What are the four strokes of a typical four-stroke engine cycle, in their correct order?

• A. Intake, Exhaust, Compression, Power
• B. Intake, Compression, Power, Exhaust
• C. Compression, Power, Intake, Exhaust
• D. Exhaust, Intake, Compression, Power

Answer: (B)

The sequence tested is the order in which a four-stroke engine completes its cycles: intake, compression, power, exhaust. Start with the intake stroke, where the intake valve opens and the piston moves downward, drawing in the air-fuel mix. Then the compression stroke has both valves closed as the piston moves upward, squeezing the charge to a higher pressure. Next is the power stroke, where the mixture is ignited (spark plug in gasoline engines) and the rapidly expanding gases push the piston downward, delivering power. Finally, the exhaust stroke has the exhaust valve open while the piston moves upward, pushing the burnt gases out of the cylinder.

The other sequences don’t fit because they would either try to compress without a charge having entered, or expel exhaust before combustion has occurred, or start with exhaust with no intake yet. This order—intake, compression, power, exhaust—allows continuous, controlled operation of a four-stroke engine.