Most four-stroke
piston engines today employ one or more
camshafts to operate
poppet valves. The lobes on the camshafts operate cam followers which in turn open the poppet valves. A
camless (or, free valve engine) uses electromagnetic, hydraulic, or pneumatic actuators to open the poppet valves instead. Actuators can be used to both open and close the valves, or an actuator opens the valve while a spring closes it.