Method of operating a system comprising a diesel engine and catalyst for exhaust gas purification

A method of operating a drive system comprising an engine and an exhaust gas purification unit containing a catalyst, where the engine emits an exhaust gas having an exhaust gas temperature and the catalyst has a catalytic activity for the purification of the exhaust gas, wherein an aging-induced decrease in the catalytic activity of the catalyst is compensated at least part of the time by increasing the exhaust gas temperature of the engine, wherein the aging-induced decrease in the catalytic activity of the catalyst is compensated by increasing the exhaust gas temperature of the engine by a factor for all operating points of the engine whereby said factor is determined as follows:

• a) the heat evolved in the combustion of a given amount of hydrocarbons over the catalyst at a certain operating point of the engine is measured after a defined number of hours of operation,
• b) the heat evolved is compared with the heat evolved over a fresh catalyst at this operating point,
• c) in case of a reduced amount of heat evolved compared to the fresh catalyst, the exhaust gas temperature of the engine prevailing at this operating point is increased without altering the torque by engine measures until the newly measured heat evolved over the aged catalyst corresponds to the heat evolved over the fresh catalyst,
• d) determining said factor as the ratio of the exhaust gas temperature of the engine after temperature increase in step c) relative to the exhaust gas temperature before temperature increase.