## 3 Discussion of the Results
### Light-Off Curves Sensitivity Analysis  
Light-off curves of CO and HC were computed in order to evaluate the impact of the dwell time and the reactants partial pressure on the conversion efficiency separately. A set of light-off simulations were carried out for two exhaust mass flows (40 and 70 kg/h), imposing a DOC inlet temperature ramp of \(1\,^{\circ}\)C/min from \(100\,^{\circ}\)C to \(300\,^{\circ}\)C. This small temperature ramp makes the efficiencies shown in the light-off curve equivalent to those that would be obtained when the catalyst operates under steady state conditions at every temperature. The gas composition was taken from the contour maps for every reactant. Figure 11: Engine-out HC partial pressure as a function of the exhaust mass flow and catalyst inlet temperature: (**a**) 0 m and 20 \(\,^{\circ}\)C; (b) 2500 m and \(-7\,^{\circ}\)C.  
Figure 12: Inhibition term as a function of the DOC inlet temperature and ambient conditions for (**a**) 40 kg/h and (**b**) 70 kg/h in exhaust mass flow. Figure 10: Engine-out CO partial pressure as a function of the exhaust mass flow and catalyst inlet temperature: (**a**) 0 m and 20 \(\,^{\circ}\)C; (**b**) 2500 m and \(-7\,^{\circ}\)C.  
Figure 13 represents the CO and HC light-off curves. In the case of HC, the adsorption mechanism was canceled, since it depends not only on temperature, but also time through the instantaneous surface coverage. Therefore, only the HC oxidation light-off curve was computed. Figure 13a,b represent the CO and HC light-off curves, respectively, corresponding to 40 kg/h, whilst plots (c) and (d) correspond to 70 kg/h. Four modeled series are compared in every plot to analyze, step-by-step, the change in conversion efficiency from warm sea-level to cold altitude conditions:  
* The black series corresponds to the baseline warm sea-level case, i.e., experimental dwell time and gas composition at 0 m and 20 \({}^{\circ}\)C.
* As a first step, the dark gray series represents the results of a simulation where the warm sea-level setup was modified, imposing the dwell time that corresponds to cold altitude. * Next, the light gray series was obtained from warm sea-level case, but imposing both the dwell time and O\({}_{2}\) partial pressure of the cold altitude case. * Finally, the red series corresponds to the baseline cold altitude, i.e., dwell time, O\({}_{2}\) partial pressure, and pollutants partial pressure (kinetic term and inhibition) impact with respect to warm sea-level results. As a common trend in every plot of Figure 13, the decrease of dwell time in altitude conditions produced a shift of the light-off curves toward higher temperatures (\(\sim\)5.5 \({}^{\circ}\)C, on average). The decrease of the O\({}_{2}\) partial pressure also shifted the curves toward higher temperature in the light-off region. In this case, the penalty depended on the engine operating conditions, due to the variation of the O\({}_{2}\) partial pressure difference between warm sea-level and cold altitude as a function of the exhaust mass flow. The light-off temperature was further delayed \(\sim\)4.5 \({}^{\circ}\)C at 40 kg/h increasing the penalty until \(\sim\)7 \({}^{\circ}\)C at 70 kg/h. Nevertheless, the higher O\({}_{2}\) partial pressure at high temperature in cold altitude canceled its negative impact and slightly increased the HC conversion efficiency above \(\sim\)225 \({}^{\circ}\)C, as observed in Figure 13b. Figure 13: Progressive CO and HC light-off curves variation from warm sea-level to cold altitude case considering dwell time and reactants partial pressure separately: (**a**) CO light-off curve at 40 kg/h; (**b**) HC light-off curve at 40 kg/h; (**c**) CO light-off curve at 70 kg/h; and, (**d**) HC light-off curve at 70 kg/h. Despite the deterioration of the light-off temperature due to dwell time and O2 contributions in cold altitude, the red series evidenced that the most relevant parameters are the pollutants partial pressures due to the inhibition effect.