Abstract:
A method and apparatus for controlling fuel injection in an engine is described. The apparatus comprises at least one fuel injector which is connected in an injector drive circuit powered by a power source. The method comprises determining an injection event sequence of the at least one fuel injector based on at least one engine operating parameter, determining a magnitude of a load parameter of the power source, comparing the magnitude to a predetermined threshold level for the load parameter, and determining a modified injection event sequence in the event that the magnitude is substantially equal to or greater than the predetermined threshold level.

Description:
TECHNICAL FIELD 
     The invention relates to a method of controlling fuel injection in an engine. More specifically, the invention relates to a method of controlling piezoelectrically actuated fuel injectors in order to improve the performance of a vehicle engine, particular at high engine speeds. The invention also relates to an apparatus for implementing the method of the invention. 
     BACKGROUND OF 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. Such an injector allows precise control of the timing and total delivery volume of a fuel injection event. This permits improved control over the combustion process which is beneficial in terms of exhaust emissions. 
     A known piezoelectric injector  2  and its associated control system  4  are shown schematically in  FIG. 1 . The piezoelectric injector  2  is connected to an injector drive circuit  6  by way of first and second power leads  8 ,  10 . The piezoelectric injector  2  includes a piezoelectric actuator  12  that is operable to control the position of an injector valve needle  14  relative to a valve needle seat  16 . The piezoelectric actuator  12  includes a stack  18  of piezoelectric elements that expands and contracts in dependence on a differential voltage supplied by the injector drive circuit  6 . 
     The axial position, or ‘lift’, of the valve needle  14  is controlled by varying the differential voltage across the actuator  12 . By application of an appropriate voltage differential across the actuator  12 , the valve needle  14  is either caused to disengage the valve seat  16 , in which case fuel is delivered into an associated combustion chamber (not shown) through a set of nozzle outlets  20 , or is caused to engage the valve seat  16 , in which case fuel delivery through the outlets  20  is prevented. 
     Piezoelectric injectors  2  are typically grouped together in banks. As described in EP1400676, each bank of piezoelectric injectors  2  has its own drive circuit  6  for controlling operation of the piezoelectric injectors  2 . The circuitry typically includes a power source, which steps-up the voltage generated by a nominal-voltage power source (e.g., an automobile battery) from its nominal voltage level (e.g., 12 Volts) to a higher voltage, and storage capacitors for storing charge and, thus, energy. The higher voltage is applied across the storage capacitors, which are used to power the charging and discharging of the piezoelectric injectors  2  for each injection event. 
     As shown in  FIG. 1 , piezoelectric injectors  2  are 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  monitors a plurality of engine operating parameters  30 , and calculates an engine power requirement signal (not shown), which is input to the ICU  22 . Examples of the engine operating parameters  30  include engine speed, driver torque demand, manifold inlet pressure and manifold inlet temperature. In turn, the ICU  22  calculates a required injection event sequence to provide the required power for the engine and operates the injector drive circuit  6  accordingly. 
     Each piezoelectric injector  2  is operable to deliver one or more injections of fuel within an injection event sequence. For example, an injection event sequence may include one or more so-called ‘pre-’ or ‘pilot’ injections, one or more main injections, and one or more ‘post’ injections. The use of several such injections within an injection event sequence can increase the combustion efficiency of the engine in order to meet emissions, fuel consumption and NVH (Noise Vibration Harshness) targets. 
     A problem can occur when an engine is run at high speeds and/or loads, wherein the ICU may calculate certain injection event sequences that can overload the power source as it provides power to the injector drive circuit  6 . If this occurs, the injector drive circuit  6  is unable to provide sufficient power to operate the piezoelectric injectors  2  according to the required injection event sequence. This may cause the piezoelectric injectors  2  to deliver less fuel than is required, which in turn may result in an undesirable and unexpected loss of power to the vehicle engine, or the engine misfiring. 
     It is an object of the present invention to provide an improved method of operating fuel injection equipment, which prevents the aforementioned problem from occurring. The invention also aims to provide an improved apparatus for operating fuel injection equipment. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of controlling fuel injection in an engine, the engine comprising at least one fuel injector which is connected to an injector drive circuit powered by a power source, and the method comprising: 
     determining an injection event sequence of the at least one fuel injector in dependence upon at least one engine operating parameter; 
     determining a magnitude of a load parameter of the power source, said magnitude indicative of a power output of said power source; 
     comparing the magnitude to a predetermined threshold level of said load parameter; and 
     determining a modified injection event sequence in the event that the magnitude of the load parameter is substantially equal to or greater than the predetermined threshold level of the load parameter. 
     Preferably, the at least one fuel injector is a piezoelectric fuel injector. An injection event sequence may comprise one or more injection events. 
     The first aspect of the present invention provides a method of controlling fuel injection in an engine such that the risk of overloading of the power source is reduced. Upon determining an injection event sequence for controlling the one or more injectors in the engine, a parameter related to the load drawn from the power source is determined and then compared to a predetermined threshold level. In the event that the calculated load equals or exceeds the threshold level then a modified injection event sequence can be determined that does not overload the power source. 
