In the Miller-cycle engine, the piston begins to compress the fuel-air mixture only after the intake valve closes; and the intake valve closes after the piston has traveled a certain distance above its bottom-most position: around 20 to 30% of the total piston travel of this upward stroke. So in the Miller cycle engine, the piston actually compresses the fuel-air mixture only during the latter 70% to 80% of the compression stroke. During the initial part of the compression stroke, the piston pushes part of the fuel-air mixture through the still-open intake valve, and back into the intake manifold.