GB1515346A - Circuits for controlling fuel injection systems - Google Patents

Abstract

1515346 Pulse width modulators ROBERT BOSCH GmbH 22 May 1975 [7 Nov 1974] 22154/75 Heading H3T A circuit arrangement for inhibiting the injection of fuel to an I.C. engine under conditions of excessive speed or overrun comprises a pulse width discriminator including a first circuit SV connectible to an engine speed transducer providing input pulses whose duration is proportional to engine speed and operable to simultaneously generate a first pulse of constant amplitude and a second pulse whose amplitude is proportional to engine speed and further operable to generate an inhibiting signal when a comparison of the amplitudes of the pulses indicates that a predetermined engine speed is exceeded. The input pulse train applied to terminal P2 causes negative needle pulses to be developed at point P5 having an amplitude dependent on engine speed and on the time constant of the first circuit comprising resistors R7, R27, R4, R5 and capacitor C1. The negative pulses are applied to the base of transistor T2 to which is applied a positive pulse generated by the RC circuit C3, R15 and fed back from transistor T3 through resistor R12. When the amplitude of the negative pulses predominates, i.e. engine speed is excessive, the transistor T3 conducts to short circuit the injection pulses which are present at terminal P3. A switch S 1 connected to the throttle valve of the engine operates in the idle position to cause transistor T5 to conduct thereby short circuiting resistance R4 and thus changing the time constant of the first circuit. Thus the amplitude of the negative pulses is increased so that transistor T3 will now become conductive at, or about, idling speed. A feedback connection RL is used to provide a hysteresis effect and prevent oscillation between the two states.