Syllabus
UNIT-I : BASICS OF IC ENGINES (5)
Heat Engines, IC and EC engines, IC engines constructioncomponents and materials, engine nomenclature, valve diagrams,
intake and exhaust systems, engine classification, applications
STANDARD CYCLES & FUEL-AIR CYCLES (5)
Fuel-air cycle, assumptions, comparison with air standard cycle,
effect of variables on performance, actual engine cycle and various
losses
Unit I [10 hrs]
Basics of IC Engines, Fuel Air cycles and actual cycles
Part A] Basics of ICE [5 hrs]
- What is meant by engine and heat engine
- Difference between IC & EC engine with examples
- Construction: Different components of I. C. Engines and names
of the materials used for different components (Spark plug, engine
block, crank case, intake, exhaust manifold etc) there function,
location.
- Nomenclature: (TDC, BDC, Stroke, Swept vol, CC etc.)
- Working of 4 stroke I. C. Engine –SI and CI, Difference
between SI & CI engine
- Working of 2 stroke IC Engine (Petrol)
- Difference between two-stroke and four stroke engine.
- Classification of I. C. Engines (Inline, radial, V-type, SI & CI, twostroke & four stroke etc.)
- Applications of IC Engines
-Theoretical and actual valve timing diagrams.
- Intake and Exhaust Systems (Manifolds and their requirements)
Part B] Fuel air cycle and actual cycle [05 hrs]
Give reference of air standard cycle and start the topic
- What is Fuel air cycle, its assumptions
- Comparison with air standard cycle
- Factors affecting fuel air cycle analysis (Composition of
cylinder gases, specific heat variation, dissociation and
molecular change)
- Effect of operating variables (CR, AF ratio) on cycle analysis
performance
- Actual cycle and various losses.
Unit I [10 hrs]
Online exam (Compulsory weight age 13 marks):
- Only theoretical treatment.
- No numerical would be asked on unit 1 for online exam.
- Offline exam (Compulsory weight age 6 marks):
- Q 1 a or Q 2a (6 marks).
- Only theoretical question for 6 marks for offline exam.
- No mathematical treatment.
- Total weight age (out of 100 marks): 19 marks.
Lecture No 1
Learning Objectives:
• To learn basics of heat engines, IC and EC engines, IC engines
construction- components and materials
Parts of an Engine
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
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
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
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
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
Parts of an IC
Engine
Name as many
parts as you can
CROSS SECTION OF OVERHEAD VALVE FOUR STROKE SI ENGINE
Cylinder head
Air cleaner
Rocker arm
Valve spring
Choke
Throttle
Intake manifold
If you scored:
25 – 32- Excellent
15 – 24- Good
10 – 14- OK
<10- Change your
lubricating oil
Breather cap
Exhaust manifold
Piston rings
Piston
Wrist pin
Cylinder block
Connecting rod
Oil gallery to piston
Oil gallery to head
Crankcase
Valve guide
Pushrod
Sparkplug
Combustion chamber
Tappet
Dipstick
Cam
Camshaft
Water jacket
Wet liner
Connecting rod bearing
Crankpin
Crankshaft
Parts of an IC
Engine
Main bearing
Oil pan or sump
Naming Engine Components
Cylinder Block, Cylinder Head, Rocker Arms,
Rocker Shaft, Push Rods, Engine Valves, Inlet &
