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Home >   How To >   How to Repair Diesel Parts >   What is a Glow Plug?

What is a Glow Plug?


Why Do Diesel Engines Need Glow Plugs?


Gasoline engines employ spark plugs to ignite the air-fuel mixture in the combustion chamber during the power stroke of the engine. Whereas, diesel engines do not have spark plugs. The air-fuel mixture in these engines are combusted the following way: The intake air is compressed and when the piston reaches close to the top dead center (TDC), the diesel is injected into the combustion chamber. The injected fuel mixes with the compressed air, evaporates, and ignites almost simultaneously. The combustion of the diesel is made possible due to the following reasons: the intake air heats up during compression and diesel has a relatively low ignition temperature. No external ignition device, like a spark plug, is needed. However, with a cold diesel engine, the compression temperature might not be high enough to ensure the proper ignition of the injected fuel, resulting in increased exhaust emissions. The glow plug- an electrical heating device - helps solve this problem.



What is a glow plug?



How Does a Glow Plug Work?


A glow plug typically comprises a heating coil in a metal tube closed at one end and filled with electrically insulating ceramic powder. The closed end of the metal tube with the heating coil protrudes through a hole in the cylinder head, into the combustion chamber. When the glow plug is electrically energized, its heated portion reaches a surface temperature of more than 1000º Celsius within a few seconds. The air-fuel spray generated by the injector ignites close to the glow plug, which initiates combustion.


Advanced glow control modules like BERU’s Instant Start System (ISS) can be integrated into the engine management system to provide gasoline-like turn-key starting performance and additional glow functions for highly sophisticated emission reduction strategies.



What is a glow plug?



What Can Go Wrong with a Glow Plug?


Generally, the glow plug can wear out due to age and prolonged use. They can also fail due to continuous exposure to heat and corrosion. However, other problems can also cause the glow plug to go bad like a faulty glow plug relay.


The following are some of the common symptoms of a defective glow plug:


Hard Starting


While hard starting generally indicates problems in the fuel system and battery, sometimes, it also indicates a bad glow plug. If the fuel system and battery are working fine, it is likely that the glow plug is at fault.


Poor Engine Power


After experiencing a hard start, the engine will still struggle to run properly and can exhibit symptoms such as misfiring or rough running. As a defective glow plug will fail to ignite the fuel properly, it results in improper combustion. This reduces the engine power and efficiency.


White Exhaust Smoke


A bad glow plug will cause the diesel oil to leak into the exhaust. The fuel again burns in the exhaust system which results in the emission of white exhaust smoke.


As soon as you detect any symptoms of failure, have your glow plugs checked and replaced, if required. While inspecting your glow plug, do not forget to check the glow plug relay and the other fuel injection components, as well.


The Latest Innovations in Glow Plugs


With emission standards becoming increasingly strict, it is essential to obtain a better understanding of combustion processes inside the cylinder. Glow plugs offer mechanical access to the combustion chamber. Engineering efforts have been expended to utilize the glow plug for advanced sensor functions.


Hope you found this article about glow plugs helpful. For more automotive articles, you can go to our how-to section. If you are looking to replace your worn-out glow plugs, purchase one from We stock high-quality vehicle parts at the lowest prices online.


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