## 3 Discussion of the Results
### CO and HC Conversion Efficiency
In addition, the differences in the equivalence ratio were also influenced by the huge variation in LP-EGR rate between the sea-level and altitude cases. The engine control closed the LP-EGR valve, which, besides the increase of the boost pressure, offset the air mass flow reduction with respect to sea-level driving [12] and contributed to setting an adequate equivalence ratio [13]. Figure 9c,d show that the fields of LP-EGR are equivalent in trend in warm sea-level and cold altitude. The LP-EGR rate increases with the exhaust mass flow (engine speed), but the increasing rate is lower as the mass flow increases. It also contributes to explaining the lower equivalence ratio and higher O\({}_{2}\) partial pressure at high exhaust mass flow. Regarding the CO conversion efficiency, Figure 10 represents the engine-out CO partial pressure for the two tested ambient conditions. A change in the sensitivity of the CO partial pressure with exhaust mass flow and catalyst inlet temperature is observed between the warm sea-level and cold altitude cases. In the warm sea-level condition, the CO partial pressure was highly dependent on the temperature at low exhaust mass flow. However, the CO emission became increasing as the exhaust mass flow did. By contrast, the CO partial pressure was mostly dependent on the DOC inlet temperature in cold altitude operation. It is very interesting to note that, despite the lower ambient pressure, the highest CO partial pressure was found in these conditions at a low DOC inlet temperature. When considering the engine-out HC partial pressure, which is represented in Figure 11, the trends are very similar to those that are found for CO. Again, the maximum HC partial pressure took place at low DOC inlet temperature in the case of altitude operation. The fact that the CO and HC partial pressures were higher in a cold altitude than the warm sea-level counterparts evidenced higher engine-out emission (mole fraction). This trend covers the entire engine-out HC field but only the medium-low DOC inlet temperature range for CO. In the case of HC, it is also remarkable that the high engine-out  
Figure 8: O\({}_{2}\) 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.  
HC emission at low temperature determined the adsorption efficiency along time. Higher HC emission at altitude conditions meant higher HC adsorption conversion efficiency at the beginning of the test. However, the faster increase of the HC surface coverage as time went by made the desorption rate increase. This is the reason why the HC adsorption conversion efficiency was lower in cold altitude than in warm sea-level at the end of the tests (Figure 6c,d), especially as the exhaust mass flow increased, since it caused greater HC surface coverage. Regarding a first-order reaction kinetic term, high CO/HC partial pressures at low temperature in altitude operation should contribute to offset the decrease in conversion efficiency caused by the lower dwell time and O\({}_{2}\) partial pressure. However, a high partial pressure of pollutant species also produces a penalty in inhibition [25]. Figure 12 depicts the oxidation inhibition term for two tested exhaust mass flows as a function of the DOC inlet temperature and ambient condition. The high CO and HC partial pressures at low temperature caused high inhibition in the cold altitude conditions. In both of the tested cases, the inhibition decreased as the temperature was increased because of the reduction in CO and HC partial pressures. The reduction of the engine-out emission combined with the lower ambient pressure gave, as a result, even lower inhibition than at sea-level. Therefore, the trends in partial pressures at low temperature (lower O\({}_{2}\), and higher CO and HC partial pressures in altitude) explain the decrease of the CO and HC conversion efficiency at high altitude, along with the dwell time decrease.