A glowplug (alternatively spelled as glow plug or glow-plug) is a heating device used to aid starting diesel engines. In cold weather, high speed diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition (which relies on that heat). Pre-chambered engines make use of small electric heaters inside the pre-chambersglowplugswhile the direct-injected engines have these glowplugs in the combustion chamber. The glowplug is a pencil-shaped piece of metal with a heating element at the tip. This heating element, when electrified, heats due to its electrical resistance and begins to emit light in the visible spectrum, hence the term "glow"-plug. The visual effect is very similar to that of a toaster. The fuel injector spray pattern then impinges directly upon the hot tip of the glow plug during the injection of fuel at top dead center. This enables the fuel to ignite even when the engine is insufficiently hot for normal operation. This reduces the cranking time needed to start the diesel engine.