## 3 Results and Discussion
### Test Experiments  
Additional experiments were carried out using the proposed test bench to assess, by way of example, the performance of the cascaded closed-loop control scheme for the driver's zone shown in Figure 11, tuned by the authors under different conditions; specifically, temperature transient and steady-state responses in the presence of setpoint changes both in driver and passenger zones, outside temperature and solar irradiance were analysed. Time responses of the interior air temperature of driver and passenger zones (\(T_{i1}\) and \(T_{i2}\), respectively) are shown in Figure 16 for the first proposed experiment, where the driver's zone setpoint was initialised at 21 \({}^{\circ}\)C and, subsequently, was changed to 23 \({}^{\circ}\)C around time = 166 min. The temperature setpoint for the passengers' zone was kept constant at 22 \({}^{\circ}\)C, the outside temperature was between 6.5 \({}^{\circ}\)C and 11.5 \({}^{\circ}\)C and a low, uniformly distributed (the same for all sides) solar irradiance was set to 50 W. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experiment** & \multicolumn{2}{c}{**Driver’s Zone**} & \multicolumn{2}{c}{**Passengers’ Zone**} \\ \cline{2-5}
**Number** & **Max. Error (\({}^{\circ}\)C)** & **MAE (\({}^{\circ}\)C)** & **Max. Error (\({}^{\circ}\)C)** & **MAE (\({}^{\circ}\)C)** \\ \hline
1 & 2.35 & 0.57 & 2.39 & 0.46 \\
2 & 1.58 & 0.28 & 0.91 & 0.17 \\
3 & 1.13 & 0.35 & 1.13 & 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Performance indices of selected validation tests from hybrid test bench and actual experimental coach data. Figure 15: Absolute errors between temperatures of the hybrid test bench and the actual experimental coach data (each column represents experiments 1, 2 and 3). Suitable setpoint tracking for \(T_{i1}\) and \(T_{i2}\) can be observed in Figure 16, where temperature changes in the driver's zone barely have an impact on the passengers' zone; additionally, the time response is slower in the former, primarily due to the strong influence from the latter, with a larger air volume, which is a well-known fact from previous experience. Complementarily, the passengers' zone setpoint changed from 21 \({}^{\circ}\)C to 23 \({}^{\circ}\)C around time = 166 min in the second proposed test experiment, keeping the setpoint constant for the driver's zone to 22 \({}^{\circ}\)C (Figure 17); all other conditions were almost the same as in the first experiment, with minimal variation regarding the outside temperatures. It can be observed from Figure 17 that there are significant impacts of the passengers' zone temperature changes on the driver's zone and how the control system is able to bring its interior air temperature back to its setpoint. This effect completely reproduces the real coach dynamics, showing this unidirectional strong interaction. For the third proposed test experiment, the temperature setpoint was set to 21 \({}^{\circ}\)C both in the driver and passenger zones and the outside temperature was gradually reduced from 10.5 \({}^{\circ}\)C to 2.8 \({}^{\circ}\)C beginning at time = 170 min and ending at time \(\approx\)200 min, while maintaining a uniform solar irradiance of 50 W. The results obtained are shown in Figure 18.  
Figure 16: Transient and steady-state responses of \(T_{i1}\) and \(T_{i2}\) from the first test experiment using the proposed hybrid system. As can be seen, the applied disturbance was effectively rejected in both zones, but with a much slower response for \(T_{i1}\), even more than in other experiments. In addition to the lower volume present in the driver's zone, in this case, the outside air also cools down the water coming from the engine, which is the heat source of the heating system, slowing down its reaction. Figure 17: Transient and steady-state responses of \(T_{i1}\) and \(T_{i2}\) from the second test experiment using the proposed hybrid system. Figure 18: Transient and steady-state responses of \(T_{i1}\) and \(T_{i2}\) from the third test experiment using the proposed hybrid system.