Glow plug

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.

Design

Glowplugs are typically used as starting-aid devices, when the engine is being cranked by a starter motor, and in most cases switched off when the engine has reached a defined minimum operating temperature. Diesel engines, unlike common spark-ignition engines, do not use spark plugs to ignite the air-fuel mixture. Instead, they rely solely on compression to raise the temperature of the air to a point where the fuel combusts spontaneously when introduced to the hot, high pressure air. When the engine's surroundings are cold, the air's thermal energy – that it has previously received from the starter motor's compression work – is absorbed by the cylinder head and engine block, and eventually transferred to the air surrounding the engine. When the temperature of the air surrounding the engine comes below a certain value, depending upon cylinder head design, (40 °C for precombustion chamber injected, 20 °C for swirl chamber injected, and 0 °C for direct injected engines), the engine loses too much compression heat to reliably initiate combustion, and the engine fails to start.