GB2300306A

GB2300306A - Drive circuits for fuel injector solenoids

Info

Publication number: GB2300306A
Application number: GB9508321A
Authority: GB
Inventors: Michael Lancaster
Original assignee: PECTEL CONTROL SYSTEMS Ltd
Current assignee: PECTEL CONTROL SYSTEMS Ltd
Priority date: 1995-04-25
Filing date: 1995-04-25
Publication date: 1996-10-30
Anticipated expiration: 2015-04-25
Also published as: GB2300306B; GB9508321D0

Abstract

The drive circuit for the solenoid S of a fuel comprises a first control transistor TR1 operable for a predetermined period for supplying an initial current to the solenoid S to cause the injector to be turned ON, and a second control transistor TR2 operable after the expiration of the time period for supplying a further reduced current to the solenoid S for causing the injector to be held ON. The second control transistor TR2 preferably operates at a reduced voltage relative to transistor TR1, and transistor TR1 preferably operates in switch mode and transistor TR2 in linear mode.

Description

Fluid Iniector Svstems This invention relates to fluid injector systems and is especially applicable to fuel injector systems for use on internal combustion engines.

Electrically operated fuel injectors for use on internal combustion engines are usually offered in two basic electrical configurations. High impedance types (typically 15 ohms) and Low impedance types (typically 1 - 4 ohms). The former type has the advantage of simple drive circuitry to operate the injector.

The total current flowing in the injector at turn on is not great enough for the injector to overheat. The low impedance injector on the other hand allows a much larger current through its windings which affords a faster operating action, but which if not limited will cause the injector to quickly overheat. The most simple method for the prevention of overheating is to include a power resistor in series with the injector to limit the overall current flow. This of course has the obvious effect of negating any advantage that may be gained from the low impedance injector in the first place, i.e. rapid armature movement due to increased current flow. The requirement therefore is to provide the injector with the maximum available current to enable it to open quickly, and then to limit the current for the remaining duration of the opening time.This process is traditionally achieved using a large semiconductor element (bi-polar transistor or mosfet device) with current sensing feedback to force the element into a linear region in order to control the current flow. This method is very effective at injector control, but there is a drawback. As the semiconductor element is used in its linear region to perform current regulation it will have to dissipate the unwanted current energy as heat. Because of the heat (and for no other reason) the semiconductor element needs to be not only physically large but also able to pass the heat energy away from itself and into a heat dissipation device (external metal case or heat sink). The size, and to a large extent the shape of an engine management system is dictated by the requirement for heat dissipation in the injector and ignition coil driver circuitry.

It is an object of the present invention to provide an improved fuel injector system in which the heat dissipation requirements are greatly reduced.

According to the present invention there is provided a fluid injector system comprising a solenoid operated injector and a drive circuit for causing said injector to be operated, said drive circuit comprising a first control device operable for a predetermined period for supplying an initial current to said injector to cause it to be turned ON, and a second control device operable subsequent to said time period for supplying a further current to said injector for causing said injector to be held ON.

In a preferred arrangement according to the invention, said initial current is derived from a first voltage supply, and said further current is derived from a second voltage supply which has a voltage which is less than that of said first voltage supply.

In a further preferred arrangement according to the invention, said first control device is operated in switch mode, and said second control device is operated in substantially linear mode.

In carrying out the invention it may be arranged that said drive circuit comprises an input for receiving an input operating pulse for said injector, and a timer circuit operable from said input for causing said first control device to be operated for said predetermined period, said predetermined period being less than the period of said input operating pulse.

It may be arranged that said drive circuit comprises a control circuit for said second control device, said control circuit having an input for said input operating pulse, current sensing means for affording an output in dependence upon said further current, and a further control device responsive to said sensed current for controlling the operation of said second control device.

Advantageously, said second voltage supply is constituted by a switch mode power supply, and typically a switch mode negative buck regulator.

The fluid injector system according to the present invention is particularly applicable to a fuel injector system for an internal combustion engine, and may form part of an internal combustion engine management system.

An exemplary embodiment of the invention will now be described, reference being made to the accompanying drawings, in which: Fig. 1 is a partially block schematic diagram of a fuel injector system in accordance with the present invention; Fig. 2 is a partially block schematic diagram of a switch mode power supply for use in the system of Fig. 1; and Fig. 3 is a partially block schematic diagram of an alternative form of switch mode power supply for use in the system of Fig. 1.

