Detection of faults in an injector arrangement

A method of detecting faults in an injector arrangement in an engine. The injector arrangement comprises at least one fuel injector having a piezoelectric actuator, and the method comprises: charging the piezoelectric actuator during a charge phase (tC); attempting to recharge the piezoelectric actuator during a test phase (tT) which commences after a time interval (Δt) following the end of the charge phase (tC); sensing a current (IS) that flows through the piezoelectric actuator during the test phase (tT); and generating a short circuit fault signal if the sensed current (IS) reaches a first predetermined threshold current (ISC) which is indicative of a short circuit in the piezoelectric actuator.

The present invention relates to a method and an apparatus for detecting faults in a fuel injector arrangement, and particularly to a method and an apparatus for detecting short circuit and open circuit faults in a piezoelectric actuator of a fuel injector arrangement.

Automotive vehicle engines are generally equipped with fuel injectors for injecting fuel (e.g., gasoline or diesel fuel) into the individual cylinders or intake manifold of the engine. The engine fuel injectors are coupled to a fuel rail which contains high pressure fuel that is delivered by way of a fuel delivery system. In diesel engines, conventional fuel injectors typically employ a valve needle that is actuated to open and to close in order to control the amount of fluid fuel metered from the fuel rail and injected into the corresponding engine cylinder or intake manifold.

One type of fuel injector that offers precise metering of fuel is the piezoelectric fuel injector. Piezoelectric fuel injectors employ piezoelectric actuators made of a stack of piezoelectric elements arranged mechanically in series for opening and for closing an injection valve needle to meter fuel injected into the engine. Piezoelectric fuel injectors are well known for use in automotive engines.

The metering of fuel with a piezoelectric fuel injector is generally achieved by controlling the electrical voltage potential applied to the piezoelectric elements to vary the amount of expansion and contraction of the piezoelectric elements. The amount of expansion and contraction of the piezoelectric elements varies the travel distance of a valve needle and, thus, the amount of fuel that is passed through the fuel injector. Piezoelectric fuel injectors offer the ability to meter precisely a small amount of fuel.

Typically, the fuel injectors are grouped together in banks of one or more injectors. As described in EP1400676, each bank of injectors has its own drive circuit for controlling operation of the injectors. The circuitry includes a power supply, such as a transformer, which steps-up the voltage generated by a power source, i.e. from 12 Volts to a higher voltage, and storage capacitors for storing charge and, thus, energy. The higher voltage is applied across the storage capacitors which are used to power the charging and discharging of the piezoelectric fuel injectors for each injection event. Drive circuits have also been developed, as described in WO 2005/028836A1, which do not require a dedicated power supply, such as a transformer.

The use of these drive circuits enables the voltage applied across the storage capacitors, and thus the piezoelectric fuel injectors, to be controlled dynamically. This is achieved by using two storage capacitors which are alternately connected to an injector arrangement. One of the storage capacitors is connected to the injector arrangement during a discharge phase when a discharge current flows through the injector arrangement, initiating an injection event. The other storage capacitor is connected to the injector arrangement during a charge phase, terminating the injection event. A regeneration switch is used at the end of the charge phase, before a later discharge phase, to replenish the storage capacitors.

Like any circuit, faults may occur in a drive circuit. In safety critical systems, such as diesel engine fuel injection systems, a fault in the drive circuit may lead to a failure of the injection system, which could consequentially result in a catastrophic failure of the engine. Examples of such faults include short circuit or open circuit faults in the piezoelectric actuators of the fuel injectors. A robust diagnostic system is therefore required to detect such faults in the piezoelectric actuators, particularly whilst the drive circuit is in use.

An aim of the invention is therefore to provide a diagnostic tool that is capable of detecting critical failure modes, or fault response characteristics, of an injector arrangement, and a method of operating the diagnostic tool.

According to a first aspect of the invention, there is provided a method of detecting faults in an injector arrangement in an engine, the injector arrangement comprising at least one fuel injector having a piezoelectric actuator, and the method comprising: charging the piezoelectric actuator during a charge phase; attempting to recharge the piezoelectric actuator during a test phase commencing after a time interval following the end of the charge phase; sensing a current that flows through the piezoelectric actuator during the test phase; and generating a short circuit fault signal if the sensed current reaches a first predetermined threshold current which is indicative of a short circuit in the piezoelectric actuator.

The method may comprise generating a first control signal during the test phase. The first control signal may be variable between a first state and a second state in response to the sensed current. The first control signal may be chopped between the first state and the second state if the sensed current reaches the first predetermined threshold current, and the short circuit fault signal may be generated when a chop occurs in the first control signal during the test phase.

Open circuit faults may also be detected. To detect open circuit faults, the method may comprise discharging the piezoelectric actuator during a discharge phase, and sensing the current that flows through the piezoelectric actuator during the discharge phase. An open circuit fault signal may be generated if the sensed current during the discharge phase does not reach a second predetermined threshold current.

