Conditions That Limit Oil Circulation in a Mobile Air-Conditioning System

An important design objective for an automotive air-conditioning system is ensuring sufficient oil return to the compressor. Maintaining proper lubrication of the compressor extends compressor life and thus minimizes warranty and consumer replacement costs.

An automotive air-conditioning test stand capable of monitoring transient system parameters was utilized. Real-time oil concentration data were obtained using an optical, oil-concentration sensor installed in the liquid line. The mass flow rate of the refrigerant-oil mixture and the evaporator superheat were varied with an electronic expansion valve. Steady-state oil concentration data are presented for both saturated and superheated evaporator exit conditions. Experimental oil concentration data taken during system startup and quasi-steady compressor-clutch-cycling conditions are also presented.

Steady-state results show that oil return rate is maximized by maintaining a two-phase condition at the evaporator exit. Doing so prevents the accumulation of oil in the evaporator and thus provides better lubrication of the compressor. The observed behavior can be explained in terms of the viscosity of the refrigerant-oil film. Analysis shows that the viscosity of the oil film in a superheated section of the evaporator is quite high creating a significant resistance to oil flow. Adding only a small amount of liquid refrigerant to the film reduces the viscosity substantially thus allowing the oil to more freely return to the compressor.

Compressor start-up and quasi-steady tests reveal a significant peak in oil concentration approximately 10 s after clutch engagement. This time delay corresponds to the transport time between the compressor and the oil concentration sensor as calculated by a numerical model of the mobile air-conditioning system. Thus, the data suggest that a significant amount of oil is ejected when the compressor is first started although additional studies are needed to confirm and refine this conclusion.