Unlike the petrol engine, the diesel engine is a self-igniter. The air suctioned into the cylinders is heated up to a temperature of approx.
700-900 °C through compression, which results in self-ignition when fuel is injected. Therefore, a diesel engine requires higher compression (compression ratio 20-24:1) and a correspondingly more stable construction than the petrol engine. In order to guarantee that the necessary temperature is reached even in unfavourable conditions such as cold-starting or frost, additional heat must be introduced to the combustion chamber.
The role of the glow plug in the diesel engine
In principle, the glow plug functions like an immersion heater: Electrical energy is introduced through a coil resistor, which heats up to very high temperatures (up to 1000 °C).
However, in practice this simple principle causes some difficulties in regard to service life, overheating protection and current consumption. As a result, starting processes in the 1960s still lasted up to 30 seconds. In the 1980s this was already reduced to starting times of 3-5 seconds. With the introduction of TDI engines, differences from petrol engine became almost imperceptible with outside temperatures above 0 °C. Only temperatures below 0 °C require the additional pre-glowing.
Most recent common rail diesel engines with ceramic glow plugs even offer cold-starting behaviour almost identical to that of petrol engines. Only at temperatures below - 10 °C is a short pre-glowing period noticeable.
Glow plugs support exhaust gas cleaning
The role of glow plugs is increasingly shifting from the traditional function as cold-starting aid to becoming an integral part of a complex system for exhaust gas treatment. This is apparent in the fact that some engines will signal a defective glow plug as a fault in the emission control. This is because glow plugs more and more frequently support DPF regeneration and help avoid EGR clogging by means of intermediate glowing.