Transcript two stroke engines

BASIC MECHANICAL ENGINEERING
Unit-1 part-1
Thermodynamics
BASIC CONCEPTS OF THERMODYNAMICS
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The term Thermodynamics is derived from the Greek words ‘Thermic’ which
means Heat and ‘Dynamics’ which means Force.
Thermodynamics is the science that deals with the relationship of heat and
mechanical energy and conversion of one into the other.
Eg: Human body, solar heaters, sun, iron box etc..
TD began in 19th Century to exploit the motive power of heat – the capacity of
hot bodies to provide work.
The study of TD is based on two general laws of nature, the First law and the
second law of TD.
First Law : Heat and work are two mutually convertible forms of energy.
Heat never flows on its own from an object at low temperature to an object at
higher temperature, this statement is basis for the second law.
Second Law : Heat energy of the source cannot be converted continuously to
work but part of it has to be rejected to sink at lower temperature.
MICROSCOPIC & MACROSCOPIC
ANALYSIS
Behavior of matter can be studied in two viewpoints:
Microscopic Analysis : Behavior of individual atoms or molecules.
Behavior of gas is described by summing up behavior of each
molecule (Statistical Thermodynamics).
Eg: Study of Atomic Structure in nuclear physics.
Macroscopic Analysis : Behavior of more number of molecules is
taken into account. It is also known as Engineering
Thermodynamics.
Eg : Force on a given area can be measured by Pressure gauge,
Measurement of pressure, volume and temperature.
WORKING SUBSTANCES: Conversion of heat energy in to work and vice-versa takes
place through the agency called working substances.
PHASE:
A quantity of matter, homogenous in chemical composition and physical structure, is
called a Phase.
PURE SUBSTANCE:
A pure substance is the one that has a homogeneous and invariable chemical
composition even though there is change in phase.
Eg: Steam or water or a mixture of steam and water.
HOMOGENEOUS: A system which consists of a single Phase.
Eg: Air and water vapour.
HETEROGENEOUS: A system which consists of two or more phases.
Eg: Ice and water, water and oil.
SYSTEM:
A system is a finite quantity of matter or prescribed region of space.
BOUNDARY:
It is a real or imaginary envelope enclosing a system.
SURROUNDINGS:
Everything external to the system.
UNIVERSE:
System and its surroundings together comprise a system.
There are three types of System:
•Closed System
•Open System
•Isolated System
•A closed system can exchange energy, but not mass, with its surroundings.
Eg: Piston-cylinder
•An open system can exchange both mass and energy with its surroundings.
Eg: Air Compressor.
•An isolated system cannot exchange mass or energy with its surroundings.
Eg- Universe, Flask…. etc….
THERMODYNAMIC PROPERTIES, PROCESSES AND
CYCLES
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STATE: It is defined as condition of the system at a point.
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PROPERTIES: It is defines as the characteristics of the system. (OR)
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A property is any observable or measurable characteristics of a substance.
Eg:- Mass, volume, density, pressure, temperatures, height, width, etc……
TYPES OF PROPERTIES: 3Types: Intensive, Extensive and Specific Property.
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INTENSIVE PROPERTY: This type of Property doesn’t depends on mass.
Eg:- Pressure, Temperature, Density, Sp. Volume, Sp. Entropy, Area, Velocity,
Viscosity….etc…..
EXTENSIVE PROPERTY: This type of Property depends on mass.
Eg:- Volume, Energy, Entropy, Weight….etc………
SPECIFIC PROPERTY: The Extensive property per unit mass is called as
specific
properties.
Eg:- Sp. Volume, Sp. Gravity, Sp. Energy, Sp. Density….etc…….
PROCESS
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Change in state (OR)
A process occurs whenever a system changes from one state to
another state (OR)
When a system changes from one equilibrium state to another
equilibrium state, it is said to have undergone a process.
 No System can be in true equilibrium during the process, since
the properties are changing.
 The continuous series of equilibrium states through which the
system passes for reaching from initial state to final state is
known as PATH. So, when a path is completely specified, it is
called as process.
Types: Reversible Process and
Irreversible Process
QUASI-STATIC PROCESS
Quasi-static process is one that takes place so slowly that the
system may be considered as passing through a succession of
equilibrium states.
 This process may be represented by a path (or line) on the
equation of state surface.
 For non-quasi-static, only the end-points can be shown.
