Diesel engines, unlike gasoline or petrol engines , do not use
spark plugs to induce combustion. Instead, they rely solely on
compression to raise the temperature of the air to a point where
the diesel will combust spontaneously when introduced to the hot
high pressure air. The high pressure and spray pattern of the
diesel ensures a controlled, complete burn.
The piston rises, compressing the air in the cylinder; this causes
the air's temperature to rise. By the time the piston reaches the
top of its travel path, the temperature in the cylinder is very
high. The fuel mist is then sprayed into the cylinder; it instantly
combusts, forcing the piston downwards, thus generating power.
The pressure required to heat the air to that temperature, however,
necessitates the use of a large and very strong engine block.
The problem solved by the glow plug occurs when starting a diesel
engine from cold, especially in cold ambient temperatures, when the
thermal mass of the metal comprising the combustion chambers
(cylinder block, cylinder head, piston) more readily absorbs the
heat energy created by the friction of the piston/cylinder
interface and of compressing the incoming air such that the
combustion chamber temperature is insufficient to support
self-combustion. In these conditions, the glowplug is temporarily
activated to add a hotspot within the combustion chamber until the
residual temperature of the combustion chamber achieves the level
required to support self-combustion.
For that reason indirect injected diesel engines are manufactured
with glow-plugs in each prechamber, and direct injected diesel
engines are manufactured with glow-plugs in each combustion
In older generation diesel-engine vehicles, unlike in a
gasoline-engine vehicle, the operator did not simply turn the key
to the "start" position and have the engine immediately start.
Instead, the operator turned the key to the "on" position for a
long duration; the glowplug relay switches the glowplugs on, and a
light (see picture at right) on the instrument cluster illuminates. This process is called
"pre-heating" or "glowing". According to Bosch: "Older engines used
a glow period of up to 21 seconds whereas more modern engines use
around a 6 to 8 second heat period and provide after glow at a
When internal sensors detect that the core of the engine block has
reached a certain designated temperature, or when a certain amount
of time elapses, the glowplug relay switches off the
"wait-to-start" light. The operator then proceeds to turn the key
to the "start" position, as in a gasoline engine.
The glowplug relay switches off the glowplugs after the engine is
running (or, in older cars, at the same time the "wait to start"
light goes out). In some newer cars, glow plugs continue to operate
for up to 180 seconds after engine start to keep the engine within
emissions regulations, as combustion efficiency is greatly reduced
when the engine is very cold.