4 Ignition System

8/13/2019 4 Ignition System

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Ignition Systems in Spark-Ignition Engines

By

Dr. Amr Ibrahim

Dr. Amr Ibrahim, 2010, all rights reserved. The commercial use of these slides without

permission is strictly prohibited.


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What is the main function of an ignition system?

The ignition system is responsible for generating the spark at the

optimum time in order to initiate the combustion of the air-fuel

mixture.

What is the optimum spark timing?

The spark is usually produced before the

piston reaches the TDC position by the end of

compression stroke

The spark timing may vary from 5 to 40

degrees before TDC

The spark timing is usually experimentally

selected in order for the engine to produce

the maximum torque. This timing also gives

maximum brake power & minimum brake

specific fuel consumption.


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Types of ignition systems:

mechanical (contact point) ignition system

electronic (transistorized) ignition system

distributor less ignition system

Retarded spark timing is used atengine idlingto bring the ignition

point closer to the TDC to avoid misfire and ensures engine smooth

operation

The optimum spark timing varies with engineload and speed


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The Mechanical ignition system


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battery & alternator

ignition switch

Ignition coil

ignition distributor

spark plugs

cables

The mechanical ignition system consists of:


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Engine battery

it is an electrochemical device

it produces electric current for starting motor and ignition

system while starting the engine

it also supplies current to electrical accessories when the

alternator is not handling the load


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The chemicals in the battery are:

o sponge lead (a solid)

o lead oxide (a paste)

o sulfuric acid mixed with water (electrolyte)


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PbO2 + Pb + 2H2SO4 2PbSO4 + 2H2O

Chemical reaction during discharging process:

2PbSO4 + 2H2O PbO2 + Pb + 2H2SO4

Chemical reaction during charging process:

the specific gravity of the battery electrolyte

ranges from about 1.265 for fully charged battery

to 1.11 for completely discharged battery

the state of charge of a battery can be

determined using a hydrometer


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The spark plug

it generates the spark within the cylinder

it has two metal electrodes (central and ground

electrodes)

there is an air gap between the central and ground

electrodes

when the central electrode is supplied with a high

voltage current, the current passes from the central to

ground electrode in the form ofa spark


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the high voltage current is supplied to the terminal from the

ignition coil

most spark plugs have electrodes made of nickel and chrome alloysthe wider the gap, the higher the voltage required to produce it

the gap may vary from 0.9 to 2 mm


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The ignition coil

it is a step up transformer

it converts the small battery voltage

of 12 volts to a high voltage of 25,000

volts

in some electronic ignition systems,the voltage may go up to 47,000 volts

or higher


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the ignition coil consists of two coils:the primary and secondary coils

the secondary coil is made of a winding of many thousands of turns

(e.g. 20,000 turns) of a fine wire

the primary coil is made of a few hundred turns (e.g. 200 turns) of

relatively heavy wire

the secondary coil is wrapped around an iron core

the primary coil is wrapped around the secondary coil (the two coils

are electrically insulated)


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The ignition distributor

it receives the high voltage current fromthe ignition coil and distribute it to spark

plugs according to the firing order

it contains two mechanisms that can vary

the spark timing according to engine speedand load

its shaft speed is equivalent to the

camshaft speed (i.e. half the crankshaft

speed) in 4-stroke cycle engines

the distributor shaft is driven by a gear on

the camshaft


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1. The rotor

it receives the high voltage current via thecentral terminal in the cap and distribute it

via the side terminals

What is under the distributor cap?


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2. The breaker cam & contact points

the contact points are operated by a breaker camas the cam rotates, the points open & close

the cam has as many lobes as no. of cylinders

Open contact points Closed contact points


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3. The condenser


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The wiring diagram:


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How the high voltage is produced?

the basic principle behind generating the high voltage in the ignition

coil is Faradays law:

dt

dE

the contact points operate as a mechanical switch in order to

interrupt the primary current in the primary circuit and produce a

change in the magnetic field

when the contact points open the primary circuit, a high voltage isproduced in the secondary circuit and a spark is generated


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The electric circuit when the contact points are closed:


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What happens when the contact points close the primary circuit?

the current flows from the battery to the primary coil

the flowing current produces a change in the magnetic field which

cuts the turns of the primary coil and secondary coil

this produces a self electric motive force in the primary coil which

opposes the flowing current

that results in a slow growth of the primary current and a small

d/dt is produced

when the secondary coil is subjected to a small d/dt, a small

voltage is produced in the secondary coil

during this period, the rotor in the distributor is rotating betweenthe side terminals


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The electric circuit when the contact points are open:


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What happens when the contact points open the primary circuit?

when the primary circuit is suddenly open, the primary current

diminishes producing a change in the magnetic field

the primary current rushes to the condenser and instantly charges it

once the condenser is charged, it is discharged again

when the condenser is being discharged, its current flows into an

opposite direction to the primary current causing a rapid collapse of theprimary current which leads to high d/dt

the high d/dt produces a high voltage in the secondary coil

at that time, the rotor in the distributor is in contact with a side

terminal and a spark is generated


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The dwell angle

The dwell angle is the angle of rotation of the distributor shaft

through which the primary circuit is closed.

