given moment a proportion of the cells making up the trap 5 is not able to capture molecules of nitrogen oxides nox for two main reasons firstly some cells are irreversibly damaged as a result of ageing for instance because they are obstructed by solid deposits the coefficient of ageing k ag which appears in equation 1 and is updated by an adaptation algorithm described in detail below takes account of the reduction of the maximum capacity c m due to the wear of the trap 5 secondly the trap 5 can also store sulphur oxides sox as discussed above consequently a proportion of the cells of the trap 5 corresponding to the quantity of sulphur oxides stored sox st is temporarily unavailable to interact with the nitrogen oxides nox until a desulphurisation process is carried out the residual capacity c r lastly represents the cells of the trap 5 that have not captured any molecules and are therefore actually available to interact with molecules of nitrogen oxides nox the quantity of nitrogen oxides stored nox st is calculated on the basis of the following equations nox st nox old nox cap u2212 nox co u2212 nox hc 7 in equations 4 5 6 and 7 nox cap is the fraction of the upstream quantity of nitrogen oxides nox m that is captured by the trap 5 at the current moment nox old is the quantity of nitrogen.