A glowplug (alternately spelled as glow plug or glow-plug) is a heating device used to aid starting diesel engines.
One common, and very effective, method for fixing this problem is to fit each cylinder with a glowplug. For that reason, many smaller diesel engines come pre-fitted from the factory with glow-plugs.
In a diesel-engine car, unlike in a gasoline-engine car, the operator does not simply turn the key to the "start" position and have the engine immediately start. Instead, the operator turns the key to the "on" position; 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".
If the car has been running very recently, or if the ambient temperature is hot, the "wait to start" light may not come on; in this case, the operator may proceed to turn the key to the "start" position and start the engine without having to wait.
A glowplug is a pencil-shaped piece of metal with a heating element at the tip; that heating element, when electrified, heats due to electrical resistance and begins to emit light in the visible spectrum, hence the term "glow" plug; the effect is very similar to that of a toaster. The heat generated by the glowplugs is directed into the cylinders, and serves to warm the engine block immediately surrounding the cylinders. This aids in reducing the amount of thermal diffusion which will occur when the engine attempts to start.
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. A pre-heating cycle usually lasts for 10 to 20 seconds. 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.
Glowplugs have a limited lifespan. Certain factors, such as the aforementioned overheating, can greatly shorten that lifespan. The amount of heat which a glowplug produces diminishes over time; this causes the engine to become progressively harder to start in cold weather.
Glowplugs are inexpensive, and typically easy to replace -- making it worthwhile to replace them annually in areas with a cold winter season.
Modern automotive diesel engines use various electronic methods of altering the timing and style of the injection process to ensure reliable cold-starting. Glow plugs are still fitted, but are rarely used for more than a few seconds.