if this angle is too small, the stored magnetic energy in the

primary circuit will not be sufficient to produce the required high

voltage in the secondary circuit

if this angle is too high, high ignition coil temperature and

perhaps coil damage could occur

the dwell angle may range from 30 to 60 degrees


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Contact points gap

the contact points gap is the distance between the two contact

points when the are fully open

the gap may range from 0.3 to 0.4 mm

the wider the gap, the smaller the dwell angle, and vice versa

rubbing block

during engine operation, wear occursin the rubbing block and gap decreases

the gap has to be readjusted by

relocating the position of the fixed

contact point

fixed point

moving point


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Adjusting the spark timing

if the spark timing istoo advanced, the combustion will start too

early and the compression work will increaseif the spark timing istoo retarded, the combustion will complete

too late and the peak cylinder pressure will be reduced causing a

decrease in the expansion work

The optimum spark timing is usually experimentally selected in

order for the engine to produce themaximum torque

The optimum spark timing varies with engineload and speed

The ignition system must be capable of changing the spark timing

with load and speed in order for the engine to produce the

maximum torque


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During engine motoring (i.e. with no combustion), the in-cylinder

pressure increases with crank angle till it reaches its maximum at the

TDC


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the period from point 1 to point 2 is called the flame development

period or ignition delay

experimentally, it was found that the maximum torque is obtained

when the spark timing is adjusted so that the maximum pressure is

produced 10 to 16 degrees after TDC

point 1: spark

timing

point 2: rapid

combustion starts

point 3: maximum

pressure occurs

after the TDC

point 4:

combustion ends


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Setting the spark timing according to engine load

o during part load operation, less air and fuel is admitted into the

cylinder compared to the full load operation

o the decrease in the inlet mixture density during part load

operation leads to a decrease in engine flame speed

o also, during engine part load operation, the engine may use lean

air-fuel mixture

o the use of lean air-fuel mixture decreases the flame speed

compared to the use of rich mixture

The flame speed during engine part load operation is slowerthan the

flame speed during full load operation due to:


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therefore, the spark timing must be advanced during engine part load

operation in order for the combustion to complete at the appropriate

time

advancing the spark timing during part load operation ensures that

the maximum cylinder pressure occurs at the optimum time for

maximum engine torque operation

the vacuum advance mechanism is

responsible for advancing the spark timing

during engine part load operation


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The vacuum advance mechanism is connected to a small port located

abovethe throttle valve


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when the throttle valve moves past the vacuum port, the intake

manifold vacuum pulls the diaphragm

this rotates the breaker plate so the contact points open earlier and

the spark timing is advanced

the spark advance increases as the vacuum acting on the diaphragm

increases

during engine idling & wide-open-throttle operation, there is no

vacuum acting on the diaphragm


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Setting the spark timing according to engine speed

experimentally, it was found that the total combustion angle increaseswith the increase of engine speed

therefore, the spark timing must be advanced with the increase of

engine speed in order for the combustion to complete at the

appropriate time and the maximum pressure to occur at the optimum

time

the centrifugal advance mechanism is responsible for advancing the

spark timing with the increase of engine speed

the centrifugal advance mechanism is located inside the ignition

distributor


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The centrifugal advance mechanism


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as the engine speed increases, the movement of the weights under

centrifugal force advances the angular position of the cam relative to

the driveshaftin this cam new position, the contact points open earlier and the

spark is advanced


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During operation, both the vacuum advance mechanism and centrifugal

advance mechanism advance the spark timing according to engine load

and speed


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Electronic (Transistorized) Ignition System


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What is the main difference between the mechanical and

electronic ignition systems?

in the mechanical ignition system, the primary current in theprimary circuit is interrupted by a mechanical switch (the contact

breaker points)

in the electronic ignition system, the primary current is interrupted

byan electronic switch


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What is the main component of an electronic switch?

o it is the transistor

the transistor is a semiconductor and has

three terminals: a collector, a base , and an

emitter

when an electric current flows to its base, it

becomes a conductor


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The advantages of using the transistor as an electronic switch

compared to the contact points:

the transistor can last for longer time (no mechanical wear)

the transistor can interrupt higher voltage current

the transistor switches the current flow faster than the mechanical

switch

The basic principle of the transistorized ignition system:

when the current flows to the transistor base, the transistor becomes a

conductor and the primary current flows through the primary circuit

when there is no current flowing to the transistor base, the transistor

becomes an insulator and the primary circuit is switched off

when the primary circuit is switched off, a high voltage is produced in

the secondary circuit and a spark is generated


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Generating the transistor base current:

the transistor base current is generated by a signal generator

located in the ignition distributor

the signal generator type could differ from an engine to another

(e.g. magnetic, Hall effect, or optical type)


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The magnetic type signal generator


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Advancing the spark timing based on engine load & speed:

the vacuum advance mechanism is used to advance the spark timing

according to engine load by shifting the position of the pick up coil


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the centrifugal advance mechanism advances the spark timing

according to engine speed by changing the angular position of the

signal rotor relative to the driveshaft


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sometimes in electronic ignition systems, both the ignition coil

and distributor are integrated in one assembly


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Electronic ignition system using an ECU

the ECU switches on and off the primary circuit

the ECU controls the spark timing according to engine operating

conditions detected by engine sensors

the distributor mainly distributes the high voltage current to engine

cylinders


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the electronic control of the spark timing via the ECU can achieve spark

timings closer to the ideal ignition timings compared to the mechanical

ignition system

ESA = electronic spark advance


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Distributor less (or direct) ignition system

in this system, an ignition coil may be used for each cylinder or one

ignition coil may be used for each two cylinders

the ECU controls each ignition coil and the spark timing is

determined according to engine operating conditions detected by

engine sensors

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4 Ignition System