Abstract:
A drive circuit ( 20   a   ,20   b ) for an injector arrangement having at least one piezeoelectric injector ( 12   a   ,12   b ) is described. The drive circuit comprises: a first charge storage means (C 2 ) for operative connection with the injector ( 12   a   ,12   b ) during a discharging phase so as to discharge current to flow therethrough, thereby to initiate an injection event; a second charge storage means (C 1 ) for operative connection with the injector ( 12   a   ,12   b ) during a charging phase so as to cause a charging current to flow therethrough, thereby to terminate the injection event; a switch means (Q 1 ,Q 2 ) for controlling whether the first charge storage means (C 2 ) is operably connected to the injector or whether the second charge storage means (C 1 ) is operably connected to the injector; a first voltage supply rail at a first voltage level; a second voltage supply rail at a second voltage level higher than the first; a voltage supply means ( 22,36 ); and regeneration switch means (Q 5 ,Q 2 , L 1 ) operable at the end of the charging phase to transfer charge from the voltage supply means to at least the second charge storage means (C 1 ) via an energy storage device (L 1 ) prior to a subsequent discharging phase.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a drive circuit for an injector arrangement. It relates particularly, although not exclusively, to a drive circuit for an injector arrangement for an internal combustion engine, the injector arrangement including at least one injector of the type having a piezoelectric actuator for controlling injector valve needle movement.  
       BACKGROUND ART  
       [0002]     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 that is actuated to open and close to control the amount of fluid fuel metered from the fuel rail and injected into the corresponding engine cylinder or intake manifold.  
         [0003]     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 closing an injection valve to meter fuel injected into the engine. Examples of piezoelectric fuel injectors are disclosed in U.S. Pat. Nos. 4,101,076 and 4,635,849. Piezoelectric fuel injectors are well-known for use in automotive engines.  
         [0004]     The metering of fuel with a piezoelectric fuel injector is generally achieved by controlling the electrical voltage potential applied to the piezoelectric elements to thereby 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 piston and, thus, the amount of fuel that is passed through the fuel injector. Piezoelectric fuel injectors offer the ability to precisely meter a small amount of fuel. However, piezoelectric fuel injectors also generally require relatively high voltages (typically in the hundreds of volts) and high currents (tens of amps) in order to function properly.  
         [0005]     Known conventional drive circuitry for controlling a piezoelectric fuel injector is generally complicated and requires extensive energy. This energy is usually provided by a dedicated power supply such as a transformer which steps-up the voltage generated by the vehicle battery (e.g., 12 volts) to a higher voltage (e.g., 230 volts). The step-up voltage is then applied to large reservoir capacitors for powering the charging and discharging of one or more fuel injectors for each injection event. This dedicated power supply generates enough energy to maintain the reservoir capacitor voltage over the full operating load and speed range of the engine. However, a disadvantage of providing a dedicated power supply of this size is increased cost. Thus, a further disadvantage is that the controller required to control the drive circuit must be of large size.  
         [0006]     German patent application no. DE 102 45 135 A1 (Nippon Soken, Inc. et al) describes a drive circuit for controlling a plurality of piezoelectric fuel injectors. The drive circuit comprises a DC/DC voltage converter  21  for stepping up the voltage produced by a vehicle battery. The stepped-up voltage is applied to capacitors in the circuit which are then used for charging the fuel injector piezoelectric elements. The drive circuit comprises a single voltage supply rail and operates in a purely unidirectional manner (i.e., it does not provide negative voltages), and therefore cannot be used to drive bi-directional fuel injectors which require both negative and positive voltages.  
         [0007]     It has been suggested that vehicle manufacturers are planning to replace 12 volt vehicle batteries with a 42 volt charging system This change has been prompted by the move to replace mechanical and hydraulic systems with electronics (i.e. “drive-by-wire”), and will provide a way to improve fuel economy and reduce emissions. Another problem with current drive circuits is that it is difficult to control dynamically the voltage across the large reservoir capacitors should the higher 42 voltage supply (or a lower voltage supply) be required.  
         [0008]     An object of the invention is therefore to provide a drive circuit which requires less components than existing drive circuits for injector arrangements, and which is therefore cheaper and more controllable than such drive circuits. Another object of the invention is to provide a drive circuit which is suitable for use with voltage supplies having different capabilities.  
       SUMMARY OF THE INVENTION  
       [0009]     According to a first aspect of the present invention there is provided a drive circuit for an injector arrangement having at least one injector, the drive circuit comprising: 
        first charge storage means for operative connection with one of the at least one injectors during a discharging phase so as to permit a discharge current to flow therethrough, thereby to initiate an injection event;     second charge storage means for operative connection with the at least one injector during a charging phase so as to cause a charge current to flow therethrough, thereby to terminate the injection event;     switch means for controlling whether the first charge storage means is operably connected to the at least one injector or whether the second charge storage means is operably connected to the at least one injector,     a first voltage rail at a first voltage level;     a second voltage rail at a second voltage level higher than the first voltage level;     a voltage supply means; and     regeneration switch means operable at the end of the charging phase to transfer charge from the voltage supply means to at least the second charge storage means via an energy storage device, prior to a subsequent discharging phase.        
 
         [0017]     Preferably the first charge storage means is connected across the first voltage rail and ground, and the second charge storage means is connected across the first and second voltage rails.  
