The glow plug heats up the combustion chamber. This process before the actual starting of the engine is also called "pre-glowing".
Different glowing states of glow plugs

For a reliable cold start, particularly at low ambient temperatures, diesel engines depend on the support of glow plugs.

This is because when starting, the cylinders and engine have cooled down significantly. They extract energy in addition to the already cold environmental air. The temperature required for self-ignition is no longer reached through compression of the air alone.

This is where the glow plug comes into play. It is screwed into the cylinder head. Its glow tube projects into the combustion chamber and heats it up as soon as it is supplied with current. Depending on the glow plug, it can reach over 1000°C, and in this manner it heats up the combustion chamber. This process before the actual starting of the engine is also called "pre-glowing".

At the beginning of the pre-glowing, current flows through the connecting bolt and the regulating coil to the heating coil, which heats up rapidly and causes the point of the rod to glow. The glowing points heat up at various speeds depending on the engine type. In doing so, the temperature is additionally increased by the current of the already heated regulating coil. As a result, its electrical resistance increases and the current is reduced to the extend that the glow rod cannot be damaged.

Different glowing states of glow plugs

After-glowing - What does it mean?

Glowing after the start, while the engine is running, reduces white/blue-smoke and eliminates cold start knocking. The glow system itself consists of the self-regulating glow plugs made of metal, or ceramic, and an electronic glow-time control unit and a temperature sensor.

The self-regulation glow rod plugs protect against overheating by limiting the current from the battery to the plug as the temperature increases. However, the voltage increases while the engine is running. In the process, some glow plugs can burn through. In addition, after start-up the plugs are also exposed to the combustion temperatures, from the inside, and are heated up from the outside.

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