The document is an educational workbook on the basic principles of internal combustion engines. It was prepared by Engineer Nazar Faisal Ouda Al-Obaidi for the Technical Training Institute's Machinery and Equipment Department. The workbook contains information on key concepts related to internal combustion engines like: - Conversion of thermal energy to mechanical energy in heat engines through thermodynamic power cycles. - Analyzing engine cycles using pressure-volume and temperature-entropy diagrams. - Characterizing the ideal Otto cycle that models spark-ignition engines and the ideal Diesel cycle that models compression-ignition engines. - Explaining engine parameters like compression ratio, mean effective pressure, and thermal efficiency.

1. ‫معهد اعداد المدربين التقنيين‬
‫قسم المكائن والمعدات‬
‫الحقيبة التعليمية‬
‫االساسية لمحركات االحتراق الداخلي‬
‫اعداد‬
‫المهندس : نزار فيصل عودة العبيدي‬
‫1‬

2. Conversion of Thermal Energy
•Almost all of the mechanical energy produced
today is produced from the conversion of thermal
energy in some sort of heat engine.
• The operation of all heat-engine cycles can usually
be approximated by an ideal thermodynamic power
cycle of some kind.
•A basic understanding of these cycles can often
show the power engineer how to improve the
operation and performance of the system.
2

3. Conversion of Thermal Energy
•Almost all of the mechanical energy produced
today is produced from the conversion of thermal
energy in some sort of heat engine.
• The operation of all heat-engine cycles can usually
be approximated by an ideal thermodynamic power
cycle of some kind.
•A basic understanding of these cycles can often
show the power engineer how to improve the
operation and performance of the system.
3

4. P- and T-s Diagrams of Power Cycles
The area under the heat addition process on a T-s
diagram is a geometric measure of the total heat
supplied during the cycle qin, and the area under the
heat rejection process is a measure of the total heat
rejected qout. The difference between these two (the
area enclosed by the cyclic curve) is the net heat
transfer, which is also the net work produced during
the cycle.
4

5. Reversible Heat-Engine Cycles
•The second law of thermodynamics states that it is
impossible to construct a heat engine or to develop a
power cycle that has a thermal efficiency of 100%.
This means that at least part of the thermal energy
transferred to a power cycle must be transferred to a
low-temperature sink.
•There are four phenomena that render any
thermodynamic process irreversible. They are:
 Friction
 Unrestrained expansion
 Mixing of different substances
 Transfer of heat across a finite temperature difference
5

6. Categorize Cycles
• Thermodynamic cycles can be divided into two
general categories: Power cycles and refrigeration
cycles.
• Thermodynamic cycles can also be categorized as
gas cycles or vapor cycles, depending upon the phase
of the working fluid.
• Thermodynamic cycles can be categorized yet
another way: closed and open cycles.
• Heat engines are categorized as internal or external
combustion engines.
6

7. Air-Standard Assumptions
To reduce the analysis of an actual gas power cycle to a
manageable level, we utilize the following
approximations, commonly know as the air-
standard assumptions:
1. The working fluid is air, which continuously circulates
in a closed loop and always behaves as an ideal gas.
2. All the processes that make up the cycle are internally
reversible.
3. The combustion process is replaced by a heat-
addition process from an external source.
4. The exhaust process is replaced by a heat rejection
process that restores the working fluid to its initial
state.
7

8. Air-Standard Cycle
Another assumption that is often utilized to simplify
the analysis even more is that the air has constant
specific heats whose values are determined at room
temperature (25oC, or 77oF). When this assumption is
utilized, the air-standard assumptions are called the
cold-air-standard assumptions. A cycle for which the
air-standard assumptions are applicable is frequently
referred to as an air-standard cycle.
The air-standard assumptions stated above provide
considerable simplification in the analysis without
significantly deviating from the actual cycles.
The simplified model enables us to study qualitatively
the influence of major parameters on the performance
of the actual engines.
8

9. Bore and
stroke of a
cylinder
9

10. Mean Effective Pressure
The ratio of the maximum volume formed in the cylinder to
the minimum (clearance) volume is called the compression
ratio of the engine.
V V
r  max  BDC
Vmin VTDC
Notice that the compression ratio is a
volume ratio and should not be
confused with the pressure ratio.
Mean effective pressure (MEP) is a
fictitious pressure that, if it acted
on the piston during the entire
power stroke, would produce the
same amount of net work as that
produced during the actual cycle. MEP 
Wnet
Vmax  Vmin
10

11. Three Ideal Power Cycles
•Three ideal power cycles are completely reversible
power cycles, called externally reversible power
cycles. These three ideal cycles are the Carnot cycle,
the Ericsson cycle, and the Stirling Cycle.
11

12. Three Ideal Power Cycles
•The Carnot cycle is an externally reversible power cycle
and is sometimes referred to as the optimum power
cycle in thermodynamic textbooks. It is composed of
two reversible isothermal processes and two reversible
adiabatic (isentropic) processes.
•The Ericsson power cycle is another heat-engine cycle
that is completely reversible or “externally reversible.” It
is composed of two reversible isothermal processes and
two reversible isobaric processes (with regenerator).
•The Stirling cycle is also an externally reversible
heat-
engine cycle and is the only one of the three ideal power
cycles that has seen considerable practical application.
It is composed of two reversible isothermal processes
and two reversible isometric (constant volume)
processes.
12

