Abstract: Two magnetic valves, one of which reduces the pressure and the other one of which increases the pressure by which the displacement of a positioning member is controlled, only one of them operated at a time in accordance to the sign of the error signal, are operated in a pulsed mode with variable pulse width when the error signal is within a range of low absolute value and in a continuous mode of operation when the error signal has a higher absolute value. Variation of the magnitude of the error signal within the intermittent valve operation range controls pulse width variation. Hysteresis is provided in a transition between operating modes of a valve by a time delay of the mode change after the error signal has risen or fallen past a threshold value of signal. High displacement resolution is obtained as well as independence from effect of temperature drifts and magnetic valve characteristics.

Abstract: In order to produce a transition mixture enrichment during acceleration of an internal combustion engine, a carburettor or other mixture generator 1 of the engine with a mixing chamber 2, a main throttle 4 and a choke valve 5, has the choke valve 5 operated by a quick-acting electric drive 6, for example a two-coil rotary setter, which is controlled by a microprocessor 8. The microprocessor is fed with signals which sense the opening of the main throttle 4 and preferably also other engine operating parameters such as temperature and speed. When the main throttle 4 is opened to accelerate the engine, the microprocessor 8 causes the electric drive 6 to close the choke valve 5 abruptly and temporarily. The extent of closure of the choke valve 5 is dependent upon the extent of the throttle opening and on the other operating parameters to give the required mixture enrichment. This arrangement avoids the necessity to provide an accelerator pump.