In a diesel engine, a glow plug (also spelled glowplug) is a heating device used to aid starting of the engine in cold weather. This device is a pencil-shaped piece of metal with an electric heating element at the tip.
A glowplug system consists of either a single glowplug in the inlet manifold, or one glowplug per cylinder. In older systems, the driver is required to manually activate the glowplug system and wait approximately 20 seconds before starting the engine. Newer systems automatically activate the glowplug(s) before the engine is started and have a quicker warm-up time.