the deflection of the air measuring member and the resulting cross sectional change at the valve 13 are preferably linearly related to avoid the necessity for corrective steps thus if the pressure difference in the suction line 14 is changed by the valve 13 then the ratio of the air quantity to the fuel quantity is also changed therefore the pressure difference is used as the controlled variable whose value defines a particular value of the ratio of the fuel quantity to the air quantity for this purpose the fuel pressures prevailing respectively ahead of and behind the valve 13 are fed to a differential pressure valve 18 which in turn controls a servo motor 19 that determines the fuel quantity delivered to the injection pump 8 by actuating a fuel metering member which in the case of a serial injection pump would be for example the control rod the exemplary embodiment of fig1 has a suction throttle injection pump 8 in which the servo motor 19 controls the suction throttle valve 20 which constitutes a second fuel control means in contrast to this example cited to explain the invention a simplified apparatus could be provided in which the fuel metering valve 13 was immediately used as the suction throttle the servo motor is embodied as a hydraulic servo motor actuated by fluid whose pressure is determined by the differential pressure valve 18 the differential pressure valve 18 has a diaphragm 21 whose one side is actuated by the fuel pressure prevailing ahead of the valve 13 and whose other side is actuated by the fuel pressure prevailing behind the valve 13 to equalize.