## 3 WLTC-Based Analysis of Study Results
### Energy Performance
Under these conditions, the engine operates with the following ignition timings: \(\Theta\)\(\approx\) 19\({}^{\circ}\) BTDC for EUR, \(\Theta\)\(\approx\) 21\({}^{\circ}\) BTDC (advanced by -2\({}^{\circ}\)) for EUR, and \(\Theta\)\(\approx\) 25\({}^{\circ}\) BTDC (advanced by -6\({}^{\circ}\)) for EUR. It is estimated that, over the entire cycle, switching from EUR to EUR and EUR fuels results in an ignition timing advance of approximately 0.8% and 4.3%, respectively. This adjustment in ignition timing by the ECU is attributed to the increased resistance to knocking provided by bioethanol, which enhances the engine's thermal efficiency. Figure 4: Air–fuel ratio and ignition timing are used using different fuels: (**a**) all WLTCs; (**b**) extra high-speed mode. For all WLTC test cases, the vehicle mileage increased consistently over the 1800-second test period, with a total distance of \(S_{veh\_\_\Sigma\_WITC}\approx 23.22\)km covered after completing the four driving modes (Figure 1b). The trends in instantaneous fuel mass consumption (\(B_{f_{m}}\)) over the driving cycle for E0, E10, and E70 were similar, though the absolute values differed (Figure 4a). The data indicate that the total fuel consumption of E10 (\(B_{f_{m}\_\Sigma\_WITC}\)) increased at a slower rate during low- and medium-speed modes but accelerated more rapidly during high- and very-high-speed modes, reaching levels comparable to E0 (Figure 4b). This suggests that E10 exhibits higher thermal efficiency at lower loads, as evidenced by the lower exhaust gas temperatures (Figure 3b), while it does not provide an energy advantage at higher speeds. The total fuel mass consumption, \(B_{f_{m}\_\Sigma\_WITC}\), during engine operation was 1.054 kg with E0, 1.055 kg with E10 (an increase of 0.1%), and 1.308 kg with E70 (an increase of 23.9%). The higher mass consumption of E70 fuel over the test cycle, compared to E0, is attributed to the reduced net calorific value associated with the 70% bioethanol concentration. Mass consumption, fuel density, and distance travelled were used to calculate the volumetric fuel consumption per 100 km. During the WLTC cycle, E0 consumed 6.08 L/100 km, E10 consumed 6.03 L/100 km (a decrease of approximately 0.8%), and E70 consumed 7.24 L/100 km (an increase of approximately 19.1%). The use of fuels with low ethanol concentrations (up to 10%) led to approximately 20% higher fuel consumption compared to the vehicle's technical specifications (Table 1). This discrepancy could be due to two primary factors. Firstly, the vehicle specifications were based on the New European Driving Cycle (NEDC), an older testing methodology. Secondly, the tested Toyota Prius II had a mileage of 200,000 km. The brake specific fuel consumption per cycle (\(BSFC\_WITC\)) was calculated by considering the fuel mass consumption and the work performed: E0--238.9 g/(kW-h); E10--239.1 g/(kW-h) (an increase of approximately 0.1%); E70--296.5 g/(kW-h) (an increase of approximately 23.9%). The evaluation of \(BSFC\), along with the LHV of the fuel, yielded the brake thermal efficiency per WLTC (\(BTE\_WITC\)) for the ICE as follows: E0--0.346; E10--0.356 (an increase of approximately 2.8%); E70--0.376 (an increase of approximately 9.0%). Increasing the bioethanol concentration in the fuel blend from 0% to 10% and 70% led to an increase in the \(BTE\), as the lower calorific value of the fuel decreased more significantly (by 3.8% and 25.7%, respectively) than the corresponding increase in effective fuel consumption (approximately 0.1% and 23.9%, respectively). The rise in efficiency with an increasing bioethanol concentration can be attributed to the advancement of ignition timing, as the engine-control system adjusted for the higher resistance of the fuel to knocking. However, excessively advanced ignition timing \(\Theta\) can also reduce \(\eta_{e}\) by increasing the fraction of combustion energy dissipated through the cooling system.