13. Carnot Cycle and Its Value in Engineering
The Carnot cycle is composed
of four totally reversible
processes: isothermal heat
addition, isentropic expansion,
isothermal heat rejection, and
isentropic compression (as
shown in the P- diagram at
right). The Carnot cycle can be
executed in a closed system (a
piston-cylinder device) or a
steady-flow system (utilizing
two turbines and two TL
th ,Carnot 1
compressors), and either a gas TH
or vapor can be used as the
working fluid.
13

14. Internal-Combustion Engine Cycles
• Internal-combustion (IC) engines cannot operate on
an ideal reversible heat-engine cycle but they can be
approximated by internally reversible cycles in which
all the processes are reversible except the heat-
addition and heat-rejection processes.
•In general, IC engines are more polluting than
external-combustion (EC) engines because of the
formation of nitrogen oxides, carbon dioxide, and
unburned hydrocarbons.
•The Otto cycle is the basic thermodynamic power
cycle for the spark-ignition (SI), internal-
combustion engine.
14

15. The Ideal Air Standard Otto Cycle
15

16. Otto Cycle: The ideal Cycle for Spark-Ignition Engines
Figures below show the actual and ideal cycles in spark-
ignition (SI) engines and their P- diagrams.
16

17. Ideal Otto Cycle
The thermodynamic analysis of
the actual four-stroke or two-
stroke cycles can be simplified
significantly if the air-standard
assumptions are utilized. The T-
s diagram of the Otto cycle is
given in the figure at left.
The ideal Otto cycle consists of four internally
reversible processes:
12 Isentropic compression
23 Constant volume heat addition
34 Isentropic expansion
41 Constant volume heat rejection
17

18. Thermal Efficiency of an Otto Cycle
The Otto cycle is executed in a closed system, and
disregarding the changes in kinetic and potential
energies, we have
qin  qout   win  wout   u
 qin  u3  u 2  Cv T3  T2 
 qout  u 4  u1  Cv T4  T1 
wnet qout T4  T1
th ,Otto  1 1
qin qin T3  T2
T1 T4 / T1  1 T 1
1  1  1  1  k 1
T2 T3 / T2  1 T2 r
k 1 k 1
T1   2   3  T4 Vmax V1 1
Where,      ;and r   
T2  1 
 
 
 4 T3 Vmin V2  2
18

19. Example IV-4.1: The Ideal Otto Cycle
An ideal Otto cycle has a
compression ratio of 8. At the
beginning of the compression
process, the air is at 100 kPa and
17oC, and 800 kJ/kg of heat is
transferred to air during the
constant-volume heat-addition
process. Accounting for the variation
of specific heats of air with
temperature,
determine a) the maximum temperature and pressure
that occur during the cycle, b) the net work output, c)
the thermal efficiency, and d) the mean effective
pressure for the cycle. <Answers: a) 1575.1 K, 4.345
MPa, b) 418.17 kJ/kg, c) 52.3%, d) 574.4 kPa>
Solution: 19

20. a  Maximum temperatur e and pressure in an Otto cycle:
T1  290K  u1  206.91kJ / kg, vr1  676.1
Process 1- 2 (isentropic compressio n of an ideal gas) :
vr 2 v2 1 vr1 676.1
   vr 2    84.51  T2  652.4 K , u 2  475.11kJ / kg
vr1 v1 r r 8
P2 v2 P v1  T2  v1  652.4
 1
 P2  P     100
1  v   8  1799.7 kPa
T2 T1  T1  2  290
Process 2 - 3 (constant volume heat addition) :
qin  u3  u 2  u3  qin  u 2  800  475.11  1275.11kJ / kg  T3  1575.1 K
P3v3 P2 v2  T3  v2  1575.1
     1.797 MPa 
 P3  P2     1  4.345MPa
T3 T2  T2  v3  652.4
Note : The property vr (relative specific volume) is a dimensionl ess
quantity used in the analysis of isentropic processes,and should not
be confused w ith the property specific volume.
20

21. b  The net w ork output :
Process 3 - 4 (isentropic expansion of an ideal gas) :
vr 4 v4
  r  vr 4  rv r 3  8  6.108  48.864
vr 3 v3
 T4  795.6 K , u 4  588.74 kJ / kg
Process 4 - 1 (constant volume heat rejection) :
 qout  u1  u 4  qout  u 4  u1  588.74  206.91  381.83 kJ / kg
Thus, wnet  qnet  qin  qout  800  381.83  418.17 kJ / kg
c  The thermal efficiency:
 0.523 or 52.3% 
wnet 418.17
th  
qin 800
Under the cold - air - standard assumption s :
th  1  k 1  1  r1 k  1  811.4  0.565 or 56.5% 
1
r
Care should be exercised in utlizing this assumption s.
21

