Why Do Diesel Engines Need Glow Plugs?

Unlike gasoline engines in which spark plugs ignite a premixed air-fuel mixture in the combustion chamber, diesel engines work according to the so-called self-ignition process. With this process, intake air is compressed and close to top dead center diesel fuel is injected into the combustion chamber. The injected fuel mixes with the compressed air, evaporates and ignites almost simultaneously because the intake air heats up during compression and diesel fuel 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 proper ignition of the injected diesel fuel, causing inconvenience to the driver and increased exhaust emissions. The glow plug - an electrical heating device - helps to solve this problem.

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 1000°C within a few seconds. The air-fuel spray generated by the injection ignites close to the glow plug and 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.

The Latest Innovations In Glow Plugs

Driven by ever stricter emission standards, it will become 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.

Federal-Mogul and Siemens VDO Automotive have announced that they will jointly develop a combustion sensor that is integrated into the glow plug.

BERU has developed an advanced piezo-resistive Pressure Sensor Glow Plug (PSG). This combines an electromechanical glow plug with sophisticated sensor functions and electronics. Both sensor concepts aim to monitor combustion pressure directly in the cylinder.

 

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