Experimental evaluation of electric vehicle compressor lubricating oil robustness for heat pump application

Abstract

Lubricating oil is the key to electric compressor performance, reliability, and durability in electric vehicle (EV) heat pumps. Previous investigations have revealed the lubricant's impact on fluid properties, component and system performance in vehicle climate control systems, but there have been few studies on heat pump application from the “robustness” perspective. In this study, we experimentally evaluated the robustness of two commercially available lubricants, polyalkylene glycol (PAG) and polyol ester (POE), for an electric scroll compressor in an electric vehicle heat pump by lubricant-refrigerant miscibility test, heat pump bench test and retrieved oil composition analysis. We showed that even if the PAG-R134a mixture kept miscible at low temperature, some insoluble substances would appear in the suction side accumulator below −10 °C, resulting in insufficient oil-return, increased compressor discharge temperature and superheat, and thus poor compressor lubrication. The mass spectrum analysis showed that the insoluble substances were reaction products of moisture and additives in the PAG oil. However, the POE oil used in this study did not show any insoluble substance under the same test condition. By comparison, the POE oil showed better robustness than the PAG. The experimental results indicate that the chemical composition of the lubricant should be carefully considered to ensure the robustness for compressor lubrication in EV heat pumps.