REVERSIBLE PROCESS: The system of the surrounding restore to their original state. (OR)
 A process is reversible if the system is in thermal equilibrium at
all states.(OR)
 In other words, a process is reversible, when the initial state
together with all energies transformed during the process can be
completely restored in both system and surroundings.
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REVERSIBLE PROCESS
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It is also called Quasi-static process.
A Quasi-static Process is one which can be stopped at any state and
reversed, so that the system and surroundings are exactly restore to
their initial states.
The process has following characteristics:
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It must pass through the same states on the reversed path as were initially
visited on the forward path.
It must pass through a continuous series of equilibrium states.
The process must be free from friction (internal and mechanical friction)
Working fluid should be ideal
There should be no heat transfer across a finite temperature difference.
At the end of the process, both system and surrounds should reach to the
initial state.
Eg: Elastic deformation, isothermal expansion (or) compression,
expansion and compression of spring.
IRREVERSIBLE PROCESS:A process is irreversible when the system passes through a
sequence of non-equilibrium states at which the process will not
have a unique value.
 An irreversible process is one in which heat is transferred to a
finite temperature or finite gradient.
 The irreversible process are represented by a broken line and the
end states are assumed to be in equilibrium.
 It is irreversible as the surroundings will not reach the initial state
accordingly.
Eg: fluid flow in a pipe, combustion of air and fuel, diffusion of
gases, free expansion….etc….
THERMODYNAMIC CYCLE:
Cycle is defined as series of process whose end states should be
identical.
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EQUILIBRIUM
A system is said to be in equilibrium if it does not tend to undergo
any change. (OR)
 A system is said to be in thermal equilibrium if the temperature
and pressure at all points are same.
 Systems under temperature and pressure equilibrium but not under
chemical equilibrium are said to be in meta-stable equilibrium
condition.
STAGES OF EQUILIBRIUM:
 Mechanical equilibrium
 Chemical equilibrium
 Thermal equilibrium and
 Thermodynamic equilibrium.
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MECHANICAL EQUILIBRIUM:
For a system to be in mechanical equilibrium there should be any pressure
unbalancing either in the interior of the system or between the system and the
surroundings.
CHEMICAL EQUILIBRIUM:
For a system to be in chemical equilibrium there should be equality of chemical
potential, i.e., there should not be any chemical reactions.
THERMAL EQUILIBRIUM:
For a system to be in thermal equilibrium there should not be any temperature
gradient in the system.
For a system to be in thermal equilibrium it is not necessary that the system
should be in mechanical and chemical equilibrium.
THERMODYNAMIC EQUILIBRIUM:
When a system satisfies the conditions of mechanical equilibrium, chemical
equilibrium and thermal equilibrium, it is said to be in a state of thermodynamic
equilibrium.
POINT FUNCTION:
When two properties locate a point on the graph (coordinate axis) then those properties are
called as Point Function.
Eg: Pressure, temperature, volume, etc..
PATH FUNCTION:
There are certain quantities which cannot be located on a graph by a point but are given by
the area under the process. Such quantities are called path functions.
Eg: Work and Heat.
HEAT:
It is defined as the form of energy that is transferred between two systems due to
temperature difference between them.
The transfer of heat in to the system is called heat addition (+ve) and heat out a system is
called heat rejection (-ve).
WORK:
Work is defined as energy expanded by a force through a displacement .
The work transferred in to the system is indicated by (-ve sign) and the work output by a
system is indicated by (+ ve sign).
W = F.S
HEAT & WORK
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Similarities between work and heat:
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Boundary Phenomenon
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Both are Path Functions
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Both are interchangeable
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Both are inexact differentials and hence are not thermodynamic properties.
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Differences between work and heat :
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There cannot be reversible work transfer but there is no restriction for transfer of heat.
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For transfer of heat, temperature difference is needed.
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Sign Convection is +ve for Qin and Wout
-ve for Qout and Win
PDV WORK OR DISPLACEMENT WORK
Displacement work (pdVwork)
Force exerted, F= p. A
Work done
dW= F.dL= p. A dL= p.dV
If the piston moves through a finite distance say 1-2,Then work done has to be evaluated
by integrating δW=∫pdV
TEMPERATURE:
It is an intensive thermodynamic property related to the “hotness” or “coldness”
of a body measured on a definite scale.
Eg: Thermometer.
ADIABATIC PROCESS:
A process in which no heat crosses the boundary of the system is called an adiabatic
process.
A wall which is impermeable to the flow of heat is an adiabatic wall, where as a wall
which permits the flow of heat is a diathermic wall.
