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-chambers—glowplugs—while the direct-injected engines have these glowplugs in the combustion chamber.