2. REVIEW OF IDEAL CYCLES
1. CARNOT CYCLE 2. STIRLING CYCLE 3.ERICSSON CYCLE
1. CARNOT CYCLE( SADI CARNOT, FRENCH ENGINEER, 1824)
• Idealized thermodynamic cycle consisting of four reversible processes (working fluid can be any substance):
• The four steps for a Carnot Heat Engine are:
Reversible isothermal expansion (1-2, TH= constant)
Reversible adiabatic expansion (2-3, Q = 0, THTL)
Reversible isothermal compression (3-4, TL=constant)
Reversible adiabatic compression (4-1, Q=0, TLTH)
Applications: Vehicles like cars, motor
cycles, trucks, ships, aeroplane, etc.
Refrigerators, deep freezers, industrial
refrigeration systems, air-conditioning
systems, heat pumps, etc
3. Ideal or Air Standard Cycles
Air standard cycles are defined as cycles using a perfect
gas as the working fluid/ medium.
• Working medium is AIR and behaves like ideal/
perfect gas throughout ( follows the Law pV=mRT )
• Working fluid is a fixed mass of air either contained
in a closed system or flowing at a constant rate
round a closed circuit
Air is invariably used as the working fluid in IC Engines
and assumed to behave as a perfect gas
Following simplifying assumptions are made in the
analysis of air standard cycles:
4. Assumptions of Ideal or Air Standard Cycles
• Working medium has constant specific heats
• Heat addition & rejection processes take place in
reversible manner and if required, instantaneously
(at constant volume)
• Compression & Expansion processes are reversible
adiabatic (Isentropic); (no heat transfer)
• All dissipative effects like friction, viscosity etc, are
neglected
• Kinetic & PE of the working fluid are neglected
• Physical constants of working medium are the same
as that of air at standard atmospheric conditions;
Cp=1.005, Cv=0.718 & γ=1.4
5. Useful Thermodynamic Relations (Perfect Gas)
• pV = mRT or pv = RT and p1V1/T1 = p2V2/T2
• Cp – Cv = R
• For reversible adiabatic process : pVγ = Const
• For Const Volume(Isochoric) process: p/T = Const
(Gay Lussac Law)
• For Const Pressure (Isobaric) process : V/T = Const
(Charle’s Law)
• For Const Temp (Isothermal) process: pV = Const
(Boyle’s Law)
• In Compression process, if p1, V1 and T1 represent
initial conditions & p2, V2 and T2 the final conditions;
n
n
n
p
p
V
V
T
T
1
1
2
1
2
1
1
2
Where n=γ for reversible
adiabatic (isentropic)
process
6. Some Useful Standard Values for Perfect Gas/Air
Specific Heat at Const Pressure Cp=1.005 kJ/kgK
Specific Heat at Const Volume Cv=0.718 kJ/kgK
Gas Constant R=0.287 kJ/kgK
Ratio of Cp/Cv=γ=1.4 (Constant)
Pascal Pa=N/m2
1 bar = 105 Pa =105 N/m2 =100 kPa =1.03 kg/cm2
1 MPa = 106 Pa = 10 bar
Pressure:
Volume:
1 lit = 1000cc = 10-3m3
7. Review of few ideal thermodynamic Cycles
• A cycle is defined as series of processes which end in the same final state of the
substance as the initial.
• Examples of some air standard cycles:
• Carnot cycle
• Otto cycle
• Diesel cycle
• Dual combustion cycle
• Brayton cycle and so on
8. CARNOT CYCLE
• Carnot was the first to study the performance of heat engine.
• The cycle consists of four processes.
• 1-2-isothermal expansion
• 2-3-adiabatic expansion
• 3-4-isothermal compression
• 4-1-adiabatic compression
10. Efficiency of reversible heat engine can be given as;
L
H
H
L
H
T
T
T
T
T
or
T
T
T
1
– W
W
Net work
Here,
supplied
Heat
Net work
HE
,
1
3
1
com
pr
expn
rev
Also ,
12. Efficiency of Otto Cycle
Net work = Heat added – Heat rejected
suppied
Heat
rejected
Heat
1
supplied
Heat
Net work
HE
,
rev
Compression Ratio,
13.
