The combustion cycles of a diesel engine basically correspond to those of a petrol engine. However, no spark plugs are required for the ignition of the air-fuel mixture.
The reason: diesel engines are self-igniting. Environmental air is suctioned in and compressed heavily in the cylinders. The compression ratio in this process is up to 25:1.
Through this compression the suctioned air reaches a temperature between 700 and 900 °C. If fuel is now added, it ignites solely on the basis of this heat.
For a reliable cold start, even with low outside temperatures, diesel engines depend on the support of glow plugs.
The reason: 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 extends into the combustion chamber and heats up as soon as it is energised. Depending on the glow plug model, the temperature reaches over 1000 °C, thus heating up the combustion chamber.
This process before the actual starting of the engine is called "pre-glowing".