ZEROTH LAW OF THERMODYNAMICS:
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When a body ‘A’ is in thermal equilibrium with body
‘B’ and also separately with body ‘C’ then B and C will
be in thermal equilibrium with each other.
If two systems are in thermal equilibrium with a third
system, they must be in thermal equilibrium with
each other.”
PROPERTIES OF IDEAL GASES:
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A gas is a substance which cannot be liquefied by application
of pressure at constant temperature.
A vapour is a gaseous substance which can be liquefied by
applying pressure at constant temperature.
The laws of perfect gas does not apply to vapour. Substance
like air, dry or superheated steam are treated as gas.
Boyle’s Law:
Boyle’s Law states that when any gas is heated at constant
temperature, the pressure and volume of the gas are inversely
proportional.
Charle’s Law:
Charle’s law states that when any gas is heated at constant pressure, its change in
volume varies directly with the absolute temperature change.
FIRST LAW OF THERMODYNAMICS
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Law of conservation of energy.
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First law of TD:
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“During any cycle that a closed system undergoes, the
net work transfer is equal to the net heat transfer.”
Specific Heat.
Joule’s Law:
Joules law states that “ The internal energy of a perfect
gas is a function of the absolute temperature.”
Relation ship between two specific heats.
Enthalpy:
Sum of internal energy and pressure volume product (pv)
is called Enthalpy (h).
h=u+pv
Ratio of specific heats
APPLICATION OF FIRST LAW OF TD TO CLOSED
SYSTEM OR NON-FLOW SYSTEM
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Reversible constant volume or Isochoric process.
Reversible constant Pressure or Isobaric Process.
Reversible constant Temperature or Isothermal Process.
Reversible Adiabatic Process or Isentropic Process.
PERPETUAL MOTION MACHINE OF FIRST KIND OR PMM1
o There can be no machine which would continuously supply mechanical
work without some form of energy disappearing simultaneously such a
fictitious machine is called a perpetual motion machine of first kind or
PMM1.
o The converse of is also true, that there can be no machine which would
continuously consume work without some other form of energy appearing
simultaneously.
FLOW PROCESS (OPEN SYSTEM)
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In flow process there is continuous flow of mass and energy in and
out of the system. Eg: flow of air, gas through a compressor, flow of
fluid through a pipe, etc..
Flow process is classified as : 1) Steady flow process
2) unsteady flow process
STEADY FLOW PROCESS: Flow rate of mass and energy
doesn’t vary with time.
UNSTEADY FLOW PROCESS: Flow rate of mass and energy
vary with time.
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STEADY FLOW ENERGY EQUATION (S.F.E.E)
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LIMITATIONS OF THE FIRST LAW OF THERMODYNAMICS
SECOND LAW OF THERMODYNAMICS
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According to second law of thermodynamics the whole
heat energy cannot be converted into work and part of
energy must be rejected to the surroundings.
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Complete conversion of low grade energy into high grade
energy in a cycle is impossible.
CLAUSIUS STATEMENT
It is impossible for a self acting machine working in a
cyclic process unaided by any external agency, to convey
heat from a body at lower temperature to a body at
higher temperature.
Ex: For a heat pump, it cannot operate without input of
work.
Refrigerator.
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KELVIN PLANCK STATEMENT
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It is impossible to construct an engine, which while operating in a
cycle produce no other effect except to extract heat from a single
reservoir and produce work.
Ex: Heat Engine.
PERPETUAL MOTION MACHINE OF SECOND
KIND OR PMM2
Without violating the first law, a machine can be imagined which
would continuously absorb heat from a single thermal reservoir
and would convert this heat completely into work, the efficiency
of such a machine would be 100%. This machine is called
perpetual motion machine of second kind.
 A machine of this kind will evidently violates the second law of
thermodynamics.
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CARNOT CYCLE
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The Carnot cycle is a reversible cycle. It was proposed by a
French military engineer Nicolas Sadi Carnot. It comprises four
reversible process namely
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Reversible isothermal expansion
 Reversible adiabatic expansion
 Reversible isothermal compression
 Reversible adiabatic compression
It is the most efficient cycle, as it involves no losses. The theoretical heat engine that
operates on this cycle is called Carnot engine.
Process 1-2:Reversible isothermal heat addition at high temperature,
TH> TC, to the working fluid in a piston-cylinder device that does
some boundary work.
Process
2-3:Reversible adiabatic expansion during which the
system does work as the working fluid temperature decreases from
TH to TC.
