Abstract:
A method and apparatus for detecting faults in an injector arrangement is described. The injector arrangement comprises a plurality of piezoelectric injectors that are located in parallel branches of an injector bank circuit of an injector drive circuit. Each branch of the injector bank circuit comprises a high side isolation switch. The high side isolation switches are each operable to enable an associated piezoelectric injector in the injector bank circuit when closed, and disable the associated piezoelectric injector in the injector bank circuit when open. The fault detection method comprises the steps of operating the high side isolation switches so as to enable one of the piezoelectric injectors and disable the other piezoelectric injector(s), and performing diagnostics to detect the presence or absence of faults on the enabled piezoelectric injector.

Description:
TECHNICAL FIELD 
     The present invention relates to a method and apparatus for detecting faults in a fuel injector arrangement, and particularly to a method and apparatus for detecting short circuit faults in piezoelectric fuel injectors. 
     BACKGROUND TO THE INVENTION 
     In a direct injection internal combustion engine, a fuel injector is provided to deliver a charge of fuel to a combustion chamber prior to ignition. Typically, the fuel injector is mounted in a cylinder head with respect to the combustion chamber such that its tip protrudes slightly into the chamber in order to deliver a charge of fuel into the chamber. 
     One type of fuel injector that is particularly suited for use in a direct injection engine is a so-called piezoelectric injector. A piezoelectric injector  12  and its associated control system  14  are shown schematically in  FIG. 1 . 
     The piezoelectric injector  12  includes a piezoelectric actuator  16  that is operable to control the position of an injector valve needle  17  relative to a valve needle seat  18 . The piezoelectric actuator  16  includes a stack  19  of piezoelectric elements, having the electrical characteristics of a capacitor. The stack  19  may be charged or discharged by application of a differential voltage to positive and negative terminals of the actuator  16 , which causes the stack of piezoelectric elements to expand or contract. The expansion and contraction of the piezoelectric elements is used to vary the axial position, or ‘lift’, of the valve needle  17  relative to the valve needle seat  18 . 
     The piezoelectric injector  12  is controlled by an injector control unit  22  (ICU) that forms an integral part of an engine control unit  24  (ECU). The ICU  22  typically comprises a microprocessor  26  and memory  28 . The ECU  24  also comprises an injector drive circuit  30 , to which the piezoelectric injector  12  is connected by way of first and second power supply leads  31 ,  32 . In a so-called ‘discharge to inject’ injector, in order to initiate an injection event, the injector drive circuit  30  causes the differential voltage applied to the injector  12  to transition from a high voltage (typically 200 V), at which no fuel delivery occurs, to a relatively low voltage (typically −55 V), which causes the valve needle  17  to lift away from the valve needle seat  18 . 
     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 that could consequentially result in a catastrophic failure of the engine. Diagnostic systems for detecting short circuit faults in piezoelectric actuators of piezoelectric injectors are disclosed in applicant&#39;s co-pending patent applications EP1843027, EP1860306, EP1927743, and EP07252534.8, EP07254036.2 the contents of each document being incorporated herein by reference. 
     Of particular relevance to this application is co-pending application EP 07252534.8, which describes a diagnostic method for detecting three main types of short circuit fault, these are: 
     i) a short circuit between the terminals of a piezoelectric actuator; otherwise referred to as a ‘stack terminal’ short circuit; 
     ii) a short circuit from the positive terminal of a piezoelectric actuator to a ground potential; the positive terminal is also referred to as the ‘high’ terminal, and this type of short circuit is generally referred to as a ‘high side to ground’ short circuit; and 
     iii) a short circuit from the negative terminal of a piezoelectric actuator to a ground potential; the negative terminal is also referred to as the ‘low’ terminal, and this type of short circuit is generally referred to as a ‘low side to ground’ short circuit. 
     Referring also to  FIG. 2 , this shows the injector drive circuit  30  described in EP 07252534.8. The injector drive circuit  30  comprises an injector bank circuit  33 , in which a pair of piezoelectric injectors  12   a ,  12   b  are connected. It should be appreciated that although the respective injectors  12   a ,  12   b  are shown as integral to the injector bank circuit  33  in  FIG. 2 , in practice the injector bank circuit  33  would be remote from the injectors  12   a ,  12   b  and connected thereto by way of power supply leads. 