Outlet Manifolds, Carburetor, Air Filter, Tappet Cover
Piston, Piston Rings, Connecting Rod, Cylinder/
Cylinder Liners, Gudgeon/Piston Pin, Crank Shaft,
Big end Bearings, Main/Journal Bearings, Cam Shaft,
Cam Followers, Oil Pan, Oil Pump, Oil Filter, Water
Pump, Fan Belt, Radiator/HE, Fuel Injector, Fuel
Pump, Governor, Spark Plug, Distributor, Ignition
Coil, Battery, Dynamo/Alternator, Flywheel, Vibration
Damper, Muffler/Silencer, Gaskets, Thermostat,
Aux Gears, Super Charger/ Turbo Charger, Fuel Tank
Fuel Lines, Fuel Filter, HT & LT Wires/Cables etc
Lecture No 2
Learning Objectives:
•To understand working of 4-stroke SI and CI engines and
2-stroke SI engines
The four-stroke engine
Spark plug
Exhaust valve
Inlet valve
Cylinder
Piston
The four-stroke engine
Inlet valve
opens
INDUCTION STROKE
The four-stroke engine
Inlet valve
open
INDUCTION STROKE
Piston down
The four-stroke engine
Air/Fuel Mixture In
Inlet valve
open
INDUCTION STROKE
Piston down
The four-stroke engine
Inlet valve
closes
COMPRESSION STROKE
Piston up
The four-stroke engine
Inlet valve
closed
Piston up
COMPRESSION STROKE
The four-stroke engine
Inlet valve
closed
BANG
POWER STROKE
The four-stroke engine
Inlet valve
closed
Piston down
powerfully
POWER STROKE
The four-stroke engine
Inlet valve
closed
Piston down
powerfully
POWER STROKE
The four-stroke engine
Inlet valve
closed
POWER STROKE
The four-stroke engine
Inlet valve
closed
Exhaust valve
open
EXHAUST STROKE
The four-stroke engine
Exhaust valve
open
Inlet valve
closed
Exhaust gases
out
Piston up
EXHAUST STROKE
The four-stroke engine
Exhaust valve
open
Inlet valve
closed
Exhaust gases
out
Piston up
EXHAUST STROKE
The four-stroke engine
Inlet valve
opens
Exhaust valve
closed
INDUCTION STROKE
And so the
cycle
continues!!
The four-stroke cycle
The four stroke
combustion cycle
consists of:
– 1.
– 2.
– 3.
– 4.
Intake
Compression
Combustion
Exhaust
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.
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.
4 Processes Cycle
1
Intake Valve
Intake
Manifold
2
Exhaust Valve
Exhaust
Manifold
Cylinder
3
4
Spark
Plug
Piston
Connecting
Rod
Intake Stroke
Intake valve opens,
admitting fuel and
air. Exhaust valve
closed for most of
stroke
Crank
Crankcase
Compression Stroke
Both valves closed,
Fuel/air mixture is
compressed by rising
piston. Spark ignites
mixture near end of
stroke.
Power Stroke
Exhaust Stroke
Fuel-air mixture burns,
Exhaust valve open,
increasing temp and
exhaust products are
pressure, expansion of
displaced from
combustion gases
cylinder. Intake valve
drives piston down. Both
opens near end of
valves closed, exhaust valve
stroke.
opens near end of stroke
Lecture No 3
Learning Objectives:
• To understand engine terminology
• To learn about classification of engines
• To learn about theoretical and actual valve timings of SI and
CI engines
• To learn about engine induction and exhaust systems
Engine Terminology
TDC, BDC
Stroke/Swept /Displacement Volume (Vs)
Clearance Volume (Vc)
Compression Ratio CR (r)
Engine Terminology
More Terminology
Terminology
•
•
•
•
•
Bore = d
Stroke = s
Displacement volume =Vs =
Clearance volume = Vc
Compression ratio = r
r = Vs + Vc
Vc
 d 2 