Process 3-4:The system is brought in contact with a heat reservoir
at TC< TH and a reversible isothermal heat exchange takes place
while work of compression is done on the system.
Process 4-1:A reversible adiabatic compression process increases the
working fluid temperature from TC to TH.
AIR STANDARD CYCLES
The power cycles can be classified into two important fields.
 The first is the power generation which the work done output of
the system such as Heat Engine.
 The second is the refrigeration and air conditioning which the
work done input to the system such as Heat Pump.
ASSUMPTIONS:
 The working fluid is air, which continuously circulates in a closed
loop and always behaves as an ideal gas.
 All the processes that make up the cycle are internally reversible.
 The exhaust process is replaced by a heat-rejection process that
restores the working fluid to its initial state.
 The combustion process is replaced by a heat-addition process
from an external source.
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OTTO CYCLE
Process 1–2 is an isentropic compression of the air as the piston
moves from bottom dead center to top dead center.
 Process 2–3 is a constant-volume heat transfer to the air from an
external source while the piston is at top dead center. This process
is intended to represent the ignition of the fuel–air mixture and the
subsequent rapid burning.
 Process 3–4 is an isentropic expansion (power stroke).
 Process 4–1 completes the cycle by a constant-volume process in
which heat is rejected from the air while the piston is at bottom
dead center.
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DIESEL CYCLE
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Process 1–2 is an isentropic compression of the air as the
piston moves from bottom dead center to top dead center.
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Process 2–3 is a constant-Pressure heat transfer to the air from
an external source while the piston is at top dead center.
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Process 3–4 is an isentropic expansion (power stroke).
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Process 4–1 completes the cycle by a constant-volume process
in which heat is rejected from the air while the piston is at
bottom dead center.
BASIC MECHANICAL ENGINEERING
INTERNAL COMBUSTION ENGINES
UNIT-1 PART-2
HEAT ENGINE
Input Energy
Energy
Chemical
Energy/
Heat
Device
Output
(Mechanical Power)
HEAT ENGINES
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Heat Engine converts thermal energy into mechanical
energy.
Heat Engine can be classified as:
Internal Combustion Engine (I.C Engine)
External Combustion Engine (E.C Engine)
IC ENGINES CLASSIFICATION
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Design
Cycle of operation
Number of strokes
Type of Ignition
Type of cooling
Method of charging
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Number of cylinders
Type of cylinder
Type of Fuel used
Fuel Supply
Speed Required
Application
IC ENGINES CLASSIFICATION
1) Based on Design: Reciprocating engines, Rotary engines
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Internal combustion engine, External combustion engine
2) Based on cycle of operation:
Otto cycle, Diesel Cycle, Dual Cycle
3) Based on number of strokes:
2 stroke, 4 Stroke
4) Type of Ignition:
Spark Ignition, Combustion Ignition
5) Type of Cooling:
Air Cooling, Water Cooling
6) Method of Charging:
Naturally aspirated, Super Charged
IC ENGINES CLASSIFICATION
7) Number of cylinders:
Single Cylinder, Multi Cylinder
8) Type of Cylinder:
Inline, Radial, U-type, H-type etc.
9) Type of Fuel Used:
Petrol, Diesel, Dual, Gas
10) Fuel Supply:
carburetor, Fuel Injector
11) Speed:
Low, Medium, High
12) Applications
Marine, Auto mobile, locomotives, aircrafts..etc...
RECIPROCATING & ROTARY ENGINES
IN-LINE ENGINE :-
V-TYPE :-
OPPOSITE CYLINDER :-
RADIAL ENGINE :-
Different Cylinder Arrangements in Multi-Cylinder Engines
BASIC ENGINE COMPONENTS
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Cylinder
Piston
Cylinder Head
Connecting Rod
Crankshaft
Piston Rings
Gudgeon Pin
Inlet& exhaust Valve
Spark Plug
Crankcase
Camshaft
Cam
NOMENCLATURE
 Cylinder
Bore (d)
 Piston Area (A)
 Stroke (L)
 Dead Centers
T.D.C and B.D.C
 Displacement or Swept Volume (Vs)
 Clearance Volume (Vc)
 Compression Ratio (r)
WORKING PRINCIPLES OF IC ENGINES
FOUR STROKE ENGINES :
Four Stroke Spark Ignition (SI) Engine
Four Stroke Compression Ignition (CI) Engine
 TWO STROKE ENGINES:
Two Stroke SI Engines
Two Stroke CI Engines
SPARK IGNITION (SI) ENGINES
 Spark Is Generated Through An External Source
COMPRESSION IGNITION (CI) ENGINES
 Air is heated to a sufficiently high temperature because of high
compression ratio. the fuel gets self-ignited on injection as finely
atomized spray.
