## 5 Characteristic Analysis of Bench Test Results during Engine Starting Process
The preset engine ignition advance angle shown in Figure 6f rises from the ignition delay angle of 1.5\({}^{\circ}\) to the ignition advance angle of 21.75\({}^{\circ}\) during the initial combustion process, contrary to the usual trend of engine fuel injection pulse width. Figure 7 shows the engine speed and torque characteristics, including the engine starting process. In the case of Figure 7a,b, the actual speed and torque of the engine output shaft (which is directly connected to the planetary frame) collected from the HBM T40B torque sensors are very different from those of the CAN network. In Figure 7a, the engine speed appears at an obvious low-frequency fluctuation, where the collected engine speed fluctuation is more evident than the engine speed fluctuation in the CAN network. Moreover, the speed characteristics of the latter are never below 300 rpm. In Figure 7b, the collected engine torque fluctuation is much more obvious than that shown in the CAN network. The pulsating torque fluctuation of the engine is most distinct during the electric start phase of the engine, and it then declines with an increase in the engine speed. Therefore, in the mode switching process involving the engine start-up, the engine speed and torque of the CAN network cannot satisfy the requirements necessary to control a PSHEV. In actual control conditions, the engine output shaft speed and torque need to be estimated online to suit the requirements of actual control accuracy. Figure 8a illustrates the collected torque of the actual driveshaft. The torque of the driveshaft in the bench test mainly comes from the superposition of the torque at the output of the reducer, the torque of the power dynamometer, and the inertia resistance torque of the system flywheel. The driveshaft torque keeps a constant positive torque in the EV stage. In the electric engine starting process, the torque of the driveshaft shows a large positive torque mutation and then remains constant. After ignition at 1.1 s, the driveshaft torque fluctuates and starts to trend towards negative torque. Figure 8b shows the equivalent speed without considering the influence of flywheel inertia. In the EV stage, the equivalent speed basically remains at about 30 km/h. Once the electric engine starting process occurs, the equivalent speed decreases before it starts to fluctuate. Then, the fluctuation ranges of speed decrease gradually after ignition. As shown in Figure 9a, the equivalent vehicle jerk is the second derivative of equivalent speed. In the EV stage, the equivalent jerk is less than 10 m/s\({}^{3}\). When entering the electric engine starting process, the equivalent jerk presents a wide fluctuation. The maximum value of the equivalent jerk reaches 92.12 m/s\({}^{3}\). Figure 9b denotes the data results of vibration acceleration in the x-direction, which are collected by the vibration acceleration sensors mounted on the transmission housing. It shows that the vibration acceleration fluctuates slightly in the EV stage and is less than 0.9 m/s\({}^{2}\). Again, when entering the electric engine starting process, the vibration acceleration fluctuates, and the maximum value of the absolute value of the vibration acceleration in this process is 4.077 m/s\({}^{2}\). After the ignition and fuel injection, the vibration acceleration fluctuates considerably, and its maximum value reaches 4.78 m/s\({}^{2}\). After this, the fluctuation is maintained at 2.5 m/s\({}^{2}\). It should be noted that the maximum vibration amplitude after ignition is caused by the combustion heat accumulation explosion of the engine transient ignition injection cylinder. Figures 6c and 7b show that the engine torque drops as the intake manifold pressure decreases during the electric start phase. Since the inlet manifold pressure of the engine directly affects the pressure change in the engine cylinder before ignition and injection, the excessive inlet manifold pressure before ignition and injection is one of the main reasons for the large output torque ripple of the engine.