Refer to figure.
Diesel engines are sometimes labeled as ‘compression ignition’ engines as it uses the heat generated during the compression stroke to ignite the mixture. For this reason, diesel engines are not fitted with spark plugs and are not heated (except in icing conditions) and require less maintenance and spare parts which ultimately reduces costs. The mixture creation is also different from the standard petrol engines as the diesel engines use direct fuel injection systems to mix the fuel and the air in a pre-chamber before supplying it to the cylinder. The fuel injection system consists of atomizers that will spray the fuel into the pre-chamber, but because not all the fuel droplets burn completely, residual soot will remain in the cylinder.

To produce enough heat for the mixture to ignite, the diesel engines operate at a much higher compression ratios than the normal petrol engine: from 14:1 up to 28:1 for the most powerful compared to 9:1 for a petrol engine. Because of this high compression ratio, diesel engines also benefit from a higher thermal efficiency. The amount of power delivered will only be determined by the fuel flow. In the same way as for car engines, diesel engines are less powerful than a petrol engine for the same swept volume.