     The drive circuit  30  includes three voltage rails: a high voltage rail VH (typically 255 V), a mid voltage rail VM (typically 55 V), and a ground voltage rail VGND (i.e. 0 V). The drive circuit  30  is generally configured as a half H-bridge with the mid voltage rail VM serving as a bi-directional middle current path  34 . The injector bank circuit  33  is located in the middle current path  34  of the drive circuit  30  and comprises a pair of parallel branches  33   a ,  33   b , in which the piezoelectric actuators  16   a ,  16   b  (hereinafter referred to simply as ‘actuators’) of the injectors  12   a ,  12   b  are respectively connected. The injector bank circuit  33  further comprises a pair of injector select switches SQ 1 , SQ 2  connected in series with the respective injectors  12   a ,  12   b  in the respective branches  33   a ,  33   b  of the injector bank circuit  33 . Each injector select switch SQ 1 , SQ 2  has a respective diode D 1 , D 2  connected across it. The injector bank circuit  33  is located between, and coupled in series with, an inductor L 1  and a current sensing and control means  35 . 
     A voltage source VS is connected between the mid voltage rail VM and the ground rail VGND of the drive circuit  30 . The voltage source VS may be provided by the vehicle battery (not shown) in conjunction with a step-up transformer (not shown), or other suitable power supply, for increasing the voltage from the battery to the required voltage of the mid voltage rail VM. 
     A first energy storage capacitor C 1  is connected between the high and mid voltage rails VH, VM, and a second energy storage capacitor C 2  is connected between the mid and ground voltage rails VM, VGND. The first capacitor C 1 , when fully charged, has a potential difference of about 200 Volts across it, whilst the potential difference across the second capacitor C 2  is maintained at about 55 Volts. A charge switch Q 1  is located between the high and mid voltage rails VH, VM, and a discharge switch Q 2  is located between the mid voltage and ground rails VM, VGND. 
     In essence, the drive circuit  30  comprises a charge circuit and a discharge circuit. The charge circuit comprises the high and mid voltage rails VH, VM, the first capacitor C 1  and the charge switch Q 1 , whereas the discharge circuit comprises the mid and ground rails VM, VGND, the second capacitor C 2  and the discharge switch Q 2 . The charge switch Q 1  is operable to connect the injectors  12   a ,  12   b  to the first capacitor C 1  causing a current to flow in the charge circuit, in the direction of the arrow ‘I-CHARGE’, to charge the actuators  16   a ,  16   b  to a known voltage. The diodes D 1 , D 2  connected across the injector select switches SQ 1 , SQ 2  allow the injectors  12   a ,  12   b  to charge in parallel when the charge switch Q 1  is closed. To initiate an injection event from a selected injector  12   a  or  12   b , a current is caused to flow in the discharge circuit, in the direction of the arrow ‘I-DISCHARGE’. This is achieved by closing both the discharge switch Q 2  and an injector select switch SQ 1 , SQ 2  to connect the selected injector  12   a  or  12   b  to the second capacitor C 2 . 
     The drive circuit  30  further includes a resistive bias network  36  connected between the high voltage rail VH and ground rail VGND, and intersecting the middle circuit branch  34  at a bias point PB. The restive bias network  36  is used to determine the voltage VB at the bias point PB in order to detect short circuit faults on the injectors  12   a ,  12   b.    
     The resistive bias network  36  includes first, second and third resistors R 1 , R 2 , R 3  connected together in series. The first resistor R 1  is connected between the high voltage rail VH and the bias point PB, and the second and third resistors R 2  and R 3  are connected in series between the bias point PB and the ground rail VGND. The first, second and third resistors R 1 , R 2 , R 3  each have a known resistance of a high order of magnitude, typically of the order of hundreds of kiloohms. For convenience, R 1 , R 2  and R 3  are used herein to refer to both the resistors and to the resistances of the resistors R 1 , R 2 , R 3 . 
     To determine the voltage VB at the bias point PB, a voltage VS is sampled between the second and third resistors R 2 , R 3  in the resistive bias network  36  using an analogue to digital (A2D) module of the microprocessor  26  ( FIG. 1 ). The resistors R 2  and R 3  form a potential divider, and so the voltage VB at the bias point PB is calculated according to equation 1 below. 
     
       
         
           
             
               
                 
                   
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     To detect high and low side to ground short circuit faults on the injectors  12   a ,  12   b , a so-called ‘unselected voltage reading’ technique can be employed. The unselected voltage reading technique involves determining the voltage VB at the bias point PB with neither of the injectors  12   a ,  12   b  selected, i.e. with both injector select switches SQ 1 , SQ 2  open. When both injector select switches SQ 1 , SQ 2  are open, a voltage V Bpred  at the bias point PB can be predicted from the high rail voltage VH, and the value of the resistors R 1 , R 2 , R 3  in the resistive bias network  36 , according to equation 2 below. 
     
       
         
           
             
               
                 
                   
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     In the event that either of the injectors  12   a  or  12   b  has a high side to ground short circuit, then this short circuit behaves as a resistor connected in parallel with the resistors R 2  and R 3  in the resistive bias network  36 . If the voltage VB is measured at the bias point PB when there is a high side to ground short circuit, then the measured voltage will be lower than the predicted voltage V Bpred  according to equation 2 above. However, if one of the injectors  12   a ,  12   b  has a low side to ground short circuit, then the measured voltage at the bias point PB will be higher than the predicted voltage V Bpred  according to equation 2, and will depend upon the inherent resistance of the low side to ground short circuit. Hence, by measuring the voltage at the bias point PB and comparing it to the predicted voltage V Bpred  according to equation 2 above, high and low side to ground short circuit faults on the injector bank  33  can be detected. 