14. DEISEL CYCLE
• Diesel cycle is modified form of Otto cycle. Here heat addition process is replaced from
constant volume type to constant pressure type.
• Compression ignition engines work based on Diesel cycles.
• Thermodynamic processes constituting Diesel cycle are :
• 1 – 2 = Adiabatic compression
• 2 – 3 = Heat addition at constant pressure
• 3 – 4 = Adiabatic expansion
• 4 – 1 = Heat rejection at constant volume
17. DUAL CYCLE
It is also called ‘mixed cycle’ or ‘limited pressure cycle
• Dual cycle is the combination of Otto cycle and Diesel cycle in which heat
addition takes place partly at constant volume and partly at constant pressure.
• Thermodynamic processes involved in Dual cycle are given as under.
• 1 – 2 = Adiabatic compression
• 2 – 3 = Heat addition at constant volume
• 3 – 4 = Heat addition at constant pressure
• 4 – 5 = Adiabatic expansion
• 5 – 1 = Heat rejection at constant volume
18. P –v and T–s diagrams for Dual cycle
DUAL CYCLE
19. Brayton Cycle
• A thermodynamic cycle (also variously called the Joule or complete
expansion diesel cycle) consisting of two constant-pressure (isobaric)
processes interspersed with two reversible adiabatic (isentropic)
processes.
• Now, the Brayton cycle is used for gas turbines only where both the
compression and expansion processes take place in rotating machinery
22. Introduction
Ideal GasCycle(Air Standard Cycle)
Idealized processes
Idealize working Fluid
Fuel-Air Cycle
Idealized Processes
AccurateWorking Fluid Model
Actual Engine Cycle
AccurateModels of Processes
AccurateWorking Fluid Model
2
Fuel-Air Cycle
Idealized Processes
Accurate Working Fluid Model
23. Theoretical Fuel-Air Cycles
Cycles, which take in to account the variations of specific
heats, effects of molecular structure, effects of composition of
mixture of fuel, air & residual gases approximating to working
substance, are called Fuel-Air Cycles
Fuel-air cycles largely take the following in to
consideration:
• Actual composition of cylinder gases i,e. fuel, air,
water vapor and residual gases
• Variation (increase) of specific heats with temp
Specific heats vary (increase) with increase in temp
(hence γ = Cp/Cv ↓with ↑T)
Cp = a + bT + cT2 + dT3
Cv = a1 + bT + cT2 + dT3; a1 > a
24. Theoretical Fuel-Air Cycle
• After combustion process, mixture is in chemical
equilibrium (No dissociation )
• Intake and exhaust processes take place at
atmospheric pressure
• Compression & expansion processes are adiabatic
without friction
• In case of Otto Cycle, mixture of air & fuel is
homogenous and it burns at constant volume
• Change in KE is negligible
• No heat exchange between gases and cylinder walls
• Mixture of fuel & air (A/F ratio)
25. Theoretical Fuel-Air Cycle
1. Effect of Composition of Fuel and Air (A/F Ratio):
• Leaner mixture has higher thermal efficiency
• Richer mixture will have lower efficiency as unburnt
fuel will go to exhaust
• Efficiency increases with CR
1
1
1
1
1
1
1
1
r
OR
r
diesel
otto
27. Theoretical Fuel-Air Cycle
2. Effect of Variation Specific Heats :
• Cp=a+bT+cT2 & Cv=a1+bT+cT2
• During adiabatic compn process 1-2, as the temp
increases, Cp & Cv increase and γ decreases
'
, 2
1
2
1
1
2 T
temp
to
down
comes
V
V
T
T
temp
Therefore
• During process 2-3, for a
given heat supplied Qs,
temp T3 will lower down
to T3’ as per the expression
Qs=mCv(T3-T2’)
Qs
28. Theoretical Fuel-Air Cycle
2. Effect of Variation Specific Heats (Contd) :
• And, therefore, process 3-4 will now become 3’-4’
• But process 3’-4’ represents process with const γ.
