## 4 Diurnal Dynamic Numerical Calculation and Verification  
Previously, the research team carried out massive analysis and verification on the maximum temperature of the auto-parts during midday period on several working days; however, to further predict and master the diurnal dynamic variation regularity of heat transfer and temperature of automobile exposed to natural environment, the following dynamic numerical calculation and analysis of the automobile heat transfer model are conducted.  
The size of the model corresponding to the test car is \(3.9\,\text{m}\times 2.1\,\text{m}\times 1.5\,\text{m}\), and the material physical parameters of each part are given according to the references and databases [45; 46; 47; 48]. The discrete model and physical property parameters are shown in Figure 5 and Table 4, respectively. The environmental weather parameters such as solar radiation, air temperature, wind speed, wind direction and atmospheric pressure continuously monitored in the natural exposure test, as well as the geographical parameters such as longitude, latitude, time zone and altitude where the test site is located, are taken as the definite solution conditions of the heat transfer model. Combined with the dynamic heat transfer Equations (1) and (10)-(13) in the above composite heat transfer model calculation method, the compound heat transfer process of the model is analyzed in detail, including the radiation, conduction and convective heat transfer, and then the dynamic simulation temperature of the complete car during day and night is calculated in real time. In the process of calculation, the discrete model and dynamic environment parameters are corrected in real time until the dynamic simulation results are consistent with the experimental data.  
The simulation verification and relative error analysis of key parts inside and outside the car are shown in Figure 6. The numerical simulation values of key parts, such as dashboard, lower dashboard decoration, bumper, engine cover, etc., are all highly consistent with the temperature values in the diurnal dynamic test, and the relative errors of some measured points are even less than 5%.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Components** & **Material** & **Density/kg\(\cdot\)m\({}^{-3}\)** & \
\begin{tabular}{c} **Thermal** \\ **Conductivity/W/(m\(\cdot\)K)** \\ \end{tabular} &
\begin{tabular}{c} **Specific** \\ **Heat/J/(Kg\(\cdot\)K)** \\ \end{tabular} \\ \hline Dashboard & PP & 908.3 & 0.26 & 1882.8 \\ Lower dashboard decoration & ABS & 1120.0 & 0.21 & 1591.0 \\ Cigar lighter cover & ABS & 1120.0 & 0.21 & 1591.0 \\ Body frame & Steel & 7768.9 & 52.02 & 460.9 \\ Window & Glass & 2529.6 & 1.17 & 754.0 \\ Bumper & PC & 1200.0 & 0.24 & 1170.0 \\ Headlight & PMMA & 1190.0 & 0.18 & 1464.6 \\ Exterior A-pillar triangle board & PA & 930.0 & 0.30 & 1502.2 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Physical property parameters of some key parts.  
Figure 5: Discrete model of sedan with three compartments.  
Figure 6: Comparison and verification between dynamic simulated and measured temperature of key parts: (**a**) dashboard temperature; (**b**) lower dashboard decoration temperature; (**c**) cigar lighter cover temperature; (**d**) bumper temperature; (**e**) engine cover temperature; (**f**) headlight temperature; (**g**) relative error of dashboard temperature; and (**h**) relative error of rear bumper temperature.