A super – charged or turbo – charged engine has an enhanced volumetric efficiency in assessment with an engine which does not use forced induction. The best volumetric efficiency is generally attained somewhere around a maximum torque of 2500 – 3000 RPM for many engines. Volumetric Efficiency should not be confused with specific power. Volumetric efficiency in the internal combustion engine design denotes the efficiency with which the engine can move the charge in and out of the cylinders. More specifically, volumetric efficiency is the ratio (it can be percentage also) of the quantity of air which is trapped by the cylinder during induction over the brushed off volume of the cylinder under still conditions.
Volumetric Efficiency can be enhanced in a number of ways; most efficiently this can be attained by compressing the induction charge (forced induction) or through forced cam phasing in naturally aspirated engines as seen in at times of racing. Volumetric efficiency can exceed 100% in the case of forced induction. There are many ways by which one can improve volumetric efficiency other than a wide system approach must be used to fully understand it’s potential. Several high performance cars use carefully set air intakes and tuned exhaust systems which use pressure waves to push air in and out of the cylinders while bringing resonance of the system into use. The Two – stroke engines are very susceptible of this concept and can use expansion chambers which return the air – fuel mixture back to the cylinder which was diffusing out. A more modern technique for 4 stroke engines, variable valve timing, makes an attempt to bring the changes in volumetric efficiency with changes in speed of the engine into notice, i.e., at higher speeds the engine requires the valves open for a greater percentage of the cycle time to move the charge into and out of the engine.