What are the four strokes of a spark-ignition piston engine?

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

Answer: (B)

In a spark-ignition piston engine, the cycle that produces power runs in four steps in a specific order: intake, compression, power, exhaust. The intake stroke opens the intake valve and the piston moves downward, drawing in the air-fuel mixture. Then the compression stroke has both valves closed as the piston moves upward, compressing that charge to a high pressure. At the top of that compression, the spark plug fires, igniting the mixture and driving the piston downward during the power stroke to convert the combustion energy into usable work. Finally, the exhaust stroke opens the exhaust valve while the piston moves upward again, pushing the burnt gases out of the cylinder. This sequence repeats, forming the standard four-stroke cycle. Starting with exhaust or skipping the intake would not allow the engine to draw in fresh charge or sustain the cycle, which is why the intake–compression–power–exhaust order is the correct one.