During which stroke of a four-stroke internal combustion engine does air charge the cylinder?

In a four-stroke internal combustion engine, the intake stroke is the phase where air (and often fuel) enters the cylinder. During this stroke, the intake valve opens, and as the piston moves down in the cylinder, it creates a vacuum that draws the air charge into the combustion chamber. This is crucial for the engine's operation since a proper air-fuel mixture is necessary for combustion to occur in the subsequent compression and power strokes.

The intake stroke is significant because it sets up the conditions for efficient combustion, which is essential for the engine's performance. Understanding this stroke is fundamental for diagnosing engine issues, tuning performance, and optimizing fuel efficiency.