## 8 Results and Discussion
In addition, as the BMEP values increase at the mid-load range (6 bar), the average percentage reduction is approximately 7%, which is higher than the set target. This region is the one of highest interest as the vehicle spends most of the driving time in this region, so the 7% decrease in fuel consumption can highly benefit the hybrid vehicle. At higher engine loads and BMEP values of 9 bar, the BSFC reduction percentage is lower, but always above 5%. One final parameter that can be examined regarding the ORC system is the overall efficiency of the system that produced the above reduction in BSFC. The efficiency of the ORC system is defined as the fraction of the turbine power output over the evaporator energy rate, showing how efficiently the available energy is used inside the ORC WHR system. However, as for this investigation a four-cylinder 2000 cc engine was selected, the available energy rate is limited and the constraints inside the system are greater when compared to a larger (commercial-type) diesel engine (which can exploit the full potential of the system and reach higher efficiencies). In order to mitigate this drawback, the ORC system has been modified to give the higher possible efficiency. Figure 5g shows the distribution of the ORC efficiency superimposed on the engine map. It can be seen that, overall, the ORC efficiency fluctuates at different engine speeds and loads, as well as the efficiency values are not very high. At lower engine speeds and BMEP values the ORC efficiency reaches a maximum of 27%, which is expected because the energy rate is fairly low, while at least the simulated turbine power has already reached near-maximum efficiencies even from lower loads and speeds. As the engine speeds and loads rise, the evaporator energy rate increases, resulting in the decrease of the efficiency because the turbine power is not increasing accordingly due to restrictions in the ORC system design and in the exhaust mass flow rate and temperature. The ORC efficiency reaches an average value of 10.5% at the engine speed of 3000 rpm and BMEP of 6.5 bar, which is an important region of the powertrain operating points. A further increase in engine speed and BMEP values show an efficiency of 9%, which is the minimum percentage obtained in this study and for the entire engine operating range. In general, the efficiency of the ORC is significant, but it is not reaching its full potential due to several inherent limitation of the smaller capacity powertrain. However, with this efficiency distribution, the ORC system is able to produce an average decrease in BSFC values of 7%, which is significant compared to a conventional HEV configuration. Hereinafter, the overall results of the project are further interpreted in order to obtain a clear ORC WHR system characterization of its performance and benefit that it brings to the conventional HEV architecture. In addition, the discussion indicates how the different hybrid modes of the vehicle should be chosen in order to further decrease the fuel consumption of the vehicle; specifically, the discussion identifies which hybrid mode should be chosen in the different driving scenarios, such that the vehicle can operate more efficiently, while consuming the minimum amount of fuel. During the various driving cycle tests that were performed in the engine simulation software, the vehicle model was run with and without the ORC configuration, so that the absolute difference between these two setups could be distinguished. Moreover, the three different driving cycle tests can validate the efficiency of the whole powertrain in different driving conditions, providing useful information about the hybrid mode which emerges in each case. Finally, these tests revealed possible areas where the ORC system could be further optimized for greater HEV vehicle benefit. For the FTP-75 driving cycle test the obtained data from the analysis showed that the fuel consumption of the vehicle without the ORC system was 9.74 L/100 km.