Process for controlling the boost pressure in an internal-combustion engine supercharged by an exhaust-gas turbocharger of adjustable turbine geometry

A process for controlling the boost pressure in an internal-combustion engine, supercharged by an exhaust-gas turbocharger with adjustable turbine diffuser, to a predetermined desired boost-pressure value dependent on the operating point, comprising:

• after a positive change of the internal-combustion engine load, transferring the turbine diffuser into a position reducing a flow cross-section onto the turbine;
• after the positive load change during a non-steady-state mode, regulating boost pressure according to a first characteristic, corresponding to the instantaneous true deviation of the actual boost-pressure value from the desired boost-pressure value, for the regulating variable for controlling the turbine diffuser, the regulation of the boost pressure taking place according to the first characteristic only below a predetermined threshold value for the exhaust-gas counterpressure upstream of the turbine;
• and
• after the predetermined threshold value has been exceeded, regulating the boost pressure according to a second characteristic, the second characteristic being based on a hypothetical deviation of the actual boost-pressure value from the desired boost-pressure value, the amount of which is larger than the instantaneous true deviation.