Method for operating an exhaust system having an SCR catalytic converter for an internal combustion engine, internal combustion engine having an exhaust system, and a motor vehicle having an internal combustion engine

A method for operating an exhaust system for an internal combustion engine of a motor vehicle comprising:

• following a cold start of the internal combustion engine, and making use of at least one preoperational state of at least a first selective catalytic reduction (“SCR”) catalytic converter that is calculated by a computer and present prior in time to the cold start:
• admitting exhaust gas emitted by the internal combustion engine to the first SCR catalytic converter in the exhaust system, whereby nitrogen oxides are adsorbed by the first SCR catalytic converter while water is condensed out of the exhaust gas in at least one exhaust gas guide tube which is part of the exhaust system and is arranged upstream of the first SCR catalytic converter and which supplies the exhaust gas to the first SCR catalytic converter;
• heating the exhaust gas guide tube, whereby the water condensed out of the exhaust gas evaporates upstream of the first SCR catalytic converter, is supplied at least in part with the exhaust gas to the first SCR catalytic converter, and is adsorbed on a first catalyst surface of the first SCR catalytic converter;
• at least partly reducing nitrogen oxide constituents stored on the first catalyst surface as a consequence of the catalyst surface being heated by an exothermic reaction of the water on the catalyst surface and by an increase in an exhaust gas temperature as a consequence of fueled operation of the internal combustion engine;
• determining, by a computer, an actual amount of reducing agent associated with the first SCR catalytic converter that was adsorbed at the first catalyst surface dependent on:
• an amount of water on the first catalyst surface,
• a mass flow of the exhaust gas,
• a catalyst temperature of the first SCR catalytic converter,
• an amount of nitrogen oxide constituents adsorbed in the first SCR catalytic converter, and
• dependent on a preliminary amount of reducing agent assigned to the preoperational state and adsorbed at the first catalyst surface;
• comparing a target amount of reducing agent associated with the first SCR catalytic converter with the actual amount of reducing agent using the computer;
• and
• adding a metered amount of compensation reducing agent, which characterizes a difference in value between the target amount of reducing agent and the actual amount of reducing agent, using a first metering device that is part of the exhaust system, is controlled by the computer, and as a consequence of being controlled by the computer, admixes the amount of compensation reducing agent to the exhaust gas upstream of the first SCR catalytic converter.