On the compression stroke, what does a diesel engine compress?

During the compression stroke in a diesel engine, only air is compressed. This is a critical aspect of diesel engine operation because, unlike gasoline engines that use a spark plug to ignite a mixture of air and fuel, diesel engines rely on the heat generated from compressing air to ignite the fuel.

As the piston moves upward in the cylinder during the compression stroke, the volume of the cylinder decreases, causing an increase in pressure and temperature of the air. This high temperature is essential for the subsequent introduction of fuel through fuel injectors at the end of the compression cycle. The fuel then ignites spontaneously due to the elevated temperatures in the cylinder.

This process distinguishes diesel engines from other types of internal combustion engines, where an air-fuel mixture is compressed. In this case, the diesel engine's methodology ensures higher efficiency and enables it to operate under heavier loads and conditions.