Load Dependent Control Circuit For A Gasoline Fuel Injection Unit

Abstract

A monostable multivibrator produces fuel injection pulses, the length of which are made dependent on the engine rpm by a control voltage fed to a first voltage divider at the input of the multivibrator. Whenever the pulse exceeds a predetermined length, corresponding to full-load operation of the engine, a time delay circuit connects the second voltage divider in parallel with the first voltage divider so as to diminish the effect of the rpm dependent correction on the pulse length.

Description

BACKGROUND OF THE INVENTION

The invention relates to a control circuit for operating at least one electromagnetic fuel injection spray valve of an internal combustion engine. The circuit includes a monostable multivibrator having an input transistor and an output transistor, the rectangular pulse output of the multivibrator determining the open time of the fuel injection valve. The control voltate at the base of the input transistor varies the pulse length in dependence on the engine rpm. The control voltage periodically varies in time with the pulses and is fed to a voltage divider at the input of the multivibrator.

With injection units of this kind, the amount of fuel for each stroke is metered by the open period of the respective fuel injection valves, which latter receives fuel at a practically constant pressure. The length of the fuel injection pulse is controlled by the feedback circuit of the multivibrator, which advantageously has an iron core inductor, the inductance of which is changed in dependence on the pressure in the intake manifold behind the throttle valve. Rpm-dependent correction is obtained (the feedback remaining unchanged) by producing a control voltage that lengthens or shortens the period of the unstable state of the multivibrator. The control voltage is generated at the end of an injection pulse by a regulating circuit having two or more switching transistors.

In a control circuit of the kind described in the first paragraph of this section, an automatic, rpm-dependent, control voltage is fed to the tap of the voltage divider. The control voltage controls the length of the injection pulse. A transformer, acting as a timing component, has one end of its secondary winding connected to this tap and the other end to the base of the input transistor of the multivibrator.

With this known circuit, the rpm-dependent corrections are equally effective for all loadings of the internal combustion engine. For various designs of internal combustion engines, it is, nevertheless, desirable and frequently necessary to alter, in dependence on load, those corrections usually obtained at loads of 60-80 percent of maximum load and giving a minimum of exhaust.

SUMMARY OF THE INVENTION

An object of the invention is an internal combustion engine that enables modification of the rpm-dependent corrections so as to take into account the load on the engine.

Briefly, the control circuit of the invention consists of trigger circuit means for producing electric injection pulses for opening the fuel injection valve, regulating circuit means, connected to the trigger circuit means, for delivering a control voltage that periodically varies in time with the pulses for varying the length of the pulses in dependence on engine speed, and time delay circuit means, connected to the trigger circuit means, for so negating the effect of the control voltage when the engine is working at full load that the open time of the fuel injection valve is suited to the full-load operation of the engine.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an embodiment of the invention;

FIG. 2 is a graph showing the variation in the length of the injection pulses in dependence on engine speed;

FIGS. 3a through 3e plot voltage against time at several important points in the circuit shown in FIG. 1; and

FIG. 4 is a circuit diagram showing a modification of part of the circuit shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, the fuel injection unit is intended for a four cylinder internal combustion engine 1 of which the spark plugs 2 are connected to a high voltage ignition system, not shown. The electromagnetic fuel injection spray valves 4 are mounted, immediately adjacent the inlet valves (not shown), on the branches of the intake manifold 3 leading to the respective cylinders. Each valve 4 is connected by a respective fuel line 5 to a distributor 6. A pump 7, driven by an electric motor (not shown), keeps the fuel in the distributor 6 and the fuel lines 5 at an approximately constant pressure of about 2 atmospheres.

Each of the fuel injection spray valves 4 comprises a solenoid (not shown), one end of which is connected to ground and the other end of which is connected by a respective lead 8 to one of the four resistors 9. These resistors are connected in pairs to the collector of a respective one of the two power transistors 10 and 11, which are part of an electronic control circuit that will be described.

In addition to the power transistors 10 and 11, the control circuit includes a monostable multivibrator 12 (enclosed within the dashed-line box). The multivibrator comprises an input transistor T1, an output transistor T2, and an iron core inductor 13, the latter serving as a timing component.

