An experimental analysis of cycling in an automotive air conditioning system

Abstract

In the majority of automotive air conditioning systems, the compressor continuously 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 vehicle speed and thus with increasing compressor cycling. This paper identifies and quantifies individual losses in an automotive vapor-compression refrigeration system during compressor cycling. The second law of thermodynamics, in particular, nondimensional entropy generation, is used to quantify the thermodynamic losses of the refrigeration system’s individual components under steady driving conditions at idle, 48.3 kph (30 mph), and 96.6 kph (60 mph). A passenger vehicle containing a cycling-clutch orifice-tube vapor–compression refrigeration system was instrumented to measure refrigerant temperature and pressure, and air temperature and relative humidity. Data were collected under steady driving conditions at idle, 48.3 kph (30 mph), and 96.6 kph (60 mph). A thermodynamic analysis is presented to determine the refrigeration system’s performance. This analysis shows that the performance of the system degrades with increasing vehicle speed. Thermodynamic losses 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 vehicle speed, thus increasing the refrigeration system’s losses. The component with the greatest increase in thermodynamic losses as a result of compressor cycling is the compressor itself. Compressor cycling reduces the compressor’s isentropic efficiency, and thus the system’s thermodynamic performance. The individual component losses of the refrigeration system are quantified. The redistribution of these losses is also given as a function of increasing vehicle speed (i.e. increasing compressor cycling). At 96.6 kph (60 mph), 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 condenser, the evaporator-accumulator, and the orifice tube, respectively. The compressor losses dominated the overall system performance. The overall system efficiency could be significantly improved by increasing the compressor’s efficiency. The compressor’s efficiency could be improved by reducing or eliminating cycling, such as could be accomplished by using a variable capacity compressor or by not directly coupling the compressor to the engine. Another way to increase the compressor’s volumetric efficiency during cycling would be to reduce the compressor operating range. This could be accomplished by using two compressors such as is done in two-stage cascade refrigeration systems.