     The injection event sequence may be modified so as to reduce the load and/or speed at which the engine is presently operating. Modifying the injection event sequence prevents overloading of the power source in the event that the engine power demand is too high, and hence avoids the undesirable and unexpected loss of power to the vehicle engine, or the misfiring, which can otherwise result from overloading the power source. 
     The step of modifying the injection event sequence may include disabling one or more of the low-priority injection events, for example one or more of the pre- or post-injection events. Such a modified injection event sequence is also referred to hereinafter as a ‘reduced-load injection event sequence’. Alternatively, or additionally, the step of modifying the injection event sequence may involve taking any other suitable action that would ultimately result in the engine running under a reduced-power regime, for example reducing the duration and/or frequency of the injection events. 
     The load parameter may represent the output power of (i.e., load drawn from) the power source, in which case the step of determining the magnitude of the load parameter may comprise monitoring a variable related to the output power (load) of the power source, and deriving the output power of the power source from the monitored variable. The method may further comprise using a function map to determine the output power of the power source from the monitored variable. Alternatively, the load parameter may be a variable indicative of, or related to, the output power of the power source. 
     The variable related to the output power of the power source may be the current drawn by the power source, for example from the vehicle battery to which the power source is connected. In order to determine the current drawn by the power source, the method may comprise determining the potential difference across a sense resistor connected between the power source and a vehicle battery. The sense resistor preferably has a low tolerance, such that its resistance is known to a suitably high degree of accuracy. The current drawn by the power source may be determined from the potential difference across the sense resistor. The method may further comprise determining the output power of the power source using a function map relating the current drawn by power source and the output power of power source. 
     Alternatively, the variable related to the output power of the power source may be a duty-cycle or an average voltage of a drive signal of the power source. The power source may comprise a DC-DC converter. The drive signal of the DC-DC converter may be connected to an input of the microprocessor of the engine control unit ECU, and the microprocessor may be configured to monitor the drive signal and determine the output power of the power source. The cost associated with connecting the drive signal of the DC-DC converter to the microprocessor is negligible, which makes this method particularly advantageous. 
     The power source may comprise a multiphase DC-DC converter, in which case the method may comprise monitoring the drive signal in a single phase of the DC-DC converter. The duty-cycle and average voltage of this signal is indicative of the output power of the multiphase DC-DC converter, and suitable scaling in accordance with standard techniques enables the output power of the multiphase DC-DC converter to be determined. 
     As mentioned above, the method may comprise using a function map, or a suitable look-up table, to determine the output power of the power source. The function map or look-up table may relate the output power of the power source to the monitored variable, for example the duty-cycle or average voltage of the drive signal of the power source, or the current drawn by the power source. The method may be performed by a motor vehicle engine control unit (ECU). The function map or look-up table may be stored in the memory of the ECU, which may be accessed by the microprocessor. 
     As described in the background to the invention, the process of calculating an injection event sequence may include monitoring at least one engine operating parameter such as engine speed, driver torque demand, manifold inlet pressure and manifold inlet temperature, calculating an engine power requirement based on the at least one engine operating parameter, and determining an injection event sequence to provide the required power for the engine. Preferably the method outlined above is performed by the ECU. 
     According to a second aspect of the invention there is provided an apparatus for controlling fuel injection from at least one fuel injector of an engine, the apparatus comprising: 
     an injector drive circuit for connection to the at least one fuel injector; 
     power source means for supplying power to the injector drive circuit; and 
     processing means arranged to:
         (i) determine an injection event sequence of the at least one fuel injector in dependence upon at least one engine operating parameter;   (ii) determine a magnitude of a load parameter of the power source means;   (iii) compare the magnitude to a predetermined threshold level for the load parameter; and   (iv) determine a modified injection event sequence in the event that the magnitude of the load parameter is substantially equal to or greater than the predetermined threshold level.       

     The apparatus is preferably part of an engine control unit (ECU) of a vehicle. The power source means may comprise a switched-mode power source, preferably a DC-DC converter. The DC-DC converter may be controlled by a drive signal of a switching or control circuit internal to the DC-DC converter. 
     In a first embodiment of the invention, the apparatus comprises a connection between the DC-DC converter and an input of the processing means, such that the drive signal of the DC-DC converter is provided to the processing means through the connection. The DC-DC converter may be a multiphase DC-DC converter having a plurality of phases. Each phase of the DC-DC converter may comprise a switch, and the connection may extend between a switch terminal of a single switch and the input of the processing means. Monitoring the drive signal of the DC-DC converter utilises only a single input on the processing means, which is advantageous and does not require additional analogue to digital inputs to be included on the microprocessor, which can be expensive. 
