Fuel Injection Systems

Abstract

A fuel injection system for an internal combustion engine has at least one electromagnetically operable fuel injection valve and a pulse generator circuit arranged to produce electrical pulses for energising the valve so as to open it for a period dependent upon the duration of the pulse by which it is energised. The system inlcudes an overrun control circuit which is responsive to an engine overrun condition and having a timing circuit connected to be started by overrun responsive means and arranged to inhibit the operation of the pulse generator for a predetermined time interval after the start of an engine overrun condition or until the overrun condition ceases.

Description

This invention relates to fuel injection systems for internal combustion engines.

According to the present invention, a fuel injection system for an internal combustion engine comprises at least one electromagnetically operable fuel injection valve, a pulse generator circuit arranged to produce electrical pulses for energising the valve so as to open it for a period dependent upon the duration of the pulse by which it is energised, said duration being controllable in dependence upon at least one operating parameter of the engine, trigger means arranged to initiate the pulses at a frequency dependent upon the rotational speed of the engine, and an overrun control circuit comprising means responsive to an engine overrun condition, and a timing circuit connected to be started by the overrun responsive means and arranged to inhibit the operation of the pulse generator for a predetermined time interval after the start of an engine overrun condition or until the overrun condition ceases.

The overrun responsive means may comprise a switch operatively connected to a throttle control of the engine and adapted to be closed when the throttle control is closed or substantially closed.

Alternatively and preferably, the overrun responsive means may comprise means for sensing the position of the throttle control of the engine, for example a potentiometer whose wiper is mechanically connected to the throttle control, and may further comprise differentiator means connected to receive and differentiate the voltage on said wiper.

The timing circuit may include a capacitor arranged to be charged via a resistor, and may further include a normally non-conductive transistor which is arranged to be driven conductive by the overrun responsive means and which is arranged to be driven non-conductive when the capacitor is charged to a predetermined level or the overrun condition ceases.

The predetermined time interval preferably lies in the range 0.25 to 1.0 seconds, for example 0.5 seconds.

In a preferred embodiment of the invention, the overrun control circuit includes speed responsive means adapted to prevent its operation below a predetermined engine speed, for example 1500 RPM.

The speed responsive means may comprise an integrator circuit connected to receive pulses from the trigger device, and voltage level responsive means such as a Schmitt trigger circuit connected to receive the voltage produced by the integrator circuit.

The invention will now be described, by way of non-limitative example only, with reference to the accompanying drawings, of which :

FIG. 1 is a block circuit diagram of a fuel injection system in accordance with the present invention for an internal combustion engine, and

FIGS. 2 and 3 are circuit diagrams of alternative embodiments of part of the system of FIG. 1.

The fuel injection system shown in FIG. 1 is intended for a six-cylinder engine and includes six fuel injection valves 10 arranged in two groups of three. In use, the valves 10 are screwed into housings in the engine induction manifold, just upstream of the inlet valve of the corresponding cylinder, and fuel is supplied at a controlled pressure to each valve 10, for example as described in United Kingdom Pat. No. 1,038,541. The fuel injection valves 10 are electromagnetically operated and may be as described in United Kingdom Pat. No. 1,064,679.

Each group of the valves 10 is connected to be energised by electrical pulses produced at the output of a respective pulse generator 12, which comprises a monostable circuit 14 having its output connected via a power amplifier 16 to its respective group of the valves 10. The monostable circuits 14 are connected to be triggered by a trigger device 18, which is in effect an engine-driven switch which is operated once per engine cycle for each group of the valves 10.

Each monostable circuit 14 includes a timing circuit (not shown) for controlling the duration of the main pulses in dependence upon the values of two D.C. voltages V1 and V2, which in turn depend upon the values of several engine operating parameters. In the example shown, the voltage V1 depends on the engine manifold pressure which is sensed by a pressure transducer forming part of a manifold law control circuit 20 and connected to the induction manifold between a throttle valve and the inlet valves of the engine, and also depends upon the rotational speed of the engine which is sensed from the trigger device 18 by means of an engine speed discriminator 22. The voltage V2 depends on the engine water temperature and the ambient air temperature, which temperatures are sensed, respectively, by a water temperature transducer connected into the water cooling system of the engine and by an air temperature transducer, both of which form part of a start and warm up control circuit 24. These various transducers may incorporate variable resistance elements, the resistances of which vary in accordance with the value of the respective operating parameters, and the resistance elements may be incorporated in voltage dividers forming part of associated circuits, namely the manifold law control circuit 20, the engine speed discriminator circuit 22, and the start and warm up control circuit 24. Since the present invention is not concerned with the means by which the main pulses or the voltages V1 and V2 are produced, the pulse generators 12 and the circuits 20, 22, 24 will not be described or illustrated in detail. It is to be understood, however, that each pulse generator 12 may, for example, take the form of, and operate in the manner of, the pulse generator 8 described in United Kingdom Pat. No. 1,107,989, and the said circuits and transducers may also take the forms described in this Patent, and for further details of the construction and operation of these components, reference should be made to this Patent.

