Which engine stroke is characterized by the compression of the air-fuel mixture?

The correct answer is the compression stroke, which is pivotal in a four-stroke engine's operation. During this phase, the piston moves upward in the cylinder after the intake stroke, pressing the air-fuel mixture into a smaller volume. This reduction in volume increases both the pressure and temperature of the mixture, preparing it for ignition.

The compression stroke is crucial because it allows for more efficient combustion when the fuel mixture is ignited by the spark plug. A higher pressure during ignition leads to a more powerful explosion, thereby maximizing the engine's power output. This concept is fundamental in understanding how internal combustion engines achieve their efficiency and performance, as the proper compression can directly impact the engine's power and efficiency.

In contrast, the intake stroke is focused on drawing in the air-fuel mixture, the power stroke is when the explosion occurs and drives the piston down, and the exhaust stroke clears the burnt gases from the cylinder. Understanding these different strokes is essential for grasping the complete cycle of four-stroke engine operation.