THE REVISED HOUSE OF AM-LV (TRHOA) CHAP 3

THE IGNITION SYSTEM

OBJECTIVES:

At the end of this lesson, the students should be able to:

 explain the operation of the four-stroke engine;

 give the role of ignition;
 state the principle of ignition;
 identify the elements of the primary circuit and high voltage; explain the operation of the
ignition system;

INTRODUCTION

spark ignition engines (petrol engines), a device is required to ignite the compressed air-fuel mixture at
the end of compression stroke. Ignition system fulfills this requirement. It is a part of electrical system
which carries the electric current at required voltage to the spark plug which generates spark at correct
time. It consists of a battery, switch, distributor ignition coil, spark plugs and necessary wiring. A

compression ignition engine, i.e. a diesel engine does not require any ignition system because; self

ignition of fuel air mixture takes place when diesel is injected in the compressed air at high temperature

at In the end of compression stroke.

2-1 ROLE OF THE IGNITION CIRCUIT

The fundamental purpose of the ignition system is to supply a spark inside the cylinder, near the end of

the compression stroke, to ignite the compressed charge of air–fuel vapour.

2-2 OPERATING PRINCIPLE OF IGNITION

The function of the ignition system is to produce a spark in the cylinders of the engine in order to

ignite the combustible mixture there. The production of these sparks must respect a sequence and a

variable rate/rhythm according to the speed of the engine and the effort which is required of it. The

correct operation of ignition is thus a condition necessary to a good combustion and the performance of

the petrol engine.

All the ignition circuits have same operation: they stop the current in the primary winding of the
coil thus producing a high voltage in the secondary winding, high voltage which causes the ignition spark

of the plug.

The way in which the interruption of current in the primary winding of the coil is obtained makes

it possible to distinguish various or different types of ignition systems:

The distributor ignition system with mechanical breaker


The electronic distributor ignition system or electronic control ignition system

The ignition system without distributor where the interruption of current is controlled

by
computer.

2-3 DESCRIPTION OF THE IGNITION CIRCUIT

I don’t expect these to be your first time in studying the ignition system
So I urge you to look at the diagram below and give me feedback inside our group with the
name of all the parts required below

I know it may look different from your diagrams but you are required to give me the name of all
the parts of the diagram above(first diagram)

2-4 STUDY OF COMPONENTS OF THE IGNITION CIRCUIT

2-4.1 Battery:

It supplies the necessary current to the primary winding of ignition coil which is converted into high
voltage current to produce spark. (See chapter battery)

2-4.2 Ignition switch:

It is a multifunction switch (Ignition-starting-locking of the steering column). Inter connected in the

primary circuit of the ignition coil, permitting it energized. It puts on and off the ignition system

2-4.3 Ignition coil :

Comparable to a transformer, ensures the production of high voltage.

It steps up the12 volt from the battery to a higher voltage (between 20000-30000 volts). It consists

of a primary winding and secondary winding wound on laminated soft iron core (see figure 2

below). Primary winding contains about 300 turns made of thick wire and Secondary consists of

about 20000 turns of thin wire

The principle of operation of the ignition coil is based on the laws of electromagnetic induction and
more precisely on the FARADAY and LENZ laws which stipulates that a winding traversed by an electrical
current produces a magnetic field, and then the winding is crossed by a magnetic flux.

2-4.4 Resistor :

It lowers the voltage the ignition coil, protects the coil from under tension.

2-4.5 Spark Plug :

Situated at the end of the secondary circuit, its function is to produce a spark between its electrodes

which ignites air/fuel mixture in the cylinder. Figure 4 below shows a typical spark plug for an

automobile engine.
2-4.6 conducting high tension leads and spark plug leads:

Ensure the transportation of voltage in the circuit

2-4.7 Distributor :

Constitutes of several parts, opens and closes the primary circuit and distributes high voltage

produced by the coil to each spark plug

I don’t know if these diagram is clear with you, but if it doesn’t then get to me for clarification

2-4.8 Distributor cap:

Collect the high voltage coming from the ignition coil and distributes it to each plug

2-4.9 Condenser:

Its function in the ignition system is to absorb and store the inductive current generated in the coil.

