In a method and an apparatus which more accurately reach a predefined setpoint value of a combustion characteristic variable for an internal combustion engine (1) with an injection system (30), a first injection signal which triggers an injection of fuel into a combustion chamber (53) of the internal combustion engine (1) is generated using at least one injection signal parameter. A combustion signal which represents a combustion of the injected fuel is generated by a detector (40). The combustion characteristic variable is determined using the combustion signal. The injection signal parameter is corrected using a deviation of the combustion characteristic variable from a setpoint value, in such a way that the deviation of a second injection signal which is generated using the corrected injection parameter is reduced for an injection which is to be performed later.