     Stack terminal short circuits can also be detected using the resistive bias network  36 . If an injector  12   a ,  12   b  has a stack terminal short circuit, then it will not hold its charge following a charge event on the bank  33 . Instead, the injector  12   a ,  12   b  will discharge through the stack terminal short circuit at a rate governed by the inherent resistance of the stack terminal short circuit. Stack terminal short circuits of suitably high resistance may not be detrimental to the normal operation of the system, and so a maximum acceptable rate of discharge may be predetermined, corresponding to a minimum acceptable resistance of a stack terminal short circuit. 
     To detect a stack terminal short circuit, a so-called ‘selected voltage reading’ technique can be employed. The selected voltage reading technique involves determining the voltage VB at the bias point PB with an injector  12   a  or  12   b  selected, i.e. with an injector select switch SQ 1  or SQ 2  closed. When an injector select switch SQ 1  or SQ 2  is closed, the voltage VB measured at the bias point PB is related to the voltage on the selected injector  12   a  or  12   b . The voltage on the selected injector  12   a  or  12   b  can be obtained by subtracting the voltage on the mid voltage rail VM (55 V in this example) from the voltage VB at the bias point PB. 
     In the selected voltage reading technique, the voltage measurement is performed after a predetermined period following a charge event on the bank  33 . The voltage on an injector  12   a ,  12   b  at the end of a charge event is known. If the voltage VB at the bias point PB is less than a predetermined voltage level, then this is indicative of a stack terminal short circuit, having a resistance below a predetermined minimum acceptable value, on one or both of the injectors  12   a ,  12   b . It should be appreciated that the expression ‘voltage on an injector’ is used for convenience and refers to the voltage on the piezoelectric stack of the injector actuator  16   a ,  16   b.    
     A disadvantage of using the selected voltage reading as described above to determine stack terminal short circuits on the injectors  12   a ,  12   b , is that this technique can entail a charge share between the injectors  12   a  and  12   b  in the event of a stack terminal fault. Charge sharing occurs when a non-faulty injector  12   a ,  12   b  is selected causing it to discharge into a faulty injector  12   a ,  12   b.    
     For example, referring to  FIG. 2 , if the second injector  12   b  has a stack terminal short circuit, then selecting the first injector  12   a  by closing the first injector select switch SQ 1  will result in a closed loop in the injector bank circuit  33 . The closed loop includes the diode D 2  connected across the second injector select switch SQ 2 , and the closed first injector select switch SQ 1 . An uncontrolled current will flow from the non-faulty first injector  12   a , around the closed loop to charge the discharged faulty second injector  12   b , in turn resulting in the non-faulty first injector  12   a  discharging. Charge sharing can also occur if one of the injectors  12   a ,  12   b  has a stack terminal short circuit, when an injector  12   a  or  12   b  is selected for discharge by closing the associated injector select switch SQ 1  or SQ 2 . Whilst the selected voltage reading technique is able to determine stack terminal short circuit faults on the injector bank  33 , charging sharing prevents this technique from being able to determine which of the individual injectors  12   a ,  12   b  is at fault. 
     An alternative diagnostic technique for detecting stack terminal faults is a so-called ‘charge pulse’ technique, as described in EP 06256140.2 and EP 07252534.8. The charge pulse technique comprises performing a first ‘charge pulse’ on the injectors  12   a  and  12   b  by closing the charge switch Q 1  for a short period of time; opening the charge switch Q 1  and allowing a predetermined period of time to elapse before closing the charge switch Q 1  again for another short period of time to perform a second charge pulse on the injectors  12   a ,  12   b . If either of the injectors  12   a ,  12   b  has a stack terminal short circuit, then it will discharge to an extent during the predetermined period prior to the second charge pulse being performed. Hence, when the second charge pulse is performed, a current will flow in the charge circuit to recharge the discharged faulty injector  12   a  or  12   b.    
     If neither of the injectors  12   a ,  12   b  has a stack terminal short circuit, then both injectors  12   a ,  12   b  should substantially hold their charge during the predetermined period prior to the second charge pulse being performed, in which case substantially no current will flow in the charge circuit when the second charge pulse is performed. The current sensing and control means  35  is arranged to monitor current flow during the second charge pulse. The presence of a current during the second charge pulse above a predetermined threshold current level is indicative of a stack terminal short circuit on one or both of the injectors  12   a ,  12   b  on the bank  33 . The predetermined threshold current level is based on a minimum acceptable resistance of stack terminal short circuit and the duration of the predetermined period prior to the second charge pulse being performed. 
