During which stroke does the piston compress the air/fuel mixture?

The compression stroke is the phase in a four-stroke engine cycle where the piston moves upward in the cylinder, compressing the air-fuel mixture that has been drawn in during the previous intake stroke. This upward movement of the piston creates higher pressure and temperature conditions, which are crucial for effective ignition and combustion.

As the piston approaches the top of its travel, the mixture is compressed to a smaller volume, making it more reactive when the spark plug ignites it at the end of this stroke. This increased pressure and temperature improve the combustion efficiency and performance of the engine during the subsequent power stroke.

The other strokes play different roles in the engine operation: the intake stroke brings in the air-fuel mixture, the power stroke is when the combustion occurs and drives the piston down, and the exhaust stroke expels the spent gases from the cylinder. Each of these strokes has a distinct function, but it is specifically during the compression stroke that the air-fuel mixture is prepared for ignition.