Hunting Phenomena Of Automotive Air Conditioning Systems With Variable Displacement Compressor

Automotive air conditioning (AAC) system with a variable displacement compressor (VDC) sometimes tends to experience hunting problems, where the system undergoes oscillation. The purpose of this paper is to study the hunting phenomena in this system and to find out the factors to cause the system stability by experimental investigation. In this paper, the test bench and testing method are described and four kinds of hunting phenomena summarized from our experiments are introduced in detail, i.e., hunting of the evaporator and thermal expansion valve (TEV) control loop, hunting in the AAC system with a VDC and fixed-area throttling device, hunting in the AAC system with a AAC and TEV, and hunting of ungoverned AAC system. Some analyses are made on this four hunting phenomena. These conclusions will give a further understanding for hunting phenomena of the automotive air conditioning system with a variable displacement compressor. INTRODUCTION The compressor capacity in an automotive air conditioning system with a constant displacement compressor is controlled by cycling off and on thereby creating an engine disturbance, which is becoming more noticeable as vehicles and engines become smaller. The infinitely variable displacement wobble plate compressor (here-after called “the variable displacement compressor” or VDC) was developed to address this problem. This type of compressor changes the angle of the wobble plate, and the piston stroke length, and the displacement consequently according to an increase or decrease of the pressure differential between the wobble-plate case and the suction chamber of the cylinder head to exactly match the vehicle air conditioning demand. The system has the advantages such as smooth continuous compressor operation, more comfortable environment inside the car, no frost formation in the evaporator and improved fuel economy compared to a system that cycles the compressor off and on to control capacity (Skinner, T.J 1985 and Takai, K 1989). However in the automotive air conditioning system with a variable displacement compressor, there is the hunting phenomenon sometimes, i.e., all parameters such as the refrigerant mass flow rate, refrigerant pressures and temperatures at different points, and the supply air temperature oscillate with constant amplitude and constant period. The instability can lead lower safety, lower life span and higher energy consumption and restrains its broad application. Inoue, A. et al (1988) had made an experimental investigation on evaluation of VDC with TEV in an AAC system, the interrelationship of refrigerant flow rate adjustment of VDC and TEV and the influence of VDC and TEV on system hunting were analyzed. Tian, C.Q et al (2000, 2001) had analyzed the factors that influence system stability by the experimental investigation into an AAC system with a VDC and TEV. But above researches just dealt with the AAC system with a VDC and TEV and many problems remain unresolved. The paper tries to study the hunting phenomena in the AAC system with a VDC this system and to find out the causes to raise the system stability by experimental investigation. TEST SYSTEM The test bench, shown in Figure 1, was built inside two environmental chambers. The compressor, the condenser and the receiver were placed in the outdoor environmental chamber, the evaporator and the throttling device in indoor one. Both chambers can keep constant temperature and humidity needed in our experiments. The whole system includes main experimental body, the control system and the measure system. The main body in the test system contains the compressor, the condenser, the receiver, the evaporator and the throttling device. The compressor used here is a 5 cylinder; wobble plate type, infinitely variable displacement compressor, whose displacement range is from 10 to 156 cc (cubic centimeter per revolution). The condenser is parallel type, the evaporator core is tube-fin type, the evaporator fan is driven by a four-speed motor. There are two throttling devices installed in parallel in this test system, the one throttling device is the thermal expansion valve and the other is the electronic expansion valve (pulse motor controlled) that is manually controlled and can be regarded as a fixed area throttling device when not be adjusted. So the test system can be used to observe the hunting phenomena of the AAC systems with a VDC and TEV or with a VDC and fixed area throttling device like orifice tube.