22. d  The mean effectivepressure is determined from its definition :
kPa.m 3
0.287  290K
RT1 kg .K m3
v1    0.832
P1 100kPa kg
wnet wnet 418.17  1kPa.m 3 
   574.4 kPa
Thus, mep   
v1  v2 v  v1 0.832  0.832  1 kJ 
1

r 8
Therefore, a constant pressure of 574.4 kPa during the pow er stroke
w ould produce the same net w ork output as the entire cycle.
Note that this problem could be solved by using equations show non
Slide #17 w ith given constant specific heats c p , cv (at room temperatur e).
22

23. Diesel Cycle: The Ideal Cycle for Compression-Ignition Engines
The diesel cycle is the ideal cycle for CI (Compression-
Ignition) reciprocating engines. The CI engine first
proposed by Rudolph Diesel in the 1890s, is very
similar to the SI engine, differing mainly in the method
of initiating combustion. In SI engines (also known as
gasoline engines), the air-fuel mixture is compressed
to a temperature that is below the autoignition
temperature of the fuel, and the combustion process is
initiated by firing a spark plug. In CI engines (also
known as diesel engines), the air is compressed to a
temperature that is above the autoignition temperature
of the fuel, and combustion starts on contact as the
fuel is injected into this hot air. Therefore, the spark
plug and carburetor are replaced by a fuel injector in
diesel engines.
23

24. The Ideal Air Standard Diesel Cycle
24

25. Ideal Cycle for CI Engines (continued)
In diesel engines, ONLY air is compressed during the
compression stroke, eliminating the possibility of
autoignition. Therefore, diesel engines can be designed
to operate at much higher compression ratios, typically
between 12 and 24.
The fuel injection process in diesel engines starts when
the piston approaches TDC and continues during the
first part of the power stroke. Therefore, the
combustion process in these engines takes place over a
longer interval. Because of this longer duration, the
combustion process in the ideal Diesel cycle is
approximated as a constant-pressure heat-addition
process. In fact, this is the ONLY process where the
Otto and the Diesel cycles differ.
25

26. Ideal Cycle for CI Engines (continued)
qin  wb ,out  u3  u2  qin  h3  h2  C p T3  T2 
qout  u4  u1  Cv T4  T1 
wnet qout T4  T1 1  rck  1 
th ,Diesel  1 1  1  k 1  
qin qin k T3  T2  r  k rc  1 
 
Where,
1
r
2
and

rc  3
2
26

27. Thermal efficiency of Ideal Diesel Cycle
Under the cold-air-standard assumptions, the
efficiency of a Diesel cycle differs from the efficiency of
Otto cycle by the quantity in the brackets. (See Slide
#26)
The quantity in the
brackets is always greater
than 1. Therefore, th,Otto
> th, Diesel when both
cycles operate on the
same compression ratio.
Also the cuttoff ratio, rc
decreases, the efficiency
of the Diesel cycle
increases. (See figure at
right)
27

28. Internal-Combustion Engines
The two basic types of ignition or firing systems are
the four-stroke-cycle engines, commonly called four-
cycle engines, and the two-stroke-cycle engines,
commonly called two-cycle engines.
The four-cycle engines has a number of advantages
over the usual two-cycle engine, including better fuel
economy, better lubrication, and easier cooling.
The two-cycle engine has a number of advantages,
including fewer moving parts, lighter weight, and
smoother operation. Some two-cycle engines have
valves and separate lubrication systems.
28

29. Cylinder Arrangements for Reciprocating Engines
Figure below shows schematic diagrams of some of the
different cylinder arrangements for reciprocating
engines.
29

30. • Vertical in-line engine is commonly used today in
four- and six-cylinder automobile engines.
• The V-engine is commonly employed in eight-
cylinder (V-8) and some six-cylinder (V-6) automobile
engines.
• The horizontal engine is essentially a V-engine with
180o between the opposed cylinders. This system was
used as the four-cylinder, air-cooled engine that
powered the Volkswagon “bug”.
• The opposed-piston engine consists of two pistons,
two crankshafts, and one cylinder. The two crankshafts
are geared together to assure synchronization. These
opposed-piston systems are often employed in large
diesel engines.
30

31. • The delta engine is composed of three opposed-
piston cylinders connected in a delta arrangement.
These systems have found application in the petroleum
industry.
• The radial engine is composed of a ring of cylinders
in one plane. One piston rod, the “master” rod, is
connected to the single crank on the crankshaft and all
the other piston rods are connected to the master rod.
Radial engines have a high power-to-weight ratio and
were commonly employed in large aircraft before the
advent of the turbojet engine.
• When the term “rotary engine” is used today, it
implies something other than a radial engine with a
stationary crank.
31

32. Engine Performance
There are several performance factors that are common
to all engines and prime movers. One of the main
operating parameters of interest is the actual output of
the engine. The brake horsepower (Bhp) is the power
delivered to the driveshaft dynamometer.
The brake horsepower is usually measured by
determining the reaction force on the dynamometer
and using the following equation:
2FRNd
Bhp 
33,000
Where F is the net reaction force of the dynamometer,
in lbf, R is the radius arm, in ft, and Nd is the angular
velocity of the dynamometer, in rpm.
32

33. Horsepower
For a particular engine, the relationship between the
mean effective pressure (mep) and the power is:
mepVdis N p 
Bhp 
33,000
Wnet
w here mep 
Vmax  Vmin
 bore2 stroke 
Vdis 
4
CN e
and N p  is the number of pow er strokes per minute.