The iron core inductor 13 is constructed as a transformer and has a movable core part 14, which is connected to an adjusting rod 15 that, in turn, is connected to the diaphragm (not shown) of a pressure box 16. The suction side of the pressure box is connected to the intake manifold 3 just behind the engine throttle valve 18, which is controlled by the foot pedal 17. When the pressure in the intake manifold falls, the diaphragm of the pressure box 16 raises the movable core part 14 in the direction of the arrow, thereby enlarging the air gap of the iron core (not shown) of the transformer 13, so that the inductance of the primary winding 19 of the transformer 13 is smaller the lower the pressure in the intake manifold 3.

One end of the secondary winding 20 of the transformer 13 is connected to the base of the input transistor T1 and to a resistor R3 connected to a common positive rail 21, whereas the other end of this winding is connected by the junction H with the tap of a voltage divider composed of two resistors R1 and R2. The resistor R2 is connected to the positive rail 21, and the resistor R1 is connected to the common negative rail 30, which is connected to ground and to the negative pole of a 12-volt battery, not shown. The emitters of the transistors T1 and T2 are directly connected to the negative rail 30. The collector of the input transistor T1 is connected by a resistor R4 to the positive rail 21, whereas the collector of the output transistor T2 is connected to the same rail by the primary winding 19 of the transformer 13 and a resistor R6 connected in series with this winding. A coupling resistor R5 connects the base of transistor T2 to the collector of transistor T1. A differentiating capacitor C1 connects the base of this latter transistor to the stationary contact 23 of a switch of which the movable contact 24 is connected to the negative rail 30. The movable contact 24 is operated by a two-lobed cam 28, which a cam shaft (not shown) connects to the crank shaft 27 of the engine. Each complete rotation of the crank shaft closes the switch once and thereby turns off the transistor T1. To permit charging and discharging of the capacitor C1, the capacitor plate connected to the switch contact 23 is connected by a resistor 29 to the positive rail and the other capacitor plate is connected to this same rail by the resistor R3 and to the junction H by the secondary winding 20.

Before describing the remaining components of the control circuit, it will be explained how the varying pressure in the intake manifold 3 changes the length of the current pulses J, which determines the open period of the fuel injection spray valves 4. A current pulse J is produced each time that the switch 23,24 is closed.

The transistor T1 is conductive and therefore keeps transistor T2 cut off during the time immediately preceding the closing of the movable contact 24. As soon as the cam 28 moves the movable contact 24 against the contact 23, the charge held by the capacitor C reduces the voltage at the C1 of transistor T1 to below the potential of the negative rail 30, the base becoming negative. The transistor T1 is consequently cut off, and the multivibrator 12 is triggered to its unstable state in which the transistor T2 is conductive. The collector current of this transistor, flowing to the primary winding 19, increases exponentially and causes a strengthening magnetic field in the iron core (not shown) and in the movable core part 14 of the transformer 13. The larger the air gap, and consequently the smaller the inductance of the primary winding 19, the faster the collector current increases. The increasing current flow induces a feedback voltage in the secondary winding 20. The maximum value of this feedback voltage occurs at the instant the switch 23,24 is closed and is determined by the amount of inductance. The feedback voltage falls exponentially and is of such a polarity that it tends to keep the transistor T1 cut off by opposing the positive bias voltage of the resistor R3, which acts to return the input transistor T1 to its stable, conductive state. The transistor T1 returns to this state when the feedback voltage induced in the secondary winding 20 is smaller than the bias voltage.

The power transistor 10 or 11, connected to transistor T2 by an amplifier 32, is held conductive by the transistor T2 as long as the transistor T1 is non-conductive. However, as soon as the transistor T1 returns to its stable, conductive, state, the transistors T2 and 10 or 11 are again cut off. The pulse J, which opens the valves 4, consequently lasts for a period of time that extends from the moment that the switch 23,24 is closed until the moment at which the transistor T1 is again conductive and the transistor T2 is cut off. When the inductance of the primary winding 19 becomes smaller with falling pressure in the intake manifold 3 (thereby permitting the collector current of transistor T2 to increase more rapidly), the feedback voltage also falls more rapidly; and the transistor 71 returns sooner to its conductive state. The fuel injection valves 4 are therefore closed earlier than when the pressure in the manifold 3 is higher and therefore the inductance of the primary winding 19 is greater.

