and with the help of the subsequent assumptions. The shape of the cold instantaneous emissionfcold (t) along time, for a certain pollutant, can be split up into three different phases: A first phase characterized by the maximum cold-start emissions due to the lower temperatures of engine and catalyst and to the greatest enrichments of the fuel–air mixture, a second phase with declining cold emissions for the progressive rise of engine and catalyst temperatures and for the lower enrichments, and the last quite stable phase with the smallest cold emissions when the normal temperatures are reached and the mixture ratio is very close to the stoichiometric value. This function, then, must be such as to take zero at the instant Treg , that is, the cold transient duration (1); it is also conceivable that the function always decreases during the transient duration and that its first derivative takes zero value at the end of the cold transient (2). A possible function that satisfies these two boundary conditions is represented by the Eq. (3), which is expressed through two parameters (Treg andf0).