A dual fuel engine operates on a gaseous fuel and a liquid fuel. The engine comprises a supply of gaseous fuel controlled by a first valve, a supply of compression ignitable liquid fuel controlled by a second valve, a control unit for controlling the supply of gaseous fuel and liquid fuel to each combustion chamber in the engine, and at least one knock sensor arranged to detect knock in each combustion chamber and to transmit an output signal proportional to the detected level of knock to the control unit. If knock is detected, the amount of gaseous fuel injected will be reduced while the amount of liquid fuel will be increased. The engine control system will subsequently adapt to the lower grade fuel and perform a calibration to operate as close as possible to the knock limit for the particular fuel.