spark egnition engine

INTERNAL COMBUTION ENGINE
Made by:
Assistant Professor : NAPHIS AHAMAD
MECHANICAL ENGINEERING
6/10/2017 Naphis Ahamad (ME) JIT 1

6/10/2017 Naphis Ahamad (ME) JIT 2
UNIT II

SPARK IGNITION ENGINE
Air-fuel ratio requirements, Design of carburetor –fuel jet size and
venture size, Stages of combustion-normal and abnormal
combustion, Factors affecting knock, Combustion chambers,
Introduction to thermodynamic analysis of SI Engine combustion
process.
6/10/2017 NAPHIS AHAMAD (ME) JIT 3

Air Fuel ratio requirement

Air-fuel ratio requirements

Variation of Power output , BSFC with A/F
ratio.

Automotive A/F ratio requirements

Carburetor

Stages of combustion

Normal Combustion
Under ideal conditions the common internal combustion engine
burns the fuel/air mixture in the cylinder in an orderly and
controlled fashion.

Abnormal Combustion
When unburned fuel/air mixture beyond the boundary of the flame front
is subjected to a combination of heat and pressure for a certain duration
(beyond the delay period of the fuel used), detonation may occur.
Detonation is characterized by an instantaneous, explosive ignition of at
least one pocket of fuel/air mixture outside of the flame front. A local
shockwave is created around each pocket and the cylinder pressure
may rise sharply beyond its design limits.

Knocking
Knocking (also called knock, detonation, spark knock, pinging or pinking) in
spark-ignition IC engine occurs when combustion of the air/fuel mixture in
the cylinder starts off correctly in response to ignition by the spark plug, but
one or more pockets of air/fuel mixture explode outside the envelope of the
normal combustion front.
The fuel-air charge is meant to be ignited by the spark plug only, and at a
precise point in the piston's stroke. Knock occurs when the peak of the
combustion process no longer occurs at the optimum moment for the 4
stroke cycle. The shock wave creates the characteristic metallic "pinging"
sound, and cylinder pressure increases dramatically. Effects of engine
knocking range from inconsequential to completely destructive.

Factors affecting knock
1. Density Factors

1. Density Factors

2. Time Factors

3. Composition Factors

Variables affecting Knocking in SI engine.

Combustion Chambers

Combustion Chambers
1. Smooth Engine Operation

Combustion Chambers
2. High Power Output and thermal Efficiency

Combustion Chambers
Typical Combustion Chamber

Combustion Chambers

Definition of Carburetion:
The process of formation of a combustible fuel-air mixture by mixing the proper
amount of fuel with air before admission to engine cylinder is called carburetion
and the device which does this job is called a carburettor.
Factors Affecting Carburetion:
Of the various factors, the process of carburetion is influenced by
a) The engine speed
b) The vaporization characteristics of the fuel
c) The temperature of the incoming air, and
d) The design of the carburetor

Principle of Carburetion:
Both air and gasoline are drawn through the carburetor and into the engine
cylinders by the suction created by the downward movement of the piston. This
suction is due to an increase in the volume of the cylinder and a con¬sequent
decrease in the gas pressure in this chamber. It is the difference in pressure
between the atmosphere and cylinder that causes the air to flow into the
chamber. In the carburetor, air passing into the combustion cham-ber picks up
fuel discharged from a tube. This tube has a fine orifice called carburetor jet
which is exposed to the air path. The rate at which fuel is discharged into the
air depends on the pressure difference or pressure head between the float
chamber and the throat of the venturi and on the area of the outlet of the tube.
In order that the fuel drawn from the nozzle may be thoroughly atomized, the
suction effect must be strong and the nozzle outlet comparatively small. In
order to produce a strong suction, the pipe in the carburetor carrying air to the
engine is made to have a restriction. At this restriction called throat due to

Simple Carburetor:
The simple carburetor mainly consists of a float chamber, fuel discharge
nozzle and a metering orifice, a venturi, a throttle valve and a choke. The float
and a needle valve system maintain a constant level of gasoline in the float
chamber. If the amount of fuel in the float chamber falls below the designed
level, the float goes down, thereby opening the fuel supply valve and admitting
fuel. When the designed level has been reached, the float closes the fuel
supply valve thus stopping additional fuel flow from the supply system. Float
chamber is vented either to the atmosphere or to the upstream side of the
venturi.During suction stroke air is drawn through the venturi. As the air passes
through the venturi the velocity increases reaching a maximum at
the venturi throat. Correspondingly, the pressure decreases reaching a
minimum. From the float chamber, the fuel is fed to a discharge jet, the tip, of
which is located in the throat of the venturi. Because of the differential
pressure between the float chamber and the throat of the venturi, known as
carburetor depression, fuel is discharged into the air stream. The fuel6/10/2017 NAPHIS AHAMAD (ME) JIT 29

Essential Parts of a Carburettor:
A carburettor consists essentially of the following parts, viz.
a) fuel strainer
b) float chamber
c) main fuel metering and idling nozzles
d) choke and throttle
The function of a carburetor is to vaporize the petrol (gasoline) by means of
engine suction and to supply the required air and fuel (petrol) mixture to the
engine cylinder. During the suction stroke, air flows from atmosphere into the
cylinder. As the air passes through the venturi, velocity of air increases and
its pressure falls below the atmosphere. The pressure at the nozzle tip is also
below the atmospheric pressure. The pressure on the fuel surface of the fuel
tank is atmospheric. Due to which a pressure difference is created, which
causes the flow of fuel through the fuel jet into the air stream. As the fuel and
air pass ahead of the venturi, the fuel gets vaporized and required uniform