Where C is the number of cylinders in the engine, Ne is
the rpm of the engine, and  is equal to 1 for a two-
stroke-cycle engine and 2 for a four-stroke-cycle
engine. 33

34. Brake Thermal Efficiency
The brake thermal efficiency of an engine, th, unlike
power plants, is usually based on the lower heating
value (LHV) of the fuel. The relationship between
efficiency and the brake specific fuel consumption
(Bsfc) is:
2545
th 
Bsfc LHV 
w here
Bsfc 
fuel rate, lbm/h 
Bhp
Note that the brake specific fuel consumption (Bsfc) of
an engine is a measure of the fuel economy and is
normally expressed in units of mass of fuel consumed
per unit energy output.
34

35. External-Combustion Systems
External-combustion power systems have several
advantages over internal-combustion systems. In
general, they are less polluting. The primary pollutants
from internal-combustion engines are unburned
hydrocarbons, carbon monoxide, and oxides of
nitrogen.
In external-combustion engines, the CHx and CO can
be drastically reduced by carrying out the combustion
with excess air and the NOx production can be
markedly reduced by lowering the combustion
temperature. By burning the fuel with excess air, more
energy is released per pound of fuel.
There are three general ideal external-combustion
engine cycles, the Stirling and Brayton are ideal gas-
power, and vapor power cycles. 35

36. Brayton Cycle:
The Ideal Cycle for Gas-Turbine Engines
The Brayton cycle was first proposed by George Brayton
for use in the reciprocating oil-burning engine that he
developed around 1870.
Fresh air at ambient conditions is drawn into the compressor,
where its temperature and pressure are raised. The high-
pressure air proceeds into the
combustion chamber, where
the fuel is burned at constant
pressure. The resulting high-
temperature gases then enter
the turbine, where they
expand to the atmospheric
pressure, thus producing
power. (An open cycle.)
36

37. Brayton Cycle (continued)
The open gas-turbine cycle can be modeled as a closed
cycle, as shown in the figure below, by utilizing the air-
standard assumptions.
The ideal cycle that the working
fluid undergoes in this closed
loop is the Brayton cycle, which
is made up of four internally
reversible processes:
12 Isentropic compression (in a
compressor)
23 Constant pressure heat addition
34 Isentropic expansion (in a
turbine)
41 Constant pressure heat rejection
37

38. T-s Diagram of Ideal Brayton Cycle
Notice that all four processes
of the Brayton cycle are
executed in steady-flow
devices (as shown in the
figure on the previous slide,
T-s diagram at the right), and
the energy balance for the
ideal Brayton cycle can be
expressed, on a unit-mass
basis, as
qin  qout   win  wout   hexit  hinlet
w here qin  h3  h2  C p T3  T2 
and qout  h4  h1  C p T4  T1 
38

39. P- Diagram and th of Ideal Brayton Cycle
Then the thermal efficiency of
the ideal Brayton cycle under
the cold-air-standard
assumptions becomes
wnet qout
th ,Brayton  1
qin qin
C p T4  T1  T1 T4 / T1  1
1 1
C p T3  T2  T2 T3 / T2  1
1
1
rp k 1 / k
k 1 / k k 1 / k
T P  P  T3 P
w here 2   2   3   , and rp  2 is the pressure ratio.
T1  P 
 1
P 
 4 T4 P1
39

40. Thermal Efficiency of the Ideal Brayton Cycle
Under the cold-air-standard
assumptions, the thermal
efficiency of an ideal Brayton
cycle increases with both the
specific heat ratio of the
working fluid (if different from
air) and its pressure ratio (as
shown in the figure at right) of
the isentropic compression
process.
The highest temperature in the cycle occurs at the end
of the combustion process, and it is limited by the
maximum temperature that the turbine blades can
withstand. This also limits the pressure ratios that can
be used in the cycle.
40

41. With the demise of the steam powered tractor in the
late 1800’s, most modern tractors are equipped with
internal combustion engines.
Internal combustion engines are identified by the
number of strokes
in the cycle and by the fuel that is used to run them.
Common Tractor Classifications:
4 stroke cycle
- gasoline
- diesel
- LP
41

42. 42

43. 43

44. Intake
Exhaust
Lubricating
Electrical
Cooling
Fuel
Hydraulic
Drive Train
44

45.  Parts:
1. Pre-Cleaner
2. Air Cleaner
3. Intake Manifold
4. Intake Valve
5. Turbocharger (if
used)
6. Intercooler (if used)
45

