Based on the VVT-i system, the VVTL-i system has adopted a cam changeover mechanism that varies the amount of lift of the intake and exhaust valves while the engine is operating at high speeds. In addition to achieving higher engine speeds and higher outputs, this system enables the valve timing to be optimally set, resulting in improved fuel economy. When the engine is operating in the low- to mid-speed range, the low- and medium-speed cams of the camshafts operate to move the two valves via the rocker arms. Then, when the engine is operating in the high-speed range, the signals from the sensors cause the engine's control module (ECM) to change the hydraulic passage of the oil control valve (for the variable valve lift), thus changing to the operation of the high-speed cams. Now the lift of the intake and exhaust valves increases, allowing the introduction of a greater volume of air-fuel mixture, as well as the discharge of a greater volume of exhaust gases. As a result, the engine operates at higher speeds and higher outputs when the engine is under more strain.