Spark Plugs
Chapter 2
Study Tips
The spark plugs are critical parts that dominate
the engine combustion and bear a major
responsibility for higher engine performance.
Spark Plugs
Chapter 2 Spark Plugs
11
The required voltage changes drastically
with various conditions.
The three factors with
particularly large impact are
the spark plug gap, the
compression pressure, and the
air-fuel mixture temperature.
The required voltage rises in proportion
to the spark plug gap.
The spark plug gap widens bit by bit as
the electrode wears, so high required
voltage becomes necessary and misfire
occurs more easily.
Spark Plug Gap
1
●
Conditions that affect
the required voltage
G: Spark plug gap V: Required voltage
Even for the same spark plug gap, if
there are edges on the electrode,
discharge occurs more easily.
Older spark plugs have electrodes that
have rounded, so discharge becomes
more difficult and misfire more likely.
Electrode shape
2
Easy discharge
Difficult spark
Electrode shape
3
3
2
2
1
1
The required voltage rises in proportion to the
compression pressure.
The compression pressure is higher for low speed
and high load. Sudden acceleration for starting
out fits these conditions, so higher voltage is
required then and misfire occurs more easily.
Compression pressure
3
P: Compression pressure T: Air-Fuel mixture
temperature V: Required voltage
The required voltage drops as the air-
fuel mixture temperature rises.
The lower the engine temperature, the
higher the required voltage, so misfire
occurs more easily at low temperature.
Air-Fuel mixture temperature
4
P: Compression pressure T: Air-Fuel mixture
temperature V: Required voltage
As the humidity rises, the electrode
temperature decreases, so the required
voltage becomes slightly higher.
Humidity
7
H: Relative humidity V: Required voltage
The required voltage drops as the
electrode temperature rises.
The electrode temperature rises in
proportion to the engine speed, so
misfire occurs more easily at low speed.
Electrode temperature
5
T: Electrode temperature V: Required voltage
There is a tendency for the required
voltage to be higher the leaner the air-
fuel mixture (the larger the air-fuel ratio).
If the air-fuel mixture becomes leaner
due to fuel system trouble misfire occurs
more easily.
Air-Fuel ratio
6
A/F: Air-fuel ratio V: Required voltage