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FOUR STROKE SPARK IGNITION (SI) ENGINE
In four stroke engine the cycle of operation is completed in 4
strokes of piston or 2 revolutions of crankshaft.
 During four strokes five events to be completed
1)Suction
2)Compression
3)Combustion
4)Expansion
5)Exhaust
 Each stroke consists of 180° of crank shaft rotation. Hence four
stroke cycle is completed through 720° of crank shaft rotation.
 Ideal four stroke engine consists of following 4 strokes.
1. Suction or Intake stroke
2. Compression stroke
3. Expansion or power stroke
4. Exhaust stroke
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The SI engines (petrol and gas engines) are used in passenger Cars,
Motor Cycles, Air crafts, Agricultural equipments…etc…
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As piston moves for TDC to BDC, the inlet valve gets opened while exhaust
valve remains closed and fresh air-fuel mixture enters the cylinder.
As piston moves for BDC to TDC, both inlet and exhaust valves remain closed
and air-fuel mixture inside cylinder gets compressed.
Highly compressed air-fuel mixture is available inside the cylinder and the spark
plug is activated and it releases spark for igniting air-fuel mixture.
Sudden increase in pressure and temperature, the combustion products try to
expand and piston moves from TDC to BDC.
During this travel the inlet and exhaust valves remain closed.
This is the stroke accompanied by positive work available at shaft.
While piston is at BDC the exhaust valve gets opened and combustion products
are exhausted out while piston travels from BDC to TDC.
Out of suction, compression, expansion and exhaust strokes only expansion
stroke is accompanied by the production of positive work,
Rest three strokes are work absorbing strokes.
Work requirement for the three strokes is met from the work available during
expansion stroke.
Cycle gets completed in two revolutions of crankshaft.
4 STROKE ENGINE
WORKING PRINCIPLE OF 4 STROKE SI ENGINE
Process in Otto Cycle
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Isentropic Compression
Constant Volume Heat
Addition
Isentropic Expansion
Constant Volume Heat
Rejection
Process in SI Engine
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Suction/ Intake Stroke
Compression Stroke
or
Isentropic Compression
Combustion at Constant
Volume
Power Stroke or Isentropic
Expansion
Exhaust Stroke
1.
2.
3.
4.
Isentropic Compression
Constant Volume Heat Addition
Isentropic Expansion
Constant Volume Heat Rejection
• Process in Otto Cycle
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Suction Or Intake Stroke (0  1)
Compression Stroke ( 1  2)
+ Burning (2  3)
Expansion Or Power Stroke (3  4)
Exhaust Stroke (4 5).
FOUR STROKE COMPRESSION IGNITION
(CI) ENGINE
The four stroke CI engine is similar to four stroke SI engine, but
operates at high compression ratio.
 The compression ratio of SI engine varies from 6 to 10, while the
compression ratio of CI engine varies from 16 to 22.
 In the CI engine during suction stroke air instead of fuel air
mixture is inducted.
 The spark plug of SI engine is replaced with fuel injector.
 The sequence in a 4-stroke CI engine (Diesel engine) are:
1. Suction or Intake stroke
2. Compression stroke
3. Expansion or power stroke
4. Exhaust stroke
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Piston travels from TDC to BDC and air is sucked into the
cylinder. Here inlet valve is open and exhaust valve is closed.
 Piston travels from BDC to TDC, while air is compressed with
inlet and exhaust valves closed.
 Fuel injector injects fuel into compressed air for certain duration.
Ignition of fuel also takes place simultaneously as air temperature
is much higher than self-ignition temperature of fuel.
 Burning of fuel results in release of chemical energy, increasing
the pressure, which forces piston to travel from TDC to BDC.
 This process is expansion process and piston comes down to BDC
with both inlet and exhaust valves closed.
 Piston travels up to TDC with exit valve open. During this piston
travel burnt gases are expelled out of cylinder. This stroke is
Exhaust stroke.
 General arrangement in CI engine is similar to that of SI engine
with spark plug replaced by fuel injector.
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WORKING PRINCIPLE OF 4 STROKE CI ENGINE
Process in Diesel Cycle
1.