By changing the inductance of the primary winding 19 in the manner described, it is possible to vary the length of the current pulses J in dependence on the instantaneous pressure in the manifold 3. Tests in driving and on the stand have shown, however, that the amount of fuel to be injected must be dependent not only on the pressure in the intake manifold, but also on the rpm of the engine. Since the lengths of the pulses J are not affected by the engine rpm, the control circuit has a further unit, which varies the voltage between the negative line 30 and the junction H in time with the fuel injection. This unit produces a voltage U.sub.S that varies in accordance with the graph shown in FIG. 3d.

The input transistor T1 returns to its conductive state when the voltage between its base and the ground rail 30 is approximately 0.3 volts (germanium) or approximately 0.5 volts (silicon). The length of the pulses J are greater for less positive values of U.sub.H, where U.sub.H is the voltage measured between the negative rail 30 and the junction between the resistors R1 and R2. The voltage U.sub.H is superimposed on the control voltage U.sub.S from transistor T6. The voltage at the base of transistor T1 is U.sub.B = U.sub.H -U.sub.S -U.sub.T, where U.sub.T is the voltage across the secondary winding 20. At higher and higher rpms, the beginnings of the pulses J shift more and more to the left (as seen FIG. 3), the length of a pulse J therefore being determined by the instantaneous value of the voltage U.sub.S at the end of the pulse. The length of the period T.sub.p accordingly lies between the beginning of the control voltage U.sub.S and that moment at which the voltage U.sub.S determines the length of the pulse J. There is consequently a constant relationship between the length T.sub.i of a pulse J and the period T.sub.p and therefore the rpm n of the engine.

The circuit A in FIG. 1 controls the pulse length T.sub.i in dependence on the engine rpm. As shown in FIG. 2, the length T.sub.i should increase steadily with increasing engine rpm n until the value n.sub.1 =2,500 revolutions per minute, then remains constant until the value n.sub.2 =3,300 revolutions per minute, then declines steadily until the value n.sub.3 =4,000 revolutions per minute, and then, for higher rpms, remains constant at a value considerably below the value between n.sub.1 and n.sub.2.

The auxiliary regulating circuit A includes a first switching transistor T3, the base of which is connected by a coupling capacitor C2 and a resistor R7 to the junction G of the collector of transistor T1. The capacitor C2 provides a constant time delay V1. The emitter of the first switching transistor T3, as well as the emitters of two further switching transistors T4 and T5, is directly connected to the negative rail 30. A resistor R8 connects the base of transistor T3 to the positive rail 21. The base of the second switching transistor T4 is connected to the positive rail by a resistor R10. The two transistors T3 and T4 are normally conductive. A capacitor C3, which also provides a constant time delay V2, connects the base of transistor T4 to the collector of transistor T3. A resistor R12 connects the base of the following switching transistor T5 to the collector of transistor T4. Transistor T5 is normally non-conductive, and when conductive it quickly charges a capacitor C4 through diode D1. The capacitor C4 and its parallel-connected discharged resistor R15 are shunted across the collector resistor R14 of the transistor T5. The voltage appearing across the capacitor C4 during the charging and discharging of the latter is used to form the control voltage U.sub.S. A transistor T6, connected emitter-follower, conducts this voltage to the junction H of the voltage divider R1 and R2. The collector of the transistor T6 is connected directly to the positive rail 21 and the base is connected by a diode D2 to the capacitor C4.

The components of circuit A thus far described operate in the following manner. As soon as the transistor T1 returns to its conductive state (which is the stable state of the multivibrator) at the end of a pulse J (as at the moment t.sub.2 : see FIG. 3), the collector of this transistor abruptly becomes less positive. This sudden change in voltage is conducted by the resistor R7 and the capacitor C2 to the base of transistor T3, and turns off the latter. The negative voltage appearing at the base of the transistor T3 dies away in accordance with an exponential function across the resistor R8 until, in dependence on the time delay V1 introduced by the capacitor C2, the transistor T3 is again conductive. Consequently, the collector of this transistor becomes abruptly less positive, this change in voltage being conducted by the capacitor C3 to the base of the normally conductive switching transistor T4. The transistor T4 cuts off until the capacitor C3 can sufficiently discharge through the resistor R10 so that the base of transistor T4 is positive with respect to the emitter. When either transistor T4 or T5 is conductive, the other transistor is non-conductive. When the transistor T5 is conductive at the moment t.sub.3 (see FIG. 3), the capacitor C4 can charge through the diode D1 and the resistor R13 very quickly to a voltage that is determined by the voltage divider R13,R14. When the transistor T5 cuts off, the capacitor C4 discharges with a long time constant beginning at the moment t.sub.4 through the resistor R15 and the very high input impedance of the emitter-follower connected transistor T6. The voltage P.sub.1 across the resistor R15 varies in accordance with the dashed line shown in FIG. 3d. The transistor T6 is conductive at all times.

