The development of a high-efficiency, two-capacity, three-cylinder, reciprocating-piston, hermetic compressor for the advanced electric heat pump is described. The patented capacity-modulation concept employs an eccentrically bored circular cylinder cam between crankpin and connecting rod, which acts to change the stroke of the compressor upon a change in the direction of rotation of the crankshaft. Complementary design features that result in high efficiency include (1) a circulating oil system that cools the drive motor and the compressor and rejects heat external to the compressor shell, (2) a suction piping system that routes gas into the cylinders in a manner to impede heat transfer, (3) a discharge piping scheme that impedes heat transfer and reduces pressure loss, (4) an improved valve and valve plate system that reduces flow impedance and in conjunction with a crowned piston reduces clearance volume, thereby improving both capacity and efficiency, and (5) a high-efficiency, dual-strength drive motor (not detailed here).