Variable Valve Timing and Valve Lift

The fuel economy of a conventional spark-ignited gasoline engine falls short of that of a direct injection diesel engine at part loads mainly due to the heavy and irreversible pumping losses, introduced by the throttle. Also, the valve timings are optimum only at full load and mid-speed. At all other operating points, many important parameters like residual gases, volumetric efficiency, and thermal efficiency are compromised. Variable valve timings (VVT) systems are, therefore, implemented in different variations taking into consideration the system’s complexity, durability, and cost. However, VVT itself results in some throttling in the low-load range affecting the thermodynamic efficiency. Incorporating variable valve lift (VVL) could almost eliminate the throttle, reducing the pumping losses and improving combustion efficiency due to turbulence enhanced by higher flow velocities at the valves. VVL could be achieved by a relatively simpler switching between two cams with larger and smaller lifts or a more complicated continuously variable lift by changing the fulcrum of the valve rocker arm. By experiments and computer simulation using commercial software packages, the optimum timings and lifts can be determined for VVT and VVL. It has been found that varying inlet valve timings are more advantageous over exhaust valve timing variation either in conjunction with the inlet timings or alone. Therefore, VVT and VVL are applied for the inlet only in the majority of engines.