## 4 Transient Simulation of the ICE-ORC System  
The performances of the ORC under various operation conditions of the engine were analyzed and the optimal operation conditions of the ORC with regard to the engine speed and the engine torque were obtained. When a vehicle is running on the road, most of the operation conditions of the engine are transient. Therefore, it is important to investigate the operation characteristics of the ORC system when the engine operates under practical transient conditions. The pump speed and the expander speed were controlled during the transient working process to get a high net power output. Based on the results of Figure 10, a feedforward control method was used to determine these two speeds according to the engine speed and the engine torque. The model for the ICE-ORC combined system and the control strategy were integrated in GT-POWER together with MATLAB as shown in Figure 4. The ICE-ORC model was built in GT-POWER and the control strategy was designed in MATLAB/Simulink. The interface block \"SimulinkHarness\" was used to connect the signals between these two different programs. In this study, the ECE-EUDC driving cycle was selected as a case study for the transient operation conditions of the engine. The corresponding vehicle speed is shown in Figure 11a. The ECE test simulated the driving conditions in urban areas while the EUDC test was used for the highway conditions. First, a truck model was built in the software Advisor. Then, the ECE-EUDC driving cycle was used as an input and the required engine speed and torque were determined shown as Figure 11b,c. Subsequently, the engine speed and torque data were input to the control strategy in MATLAB. In the designed control strategy, the injected fuel amount was controlled according to the required engine torque. A feedback PID controller was employed to ensure the actual engine torque output could follow the target one. The actual torque output was measured by a torque sensor in GT-POWER. Then, the PID controller calculated the difference between the required and measured torques and the injected fuel amount was regulated accordingly. The working parameters of the ICE such as the speed, the torque output, the temperature and mass flow rate of the exhaust gas were measured by the sensors in GT-POWER and then were transmitted to the control strategy in MATLAB/Simulink via the interface. In the Simulink model, the control strategy calculated the optimal speeds of the pump and the expander according to the engine speed and the engine torque by looking up two 2D tables. Then, the manipulated variables such as the injected fuel amount, the pump speed, and the expander speed were output from the Simulink model to the actuators in the GT-POWER model via the interface. The vehicle speed underwent a quick acceleration and deceleration frequently under the urban driving conditions. Therefore, the transmission needs to shift frequently. Accordingly, both the engine speed and the engine torque showed great fluctuation during the shifting process shown as Figure 11b,c. The exhaust energy may be not high enough to start the ORC system according to the results in Figure 9 because both the engine speed and the engine torque were small. Thus, the power output of the expander of the ORC showed a series of spikes ranging from 2.8 kW to 5.5 kW shown as Figure 11d. This pulsing operation was not helpful for the durability of the ORC system. When the vehicle was running on the highway conditions, the vehicle speed was increased greatly and the exhaust energy was large. Therefore, the ORC system can operate for most of the time except during the braking processes where the engine is normally shut off. The power output of the expander was in the range from 0.8 kW to 6 kW. The results indicated that the ORC system had a better performance under the highway driving conditions than that of the urban conditions.