         [0018]     In a first embodiment of the present invention, the regeneration switch means is preferably operable at the end of the charging phase to transfer charge from the voltage supply means to the first charge storage means, and then to the second charge storage means from the first charge storage means via the energy storage device. Most preferably, the regeneration switch means is used to transfer charge from the voltage supply means to both the first and second voltage rails such that the voltage across the first and second charge storage means is increased. In this embodiment, the voltage supply means advantageously comprises a vehicle battery and a transformer to step-up the voltage generated by the vehicle battery to a higher voltage suitable for applying to the first charge storage means. An advantage of this embodiment of the present invention is that the voltage supply means is only used to increase the charge on the first charge storage means, and therefore a smaller and cheaper voltage supply means may be used than in known drive circuits. A further advantage of this embodiment of the present invention is that if the voltage supply means provides a 42 Volt charging system, and the injectors are operable at a similar voltage, then a transformer may not be required, thereby leading to a further reduction in the size and cost of the voltage supply means.  
         [0019]     In a second embodiment of the present invention, the regeneration switch means is preferably operable at the end of the charging phase to transfer charge from the voltage supply means to the first charge storage means and also to the second charge storage means prior to the subsequent discharging phase. Most preferably, the regeneration switch means is used to transfer charge from the voltage supply means to the second voltage rail such that the voltage across the second charge storage means is increased (the first voltage rail being supplied by the voltage supply means). Preferably the voltage supply means comprises a vehicle battery, and advantageously no transformer is required to step up the voltage generated by the vehicle battery. The advantage of this embodiment is that there is no need to provide a dedicated power supply (such as a transformer) which leads to a cheaper and more controllable drive circuit than those known in the prior art  
         [0020]     If injectors which are operable at approximately −12 Volts are utilised in the drive circuit, then the ratio of the capacitance of the first charge storage means to the second charge storage means is preferably selectable to achieve the required injector negative operating voltage.  
         [0021]     In both embodiments of the present invention, the drive circuit may further comprise a switch means including a first switch (such as a “charge” switch) operable to close to activate the charging phase, and a second switch (such as a “discharge” switch) operable to close to activate the discharging phase. Thus, in the first embodiment, the regeneration switch means is preferably arranged to transfer charge from the voltage supply means to the second charge storage means in response to the operation of the second switch, as charge is supplied to the first charge storage means by the voltage supply means. However, in the second embodiment, the regeneration switch means is preferably arranged to transfer charge from the voltage supply means to the first and second charge storage means in response to the operation of the second switch.  
         [0022]     The regeneration switch means need not be operable between all injection events, but may be selectively operable between only some injection events.  
         [0023]     Preferably the first and second charge storage means comprise capacitors, and the energy storage device is an inductor.  
         [0024]     Preferably the drive circuit includes first and second injectors which are arranged in parallel and operatively connected to the switch means, the regeneration switch means, and a further switch means for controlling independent selection of the first or second injector to permit a discharging current to be supplied to the selected injector during a discharge phase so as to initiate an injection event  
         [0025]     The drive circuit is preferably configured as a half H-bridge circuit having a middle circuit branch, with the first and second injectors being arranged in parallel in the middle circuit branch.  
         [0026]     The drive circuit may also include voltage sensing means for sensing the voltage across the selected injector (and also the unselected injector, if desired), and control means for receiving a signal indicative of the sensed voltage and providing a termination control signal to the further switch means to terminate the charging phase of the selected injector once a predetermined charge threshold voltage is sensed. The control means may also be arranged to provide an initiate signal to the switch means to initiate the charging phase of the selected injector. The control means may also be arranged to provide an initiate control signal to the switch means to initiate the discharge mode of the selected injector, and to provide a terminate control signal to the switch means to terminate the discharge phase once a predetermined threshold discharge voltage is sensed.  
         [0027]     The drive circuit may also include sensing means for sensing the voltage on the first and second capacitors. The control means may also be arranged to provide an initiate signal to initiate the regeneration phase of the circuit, and to provide a terminate signal to terminate the regeneration phase.  
         [0028]     The control means may further be arranged to provide a pulse width modulated signal to alternately enable and disable the discharge switch (i.e. to pulse the discharge switch on and off) during the regeneration phase.  
         [0029]     By “enabling” the discharge switch, it is meant that the discharge switch is put in a state so that it may be activated (i.e. closed), whether under the direct control of the microprocessor via a pulse width modulated signal, by the regeneration current falling below a predetermined current level, or via any other suitable method. Similarly, by “disabling” the discharge switch, it is meant that the discharge switch is put in a state so that it cannot be activated without first being enabled.  
         [0030]     The drive circuit of the present invention is appropriate for controlling a bank of at least two injectors, with each injector being arranged to inject fuel to an associated combustion space or engine cylinder. The bank may include any number of injectors, and an engine may have more than one injector bank, depending on the number of engine cylinders. The drive circuit is equally applicable, however, to controlling just a single injector. Due to the drive circuit of the present invention having first and second voltage rails, the drive circuit operates in a bi-directional manner and is therefore suitable for driving bi-polar fuel injectors which require both positive and negative voltages for their operation.  
         [0031]     According to a second aspect of the present invention there is provided a control method for an injector arrangement having at least one injector, the method comprising: operably connecting a first charge storage means to one of the at least one injectors during a discharging phase so as to cause a discharge current to flow therethrough, thereby to initiate an injection event; operably connecting a second charge storage means with the at least one injector during a charging phase so as to cause a charge current to flow therethrough, thereby to terminate the injection event; activating a regeneration switch means at the end of the charging phase to initiate a regeneration phase wherein charge is transferred from a voltage supply means to an energy storage device, and transferred from the energy storage device to at least the second charge storage means prior to the subsequent discharging phase; and deactivating the regeneration switch means to terminate the regeneration phase.  
         [0032]     In one embodiment of the present invention, during the activating step charge is transferred from the voltage supply means to the energy storage device, and subsequently transferred from the energy storage device to the first and second charge storage means. In another embodiment of this aspect of the invention, during the activating step charge is transferred from the voltage supply means to the first charge storage means, and subsequently transferred from the first charge storage means to the energy storage device for transfer to the second charge storage means.  
