## 3 Operating Characteristics of the ICE-ORC System
### Operation Characteristics Under Various Conditions  
The high temperature exhaust gas was delivered to the ORC as the heat source. The temperature and mass flow rate of the exhaust gas varied with the engine speed and the engine load. Therefore, the operation characteristics of the ICE had a great influence on the performance of the ORC. Normally, a vehicular ICE would operate under various conditions such as high-speed or high-load conditions during a journey. It is necessary to investigate the performance of the ORC when the ICE operates over a wide range. First, the waste energy of the exhaust gas over the entire engine's operation range was analyzed. The mass flow rate of the exhaust gas increased as the engine speed rose. The simulated mass flow rate ranged from 35.94 g/s to 185.59 g/s. The exhaust temperature increased as the engine load rose. When the engine speed was fixed, the engine load was essentially proportional to the injected fuel amount. A higher engine load resulted in a higher exhaust temperature because the compression ratio of the engine was constant, leading to a larger amount of waste energy. The simulated exhaust temperature was in the range of 599-972 K. In this study, the minimum exhaust temperature at the outlet of the  
Figure 7: Influence of the expander efficiency: (**a**) The net power and (**b**) the thermal efficiency. evaporator was fixed as 350 K. Accordingly, the maximum waste energy for each operation point of the engine was determined according to the mass flow rate of the exhaust and the temperature decrease through the evaporator. The obtained exhaust energy was approximately proportional to the power output of the engine, increasing as the engine speed or the engine load increased. The maximum exhaust energy reached 75.55 kW under the rated power condition of the engine. Different engine operation points based on the engine speed and the engine torque were selected over the entire engine's operation range. The ICE-ORC model was simulated under each operation point and the results were analyzed. Figure 8 shows the results for the performance of the ORC under a constant engine load while the engine speed increased gradually. The engine load was fixed at 100% while the engine speed was set to 1600, 2600, and 3600 r/min, respectively. The variation tendencies of the net power as a function of the pump speed and the expander speed under different engine speeds were similar. However, the optimal operation point of the ORC with a maximum net power output varied with the engine speed. The optimal pump speed increased apparently as the engine speed rose. The results for the net power of the ORC under different engine loads with an engine speed of 3600 r/min are shown in Figure 9. The optimal pump speed increased as the engine load rose. Because the exhaust energy increased as the engine load increased, the mass flow rate must be increased accordingly to utilize the waste energy comprehensively, resulting in an increase of the optimal pump speed. Figure 8: Results for the net power of the ORC under various engine speeds with a 100% load. The optimal operation point of the ORC under various operation conditions of the engine were determined by the ICE-ORC model and the results are shown in Figure 10. Figure 10a shows the results for the optimal pump speed. The optimal pump speed reached a peak of 2350 r/min. Meanwhile, the engine operated at the rated power point. The optimal pump speed decreased as the engine power dropped. When the exhaust energy declined to 16-17 kW, the pump speed reduced to a minimum to keep the superheat degree of the working fluid at the outlet of the evaporation greater than 0. In the lower-left region of Figure 10a, the exhaust energy was too small to drive the ORC system. Hence, there existed a boundary line, below which the ORC system must stop. Figure 10: Results for the optimal operation point of the ORC under various engine operation conditions: **(a)** The pump speed and **(b)** the expander speed.