     Whilst the charge pulse technique described above does not suffer from the charge share problems of the selected voltage reading technique (because both injector select switches SQ 1 , SQ 2  remain open), in common with the other diagnostic techniques described above, the charge pulse technique is not able to determine which of the individual injectors  12   a ,  12   b  is at fault. 
     As mentioned above, in each of the diagnostic techniques described above, faults can be traced as far as the injector bank  33 , but faulty injectors  12   a ,  12   b  cannot be identified. In such circumstances, the recovery action on detection of a fault is to shut down the entire injector bank  33 . In the case of a four-cylinder engine, this would result in the engine running on only two cylinders, when the fault may only be associated with one of the injectors  12   a ,  12   b  on the bank  33 . This can cause associated problems at engine service, because further tests must be performed to identify the injector  12   a ,  12   b  at fault. 
     SUMMARY OF INVENTION 
     It is against this background that the invention provides, in a first aspect, a method of detecting faults in an injector arrangement comprising a plurality of piezoelectric injectors, the piezoelectric injectors being located in parallel branches of an injector bank circuit of an injector drive circuit and each branch of the injector bank circuit comprising a respective high side isolation switch operable to enable an associated piezoelectric injector in the injector bank circuit when closed, and disable the associated piezoelectric injector in the injector bank circuit when open, wherein the method comprises the steps of: operating the high side isolation switches so as to enable one of the piezoelectric injectors and disable the other piezoelectric injector(s); and performing diagnostics to detect the presence or absence of faults on the enabled piezoelectric injector. 
     The injector drive circuit is operable to selectively connect the injector bank circuit to a first voltage source to charge the piezoelectric injectors and to a second voltage source to discharge the piezoelectric injectors, the first voltage source being of higher voltage than the second voltage source. Each high side isolation switch is connected between a piezoelectric injector and the first voltage source in the respective branches of the injector bank circuit. 
     The use of high side isolation switches provides improvements in the diagnostics of short circuits on an injector bank. This is because the injectors can be tested for faults individually, one by one, so that a single faulty injector can be identified. This provides advantages when the engine is serviced, because the faulty injector can immediately be replaced without further tests being required to identify which injector on the bank is at fault. As such, the method may provide recording the location or address of a faulty injector in a memory device. The memory device can be read at engine service so that a service engineer can readily locate and replace the faulty injector. 
     Once a faulty injector has been identified by the diagnostics, the associated high side isolation switch may be opened to disable the faulty injector from the injector bank so that the engine can continue to run on all the remaining non-faulty injectors on the bank. Accordingly the method may provide the additional step of operating the associated high side isolation switch so as to disable the enabled injector in the event that a fault is determined on the enabled injector. Disabling the faulty injector results in the faulty injector being electrically isolated from the injector bank so that the faulty injector does not interfere with the normal operation of the remaining non-faulty injectors on the bank. A significant advantage of the high side isolation switches is that they enable high side to ground faults to be electrically isolated, which is not otherwise possible using switches located on the low sides of the injectors, which are commonly found in prior art injector drive circuits. 
     The diagnostics may include testing the enabled injector for high and low side to ground short circuit faults. This may be achieved by determining a bias voltage at a bias point in the injector drive circuit, and determining the presence of a high or low side to ground short circuit on the enabled piezoelectric injector if the bias voltage is not within a predetermined tolerance of a predicted bias voltage. A high side to ground short circuit may be determined if the bias voltage is lower than the predicted bias voltage by more than a first predetermined tolerance value. A low side to ground short circuit may be determined if the bias voltage is more than the predicted bias voltage by more than a second predetermined tolerance value. The unselected voltage reading technique, as described above by way of background to the invention, may be performed on the enabled injector to determine high and low side to ground short circuits. 
     Alternatively or additionally, the diagnostics may include testing the enabled injector for stack terminal short circuit faults. To test the enabled injector for a stack terminal fault, the method may comprise measuring a voltage indicative of the voltage on the enabled injector, comparing the measured voltage to a predetermined threshold voltage level, and determining the presence of a stack terminal short circuit if the measured voltage is less than the predetermined threshold voltage level. The selected voltage reading technique, as described above by way of background to the invention, may be performed on the enabled injector to determine stack terminal short circuits. As an alternative to using the selected voltage reading technique, the charge pulse technique, also described above by way of background to the invention, may be performed on the enabled injector. 
     In one embodiment of the invention, the high side isolation switches are predominantly open, such that the operating step comprises closing a high side isolation switch so as to enable the associated piezoelectric injector. With the high side isolation switches being predominantly open, the piezoelectric injectors are always electrically isolated from each other. This eliminates the possibility of charge sharing occurring between faulty and non-faulty injectors. Furthermore, this technique allows a faulty injector to be identified immediately and disabled without any post-processing steps being required to identify the injector at fault once a fault on the injector bank is detected. Relatively high speed high side isolation switches are required in this embodiment. 