2-4.10 Contact breaker:

The main purpose of a contact breaker in an ignition system is to switch ON and OFF the primary circuit
of an ignition coil. And it is operated by the cam, the battery supply current to the ignition coil, primary
coil, due to the principal of induction, these battery current is induced in the secondary coil which
connected to the contact breaker

Operating principle of contact breaker

As the camshaft rotates, so thus the cam in a contact breaker, hence the cam is forced to open and
close, as the cam Opens, it switches off the primary circuit of the ignition coil, for these reason, magnetic
field collapse and high voltage from the coil is send to the distributor, and further to the spark plug.

Test

1. What will happen if cam angle too

A. Big
B. Small
2. Defined dwell angle
3. State the formula to calculate dwell angle

2-4.12 Distribution :

The purpose of it is to distribute at quite precise moments (moments of ignition) the sparks towards the
cylinders by taking account of:

 The position of the crankshaft;

 The opening of the breaker;
 The firing order (distribution of spark plug)
 2-6 IGNITION POINT

The moment of ignition must be precise in order to obtain a maximum pressure on the piston.

2-6.1 Initial or Static timing

The initial timing is appropriate at idling speed.

2-6.2 Inconveniences of a poor initial or static timing

 Rise in temperature ;
 Poor efficiency;  Rattling;
 Pollution.

2-7 AUTOMATIC ADVANCE MECHANISMS

The purpose of spark advance mechanism is to assure that under every condition of engine
operation, ignition takes place at the most favorable instant in time i.e. most favorable from a
standpoint of engine power, fuel economy and minimum exhaust dilution. By means of these
mechanisms the advance angle is accurately set so that ignition occurs before TDC point of the
piston. The engine speed and the engine load are the control quantities required for the automatic
adjustment of the ignition timing. Most of the engines are fitted with mechanisms which are integral
with the distributor and automatically regulate the optimum spark advance to account for change of
Speed and load

2-7.1 Centrifugal advance mechanism

The centrifugal advance mechanism controls the ignition timing for full- load operation. The Adjustment
mechanism is designed so that its operation results in the desired advance of the spark. The cam is
mounted, movably, on the distributor shaft so that as the speed increases, the flyweights Which are
swung farther and farther outward, shaft the cam in the direction of shaft rotation. As a Result, the cam
lobes make contact with the breaker lever rubbing block somewhat earlier, thus Shifting the ignition
point in the early or advance direction. Depending on the speed of the engine, And therefore of the
shaft, the weights are swung outward a greater or a lesser distance from the Center. They are then held
in the extended position, in a state of equilibrium corresponding to the Shifted timing angle, by a
retaining spring which exactly balances the centrifugal force. The weights Shift the cam either or a
rolling contact or sliding contact basis; for this reasons we distinguish Between the rolling contact type

and the sliding contact type of centrifugal advance mechanism.

2-7.2 Vacuum advance mechanism

Vacuum advance mechanism shifts the ignition point under partial load operation. The adjustment
system is designed so that its operation results in the prescribed partial load advance Curve. In this
mechanism the adjustment control quantity is the static vacuum prevailing in the Carburetor, a pressure
which depends on the position of the Throttle valve at any given time and Which is at a maximum when
this valve is about half open. This explains the vacuum maximum. The diaphragm of a vacuum unit is
moved by changes in gas pressure. The position of this Diaphragm is determined by the pressure re
differential at any given moment between the prevailing Vacuum and atmospheric pressure. The
beginning of adjustment is set by the pre -established tension
The filling or admission of fresh mixture of the engine varies according to the position of the

accelerator. This implies:

Combustion speed varies (it is all the more slow as admission of mixture is poor). NB: it is

then necessary to correct the ignition point in function of:

 Engine speed by a centrifugal advance mechanism;

 Engine admission of air/fuel mixture by a vacuum advance mechanism.

It is an assembly of these two advance mechanisms which determines the point of ignition to a valve
being appropriate for each position

Assignment

Write short notes on distribution timing and submit to the

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