A second control signal may be generated during the discharge phase, the second control signal may be variable between a first state and a second state in response to the sensed current during the discharge phase. The second control signal may be chopped between the first state and the second state if the sensed current exceeds the second predetermined threshold current, and an open circuit fault signal may be generated if a chop does not occur in the second control signal during the discharge phase. The open circuit fault signal may be generated if a chop has not occurred in the second control signal after a predetermined time interval following the start of the discharge phase.

The time interval may depend on an angle of rotation of a crankshaft of the engine, which may in turn depend on the engine speed and/or load.

According to a second aspect of the invention, there is provided an apparatus for detecting faults in an injector arrangement, the injector arrangement comprising at least one fuel injector having a piezoelectric actuator, and the apparatus comprising: charge means for charging the piezoelectric actuator; current sensing means for sensing a current through the piezoelectric actuator; and control means arranged to cause the charge means to connect to the piezoelectric actuator during the charge phase and re-connect to the piezoelectric actuator during a test phase, the test phase commencing after a time interval following the charge phase; wherein the control means is further arranged to generate a short circuit fault signal if the sensed current during the test phase reaches a first predetermined threshold current.

The apparatus may comprise means for generating a first control signal which is chopped between a first state and a second state when the sensed current during the test phase reaches the first predetermined threshold current. The control means may be arranged to generate the short circuit fault signal if a chop occurs in the first control signal during the test phase.

The apparatus may also comprise discharge means for discharging the piezoelectric actuator during a discharge phase, and the control means may be arranged to generate an open circuit fault signal if the sensed current during the discharge phase does not exceed a second predetermined threshold current.

The apparatus may further comprise means for generating a second control signal which is chopped between a first state and a second state if the sensed current during the discharge phase exceeds the second predetermined threshold current, and the control means may be arranged to generate the open circuit fault signal if a chop does not occur in the second control signal during the discharge phase. The control means may further be arranged to generate the open circuit fault signal if a chop has not occurred in the second control signal after a predetermined time interval following the start of the discharge phase.

According to a third aspect of the invention, there is provided a method of detecting faults in an injector arrangement of an engine, the injector arrangement comprising at least one fuel injector having a piezoelectric actuator which is connected in a drive circuit, and the method comprising: charging the piezoelectric actuator during a charge phase; selecting the piezoelectric actuator into the drive circuit and determining the voltage on the selected piezoelectric actuator at the end of the charge phase; deselecting the piezoelectric actuator from the drive circuit and allowing a time period to elapse before selecting the piezoelectric actuator into the drive circuit again and determining the voltage on the selected piezoelectric actuator; calculating a voltage drop or a voltage gradient; and generating a short circuit fault signal if:

(a) the voltage drop is greater than a predetermined voltage drop value, or

(b) the voltage gradient is greater than a predetermined voltage gradient value.

The time interval may depend on an angle of rotation of a crankshaft of the engine, which may in-turn depend on an engine speed and/or load.

According to a fourth aspect of the invention, there is provided a drive circuit for detecting faults in an injector arrangement, the injector arrangement comprising at least one fuel injector having a piezoelectric actuator, and the drive circuit comprising: charge means for charging the piezoelectric actuator; injector select means for selecting the piezoelectric actuator into the drive circuit; means for determining a first voltage on the selected piezoelectric actuator immediately after the piezoelectric actuator has been charged, and for determining a second voltage on the selected piezoelectric actuator after a time period following the charging of the piezoelectric actuator; and processing means programmed to calculate a voltage drop or a voltage gradient, and generate a short circuit fault signal if:(a) the voltage drop is greater than a predetermined voltage drop value, or(b) the voltage gradient is greater than a predetermined voltage gradient value.

The inventive concept encompasses a computer program product comprising at least one computer program software portion which, when executed in an executing environment, is operable to implement the methods described above. The inventive concept also encompasses a data storage medium having the or each computer software portion stored thereon, and a microcomputer provided with said data storage medium.

Referring toFIG. 1, an engine10, such as an automotive vehicle engine, is generally shown having a fuel injector arrangement comprising a first fuel injector12aand a second fuel injector12b. The fuel injectors12a,12beach have an injector valve needle14and a piezoelectric actuator16a,16brespectively. The piezoelectric actuators16a,16bare operable to cause the injector valve needle14to open and close to control the injection of fuel into an associated cylinder of the engine10. The fuel injectors12a,12bmay be employed in a diesel internal combustion engine to inject diesel fuel into the engine10, or they may be employed in a spark ignited internal combustion engine to inject combustible gasoline into the engine10.

