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Automotive glossary: What is a Heater Plug?

Definition of Heater Plug: Diesel engines ignite by the heat build-up of the gas in the cylinder caused by compression. When starting cold, a portion of the heat gets lost through the still cold cylinder and piston surfaces, so that the self combustion does not work. For this reason, heater plugs are inserted in the cylinders, to heat up the inner rooms of the cylinders before starting. While preheating, the heater plugs are provided with high amperage, in order to anneal. In modern car diesel engines, some seconds of preheating are sufficient, before the engine can be started. Diesel direct injectors need preheating only when starting cold at very low temperatures.

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