Four-Stroke Engine, the Otto Cycle

A four-stroke engine completes its cycle in four piston strokes, which takes two crankshaft revolutions, so each cylinder fires only once every 720 degrees of crank rotation. The camshaft therefore has to turn at half crankshaft speed, which is why the timing gears are in a two to one reduction.

On the intake stroke the inlet valve is open and the descending piston draws in the charge. On compression both valves are closed and the rising piston compresses it, raising both pressure and temperature. Ignition occurs shortly before top dead centre, and the power stroke follows as the burning gases drive the piston down with both valves still closed. Finally the exhaust stroke pushes the products out through the open exhaust valve.

Only the power stroke does useful work, so the flywheel carries the engine through the other three. Raising the compression ratio raises thermal efficiency, because the expansion extracts more of the heat released, but it is limited in practice by detonation: too high a ratio, or too low a fuel grade, and the charge self-ignites violently instead of burning progressively.

• Try this: Step through the strokes one at a time and note the valve positions. In which two strokes are both valves closed, and why must they be?
• Try this: Raise the compression ratio and watch the efficiency. Why can this not simply be increased without limit?
• Try this: Watch the crank angle over a full cycle. How many degrees of crank rotation pass between one firing and the next, and what does that mean for camshaft speed?