A low compression ratio is desirable because of its direct effect on the volumetric efficiency of the compressor. The clearance volume of vapor in the compression stroke is not expelled from the cylinder. Power used to compress this gas does not do any useful work. Wasted power may vary from 10 to 50% of total power applied. Ratio of actual volume pumped to calculated volume is called volumetric efficiency. A lot of time has been spent trying to improve on valve plate and piston design in an attempt to decrease the amount of gas wasted on each compression stroke.
Excerpt from the Technical Institute Manual #1 of the Refrigeration Service Engineers Society online store. Visit RSES.ORG for more details.