A glow plug helps start a cold diesel engine by heating the combustion chamber. Its electrically heated tip supplies extra heat when compression alone cannot reliably raise the air-fuel mixture to ignition temperature.
Diesel engines do not normally use spark plugs. They compress air until it becomes hot, then inject fuel into that high-pressure air; the fuel ignites spontaneously. In cold conditions, heat is lost to the cylinder head, block, and surrounding environment, so the compressed air may not become hot enough. A glow plug compensates during starting and may continue operating briefly afterward.
Older systems often required a driver to wait for a dashboard indicator before cranking, sometimes for roughly 20 seconds. Modern controllers measure temperature and manage the plugs automatically, making preheating faster and allowing “after-glow” operation to smooth early running and reduce emissions.
A glow plug is therefore a starting aid, not the diesel equivalent of a spark plug in normal operation. Some diesel engines start without one in warm conditions, while indirect-injection engines generally depend on glow plugs more heavily in cold weather.