## 4 Conclusions  
In order to enhance the performance of electric vehicle R744 air conditioning units, the effects of the gas cooler air volume flow rate on the key parameters have been experimentally studied. Furthermore, the coupling influence of operating conditions and air flow rate on the system performance have been studied in detail. As a result, a novel approach to evaluate the appropriateness of the gas cooler air volume flow rate has been implemented. The main conclusions of this work can be summarized as follows:  
1. the new approach suggests that the optimal gas cooler air volume flow rate can be selected as the one, which results in the discharge temperature, gas cooler outlet temperature and discharge pressure change rates below 2%;
2. high compressor speeds are recommended in order to promote optimal gas cooler air volume rates;
3. the ambient temperature does not influence the optimal gas cooler air volume rate significantly;
4. for the single-row and dual-process microchannel heat exchanger used in the experiment, the recommended value of the optimal air flow rate is about 2500 m\({}^{3}\cdot\)h\({}^{-1}\).  
Since the real time power consumption of the fan on the gas cooler side has not been measured to calculate the overall performance of the air conditioning system, there is still great potential for the enhancement of the evaluation criteria proposed in this article. In future work, the real time power consumption of the fan will be simultaneously assessed along with that of the compressor to evaluate the optimal air volume flow rate on the gas cooler side more accurately.  
Figure 10: Influence of ambient temperature on the optimal gas cooler air volume flow rate.  
**Author Contributions:** Conceptualization, A.W. and X.Y.; methodology, A.W.; software, J.F.; validation, Y.S., F.C.; formal analysis, X.Y.; investigation, A.W. and J.F.; resources, F.C.; data curation, A.W.; writing--original draft preparation, A.W.; writing--review and editing, Y.S., J.F. and P.G.; visualization, P.G.; supervision, F.C.; project administration, F.C.; funding acquisition, F.C. and X.Y. All authors have read and agreed to the published version of the manuscript.  
**Funding:** A.W., J.F., X.Y., Y.S. and F.C. are grateful to the National Natural Science Foundation of China [grant number 51976153], the National Natural Science Foundation of China [grant number 52006162] and the National Science and Technology Major Project [grant number 2017-III-0010-0036] for funding this research.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclature**  
COP  
Coefficient of performance  
D  
Depth, mm  
EXV  
Expansion valve  
GWP  
Global warming potential  
h  
Specific enthalpy, kJ-kg\({}^{-1}\)  
H  
Height, mm  
HVAC  
Heating, ventilation and air conditioning  
HVAC  
Internal heat exchanger  
L
Length, mm  
ODP  
Ozone depletion potential  
P
Pressure, MPa  
PBE  
Pressure upstream of expansion valve  
Q  
Cooling capacity, W  
SGC  
Series gas cooler  
T  
Temperature, \({}^{\circ}\)C  
V  
Volume flow rate, m\({}^{3}\).h\({}^{-1}\)  
W  
Power input, W  
x  
Uncertainty of independent variable  
**Greek symbols**  
\(\omega\)  
Resulting uncertainty  
\(\rho\)  
Density, kg\(\cdot\)m\({}^{-3}\)  
\(\eta\)  
efficiency  
**Subscripts**  
air  
Air  
airW  
Wet bulb temperature of air  
airD  
Dry bulb temperature of air  
dis  
Discharge  
eva  
Evaporation  
eva i  
Evaporator inlet  
eva o  
Evaporator outlet  
gasc_o  
Gas cooler outlet  
opt  
Optimal  
suc  
Suction  
V  
Volumetric  
I  
Isentropic  
R  
Representation of the parameter