         [0033]     Preferably the steps of transferring charge to and from the energy storage device are carried out periodically, most preferably under the control of a pulse-width modulated (PWM signal.  
         [0034]     The efficiency of the fuel injectors determines how much energy is removed from the first and second charge storage means, and also determines a peak current in the energy storage device over a period of time to regenerate the charge stored on the first and second charge storage means. In other words, the more efficient the injector, the less current in the energy storage device, and the shorter regeneration time required. It is therefore a preferable feature of the present invention that the regeneration time is controllable.  
         [0035]     Preferably the method includes the further step of varying the characteristics (such as duty-cycle and modulating frequency) of the PWM signal. The duty-cycle required may depend upon the voltage of the voltage supply means. For example, if the voltage of the voltage supply means is low, then longer PWM ON times are required and, conversely, if the voltage of the voltage supply means is high, then shorter PWM ON times are necessary. The duty-cycle and/or modulating frequency of the PWM signal are optionally varied by the microprocessor, for example to directly actuate the regeneration switch means. Preferably, the characteristics of the PWM signal may be controlled by allowing the drive circuit to detect the current in the energy storage device by starting a normal discharge event, but selecting the regeneration switch means rather than an injector.  
         [0036]     The method may include the further step of controlling whether the first or second charge storage means is operably connected to the injector.  
         [0037]     The method may also include the steps of providing a regeneration initiate signal to activate the regeneration switch means and hence begin the regeneration phase, and providing a regeneration terminate signal to deactivate the regeneration switch means and thus terminate the regeneration phase.  
         [0038]     Preferably the regeneration initiate signal may be provided after each injection event. Alternatively, the regeneration initiate signal may be provided after a predetermined number of injection events. That is, the regeneration phase may be carried out between injection events. Advantageously, the regeneration phase is carried out for a period necessary to maintain a constant charge on the first and second charge storage means.  
         [0039]     It will be appreciated that although the present invention is particularly applicable to an injector system in which the injectors have piezoelectric actuators, it is equally applicable to any system in which the injectors have capacitive-like properties, for example motor-driven injectors. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0040]     Preferred embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0041]      FIG. 1  is a block diagram illustrating a drive circuit according to preferred embodiments of the present invention for controlling a piezoelectric fuel injector in an engine;  
         [0042]      FIG. 2   a  is a circuit diagram illustrating the piezoelectric drive circuit of  FIG. 1 , according to a first embodiment of the present invention;  
         [0043]      FIG. 2   b  shows the circuit diagram of  FIG. 2   a  indicating the current flow path during a regeneration phase of operation of the circuit;  
         [0044]      FIG. 3   a  is a graph to illustrate the energy levels in a first capacitor in the drive circuit of  FIG. 2  during operation of the drive circuit;  
         [0045]      FIG. 3   b  is a graph illustrating the energy levels in a second capacitor in the drive circuit of  FIG. 2  during operation of the drive circuit;  
         [0046]      FIG. 3   c  is a graph showing the current in an inductor, the pulsing of a discharge switch, and the activation of a regeneration switch in the drive circuit of  FIG. 2  during operation of the drive circuit;  
         [0047]      FIG. 3   d  is a graph to illustrate a drive pulse applied to a fuel injector to initiate and terminate an injection event;  
         [0048]      FIG. 3   e  illustrates the enabling/disabling and activation/deactivation of a switch during operation of the drive circuit of  FIG. 2 ;  
         [0049]      FIG. 4  is a circuit diagram illustrating the piezoelectric drive circuit of  FIG. 1 , and the current flow path through the circuit during the regeneration phase of operation of the circuit, according to a second embodiment of the present invention;  
         [0050]      FIG. 5   a  is a graph illustrating the energy levels in the first capacitor in the drive circuit of  FIG. 4  during operation of the drive circuit;  
         [0051]      FIG. 5   b  is a graph illustrating the energy levels in the second capacitor in the drive circuit of  FIG. 4  during operation of the drive circuit; and  
         [0052]      FIG. 5   c  is a graph illustrating the current in the inductor, and the drive pulse applied to a fuel injector to initiate and terminate an injection event. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0053]     Referring to  FIG. 1 , an engine  10 , such as an automotive vehicle engine, is generally shown having a first and second piezoelectric fuel injectors  12   a  and  12   b  for metering and injecting fuel into individual cylinders or an intake manifold of the engine  10 . The piezoelectric fuel injectors  12   a  and  12   b  control the amount of fluid (e.g., liquid) fuel injected from a fuel rail of a fuel delivery system into an engine during each fuel injection stroke of the engine  10 . The piezoelectric fuel injectors  12   a  and  12   b  may be employed in a diesel engine to inject diesel fuel into the engine, or may be employed in a spark ignited internal combustion engine to inject combustible gasoline into the engine. While two piezoelectric fuel injectors  12   a  and  12   b  are shown and described in the embodiment of  FIG. 1 , it should be appreciated that the engine  10  may include more piezoelectric fuel injectors, all of which could be controlled by a common drive circuit.  
         [0054]     The engine  10  is generally controlled by an engine control module (ECM)  14 . The ECM  14  generally includes a microprocessor and memory  16  for performing various control routines for controlling the operation of the engine  10 , including control of the fuel injection. The ECM  14  may monitor engine speed and load and control the amount of fuel and injection timing for injecting fuel into the engine cylinder. Also included in the microprocessor and memory  16  is a pulse-width modulated signal generator  24  for generating pulse-width modulated (PWM signals  26 , the purpose of which will be described in detail later.  