The fuel injectors12a,12bform an injector bank18and are controlled by means of a drive circuit20,20A. In practice, the engine10may be provided with more than one injector bank18, and each injector bank18may have one or more fuel injectors12a,12b. For reasons of clarity, the following description relates to only one injector bank18. In the embodiments of the invention described below, the fuel injectors12a,12bare of a negative-charge displacement type. The fuel injectors12a,12bare therefore opened to inject fuel into the engine cylinder during a discharge phase and closed to terminate injection of fuel during a charge phase.

The engine10is controlled by an Engine Control Module (ECM)22, of which the drive circuit20,20A forms an integral part. The ECM22includes a microprocessor24and a memory26which are arranged to perform various routines to control the operation of the engine10, including the control of the fuel injector arrangement. Signals are transmitted between the microprocessor24and the drive circuit20,20A, and data which is comprised in the signals received from the drive circuit20,20A is recorded in the memory26. The ECM22is arranged to monitor engine speed and load. It also controls the amount of fuel supplied to the injectors12a,12band the timing of operation of the injectors12a,12b. The ECM22is connected to a vehicle battery (not shown) which has a battery voltage of about 12 Volts. Further detail of the operation of the ECM22and its functionality in operating the engine10, particularly the injection cycles of the injector arrangement, is described in detail in WO 2005/028836A1.

The drive circuit20,20A operates in four main phases: a discharge phase, a charge phase, a test phase, and a regeneration phase. During the discharge phase, the drive circuit20operates to discharge the piezoelectric actuator16aor16bof one of the fuel injectors12aor12bto open the injector valve needle14to inject fuel into the associated engine cylinder. During the charge phase, the drive circuit20operates to charge the previously discharged piezoelectric actuator16aor16bto close the injector valve needle14of the associated injector12aor12bto terminate the injection of fuel. During the test phase, the drive circuit20operates to test if there is a short circuit in any of the piezoelectric actuators16a,16b, and during the regeneration phase, energy in the form of electric charge is replenished to a first storage capacitor C1and a second storage capacitor C2(as shown inFIG. 2a), for use in subsequent injection cycles. Each of these phases of operation is described in further detail below with reference toFIG. 2a.

FIG. 2ashows a drive circuit20in accordance with a first embodiment of the invention. The drive circuit20includes high, low and ground voltage rails VH, VLand VGNDrespectively. The drive circuit20is generally configured as a half H-bridge with the low voltage rail VLserving as a bi-directional middle current path21. The piezoelectric actuators16aand16bof the injectors12a,12b(FIG. 1) are connected in the low voltage rail VL. The piezoelectric actuators16aand16bare located between, and coupled in series with, an inductor L1and a current sensing and control means28which are also connected in the low voltage rail VL.

The piezoelectric actuators16aand16b(hereinafter referred to simply as ‘actuators’) are connected in parallel. Each actuator16a,16bhas the electrical characteristics of a capacitor and is chargeable to hold a voltage which is the potential difference between its charge (+) and discharge (−) terminals. Each actuator16a,16bis connected in series with a respective injector select switch SQ1, SQ2, and each injector select switch SQ1, SQ2has a diode D1, D2connected across it.

The injector bank18includes a regeneration branch30in parallel with the actuators16a,16b. The regeneration branch30includes a regeneration switch RSQ, a first diode RSD1connected across the regeneration switch RSQ and a second diode RSD2connected in series with the regeneration switch RSQ. The first and second diodes RSD1, RSD2are opposed to one another so that current can only flow one way through the regeneration branch30and then only when the regeneration switch RSQ is closed.

The drive circuit20includes a voltage source32connected between the low voltage rail VLand the ground rail VGND. The voltage source32may be provided by the vehicle battery (not shown) in conjunction with a step-up transformer (not shown) for increasing the voltage from the battery to the required voltage of the low voltage rail VL. In this example, the voltage on the low voltage rail VLis about 55 volts, and the voltage on the high voltage rail is about 255 volts, however the skilled person would realise that other voltages can be used to similar effect. In general, it is preferred that VHis about 200 volts in excess of VL. The voltage on the high voltage rail VHis achieved during the regeneration phase as described in more detail later

A first energy storage capacitor C1is connected between the high and low voltage rails VH, VL, and a second energy storage capacitor C2is connected between the low and ground voltage rails VL, VGND. The capacitors C1, C2store energy which is used to charge and discharge the actuators16a,16bduring the charge and discharge phases respectively. A charge switch Q1is connected between the high and low voltage rails VH, VL, and a discharge switch Q2is connected between the low voltage and ground rails VL, VGND. Each switch Q1, Q2has a respective diode RD1, RD2connected across it for allowing current to return to the capacitors C1, C2during the regeneration phase.

