A diesel engine is also known as a compression-ignition engine. Unlike a gasoline engine, which ignites the compressed fuel-air mixture with an electric spark from a spark plug, the fuel in a diesel cylinder ignites spontaneously, solely from the heat generated by the process of compression.
If the engine is cold and the ambient temperature of the air intake to the diesel engine is too low, much of the heat generated by the initial rotations of the engine will be conducted away into the engine block and other components, and into the environment. If that happens, the air-fuel mixture temperature in the cylinder may not be raised sufficiently by engine compression for ignition to occur.
Glow plugs pre-heat the incoming air so that ignition can more easily occur. This assists a cold engine to start more reliably, and once the engine is running, the glow plugs are no longer required.
A glow plug consists of a resistor attached to the end of a plug which penetrates the combustion chamber. When the glow plug is activated, a current flows through the resistor causing it to heat up, which then raises the temperature of the incoming air.
The glow plugs are activated by the driver of a typical modern diesel engine for somewhere between 10 and 20 seconds prior to the engine firing. Older and less efficient diesel engines, or worn engines, may require pre-heating times several times longer than that prior to ignition occurring.
There is a variety of methods for altering the timing and style of the electronic fuel injection process in modern automotive diesel engines to ensure reliable cold-starting. Even though glow plugs are fitted, they are rarely used for more than a few seconds.
Glow plug resistor elements, also called filaments, must be made of materials that are resistant both to heat and to oxidation. Examples of such materials are platinum and iridium.