Power Stroke: One of the four 180-degree full "sweeps" of the piston moving in the cylinder of a four-stroke, internal-combustion engine (originally devised by Nikolaus Otto in 1876). During the power stroke, the piston moves from TDC to BDC as the burning air/fuel mixture forces the piston down the cylinder. Note: The 180-degree duration of the power stroke is commonly longer than the duration between top dead center and the exhaust-valve opening point, sometimes referred to as the true "Power Cycle." The power stroke is followed by the exhaust stroke.