In order to comply with the legal emission limits, modern motor vehicles with a petrol engine have a tank venting device by means of which fuel vapors escaping from the fuel tank are collected and stored in a suitable storage tank (generally an activated carbon canister). From time to time, it is necessary to regenerate the storage container. To this end, the storage container is connected to an inlet manifold of the internal combustion engine by means of a venting pipe. The controlled opening of a tank venting valve located in the venting pipe pneumatically connects the storage container with the inlet manifold. Due to the negative pressure prevailing in the inlet manifold, the fuel vapors stored in the storage container are sucked into the inlet manifold and then participate in the combustion process.
The combustible mixture changes depending on the concentration of the hydrocarbons in this regeneration gas flow. It is however necessary to set the air/fuel ratio of the combustible mixture to a defined value in order to ensure a sufficient combustion quality (running smoothness) and optimum exhaust gas treatment. Without further measures, the introduction of the regeneration gas into the inlet manifold would result in a deterioration of the exhaust gas quality or impairment of the combustion stability. In order to prevent this, the quantity of fuel supplied to the internal combustion engine by means of an injection valve is adapted to the quantity of fuel that is additionally supplied by the regeneration gas flow. For this, however, the fuel concentration in the regeneration gas flow must be known as precisely as possible.
According to a known method, the fuel concentration in the regeneration gas flow can be determined by measuring the exhaust gas composition with a lambda sensor when the tank venting valve is closed and storing this as a reference variable. The tank venting valve is then gradually opened and the change that this causes in the exhaust gas composition is determined. Based on the difference in the exhaust gas composition, the fuel concentration in the regeneration gas flow can be determined. If it is determined during an executed tank venting process that the value for the fuel concentration in the regeneration gas flow has been incorrectly ascertained or has changed, the tank venting valve must be closed, a constant operating point of the internal combustion engine waited for and the fuel concentration in the regeneration gas flow determined once again. This very time-consuming process considerably restricts the number of possible tank venting processes and the flexibility of their execution.