     However, in another embodiment of the invention, the high side isolation switches are predominantly closed, such that the operating step comprises opening at least one high side isolation switch in order to leave a single high side isolation switch closed, and hence the associated piezoelectric injector enabled. With the high side isolation switches being predominantly closed, there remains a risk of charge sharing occurring between faulty and non-faulty injectors because the injectors are not always electrically isolated from each other. However, this technique allows relatively slow speed high side isolation switches to be used, which may provide a cost benefit. 
     In the case where the high side isolation switches are predominantly closed, the method may comprise performing a set of initial diagnostics on the injectors with all of the injectors enabled, i.e. with all of the high side isolation switches closed. The initial diagnostics enable the presence or absence of a fault on the injector bank to be determined, but do not locate the injector at fault. In the event that a fault is detected on the injector bank, one of high side isolation switches remains closed whilst the other high side isolation switches are opened so that only a single injector remains enabled on the bank. The enabled injector is then tested for faults as described above. 
     If the enabled injector is found to be at fault, then the associated high side isolation switch is opened to disable the faulty injector from the injector bank circuit. However, if the enabled injector is not found to be at fault, then in the case where there are only two injectors on the injector bank, the fault determined by the initial diagnostics can be attributed to the other injector. Alternatively, if the injector bank comprises more than two injectors, the remaining injectors are tested individually one at a time by closing and opening the high side isolation switches in the appropriate combinations. In either case, once the injector at fault has been determined, the high side isolation switch associated with the faulty injector is opened to disable the faulty injector, whilst the high side isolation switches associated with the injectors found to be non-faulty are closed to enable the non-faulty injectors so that the engine can run on all of the non-faulty injectors. In the unlikely event that more than one injector is found to be faulty, each faulty injector is disabled. 
     The initial diagnostics may comprise testing the injector arrangement for stack terminal faults using the charge pulse technique described above by way of background to the invention. Initially the charge pulse technique may be performed on all of the injectors, i.e. with each high side isolation switch closed so that each injector is enabled. In the event that a stack terminal short circuit is detected, then the method may comprise performing the charge pulse technique on individually enabled injectors to locate the injector at fault. However, once a stack terminal fault has been detected generally on the injector bank, and the injectors electrically isolated from one another, the selected voltage reading technique may be used, as described above by way of background to the invention, to determine which of the injectors is at fault. The selected voltage reading technique is of higher resolution than the charge pulse technique and the risk of charge sharing is eliminated when the injectors are electrically isolated from one another. 
     Additionally or alternatively, the initial diagnostics may include testing the injector arrangement for high side and low side to ground short circuits using the unselected voltage reading technique, also described above by way of background to the invention. In the event that a high or low side to ground short circuit is detected, then the method may comprise performing the unselected voltage reading technique on individually enabled injectors to locate the injector at fault. 
     According to a second aspect of the present invention, there is provided an apparatus for detecting faults in an injector arrangement comprising a plurality of piezoelectric injectors, the piezoelectric injectors being located in parallel branches of an injector bank circuit of an injector drive circuit and each branch of the injector bank circuit comprising a high side isolation switch operable to enable an associated piezoelectric injector in the injector bank circuit when closed, and disable the associated piezoelectric injector in the injector bank circuit when open, the apparatus further comprising diagnostic means for determining faults on the enabled piezoelectric injectors. 
     The injector drive circuit may be operable to selectively connect the injector bank circuit to a first voltage source to charge the piezoelectric injectors and to a second voltage source to discharge the piezoelectric injectors, wherein the first voltage source is of higher voltage than the second voltage source. The injectors are preferably discharge to inject injectors. 
     The injector bank circuit preferably includes a plurality of injector select switches individually associated with the respective injectors and connected on the low sides of the injectors. The injector select switches may be operated to select the individual piezoelectric injectors to perform an injection event. In this configuration, the piezoelectric injectors are each connected between a pair of switches: an injector select switch on the low side, and a high side isolation switch on the high side. 
     It will be appreciated that optional features of the method aspect of the invention are equally applicable to the apparatus aspect and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference has already been made to  FIGS. 1 and 2  by way of technical background to the present invention. 
         FIG. 1  is a schematic representation of a known piezoelectric injector and its associated control system comprising injector drive circuit, and 
         FIG. 2  is a circuit diagram of the injector drive circuit in  FIG. 1 . 
