How does a diesel engine generate a power stroke?

A diesel engine generates a power stroke primarily through the process of compression ignition. In this process, air is compressed within the cylinder to a very high temperature, often exceeding 500 degrees Fahrenheit. Once the air is compressed, diesel fuel is injected directly into the combustion chamber. The high temperature of the compressed air ignites the fuel spontaneously without the need for spark plugs, which distinguishes diesel engines from gasoline engines that use spark ignition.

The role of compressed air is crucial here because it creates the ideal conditions for ignition. The compression increases the pressure and temperature of the air to the point that when the diesel fuel is injected, it ignites almost immediately, resulting in a power stroke as the rapidly expanding gases push down the piston.

The other options do not accurately represent diesel engine operation. Gasoline is primarily used in gasoline engines, and fuel vapor does not ignite in diesel engines in the same manner as it does in gasoline engines. Spark ignition is specific to gasoline engines, not diesel. Thus, the correct answer captures the essence of how a diesel engine operates during the power stroke.