How does a compression ignition engine ignite the fuel?

A compression ignition engine ignites fuel through the process of compressing air within the cylinder, which significantly increases the temperature of the air. When diesel fuel is injected into this highly compressed and heated air, it ignites spontaneously without the need for an external spark. This ignition process is fundamental to how diesel engines operate, differentiating them from spark-ignition engines that rely on spark plugs to ignite a fuel-air mixture.

In a compression ignition engine, the high compression ratios—typically between 14:1 and 25:1—lead to temperatures that exceed the auto-ignition point of the diesel fuel. This self-ignition capability is what allows compression ignition engines to operate efficiently and effectively on fuels that necessitate such conditions to combust. The other options involve methods of ignition that are not used in compression ignition engines, such as the use of electrical sparks or external spark plugs, which are characteristic features of gasoline or spark-ignition engines.