Since eng is in expansion stroke, the temp of gases
decreases, Cp & Cv decrease and hence γ increases
'
'
'
'
'
' 4
1
3
1
1
2
3
1
4
3
3
4 T
to
decreases
r
T
V
V
T
V
V
T
T
Temp
• Hence, actual process
becomes 3’-4’’ from 3’-4’
• Therefore, actual cycle
becomes 1-2’-3’-4’’
although ideal Otto Cycle
was 1-2-3-4
29. Theoretical Fuel-Air Cycle
3. Effect of Molecular Structure :
• Pressure of gases in comb chamber is proportional to
number of moles for given temp and volume by the
relation pV=nR˚T; where n is the no of moles
• If the no of moles before and after combustion are
different, pressure will change accordingly
• Take example of combustion :
C + O2 = CO2
1 mole 1 mole 1 mole
2H2 + O2 = 2H2O
2 moles 1 mole 2 moles
Molecular
Contraction
C8H18 + 12.5O2 = 8CO2 + 9H2O
1 mole 12.5 moles 8 moles 9 moles
Molecular
Expansion
30. Theoretical Fuel-Air Cycle
• From the foregoing, it is clear that no of moles may
be more or less after the combustion
• This phenomenon is called molecular contraction or
molecular expansion
• Therefore, actual pressure in combustion chamber
will be different compared to theoretical cycle
• Actual pressure in combustion chamber shall be more
in case of molecular expansion and lesser in case of
molecular contraction compared to theoretical cycle
31. Theoretical Fuel-Air Cycle
4. Dissociation Losses:
• Products of combustion dissociate in to its
constituents at higher temp beyond 1000˚C
• Rate of dissociation increases with increase in temp
• Dissociation process absorbs heat energy from comb
gases being chemically endothermic reaction and
association releases energy being exothermic reaction
2CO2=2CO+O2 : (Dissociation) Endothermic Reaction
2CO+O2=2CO2 : (Association) Exothermic Reaction
32. Theoretical Fuel-Air Cycle
• This results in lowering of temp and hence pressure
which in turn reduces power output and thermal
efficiency
• However, at the end of expansion stroke,
temperatures become low and dissociated gases
start combining releasing heat energy.
• But, it is too late as most
of this heat energy is
carried away by exhaust
gases. This loss of power
is called dissociation loss
• Dissociation losses have
been shown in Fig
33. Comparison of Fuel-Air Cycles with Air Standard Cycle
• Air std cycle has highly simplified approximations
• Therefore, estimate of engine performance is much
higher than the actual performance
• For example, actual indicated thermal efficiency of
a petrol engine for CR 7, is around 30% whereas
air std efficiency is around 55%.
• This large difference is due to non-instantaneous
burning of charge, incomplete combustion and
largely over simplifications in using values of
properties of working fluid for analysis
• In air std cycle, it was assumed that working fluid
was air, which behaves like perfect gas and had
constant specific heats
34. • In actual engine , working fluid is not air but a
mixture of air, fuel and residual gases
• Also, specific heats of working fluid are not constant
but increase as the temp rises
• And, products of combustion are subjected to
dissociation at high temperatures
• Engine operation is not frictionless
Comparison of Fuel-Air Cycles with Air Standard Cycle
35. Actual/Real Fuel-Air Cycles
Actual cycle efficiency is much lower than the air std
efficiency due to various losses occurring in actual
engine operation. These are:
1. Losses due to variation of specific heats with temp
2. Dissociation or chemical in-equilibrium losses
3. Time losses
4. Incomplete combustion losses
5. Direct heat losses from comb gases to surroundings
6. Exhaust blow-down losses
7. Pumping losses
8. Friction losses
36. Actual/Real Fuel-Air Cycle
• Working substance is mixture of fuel, air & residual
gases (not air or perfect gases)
• Specific heats vary (increase) with temp
(hence γ = Cp/Cv ↓with ↑T)
Cp = a + bT + cT2 + dT3
Cv = a1 + bT + cT2 + dT3; a1 > a
• Effect of molecular structure due to comb of fuel.
