Variable Valve Timing-A Possibility to Control Engine load without Throttle

A spark ignition engine's pumping loss increases over-proportionally when the engine load is reduced. Without throttling, control of the charge of the S. I. engine can be realized by variation of the intake valve opening period. Therefore, the variable valve timing has great potential for reducing pumping losses. A mechanically operating variable valve timing system has been implemented on a Porsche 924 TOP. Under both full load and part load, improvement is obtained with optimization of intake valve closing at any load and speed condition. Under low speed wide open throttle conditions, intake valve closing at the correct time causes an increase in volumetric efficiency and therefore a torque increase of up to 8%. Under lower part load conditions, fuel consumption can be improved by as much as 4%, and at idle by 10%. These improvements are the result of a 20%-50% reduction of pumping losses and of an improved combustion process due to less residual gas fraction.