When the internal combustion engine turns over slowly and the period T.sub.p of the injection pulses J is consequently long, the capacitor C4 has sufficient time to discharge. Another injection pulse J.sub.1 is not generated until the moment t.sub.11, when the switch 23,24 is again closed, the end of the pulse being determined by the absolute value of the control voltage U.sub.S at that time. The end of this pulse is denoted in FIG. 3d by t.sub.12, and the effective value of the control voltage U.sub.S is denoted by U.sub.1.

As the rpm n of the internal combustion engine increases, the period T.sub.p = 1/n is shorter, and the end of the next injection pulse is shifted leftward (as seen in FIG. 3) towards the moment t.sub.4 : in other words, it is shifted to a more negative value for P.sub.1. The pulse length T.sub.i increases with the rpm until n= n.sub.1 (see FIG. 2); at higher rpms, the end of the next injection pulse occurs during the time of the constant value U.sub.S = U.sub.o, which is conducted by the diode D3 from the voltate divider R16,R17 to the base of transistor T6. Consequently, the pulse length T.sub.i remains constant until n=n.sub.2, assuming that all other factors remain unchanged.

In order to obtain the desired reduction in the pulse length in the range between n.sub.2 and n.sub.3, a diode D5 connects a capacitor C5 to the collector of transistor T4. This capacitor, which is connected to the negative rail 30, quickly charges between moments t.sub.3 and t.sub.4 and slowly discharges between moments t.sub.4 and t.sub.14 through resistor R19. A diode D4 ensures that the capacitor C5 influences the transistor T6 only so long as the voltage P.sub.2 at the base of this transistor is more positive than the voltage U.sub.o : in other words, with reference to FIG. 3d, P.sub.2 is nearer to the zero line. When the engine rpm exceeds n.sub.3, the end of the next injection pulse occurs before the moment t.sub.4 or t.sub.14 ; the capacitor C5 is still at nearly full charge, the consequence of which is that the pulse length T.sub.i remains virtually constant above n.sub.3.

The auxiliary regulating circuit A and the thus rpm-corrected (see FIG. 2), intake manifold pressure dependent, lengths T.sub.i of the injection pulses J are given by way of example only. Both the circuit and the resulting rpm-correction of the pulse lengths can be modified to suit internal combustion engines of other designs. The curve of FIG. 2 would, in this case, have a different shape.

Aside from the question as to the most favorable shape for the curve of FIG. 2, it is frequently necessary-- to obtain, for example, the maximum power output from the engine-- to suit the amount of fuel injected to the rpm at full load operation in a way different from the way in which it is suited to the rpm at less than full load (partial load) operation. In order to permit rpm-corrected fuel injection at full engine load, but with diminished or strengthened action, there is provided a second voltage divider R21,R22, which is connected in parallel with the first voltage divider R1,R2, when the engine operates under full load. The second voltage divider is switched in parallel with the first voltage divider by a transistor T7 of which the emitter-collector path, connected in series with resistor R23, is connected between the junction H (the tap of the first voltage divider) and the tap of the second voltage divider.

In the embodiment shown in FIG. 1, the criterion for full load is the length of the injection pulse J delivered by the monostable multivibrator. If the length of this pulse exceeds that of a comparison pulse having a length S, delivered by a delay circuit having two transistors T8 and T9, full load is indicated.

The two transistors T8 and T9 --both NPN types-- have their emitters directly connected to the negative rail 30. The base of transistor T9 is connected by a diode D6 and a resistor R28 to the positive rail 21 and by a resistor R27 to the negative rail 30. The predetermined delay time S is provided by a capacitor C6, which is connected to the junction between the diode D6 and the resistor R28 and to the junction between a resistor R29, connected to the positive rail 21, and a diode D7 of which the cathode is connected to a junction K. The voltage at the junction K is at or near to the potential of the positive rail 21 between injection pulses J, and during an injection pulse the voltage of this junction is appreciably lower. In the embodiment illustrated, the voltage of the junction K is the same as that of the collector of the multivibrator output transistor T2. The circuit block including transistors T7, T8 and T9 constitutes compensating means in this embodiment.

