## 3 Results
### Emission-Centered Scenario
(**a**): intake pressure; (**b**): relative spark advance; (**c**): air to fuel equivalence ratio. Figure 14: Power sharing with the emission-centered strategy in the WLTC driving cycle. Figure 15: Operating points for the emission-centered strategy in the WLTC driving cycle. (red squares mean that the catalyst temperature is higher than the light-off temperature; blue squares mean that it is lower). (**a**): engine; (**b**): electric motor. Figure 15 shows how the strategy operates regarding the operating points of the ICE and EM. First, we notice that a cloud of points is located near the full load of the ICE, which also matches with the best efficiency area. The blue points denote the operating point used with a cold and thus low efficiency catalyst. Although these points correspond to full open throttle (see intake pressure in Figure 16, plot a), as the spark advance is delayed by 20 to 30 degrees (plot b), the resulting torque is low. During this stage, the strategy increases exhaust enthalpy in order to quickly warm the catalyst. Of course, these operating points present a low thermal efficiency, but this behavior last only a few seconds over the WLTC driving cycle, thus producing a minor impact on global fuel consumption. At the same time, engine admits a lean mixture (air to fuel equivalence ratio lower than 1, see Figure 16, plot c), but as the catalyst is cold, it has no impact on its behavior. This has a small effect, but increases engine efficiency and exhaust temperature while reducing CO emissions at the engine outlet. As soon as the catalyst is primed, the strategy has to conciliate conflicting constraints:
* For the fuel, the major parameters will be the relative spark advance, which has to be close to its optimal value (\(\Delta_{SA}=0\)). It is also clear that an intake pressure close to atmospheric pressure eliminates pressure drop at the intake valve, thus maximizing efficiency;
* CO is not influenced by the spark advance, but only by the equivalent air to fuel ratio. A low value of \(\phi\) is better for CO engine emission. That is not what we observed. Several factors explain this result:
* In this narrow window, CO emissions variation are low;
* CO efficiency is very high when catalyst is primed;
* The strategy greatly reduces CO emission before catalyst light-off (see Figure 10). * The HC level is very low, and its performance index, \(\frac{\dot{m}_{HC}}{\dot{m}_{HC,ref}}\) is less than 0.1 (see Figure 12), so it has no impact on the control parameters;
* NO\({}_{\rm x}\) clearly deeply influences the strategy; we observe a slight enrichment of the mixture (\(\phi=1.008\)) that drives the catalyst efficiency to 0.995 (see Figure 4). Conversely, we notice an alternation of the relative spark advance between 0 and \(-10^{\circ}\). This last value means firing later than is optimal, thus reducing the temperature in the combustion chamber and the feedgas NO level. Finally, the temporal distribution of the spark advance values is questionable. It seems to be correlated with the driving power; the strategy adopts a null relative spark advance when the driving power is high, and delays ignition when the wheel power decreases. This interpretation is highlighted in Figure 18.  
Figure 17: Pollutant emissions with the emission-centered strategy in the WLTC driving cycle. (**a**): Carbon monoxyde; (**b**): unburned Hydrocarbons; (**c**): Nitrogen oxides.