46.  Parts:
1. Exhaust Valve
2. Exhaust Manifold
3. Muffler
4. Cap
46

47.  Parts:
1. Crankcase Oil Reservoir (Oil
Pan)
2. Oil Pump
3. Oil Filter
4. Oil Passages
5. Pressure Regulating Valve
 Oil goes to:
1. Camshaft Bearings
2. Crankshaft Main Bearings
3. Piston Pin Bearing
4. Valve Tappet Shaft
47

48.  Parts:
1. Battery
2. Ground Cable
3. Key Switch
4. Ammeter
5. Voltage Regulator
6. Starter Solenoid
7. Starter
8. Distributor * Gasoline
Only
9. Coil
10. Alternator
11. Spark Plug
12. Power Cable
48

49. Cooling System
Liquid & Air
Parts:
1. Radiator
2. Pressure Cap
3. Fan
4. Fan Belt
5. Water Pump
6. Engine Water Jacket
7. Thermostat
8. Connecting Hoses
9. Liquid or Coolant
49

50. Cooling System
Air cooled
Fins are used to dissipate heat
Liquid cooled
Coolant is used to dissipate heat.
50

51.  Gasoline
 Diesel
 Liquid Propane (LP)
 Alternate Fuels
51

52.  Parts:
 Fuel Tank
 Fuel Pump
 Carburetor
 Fuel Filter
 Fuel Lines
52

53. Diesel Fuel System
Parts: 1. Fuel Tank
2. Fuel Pump
3. Fuel Filters
4. Injection Pump
5. Injection Nozzles
53

54. Power Transmission
Mechanical & Hydraulic
Parts:
1. Clutch Pedal
2. Clutch
3. Shift Controls
4. Transmission
5. Differential
6. Differential Lock Pedal
7. Final Drives
8. Power Take Off (PTO)
54

55. CONVENTIONAL INTERNAL
COMBUSTION ENGINES
TWO STROKE ENGINES
Migrating Combustion Chamber Engine (MCC)
FOUR CYCLE ENGINES
Conventional Four Cycle (OTTO ENGINE)
Rotary Engine (WANKEL)
Rotating Cylinder Valve Engine (RCV)
55

56. TWO STROKE ENGINES
Two-stroke engines do not have valves,
which simplifies their construction and
lowers their weight.
Two-stroke engines fire once every
revolution, while four-stroke engines fire
once every other revolution. This gives
two-stroke engines a significant power
boost.
56

57. TWO STROKE ENGINES
These advantages make two-stroke engines lighter, simpler and less
expensive to manufacture.
Two-stroke engines also have the potential to pack about twice the
power into the same space because there are twice as many power
strokes per revolution.
The combination of light weight and twice the power gives two-stroke
engines a great power-to-weight ratio compared to many four-stroke
engine designs.
57

58. TWO STROKE ENGINES
Two-stroke engines don't last nearly as
long as four-stroke engines. The lack of
a dedicated lubrication system means
that the parts of a two-stroke engine
wear a lot faster.
Two-stroke oil is expensive, and you
need about 4 ounces of it per gallon of
gas. You would burn about a gallon of
oil every 1,000 miles if you used a two-
stroke engine in a car.
58

59. TWO STROKE ENGINES
Two-stroke engines do not use fuel
efficiently, so you would get fewer miles
per gallon.
Two-stroke engines produce a lot of
pollution
so much, in fact, that it is likely that you won't
see them around too much longer.
59

60. FUEL
INTAKE
60

61. COMPRESSION
61

62. COMBUSTION
&
EXHAUST
62

63. TWO STROKE
OPERATION
TWO
STROKE
OPERATI
ON
63

64. FOUR CYCLE ENGINES
conventional Otto engines
64

65. FOUR CYCLE ENGINE OPERATION
65

66. FOUR CYCLE ENGINE
CHARACTERISTICS
FOUR STROKE ENGINES LASTS LONGER THAN TWO STROKE ENGINES. The
lack of a dedicated lubrication system means that the parts of a two-stroke engine
wear a lot faster.
FOUR STROKE ENGINES DON’T BURN OIL IN COMBUSTION CHAMBER. Two-
stroke oil is expensive, and you need about 4 ounces of it per gallon of gas. You
would burn about a gallon of oil every 1,000 miles if you used a two-stroke engine in
a car.
FOUR STROKE ENGINES ARE MORE FUEL EFFICIENT. Two-stroke engines do
not use fuel efficiently, so you would get fewer miles per gallon.
FOUR STROKE ENGINES ARE CLEANER. Two-stroke engines produce a lot of
pollution
INVERTED FLIGHTS MAY NOT BE EASY IN FOUR STROKE ENGINES. Two-
stroke engines can work in any orientation, which can be important in acrobatic
flights. A standard four-stroke engine may have problems with oil flow unless it is
upright, and solving this problem can add complexity to the engine.
66

67. Unusual Four stroke engines
applications
ROTARY ENGINES WANKEL ENGINE
ROTARY CYLINDER VALVE ENGINE RCV ENGINE
67

68. ROTARY ENGINES
Wankel Engine
Rotary engines use the four-stroke combustion
cycle, which is the same cycle that four-stroke
piston engines use. But in a rotary engine, this is
accomplished in a completely different way. 68