         [0055]     According to the present invention, a piezoelectric half H-bridge drive circuit  20   a , 20   b  is shown integrated into the engine control module  14 . The drive circuit  20   a , 20   b  is arranged to monitor and control the injector high side voltages INJ 1 HI, INJ 2 HI and injector low side voltages INJ 1 LO, INJ 2 LO to control actuation of the piezoelectric fuel injectors  12   a  and  12   b  to open and close the injectors. The piezoelectric drive circuit  20   a , 20   b  may be integrated in the engine control module  14  as shown, or may be provided separate therefrom. The microprocessor and memory  16  provide various control signals  18 , 26  to the drive circuit  20   a , 20   b .  
         [0056]     The piezoelectric drive circuit  20   a , 20   b  as shown and described herein operates in a discharge phase which discharges an injector  12   a , 12   b  to open the injector valve to inject fuel, and further operates in a charge phase which charges an injector  12   a , 12   b  to close the injector valve to prevent injection of fuel. In this case, the injectors are of the negative-charge displacement type. However, the drive circuit  20   a , 20   b  and injectors  12   a  and  12   b  could be otherwise configured to open during a charge phase and close during a discharge phase, wherein the injectors are of the positive-charge displacement type.  
         [0057]     The piezoelectric drive circuit  20   a , according to a first embodiment of the present invention, is illustrated in detail in the block/circuit diagram of  FIG. 2   a  The drive circuit  20   a  includes first and second voltage supply rails V 0  and V 1 , and is generally configured as a half H-bridge having a middle circuit current path  32  which serves as a bi directional current path. The middle circuit branch  32  includes an inductor L 1  coupled in series with a parallel connection of the injectors  12   a  and  12   b  and associated switching circuitry. Each injector  12   a  and  12   b  has the electrical characteristics of a capacitor, with its piezoelectric actuator stack being chargeable to hold a voltage which is the potential difference between the charge (+) and discharge (−) terminals of the injector  12   a  and  12   b . Charging and discharging of each injector  12   a , 12   b  is achieved by controlling the flow of current through the bidirectional current path  32  by means of the microprocessor  16 .  
         [0058]     The drive circuit  20   a  further includes first and second switches Q 1  and Q 2  for controlling the charge and discharge operations of the injector  12   a  and/or  12   b . The switches Q 1  and Q 2  may each include an n-channel insulated gate bi-polar transistor (IGBT) having a gate controlling current flow from the collector to the emitter. Each of switches Q 1  and Q 2  allows for unidirectional current flow from the collector to the emitter when turned on, and prevents current flow when turned off. Each switch Q 1 ,Q 2  has a respective recirculation diode D 1 ,D 2  connected across it to allow a recirculation current to return to energy storage capacitors C 1 ,C 2  during an energy recovery or recirculation phase of operation of the circuit, and also a regeneration phase, as described in detail below. The first energy storage capacitor C 1  is connected across the first and second voltage supply rails V 0  and V 1 , whereas the second energy storage capacitor C 2  is connected across the first voltage supply rail V 0  and ground.  
         [0059]     The drive circuit  20   a  also includes a voltage source  22  such as a vehicle battery. However, unlike known drive circuits for injector arrangements, the drive circuit  20   a  of the first embodiment of the present invention does not include a dedicated power supply for supplying energy to the first C 1  and second C 2  energy storage capacitors, such as that indicated by the dashed lines  85  in  FIG. 2   a.    
         [0060]     Each of the injectors  12   a , 12   b  is connected in series with an associated selector switch Q 3 ,Q 4 . Each selector switch Q 3 ,Q 4  typically takes the form of an IGBT having a gate coupled to a gate drive which is powered at a bias supply input When the selector switch Q 3  associated with the first injector  12   a , for example, is activated (i.e. switched on), current flow (I DISCHARGE ) is permitted in a discharge direction through the selected injector. A diode D 3  is connected in parallel with the selector switch Q 3  to allow current (I CHARGE ) to flow in the charge direction during the charging phase of operation of the circuit. Similarly, a diode D 4  is connected in parallel with the selector switch Q 4  for the second injector  12   b.    
         [0061]     A regeneration switch Q 5  is included in the circuit  20  between the inductor L 1  and the vehicle battery  22 , for connecting (and disconnecting) the battery to the first C 1  and second C 2  capacitors. The regeneration switch Q 5  typically takes the form of an IGBT having a gate coupled to a gate drive which is powered at a bias supply input. A diode D 5  is connected in series with the regeneration switch Q 5  for preventing current from flowing therethrough during the charge phase.  
         [0062]     The middle circuit path  32  further includes a current sensing and control means 34 arranged to sense the current in the path  32 , to compare the sensed current with predetermined current thresholds I P  and I R , and to generate output signals, where I P  is the peak current threshold, and I R  is the recirculation current threshold. Predetermined values for I P  and I R  are stored in the microprocessor and memory  16 , along with a charge voltage threshold (V CHARGE ), and a discharge voltage threshold (V DISCHARGE ). Predetermined voltage levels V gc1  and V gc2  across capacitors C 1  and C 2 , for determining when the regeneration phase is to be terminated, may also be stored in the microprocessor and memory  16 . If required, the current thresholds I P  and I R , the voltage thresholds, V CHARGE  and V DISCHARGE , and the voltage levels, V gc1  and V gc2 , may be adjustable.  
         [0063]     A voltage sensing means (not shown) is also provided to sense the voltage, V SENCE , across the injector  12   a , 12   b  that is selected for injection. The voltage sensing means may also be used to sense the voltages V C1  and V C2  across the first C 1  and second C 2  capacitors, and the vehicle battery  22  voltage. The microprocessor and memory  16  further provides a charge/discharge signal C/D (which may be used to enable and disable a switch), an injector selector for selecting one of the injectors during the discharge operation, and a control signal for activating the regeneration switch Q 5 .  