2.
3.
4.
Isentropic Compression
Constant Pressure Heat
Addition
Isentropic Expansion
Constant Volume Heat
Rejection
Process in CI Engine
1.
2.
3.
4.
5.
Suction/ Intake Stroke
Compression Stroke
or
Isentropic Compression
Combustion at Constant
Pressure
Power Stroke or Isentropic
Expansion
Exhaust Stroke
• Process in Diesel Cycle
1.
2.
3.
4.
Isentropic Compression
Constant Pressure Heat Addition
Isentropic Expansion
Constant Volume Heat Rejection
• Process in CI Engine
i) Suction Stroke Air Alone Inducted (0  1)
ii) Compression Stroke  Air Compressed Into Clearance Volume (1  2)
iii) Expansion Stroke  Fuel Injection Maintaining Constant Pressure During
Combustion (2  3) + Expansion (3 4)
iv) Exhaust Stroke  Exhaust Gases Pushed Out (4  5) .
TWO STROKE SI ENGINE
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Douglas Clarke Invented The 2 Stroke Engine In 1878
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In this 2 stroke SI engine the cycle Is completed In one revolution of
the Crank Shaft or in two stroke of the piston.
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The main difference between 2-stroke and 4-stroke engine is in the
method of filling the fresh charge and removing burnt gases from the
cylinder.
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In the 4 stroke engine these operations are performed by the piston
during the suction and exhaust strokes respectively.
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In a 2 stroke, the filling process is accomplished by the charge
compressed in the crank case. The induction of the compressed
charge moves out the product of combustion through exhaust ports.
Therefore, no piston strokes are required for these two operations.
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Two strokes are sufficient to complete the cycle, one for compressing
the fresh charge and the other for expansion or power stroke.
Construction of 2 Stroke SI Engine
a. Compression/ Ignition b. Expansion
and
c. Exhaust
Working of a Two-stroke Gasoline Engine
FOUR EVENTS OF 2-S ENGINE
2 STROKE ENGINE
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When piston moves from BDC to TDC then the suction port gets
uncovered and fresh mixture enters and goes into crank case.
Piston moving from TDC to BDC and during covered position of
suction port the mixture gets transferred to the top of piston through
transfer port.
During piston travelling from BDC to TDC, the air fuel mixture on
top of piston gets compressed and subsequently gets ignited by spark
from spark plug.
Release of excessive energy which forces piston to move from TDC
to BDC and simultaneously as piston uncovers exhaust port the burnt
gases go out through exhaust port.
Suction and compression, both processes get completed during travel
of piston from BDC to TDC
Expansion and exhaust processes occur during travel of piston from
TDC to BDC along with transfer of fresh fuel air mixture from
crankcase to top of piston.
Here all four processes occur during two strokes and one revolution
of crank shaft.
IDEAL INDICATOR DIAGRAM OF A TWO STROKE SI ENGINE
TWO STROKE CI (OR) DIESEL ENGINE
• More Advantageous Than Two Stroke SI Engine.
• No Loss Of Fuel With Exhaust Gases As The Intake Charge
Is Only Air.
• Hence Many Of The High Output Diesel Engines Work On
This Cycle.
• A General Disadvantage Common To Both Two Stroke
Gasoline And Diesel Engines Is Greater Cooling And
Lubricating Oil Requirements Due To One Power Stroke Per
Crank Shaft Rotation And Higher Temperatures.
• Results In Higher Consumption Of Lubricating Oil.
TWO STROKE DIESEL ENGINE
During piston travel from BDC to TDC air enters crankcase.
 When piston reaches TDC and reverses its motion to BDC, air in
crankcase gets partly compressed and is transferred from
crankcase to top of piston through transfer port.
 Piston motion from BDC to TDC, the compression of air occurs
by the top side of piston while on the bottom side of piston air
again enters into crankcase
 Upon piston reaching TDC fuel is injected into compressed air
which is at high temperature and pressure.
 The energy released causes piston to go back from TDC to BDC,
i.e. the expansion process.
 As piston reaches BDC it simultaneously forces air in crank case
to get transferred to cylinder space and forces burnt gases out of
cylinder i.e. exhaust process.
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top of the piston has usually a projection to deflect the
fresh charge towards the top of the cylinder before flowing
through the exhaust ports.
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has the dual purpose of
A. Scavenging
the upper part of the cylinder of the
combustion products and
B. Preventing
the fresh charge from flowing directly
towards the exhaust ports.