       In order that it may be more readily understood, the present invention will now be described with reference to the following figures, in which: 
         FIG. 3  is a circuit diagram of an injector drive circuit for a pair of piezoelectric injectors, the injector drive circuit including a pair of high side isolation switches that are both open; 
         FIG. 4  is a circuit diagram of the injector drive circuit of  FIG. 3 , in which both of the high side isolation switches are closed; 
         FIG. 5  is a flow chart of a first diagnostic routine for detecting faults on the piezoelectric injectors connected in the injector drive circuit of  FIGS. 3 and 4 , with the default state of the high side isolation switches being closed as shown in  FIG. 4 ; and 
         FIG. 6  is a flow chart of a second diagnostic routine for detecting faults on the piezoelectric injectors connected in the injector drive circuit of  FIGS. 3 and 4 , with the default state of the high side isolation switches being open as shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 3 , this shows an injector drive circuit  30  a similar to the drive circuit  30  in  FIG. 2 , but comprising a modified injector bank circuit  33 . The modified injector bank circuit  33  is similar to the injector bank circuit  33  in  FIG. 2 , but also includes a pair of high side isolation switches QHS 1 , QHS 2  connected in respective branches  33   a ,  33   b  of the injector bank circuit  33 , on the high sides of the respective injectors  12   a ,  12   b . Hence, each injector  12   a ,  12   b  is connected between an injector select switch SQ 1 , SQ 2  on the low side, and a high side isolation switch QHS 1 , QHS 2  on the high side. 
     There now follows a description of two different diagnostic routines for detecting short circuits on the injectors  12   a ,  12   b , in which the high side isolation switches QHS 1 , QHS 2  are utilized to determine which injector  12   a ,  12   b  is at fault and to disable the faulty injector  12   a ,  12   b . As explained in further detail below, in the first diagnostic routine the default state of the high side isolation switches QHS 1 , QHS 2  is closed as shown in  FIG. 4 , whilst in the second diagnostic routine the default state of the high side isolation switches QHS 1 , QHS 2  is open as shown in  FIG. 3 . 
     Referring to  FIG. 4 , this shows the drive circuit  30   a  of  FIG. 3  with both of the high side isolation switches QHS 1 , QHS 2  closed. As described in further detail below, if a short circuit fault on the injector bank  33  is detected with the high side isolation switches QHS 1 , QHS 2  closed, then the high side isolation switches QHS 1 , QHS 2  are opened in turn and further tests conducted to determine which of the injectors  12   a ,  12   b  is at fault. 
       FIG. 5  is a flow diagram showing the steps of the first diagnostic routine, with the default state of the high side isolation switches QHS 1 , QHS 2  being closed as shown in  FIG. 4 . Referring to  FIG. 5  and also to  FIG. 4 : 
     [Step A 1 ] With both high side isolation switches QHS 1 , QHS 2  closed, and both injectors  12   a ,  12   b  deselected, i.e. with both of the injector select switches SQ 1 , SQ 2  open, a voltage VB 1  at the bias point PB is determined by sampling the voltage VS between the second and third resistors R 2 , R 3  in the resistive bias network  36  and calculating VB 1  according to equation 1 above. 
     [Step A 2 ] The voltage VB 1  at the bias point PB is compared to a set of predetermined voltage limits. As described earlier, if either or both of the injectors  12   a ,  12   b  has a high side to ground short circuit, then the voltage VB 1  at the bias point PB will be lower than the predicted voltage V Bpred  according to equation 2. Conversely, if either or both of the injectors  12   a ,  12   b  has a low side to ground short circuit, then the voltage VB 1  at the bias point PB will be higher than the predicted voltage V Bpred  according to equation 2. This allows suitable voltage limits to be set, respectively, above and below the predicted voltage V Bpred . 
     [Step A 3 ] If the voltage VB 1  at the bias point PB is within the predetermined voltage limits, there are no high- or low-side to ground faults on the injector bank  33 . Further tests are then performed as described later to determine if either or both of the injectors  12   a ,  12   b  has a stack terminal short circuit fault. 
     [Step A 4 ] If the voltage VB, is not within the predetermined voltage limits, this is indicative of a high or low side to ground short circuit on the bank  33 . When a high or low side to ground short circuit is detected on the injector bank  33 , subsequent injections on both of the injectors  12   a ,  12   b  on the bank  33  are suspended and further tests are performed [Steps A 5 -A 9 ] to determine which of the injectors  12   a ,  12   b  is at fault. 
     [Step A 5 ] The second high side isolation switch QHS 2  is opened to disconnect or electrically isolate, the second injector  12   b  from the injector bank  33 . 
     [Step A 6 ] A voltage VB 2  at the bias point PB is determined with the second injector  12   b  disconnected from the injector bank  33 . At this point, the first high side isolation switch QHS 1  is closed, the second high side isolation switch QHS 2  is open, and both injector select switches SQ 1 , SQ 2  are open. 
     [Step A 7 ] The voltage VB 2  at the bias point PB determined at step A 6  above is compared to the voltage limits described in Step A 2 . 
