**8. Results and Discussion**

In this section the results from the ORC simulations are presented, analyzing how efficiently the integrated system operated and where room for improvement could be identified. Lastly, the exact figures of the vehicle's fuel consumption are shown, respectively, for each of the three different driving cycles and constitute the deciding factor for the selection of the optimum hybrid profile for the integrated powertrain. Several aspects that control the efficiency of the ORC system and how this system is combined with the IC engine to provide more power to the wheels are analyzed in this section. It is worth stating at this stage that the global assumptions considered for the ORC configuration of the HEV powertrain, namely, that even though the backpressure caused by the ORC's evaporator placed in the exhaust pipe, and also all of the extra weight imposed by ORC components to the vehicle system, deteriorate the overall performance and the fuel consumption of the vehicle to some extent. In this early evaluation study of ORC WHR systems for HEVs, it has been assumed that the effects of these are negligible, but should be borne in mind by the reader when considering the results.

The main target set for the results of the ORC analysis is that the maximum efficiency of the turbine and the pump does not have a percentage difference of higher than 15%. Moreover, the power output of the turbine is critical, as it is added to the power output of the IC engine to provide the total power of the powertrain, so it is required to be as high as possible. Another parameter that is tested in this analysis is the pressure rise in the pump which indicates the degree to which the pump ensures the successful provision of the required pressure level throughout the entire cycle. Finally, this section concludes with the presentation of the new BSFC map of the upgraded powertrain (with ORC). The distribution of the turbine and pump efficiency is illustrated in Figure 5a,b, respectively, over the tested engine load and speed range. In the figures it can be observed how the efficiencies of these two components fluctuate with engine speed and the BMEP, as well as how the contours of constant efficiency are formed inside the plot. The maximum value of turbine efficiency is 61% and it is obtained at part-load, while the maximum efficiency of the pump is 72% at higher BMEP values and engine speeds. Great effort was expended to reach the highest efficiencies of these two components and resulted in an efficiency improvement of almost 10% from the initial model.

**Figure 5.** Schematic view of the engine system model in GT-Power code: (**a**) turbine efficiency distribution; (**b**) pump efficiency distribution; (**c**) turbine power distribution; (**d**) EVAPORATOR energy rate; (**e**) Pump pressure rise; (**f**) BSFC percentage reduction distribution; (**g**) ORC efficiency distribution; and (**h**) fuel consumption per driving cycle.

Figure 5c represents the allocation of the turbine power output over the tested range of engine loads and speeds. It can be observed that the maximum power is obtained at full load and high engine speeds because the exhaust gases have greater temperature and mass flow at these speeds. The plot shows that the maximum power output that the turbine produces is almost 5 kW at high engine speed and load. At part-load (around the median of 6 bar) and lower engine speeds the turbine can produce typical values of around 2.5 kW. These values may seem small at first sight, but they proved to be high enough to cause a decrease in BSFC of almost 7% under the same operating conditions.

Figure 5d,e provides information about the evaporator energy rate and the pump pressure rise, respectively. Figure 5f illustrates the BSFC reduction contour plot superimposed on the engine map. It can be interpreted from the plot that the BSFC percentage reduction differs for different engine loads and speeds with the maximum values of the reduction being obtained at the lower BMEP values, reaching a maximum 12% decrease. 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. When the ORC system was included in the powertrain, the new fuel consumption was calculated to be 8.76 L/100 km, which is almost 1 L less per 100 km.

On the other hand, on the NEDC, the results showed that the fuel consumption without the ORC system was 8.79 L/100 km, while when the ORC system was included in the analysis the new fuel consumption was 8.07 L/100 km. The difference between these two values is almost 0.8 L/100 km.

The final driving cycle test was the US06, giving a fuel consumption of 10.57 L/100 km without the ORC system. When the ORC system was added in the analysis, the improved fuel consumption was decreased to 9.84 L/100 km, which is almost 0.7 L/100 km less. This driving cycle is more aggressive than the previous two, which is why the powertrain reached higher fuel consumption values in comparative terms to NEDC and FTP-75.

Moreover, the results from the driving cycle analysis can assist the understanding of how the ORC system cooperates with the engine under different loads, accelerations, and decelerations. The three selected driving cycles constitute a representative example of driving regulations in use today and which were required to validate HEV performance with the novel component (ORC WHR system).

In Figure 5h the bar charts illustrate the differences in fuel consumption between the original HEV engine and the ORC-equipped equivalent in order to minimize fuel consumption. Furthermore, the percentage difference is calculated, so that the absolute difference of the improved powertrain can be differentiated. This bar chart includes information for all three of the driving cycle tests and presents the exact amount of burned fuel per 100 km of driving. It can be noticed that in all of the driving cycle scenarios, the fuel consumption was decreased after the implementation of the ORC system. The largest reduction was found to be in the FTP-75 driving cycle and the calculation of the absolute percentage difference showed a 10% lower fuel requirement for 100 km. The NEDC required the least amount of fuel, as it is not as aggressive as the other two cycles. Moreover, in this driving cycle the ORC system improved the fuel consumption by 8%, which is a significant reduction if the strict European regulations are taken into consideration. Finally, the US06 driving cycle required the most fuel overall, but the ORC system managed to lower the fuel consumption of the vehicle by 7%, which resulted in a drop of fuel consumption below 10 L/100 km.

A closer look into the three driving cycles was required to reveal the driving strategy needed in order to maximize the powertrain performance in the electric hybrid vehicle. Taking into account the three major hybrid operation modes, which are the traction mode, braking mode, and coasting mode, the driver can be guided in how to handle the accelerator pedal, the braking pedal, and the clutch pedal to maximize the efficiency of the overall vehicle.

As far as the NEDC is concerned, it can be identified that it consists of numerous accelerations and decelerations, and the average cruising speed is 60 km/h. This driving cycle profile can exploit the braking and coasting operation modes of the hybrid configuration to gain advantages regarding the fuel consumption of the vehicle. During the decelerations the brake should be lightly applied and this will lead to the battery being charged by dissipating the braking energy and, hence, more power can be delivered to the wheels during the acceleration zones. Moreover, the driver can use the coasting mode, while no brakes or throttle are applied, before reaching the deceleration zones and prevent, in that way, the engine from burning fuel in a non-beneficial manner.

The FTP-75 driving cycle has a complicated profile of accelerations and decelerations, which makes it difficult for the driver to adopt the optimum driving strategy. However, as there are multiple fluctuations in the speed profile of this cycle, the driver can exploit the braking mode to charge the battery of the hybrid setup. This gained power can be used during steep acceleration gradients to assist the work of the IC engine, leading to less fuel being burned during the numerous accelerations. As long as there are no flat speed regions in this driving cycle, the coasting mode can hardly be applied and cannot influence a further reduction of the fuel consumption.

Finally, the US06 driving cycle results exhibit lower fluctuations in the speed profile compared to the FTP-75, which means that the coasting mode can be applied, offering significant benefits in fuel consumption reduction. This driving cycle pushes the vehicle to reach high speeds of more than 120 km/h, which undoubtedly increases the consumption of fuel of the vehicle. In order to counteract this, judicious use of the accelerator pedal would be recommended, along with use of the power of the electric motor to assist with significant accelerations. On the other hand, the braking mode can be used to gather the essential electric power in the battery and the coasting mode can be used when the car needs to decrease its speed without the strict application of the brakes.