The fuel injection system also includes an acceleration enrichment circuit 26, which may take either of the forms described in our co-pending United Kingdom Pat. applications nos. 43,503/68 or 42813 /70, for supplying additional pulses to all of the valves 10 when acceleration of the engine is required. The circuit 26 is connected to receive a manifold pressure signal V(P) from the manifold law control circuit 20, and its output is connected to both of the power amplifiers 16.

The fuel injection system further includes an overrun control circuit 30 which comprises a switch 32 having a movable contact 34 mechanically connected to a suitable point in the linkage between the throttle pedal and the throttle valve of the engine. The movable contact 34 is connected via a capacitor C1 to a resistor R1, and moves from a contact 36 to a contact 38 when the throttle valve closes. The contact 36 is connected via a resistor R2 to the junction of C1 and R1, while the contact 38 is connected to a positive supply rail 39.

The capacitor C1 and the resistors R1 and R2 form part of a timing circuit, generally indicated by 40, which further comprises an NPN transistor TR1 whose base is connected to the resistor R1 and, via a resistor R3, to a negative supply rail 41. The emitter of TR1 is connected to the supply rail 39 via a resistor R4, and to the supply rail 41 by a resistor R5, while the collector thereof is connected to the supply rail 39 via resistors R6 and R7 in series. A PNP transistor TR2 having a collector resistor R8 is connected in the grounded-emitter configuration to the junction of the resistors R6 and R7.

The output at the collector of TR2 is connected to one input 42 of a two-input AND gate 44 whose other input 46 is connected to the output of a Schmitt trigger circuit 48. The input of the Schmitt trigger circuit 48 is connected to the output of an integrator circuit 50 which is connected in turn to the output of a monostable circuit 52. The monostable circuit 52 is in turn connected to be triggered by both outputs of the trigger device 18 via respective diodes D1 and D2 and a pulse shaping amplifier 53.

The engine speed discriminator 22 produces a voltage V(S) which varies as a function of engine speed, and for most applications of the fuel injection system this voltage would be increasing with increasing engine speed in the range of speeds around 1,500 RPM. If desired, therefore, the diodes D1 and D2 and the circuits 50, 52, 53 may be dispensed with, and the Schmitt trigger 48 may be connected to receive V(S) as its input voltage (as shown by the dotted line in FIG. 1).

The output of the AND gate 44 is connected via an amplifier 54 and respective diodes D3, D4 to respective inhibit inputs of the monostable circuits 14. In a typical example, the inhibit inputs might be constituted by the trigger inputs of the monostable circuits 14, the arrangement being such that the trigger inputs are short-circuited by the signal at the output of the AND gate 44.

In operation, suppose that the engine rotational speed is above 1,500 RPM and the throttle valve is partly open. The threshold voltage of the Schmitt trigger circuit 48 is arranged to be less than the output voltage of the integrator circuit 50 (or V(S)) at engine rotational speeds above 1500 RPM, so the Schmitt trigger circuit 48 is in its triggered state, which is arranged to enable the input 46 of the AND gate 44.

The movable contact 34 of the switch 32 is in contact with the contact 36, so the capacitor C1 is held discharged by the resistor R1 and no current is supplied to the base of TR1, which is therefore in its non-conductive state. TR2 is therefore also in its non-conductive state, so no signal is applied to the input 42 of the AND gate 44. The inhibit inputs of the monostable circuits 14 are thus not energised, and the valves 10 are energised by the pulse generators 12 in the normal manner to supply fuel to the engine.

If the throttle valve is then closed, initiating an engine overrun condition, the movable contact 34 of the switch 32 moves to the contact 38, triggering TR1 into its conductive state via C1. TR2 in turn is rendered conductive, and the inhibit inputs of both of the monostable circuits 14 are energised to prevent further pulses being supplied to the valves 10. The fuel supply to the engine is thus completely cut off.

The capacitor C1 starts to charge up via R1 and R3 until after a predetermined time interval, typically 0.5 seconds, it reaches a voltage level at which TR1 is rendered non-conductive again. TR2 again becomes non-conductive, and the inhibit inputs of the monostable circuits 14 are de-energised, thus restoring a normal fuel supply to the engine.