In essence, the drive circuit20comprises a charge circuit and a discharge circuit. The charge circuit comprises the high and low voltage rails VH, VL, the first capacitor C1and the charge switch Q1, whereas the discharge circuit comprises the low voltage and ground rails VL, VGND, the second capacitor C2and the discharge switch Q2. There now follows a brief description of the discharge, charge and regeneration phases of operation of the drive circuit20.

To open an injector valve needle14(FIG. 1) and commence injection from one of the injectors12aor12b, the drive circuit20operates in the discharge phase, wherein one of the actuators16a,16bis discharged. During the discharge phase, an injector12aor12b(FIG. 1) is selected for injection by closing the associated injector select switch SQ1or SQ2respectively, the discharge switch Q2is closed and the charge switch Q1remains open. For example, to inject from the first injector12a, the first injector select switch SQ1is closed and current flows from the positive terminal of the second capacitor C2, through the current sensing and control means28, through the actuator16aof the selected first injector12a(from the low side − to the high side +), through the inductor L1(in the direction of the arrow ‘I-DISCHARGE’), through the discharge switch Q2and back to the negative side of the second capacitor C2. No current is able to flow through the actuator16bof the unselected second injector12bbecause of the diode D2and because the associated injector select switch SQ2remains open.

To charge the actuators16a,16bduring the charge phase, the charge switch Q1is closed and the discharge switch Q2remains open. The first capacitor C1, when fully charged, has a potential difference of about 200 volts across it, and so closing the charge switch Q1causes current to flow around the charge circuit, from the positive terminal of the first capacitor C1, through the charge switch Q1and the inductor L1(in the direction of the arrow ‘I-CHARGE’), through the actuators16aand16b(from the high sides +to the low sides −) and associated diodes D1and D2respectively, through the current sensing and control means28, and back to the negative terminal of the first capacitor C1. In the charge phase, the previously discharged actuator16ais charged which causes the injector valve needle14(FIG. 1) of the injector12ato close to terminate the injection of fuel into the associated cylinder (not shown).

Energy is replenished to the capacitors C1, C2during the regeneration phase so that the capacitors C1, C2are ready for use in further charge and discharge phases. To commence the regeneration phase, the regeneration switch RSQ and the discharge switch Q2are closed whilst the charge switch Q1remains open. Current from the vehicle battery (not shown) flows around the discharge circuit to charge the second capacitor C2. The discharge switch Q2is then opened, and because of the inductance of the inductor L1, some current continues to flow through the middle current path21for a short while after the discharge switch Q2is opened. This current flows through the diode RD1connected across the charge switch Q1and into the positive terminal of the first capacitor C1to partially charge the first capacitor C1. The discharge switch Q2is repeatedly closed and opened to further charge the first capacitor C1until the potential difference across the first capacitor C1is increased to about 255 volts. The regeneration process is described in more detail in WO 2005/028836A1.

The drive circuit20operates under a “charge-control” method as described in detail in co-pending patent application EP 06254039.8, the contents of which is incorporated herein by reference. The charge-control method involves controlling the current supplied to the actuators16a,16bduring the charge and discharge phases, and controlling the duration of the charge and discharge phases; the charge added to the actuators16a,16bduring the charge phase, and the charge removed from the actuators16a,16bduring the discharge phase is controlled under the relationship charge=current×time (Q=It).

In practice a varying current is driven through the actuators16a,16bduring the charge and the discharge phases. The varying current is achieved by the presence of the inductor L1, and by repeatedly opening and closing the charge switch Q1during the charge phase, and repeatedly opening and closing the discharge switch Q2during the discharge phase; the switches Q1and Q2are opened and closed under the control of the microprocessor24, in response to signals received from the current sensing and control means28.

The inductor L1opposes changing currents. Therefore, during the charge phase, the inductor L1delays the rise in current flowing around the charge circuit when the charge switch Q1changes from an open position to a closed position. Similarly, the inductor L1delays the fall in current when the charge switch Q1changes from a closed position to an open position; i.e. current continues to flow for a short while after the charge switch Q1is opened. The inductor L1has a similar effect during the discharge phase. Opening and closing the charge and discharge switches Q1, Q2therefore results in a varying current in the charge and discharge circuits respectively.

The control of current during the discharge phase and during the charge phase is described below with reference toFIG. 3(a) which shows an ideal graph of a varying current34generated during the discharge and the charge phases, tDand tCrespectively, of an actuator16aor16b. The current34is shown as positive during the charge phase tCand negative during the discharge phase tDbecause the current flows in opposite directions through the middle current path21(FIG. 2a) in these two phases. Reference is also made toFIGS. 3(b), (c) and (d) which show, respectively, a discharge enable signal36, a charge enable signal38, and a control signal40. The discharge enable signal36and the charge enable signal38are output directly from the microprocessor24, whereas the control signal40is output from the current sensing and control means28(FIG. 2a).

