From stage 1 to stage 2, the enthalpy of the refrigerant stays approximately constant, thus h 1 ~ h 2. From stage 2 to stage 3, heat is put into the system, thus q in = h 3 – h 2 = h 3 – h 1. From stage 3 to stage 4, work is put into compressor, thus 4 – h 3. JDM Japanparts.com - JDM Parts,Performance Auto Parts. RA-R MIRROR COVER PAIR.