69. The heart of a rotary engine is the rotor. This is roughly the
equivalent of the pistons in a piston engine. The rotor is
mounted on a large circular lobe on the output shaft. This
lobe is offset from the centerline of the shaft and acts like the
crank handle on a winch, giving the rotor the leverage it
needs to turn the output shaft. As the rotor orbits inside the
housing, it pushes the lobe around in tight circles, turning
three times for every one revolution of the rotor. 69

70. How Rotary Engines Work
For every three
rotations of the
engine shaft
corresponds to
one complete
piston rotation
(360 degrees)
WANKEL ENGINE OPERATION
70

71. How Rotary Engines Work
If you watch carefully, you'll see the offset
lobe on the output shaft spinning three times
for every complete revolution of the rotor.
As the rotor moves through the
housing, the three chambers
created by the rotor change size.
This size change produces a
pumping action. Let's go through
each of the four stokes of the
engine looking at one face of the
rotor.
71

72. Four Stroke Gas Engines
The four strokes of a internal combustion engine are:
•Intake
•Compression
•Power
•Exhaust
Each stroke = 180˚ of
crankshaft revolution.
Each cycle requires two revolutions
of the crankshaft (720˚ rotation), and
one revolution of the camshaft to complete
(360˚ rotation).
72

73. Intake Stroke
First Stroke
The piston moves down the cylinder
from TDC (Top Dead Center) to BDC
(Bottom Dead Center).
This movement of piston causes low
air pressure in the cylinder (vacuum)
Mixture of Air and Fuel in the ratio
of 14.7 : 1 (air : fuel) is drawn into
the cylinder.
Intake valve stays open and the
Exhaust valve stays closed during
this stroke.
73

74.  This starts at the
highest point
known as top
dead center and
ends at bottom
dead center
 The intake stroke
allows the piston
to suck fuel and
air into the
combustion
chamber through
the intake valve 74

75. Compression stroke
Second stroke
The piston moves from BDC to TDC
Intake and exhaust valves stay closed
Air and fuel mixture is compressed
8:1 to 12:1
The pressure in the cylinder is raised
75

76.  Compression starts
at bottom dead
center and ends at
top dead center.
 The second motion of
the stroke takes all
the fuel and air that
was stored and
compresses it into
one tenth its original
sizes. Making the
air/fuel mixture
increase in
temperature
preparing it for the
next stage in its
combustion cycle. 76

77. Power stroke
Third stroke
At the end of compression stroke
the sparkplug fires, igniting the air/fuel
mixture.
Both the valves stay closed in
this stroke.
The expanding gases from the
combustion in the cylinder
(with no escape) push the piston
down.
The piston travels from TDC to BDC.
77

78. Force acting from pressure
Pr e s s u r e
• In engines the
amount of force
exerted on the top of
a piston is A re a
determined by the
cylinder pressure
during the
combustion process.
78

79.  The power stroke starts as
soon as the piston reaches
top dead center allowing the
spark plug to ignite.
 This electric current created
by the spark plug ignites the
fuel and air mixture sending
the piston back down the
cylinder with a pressure
reaching high as 600 PSI.
79

80. Exhaust stroke
Fourth and last stroke
The momentum created by the
Counter-weights on the crankshaft,
move the piston from BDC to TDC.
The exhaust valve opens and
the burned gases escape into the
exhaust system.
Intake valve remains closed.
80

81.  The final stage of the
stroke releases all the
burned fuel through the
exhaust valve.
 As the piston moves
from bottom dead center
to top dead center it
takes all the burned fuel
and pushes it out of the
cylinder, preparing it for
the next cycle of strokes.
81

82. Indicator Diagrams and Internal Combustion Engine
Performance Parameters
• Much can be learned from a record of the cylinder pressure
and volume. The results can be analyzed to reveal the rate at
which work is being done by the gas on the piston, and the rate
at which combustion is occurring. In its simplest form, the
cylinder pressure is plotted against volume to give an indicator
diagram.
82

83. Pressure-Volume Graph 4-stroke SI engine
One power stroke for every two crank shaft revolutions
Pressure Spark
Exhaust valve
Exhaust opens
valve
closes
1 atm Intake valve
closes
Intake
valve
opens
TC BC
Cylinder volume
83

84. Exhaust Valve : Valve Timing Diagram
Pcyl
Patm
84

85. Inlet Valve : Valve Timing Diagram
Pcyl
Patm
85

86. Valve Timing for Better Flow
86

87. Efficiency
• In general, energy conversion efficiency is the ratio between
the useful output of a device and the input. For thermal
efficiency, the input, to the device is heat, or the heat-content
of a fuel that is consumed. The desired output is mechanical
work, or heat, or possibly both. Because the input heat
normally has a real financial cost, a memorable, generic
definition of thermal efficiency is;
87

88. • When expressed as a percentage, the thermal efficiency must be
between 0% and 100%. Due to inefficiencies such as friction,
heat loss, and other factors, thermal engines' efficiencies are
typically much less than 100%. For example, a typical gasoline
automobile engine operates at around 25% efficiency. The largest
diesel engine in the world peaks at 51.7%.
88

