According to the invention, prior to or during an intermediate compression around the upper dead center of a piston of an internal combustion engine during a change in charge a predetermined first fuel mixture is metered into the combustion chamber of the respective cylinder. During the intermediate compression, measured values of pressure in the combustion chamber are measured in a predetermined first crankshaft angle window using cylinder pressure sensor. A measurement for a conversion rate of the first fuel mixture is determined based on the detected measured values of the pressure and reference values of the pressure, which characterize the corresponding pressure curve in the combustion chamber when no fuel is metered. A control signal for at least one actuator of the internal combustion engine is determined based on the conversion rate.