During the interval between two fuel injection pulses J, the transistors T7 and T9 are conductive, and the transistor T8 is non-conductive. The plate of the capacitor C6 connected to the base of transistor T9 is charged strongly negative with respect to the other plate of the capacitor. As soon as the multivibrator 12 is triggered to its unstable state at the moment t.sub.1 (or t.sub.11) and causes the beginning of a fuel injection pulse J, the multivibrator output transistor T2 is conductive and its collector is brought nearly to ground potential, which fact causes the charge on the capacitor C6 to cut off the transistor T9, thereby rendering transistor T8 conductive and the switching transistor T7 also non-conductive. This condition holds true until, at the end of the interval S, the capacitor C6 has discharged and permits the transistor T9 to return to its conductive state at time t.sub.5 (or t.sub.15).

When, because the throttle valve 18 is set at partial load, the absolute pressure in the intake manifold 3 is very low and the length T.sub.i of the injection pulse J is consequently short and ends at the moment t.sub.2 (which occurs before the moment t.sub.5 at the end of the time delay S), the voltage divider R21,R22 does not affect the circuit, because the switching transistor T7 is stilll non-conductive at time t.sub.2. The rpm-dependent correction of the control voltage U.sub.S is fully effective.

On the other hand, when the pressure in the intake manifold is only slightly below the outside air pressure, because the throttle valve 18 is nearly wide open, the inductor 13 causes a correspondingly long injection pulse J that ends only after the transistors T9 and T7 have returned to their original, conductive, states. In this case, the control voltage U.sub.S has only a diminished effectiveness because of the resistor R18, since both voltage dividers R1,R2, and R21,R22 are simultaneously effective, and the increased idling current holds the predetermined direct current voltage at junction H more nearly constant.

The length of the comparator pulse S is so chosen that it is longer than the interval T.sub.i of the injection pulses J (determined by the pressure box 16 and the transformer 13) throughout the partial load range (throttle valve 18 only partly open). At full-load operation (valve 18 completely, or nearly completely, open), the pulse S is shorter than the pulses J. In this way, the lengths of the two pulses J and S are electrically compared; and it is possible to omit the auxiliary switch (of the kind shown in FIG. 4), which is operated at full load.

In accordance with the invention, it is important that the switching transistor T7 changes its state (from conductive to non-conductive, or from non-conductive to conductive) each time that a fresh injection pulse J begins. This is essential for the aforesaid electronic comparison of the pulses S and J.

The illustrated embodiment enables the curve of FIG. 2 to be influenced by a great number of factors. For example, the ratio between R21 and R22 can be chosen to be larger or smaller than that between R1 and R2, thereby raising or lowering the voltage at the junction H. The curve can also be altered by changing the value of resistor R23.

The second embodiment, shown in FIG. 4, is a simplification of the timing circuit (T7,T8,T9) of FIG. 1 because an electrical switch K1,K2 is mechanically closed when the engine changes from partial load to full load operation. This switch is connected in the base circuit of an npn transistor T10 of which the collector and collector-resistor R25 (as is the case with the transistor T8 of the first embodiment) are connected to the base of the switching transistor T7. In contradistinction to the first embodiment, the base of transistor T10 is connected by a resistor R30 to the ground rail 30 and by a diode D8 and two resistors R31 and R32 to the positive rail 21. So long as the switch K1, K2 is opened, a sufficiently large base current flows through the diode and the two resistors to keep the transistor T10 conductive. The switch K1,K2 is operated in dependence on the position of a gas pedal 17 (as illustrated in FIG. 4) or in dependence on the position of the throttle valve 18.

When the switch K1, K2 is closed, by depressing the accelerator 17, just before the valve 18 is completely opened, the transistor T10 is cut off; the transistor T7, however, receives a sufficient base current through resistor R25 to remain conductive, whereby the voltage divider R21,R22 is connected in parallel with the voltage divider R1,R2, with the aforesaid results.