89. • Work done on the piston due to
pressure
89

90. • The term indicated work is used to define the net work done
on the piston per cycle
• the indicated mean effective pressure (imep),can be defined
by;
90

91. • The imep is a hypothetical pressure that would produce the
same indicated work if it were to act on the piston throughout
the expansion stroke. The concept of imep is useful because it
describes the thermodynamic performance of an engine, in a
way that is independent of engine size and speed and frictional
losses.
• Unfortunately, not all the work done by the gas on the piston is
available as shaft work because there are frictional losses in
the engine. These losses can be quantified by the brake mean
effective pressure (bmep,), a hypothetical pressure that acts on
the piston during the expansion stroke and would lead to the
same brake work output in a frictionless engine.
91

92. Mechanical Efficiency
Some of the power generated in the cylinder is used to overcome engine
friction and to pump gas into and out of the engine.

The term friction power, W f , is used to describe collectively these power
losses, such that:
  
W f  Wi , g  Wb
Friction power can be measured by motoring the engine.
The mechanical efficiency is defined as:

Wb Wi , g  W f
  
Wf
m    1
Wi , g Wi , g 
Wi , g
92

93. Mechanical Efficiency, cont’d
• Mechanical efficiency depends on pumping losses
(throttle position) and
frictional losses (engine design and engine speed).
• Typical values for automobile engines at WOT are:
90% @2000 RPM and 75% @ max
speed.
• Throttling increases pumping power and thus the
mechanical efficiency
decreases, at idle the mechanical efficiency
approaches zero.
93

94. • Brake Specific Fuel Consumption (BSFC) is a measure of fuel
efficiency within a shaft reciprocating engine. It is the rate
of fuel consumption divided by the power produced. Specific
fuel consumption is based on the torque delivered by the
engine in respect to the fuel mass flow delivered to the engine.
Measured after all parasitic engine losses is brake specific fuel
consumption [BSFC] and measuring specific fuel consumption
based on the in-cylinder pressures (ability of the pressure to do
work) is indicated specific fuel consumption [ISFC].
94

95. • The final parameter to be defined is the volumetric
efficiency of the engine; the ratio of actual air flow to
that of a perfect engine is
• In general, it is quite easy to provide an engine with
extra fuel; therefore, the power output of an engine
will be limited by the amount of air that is admitted to
an engine.
95

96. Volumetric Efficiency
• Volumetric efficiency a measure of overall effectiveness of
engine and its intake and exhaust system as a natural
breathing system.
• It is defined as: 
2 ma
v 
r a , 0Vd N
• If the air density ra,0 is evaluated at inlet manifold conditions, the
volumetric efficiency is a measure of breathing performance of the
cylinder, inlet port and valve.
• If the air density ra,0 is evaluated at ambient conditions, the volumetric
efficiency is a measure of overall intake and exhaust system and other
engine features.
• The full load value of volumetric efficiency is a design feature of entire
engine system.
96

97. • Systems which are thermally insulated from their surroundings
undergo processes without any heat transfer; such processes
are called adiabatic. Thus during an isentropic process there
are no dissipative effects and the system neither absorbs nor
gives off heat.
• A reversible process, is a process that can be "reversed" by
means of infinitesimal changes in some property of the system
without loss or dissipation of energy.
• Isentropic process is a process which is a process is both
adiabatic and reversible .
97

98. • A closed cylinder with a locked piston contains air. The
pressure inside is equal to the outside air pressure. This
cylinder is heated to a certain target temperature. Since the
piston cannot move, the volume is constant, while temperature
and pressure rise. When the target temperature is reached, the
heating is stopped. The piston is now freed and moves
outwards, expanding without exchange of heat (adiabatic
expansion). Doing this work cools the air inside the cylinder to
below the target temperature. To return to the target
temperature (still with a free piston), the air must be heated.
This extra heat amounts to about 40% more than the previous
amount added. In this example, the amount of heat added with
a locked piston is proportional to CV, whereas the total amount
of heat added is proportional to CP. Therefore, the heat
capacity ratio in this example is 1.4
98

99. 99

100. Efficiencies of Real Engines
• The efficiencies of real engines are below
those predicted by the ideal air standard cycles
for several reasons. Most significantly, the
gases in internal combustion engines do not
behave perfectly with a ratio of heat capacities.
10
0

101. Ignition and Combustion in Spark Ignition
and Diesel Engines
• Spark ignition (SI) engines usually have pre-mixed combustion, in which a
flame front initiated by a spark propagates across the combustion chamber
through the unburned mixture. Compression ignition (CI) engines normally
inject their fuel toward the end of the compression stroke, and the
combustion is controlled primarily by diffusion.
• Whether combustion is pre-mixed (as in SI engines) or diffusion controlled
(as in CI engines) has a major influence on the range of air-fuel ratios
(AFRs) that will burn.
• In pre-mixed combustion, the AFR must be close to stoichiometric-the AFR
value that is chemically correct for complete combustion. In practice,
dissociation and the limited time available for combustion will mean that
even with the stoichiometric AFR, complete combustion will not occur.
• In diffusion combustion, much weaker AFRs can be used (i.e., an excess of
air) because around each fuel droplet will be a range of flammable AFRs.
• Typical ranges for the (gravimetric) air-fuel ratio are as follows:
10
1

