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
09
1 Spark and Ignition
Spark and Ignition
Spark
When the high voltage produced by the
ignition system is applied between the
center electrode and ground electrode of
the spark plug, the insulation between the
electrodes breaks down, current flows in
the discharge phenomenon, and an
electrical spark is generated.
This spark energy trigger ignition and
combustion in the compressed air-fuel
mixture.
This discharge is of extremely brief
duration (about 1/1000 of a second)
and is extraordinarily complex.
The role of the spark plug is to reliably
generate a strong spark between the
electrodes accurately at the specified
time to create the trigger for combustion
of the gas mixture.
Ignition
Ignition by electrical spark occurs because the fuel particles between the electrodes are activated by the spark to discharge, a
chemical reaction (oxidation) is triggered, the heat of reaction is generated, and the flame core is formed. This heat activates the
surrounding air-fuel mixture, eventually a flame core is formed that spreads the combustion to the surroundings itself.
However, if the quenching effect between the electrodes (the work of the electrodes absorbing the heat and extinguishing the flame) is
greater than the flame core heat generation action, the flame core is extingushed and the combustion stops.
If the plug gap is wide, the flame core is larger and the quenching effect is smaller, so reliable ignition can be expected, but if the gap
is too wide, a large discharge voltage becomes necessary, the limits of the coil performance are exceeded, and discharge becomes
impossible.
Formation of the flame core
Quenching effect
High melting point
Sparking wear
Center electrode
Ground electrode
Flame core
Heat absorption
Flame propagation
5
5
4
4
3
3
2
2
1
1
10,000 deg.C
1
1
2
2
3
3
4
4
4
4
5
5
Answer: 1
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