Most engines have two valves for each cylinder, one intake and one exhaust valve.
Since each of these valves operates at different times, separate operating mechanisms must be provided for each valve. Valves are normally held closed by heavy springs and by compression in the combustion chamber. The purpose of the valve-actuating mechanism is to overcome the spring pressure and open the valves at the proper time. This thrust over-comes the valve spring pressure as well as the gas pressure in the cylinder, causing the valve to open. When the lobe moves from under the tappet, the valve spring pressure reseats the valve.