         [0064]     The drive circuit  20   a  also includes control logic  30  for receiving the output of the current sensing and control means  34 , the sensed voltage, V SENCE , from the positive terminal (+) of the injectors  12   a  andl 2   b , and the various output signals from the microprocessor and memory  16 . The control logic  30  may include software executed by the microprocessor and memory  16  for processing the various inputs so as to generate control signals for each of the charge/discharge switches Q 1 , Q 2 , the selector switches Q 3 , Q 4 , and the regeneration switch Q 5 .  
         [0065]     During operation of the drive circuit  20   a , a drive pulse (or voltage waveform) is applied to the piezoelectric actuator of the fuel injectors  12   a  and  12   b . The drive pulse varies between the charging voltage, V CHARGE , and the discharging voltage, V DISCHARGE . When the injector  12   a  is in a non-injecting state, prior to injection, the drive pulse is at V CHARGE  so that a relatively high voltage is applied to the piezoelectric actuator. Typically, V CHARGE  is around 200 to 300 V. When it is required to initiate an injection event, the drive pulse is reduced to V DISCHARGE , which is typically around −100 V. To terminate injection, the voltage of the drive pulse is increased to its charging voltage level, V CHARGE  once again.  
         [0066]     The drive circuit  20   a  generally operates in three phases: (1) a discharge phase to open a selected one of the fuel injectors  12   a , 12   b ; (2) a charge phase to close the fuel injectors  12   a  andl 2   b ; and (3) a regeneration phase for re-energising the energy storage devices C 1  and C 2  in the circuit  20   a  such that a dedicated power supply is not required. Each of these phases will now be described in detail.  
         [0067]     During the discharge phase, the discharge switch Q 2  is activated (i.e. closed) and one of the selector switches Q 3  and Q 4  is activated to select one of injectors  12   a  and  12   b  for injection. So, for example, if it is required to inject with the first injector  12   a , the selector switch Q 3  is closed. The other selector switch Q 4  for the second injector  12   b  remains deactivated as the second injector  12   b  is not required to inject.  
         [0068]     Assuming that it is desired to inject using the first injector  12   a , upon activation of the discharge switch Q 2 , current is allowed to flow directly from the voltage supply  22  across the capacitor C 2 , through the current sensing and control means  34 , through the selector switch Q 3 , and into the corresponding negative side of the selected injector  12   a . A discharge current I DISCHARGE  flows from the injector load for injector  12   a , through the inductor L 1 , through the closed discharge switch Q 2 , and back to the negative terminal of the capacitor C 2 . As the selector switch Q 4  remains open, and due to the presence of the diode D 4 , substantially no current is able to flow through the second injector  12   b  into the negative side of the injector  12   b.    
         [0069]     The current sensing and control means  34  monitors the current flow through the bi-directional current path  32  as it builds up and, as soon as the peak current threshold I P  is reached, an output signal is generated to initiate de-activation (i.e. opening) of the discharge switch Q 2 . At this point, the current that is built-up in the inductor L 1  recirculates through the diode D 1  associated with the charge switch Q 1 . As a consequence, the direction of current flow through the inductor L 1  and the selected one of the injectors  12   a  and  12   b  does not change. This is known as the “recirculation phase” of the discharging phase of operation of the drive circuit  20   a.    
         [0070]     During the recirculation phase, current flows directly from the negative side of the capacitor C 1 , through the current sensing and control means  34 , through the selected switch Q 3 , through the selected injector  12   a , through the inductor L 1 , and finally through the diode D 1  and into the positive side of capacitor C 1 . During this recirculation phase, energy from the inductor L 1  and the selected one of the piezoelectric injectors  12   a  or  12   b  is transferred to the capacitor C 1  for energy storage therein.  
         [0071]     The current sensing and control means  34  monitors the recirculation current, and when the recirculation current has fallen below the recirculation current threshold IR, a signal is generated to reactivate the discharge switch Q 2 , thereby continuing the discharge operation. The voltage V inj1  or V inj2  across the selected injector  12   a  or  12   b  is also monitored by the voltage sensing means (not shown), and the cycle of current buildup and recirculation continues until the appropriate discharge voltage level (threshold V DISCHARGE ) has been achieved.  
         [0072]     In this discharge cycle, the capacitor C 2  provides energy, while the capacitor C 1  receives energy for storage. Once the appropriate discharge voltage threshold V DISCHARGE  is achieved, the half H-bridge drive circuit  20   a  is deactivated until a charge cycle is initiated.  
         [0073]     In order to charge (i.e. close) the first injector  12   a , the charge switch Q 1  is activated, thus allowing a charge current I CHARGE  to flow through the current path  32  and to the first injector  12   a . This is known as the charging phase of operation of the drive circuit  20   a  During the charging phase, the majority of the charge current I CHARGE  will flow through the previously discharged injector (i.e. the first injector  12   a ). The second injector  12   b  that was not previously discharged will receive current if the corresponding voltage V inj2  across it has dropped below the charge voltage threshold V CHARGE .  
         [0074]     The current sensing and control means  34  monitors the current buildup, and as soon as the peak current threshold I P  is reached, the control logic  30  generates a control signal to open the charge switch Q 1 . At this point, the current that is built up in inductor L 1  recirculates through the diode D 2  associated with the (open) discharge switch Q 2 . This is the recirculation phase of the charging phase of operation of the drive circuit  26 . Thus, the direction of current flow through the inductor L 1  and injectors  12   a  and  12   b  does not change.  
