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Automotive air conditioning systems are subject to constantly changing operation conditions and steady state simulations are not sufficient to describe the actual performance. The refrigerant mass migration during transient events such as clutch-cycling or start-up has a direct impact on the transient performance. It is therefore necessary to develop simulation tools which can accurately predict the migration of the refrigerant mass. To this end a dynamic model of an automotive air conditioning system is presented in this paper using a switched modeling framework. Model validation against experimental results demonstrates that the developed modeling approach is able to describe the transient behaviors of the system, and also predict the refrigerant mass migration among system components during compressor shut-down and start-up (stop-start) cycling operations.

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.

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.