A small single cylinder four stroke SI engine varying capacity from 100cc to 500 cc generally used for two wheelers has a wide range of engine speeds during operation and may vary from 1000 to 8000 rpm. Such variation in speed of engine demands variation in the valve timing and lift of engine at different speeds for optimum performance. Volumetric efficiency is the measures of performance of an engine. It is often referred as breathing capacity of engine. The power output of an engine depends directly on the amount of charge that can be inducted in the cylinder. When an engine is throttling, a plate obstructs the air intake flow and causes a pressure drop across the plate. In conventional SI engines, the throttle-based air control wastes about 10% of the input energy in pumping the air. Throttling not only reduces the amount of air induced into the engine, but also it introduces flow losses which reduce an engine’s efficiency. One of the methods of increasing the volumetric efficiency of an engine is reducing the mechanical losses associated with throttling through Continuous Variable Valve Lift (CVVL) mechanism which provides throttle free load control. In this lift is controlled by the inlet valve, according to load condition.The present work aims at analyzing the volumetric efficiency of a four stroke spark-ignition engine with CVVL mechanism. Nature of volumetric efficiency has been obtained and compared for with throttle and without throttle valve condition. The analysis has highlighted a great influence of throttle valve on volumetric efficiency and hence on valve lift. Volumetric efficiency with throttle free operation achieved with CVVL mechanism is always superior to throttled operation in SI engine.
Keywords : Volumetric Efficiency, Throttling, Continuous Variable Valve Lift (CVVL) Mechanism, Throttle Free Load Control