         [0075]     During the recirculation phase, current flows from the negative side of the second capacitor C 2 , through the diode D 2  associated with the discharge switch Q 2 , through the inductor L 1  and the injectors  12   a  and  12   b , through the diodes D 3  and D 4 , and the current sensing and control means  34 , and into the positive side of energy storage capacitor C 2 . During this recirculation phase, energy from the inductor L 1  and piezoelectric injectors  12   a  and  12   b  is transferred to the energy storage capacitor C 2 . The current sensing and control means  34  monitors the recirculation current, and when the recirculation current has fallen below the recirculation current threshold I R , a signal is generated to reactivate the charge switch Q 1  to continue the charge process. The voltage across the selected injector  12   a  is monitored, and the cycle of current buildup and recirculation continues until the appropriate charge voltage level (threshold V CHARGE ) has been achieved. In this charging phase, the energy storage capacitor C 1  provides energy, and the energy storage capacitor C 2  receives energy for storage. Once the appropriate charge voltage threshold V CHARGE  is achieved, the half H-bridge drive circuit  20   a  is deactivated until a discharge cycle is initiated.  
         [0076]     Following the charging phase, at the end of the injection event, the regeneration phase follows. During the regeneration phase, the regeneration switch Q 5  (which has remained deactivated during the charge and discharge phases) is activated, and the discharge switch Q 2  is opened and closed, under the control of the pulse-width modulated signal  26 , until the voltages across the first C 1  and second C 2  capacitors reach predetermined levels (i.e. V gc1  and V gc2  in  FIGS. 3   a  and  3   b , respectively).  
         [0077]     Referring to  FIG. 2   b , with the regeneration switch Q 5  activated, while the discharge switch Q 2  is switched on, current is drawn from the vehicle battery  22  and passes through the inductor L 1  and the discharge switch Q 2 , as illustrated by the dashed arrows  87 . When the discharge switch Q 2  is switched off, current flows from the vehicle battery  22 , through the inductor L 1 , through diode D 1  associated with charge switch Q 1 , and passes through capacitors C 1  and C 2  (from positive to negative) such that the voltage V C1  and V C2  across the capacitors C 1  and C 2  increases and the energy stored thereon increases. Thus, during the regeneration phase, the inductor L 1  elevates the battery voltage to increase the voltage on the first and second voltage supply rails V 0  and V 1  such that the voltage across the capacitors C 1  and C 2  also increases (i.e., the inductor L 1  acts as a power supply means). The path of the current during the regeneration phase is illustrated by the solid arrows  89  in  FIG. 2   b.    
         [0078]     Referring now to  FIGS. 3   a  and  3   b , the energy Ec 1  and Ec 2  stored on the capacitors C 1  and C 2  are shown during discharge, charge and regeneration phases.  
         [0079]     The energy E C1  stored on the capacitor C 1  (given by line  40 A in  FIG. 3   a ) is shown increasing via waveform  42 A having spikes  46 A during the discharge phase, and decreasing via waveform  44 A having spikes  48 A during the charge phase. Waveform  50 A shows the energy stored on the capacitor C 1  increasing during the regeneration phase while the discharge switch Q 2  is pulsed on and off. Spikes  52 A are also shown illustrating that energy is transferred to the first capacitor C 1  every time the discharge switch Q 2  is switched between activated (closed) and de-activated (open) states.  
         [0080]     The energy Ec 2  stored on the capacitor C 2  (given by line  40 B in  FIG. 3   b ) is shown decreasing via waveform  42 B having spikes  46 B during the discharge phase, and increasing via waveform  44 B having spikes  48 B during the charge phase. Waveform  50 B shows the energy stored on the capacitor C 2  increasing during the regeneration phase while the discharge switch Q 2  is pulsed on and off. Spikes  52 B are also shown illustrating that energy is transferred to the second capacitor C 2  every time the discharge switch Q 2  is switched between activated (closed) and deactivated (open) states.  
         [0081]      FIG. 3   c  shows the current I L1  through the inductor L 1 , the switching on and off of the discharge switch Q 2 , and the switching on and off of the regeneration switch Q 5  during charge, discharge and regeneration phases.  
         [0082]     The inductor current I L1  (given by line  50 ) is shown ramping down to approximately minus twenty amps (−20 A) during current buildup and decaying back to about minus five amps (−5 A) during the recirculation phase of the discharge phase as shown by spikes  56  of waveform  52 . During the charge phase, the inductor current I L1  increases from about zero amps to approximately twenty amps (+20 A) during current buildup, and ramps back down to approximately five amps (+5 A) during the recirculation phase, as shown by spikes  58  of waveform  54 . The spikes  56  and  58  of current I L1  occur for as long as the voltage V C2  or V C1  is applied to discharge or charge the injector voltage V inj1 , as shown in  FIG. 3   d . Waveform  70  illustrates the inductor current I L1  periodically decreasing from about zero amps to approximately minus 15 amps (−15 A) during the pulsing of the discharge switch Q 2  during the regeneration phase (i.e. when regeneration switch Q 5  is activated, as shown by the dashed line  78 ). The waveform  72  represents the control signal applied to the discharge switch Q 2  to activate and deactivate the switch. So, for example, the waveform  74  illustrates the pulsing of the discharge switch Q 2  during the recirculation phase of the discharge phase, while the waveform  76  represents the pulse-width modulated pulsing of the discharge switch Q 2  during the regeneration phase of the circuit operation.  
         [0083]      FIG. 3   d  shows the charge/discharge voltage V inj1  across the injector  12   a  during charge, discharge and regeneration phases. The injector voltage V inj1 , shown by line  60  in  FIG. 3   d , shows the voltage V inj1  of the first injector  12   a  decreasing in waveform  62  during the discharge phase and increasing in waveform  64  during the charge phase. Line  66  shows the voltage V inj1  of the first injector  12   a  remaining substantially constant during the regeneration phase of the circuit operation.  
