Volumetric efficiency (Ev) is an important variable in reciprocating compressor calculations, affecting the diameter, stroke, and speed for a given compressor capacity. The compressor cylinder’s actual inlet flow (ICFM) is the product of the volumetric efficiency and the cylinder’s displacement over time, expressed in cubic feet per minute (CFM).

Ev is related to the clearance volume in the cylinder (the volume to the left of the dotted lines in Figure 300-2). On the PV diagram, it is the volume between points 0 and 3. The total volume displaced by a full stroke of the piston in the single-acting cylinder illustrated in Figure 300-2 is the volume between points 1 and 3. Clearance is usually expressed as a percentage of the displaced volume as follows:

The theoretical volumetric efficiency (in percent) of a cylinder is: