In order to reduce the fuel overconsumption, new air conditioning systems fitted with externally controlled compressors have been implemented. Their operating principle consists in driving electrically (external control) the compressor displacement in order to adjust the cooling capacity delivered by the A/C system to the cooling capacity required by the passenger compartment. Therefore, it is possible to reduce the mechanical power absorbed by the compressor and by this means, the fuel overconsumption. However, this potential of fuel consumption reduction can be only achieved under the condition that the other components of the A/C system, such as the thermostatic expansion valve (TxV), are well adapted all together in order to fully take advantage of the implementation of such new type of compressors.
The paper describes the different possibilities of optimization of the TxV in regard to hunting phenomena. One of these solutions, consisting in the adjustment and in the increase of the valve setting (start opening characteristic) is described. However, it is pointed out that this solution must be considered very carefully, and other parameters such as compressor power consumption and reliability have also to be considered. A selection of a low TxV setting will lead to a too low limitation of the controllable air temperature, and thus will force the HVAC to operate in a partial reheat mode. On the contrary, a high setting will lead the TxV to feed the evaporator with a too high refrigerant flow rate: in this case, a part of the refrigerant will remain in a liquid phase at the outlet of the evaporator and will be sucked by the compressor. The consequences will be an increase of the mechanical power absorbed by the compressor in both cases, and a risk of a too rapid degradation of the compressor in case of liquid slugging.
The solution proposed will consist in selecting an intermediate TxV setting, allowing for both, minimization of the HVAC reheat mode and minimization of liquid refrigerant suction by the compressor.