         [0084]     In summary, when it is required to inject with a selected injector (e.g. the first injector  12   a ), the discharge switch Q 2  and the selector switch Q 3  of the first injector are both closed. During the discharge and recirculation phases that follow, the discharge switch Q 2  is automatically opened and closed until the voltage across the selected injector  12   a  is reduced to the appropriate voltage discharge level (i.e. V DISCHARGE , as shown in  FIG. 3   d ) to initiate injection. After a predetermined time for which injection is required, closing of the injector  12   a  is achieved by closing the charge switch Q 1 , causing a charging current to flow through the first and second injectors  12   a  and  12   b . During the subsequent charging and recirculation phases, the charge switch Q 1  is continually opened and closed until the appropriate charge voltage level is achieved (i.e. V CHARGE , as shown in  FIG. 3   d ). During the regeneration phase, the regeneration switch Q 5  is activated, and the discharge switch Q 2  is periodically opened and closed under the control of the pulse-width modulated signal  26  until the voltage across the first C 1  and second C 2  capacitors reaches a predetermined level (i.e. V gc1  and V gc2  in  FIGS. 3   a  and  3   b , respectively).  
         [0085]     Although the operation of the circuit  20   a  in the charge, discharge and regeneration phases has been explained with reference to the activation of the charge and discharge switches Q 1  and Q 2 , in practice charge, discharge and regeneration of the injectors  12   a  and  12   b  can be controlled in a number of ways. Firstly, operation of the circuit  20   a  in these phases can be carried out by enabling the charge switch Q 1  or discharge switch Q 2 , and using the peak current and recirculation current thresholds I P  and I R  to control the activation and deactivation of the charge switch or discharge switch (mode  1 ). Or, both activation and deactivation of the charge Q 1  or discharge Q 2  switches can be carried out under the direct control of the microprocessor  16  by pulsing the charge/discharge signal C/D (mode  2 ). Alternatively, the enabling of the charge switch or discharge switch can be carried out under the direct control of the microprocessor  16 , and the deactivation of the charge switch or discharge switch can occur when the current flowing in the bidirectional path  32  falls below a reduced recirculation current threshold I R  (mode  3 ).  
         [0086]     The aforedescribed modes are illustrated in  FIG. 3   e , where plot (a) firstly illustrates the current I INJ1 , flowing in the first injector  12   a  during a discharge phase (although the plot is equally applicable to the charge phase of operation). It can be seen that the current in the bidirectional path  32  is oscillating between the peak current threshold I P  and the recirculation current threshold I R . Plot (b) illustrates the C/D signal changing from low (disable) to high (enable) to enable the discharge switch Q 2  during the discharge phase. Plot (c) shows the discharge switch Q 2  switching on as the current reaches I P , and switching off when the current falls to below I R . Mode  2  is illustrated in plots (d) and (e) where the C/D signal (shown in plot (d)) is pulsed to enable and disable the discharge switch Q 2  (shown in plot (e)).  
         [0087]     A drive circuit  20   b  according to a second embodiment of the present invention is shown in  FIG. 4 . The drive circuit  20   b  is generally configured as the drive circuit  20   a  of the first embodiment of the invention, with like components having identical reference numerals. As for the first drive circuit  20   a , the second drive circuit  20   b  has first and second voltage supply rails V supply  and V 1 , and is generally configured as a half H-bridge having a middle circuit path  32  which serves as a bidirectional current path. The drive circuit  20   b  also includes an inductor L 1  coupled in series with a parallel connection of injectors  12   a  and  12   b . The second drive circuit  20   b  also includes a first (charging) switch Q 1  and a second (discharging) switch Q 2  at opposite corners of the half H-bridge arrangement, with each switch having a respective recirculation diode D 1  and D 2  connected across it to allow a recirculation current to return to the first C 1  and second C 2  energy storage capacitors during the recirculation phase, and a regeneration current I regen  to flow to the energy storage capacitors during the regeneration phase.  
         [0088]     The second drive circuit  20   b  also includes a voltage source  22 , such as a vehicle battery, which may be connected to an optional power supply unit (PSU)  36 . The power supply unit  36  (if required) is connected between ground and the voltage rail, V supply , (which is a low voltage rail) and is arranged to supply energy to the second energy storage capacitor C 2 . The first energy storage capacitor C 1  is connected across the first and second voltage supply rails V supply  and V 1 , whereas the second energy storage capacitor C 2  is connected across the first voltage supply rail V supply  and ground.  
         [0089]     Each of the injectors  12   a  and  12   b  is connected in series with an associated selector switch Q 3  and Q 4 , and each selector switch has an associated diode D 3  and D 4 . The function of the selector switches and associate diodes is as described for the first drive circuit  20   a.    
         [0090]     A regeneration switch Q 5  is included in the circuit  20   b  in parallel with the first  12   a  and second  12   b  injectors, for connecting the second energy storage capacitor C 2  to the inductor L 1 . The regeneration switch Q 5  typically takes the form of an IGBT having a gate coupled to a gate drive which is powered at a bias supply input. The regeneration switch Q 5  has an associated protection diode D 5  connected in parallel thereto. A further diode D 6  is connected in series with the regeneration switch Q 5  for preventing current flowing therethrough during the charge phase.  
         [0091]     The middle circuit path  34  further includes a current sensing and control means  34  which has the same function as in the first circuit  20   a  and will therefore not be described further. A voltage sensing means (not shown) is also provided, as previously described.  