In Fig. 1 there is depicted a fluid injector system in accordance with the present invention in the form of a fuel injector system designed for an internal combustion engine. The system comprises a solenoid operated fuel injector only the solenoid S of which is shown. The system is primarily designed for low impedance injectors (typically 1 - 4 ohms) but as will hereinafter be described may be used with high impedance injectors (typically 15 ohms).

The principle of the fuel injector system to be described is based on the appreciation that the injector requires a large current in order to turn it ON and a much lower current in order to "hold" it in its ON state. This is achieved by providing two control devices e.g. bi-polar transistors or mosfet devices, for each injector, one for effecting the initial turn ON and the other for supplying the reduced "hold" current. Because the "hold" current is so much lower than the turn ON current it can be supplied from a separate lower voltage power supply, thereby reducing the amount of heat that is generated.

In the fuel injector system of Fig. 1, the solenoid S is connected in series with a first control transistor TR1 between a first voltage supply V1 and ground.

The first voltage supply V1 may be constituted by a car battery and typically be at 13.8 V. The first control transistor TR1 is operated in switch mode i.e. it is either ON or OFF, its base being connected via a resistor R1 which limits the base drive current to the output of a "timer" circuit T, typically in the form of a monostable circuit or a "one shot" timer. The timer circuit T is connected to an input I to which is applied an injector drive pulse which determines when and for how long the injector is to be turned ON.

When an injector drive pulse is applied to the input I the timer T generates a pulse, typically of 2mS duration, which causes the transistor TR1 to be turned ON, so that the solenoid S is connected directly between the supply V1 and ground, to cause the injector to be turned ON.

The solenoid S is also connected via an isolation diode D1 to a second control transistor TR2 which is operated in "linear" mode, the emitter of which has a series connected current sensing resistor R2, the other end of which is connected to the output of a switch mode power supply P the input of which is connected to the first voltage supply V1. The power supply P is arranged to generate an output voltage V2 which is typically four volts less than that of the first voltage supply V1.

The base of transistor TR2 is connected via base limiting resistor R4 to the input I to which the injector drive pulse is applied.

If the injector drive pulse has a longer period than that of the timer T, typically 2mS, the second control transistor TR2 allows current to flow via itself, not to ground but to the potential V2 which is about 4V less than the first voltage supply V1. The current through transistor TR2 is controlled so that only sufficient current is passed through the solenoid S to "hold" the injector ON.

This is achieved by making use of the voltage developed across the current sensing resistor R2 to drive a further control transistor TR3, via a base limiting resistor R3, which has the effect of conducting current away from the base of transistor TR2 when current flows in resistor R2.

The diode D1 is provided to prevent current flowing in transistor TR2 when transistor TR1 is conducting.

As has already been mentioned, the fuel injector system of Fig. 1 has been primarily designed for use with low impedance injectors, but can be adapted for use with high impedance injectors by simply bypassing the timer T so that the first control transistor TR1 is controlled directly by the injector drive pulse applied to input I, and the "hold" current afforded by transistor TR2 is not utilised. Also, although the transistor TR1 is turned ON for a predetermined time period, typically 2mS, by the timer T, this time period may be different for different injector configurations and it is anticipated that the time period of the timer T can be changed electronically so that the system is adaptable to such different injector configurations.

Critical to the electrical efficiency of the fuel injector system of Fig. 1 is the design of the power supply P used to generate the "hold" current for the solenoid S. A suitable switch mode power supply is shown in Fig. 2 of the drawings.

The switch mode power circuit of Fig. 2 is based on a Linear Technology Lot 1170 device It 1. This device was chosen for its ease of use, high switch current supply and physical simplicity. The LT1170 and its derivatives can be operated in many different configurations depending on output voltage, output current, input voltage and voltage polarity. The mode of operation which is felt to be most suitable for this application is negative buck configuration, although other modes are possible. The use of this mode of operation allows the output voltage V2 to track the input voltage positive supply rail V1 and therefore "float" with respect to ground potential.This function ensures that a constant "hold" current supply voltage is supplied to the injector drive electronics even if the supply voltage V1 varies in potential with respect to ground. The components R22, R23, R24 and TR2 form a voltage output feedback path to the internal control circuitry of ICi. Components C1 and R21 allow a time delayed soft start of the system and can also allow some control of current limit. The size of the inductor L is dictated to a large extent by the switching frequency of IC 1, which is typically about 100Khz. Capacitors C2 and C3 are provided for smoothing purposes.

