During which stroke is the air-fuel mixture greatly compressed?

The compression stroke is the phase in the four-stroke engine cycle where the air-fuel mixture that was drawn into the cylinder during the intake stroke is significantly compressed. This compression occurs because the piston moves upward in the cylinder, decreasing the volume of the space that the mixture occupies.

During this stroke, both the intake and exhaust valves are closed, allowing the piston to compress the mixture without any loss of pressure. The compression increases the temperature and pressure of the mixture, preparing it for ignition. This is critical because a higher compression ratio leads to more efficient combustion when the spark plug ignites the mixture, resulting in greater power output during the subsequent power stroke.

The other strokes serve different purposes: the intake stroke is for drawing in air-fuel mixture, the power stroke is the phase where the actual power is produced from the combustion, and in the exhaust stroke, the burnt gases are expelled from the cylinder. Each stroke plays a vital role in the overall operation of an engine, but it is specifically the compression stroke that focuses on compressing the air-fuel mixture for efficient ignition.