     [Step A 8 ] If the voltage VB 2  at the bias point PB is outside the limits, then this indicates that the first injector  12   a  has a short circuit fault, and hence the fault detected at Step A 4  is attributable, at least in part, to the first injector  12   a . In this case, the first injector  12   a  is electrically isolated/disconnected from the injector bank  33  by opening the first high side isolation switch QHS 1  to disable further charging and discharging of the first injector  12   a  and further injections from the first injector  12   a . If the first injector  12   a  is found to be at fault, the second injector  12   b  is assumed to be non-faulty, and so the second high side isolation switch QHS 2  is re-closed, to connect the second injector  12   b  back to the injector bank circuit  33 , and injections on the second injector  12   b  are re-enabled. In the unlikely event that both injectors  12   a  and  12   b  are at fault, then the fault on the second injector  12   b  is detected by further diagnostics performed on the injector bank  33  with just the second injector  12   b  enabled i.e. with QHS 2  closed and QHS 1  open. 
     [Step A 9 ] If the voltage VB 2  at the bias point PB is within the limits, then this indicates that the first injector  12   b  is non-faulty, and hence the short circuit fault detected at Step A 4  above is on the second injector  12   b . In this case, the second high side isolation switch QHS 2  remains open and injections on the faulty second injector  12   b  are disabled, whilst injections on the non-faulty first injector  12   a  are re-enabled. 
     As mentioned at Step A 3  above, if the voltage VB 1  at the bias point PB is within the predetermined voltage limits, then further tests are preformed to determine if either of the injectors  12   a ,  12   b  has a stack terminal short circuit fault. With the default state of the high side isolation switches QHS 1 , QHS 2  being closed, the selected voltage reading technique, described above by way of background to the invention with reference to  FIG. 2 , is not utilized initially, because charge share between the injectors  12   a ,  12   b  can occur in the event of a stack terminal short circuit on one of the injectors  12   a ,  12   b . Instead, the charge pulse technique, also described above by way of background to the invention with reference to  FIG. 2 , is used initially with both high side isolation switches QHS 1 , QHS 2  closed. If a current is detected by the current sensing and control means, depicted in  FIGS. 3 and 4  as a current sense resistor  35 , when the second charge pulse is performed, and if this current exceeds a predetermined threshold level, this is indicative of a stack terminal short circuit on either or both of the injectors  12   a ,  12   b  on the injector bank  33 . 
     To determine which of the injectors  12   a  or  12   b  is at fault, the selected voltage reading technique is used, as described above by way of background to the invention. To test the first injector  12   a  for a stack terminal short circuit using the selected voltage reading technique, the second high side isolation switch QHS 2  is opened to disable the second injector  12   b , leaving just the first injector  12   a  enabled. The first injector  12   a  is selected by closing the first injector select switch SQ 1  and the voltage VB at the bias point PB is determined. If the voltage VB at the bias point PB is less than a predetermined voltage level, then this is indicative of a stack terminal short circuit on the selected first injector  12   a . However, if the voltage VB is equal to or greater than the predetermined voltage level, then it can be inferred that the second injector  12   b  has a stack terminal fault. 
     Referring now to the second diagnostic routine. As mentioned above, in the second diagnostic routine the default state of the high side isolation switches QHS 1 , QHS 2  is open, as shown in  FIG. 3 . The high side isolation switches QHS 1 , QHS 2  are only closed when an injection or diagnostic event is to be performed on the bank  33 . The high side isolation switches QHS 1 , QHS 2  are closed in turn to enable a single injector  12   a  or  12   b , and to allow diagnostics to be performed on the single enabled injector  12   a  or  12   b.    
       FIG. 6  is a flow diagram showing the steps of the second diagnostic routine to determine if a fault exists on the first injector  12   a . A similar test could be performed to determine if a fault exists on the second injector  12   b . Initially both high side isolation switches QHS 1 , QHS 2  are open as shown in  FIG. 3 . Referring now to  FIG. 6  and also to  FIG. 3 : 
     [Step B 1 ] The first high side isolation switch QHS 1  is closed to enable the first injector  12   a  on the injector bank  33 . The second high side isolation switch QHS 2  remains open such that the second injector  12   b  is disabled from the bank  33 . Both injector select switches SQ 1 , SQ 2  are open. 
     [Step B 2 ] The voltage VB at the bias point PB is determined. 
     [Step B 3 ] The voltage VB at the bias point PB is compared to a set of predetermined voltage limits, in the same way as described above for Step A 2  of the first diagnostic routine, in order to test the first injector  12   a  for high or low side to ground short circuit faults. 
     [Step B 4 ] If the voltage VB at the bias point PB is not within the predetermined voltage limits, this is indicative of a high or low side to ground short circuit on the first injector  12   a . If a high or low side to ground short circuit is detected on the first injector  12   a , then the first high side isolation switch QHS 1  is opened to disable the first injector  12   a  from the bank  33 . 
