Thermodynamic and fluid-dynamic properties of carbon dioxide with different lubricants in cooling circuits for automobile application

The application of carbon dioxide as the refrigerant in vehicle air conditi oning requires the development of a suitable lubricant for the quickly rota ting compressor. The phase behavior of different synthetic lubricants with carbon dioxide forms the basis for developing a CO2- cooling circuit. Furth ermore, the influence of carbon dioxide upon the viscosity of the oil is es sential for evaluating a lubricant. These data of the binary mixtures have been measured at temperatures between 5 and 100 degreesC under pressures of up to 150 bar. The qualitative observation of phase behavior took place in a high-pressure view cell, whose complete content is monitored: The phase equilibria have been determined in an autoclave based on a static-analytica l method. The viscosity of the lubricant which is saturated with carbon dio xide has been measured with an integrated quartz viscosimeter. The phase be havior of oils with carbon dioxide can be divided into three different type s: binary systems with closed miscibility gaps, systems with open miscibili ty gaps, and systems that shaw barotropic phenomena. Oils that show barotro pic behavior in contact with compressed carbon dioxide are not recommended as lubricants. Oils with complete or limited miscibility with carbon dioxid e may be used. The knowledge of phase behavior and the resulting viscosity of gas-saturated lubricants allow one to evaluate the applicability of lubr icants in car-climatization systems.