Focusing on the problem wherein the outdoor heat exchanger of a vehicle heat pump system is prone to frost in winter

this study designs and builds a corresponding experimental system for testing. The frost and defrost characteristics of the outdoor heat exchanger were studied. Then

the effects on defrosting time of two different electronic expansion valve (EXV) calibers

?2.1 mm and ?3.0 mm

were compared. The experimental results show that the degree of frosting on the heat exchanger can be judged by the change rate of the suction temperature or pressure relative to the non-frosted state

and the completion of defrosting can be judged by the temperature by which the outdoor heat exchanger outlet exceeds 0 ℃. For defrosting conditions with different ambient temperatures and different frost layers

the impact of EXV caliber on defrosting time is not consistent. The EXV with a large caliber has a good defrosting effect in the early stage

but in the later stage

it is not as good as the EXV with a small caliber because the compressor exhaust pressure of the large-caliber EXV rises slowly

causing the inlet temperature of the outdoor heat exchanger to increase slowly. Therefore

in real vehicle applications

a larger caliber EXV can be used in combination with intelligent control to achieve the best overall defrosting effect.