(Beyond 1000°C, products of comb dissociate &
absorb heat energy, thus lowering comb temp and
hence the power)
• Comb is not instantaneous (at const volume) as
piston continuously keeps moving resulting in time
losses
• Heat addition is not from reservoir but due to comb
of fuel, which alters composition of working fluid
37. Actual/Real Fuel-Air Cycle
• Compression & Expansion processes are polytropic
due to direct heat transfer to surroundings
• Opening and closing of valves are not
instantaneous. All 4 strokes do not take place in
180° crank rotation. Early opening of exhaust valve
causes blow down losses
• Suction stroke takes place below atmospheric
pressure and exhaust stroke above atm pressure
(Pumping losses)
• Thus, work developed in actual cycle is much less
than the theoretical cycle
• Friction losses also take place
38. Losses In Actual Cycle Other Than Fuel-Air Cycle
1. Time Losses:
• Work developed in actual
cycle is much less than
theoretical cycle as
shown in Fig (Area
enclosed by Blue Curve)
• Due to this time lag, actual max pr in comb chamber
lowers down to point x.
• In ideal cycles, heat addition is assumed at constant
volume but actually, combustion takes some finite
time while piston continues to move (30-40˚rotation
of crank shaft)
• Loss of work represents
time losses
39. Losses In Actual Cycle Other Than Fuel-Air Cycle
2. Heat Losses:
• Due to this, lot of work is lost
• There is considerable quantity of heat loss during
combustion and expansion processes
• Ideal Compression and Expansion processes are
assumed to be adiabatic but in actual processes,
heat transfer does take place from working fluid to
cylinder walls
• These work losses are called Heat Losses
40. Losses In Actual Cycle Other Than Fuel-Air Cycle
3. Exhaust Blow-down Losses:
• But due to this, lot of heat energy is carried away
by exhaust gases resulting in to loss of work
• In ideal cycle, exhaust valve is assumed to open at
BDC, when exhaust stroke starts but in actual cycle,
it opens 30 to 40˚ before BDC in power stroke itself
• This helps in reducing pressure in the cylinder during
exhaust stroke, so that work required to push out
exhaust gases, reduces
• This work losses are called Exhaust Blow-down Losses
41. Losses In Actual Cycle Other Than Fuel-Air Cycle
4. Pumping Losses or loss due to gas exchange Processes:
• The difference of work done in expelling the exhaust gases & the work done by the
fresh charge during the suction stroke is called Pumping work. Pumping loss is due to
pumping gas from lower inlet pressure to higher exhaust pressure.
•In ideal cycle, suction and exhaust processes are assumed to be taking place at
atmospheric pressure
• But in actual cycle, suction is carried out below and
exhaust above atm pressures and for these
operations, work is required to be done on gases
which comes from actual
work developed, thus
reducing over all power
output
• These work losses are
called Pumping losses
(shown in pink in Fig)
43. Losses In Actual Cycle Other Than Fuel-Air Cycle
5. Friction Losses:
• All this comes from power developed by the engine,
thus reducing actual power out put
• In ideal cycle, engine operation is considered
frictionless but in actual it is not so.
• Friction losses do occur between sliding or rotating
components like piston rings and cylinder walls,
bearings etc and it increases rapidly with speed of
the engine. Also, power is required to run various
auxiliary equipment like fans, pumps etc
• These power losses are called Friction Losses
49. 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.
50. 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.
51. 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.
52. 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.
53. 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.
55. Engines Components & Materials
1. Cylinder block/ Crank case:
• For holding major components like crankshaft,
cylinder head, liners, gears, pumps etc.
• Cooling jackets, oil passages, passages for push rods etc
• Grey CI, Al alloy
2.Cylinder head:
• For fitment of SP/ injectors, valve openings, comb
chamber, valves & valve operating mechanism
• CI , Al alloy
3. Oil pan:
• Oil sump
• Pressed steel sheet, Al alloy
56. Engines Components & Materials
4. Manifolds:
• Inlet & exhaust tubing for AF intake & exhaust
• CI
5. Gaskets:
• For leak proof sealing between two components
• Embossed steel, cork, special rubber
57. Engines Components & Materials
7. Piston:
• For transmission of force, light weight, high thermal k,
low thermal coeff of expansion
• Al alloy
8. Piston rings:
• For high pr leak proof sealing between piston &
cylinder.
