Ignition Systems page 6Ignition Systems page 6

Ignition timing Ignition timing

Maximum cylinder pressure Maximum cylinder pressure should be gained just after should be gained just after

TDC

Which generally means the Which generally means the spark must be produced before spark must be produced before

TDC. TDC.

This is to allow for the time lapse This is to allow for the time lapse while the gas builds up to its while the gas builds up to its

maximum combustion pressure.maximum combustion pressure.

The time factor for the fuel to The time factor for the fuel to burn is relatively constant, but burn is relatively constant, but

the angle moved by the the angle moved by the crankshaft varies with engine crankshaft varies with engine

speed. speed.

This means that as the engine This means that as the engine speed increases the timing of speed increases the timing of

the spark needs to be the spark needs to be ………………… (This means it ………………… (This means it happens earlier in the cycle) happens earlier in the cycle)

Advanced

The way in which this is The way in which this is achieved is by using a achieved is by using a

centrifugal advance mechanism. centrifugal advance mechanism. The pictures below illustrate the The pictures below illustrate the

advance. advance.

The distributor spins at half engine The distributor spins at half engine speed, usually anticlockwise. speed, usually anticlockwise.

The centrifugal timing advance The centrifugal timing advance mechanism is sensitive to mechanism is sensitive to

engine speed, but not to engine engine speed, but not to engine

• Load

During part engine load, when During part engine load, when the engine speed isn’t very fast, the engine speed isn’t very fast,

the centrifugal weights can’t the centrifugal weights can’t advance the timing enough; advance the timing enough; especially if the mixture is especially if the mixture is

excessively weak and slow excessively weak and slow burning.burning.

The way in which the ignition The way in which the ignition timing is advanced around part timing is advanced around part throttle openings is with the use throttle openings is with the use of a …………...... timing control. of a …………...... timing control. Vacuum

Questions Questions

1. What is used to advance the timing at high engine speeds?

2. What is used to advance the timing at lower engine speeds?

3. Why does the timing need advancing anyway?

4. If the spark plug is sparking late, what is this known as?

Fly weights

The vacuum capsule

Less time to burn the fuel, and leaner slower burning mixture

Retarded ignition timing