In Fig. 3 of the drawings there is depicted an alternative form of the switch mode power supply P of Fig. 1 which is similar to that of Fig. 2 and which is also operated in negative buck configuration, but which affords a higher efficiency than that of Fig. 2.

The switch mode power supply circuit of Fig. 3 is built using a Maxim MAX1771 high efficiency controller IC2. This device, in contrast to the Linear Technology device IC1 of Fig. 2, uses an external drive transistor TR3 as the switch element and an external resistor R25 for current sense feedback. The fundamental difference between the circuits of Figs. 2 and 3 is that the IC2 operates at a higher frequency and has the use of a more advanced switch element TR3. The higher frequency allows the use of a physically smaller inductor L, whilst the switch transistor TR3 produces less voltage drop when turned ON, reducing the heat energy dissipation.

It has been found that an engine management system using the fuel injector system of Fig. 1 in conjunction with the power supply of Fig. 3, up to twelve fuel injectors can be driven from a unit which occupies a case the size of a conventional four injector unit, with all the control transistors being mounted on a single printed circuit board, and without elaborate heat sinking arrangements being required.

It will be appreciated that the fluid injector system which has been described has been given by way of example only and may be adapted to suit any particular requirement. Although the system described is applicable to fuel injector systems for automotive applications, similar principles may be used in other fluid injector applications.

Claims

1. A fluid injector system comprising a solenoid operated injector and a drive circuit for causing said injector to be operated, said drive circuit comprising a first control device operable for a predetermined period for supplying an initial current to said injector to cause it to be turned ON, and a second control device operable subsequent to said time period for supplying a further current to said injector for causing said injector to be held ON.

2. A system as claimed in claim 1, in which said initial current is derived from a first voltage supply and said further current is derived from a second voltage supply, which has a voltage which is less than that of said first voltage supply.

3. A system as claimed in claim 1 or claim 2, in which said first control device is operated in switch mode, and said second control device is operated in substantially linear mode.

4. A system as claimed in any preceding claim, in which said drive circuit comprises an input for receiving an input operating pulse for said injector, and a timer circuit operable from said input for causing said first control device to be operated for said predetermined period, said predetermined period being less than the period of said input operating pulse.

5. A system as claimed in claim 4, in which said drive circuit comprises a control circuit for said second control device, said control circuit having an input for said input operating pulse, current sensing means for affording an output in dependence upon said further current, and a further control device responsive to said sensed current for controlling the operation of said second control device.

6. A system as claimed in claim 2, in which said second voltage supply is constituted by a switch mode power supply.

7. A system as claimed in claim 6, in which said switch mode power supply is constituted by a switch mode negative buck regulator.

8. A fluid injector system substantially as hereinbefore described with reference to the accompanying drawings.

9. An internal combustion engine management system comprising a fuel injector system as claimed in claim 8.

9. A fluid injector system as claimed in any preceding claim, in the form of a fuel injector system for an internal combustion engine.

10. An internal combustion engine management system comprising a fuel injector system as claimed in claim 9.

Amendments to the claims have been filed as follows

1. A fluid injector system comprising a solenoid operated injector and a drive circuit for causing said injector to be operated, said drive circuit comprising a first control device operable for a predetermined period for supplying an initial current to said injector to cause it to be turned ON, and a second control device operable subsequent to said time period for supplying a further current to said injector for causing said injector to be held ON, said initial current being derived from a first voltage supply and said further current being derived from a second voltage supply which has a voltage which is less than that of said first voltage supply.

2. A system as claimed in claim 1, in which said first control device is operated in switch mode, and said second control device is operated in substantially linear mode.

3. A system as claimed in any preceding claim, in which said drive circuit comprises an input for receiving an input operating pulse for said injector, and a timer circuit operable from said input for causing said first control device to be operated for said predetermined period, said predetermined period being less than the period of said input operating pulse.

4. A system as claimed in claim 3, in which said drive circuit comprises a control circuit for said second control device, said control circuit having an input for said input operating pulse, current sensing means for affording an output in dependence upon said further current, and a further control device responsive to said sensed current for controlling the operation of said second control device.

5. A system as claimed in any preceding claim, in which said second voltage supply is constituted by a switch mode power supply.

6. A system as claimed in claim 5, in which said switch mode power supply is constituted by a switch mode negative buck regulator.

7. A fluid injector system substantially as hereinbefore described with reference to the accompanying drawings.

8. A fluid injector system as claimed in any preceding claim, in the form of a fuel injector system for an internal combustion engine.