 Scavenging
is the process of pushing exhausted gascharge out of the cylinder and drawing in a fresh draught
of air or fuel/air mixture
COMPARISON OF SI AND CI ENGINES
Sl.No DESCRIPTION
SI ENGINE
CI ENGINE
1
Thermodynamic
cycle.
Otto cycle.
Diesel or dual cycle.
2
Fuel
Petrol or gas (costly
fuel).
Diesel oil (cheaper fuel).
3
Fuel supply
Requires carburetor.
Requires Fuel injector.
4
Compression ratio 6 to 10 (average 7-8).
12 to 22 (average 16-17).
5
Combustion or
Spark ignition
Ignition of charge.
Compression ignition
6
Thermal
efficiency
Lower thermal
High thermal efficiency
efficiency due to low
due to high compression
compression ratio 25%. Ratio 40%.
7
Weight
Lighter due to low
compression ratio and
low peak pressure.
Heavier due to high
compression ratio and
high peak pressure.
Sl.No
DESCRIPTION
SI ENGINE
CI ENGINE
8
Speed, operation
High speed (2000-6000
rpm)
Low speed (400rpm),
Medium speed (4001200),
High speed (1200-3500).
9
Noise and
vibration
Less
More
10
Pollution
Less
More
11
Cost
Cost is low, maintenance
is costly due to high cost
of fuels.
Initial cost is high.
12
Applications
Develops less power and
generally used for light
duty vehicles (mopeds,
cars, aircrafts..etc)
Heavy duty vehicles.
Tractors, trucks,
locomotives, buses..etc.
ADVANTAGES OF SI ENGINES OVER CI ENGINES
•
•
•
•
•
•
•
•
Initial cost is less
For a given output it is lighter in weight and occupies less
space
Due to low compression it is easy to start
Flotation of speed is minimum and thus requires light
flywheel
Limitations:
Requires costly fuels and maintenance cost is high
Chances of pre ignition of charge are more
Less dependable
Suitable only for light duty vehicles
COMPARISION OF 2-STROKE AND 4-STROKE
CYCLE ENGINE
SL.NO
2-STROKE ENGINE
4-STROKE ENGINE
1
All events are completed in 2-strokes of
the piston or 1 revolution of crankshaft
i.e., there is one power stroke for every
revolution of crank shaft.
All events are completed in 4-strokes
of the piston or 2 revolution of
crankshaft i.e., there is one power
stroke for every 2 revolution of crank
shaft.
2
Doesn’t have valves and only ports are Contains valves and the valves are
provided. There ports are closed by actuated by cam mechanism.
piston.
3
The charge first enters the crankcase, and The charge is directly admitted into
therefore crank is made gas tight.
the engine cylinder.
4
Torque is more uniform, requires lighter Torque is not uniform,
flywheel.
heavier flywheel.
5
Volumetric efficiency is low due to lesser Volumetric efficiency is more due to
time for suction.
more time for suction.
6
Thermal efficiency is low.
Thermal efficiency is high.
requires
SL.NO
2-STROKE ENGINE
4-STROKE ENGINE
7
Specific fuel consumption is more due Specific fuel consumption is less.
to fuel loss through exhaust.
8
Elimination of suction & exhaust More frictional loses.
strokes minimizes the frictional loses.
9
Consumes more lubricating oil.
Consumes less lubricating oil.
10
More noisy in operation.
Less noisy.
11
Initial cost is less.
Initial cost is more.
12
Higher rate of wear and tear.
Lower rate of wear and tear.
13
Generally employed in light duty Employed in heavy duty vehicles such
vehicles such as mopeds, scooters, as cars, buses, trucks, tractors, power
motor cycles, etc..
generation, industrial engines, aero
planes etc..
COOLING SYSTEMS

1.
2.
3.
4.
Reasons for providing cooling system
The even expansion of piston in cylinder may result in
seizure of piston.
High temperatures reduced strength of piston and
cylinder liner.
Overheated cylinder may lead to Pre ignition of the
charge, in case of SI engine.
Physical and chemical changes may occur in
lubricating oil which may cause stocking of the piston
rings and excessive wear of cylinder. of lubricant.
There are mainly two methods of cooling IC engine.
1)
Air cooling or Direct cooling.
2)
Liquid cooling or Indirect cooling.