Types of superchargers:
Supercharger is a pressure-boosting device which supplies air (or mixture) at a
higher pressure. A centrifugal or axial flow or displacement type compressor is
normally used. If the supercharger is driven by the engine crankshaft, then it is
called mechanically driven
supercharger. Some superchargers are driven by a gas turbine, which derives its
power from the engine exhaust gases. Such a supercharger is called
turbocharger. There are three types of superchargers
Centrifugal type
Root's type
Vane type

a) Centrifugal Type Supercharger:
The centrifugal type supercharger is commonly used in automotive engines and is as
shown in Fig.3.28. A V-belt from the engine pulley runs the supercharger. First, the
air-fuel mixture enters the impeller at the centre. It then passes through the impeller
and the diffuser vanes. Finally, air or mixture enters the volute casing and then goes
to the engine from the casing. The mixture will come out at higher pressure and this
condition is called supercharged condition. Because of higher pressure more air-fuel
mixture is forced into the cylinder. About 30% more air-fuel mixture can be forced
into the combustion chamber. The impeller runs at very high speeds, about 80,000
revolutions per minute. Therefore the impeller should be able to withstand the high
stresses produced at this speed. Impellers are usually made of duralumin, or alloy6/10/2017 NAPHIS AHAMAD (ME) JIT 33

b) Root's Supercharger:
The details of Root's supercharger is shown in Fig.3.29. The Root's super-charger
has two rotors of epicycloids shape, with each rotor keyed to its shaft. One rotor is
connected with the other one by means of gears. The gears are of equal size and
therefore both the rotors rotate at the same speed. The Root's supercharger
operates like a gear pump. The mixture at the outlet of this supercharger will be at
much higher pressure than the inlet

c) Vane Type Supercharger
Details of a typical vane type supercharger is shown in Fig.. A number of vanes
are mounted on the drum which is inside the body of the supercharger. The vanes
can slide in or out, against the force of the spring. Because of this arrangement,
the vanes are always in contact with the inner surface of the body. The space
between the inner surface of the body and the drum decreases from the inlet to
the outer side. In this way, the quantity of the mixture which enters at the inlet,
decreases in volume, because of which the pressure of the mixture will increase
as it reaches the exit

What is Ignition System ???
 The system in an internal-combustion engine that
produces the spark to ignite the mixture of fuel and air:
includes the battery, ignition coil, distributor, spark
plugs, and associated switches and wiring.

IGNITION FUNCTION
 Produces 30,000 volt spark across spark plug
 Distributes high voltage spark to each spark plug in correct
sequence
 Times the spark so it occurs as piston is nearing top dead center
 Varies spark timing with load, speed, and other conditions

BASIC IGNITION SYSTEM
COMPONENTS
 BATTERY
 IGNITION SWITCH
 IGNITION COIL
 SWITCHING DEVICE
 SPARK PLUG
 IGNITION SYSTEM WIRES

BASIC IGNITION SYSTEM
 Battery supplies power to entire
system
 Ignition Switch turns engine on or
off
 Coil transforms volts
 Switching device triggers ignition
coil
 Spark Plug and wires distribute
spark

IGNITION COIL
 Transformer
 2 sets of windings
◦ Primary windings
◦ Secondary windings
 Iron core
 Produces magnetic field

IGNITION SYSTEM TYPES
 Battery ignition system
 Magneto ignition system
 Distributor less ignition system

BATTERY IGNITION SYSTEM
 A battery ignition system has a 6- or 12-volt battery charged by an
engine-driven generator to supply electricity, an ignition coil to increase
the voltage, a device to interrupt current from the coil, a distributor to
direct current to the correct cylinder, and a spark plug projecting into
each cylinder.
 Current goes from the battery through the primary winding of the coil,
through the interrupting device, and back to the battery.

The primary circuit consist of the battery, ammeter, ignition switch,
primary coil winding, capacitor, and breaker points. The function of
these components are :
 Battery – Provides the power to run the system
 Ignition switch – allows the driver to turn the system on and off
 Primary coil – produces the magnetic field to create the high voltage in
the secondary coil.
 Breaker points – a mechanical switch that acts as the triggering
mechanism
 Capacitor – protects the points from burning out.

The Secondary circuit converts magnetic induction into high voltage
electricity to jump across the spark plug gap, firing the mixture at the right
time. The function of the components are –
 secondary coil – the part of the coil that creats the high voltage electricity.
 Rotor – spin around on the top of the distributor shaft, and distributes the
spark to the right spark plug.
 spark plug – Take the electricity from the wires and give it an air gap in
the combustion chamber to jump across to light the mixture.

MAGNETO IGNITION SYSTEM
 The simplest form of spark ignition is that using a magneto.
 An ignition magneto, or high tension magneto, is a magneto that provides current
for the ignition system of a spark-ignition engine, such as a petrol engine.
 The engine spins a magnet inside a coil, or, in the earlier designs, a coil
inside a fixed magnet, and also operates a contact breaker, interrupting
the current and causing the voltage to be increased sufficiently to jump a
small gap.
 The spark plugs are connected directly from the magneto output.

IGNITION SYSTEM – Magneto System
Ignition
Switch
Distribution
Contact
Breaker
Coil
Magneto
Condenser
Power
Generation
Spark GenerationMagneto Unit
Rotor Arm

IGNITION SYSTEM TROUBLESHOOTING
Problem Possible causes and/or solutions
No spark out of the coil Possible open in the ignition
switch circuit
Possible defective ignition module
(if electronic ignition coil)
Possible shorted condenser
Weak spark out of the coil Possible high-resistance coil wire
or spark plug wire
Possible poor ground between the
distributor or module and the
engine block
Engine missing Possible defective (open) spark
plug wire Possible
worn or fouled spark plugs
Possible defective pickup coil
Possible defective module
Possible poor electrical
connections at the pickup coil
and/or module

spark egnition engine

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