Spark Plugs
Chapter 2 Spark Plugs
26
5
5
4
4
3
3
2
2
1
1
S
T
400
600
800
1000
（℃）
Spark plug
temperature and
vehicle speed (1)
The relationship between the
spark plug temperature and
vehicle speed and heat range
is expressed with a graph like
that in the figure.
There are restrictions on the
temperatures at which spark
plugs can be used: the lower
limit is the self-cleaning
temperature and the upper
limit is the pre-ignition
temperature.A spark plug
only functions completely
when its center electrode
temperature is between these
temperatures of about 500
°
C
and 950
°
C.
Spark plug
temperature and
vehicle speed (2)
- self-cleaning
temperature
When the center electrode
temperature is 500
°
C or lower,
free carbon generated when
the fuel does not combust
completely is deposited on the
surface of the insulator.
Therefore, the insulation
between the insulator and the
housing falls, electricity leaks
occurs, the spark across the
gap is incomplete, causes
ignition failures.
This temperature of 500
°
C is
called the self-cleaning tem-
perature because above this
temperature the carbon is
naturally burnt away comple-
tely by combustion.
Spark plug
temperature and
vehicle speed (3)
- pre-ignition
temperature
When the center electrode
reaches 950
°
C or higher, pre-
ignition (early ignition) occurs,
meaning that the electrode
serves as a heat source and
ignition occurs without a
spark.
Therefore, output falls and
this can reach the level of
electrode wear and insulator
damage.
Spark plug
temperature and
vehicle speed (4)
Low heat range spark
plugs
have center electrode
tem-peratures that rise easily
and even at low-speed, they
easily reach the self-cleaning
temperature, so carbon is not
deposited easily on the insu-
lator section.
On the other hand,
high heat
range spark plugs
have
center electrode temperatures
that do not rise easily, so they
are unlikely to reach the pre-
ignition temperature even at
high speed.
Therefore, this type of spark
plug is generally used for high
speed, high output engines.
That is why it is necessary to
select spark plugs with the
appropriate heat range for the
engine characteristics, run-ning
conditions, etc.
Self-cleaning temperature
Pre-ignition temperature
Low heat range (hot type) spark plugs
Standard spark plugs
High heat range (cold type) spark plugs
S: Vehicle speed T: Spark plug temperature (
°
C )
5
5
4
4
3
3
2
2
1
1