If the throttle valve is re-opened before TR1 has been rendered non-conductive by the voltage across C1, the re-opening immediately renders TR1 non-conductive.

If the throttle valve is closed at an engine rotational speed below 1500 RPM, the Schmitt trigger circuit 48 is in its untriggered state, thus closing the AND gate 44 and rendering the overrun control circuit 30 inoperative.

An alternative embodiment of the overrun control circuit 30 is shown in FIG. 2 and comprises a PNP input transistor TR101 having its base connected, via a resistor R101, to receive either V(S) or a speed-dependent voltage produced by circuitry (not shown) similar to the circuits 50, 52, 53 and the diodes D1 and D2 of FIG. 1. The emitter of TR101 is connected via a diode D101 and a resistor R102 to the negative supply rail 41, and via a resistor R103 in series with the contacts 34, 38 of the throttle switch 32 to the positive supply rail 39. The contact 36 of the switch 32 is left open-circuit.

The collector of TR101, which constitutes the output thereof, is connected to the negative supply rail 41 via a resistor R104 and, via a PNP emitter-follower stage TR102, having an emitter resistor R105, to a timing circuit 60 comprising a capacitor C101 connected to the negative supply rail 41 via a resistor R106. The timing circuit 60 also includes an NPN transistor TR103 whose base is connected to the junction of C101 and R106 and whose emitter is connected to the negative supply rail 41 via a resistor R107. The collector of TR103, which constitutes the output thereof, is connected to the positive supply rail 39 via a resistor R108 and to a PNP grounded-emitter stage TR104 whose collector has a load resistor R109 and is connected to the diodes D5, D6.

The operation of the circuit of FIG. 2 is similar to the operation of the circuit of FIG. 1. However, the transistor TR101 performs the AND function of the AND gate 44 in FIG. 1, since it is arranged to be non-conductive when the switch 32 is open and to be rendered conductive by its speed-dependent input voltage when the switch 32 closes at an engine speed above 1500 RPM.

When the switch 32 closes at an engine speed in excess of 1500 RPM, therefore, a positive-going voltage step is applied via C101 to the base of TR103, rendering it conductive and thus inhibiting both monostable circuits 14 via TR104 and the diodes D3, D4. The fuel supply to the engine is thus completely cut off. The capacitor C101 then starts to charge up via R106 until, typically after 0.5 seconds, it reaches a voltage level at which TR103 is rendered non-conductive again. A normal fuel supply to the engine is thus restored.

The embodiment of the overrun control circuit 30 shown in FIG. 3 comprises a potentiometer RV201 whose movable wiper 70 is mechanically connected to a suitable point in the linkage between the throttle pedal and the throttle valve of the engine and is movable therewith. The potentiometer RV201 forms part of a potential divider chain which further includes resistors R201 and R202 and which is connected between the supply rails 39, 41. The wiper 70 of the potentiometer RV201, which thus carries a voltage indicative of the instantaneous position of the throttle valve and throttle pedal, is connected to the input of a differentiator 72 which comprises a PNP grounded-emitter transistor TR201 having a base input resistor R203, a feedback capacitor C201, an emitter feedback resistor R204 and a collector resistor R205. The output voltage on the collector of TR201 is therefore indicative of rate of change of throttle pedal position.

In practice, the potentiometer RV201, the resistors R201, R202 and the differentiator 72 may be constituted by part of the acceleration enrichment circuit 26 of FIG. 1, since this circuit also utilises a voltage indicative of rate of change of throttle pedal position. The output of transistor TR 201 is connected through a resistor R 206 and a capacitor C202 in series, to the base of an NPN grounded-emitter transistor TR202 having a collector resistor R207. The base of TR202 is also connected to the positive supply rail 39 via a resistor R208, and to the negative supply rail 41 by a normally reverse-biassed diode D201. The circuit components R206, C202, R208 and the grounded-emitter stage TR202 together constitute a timing circuit generally indicated by 80.

The collector of TR202 is connected via a further NPN grounded-emitter transistor TR203 to the input of a PNP grounded-emitter transistor TR204 arranged similarly to TR2 of FIG. 1. The output of TR204 is connected to the input 42 of the AND gate 44 of FIG. 1, the other input 46 of which is connected to receive either of the voltages specified in relation to FIG. 1.

When the throttle pedal is moved in the closing direction of the throttle valve, thus initiating an engine overrun condition, the voltage on the wiper 70 of RV201 changes, causing the voltage on the collector of TR201 to go more negative by an amount dependent upon the rate of throttle pedal movement. This negative voltage change is transmitted to the base of TR202 via R206 and C202, and when the change exceeds an amount determined by R206 it renders TR202 nonconductive. TR202, in turn, renders both TR203 and TR204 conductive.

