The Effects of Oil in Circulation on the Performance of an Automotive Air Conditioning System

An automotive air conditioning system was subjected to varying percentages of oil in circulation (OIC) to investigate its impact on the evaporator, the condenser, and cycle performance. Since automotive air conditioning systems do not typically use oil separators, the compressor lubricant circulates through the system and effects evaporator, condenser and cycle performance. The OIC of the system was controlled by means of an oil separator installed in a parallel line at the compressor outlet allowing variable amounts of refrigerant to pass through the separator. The separated oil was returned upstream of the compressor inlet. It was found that the amount of OIC had an impact on the air conditioning system’s performance. The information presented allows the designer to judge the performance merits of installing an oil separator in an actual automotive air conditioning system of the type that was being simulated. The test system was built on a workbench to facilitate installation of various sensors to measure temperatures, pressures, relative humidity, flow rates, and power consumption. The workbench was placed across two environmental chambers such that the environmental conditions at the evaporator and condenser could be independently controlled. A test matrix of varying refrigerant flow rates, air flow rates, environmental conditions, and OIC were designed to simulate a range of car speed conditions that would be encountered in practice. An analysis was performed on the data collected from the workbench and the equations used in that analysis are presented in the paper. Although the results show some increase in heat transfer performance and cycle efficiency the overall improvements were moderate.