## 3 Results  
### Consumption Centered Scenario  
The best efficiency area lies at an equivalence ratio of \(\phi\) = 0.97 (the model is from [29] and is fully developed in [24]). However, the efficiency improvement is less than 2% between \(\phi\) = 0.97 and \(\phi\) = 1.0, and the value \(\phi\) = 0.97 emits a very large amount of NOx (the NOx conversion efficiency falls from 100% to almost 0 in this narrow window; see Figure 4). Therefore, this strategy will not be studied. The consumption centered scenario is calculated for stoichiometric combustion and an optimal spark advance. Results are presented in Table 5.  
These values must be compared to the CO2 emissions targets for cars, established at 95 g CO2/km, and the EU current regulations (EURO6) as given in Table 2.  
The first analysis shows that even very efficient sustaining hybrid drive trains have difficulties achieving the new regulations in term of CO2 emissions. In this example, the WLTC driving cycle consumes 20% more than its NEDC predecessor. This new cycle must be welcomed, as it is closer to real-world conditions, but it makes the 95 g CO2/km target all the more difficult to achieve. Figure 9 shows the efficiency maps of ICE and EM (the points correspond to the hybrid mode only; i.e., we do not represent the electrical machine operating points in all-electric mode). Firstly, we notice that the best efficiency of the engine used in this article is 32% (which corresponds to a BSFC of 245 g/kWh). Better efficiency is currently possible; for example, a minimum BSFC value of 225 g/kWh is obtained with the Atkinson cycle engine that equips the Prius II (Model/Year 2004, see [30]). Secondly, we notice that the engine works in its best efficiency area, which is quite normal for this strategy. This is also the case for the electric machine. A supplementary degree of freedom that is not in the scope of this work is the gearbox. Neither the number and ratios of the gears, nor the instances where the gears change have been optimized. This latter option can bring improvements in fuel consumption without sacrificing vehicle driveability [31]. The five gear '20 DP42' gearbox used in this study is quite old and we currently see automated gearbox with six, seven, and even eight gears in today's vehicle. This greatly improves fuel consumption, especially in highway mode [32]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Optim.** & **Driving** & **Fuel Cons.** & CO2 & CO & HC & NOx \\
**Parameter** & **Cycle** & **1/100 km** & **g/km** & **g/km** & **g/km** & **g/km** \\ \hline Group A & NEDC & 3.58 & 84 & 0.213 & 0.014 & 0.171 \\ Group A & WLTC & 4.40 & 103 & 0.133 & 0.009 & 0.167 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Performance of the consumption centered scenario. In term of emissions, CO or HC are very low compared to the current regulations. However, the NOx level is almost three times higher than EURO6 limit. Figure 10 shows the cumulative emissions, while Figure 11 shows the temporal evolution of the catalyst temperature and efficiency. Before the catalyst light off, almost half of the total amount of CO and HC has already been emitted. For the total NOx, 15% has already been dispersed in the environment. Catalyst is primed after 250 s (the time when the efficiency exceeds 50%, symbolized with blue stars in Figures 10 and 11), during which half of the urban part of the driving cycle has been performed. Between 250 and 600 s, the catalyst is cooling and is not far from being inefficient due to low temperature (light-off temperature is 550 K in our model). This can happen in real usage (stops or congested areas, for example) and must be managed by the engine ECU. Figure 10: Cumulative emissions for consumption-centered strategy in the WLTC driving cycle. (**a**): Carbon monoxyde; (**b**): unburned Hydrocarbons; (**c**): Nytrogen oxides. Figure 9: Operating points for consumption-centered strategy in the WLTC driving cycle.