Understanding Compression in Diesel Engines

Diesel engines operate on a principle called the diesel cycle. A key characteristic of the diesel cycle, unlike the petrol (gasoline) cycle, is how combustion is initiated.

In a diesel engine, during the compression stroke, the piston moves upwards in the cylinder, significantly reducing the volume above it. The medium present in the cylinder at the beginning of the compression stroke is solely air.

This compression of air causes its temperature to rise dramatically. The compression ratio in a diesel engine is much higher than in a petrol engine, leading to a much higher compressed air temperature.

Once the air is highly compressed and hot, fuel (diesel) is injected into this hot air. The high temperature of the compressed air causes the fuel to ignite spontaneously without the need for a spark plug.

In contrast, petrol engines compress a mixture of air and fuel, which is then ignited by a spark.

Therefore, the medium that is compressed in a diesel engine cylinder during the compression stroke before fuel injection is primarily air.