Glow Plug
A heating element in diesel engines that preheats the combustion chamber to assist in starting the engine under cold conditions.
A glow plug is a pencil-shaped metal heating element located in each cylinder of a diesel engine. Unlike gasoline engines that use spark plugs to ignite fuel with an electrical spark, diesel engines rely on compression ignition—compressing air until it reaches a temperature hot enough to spontaneously ignite the injected diesel fuel.
In cold weather, the cold engine block and cylinder walls absorb the heat of compression, preventing the air from reaching the required ignition temperature. Glow plugs solve this by preheating the air inside the combustion chambers or pre-chambers before and during cranking, ensuring a quick and start.
Modern glow plugs can reach temperatures exceeding 1,800 degrees Fahrenheit in a matter of seconds. They continue to run for a short period after the engine starts—a process known as post-heating—to reduce engine noise, rough running, and white exhaust smoke during warm-up.
Glow plugs wear out over time, leading to starting difficulties. Signs of failing glow plugs include hard starting in cold weather, engine misfires on startup, white or grey smoke from the tailpipe, and a glow plug warning light on the dashboard. Diagnosing and replacing glow plugs is a standard part of diesel engine maintenance.