An experimental analysis of cycling in an automotive air conditioning system

In the majority of automotive air conditioning systems, the compressor cont inuously cycles on and off to meet the steady-state cooling requirements of the passenger compartment. Since the compressor is a belt-driven accessory device coupled to the engine, its cycling rate is directly related to the vehicle speed. The refrigeration system's losses increase with increasing v ehicle speed and thus with increasing compressor cycling. This paper identi fies and quantifies individual losses in an automotive vapor-compression re frigeration system during compressor cycling. The second law of thermodynam ics, in particular, nondimensional entropy generation, is used to quantify the thermodynamic losses of the refrigeration system's individual component s under steady driving conditions at idle, 48.3 kph (30 mph), and 96.6 kDh (60 mph). A passenger vehicle containing a cycling-clutch orifice-tube vapo r-compression refrigeration system was instrumented to measure refrigerant temperature and pressure, and air temperature and relative humidity. Data w ere collected under steady driving conditions at idle, 48.3 kph (30 mph), a nd 96.6 kph (60 mph). A thermodynamic analysis is presented to determine th e refrigeration system's performance. This analysis shows that the performa nce of the system degrades with increasing vehicle speed. Thermodynamic los ses increase 18% as the vehicle speed changes from idle to 48.3 kph (30 mph ) and increase 5% as the vehicle speed changes from 48.3 kph (30 mph) to 96 .6 kph (60 mph). The compressor cycling rate increases with increasing vehi cle speed, thus increasing the refrigeration system's losses. The component with the greatest increase in thermodynamic losses as a result of compress or cycling is the compressor itself. Compressor cycling reduces the compres sor's isentropic efficiency, and thus the system's thermodynamic performanc e. The individual component losses of the refrigeration system are quantifi ed. The redistribution of these losses is also given as a function of incre asing vehicle speed (i.e, increasing compressor cycling). At 96.6 kph (60 m ph), the thermodynamic losses, based on the ratio of entropy generation to entropic load, are 0.22, 0.10, 0,07, and 0.02 in the compressor, the conden ser, the evaporator-accumulator, and the orifice tube, respectively. The co mpressor losses dominated the overall system performance. The overall syste m efficiency could be significantly improved by increasing the compressor's efficiency. The compressor's efficiency could be improved by reducing or e liminating cycling, such as could be accomplished by using a variable capac ity compressor or by not directly coupling the compressor to the engine. An other way to increase the compressor's volumetric efficiency during cycling would be to reduce the compressor operating range. This could be accomplis hed by using two compressors such as is done in two-stage cascade refrigera tion systems. (C) 2000 Elsevier Science Ltd. All rights reserved.