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

Typical Combustion Chamber


Combustion Chambers Typical Combustion Chamber


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

increase in velocity of flow, a suction effect is created.


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 fuel

discharge is affected by the size of the discharge jet and it is chosen to give


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

mixture is supplied to the engine.6/10/2017 NAPHIS AHAMAD (ME) JIT 30

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 exit6/10/2017 NAPHIS AHAMAD (ME) JIT 35


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

Engineering