Referring toFIG. 3(b), the discharge phase tDis initiated at time t1. To initiate the discharge phase tDat t1, the microprocessor24generates a logic high discharge enable signal36and the current sensing and control means28outputs a logic high control signal40(FIG. 3(d)). The discharge enable signal36is combined with the control signal40through a logical AND gate in the microprocessor24, and the resultant signal (36AND40=HIGH) is output by the microprocessor24to the discharge switch Q2causing it to close.FIG. 2bis a simplified diagram of the microprocessor24showing various inputs for the signals36,38and40, and various outputs for signals to control the operation of the switches Q1, Q2, SQ1, SQ2and RSQ which are shown inFIG. 2a.

The current sensing and control means28senses the current ISas it flows through the middle current path21to discharge the actuator16aor16bof the selected injector12aor12b. The current sensing and control means28comprises a current comparator which compares the sensed current ISto a reference current and generates a logic low signal when ISrises above a predetermined upper threshold current I2, and a logic high signal when ISfalls below a predetermined lower threshold current I1; i.e. the current sensing and control means ‘chops’ the control signal40between the logic low and the logic high when the predetermined threshold currents I1and I2are sensed.

Referring toFIG. 3(a), when the discharge phase tDis initiated at t1to initiate an injection of fuel, the sensed current ISgradually increases because of the inductance of the inductor L1. This increase in current is indicated by reference numeral41onFIG. 3(a), and although this part of the graph is shown to have a negative gradient, current is increasing towards the predetermined threshold current I2. At time t2the sensed current ISreaches the predetermined upper threshold current I2, and hence the current sensing and control means28chops the control signal40(FIG. 3(d)) to a logic low. At time t2, the resultant of the combined discharge enable signal36and control signal40(36AND40=LOW) causes the discharge switch Q2(FIG. 2a) to open. The current then begins to gradually fall because of the inductance of the inductor L1until ISreaches the predetermined lower threshold current I1at a time t3. The current sensing and control means28senses that the current IShas reached the lower current threshold I1at t3, and chops the control signal40to a logic high; the resultant combined signal (36AND40=HIGH) causes the discharge switch Q2to close again. This process continues for the period tD.

The charge phase tCto terminate injection of fuel is analogous to the discharge phase tDdescribed above and is therefore not explained in detail herein. During the charge phase tC, the control signal40is combined with the charge enable signal38in the microprocessor24(FIG. 2b) and the resultant signal (38AND40) is applied to the charge switch Q1(FIG. 2a) to generate a current which varies between I3and I4over the period tCas shown inFIG. 3(a).

Look-up tables within the microprocessor's memory26produce values for the upper (more negative) current threshold I2during the discharge phase tD; the lower current threshold I1during the discharge phase tDis calculated from a ratio of the upper current threshold I2. Similarly, during the charge phase tC, the upper current threshold I4is obtained from a look-up table and the lower current threshold I3is calculated from a ratio of the upper current threshold I4. The values of I2and I4are selected depending on a number of factors including stack pressure, stack temperature, fuel demand and fuel rail pressure. The drive circuit20, and hence fuel delivery, are controlled by the ECM22. The ECM22incorporates strategies, which determine the required fueling and timing of injection pulses based on the current engine operating conditions, including torque, engine speed and operating temperature. The timing of when the injectors12a,12bopen and close is determined by the ECM and is not important to the understanding of the present invention.

A test phase tT, in which the actuators16a,16bare tested for short circuits, generally follows a charge phase tCat the end of the injection. If an actuator16aor16bdevelops a short circuit, it behaves electrically as a capacitive element with a resistive element in parallel. When the faulty actuator16aor16bis charged the capacitive element will gradually discharge itself through the resistive short circuit element. If no short circuit exists, the actuator16aor16bwill remain charged.

In the first embodiment of the invention, a ‘chop-feedback’ method is used in the test phase tTto detect short circuits in the actuators16aand16b. In the chop-feedback method, a short charge pulse is performed on the actuators16aand16bafter a predetermined time interval following the end of the charge phase tC. For properly functioning actuators16a,16bi.e. those without short circuits, no current should flow when this charge pulse is performed. If an actuator16aand/or16bhas a short circuit it will have discharged itself to a certain extent through its short circuit resistance during the predetermined time interval following the charge phase. In which case a current will flow to recharge the discharged actuator or actuators16aand/or16bwhen the charge pulse is performed during the test phase. This current can be detected using the current sensing and control means28(FIG. 2a).

In common with both charge and discharge phases tC, tD, during a test phase tTthe current sensing and control means28is programmed to output a control signal40which is variable between a high and a low state. The current sensing and control means28is further programmed to chop the control signal40if a current ISis sensed which reaches or exceeds a predetermined threshold current ISCindicative of a short circuit in one or both of the actuators16a,16b. ISCis chosen to be a value very close to zero amps because substantially no current should flow during the test phase if the injectors are all functioning correctly and none have short circuits. The control signal40is fed to an input of the microprocessor24, as shown inFIG. 2b, and if the microprocessor24detects the presence of a chop in the control signal40during the test phase, the microprocessor24generates a warning signal to indicate that there is a short circuit in the injector bank18.

