## 5 Discussion
### Influence of Main Engine Load on System Performance  
As shown in Table 2, the variation in waste heat recoverable by the ORC system is contingent upon the different loads at which the main engine operates. The influence of the main engine load on the working fluid flow and net output power of the ORC system is shown in Figure 4a. The working fluid flow of the ORC system increases with the main engine load. As the main engine load escalates from 60% to 100%, the BORC system experiences a rise in the working liquid flow speed from 7.30 kg/s to 13.30 kg/s, while the PORC system witnesses an increase in the working liquid flow speed from 8.45 kg/s to 14.91 kg/s. The increase in the main engine load leads to a corresponding rise in the waste heat recoverable by the ORC system. Consequently, the mass flow rate of the working fluid within the ORC system also increases in accordance with the heat absorption capacity. As seen in Figure 4a, the net output power of the ORC system ten rises with the increase of the main engine load. When the main engine load increases from 60% to 100%, the net output power of the BORC system increases from 237.4 kW to 445.3 kW, increasing by 87.57%. The net output power of the PORC system demonstrates a remarkable increase, rising from 270.0 kW to 491.3 kW, signifying an 81.96% improvement. Moreover, the net output power of the PORC system consistently surpasses that of the BORC system. Notably, as the main engine load escalates to 80%, the rate of net output power increment for the PORC system progressively accelerates. The influence of the main engine load on the thermal efficiency and expansion ratio of the ORC system is shown in Figure 4b. As the main engine load increases, the two systems' thermal efficiency and expansion ratio decrease first and then increase, and the change rule  
Figure 4: Variations in performance with main engine load on two systems (**a**) working fluid mass flow and net output power; (**b**) thermal efficiency and expansion ratio. is the same. The thermal efficiency of BORC and PORC systems reach the maximum value under 100% load, which is 12.84% and 12.71%, respectively. As illustrated in Figure 4b, When the main engine's load becomes 80%, the exhaust system expansion ratio is the lowest. The system expansion ratio defines the work capacity of the unit working fluid, which influences the system's net output power. In accordance with the comprehensive analysis of Figure 4a,b, when the main engine load raises from 60% to 80%, the mass flow rate of the fluid being used increases gradually, but the system expansion ratio decreases and the work capacity of the unit working fluid decreases. The net output power of the system exhibits a direct correlation with both the mass flow of the working fluid and the work capacity of the working fluid per unit mass. As a result, within the range of this load, the net output power of the system experiences a gradual and moderate increase. When the burden of the main engine rises from 80% to 100%, not only does the expansion ratio of the system increase, but also the flow rate of the ORC system working mass increases. Consequently, the net output power of the system experiences a significant surge, followed by a subsequent decline in the system's thermal efficiency. The thermal efficiency of the system is directly influenced by the net output power, while inversely affected by the heat absorption capacity. Initially, the thermal efficiency decreases, but subsequently exhibits an increase as the main engine load escalates. The effect of load changes on the difference and increase in net output power of the two systems is shown in Figure 5. When the main engine load is between 60% and 80%, the average increase in net output power of the BORC due to the addition of a preheater can reach 32.3 kW.