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This paper presents the oil circulation behavior in a CO2 climate control system operating at low evaporating temperature down to -32°C. The increase of oil circulation ratio (OCR) from 0 to 6 wt.% during steady state conditions degrades the coefficient of performance and cooling capacity by 15% and 8%, respectively. The pressure drop across the heat exchangers increases, especially in the gas cooler. In low temperature CO2 systems some fluctuations of oil and refrigerant flow rates were observed during cyclic operations when the system did not equip the oil separator, but was observed only at high oil charge when the system did equip the oil separator. These instabilities lead to a periodic compressor performance fluctuation, which caused system performance degradations. Therefore, the use of an oil separator is recommended for the low temperature operation if an ordinary metering valve is adopted as an expansion device without any special control strategy.

This paper presents the effect of the oil circulation ratio on the system performance of a CO2 climate control system and the effect of an oil separator on the oil circulation behavior using a new oil circulation ratio measurement method. In this study, the oil circulation ratio of a CO2/PAG mixture at various system conditions was measured. When the oil circulation ratio increases from 0.5 wt.% to 7 wt.%, the coefficient of performance decreases by 8% and 11% for idling and driving conditions, respectively. Without the oil separator, the oil circulation ratio increases more than 10 wt.%; with the oil separator, it was maintained at below detection level and 1.1 wt.% for idling and driving conditions, respectively.