If a warning signal is generated, the microprocessor24disables all further activity on the injector bank18; this includes the disabling of all subsequent discharge, charge and regeneration phases. The lower the level of ISC, the more robust the short circuit detection will be because higher resistance short circuits will be detectable (i.e. less current will flow during the test phase tT). This chop-feedback method of detecting short circuits is described in more detail below with reference toFIGS. 3 to 6.

Referring again toFIG. 3(c), which shows the charge enable signal38output by the current sensing and control means28, the test phase tTbegins at time t4, after a predetermined time period Δt following the end of the charge phase tC. In practice, a crank angle is measured, and the test phase tTbegins after the crank has rotated by a predetermined angle. The time period Δt therefore varies with engine speed and load, and decreases with increasing engine speed. This means that at low engine speeds, the resolution of the fault detection is maximised because there is more time available in which a charged injector can discharge through a short circuit. Therefore, higher resistance short circuits can be measured at lower engine speeds.

At time t4the microprocessor24switches the charge enable signal38(FIG. 3(c)) from a logic low to a logic high, such that a logic high signal pulse42is generated. The signal pulse42is of duration tT, which is equivalent to t5−t4(t5minus t4). The signal pulse42is also shown inFIG. 4(b) andFIG. 5(b).

FIGS. 4 and 5show ideal graphs of (a) the sensed current ISduring a test phase tT, (b) the charge enable signal pulse42shown inFIG. 3(c), and (c) the control signal40during the test phase tT.FIG. 4represents a situation where both of the actuators16a,16bin the injector bank18are functioning correctly and neither has a short circuit, whereasFIG. 5represents a situation where one or both of the actuators16a,16bhas a short circuit.

Referring first toFIG. 4, at time t4the control signal40(FIG. 3(c)) is switched from a logic low to a logic high simultaneously with the charge enable signal38(FIG. 3(b)). The control signal40is combined with the charge enable signal38and the resultant combined signal (38+40=HIGH) causes the charge switch Q1(FIG. 2)to close at time t4. It can be seen fromFIG. 4(a) that the sensed current ISduring the test phase tTis substantially zero amps and hence substantially no current flows during the test phase tTto recharge either actuator16a,16b. This is because both actuators16aand16bare still substantially fully charged at the beginning of the test phase tTbecause neither actuator16anor16bhas a short circuit.

As described earlier, the control signal40chops from high to low if the sensed current ISduring the test phase tTreaches the predetermined threshold current ISC, which is shown onFIG. 4(a) by the dashed line44. The sensed current ISinFIG. 4(a) does not reach the threshold current ISC, and hence the control signal40(FIG. 4(c)) is not chopped during the test phase tTand instead remains at a logic high. If no chop is detected in the control signal40during the test phase tT, then the actuators16a,16bare functioning correctly and there are no short circuits in the injector bank18. At t5, the charge enable signal38switches from logic high to logic low, and the resultant combined signal (38AND40=LOW) causes the charge switch Q1to open and terminate the test phase tT.

Reference is now made toFIG. 5which represents the situation where one or more of the actuators16aand/or16bhas a short circuit. As previously described with reference toFIG. 4, at the beginning of the short circuit testing phase (time t4) the charge enable signal38and control signal40are both set to high and combined, with the effect that the charge switch Q1(FIG. 2a) closes. In the case shown inFIG. 5, however, one or both of the actuators16a,16bhas discharged to a certain extent through a short circuit during the period Δt (FIG. 3) following the charge phase. The charge pulse42therefore causes a current to flow during the test phase tTto recharge the previously discharged actuator or actuators16aand/or16b.

FIG. 5(a) shows the current ISthat flows during the test phase tTwhen one or both of the actuators16a,16bhas a short circuit. At time tSCD, the current ISreaches the predetermined upper threshold current ISCwhich causes the current sensing and control means28to chop46the control signal40(FIG. 5(c)) from a logic high to a logic low. The combined signal (38AND40=LOW) causes the charge switch Q1to open at tSCDand the faulty actuator begins to discharge again through its short circuit. As shown inFIG. 5(a), the sensed current IScontinues to flow, but decreases, during a short period of time after the charge switch Q1opens at tSCD; this is because of the inductance of the inductor L1. The control signal40is fed back to the microprocessor24. The presence of the chop46in the control signal40during the test phase tTis indicative of a short circuit in the injector bank18and causes the microprocessor24to generate a warning signal. Subsequent discharge, charge and regeneration phases are then suspended on the faulty injector bank18if a short circuit is detected.

