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
25
4 Heat Range
Heat Range
1
1
2
2
80％
100％
38％
20％
38％
20％
20％
10％
10％
2％
Spark plug heat
dispersion
The heat that the electrode section of
the spark plug receives due to
combustion is dispersed through the
path in the figure.
The degree to which a spark plug
disperses the heat it receives is called
its "heat range". Spark plugs with a
high degree of heat dispersion are
called high heat range (cold type) and
those with a low degree of heat
dispersion are called low heat range
(hot type).
This is largely determined by the
temperature of the gas inside the
combustion chamber and the spark
plug design.
Water
Cooled by intake air-fuel mixture
2
2
1
1
Low heat range and high heat range
Low heat range plugs have long insulator leg sections and the surface area affected by the flame and the gas pocket capacity are large.
Also, since the heat release path from the insulator nose to the housing is long, heat dispersion is low and the temperature of the
center electrode rises easily.
On the other hand, high heat range plugs have short insulator nose and the surface area affected by the flame and the gas pocket
capacity are small.
Also since the heat release path from the insulator nose to the housing is short, heat dispersal is high and the temperature of the
center electrode does not rise easily.
Low heat range (hot type) spark plugs
High heat range (cold type) spark plugs
Gas pocket
3
3
2
2
1
1
1
1
3
3
2
2
