## 3 Discussion of the Results
### CO and HC Conversion Efficiency
batement. However, since the tested ranges are equivalent for warm sea-level and cold high altitude cases, the underlying causes explaining the change in conversion efficient must be found in additional parameters, namely dwell time and reactants partial pressure. Figure 7 represents the dwell time as a function of the exhaust mass flow and DOC inlet temperature for the tested ambient conditions. The dwell time is evidently lower when working in altitude conditions for the same exhaust mass flow and DOC inlet temperature, as expected from the lower exhaust pressure after the VGT expansion in altitude operation. In this particular study, the altitude dwell time was decreased by a \(\sim\)25% with respect to sea-level, according to the ambient pressure reduction from sea-level to 2500 m. Consequently, this parameter is negatively affecting the conversion efficiency in the cold altitude case in an homogeneous way throughout the tested range. Opposite to dwell time, the reactants partial pressures can differently affect as a function of the exhaust mass flow and DOC inlet temperature because of their relation with the engine operating point. Partial pressure provides dual sensitivity because of the exhaust pressure at the VGT outlet, which is mostly given by the altitude, and the mole fraction, which is determined by the engine management as a function of both altitude and ambient temperature. Figure 8 plots the O\({}_{2}\) partial pressure. At very low temperature, the O\({}_{2}\) partial pressure was lower in cold altitude than in the warm sea-level. This was expected from the lower ambient pressure in altitude conditions. As a consequence, it contributed to lower CO and HC reactivity in the oxidation light-off and warm-up regions during altitude operation. This result is in agreement with the trends in conversion efficiency that are described in Section 3.1. If the ratio of O\({}_{2}\) partial pressures between altitudes was exclusively related to ambient pressure variation, its ratio should be 1.356 (sea-level to 2500 m ambient pressure ratio) in the entire contour map. However, this ratio was lower everywhere and variable with exhaust mass flow at constant DOC inlet temperature. At very low temperature,  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Point**} & \multirow{2}{*}{**Condition**} & **Temp. ** & **Mass Flow** & **CO eff. ** & **HC ox. eff. ** & **HC ads. eff. ** & **Total HC eff. ** \\  & & [°C] & **[kg/h]** & **[\%]** & **[\%]** & **[\%]** & **[\%]** \\ \hline A & 0 m \& 20 \({}^{\circ}\)C & 98.17 & 66.00 & 1.09 & 0.00 & 68.83 & 68.83 \\ B & 0 m \& 20  
the O\({}_{2}\) partial pressure at sea-level was very sensitive to the exhaust mass flow, although higher than at altitude; at high temperatures, the O\({}_{2}\) partial pressures at sea-level and altitude tended to converge. In fact, the O\({}_{2}\) partial pressure resulted in being lower at sea-level above 250 \({}^{\circ}\)C for low exhaust mass flows than in the altitude counterpart. These trends evidence that additional engine variables conditioned the DOC performance. Figure 9 represents the fields of equivalence ratio and LP-EGR rate in the tested conditions. According to plots (a) and (b) shown in Figure 9, the equivalence ratio was higher for warm sea-level in the entire tested range, but especially at high DOC inlet temperature. Comparatively, it contributed to offsetting the expected differences in O\({}_{2}\) partial pressure due to altitude operation. The increasing O\({}_{2}\) partial pressure with exhaust mass flow at constant DOC inlet temperature corresponding to warm sea-level (Figure 8) was also dictated by the inverse trend in equivalence ratio. As a remark, the operation in cold altitude required higher engine speed and fuel mass flow, besides higher boost pressure (closer VGT, and higher turbine inlet pressure and temperature), for the same exhaust mass flow and DOC inlet temperature.