• Alloy CI with Si, Mn with chromium plating
9. Connecting rod:
• For transmitting force on piston to crankshaft
• I-section, drop forged from steel
• Axial and bending stresses
58. Engines Components & Materials
10. Piston pin/Gudgeon Pin:
• For connecting piston to small end of connecting rod
• Case hardened steel
11. Crankshaft:
• For converting reciprocketing motion of piston to
rotary motion of crankshaft by connecting rod,
vibration damper and fly wheel fitted
• Forged from spheroidal graphite iron
12. Main & Big end bearings:
• For facilitating holding & friction free rotation of
crankshaft
• Babbitt material- alloy steel
59. Engines Components & Materials
13. Engine Valves:
• Inlet –for air/AF intake; Silicon-chrome steel
(C+Ni +Mn+Si)
• Exhaust- for exiting burnt gases (C+Ni+Mn+Si+Mb)
14. Camshaft:
• For operating valves (rotates at half speed of C/S)
• Forged alloy steel
15. Silencer/ Mufler:
• For reducing exhaust/comb sound
• Metal sheet
60. Parts of an
IC Engine
CROSS SECTION OF OVERHEAD VALVE FOUR STROKE SI ENGINE
Name as many
parts as you can
61. Parts of an
IC Engine
Air cleaner
Choke
Throttle
Intake manifold
Exhaust manifold
Piston rings
Piston
Wrist pin
Cylinder block
Connecting rod
Oil gallery to piston
Oil gallery to head
Crankcase
Crankpin
Crankshaft
Cylinder head
Breather cap
Rocker arm
Valve spring
Valve guide
Pushrod
Sparkplug
Combustion chamber
Tappet
Dipstick
Cam
Camshaft
Water jacket
Wet liner
Connecting rod bearing
Main bearing
Oil pan or sump
78. The four-stroke cycle
The four stroke
combustion cycle
consists of:
• 1. Intake
• 2. Compression
• 3. Combustion
• 4. Exhaust
79. The four-stroke cycle
The piston starts at the
top, the intake valve
opens and the piston
moves down to let the
engine take in a full
cylinder of air and
gasoline during the
intake stroke
The piston then moves
up to compress the
air/gasoline mixture.
This makes the
explosion more
powerful.
80. The four-stroke cycle
• When the piston
reaches the top, the
spark plug emits a spark
to ignite the
gasoline/air mixture.
• The gasoline/air mixture
explodes driving the
piston down.
• The piston reaches the
bottom of its stroke, the
exhaust valve opens
and the exhaust leaves
out of the tailpipe.
• The engine is ready for
another cycle.
81. 4 Processes Cycle
Intake Stroke
Intake valve opens,
admitting fuel and
air. Exhaust valve
closed for most of
stroke
Compression Stroke
Both valves closed,
Fuel/air mixture is
compressed by rising
piston. Spark ignites
mixture near end of
stroke.
Intake
Manifold
Spark
Plug
Cylinder
Piston
Connecting
Rod Crank
Power Stroke
Fuel-air mixture burns,
increasing temp and
pressure, expansion of
combustion gases
drives piston down. Both
valves closed, exhaust valve
opens near end of stroke
1 2 3
4
Exhaust Stroke
Exhaust valve open,
exhaust products are
displaced from
cylinder. Intake valve
opens near end of
stroke.
Crankcase
Exhaust
Manifold
Exhaust Valve
Intake Valve
82.
83.
84. 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.
85. 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.
88. Terminology
• Bore = d
• Stroke = s
• Displacement volume =Vs =
• Clearance volume = Vc
• Compression ratio = r
4
d
s
2
TDC
BDC
V
V
r = Vs + Vc
Vc
89. Cylinder Orientation
There is no limit on the number of cylinders that a small engines can have,
but it is usually 1 or 2.
Vertical
Horizontal
Slanted
Multi position
91. Horizontal
Vertical
S m a l l ga s e n g i n e s u s e t h re e c ra n ks h a f t o r i e n tat i o n s :
Crankshaft Orientation
Multi-position
92.
93. Actual Valve Timings : 4 Stroke SI Engine
IVO
IVC
TDC
BDC
EVC
EVO
Suction
Stroke
Power/ Expansion
Stroke
Exhaust
Stroke
Compression Stroke
10°
20°
25°
20°
Ign Adv
94.
95. Actual Valve Timings : 4 Stroke CI Engine
IVO
IVC
TDC
BDC
EVC
EVO
Suction
Stroke
Power/
Expansion
Stroke
Exhaust Stroke
Compression Stroke
10°- 25°
10°-15°
45°
20°- 30°
FIC
FIS
15° 25°
96. 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.
