Spark Plugs
Chapter 3
Study Tips
How is the high voltage produced?
When does the spark discharge occur?
Basics of Ignition
Chapter 3 Basics of Ignition
39
3 Ignition Timing
Ignition Timing
A: Ignition B: Ignition lag C: Flame propagation D: Crankshaft angle
E: Maximum combustion pressure F: Combustion completion P: Pressure
Ignition timing and
combustion
pressure
Until the air-fuel mixture ignites and
the combustion reaches the maximum
pressure requires a certain time.
(Ignition lag, flame propagation)
Generally, the engine releases the
maximum pressure when the
maximum pressure due to combustion
comes at the position where the
crankshaft position is about 10
degrees after top dead center
(ATDC10
°
CA), so the ignition timing is
set ahead of that time by the duration
of the flame propagation and the
ignition lag.
Furthermore, since the combustion
speed until the maximum pressure is
reached depends on the engine
speed, the engine load, etc., it is
necessary to control the ignition timing
according to those factors.
4 Ignition Order
Ignition Order
In multi-cylinder
engines, in order to
make the engine run
smoothly, the cylinders
are combusted in a
specific order.
Example of the ignition
order for a 4-cycle
multi-cylinder engine
■
4 cylinders
■
6 cylinders
■
8 cylinders
1
1
4
4
3
3
2
2
