How does Engine Valve Timing help?
This air-fuel mixture (also known as charge) has both mass and motion. The entire charge cannot enter the cylinder even when the piston reaches the end of its downward stroke because the opening of the inlet valve is small. Therefore, the pressure in the combustion chamber remains below the atmospheric pressure, while the charge is still moving in the direction of the motion of the piston with high velocity.
If the inlet valve closes at this point, the cylinder will receive less charge than its requirement. Hence, the inlet valve is kept open until the piston enters its next upward stroke (i.e) the compression stroke. At this point, the pressure in the cylinder becomes almost equal to the atmospheric pressure. The engineers precisely calibrate the actual closing point of the inlet valve in such a way that it coincides with the point where the motion of the incoming charge starts to reverse.