s
 4 
VBDC

VTDC
Classification of IC Engines
Based on No of stroke per cycle:
• Four stroke
• Two stroke
Based on thermodynamic cycle:
• Otto/Constant volume cycle
• Diesel/Constant pressure cycle
• Dual Cycle
Based on No of cylinders:
• Single cylinder
• Multi-cylinders
Classification of IC Engines
Based on arrangement of cylinders:
• Inline engines
• V – engines
• Radial engines
• Opposed cylinders engines
• Opposed pistons engines
Based on ignition systems:
• SI engines
• CI engines
Based on cooling system:
• Air cooled
• Liquid cooled
4. - 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
Slanted
Horizontal
Multi position
4. - Cylinder Orientation—cont.
Three common cylinder configuration in multiple cylinder engines:
V
In-line
Horizontally opposed
5. Crankshaft Orientation
Small gas engines use three crankshaft orientations:
Multi-position
Horizontal
Vertical
Classification of IC Engines
Based on fuel used:
• Petrol engines
• Diesel engines
• Gas engines
• Bi-fuel engines
Based on fuel supply systems:
• Carburetor engines
• Solid injection engines
Based on lubrication system:
• Wet sump lubrication
• Dry sump lubrication
• Mist lubrication
Actual Valve Timings : 4 Stroke SI Engine
TDC
EVC
Ign Adv
20°
Compression Stroke
Exhaust
Stroke
IVO
Suction
Stroke
10°
IVC
20° 25°
Power/ Expansion
Stroke
EVO
BDC
Actual Valve Timings : 4 Stroke CI Engine
TDC
10°-15°
FIS
Compression Stroke
15°
25°
FIC
IVO
Suction
Stroke
10°- 25°
20°- 30°
Exhaust Stroke
EVC
IVC
45°
EVO
BDC
Power/
Expansion
Stroke
Engine Intake System
Air Filter
Carburetor
Intake Manifolds
A/F Mixture
Engine
Engine Manifolds
• Manifolds are conduits, which are connected
to engine cylinder head; one per cylinder
• Inlet manifolds are used to carry air-fuel
mixture from carburetor/ air from air filter to
cylinders through intake valve(s) in SI engs;
air from air filter to cylinders in CI engines
• Exhaust manifolds carry burnt/flue gases from
cylinder through exhaust valve(s) to silencer/
muffler and ultimately to atmosphere
• Manifolds are manufactured by casting process
of cast iron/ by die-casting of aluminum alloy
Intake Manifolds
• Intake manifolds should cause minimum
pressure loss thus ensuring max volumetric
efficiency
• Should distribute A/F mixture uniformly to
each cylinder over wide range of speeds and
loads (equal length to each cylinder)
• Should assist vaporization of fuel and mixing
with air while passing through it
• Shape and size should be to prevent
condensation of fuel without restricting air
flow
Cylinder Head
For 4 Cylinder Engine
Exhaust System
Engine
Exhaust
Manifolds
Silencer/
Muffler
Exhaust Pipe
Exhaust/Tail Pipe
Open to Atm
Exhaust Manifolds
• After power stroke, flue/ burnt gases are required to
be removed from engine cylinders
• Exhaust manifolds collect the burnt gases from each
cylinder and through silencer, pass them to atm
Requirements:
• Minimum back pressure to reduce power loss
(Increase in back pressure by 0.1 bar results in
decrease in power output by about 1.5%)
• Reduction in combustion noise as exiting gases will
expand suddenly in atmosphere making lot of noise
• Transfer of minimum possible heat to system (Vehicle)
• Reduce emissions going to atmosphere
(exhaust treatment like using Catalytic Converter)
Silencer/ Muffler
• After the power stroke, burnt gases, which
are at higher pressure, if exhausted to
atmosphere directly, make unpleasant loud
sound due to difference in exhaust pressure
and atmospheric pressure
• Muffler allows the gases to expand in it so
that pressure pulsations die down with the
result, burnt gases are discharged to
atmosphere quietly
Assignments
Q1. Explain working of 4-stroke CI engine and compare
with 4 -stroke SI engine in every aspect
Q2.Compare diesel and petrol engines
Q3.Compare 2 stroke and 4 stroke engines
Lecture No 4
Learning Objectives:
• To highlight certain aspects of Air Standard Cycles
Ideal or Air Standard Cycles
Air standard cycles are defined as cycles using a perfect
gas as the working fluid/ medium.
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:
• 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
Assumptions of Ideal or Air Standard
Cycles
• 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
• 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)
• Kinetic & PE of the working fluid are neglected
• All dissipative effects like friction, viscosity etc, are
neglected
Useful Thermodynamic Relations
(Perfect Gas)
• pV = mRT or pv = RT and p1V1/T1 = p2V2/T2
• Cp – Cv = R
• 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)
• For reversible adiabatic process : pVγ = Const
• In Compression process, if p1, V1 and T1 represent
initial conditions & p2, V2 and T2 the final conditions;
n 1
n 1
 p2  n Where n=γ for reversible
T2  V1 
   
adiabatic (isentropic)
T1 V2 
 p1 
process
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)
Pressure:
Pascal Pa=N/m2
1 bar = 105 Pa =105 N/m2 =100 kPa =1.03 kg/cm2
1 MPa = 106 Pa = 10 bar
Volume:
1 lit = 1000cc = 10-3m3
Important Cycles for Piston
Engines
1. Constant Volume or Otto Cycle
2. Constant Pressure or Diesel Cycle
3. Dual Combustion or Limited Pressure Cycle
Idealized Otto /Const Volume Cycle
3
1-2 : Adiabatic Compression
2-3 : Const Volume Heat Addition
3-4 : Adiabatic Expansion
4-1 : Const Volume Heat Rejection
p
 1 
2
4
0
V2/V3
1
V
V1/V4
1
r
 1
Air Standard Efficiency of Otto
Cycle
Ideal Diesel /Constant Pressure Cycle
1    1 
   1   1 

r     1
Some Important Aspects of Diesel
Cycle
• During heat addition at constant pressure, air
expands from volume V2 to V3 doing some work as
fuel injection commences at V2 and cuts off at V3 ,
called Cut Off Point
• In actual engine, heat addition takes place in the
form of injection of fuel, which self-ignites due to
high temp caused by high CR and burns at constant
pressure as piston moves down
• The volume ratio V3/V2 is called cut off ratio and is
denoted by ρ
• Compression Ratio and Expansion Ratio are not
equal in diesel cycle (unlike in Otto Cycle)
Air Standard Efficiencies : Otto &
Diesel Cycles
1
Otto Cycle:   1  r  1
Diesel Cycle:
1    1 
 1   1 