In addition to detecting short circuits, the current sensing and control means28and the microprocessor24are also used to detect open circuit faults. Open circuit faults are tested for during the discharge phase tDand hence it is not necessary to introduce an additional phase into the normal operation of the drive circuit to test for open circuit faults. When the discharge switch Q2(FIG. 2a) is closed, and an injector, for example the first injector12a, is selected for injection by closing the injector select switch SQ1(FIG. 2a), a current should flow through the actuator16aof the selected injector12a. If the actuator16aof the selected injector12ais open circuit, then substantially no current will flow during this discharge phase tD.

Now, as explained earlier with reference toFIG. 3, the current that flows during the discharge phase tDis controlled between the lower and upper current levels I1and I2respectively using the control signal40, such that when the upper current level I2is reached, the control signal40is chopped. If, therefore, the actuator16aof the selected injector12ais open circuit, the upper current threshold I2will not be reached during the discharge phase tDand hence the control signal40will not be chopped. The control signal40is fed back to the microprocessor24, and if no chop is present in the control signal40during the discharge phase tD, then the microprocessor24outputs an open circuit warning signal.

As an improvement to the open circuit detection method, a ‘time window’ may be introduced whereby an open circuit warning signal is generated if a chop has not occurred in the control signal40after a predetermined time interval following the commencement of the discharge phase tD. If the selected injector12ais found to be open circuit, then the injector12ais disabled. The remaining injectors12bon the injector bank18are not disabled and can continue normal operation. If all injectors12a,12bon the injector bank18are found to be open circuit, then the injector bank18is disabled entirely.

The method of detecting short circuits and open circuits using chop-feedback as described above is used during vehicle running so that any faults are detected as and when they occur. Although the detection of short circuits introduces an extra stage into the normal running of the drive circuit20, there is always a period of time between charging the actuators16a,16band the next injection from the injector bank18; the short circuit testing phase is performed immediately before this next injection, and so does not adversely affect the normal running of the vehicle. The open circuit detection does not introduce any extra stages into the normal running of the drive circuit20because it is performed during a discharge phase.

In addition to detecting short and open circuit faults during the running of the vehicle, the drive circuit20inFIG. 2ais used to detect short and open circuit faults during engine start-up. The method is slightly different, however, during start-up, and will now be explained with reference to the flow chart inFIG. 6:[step48] At start-up, a small calibratable voltage is generated on the high voltage rail VH. This voltage is typically about 75 volts, or about 20 volts above the voltage of the low voltage rail VL; this is in contrast to the situation during normal running of the engine when the high voltage rail is at about 255 volts;[step50] each actuator16a,16bon the injector bank18is charged to the same voltage as the high voltage rail VH;[step52] a calibratable period of time elapses during which any actuator16aand/or16bhaving a short circuit discharges to an extent;[step54] a charge pulse is performed on the actuators16a,16bat a calibratable current and for a calibratable period of time;[step56] the current sensing and control means28senses the current ISthat flows during the charge pulse;[step58] the sensed current ISis compared to a predetermined threshold current ISCwhich is indicative of a short circuit in at least one of the actuators16a,16b;[step60] if the sensed current ISreaches or exceeds the predetermined threshold current ISC, the current sensing and control means28chops the control signal40which is fed back to the microprocessor24—a chop in the control signal40indicates that there is a short circuit in at least one of the actuators16a,16b;[step62] if the sensed current ISdoes not reach or exceed the predetermined threshold current ISCi.e. if no chop occurs in the control signal40, then it is deemed that there is no short circuit, and the injectors12a,12bare then tested, one by one, for open circuit faults during successive discharge phases tDby selecting an injector12a,12band closing the discharge switch Q2;[step64] the current sensing and control means28senses the current ISthat flows during the discharge phases tD;[step66] the current sensing and control means28chops the control signal40if the sensed current ISreaches or exceeds a predetermined threshold current IOC. The threshold current IOCwhich is used at start-up is lower than the threshold current I2which is used for open circuit testing during running. This is because the actuators16a,16bare charged to a lower level at start-up, and hence less current flows during a discharge phase tDat start-up;[step68] if the sensed current ISdoes not reach or exceed the predetermined threshold current IOC, then a chop does not occur in the control signal40; the absence of a chop in the control signal40indicates that the actuator16aor16bof the selected injector12aor12bis open circuit, and the microprocessor24accordingly generates a warning signal; if a warning signal is generated, then the actuator16aor16bof the selected injector12aor12bis open circuit and that injector is then disabled;[step70] if the sensed current ISreaches or exceeds the predetermined threshold current IOC, then a chop occurs in the control signal40; the presence of the chop indicates that the actuator16aor16bof the selected injector12aor12bis not open circuit, and the remaining injectors12a-12N are each tested in turn until all the injectors12a-12N on the injector bank18have been tested;[step72] testing is complete once all the injectors12a-12N have been tested. The results of the tests will show if there is a short circuit in the injector bank18, and if any of the actuators16a,16bis open circuit. Additionally, the tests can determine which one (if any) of the actuators16a,16bis open circuit.

