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Automotive fixed orifice tube (FOT) systems are especially prone to cycling losses due to their clutch cycling operation. Therefore, it is important to better understand the dynamics of the refrigerant and oil migration during transient events such as cycling and start-up. To measure the refrigerant mass and oil distribution of an automotive R134a FOT breadboard system, two ball valves around each component are added. By simultaneously closing the valves, the refrigerant and oil is trapped in different sections of the system and can be measured. The transient refrigerant migration during a stop-start transient as well as the refrigerant mass distribution as a function of system charge at steady state operation is presented. A transparent accumulator and transparent tubes at the inlet and outlet of the accumulator are used to visualize the flow of the refrigerant. High speed video snapshots are presented for the first seconds after the start-up.

This paper presents experimental data from an investigation of an R744 dual evaporator system for automobile applications in a breadboard laboratory. Several expansion devices combinations were investigated, focusing on comparing fixed area and controlled area expansion devices. The role of an accumulator in an R744 dual evaporator system is demonstrated by experiments conducted with a prototype accumulator in which the outflow of liquid R744 and oil is controllable. Contrary to a conventional single R744 evaporator system, the refrigerant outlet qualities in a dual evaporator system are not fixed by the accumulator inlet/exit quality in steady state. An accumulator in an R744 dual evaporator system only guarantees that the quality of the mixed flow downstream of the evaporators has the same quality as the inlet/exit quality of the accumulator.

This paper presents one possible control strategy for a transcritical R744 automotive air conditioning system. The controller is developed for systems using clutch cycling, variable displacement, and variable speed compressors. The controller adjusts the high side pressure, blower speed, reheat, and modulates the compressor to achieve its goal. Test data is presented to support the claims and assumptions made and develop the controller.

This paper describes the performance of a transcritical R744 a/c system for a typical compact car. The prototype was designed to have heat exchanger dimensions, face velocities and air-side pressure drops nearly identical to conventional R134a systems. Heat exchangers are made of flat aluminum extruded multiport tubes; the 21 cc compressor is of swash plate design, and the system can be controlled by either a needle valve or a backpressure valve. Test matrices are defined for the purpose of developing component and system simulation models, as well as supporting data-to-data comparisons at normal, seasonal and extreme operating conditions. The design of experiments and test facility are also discussed. Results are presented for the R744 system operating in both steady state and cycling modes.

This paper presents experimental results on the performance of an a/c system with orifice tube for a typical compact car. Tests were conducted over a wide range of operating conditions, covering both steady state and cycling. The test matrices are presented and discussed. Special care was taken to understand the cycling conditions and the way to determine energy efficiency in this operating mode. The system test facilities and component instrumentation designed to achieve that goal are described. The paper also discusses issues related to the design of experiments and facilities for such comparisons.

Transcritical R744 systems are under consideration for use in mobile air conditioning. Experimental results indicate that the inherent thermodynamic shortcomings of R744 could be overcome by exploiting inherent advantages in transport properties, pressure drop, and other characteristics. This paper describes one possible way to improve both capacity and energy efficiency of the R744 system for the car air conditioning. It considers the use of a suction line heat exchanger. Four different configurations of the prototype system were tested: without heat exchanger, and with three coaxial heat exchangers having lengths 1.0, 1.5 and 2.0m. All heat exchangers were made from identical pipes. Results show that both the capacity and the coefficient of performance could be increased by up to 25%. Greater influence is registered at the higher temperatures of the air to the gas cooler.