Method for optimizing the consumption of reducing agent in a motor vehicle exhaust line

A process for optimizing consumption of a reducing agent in the form of ammonia in an exhaust line for gases resulting from the combustion of a motor vehicle combustion engine, the exhaust line comprising first and second selective catalytic reduction systems, the second reduction system being arranged downstream of the first reduction system in the exhaust line, being spaced apart by a spacing from the first reduction system, the first reduction system injecting an ammonia precursor agent into the exhaust line for the reduction of nitrogen oxides, a surplus of unused ammonia passing via the exhaust line from the first to the second reduction system, comprising measuring or estimating the temperature in the exhaust line at the second reduction system at predetermined time intervals, and:

• when a rate of temperature rise, calculated from the measured or estimated temperature in the exhaust line, is greater than a predetermined rate of temperature rise, with the proviso that the temperature in the line at the second reduction system is greater than a second maximum temperature value that is less than a first maximum temperature value,
• carrying out forced cooling in the spacing between the first and second reduction systems.