 Use of automatic valves simplified the mechanism, but "valve float" limited the speed at which the engine could run, and by about 1905 mechanically operated inlet valves were increasingly adopted for vehicle engines.Mechanical operation is usually by pressing on the end of the valve stem, with a spring generally being used to return the valve to the closed position.At high revolutions per minute (RPM), the inertia of the spring means it cannot respond quickly enough to return the valve to its seat between cycles, leading to valve float, also known as "valve bounce".