Glow Plugs

  • Part Number: Various
  • Model Number: Glow Plugs
  • OEM Part Number: Various
  • Application: Indirect and Direct Injection Diesel Engine Glow Plugs for most applications.
  • Price: Please enquire with part number if possible
  • Description:

    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 chamber.

    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 reduced voltage"

    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.

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