Abstract: A Dual-fuel combustion engine, possessing: a control device at least one combustion chamber at least one gas supply device for supplying a gaseous fuel to at least one combustion chamber, and at least one injector for injecting liquid fuel into the at least one combustion chamber, and which injector is controllable through a control device by the use of an actuator triggering signal, for which at least one injector possesses an output opening for the liquid fuel, which is closable by means of a needle (6), and for which the control device regulates through the use of the actuator triggering signal, the opening of the needle (6) in the ballistic region of the needle in a pilot operating mode of the combustion engine to which end, an algorithm is stored in the control device, which receives, as input values, at least the actuator triggering signal (?t) and, using an injector model, calculates the mass of liquid fuel introduced via the output opening of the injector, and which compares the mass calculated by