A piston displacement device for a reciprocating compressor comprises a piston linearly reciprocated in a cylinder and having a stroke length varied by a reciprocating motor, a plurality of resonance springs for guiding a resonant motion of the piston, and a piston transfer unit for moving the piston...http://www.google.ca/patents/US7934910?utm_source=gb-gplus-sharePatent US7934910 - Piston displacement device for reciprocating compressor
