An internal combustion engine has a stator passage leading to a cylinder. An intake poppet valve is disposed between the cylinder and stator passage, and a rotary valve adjoins the opposite end of the stator passage. The rotary valve has a rotor with an elongated tapered rotor passage partially in registration with the stator passage. The rotor is driven by the crankshaft and is adjustable in phase to vary the time of opening and closing of the rotary valve, and is axially shiftable to vary the valve opening duration. A control system responsive to engine speed and load independently varies the rotary valve phase and duration for optimum engine operation.