Page 265 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Figure 2.335 Heat-loss paths: 1, cold plug; 2, hot plug; 3, temperature (the cold plug is able to
transfer heat more easily so is suitable for a hot engine)

Figure 2.336 A range of spark plugs. (Source: Bosch Media)

The heat range of a spark plug is a measure of its ability to transfer heat away
from the centre electrode ( Fig. 2.335 ). A hot running engine will require plugs
with a higher thermal ability than a colder running engine. Note that hot and cold
running of an engine in this sense refers to the combustion temperature, not to
the cooling system.
Spark plug electrode gaps in general have increased as the power of the ignition
systems driving the spark has increased. The simple relationship between
plug gap and voltage required is that as the gap increases so must the voltage
(leaving aside engine operating conditions). Further, the energy available to form
a spark at a fi xed engine speed is constant, which means that a larger gap using
higher voltage will result in a shorter duration spark. A smaller gap will allow a
longer duration spark. For cold starting an engine and for igniting weak mixtures
the duration of the spark is critical. Likewise, the plug gap must be as large Key fact
as possible to allow easy access for the mixture to prevent quenching of the Plug gaps of 0.6–1.2 mm are in
fl ame. The fi nal choice is therefore a compromise reached through testing and common use.
development of a particular application. Plug gaps in the region of 0.6–1.2 mm
seem to be the norm at present ( Fig. 2.336 ).