In a second embodiment of the invention, an alternative method is used to detect short circuits in the injector arrangement. The alternative method will now be explained with reference toFIG. 7which shows a second embodiment of the drive circuit20A inFIG. 1. InFIG. 7, equivalent components have the same reference numerals as those inFIG. 2a. The drive circuit20A is essentially the same as the drive circuit20inFIG. 2a, but with the addition of a resistive bias network74which is connected across the high voltage rail VHand ground rail VGNDand which intersects the low voltage rail VLat a bias point PB. The foregoing description applies equally toFIG. 7as toFIG. 2aexcept in so far as it relates to the chop-feedback method of fault detection.

The resistive bias network74includes first, second and third resistors (R1, R2, R3) connected together in series. The first resistor R1is connected between the high voltage rail VHand the bias point PBon the low voltage rail VL, and the second and third resistors R2and R3are connected in series between the bias point PBand the ground rail VGND. The second resistor R2is connected between the bias point PBand the third resistor R3, and the third resistor R3is connected between the second resistor R2and the ground rail VGND.

The resistive bias network74is used to determine the voltage on a selected actuator16aor16bimmediately after a charge phase tC, and again after a predetermined time period ΔtAfollowing the end of that charge phase tC. The gradient of any voltage drop between the two readings will identify whether or not the selected actuator16aor16bhas a short circuit, and the extent of this short circuit. The gradient of the voltage drop should be substantially zero for an actuator16aor16bthat is functioning correctly and that does not have a short circuit. Any voltage drop gradient which is greater than a predetermined amount will indicate that the selected actuator16aor16bhas a short circuit.

The voltage on a selected actuator16aor16bis the potential VBat the bias point PBminus the voltage on the low voltage rail VL(55V in this example) when the relevant injector select switch SQ1or SQ2is closed. The resistive bias network74is used to measure the potential VMat a point PMwhich is between the second and third resistors R2and R3(by measuring the voltage across the third resistor R3) and the measured voltage VMis used to calculate the potential VBat the bias point PBas follows:

VM=VB×R3R2+R3(1)
and hence

For example, the following method is used during a test phase tTto test the actuator16aof the first injector12afor a short circuit using the resistive bias network74:Immediately after a charge phase tC, the first injector12ais selected by closing the injector select switch SQ1; the charge switch Q1and discharge switch Q2remain open, and the voltage VM1across the third resistor R3is measured;the potential at the bias point PB, and hence the voltage VBon the actuator16aof the selected first injector12aimmediately after the charge phase tC, is calculated from VM1, and the value of VBis stored in the memory26of the microprocessor24as a variable VB1;the injector12ais deselected by opening the injector select switch SQ1and a predetermined time period ΔtAis allowed to elapse following the end of the charge phase—the predetermined time period ΔtAmay depend on a crank shaft angle and hence engine speed as described previously;after the predetermined time period ΔtA, the injector12ais selected again by closing the injector select switch SQ1, and the voltage VM2across R3is measured;the value of VBafter this predetermined time period ΔtAis calculated from VM1and stored in the memory26of the microprocessor24as a variable VB2;a voltage drop VB2−VB1is calculated and compared to a predetermined voltage drop value. If the calculated voltage drop (VB2−VB1) exceeds the predetermined voltage drop value, then the microprocessor24outputs a short circuit warning signal.

The magnitude of the voltage drop (VB2−VB1) is dependent on the resistance of the short circuit and on the time period ΔtAwhich elapses between the voltage measurements. Higher resistance short circuits can be measured when the time period ΔtAis longer because the faulty actuator will have had a longer period to discharge. This means that the resolution of the short circuit detection is maximised at lower engine speeds when the time period ΔtAis longer.

As an alternative to comparing the voltage drop (VB2−VB1) to a predetermined voltage drop value, a voltage gradient may be calculated instead, as follows:

This voltage gradient does not depend on the time period ΔtAwhich elapses between the voltage measurements. The voltage gradient is compared to a predetermined voltage gradient value and, if the calculated voltage gradient exceeds the predetermined voltage gradient value, then the microprocessor24outputs a short circuit warning signal.

In either case, if the microprocessor24generates a short circuit warning signal, the selected injector12ais disabled. If the calculated voltage drop is less than the predetermined voltage drop value, or if the calculated voltage gradient is less than the predetermined voltage gradient value, then a short circuit warning signal is not generated and the drive circuit may proceed to operate as normal. The remaining actuators16b-16N are each tested for short circuits in a similar way to that just described.