## 3 Discussion of the Results
### CO and HC Conversion Efficiency  
Figure 4 shows the CO conversion efficiency contour maps for the two tested ambient conditions as a function of the exhaust mass flow and DOC inlet temperature. The upper row represents the experimental (Figure 4a) and modeled (Figure 4b) CO conversion efficiency at sea-level and 20 \({}^{\circ}\)C, whilst Figure 4c,d do the same at 2500 m and \(-\)7 \({}^{\circ}\)C. The black dots on the contours represent the measured steady state points. The comparison of Figure 4a,b reveals that the model was able to capture the trends in CO conversion efficiency in warm sea-level. The CO conversion efficiency was lower than 20% below 100 \({}^{\circ}\)C in the DOC inlet temperature, regardless the exhaust mass flow. However, as the DOC inlet temperature increased, the CO conversion efficiency and its sensitivity to the exhaust mass flow also did. The CO light-off showed a marked dependence on the exhaust mass flow, increasing from \(\sim\)120 \({}^{\circ}\)C at 40 kg/h to \(\sim\)150 \({}^{\circ}\)C at 68 kg/h, according to the experimental and modeled results. From this threshold, the light-off temperature showed even higher sensitivity to the mass flow. The model also accurately predicted the region reaching 100% of CO conversion efficiency. Again, the maximum conversion efficiency was reached earlier for minimum mass flow, increasing slowly from \(\sim\)160 \({}^{\circ}\)C in DOC inlet temperature at 40 kg/h to \(\sim\)210 \({}^{\circ}\)C at 71 kg/h. In this temperature range, a further increase of the exhaust mass flow at constant temperature also led to a faster deterioration of the CO conversion efficiency. When comparing warm sea-level results with those corresponding to cold altitude (Figure 4c), which were also reproduced with high accuracy by the model (Figure 4d), the change in CO conversion efficiency fashion is evidenced. For these extreme ambient conditions, the CO light-off temperature was delayed until \(\sim\)165 \({}^{\circ}\)C at 40 kg/h and remained almost constant until 60 kg/h. This involves an increase of light-off temperature ranging between 45 and 15 \({}^{\circ}\)C with respect to the warm sea-level. The complete CO abatement was also reached at higher temperatures, which varied linearly from \(\sim\)190 \({}^{\circ}\)C to \(\sim\)230 \({}^{\circ}\)C between 40 kg/h and 78 kg/h. The deterioration of the CO conversion efficiency in cold altitude was extensible to the entire exhaust mass flow range, although it was more penalized at low exhaust mass flows. Figure 4: CO conversion efficiency as a function of the exhaust mass flow and catalyst inlet temperature: (**a**) experimental results at 0 m and 20 \({}^{\circ}\)C; (**b**) modeled results at 0 m and 20 \({}^{\circ}\)C; (**c**) experimental results at 2500 m and \(-\)7 \({}^{\circ}\)C; and, (**d**) modeled results at 2500 m and \(-\)7 \({}^{\circ}\)C.  
Figure 5 presents the experimental and modeled contour maps of the HC conversion efficiency at each ambient condition in the same manner. An overview of the results confirms that the model was also accurate regarding HC abatement prediction, with high sensitivity to the exhaust mass flow and DOC inlet temperature for both ambient conditions. Cold altitude conditions also moved down the HC conversion efficiency, although with different trends to the CO case. The adsorption on the zeolites provided the HC with higher conversion efficiencies at low temperature than those that were found for CO. In warm sea-level, the HC conversion efficiency was over 60% and reached 80% at \(\sim\)170 \({}^{\circ}\)C. This temperature was kept almost constant until 70 kg/h, from which a decrease of the HC conversion efficiency started, as also found in CO case. The model provided similar fashion of the results, although with a slightly conversion efficiency variation with the exhaust mass flow.