In a four-stroke cycle, which sequence correctly lists the stages?

• A. Intake, exhaust, compression, power.
• B. Intake, compression, exhaust, power.
• C. Compression, intake, power, exhaust.
• D. Intake, compression, power, exhaust.

Answer: (D)

Four-stroke operation follows a fixed sequence of strokes: intake, compression, power, exhaust. During intake, the piston moves down with the intake valve open, pulling in the air-fuel mixture. Next, with the intake valve closed, the piston moves up to compress that mixture. The power stroke comes when the mixture is ignited by the spark, and the resulting explosion pushes the piston down, providing useful work. Finally, the exhaust stroke has the exhaust valve open while the piston moves up, pushing the burnt gases out of the cylinder. This order is essential because each step prepares the next: drawing in fresh charge, increasing its pressure, delivering combustion-driven force, and clearing the exhaust so the cycle can start again. A handy mnemonic is “intake, compression, power, exhaust.”