What type of combustion occurs in a diesel engine?

In a diesel engine, combustion occurs primarily through the process of compression ignition. This method relies on the engine's high compression ratio to increase the temperature of the air inside the cylinder. When fuel is injected into the highly compressed and hot air, it ignites spontaneously without the need for a spark plug. This is a fundamental principle of diesel engine operation, distinguishing it from gasoline engines, which utilize spark ignition to initiate combustion.

Compression ignition is highly efficient and allows diesel engines to operate at higher compression ratios, leading to improved fuel efficiency and torque characteristics compared to other types of engines. This process is crucial for the performance of diesel engines, making it the correct answer to the question. In contrast, the other types of combustion mentioned do not accurately describe the way diesel engines function, as they rely on different ignition methods that aren't applicable to diesel technology.