Four-Stroke Engine — the Otto Cycle

In a four-stroke (Otto-cycle) engine the crank turns twice for every power stroke. Intake: piston descends, inlet valve open, drawing in mixture. Compression: both valves shut, piston rises and squeezes the charge. Power: the spark fires, hot gas drives the piston down. Exhaust: the exhaust valve opens and the rising piston pushes the burnt gas out. Only the power stroke does useful work — the flywheel carries the engine through the other three.

• Try this: Watch which valves are open during each stroke. When are both shut?
• Try this: Raise the compression ratio. Why does that tend to improve efficiency?
• Try this: Count crank revolutions per power stroke. Why is it two, not one?