     [Step B 5 ] If the voltage VB at the bias point PB is within the predetermined voltage limits, then there are no high or low side to ground short circuits on the first injector  12   a . The first injector  12   a  is then selected by closing the first injector select switch SQ 1 , and tested for stack terminal short circuits using the selected voltage reading technique [Steps B 6  to B 9 ], which is also described above by way of background to the invention with reference to  FIG. 3 . 
     [Step B 6 ] With both the first high side isolation switch QHS 1  and the first injector select switch OS 1  closed, the voltage VB at the bias point PB is determined. 
     [Step B 7 ] The voltage VB at the bias point PB is compared to a predetermined threshold level. 
     [Step B 8 ] If the voltage VB at the bias point PB is less than the predetermined threshold level, this is indicative of a stack terminal short circuit on the first injector  12   a . If a stack terminal short circuit is determined on the first injector  12   a , then the first high side isolation switch QHS 1  is opened to disable the faulty first injector  12   a  from the bank  33 . No further injections are performed on the faulty first injector  12   a.    
     [Step B 9 ] If the voltage VB at the bias point PB is greater than the predetermined threshold level, then there is not a stack terminal short circuit fault on the first injector  12   a . In this case, normal running is continued on the first injector  12   a . The second injector  12   b  is tested by opening the first high side isolation switch QHS 1 , closing the second high side isolation switch QHS 2 , and performing steps B 1  to B 9  on the second injector  12   b.    
     With the high side isolation switches QHS 1 , QHS 2  being predominantly open as in the second diagnostic routine ( FIGS. 3 and 6 ), the injectors  12   a ,  12   b  are always electrically isolated from one another. This allows a faulty injector  12   a ,  12   b  to be identified immediately and switched off with no risk of charge share occurring between the injectors  12   a ,  12   b . This also allows the voltage on an injector  12   a ,  12   b  to be measured with no risk of charge share with the other injector  12   a ,  12   b , thereby providing added flexibility to the diagnostics. The second diagnostic routine ( FIG. 6 ) requires relatively high speed high side isolation switches QHS 1 , QHS 2 . 
     With the high side isolation switches QHS 1 , QHS 2  being predominantly closed as in the first diagnostic routine ( FIGS. 4 and 5 ), there remains a risk of charge sharing occurring and additional diagnostic steps must be performed to determine the injector  12   a ,  12   b  at fault once the presence of a fault is determined on the bank  33 . These additional diagnostic steps, which are otherwise referred to as ‘post-processing’, require both injectors  12   a ,  12   b  to be shut down until the faulty injector  12   a ,  12   b  is identified. However, this technique allows slower speed high side isolation switches QHS 1 , QHS 2  to be used, which may provide cost benefits. 
     In both the first and second diagnostic routines, described above with reference to  FIGS. 5 and 6  respectively, the use of high side isolation switches QHS 1 , QHS 2  enables a faulty injector  12   a ,  12   b  to be diagnosed and disabled from the injector bank  33 . Disabling the faulty injector  12   a ,  12   b  electrically isolates the faulty injector  12   a ,  12   b  from the other injectors  12   a ,  12   b  on the bank  33 . Once disabled, any short circuit faults associated with the faulty injector  12   a ,  12   b  will then not affect the normal operation of the remaining non-faulty injectors  12   a ,  12   b  on the bank  33 . 
     In addition to the advantages described above, the inclusion of isolation switches QHS 1 , QHS 2  on the high sides of the injectors  12   a ,  12   b  enables high side to ground faults on the injectors  12   a ,  12   b  to be electrically isolated. This has not been possible until now, because switches have traditionally been located on the low side of the injectors  12   a ,  12   b , which means that even when these switches are opened, a high side to ground short circuit is not electrically isolated and may disrupt the normal operation of non-faulty injectors  12   a ,  12   b  on the bank  33 . 
     The terms ‘open’ and ‘close’ used to described the operation of the various switches are used herein for convenience. These terms are not intended to limit the invention, and as such, the term ‘close’ is intended to mean operating a switch to allow current to pass, whereas the term ‘open’ is intended to mean operating a switch to substantially prevent current from passing. 
     It will be appreciated that the methods described above are automated under the control of the microprocessor  26  of the ECU  24  ( FIG. 1 ). It will also be appreciated that whilst two injectors  12   a ,  12   b  are shown in the injector bank circuits  33  in  FIGS. 3 and 4 , in other embodiments of the invention, the injector bank  33  may include more than two injectors connected in parallel. Furthermore, whilst only one injector bank  33  is described herein, the ECU  24  may be arranged to control more than one injector bank  33 , in which case, each injector bank  33  will have its own drive circuit similar to the drive circuit  30   a  in  FIGS. 3 and 4 .