The first stroke is called the intake stroke. The piston moves down the cylinder, creating a partial vacuum in the cylinder. A mixture of air and fuel is then forced through the inlet valve into the cylinder by atmospheric pressure, now greater than the pressure in the cylinder. The exhaust valve stays closed during the stroke. The downward stroke of the piston that draws fuel into the combustion chamber of a four stroke engine. This fuel is compressed by the the subsequent upward stroke of the piston and ignited. The resulting high pressure in the combustion chamber forces the piston downward on a power stroke, and upward on the exhaust stroke.