When the piston descends from the top dead center to the bottom dead center in the intake stroke, the piston opens and the exhaust valve closes. When the piston completes the first stroke, it moves up and down, and when it reaches the top dead center, it is called the compression stroke. At this time, the intake valve and exhaust valve are closed.
Then the piston will continue to descend to the bottom dead center, which is called the power stroke. At this point, the valve is still closed. When the work is completed, the piston will rise to the top dead center, which is called the exhaust stroke. At this time, the intake valve is closed and the exhaust valve is opened.