To see how valve-timing works in a 4-stroke engine cycle, let’s show piston motion as a circle. In this simple cycle, each stroke is shown as a semi-circle.
This intake valve opens at top dead center, and closes at bottom dead center. The blue line shows that period and it matches the intake stroke.
The exhaust valve opens at bottom dead center, then closes at top dead center before the new air-fuel mixture enters the cylinder.
In practice, the intake valve usually opens earlier than top dead center, and stays open a little past bottom dead center.
The exhaust valve opens a little before bottom dead center, and stays open a little past top dead center.
When the valves actually open and close, can be measured by angles. To make these angles easier to read, let’s use a spiral instead of a circle.
This intake valve opens 12° before the piston reaches top dead center.
And it closes 40° after bottom dead center.
The exhaust valve opens 47° before bottom dead center - and stays open - until 21° past top dead center. This gives exhaust gases more time to leave.
By the time the piston is at 47° before bottom dead center on the power stroke, combustion pressures have dropped considerably and little power is lost by letting the exhaust gases have more time to exit.
When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called lead.
When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag.
On the exhaust stroke, the intake and exhaust valve are open at the same time for a few degrees around top dead center. This is called valve overlap. On this engine, it is 33°.
Different engines use different timings. Manufacturer specifications contain the exact information.