Thus if the engine speed is above 1,500 RPM, enabling the input 46 of the AND gate 44, the output of TR204 inhibits both monostable circuits 14 and cuts off the fuel supply to the engine.

The capacitor C202 then begins to charge up via R208 until, after a time determined by the time constant of R208, C202 and the rate of throttle movement which initiated the overrun condition, TR202 is rendered conductive again, so restoring a normal fuel supply to the engine. For large and rapid throttle closing movements, therefore, the duration of the cut-off period is largely dependent upon the time constant of R208, C202 and is typically 0.5 seconds, while for small, slow movements this duration is reduced in proportion to the rate of throttle pedal movement.

The fuel injection systems hereinbefore described significantly reduce the amount of exhaust pollution produced by the engine during overrun conditions, and increase fuel economy, at engine speeds above 1,500 RPM. Below 1500 RPM, it is possible that the driver of a vehicle propelled by the engine might feel slight jerks as the fuel is cut-off; the provision of speed responsive means such as the Schmitt trigger 48 and its associated circuitry prevents this by rendering the overrun control circuits 30 inoperative at engine speeds below 1,500 RPM. The circuit of FIG. 3 further reduces exhaust pollution since it senses throttle-closing movements, and therefore cuts off the fuel supply to the engine before the throttle valve is fully closed. The engine is therefore purged with air at the start of the overrun condition.

It will be appreciated that many modifications can be made to the described circuits. For example, the timing circuits 40, 60 or 80 could have time constants greater than 0.5 seconds, and could be replaced by conventional monostable circuits, while the integrator circuit 50 of FIG. 1 could be constituted by a diode pump circuit. Further, the overrun control circuits 30 are applicable to fuel injection systems other than the one described, for example to systems having only one injection valve, or a plurality of individually energisable injection valves.

Claims

I claim:

1. A fuel injection system for an internal combustion engine comprising at least one electromagnetically operable fuel injection valve, a pulse generator circuit arranged to produce electrical pulses for energising the valve so as to open it for a period dependent upon the duration of the pulse by which it is energised, means for controlling said duration in dependence upon at least one operating parameter of the engine, trigger means arranged to initiate the pulses at a frequency dependent upon the rotational speed of the engine, and an overrun control circuit comprising means responsive to an engine overrun condition and a timing circuit connected to inhibit the operation of the pulse generator only until the end of a predetermined fixed time interval after the start of an engine overrun condition or until the overrun condition ceases, whichever occurs first the length of said time interval being independent of the duration of said overrun condition.

2. A system as claimed in claim 1, where in the timing circuit includes a capacitor arranged to be charged via a resistor, and further includes a transistor which is arranged to be changed in conductivity state by the overrun responsive means and which is arranged to return to its initial conductivity state at the end of said predetermined time interval when the capacitor has been charged to a predetermined level or when the overrun condition ceases, whichever occurs first.

3. A system as claimed in claim 2 in which said overrun responsive means comprises a potentiometer connected to be actuated by the closing of a throttle controlling said engine, said potentiometer being connected to change the state of said transistor before said throttle is fully closed.

4. A system as claimed in claim 3 which further comprises a differentiator connected to receive and differentiate a voltage from said potentiometer.

5. A system as claimed in claim 2 in which said overrun responsive means comprises a switch connected to be actuated by a throttle controlling said engine and said overrun control circuit comprises means responsive to the speed of said engine connected to prevent said timing circuit from inhibiting operation of said pulse generator when said engine is operating at less than a predetermined speed.

6. A system as claimed in claim 2 comprising means for providing a voltage responsive to the speed of said engine and in which said transistor is preceded in said timing circuit by a further transistor connected to said speed responsive means and to said overrun responsive means for functioning as an AND gate, so as to be rendered conductive by operation of said overrun responsive means whenever said speed responsive means provides a voltage indicative of an engine speed at least equal to a predetermined speed.

7. A system as claimed in claim 4, wherein the differentiated voltage which is indicative of the position of the throttle control is applied to a transistor to change its conductivity state under overrun conditions for said predetermined time interval or until the overrun condition ceases, whereafter the transistor returns to its original conductivity state.

8. A system as claimed in claim 1, wherein the predetermined time interval lies in the range 0.25 to 1.0 seconds.

9. A system as claimed in claim 5, wherein the speed responsive means comprises an integrator circuit connected to receive pulses from the trigger means, and voltage level responsive means connected to receive the voltage produced by the integrator circuit.