AIR COOLING
Heat is carried away by the air flowing over and
around the engine cylinder
 Used in two wheelers,
 Fins are cut on the cylinder

ADVANTAGES:
Simple cooling system
 No danger of coolant leakage
 No trouble of freezing of coolant
 Installation is easier
 No external components like tank and radiator

DISADVANTAGES:
 Their movement is noisy
 Non uniform cooling
 Output of engine is less
 Maintenance is not easy
 Applicable for smaller compression ratio
LIQUID COOLING
Cylinder head and wall are provided with jackets
through which the cooling liquid is circulated
 Heat is transferred by means of conduction and
convection
 Heated liquid rejects heat to air by air cooled radiator
system.

ADVANTAGES:
Fuel consumption is low
 Cooling system can be located where ever required
 Higher heat transfer rate is achieved
 Compact design of engine

DISADVANTAGES:
 Continuous supply of water is required
 In case of failure serious damage may be caused to the engine
 Cost of system is high
 Maintenance of various parts of system
LUBRICATION SYSTEM

Lubrication is the admittance of oil between two surfaces having
relative motion

Purpose of lubrication
Reduce friction and wear
Cooling of surface (heat due to friction)
Seal space between adjoining surfaces
Clean the surface by carrying away the carbon and metal
particles caused by wear
1.
2.
3.
4.
PROPERTIES:
Fire point
Lowest temp at which oil burns continuously
•
Cloud point
Temp at which change of state takes places
•
Pour temperature
Temp at which the oil will pour. Ability to move at low temp
•
Oiliness
Property which enables oil to spread over
•
Corrosion
Should not corrode the working parts
•
Physical and chemical stability
Physically and chemically stable
temperatures
•
• Adhesiveness
Oil particles stick to the metal surface
•Specific
gravity
It is the measure of density of oil
between
operating
TYPES OF LUBRICATION SYSTEM
A. Wet sump lubrication
1.
Splash
2.
Semi pressure system or Pressure Fed or Force Feed.
3.
Full pressure system or Combination Pressure Fed and
Splash.
B. Dry sump lubrication
C. Mist lubrication
1.
Gasoline Oil Premix
ENGINE PERFORMANCE PARAMETERS
Sl. No.
Parameter
Notation
i.
Indicated Thermal Efficiency
ηith
ii
Brake Thermal Efficiency
ηbth
iii
Mechanical Efficiency
ηm
iv
Volumetric Efficiency
ηv
v
Relative Efficiency/ Efficiency Ratio
ηrel
vi
Mean Effective Pressure
pm
vii
Specific Power Output
Ps
viii
Specific Fuel Consumption
sfc
Ix
Fuel-Air or Air-Fuel Ratio
F/A or A/F
x
Calorific value of the Fuel
CV (HCV/ LCV)
ENGINE PERFORMANCE PARAMETERS
The engine performance s indicated by the term η.
 Calorific value: It is the thermal energy released per unit quantity
of the fuel when the fuel is burned completely and the products of
combustion are cooled back to the initial temperature at the
combustion mixture.
 Indicated power (I.P): The total power developed by combustion
of the fuel in the combustion chamber is called indicated power.
 Break power (B.P): The power developed by an engine at the out
put shaft is called break power. The difference between I.P and B.P
is called frictional power (F.P).
F.P=I.P-B.P
 Indicated Thermal Efficiency ηith : It is the ratio of energy in the
indicated power, to the input fuel energy in appropriate units.

ENGINE PERFORMANCE PARAMETERS
Break thermal efficiency ηbth : it is the ratio of energy in the brake
power, BP to the input fuel energy in appropriate units.
 Mechanical efficiency ηm : it is defined as the ratio of brake power
(delivered power) to the indicated power (power provided to the
piston) Or
It can also be defined as the ratio of the brake thermal efficiency to
the indicated thermal efficiency.
 Volumetric efficiency ηv : it is the ratio of the volume of air
actually inducted at ambient conditions to the swept volume of the
engine.
 Relative efficiency or Efficiency ratio: it is the ratio between
actual thermal efficiency to the ideal efficiency (air standard
efficiency) of the cycle employed.

ENGINE PERFORMANCE PARAMETERS
Mean effective pressure: it is the average pressure inside the
cylinders of an I.C engine based on the measured power output.
 Specific power out put: it is defined as the break output per unit
of piston displacement.
 Fuel-air ratio: it is the ratio of the mass of the fuel to the mass of
the air in the air fuel mixture.
 Specific fuel consumption: it is the mass of the fuel consumed
per KW developed per hour and is a criterion of economical
power production.
