust gases do not leave the cylinder, the fresh
charge gets diluted and efficiency of the engine will decrease. The piston then again starts
moving from bottom dead centre (B.D.C.) to top dead centre (T.D.C.) and the charge gets
compressed when E.P. (exhaust port) and T.P. are covered by the piston ; thus the cycle is
repeated.

24. Compression Ignition (C.I.) Engines
These engines work on either four stroke cycle or two stroke cycle.
• The operation of C.I. engines (or diesel engines) is practically the same as those of
S.I. engines. The cycle in both the types, consists of suction, compression, ignition,
expansion and exhaust. However, the combustion process in a C.I. engine is
different from that of a S.I. engine as given below :
• In C.I. engine, only air is sucked during the stroke and the fuel is injected in the
cylinder near the end of the compression stroke. Since the compression ratio is very
high (between 14 : 1 to 22 : 1), the temperature of the air after compression is quite
high. So when fuel is injected in the form of a spray at this stage, it ignites and
burns almost as soon as it is introduced. The burnt gases are expanded and
exhausted in the same way as is done in a S.I. engine.

25. Valve timing diagrams
• A Valve timing diagram is a graphical representation of the exact moments in the
sequence of operations at which the two valves (inlet and exhaust) open and close
as well as firing of the fuel.
• A Valve timing diagram shows opening and closing of inlet, exhaust and fuel valve
period, in degree of crank angle.
• The theoretical valve and port timing diagrams for four stroke and two stroke
engines respectively are as shown in figure.
• The actual valve and port timing diagrams for four stroke and two stroke engines
are as follows.

26. Valve timing diagram for a four stroke cycle petrol engine.
• The IVO before the piston reaches TDC: This is to avoid throttling of intake charge. Further
the inertia of exhaust gases assist in drawing the charge inside the engine cylinder.
• The IVC after BDC: This is because, due to high speed in suction the charge has momentum.
So charge continue to flow into cylinder although the piston move upwards from BDC.
• Ignition(IGN) before TDC: This is done as the charge requires some time to ignite. The spark
advance is up to 30o to get maximum pressure just after TDC.
• The EVO before BDC: early opening of exhaust valve helps to drive out exhaust gas due to
pressure difference between exhaust and atmosphere.
• The EVC after TDC: This is done as the burnt gas continue to leave the engine cylinder
although the piston is moving downwards.
• It may be noted that for a small fraction of a crank revolution, both inlet and exhaust valves
remain open (valve overlap), during which the incoming charge helps to drive out the traces
of exhaust gas. This period is known as valve overlap period and that process is called
scavenging.(Bad for SI engine)

27. Valve timing diagram for a four stroke cycle Diesel engine.
• The IVO before the piston reaches TDC: This is to avoid throttling of intake air. Further the inertia
of exhaust gases assist in drawing the air inside the engine cylinder.
• The IVC after BDC: This is because, due to high speed in suction the intake air has momentum. So
air continue to flow into cylinder although the piston move upwards from BDC.
• FVO before TDC: This is done as some time is required for fuel injection (as per design)
• FVO after TDC: This is done as the required quantity of fuel is injected into the engine cylinder.
• The EVO before BDC: Early opening of exhaust valve helps to drive out exhaust gas due to pressure
difference between exhaust and atmosphere.( the loss in power is compensated by better scavenging
and less negative work)
• The EVC after TDC: This is done as the burnt gas continue to leave the engine cylinder although the
piston is moving downwards.
• It may be noted that for a small fraction of a crank revolution, both inlet and exhaust valves remain
open (valve overlap), during which the incoming air helps to drive out the traces of exhaust gas. This
period is known as valve overlap period and that process is called scavenging.(Good for CI engine)

28. Port timing diagram for a two stroke cycle Petrol engine
• EPO before BDC: This is done as the burnt gas start leaving the cylinder
• TPO before BDC: The fresh F/A mixture enters cylinder and also this fresh charge
helps in pushing out the burnt gases.
• As the crank moves a little beyond BDC, first the transfer port closes (TPC) and
then the exhaust port closes (EPC). This is done to suck fresh charge through the
transfer port and to exhaust the burnt gases through the exhaust port simultaneously.
• Ignition(IGN) before TDC: This is done as the charge requires some time to ignite.
• It may be noted that the exhaust and transfer port open and close at equal angles on
either side of the BDC position.

29. Port timing diagram for a two stroke cycle Diesel engine
• EPO before BDC: This is done as the burnt gas start leaving the cylinder
• TPO before BDC: The fresh air enters cylinder and also this fresh air helps in
pushing out the burnt gases.
• As the crank moves a little beyond BDC, first the transfer port closes (TPC) and
then the exhaust port closes (EPC). This is done to suck fresh air through the
transfer port and to exhaust the burnt gases through the exhaust port simultaneously.
• The fuel valve open (FVO) before TDC and Fuel Valve closes ( FVC) after TDC.
This is done as the required quantity of fuel is injected into the engine cylinder.
• It may be noted that the exhaust and transfer port open and close at equal angles on
either side of the BDC position.

30. THANK YOU
Any queries/Doubts/ Corrections may please be informed.