102. Diesel engines have a higher maximum
efficiency than spark ignition engines for three
reasons:
• The compression ratio is higher.
• During the initial part of compression, only air
is present.
• The air-fuel mixture is always weak of
stoichiometric.
10
2

103. Simple Combustion Equilibrium
• For a given combustion device, say a piston engine, how
much fuel and air should be injected in order to completely
burn both? This question can be answered by balancing the
combustion reaction equation for a particular fuel. A
stoichiometric mixture contains the exact amount of fuel
and oxidizer such that after combustion is completed, all the
fuel and oxidizer are consumed to form products.
10
3

104. • Combustion stoichiometry for a general hydrocarbon fuel, with
air can be expressed as;
• The amount of air required for combusting a stoichiometric
mixture is called stoichiometric or theoretical air.
10
4

105. Methods of Quantifying Fuel and Air Content
of Combustible Mixtures
• In practice, fuels are often combusted with an amount of air
different from the stoichiometric ratio. If less air than the
stoichiometric amount is used, the mixture is described as fuel
rich. If excess air is used, the mixture is described as fuel lean.
For this reason, it is convenient to quantify the combustible
mixture using one of the following commonly used methods:
• Fuel-Air Ratio (FAR): The fuel-air ratio, f, is given by
10
5

106. • Equivalence Ratio: Normalizing the actual fuel-air ratio by the
stoichiometric fuel air ratio gives the equivalence ratio,
• The subscript s indicates a value at the stoichiometric
condition. f <1 is a lean mixture , f¼1 is a stoichiometric
mixture, and f >1 is a rich mixture
• Lambda is the ratio of the actual air-fuel ratio to the
stoichiometric air-fuel ratio defined as
10
6

107. Fuel Requirements
• Gasoline is a mixture of hydrocarbons (with 4 to
approximately 12 carbon atoms) and a boiling point range of
approximately 30-200°C. Diesel fuel is a mixture of higher
molarmass hydrocarbons (typically 12 to 22 carbon atoms),
with a boiling point range of approximately180-380°C. Fuels
for spark ignition engines should vaporize readily and be
resistant to self-ignition, as indicated by a high octane rating.
In contrast, fuels for compression ignition engines should self-
ignite readily, as indicated by a high cetane number.
10
7

108. • Octane number is a standard measure of the anti-knock
properties (i.e. the performance) of a motor or aviation fuel.
The higher the octane number, the more compression the fuel
can withstand before detonating. In broad terms, fuels with a
higher octane rating are used in high-compression engines that
generally have higher performance.
• Knocking (also called knock, detonation, spark knock, pinging
or pinking) in spark-ignition internal combustion engines
occurs when combustion of the air/fuel mixture in the cylinder
starts off correctly in response to ignition by the spark plug,
Effects of engine knocking range from inconsequential to
completely destructive.
.
10
8

109. • Cetane number or CN is a measurement of the combustion
quality of diesel fuel during compression ignition. It is a
significant expression of diesel fuel quality among a number of
other measurements that determine overall diesel fuel quality.
10
9

110. • The octane or cetane rating of a fuel is established by
comparing its ignition quality with respect to reference fuels in
CFR (Co-operative Fuel Research) engines, according to
internationally agreed standards. The most common type of
octane rating worldwide is the Research Octane Number
(RON). RON is determined by running the fuel in a test engine
with a variable compression ratio under controlled conditions,
and comparing the results with those for mixtures of iso-
octane and n-heptane.
11
0

111. Engine Knock and thermal Efficiency of an Engine
The thermal efficiency of the ideal Otto cycle increases
with both the compression ratio and the specific heat
ratio.
 When high compression ratios
are used, the temperature of the
air-fuel mixture rises above the
autoignition temperature
produces an audible noise,
which is called engine knock.
(antiknock, tetraethyl lead? 
unleaded gas)
 For a given compression ratio, an ideal Otto cycle using
a monatomic gas (such as argon or helium, k = 1.667) as
the working fluid will have the highest thermal efficiency.
11
1

112. Charge Stratification
11
2

113. Combustion Chamber Designs
11
3

114. Combustion Chamber Design
11
4

115. Combustion Chamber Design
11
5

116. Combustion Chamber Design
11
6

117. Combustion Chamber Design
11
7

118. Combustion Chamber Design
11
8

119. Turbocharging
• A turbocharger, or turbo, is a centrifugal compressor
powered by a turbine that is driven by an engine's exhaust
gases. Its benefit lies with the compressor increasing the
mass of air entering the engine (forced induction), thereby
resulting in greater performance (for either, or both, power
and efficiency). They are popularly used with internal
combustion engines (e.g., four-stroke engines like Otto
cycles and Diesel cycles).
11
9

120. Engine Artificial Respiratory System: An Inclusion of
CV
Turbo-Charged Engine 12
0

121. Turbo -Charger
12
1