A method and device for estimating temperature of gases arising from
internal combustion of an engine at the entrance to a system for
post-treatment of the gases. The post-treatment system is downstream from
a system for treatment of the gases, which may include a nitrogen oxide
NO.sub.x trap, an oxidation catalyst, or a 4-way catalyst. The systems
connect to each other by a line, wherein the treatment system is
considered as the whole of a whole finite number n of fictive elementary
reactors R.sub.i that are perfectly agitated and serially connected,
wherein the total number n of fictive elementary reactors R.sub.i is
determined, in particular as a function of volume of the treatment
system, and the fictive elementary reactors R.sub.i are numbered from 1
to n in the gas flow direction along the treatment system. Fictive
elementary reactor R.sub.n is the last to be cross-flown by the gases
before leaving the treatment system, and the temperature at the
post-treatment system entrance downstream from the system for the
treatment of the gases is calculated from the temperature inside the last
fictive elementary reactor R.sub.n.