1.  Field of the Invention
This invention relates to air source heat pumps, and more particularly to solar augmented air source heat pump systems employing a multi-cylinder reciprocating compressor. 
2.  Description of the Prior Art
A reciprocating compressor has long been employed to compress refrigerant vapor in an air source heat pump system with the compressor in series with and between the outdoor and indoor coils which coils trade functions; the outdoor coil constituting the air source evaporator under heating mode and the indoor coil, the condenser; while during cooling mode the indoor coil becomes the system evaporator and the outdoor coil becomes the air source condenser.  When the heat pump is operating under heating mode, the system compression ratio increases as the air source heat pump system operates under colder and colder ambient.  For instance, assuming that the reciprocating compressor comprises four cylinders and assuming 100% volumetric efficiency under single stage operation would equal four flow units, at 50% volumetric efficiency the single stage operation results is equivalent to two flow units.  In higher system compression ratios, the reciprocating compressor volumetric efficiency drops to very low value and 25% volumetric efficiency under max heating conditions are common.  For a single stage four cylinder operation, the result is one flow unit at the higher compressor ratios. 
Further, it is conventional to improve system efficiency by incorporating a subcooler between the indoor and outdoor coils which functions to subcool the liquid refrigerant downstream of the coil constituting the condenser and prior to feeding the same to the coil acting as an evaporator of the system.  A portion of the high pressure liquid refrigerant is bled from the system and vaporized to further reduce the temperature of that portion of the refrigerant delivered to the coil functioning as the evaporator for the system under that particular mode.  The vapor generated in the subcooler is at a pressure which is well above the suction pressure to the reciprocating compressor.  The expansion of that refrigerant to the pressure of the refrigerant vapor passing from the downstream side of the coil functioning as the evaporator in the system and entering the inlet or suction side of the compressor, constitutes a system loss reducing the efficiency of the heat pump system. 
Solar collectors have been employed as a source of thermal energy to supplement thermal energy input to refrigeration systems, particularly heat pumps. 
It is therefore an object of the present invention to provide a simplified automatic heat pump system employing a reciprocating compressor which minimizes the reduction in real volume of suction gas pumped by the reciprocating compressor as the system compression ratio increases when the air source heat pump system operates in colder and colder ambient. 
It is a further object of this invention to provide a simplified air source heat pump system which automatically shifts from the air source evaporator to a solar source evaporator under system heating mode when the temperature of the solar source exceeds that of the air source evaporator ambient or when otherwise determined to be economical. 
It is a further object of this invention to provide an improved air source heat pump employing a multi-cylinder reciprocating compressor in which the cylinders are staged while maintaining load reversal on the wrist pins of the reciprocating compressor pistons and connecting rod assemblies under automatic staging conditions. 
It is a further object of this invention to provide an improved air source heat pump system employing an automatic staging, multi-cylinder reciprocating compressor, wherein the vapor returned from a system subcooler may be selectively directed to the suction side of the high stage cylinder or cylinders under low temperature ambient conditions.