volumetric efficiency - ratio of the weight of air drawn into the cylinder of an operating internal combustion engine to the weight of air the cylinder could hold at rest when the piston is at the bottom of the stroke and the valves are fully closed. Any restriction of air flow into the cylinder reduces volumetric efficiency, which, in turn, reduces power output. The volumetric efficiency of an automotive engine is usually slightly more than 80% at about half the rated speed of the engine, then decreases considerably at higher speed, thus limiting the power output of the engine. The air charge to the cylinder can be increased at high speeds by means of supercharging. See supercharger.