         [0092]     The operation of the second drive circuit  20   b  is generally as described for the first drive circuit  20   a , but with some differences during the regeneration phase of operation of the circuit due to the presence of the voltage supply  22  (and optionally the PSU  36 ) being connected to the V supply  rail of the circuit.  
         [0093]     As for the first embodiment of the invention, the regeneration phase follows the charging phase, at the end of the injection event. During the regeneration phase, the regeneration switch Q 5  (which has remained in its deactivated state during the charge and discharge phases) is activated, and the discharge switch Q 2  is opened and closed, under the control of the pulse-width modulated signal  26 , until the energy on the first C 1  capacitor reaches a predetermined level (i.e. E C1  in  FIG. 5   a ). As in the first embodiment of the invention, the discharge switch Q 2  may be enabled during the regeneration phase (and the charge/discharge phases) in the manner previously described.  
         [0094]     Referring again to  FIG. 4 , with the regeneration switch Q 5  activated, while the discharge switch Q 2  is on, current is drawn from the vehicle battery  22  (or the PSU  36 ) and passes through the regeneration switch Q 5 , the diode D 6 , the inductor L 1 , the discharge switch Q 2 , and through the second energy storage capacitor C 2  (as illustrated by the dashed arrows) such that the energy on the second capacitor C 2  decreases. When the discharge switch Q 2  is switched off, current flows from the first capacitor C 1 , through the regeneration switch Q 5 , the diode D 6 , the inductor L 1 , and the diode D 1  associated with the charge switch Q 1 , such that the energy on the first capacitor C 1  increases (shown by the bold arrows). Thus, during the regeneration phase in the second embodiment of the invention, the inductor L 1  transfers energy from the second energy storage capacitor C 2  to the first energy storage capacitor C 1 , and the vehicle battery  22  (or the PSU  36 ) maintains the voltage on C 2 . Thus, the regeneration phase is used to transfer battery voltage to the second voltage supply rail V 1  such that the voltage across the first energy storage capacitor C 1  increases.  
         [0095]     Referring now to  FIGS. 5   a  and  5   b , the energy E C1  and E C2  stored on the first C 1  and second C 2  capacitors is shown during the discharge, charge and regeneration phases.  
         [0096]     The energy E C1  stored on the first capacitor (given by line  40 A in  FIG. 5   a ) is shown increasing via waveform  42 A having spikes  46 A during the discharge phase, and decreasing via waveform  44 A having spikes  48 A during the charge phase. Waveform  50 A shows the energy stored on the first capacitor C 1  increasing during the regeneration phase while the discharge switch Q 2  pulses on and off. Spikes  52 A are also shown, illustrating that energy is transferred to the first capacitor C 1  every time the discharge switch Q 2  switches between its activated (closed) and de-activated (open) states.  
         [0097]     The energy E C2  stored on the second capacitor C 2  (given by line  40 B in  FIG. 5   b ) is shown decreasing via waveform  42 B having spikes  46 B during the discharge phase, and increasing via waveform  44 B having spikes  48 B during the charge phase. Waveform  50 B shows the energy stored on the second capacitor decreasing during the regeneration phase while the discharge switch Q 2  is pulsed on and off. The spikes  52 B show that energy is transferred from the second capacitor C 2  (and onto the first capacitor C 1 ) every time the discharge switch Q 2  is switched between activated (closed) and deactivated (open) states.  
         [0098]      FIG. 5   c  shows the current I L1 , through the inductor L 1 , the charge/discharge voltage V inj1  across the injector  12   a  during charge, discharge and regeneration phases. The inductor current I L1 , (given by line  50 ) is shown ramping down to approximately minus twenty five amps (−25 A) during current buildup, and decaying back to about minus five amps (−5 A) during the recirculation phase of the discharge phase, as shown by spikes of waveform  52 . During the charge phase, the inductor current I L1  increases from about zero amps to approximately twenty five amps (+25 A) during current buildup, and ramps back down to approximately five amps (+5 A) during the recirculation phase, as shown by the spikes of waveform  54 . The spikes of current I L1 , occur for as long as the voltage is applied to discharge or charge the injector voltage V inj1 . Waveform  70  illustrates the inductor current I L1 , periodically decreasing from about zero amps to approximately minus fifteen amps (−15 A) during the pulsing of the discharge switch Q 2  during the regeneration phase.  
         [0099]     The injector voltage V inj1  (given by line  60 ) shows the voltage of the first injector  12   a  decreasing in waveform  62  during the discharge phase, and increasing in waveform  64  during the charge phase. Line  66  shows the voltage V inj1  remaining substantially constant during the regeneration phase.  
         [0100]     Having described preferred embodiments of the present invention, it is to be appreciated that the embodiments in question are exemplary only and that variations and modifications such as will occur to those possessed of the appropriate knowledge and skills may be made without departure from the scope of the invention as set forth in the appended claims.  
         [0101]     For example, the piezoelectric injectors  12   a , and  12   b  described herein operate in a discharge mode which discharges an injector to open the injector valve to inject fuel, and further operate in a charge mode which charges an injector to close the injector valve to prevent injection of fuel. In this case, the injectors are of the negative-charge displacement type. However, the drive circuits  20   a  and  20   b  described herein could be otherwise configured to open during a charge mode and close during a discharge mode for an injector of the positive-charge displacement type.  
         [0102]     While two piezoelectric fuel injectors  12   a  and  12   b  are shown and described in connection with the drive circuits  20   a  and  20   b  of the present invention, it should be appreciated that the engine  10  may include one or more fuel injectors, all of which could be controlled by the drive circuits  20   a  and  20   b.    
         [0103]     The drive circuits  20   a  and  20   b  described herein maybe integrated in the engine control module  14 , or may be provided separate therefrom.