Valve timing

The intake valve is opened before the exhaust gases have completely left the cylinder, and their considerable velocity assists in drawing in the fresh charge.

Engine designers aim to close the exhaust valve just as the fresh charge from the intake valve reaches it, to prevent either loss of fresh charge or unscavenged exhaust gas.

Variable valve timing can give both maximum power at high rpm and smooth idling at low rpm by making small changes to the relative angular position of the camshafts and thereby varying the valve overlap.

If tappet clearance is less, then valve will open early and close late.

However, the duration of the intake event can be controlled (the "cut-off") using the reversing gear and this reduces steam usage under cruising conditions.

Valve timing diagram for a 4-stroke engine
Two-Stroke engine showing ports in the cylinder walls. The timing cannot be varied.