In the embodiment illustrated in FIG. 4, the switch K1,K2 is operated by means of its mechanical linkage to the accelerator 17 or to the throttle valve 18. In accordance with the invention, this switch can be operated at the change from partial load to full-load operation in still other ways. There can be utilized, for example, the change in a physical magnitude during the change to full-load operation, such as the great fall in air pressure in the intake manifold 3. To this end, a pressure box, for example, can be connected to the intake manifold, the diaphragm of the box being subjected on one face to the pressure in the manifold, and on the other face to the outside air pressure. Below a pressure difference of about 50 to 70 Torr, the switch K1,K2 can be opened or closed.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits differing from the types described above.

While the invention has been illustrated and described as embodied in a load dependent control circuit for a gasoline fuel injection unit, it is not intended to be limited to the details shown, since various modifications and circuit changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

Claims

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. A control circuit for regulating the open period of at least one fuel injection valve of an internal combustion engine, comprising, in combination trigger circuit means for producing electric valve-opening pulses for opening the fuel injection valve; regulating circuit means, connected to said trigger circuit means, for delivering a control voltage that periodically varies in time with said pulses for varying the duration of said pulses in dependence on engine speed; and time delay circuit means, connected to said trigger circuit means, operative when said pulses exceed a predetermined pulse-length, for so negating the effect of said control voltage when the engine is working at full load that the open time of said fuel injection valve is suited to full load operation of the engine.

2. A control circuit for regulating the open period of at least one fuel injection valve of an internal combustion engine, comprising, in combination, trigger circuit means for producing electric valve-opening pulses for opening the fuel injection valve; regulating circuit means connected to said trigger circuit means for varying the pulse-duration of said valve-opening pulses in dependence upon at least one engine operating condition according to a first predetermined functional relationship; and compensating means, operative only after a predetermined time delay relative to the beginning of each valve-opening pulse, for altering said regulating circuit in such a manner as to effect a changeover to a second predetermined functional relationship appropriate for full-load operation of said engine, said compensating means accordingly participating in the determination of the pulse-duration of valve-opening pulses only when such valve-opening pulses have a duration exceeding said predetermined time delay.

3. In a combustion engine as defined in claim 1, wherein said regulating circuit means comprises means for timing the lengths of said pulses as a function of airflow into the engine.

4. In a combustion engine as defined in claim 1, wherein said regulating circuit means comprises means for timing the lengths of said pulses as a function of airflow into the engine and also as a function of engine speed.

5. In a combustion engine as defined in claim 4, wherein said control means comprises means operative when said pulses exceed a predetermined pulse-length to compensate for changes in engine load by changing the functional dependence of said pulses on engine speed.

6. In a combustion engine as defined in claim 1, wherein said regulating circuit means conprises means for timing the lengths of said pulses as a function of an engine operating condition, and wherein said control means comprises means operative when said pulses exceed a predetermined pulse-length to compensate for changes in engine load by changing the functional dependence of said pulses on said operating condition.

7. In an engine as defined in claim 8, wherein said monostable multivibrator includes a first voltage divider, and time delay circuit means includes a second voltage divider and switch means for connecting said second voltage divider in parallel with said first voltage divider when said pulses exceed said predetermined pulse length.

8. In an engine as defined in claim 1, wherein said trigger circuit is a monostable multivibrator.

9. In an engine as defined in claim 7, wherein said switch means is a first switching transistor of which the emitter-collector path is connected between the tap of said first voltage divider and the tap of said second voltage divider.

10. In an engine as defined in claim 9, including a resistor connected in series with said emitter-collector path.

11. In an engine as defined in claim 10, including means for changing the conductive state of said first switching transistor at the beginning of each injection pulse and for a predetermined period that is shorter than the open time of the fuel injection valve at full engine load but longer than said open time at partial load.

12. In an engine as defined in claim 11, wherein said means is an electronic switch having at least one transistor and at least one RC network.

13. In an engine as defined in claim 12, further including first and second operating voltages, and wherein the emitter of said transistor of said electronic switch is directly connected to said first operating voltage, said electronic switch further including a resistor connected between the base of said transistor and said first operating voltage; a series-connected resistor and diode connected between said base and said second operating voltage, said diode being connected with such polarity so as to conduct the base current; a resistor connected to said second operating voltage; a capacitor connected between said last-named resistor and the junction between said series-connected resistor and diode; and a diode connected between said capacitor at the junction thereof with said last-named resistor and a point in the control circuit having a voltage at least approximately equal to that of said second operating voltage between successive injection pulses.