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Ignition in a diesel engine is accomplished by a combination of fuel injection and compression of intake air. Diesel engines normally require longer cranking periods than gasoline engines. At low ambient temperatures, a diesel engine is extremely difficult or impossible to start without adequate accessories to assist in the starting process. As the outside temperature drops, battery efficiency is reduced and cranking load becomes high. The increased load results from higher oil viscosity. The cold cylinder walls also chill the incoming air, and the air cannot reach the temperature required for combustion. The methods used for helping an engine start in cold weather include (1) heating the air in the cylinder (glow plugs); (2) heating the intake air (grid resistor); (3) adding a volatile, easily combustible fluid (ether) to the intake air; or (4) heating the coolant and/or lubricating oil (heaters).


Deriving its power from the battery, the glow plug is a low-voltage heating element that is inserted in the combustion chamber of each cylinder. The glow plug is used briefly before the cold engine is cranked. In general, the time limit for the use of the glow plug is dependent upon the ambient temperature and the design of the engine. The operating temperature of a glow plug is between 1652 and 1832F.


The grid resistor usually consists of an electrical resistance grid mounted on a frame and supported by insulating blocks in the engine air-intake manifold. The grid is preheated by current from the starting battery, before the engine is cranked, and is operated during the cranking period until the engine has reached operating speed.

The basic drawback in the use of glow plugs or the grid resistor as starting aids is that they require battery power that is also needed for cranking. The cranking power of a battery is already reduced at low temperatures. In the following examples, note how the cranking power of a battery is reduced as the ambient temperature drops:


80F 100 percent

32F 65 percent

0F 45 percent


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