Zheng, Y., "Ways to Determine Vehicle Dual AC System Charge Level," SAE Technical Paper 2014-01-0697, 2014, doi:10.4271/2014-01-0697.
This paper addresses various ways to determine vehicle dual AC system charge level. Traditionally, either checking charge level plateau and/or using the certain condenser outlet subcooling magnitude are adopted to determine AC system charge level. It is challenging to determine refrigerant charge level in the following scenarios: (1) Some AC systems do not exhibit the flatted charge plateau. (2) The condenser outlet subcooling continues to rise. (3) The system has the requirements to run both front and aux evaporators, front evaporator only and aux evaporator only.It was found that compressor compression ratio of absolute discharge pressure to absolute suction pressure always presents the bath tub curve for all AC systems. When the system reaches the optimal charge level, the evaporator air outlet temperatures show the stable trend. In addition to the traditional condenser subcooling method, few approaches are presented in the paper. One way to determine the dual evaporator system charge level is: checking compressor compression ratios vs. charge level on the tests with (1) running both front and aux evaporators, (2) running front evaporator only (aux evaporator off). In addition, checking front evaporator discharge air temperature is introduced. The second way to determine the dual evaporator system charge level is: using compressor refrigerant density ratio of discharge to suction vs. charge level. The third way to determine the dual evaporator charge level is: using AC system unit energy efficiency vs. charge level and compressor mass flowrate vs. charge level. By adding more depth analyses on system and compressor behaviors, the optimal charge of the dual evaporator system becomes an obvious selection when there is a challenge in using the traditional condenser subcooling method.