1
r


• In Diesel Cycle, bracketed
term is always > 1, hence
η for diesel cycle will
always be lower than Otto
for same CR
• With increase in CR, η initially increases at faster rate
• Diesel engs operate at much higher CR as compared
to petrol engs, hence η for diesel eng is actually higher
• η decreases as Cut off ρ increases
Dual Combustion or Limited Pressure
Cycle
1-2 : Adiabatic Compression
3
4
2-3 : Heat Addition at Const Volume
3-4 : Heat Addition at Const Pressure
4-5 : Adiabatic Expansion
5-1 : Heat Rejection at Const Volume
2
p
5
0
1
V





1
 .  1
 1   1 

r    1   .   1
Theoretical/ Air Std Efficiencies
Otto Cycle:
 1 
1
r
 1

Diesel Cycle:
Dual Cycle:
1   1 
  1   1 

r     1



1 
 .   1
 1   1 

r
   1   .   1
Q. An engine working on Otto Cycle has a clearance
volume of 17% of the total volume. Find (i) air std
efficiency of the cycle (ii) Relative efficiency
if actual efficiency is 26.85%.
Solution:
 air Std
Vs  Vc
 1   1 ; r 
Vc
r
1
Now Vc=0.17V1=0.17(Vs+Vc)
r 
V1
 5.88
0.17V1
1
  1 
 0.5076 or 50.76%
1.4 1
5.88
Actual Efficiency 26.85
 r 

x100  52.9%
Air std efficiency 50.76
Lecture No 5
Learning Objectives:
• To learn theoretical Fuel-Air Cycles
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
Theoretical Fuel-Air Cycle
• Mixture of fuel & air (A/F ratio)
• 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
• No heat exchange between gases and cylinder walls
• Change in KE is negligible
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
 otto  1 
 diesel
1
r
 1
OR


1
 1 
 1   1 

r     1 
2. Effect of Variation Specific Heats :
3
3’
p
Ideal Otto Cycle 1-2-3-4
Actual Cycle 1-2’-3’-4’’
2
4
4’
4’’
1
2’
V
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
 V1
Therefore, temp T2  T1 
 V2
• 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’)



 1
comes down to temp T2 '
Qs
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
 V3
 Temp T4 '  T3 ' 
 V4



 1
• 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
 V2
 T3 ' 
 V1



 1
T3 '
  1 decreases to T4 ' '
r
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
C8H18 + 12.5O2 = 8CO2 + 9H2O
1 mole 12.5 moles 8 moles 9 moles
Molecular
Contraction
Molecular
Expansion
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
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
2CO2=2CO+O2 : (Dissociation) Endothermic Reaction
2CO+O2=2CO2 : (Association) Exothermic Reaction
• Dissociation process absorbs heat energy from comb
gases being chemically endothermic reaction and
association releases energy being exothermic reaction
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
Lecture No 6
Learning Objectives:
• To compare air standard cycle with theoretical
Fuel-Air Cycles
• To learn actual fuel-air cycle
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
Comparison of Fuel-Air Cycles
with Air Standard Cycle
• 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
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
Actual/Real Fuel-Air Cycle
• Working substance is mixture of fuel, air & residual
gases (not air or perfect gases)
• Heat addition is not from reservoir but due to comb
of fuel, which alters composition of working fluid
• 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
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)
• Friction losses also take place
• Thus, work developed in actual cycle is much less
than the theoretical cycle
Losses In Actual Cycle Other Than Fuel-Air Cycle
1. Time Losses:
• 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)
• Due to this time lag, actual max pr in comb chamber
lowers down to point x.
• Work developed in actual
cycle is much less than
theoretical cycle as
shown in Fig (Area
enclosed by Blue Curve)
• Loss of work represents
time losses
Losses In Actual Cycle Other Than Fuel-Air Cycle
2. Heat Losses:
• 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
• There is considerable quantity of heat loss during
combustion and expansion processes
• Due to this, lot of work is lost
• These work losses are called Heat Losses
Losses In Actual Cycle Other Than Fuel-Air Cycle
3. Exhaust Blow-down Losses:
• 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
• But due to this, lot of heat energy is carried away
by exhaust gases resulting in to loss of work
• This work losses are called Exhaust Blow-down Losses
Losses In Actual Cycle Other Than Fuel-Air Cycle
4. Pumping Losses:
• 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)
Losses In Actual Cycle Other Than Fuel-Air
Cycle
5. Friction Losses:
• 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
• All this comes from power developed by the engine,
thus reducing actual power out put
• These power losses are called Friction Losses
Real/
Actual
Otto Cycle
Recent Trends in Engine Development
• More Power
• Compact Size
• Fuel Economy (VCR, VVT, MPFI, Stratified Charge, HCCI)
• Reduced Emissions/ Hybrid/ Electric vehicles
• Cheaper Fuel – Low Cetane / Octane No
• Lighter engines/ better materials for eng components
• DI combustion chambers; IDI being phased out
• Two stage boosting
• Increase in specific power output-90kW/Litre