     A low pass filter may be located in the connection between the DC-DC converter and the processing means. The low pass filter may be arranged to output a signal indicative of the duty-cycle of the drive signal of the DC-DC controller to the processing means. The processing means may be configured to determine the output power of the power source means from the duty-cycle of the drive signal. A function map or look-up table stored in the memory of the ECU and accessible by the processing means may be used for this purpose as described above in relation to the first aspect of the invention. 
     In a second embodiment of the invention, the apparatus comprises a sense resistor of substantially known resistance connected between the power source means and a vehicle battery. The processing means may be arranged to monitor the potential difference across the sense resistor and determine the current into the power source means from the known resistance of the sense resistor and the potential difference across the sense resistor. The output power of the power source means may be determined using a function map or look-up table. As described above in relation to the first aspect of the invention, the function map or look-up table may relate the current into the power source and the output power of power source. 
     According to a third aspect of the present invention, there is provided a method of controlling at least one fuel injector connected in an injector drive circuit, the method comprising: 
     determining a magnitude of a load parameter of a power source used to supply power to the injector drive circuit; 
     comparing the magnitude to a predetermined threshold value of the load parameter; and 
     determining a reduced-load injection event sequence in the event that the magnitude of the load parameter is substantially equal to or greater than the predetermined threshold level. 
     According to a fourth aspect of the present invention, there is provided an apparatus for controlling at least one fuel injector, the apparatus comprising: 
     an injector drive circuit for connection to the at least one fuel injector; 
     power source means for supplying power to the injector drive circuit; and 
     processing means for:
         (i) determining a magnitude of a load parameter of the power source means;   (ii) comparing the magnitude to a predetermined threshold level; and   (iii) determining a reduced-load injection event sequence of the at least one fuel injector in the event that the magnitude is substantially equal to or greater than the predetermined threshold level.       

     In common with the first and second aspects of the invention, the load parameter in the third and fourth aspects of the invention may be the output power of the power source, or a parameter indicative of, or related to the output power of the power source. For example, the power parameter may be the duty-cycle of a drive signal of the power source, the average voltage of said drive signal or a signal indicative of the current drawn by the power source. For example, in an alternative embodiment of the invention the duty-cycle of the drive signal of the power source could be compared to a predetermined threshold level and a reduced-load injection event sequence calculated if this parameter substantially equals or exceeds the predetermined threshold level. This method would eliminate the step of calculating the actual power output of the power source in physical units, thereby reducing the burden on the microprocessor. 
     It will be appreciated that optional features described above in relation to the various method aspects of the invention are equally applicable to the various apparatus aspects of the invention, and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference has already been made to  FIG. 1 , which is a schematic representation of a known piezoelectric injector and its associated control system. 
       In order that it may be more readily understood, the present invention will now be described with reference also to the following figures, in which: 
         FIG. 2  is a circuit diagram of an injector drive circuit, powered by a DC-DC converter, and modified in accordance with a first embodiment of the present invention; 
         FIG. 3  is a schematic diagram showing the steps performed in determining the power output of the DC-DC converter in  FIG. 2 ; and 
         FIG. 4  is a circuit diagram of a voltage sensing circuit used to determine the output power of the DC-DC converter of a drive circuit similar to the drive circuit in  FIG. 2 , in accordance with a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is implemented in an engine control unit (ECU)  24 , such as that shown in  FIG. 1 , including the injector control unit (ICU)  22  and the drive circuit  6 . In a first embodiment of the invention, the drive circuit differs from that shown in  FIG. 1 , as described below with reference to  FIG. 2 . 
       FIG. 2  shows an injector drive circuit  6   a  in accordance with a first embodiment of the present invention. The injector drive circuit  6   a  includes a switching circuit  31  in conjunction with an injector bank circuit  32  comprising first and second injectors,  34  and  36  respectively. Each of the injectors  34 ,  36  of the injector bank circuit  32  is of the type shown in  FIG. 1 , having a respective piezoelectric actuator  12 . The piezoelectric actuators are considered electrically equivalent to capacitors, and are represented as such in  FIG. 2 . 
     The switching circuit  31  includes three input voltage rails: a high voltage rail V HI  (typically 255 V), a mid voltage rail V MID  (typically 55 V), and a ground rail GND. The switching circuit  31  also includes a high side voltage output V 1  and a low side voltage output V 2  and is operable to connect the high side voltage output V 1  to either the high voltage rail V HI  or the ground rail GND, through an inductor L, by means of first and second switch means Q 1 , Q 2 . The first switch means shall be referred to as the charge switch Q 1  and the second switch means shall be referred to as the discharge switch Q 2 . A first diode D Q1  is connected across the charge switch Q 1  and a second diode is connected across the discharge switch Q 2 . 
     The switching circuit  31  is also provided with a diode D 1  that connects the high side voltage output V 1  to the high voltage rail V HI . The diode D 1  is oriented to permit current to flow from the high side voltage output V 1  to the high voltage rail V HI  but to prevent current flow from the high voltage rail V HI  to the high side voltage output V 1 . 
     The injector bank circuit  32  comprises first and second branches  38 ,  40 , each of which is connected in parallel between the high side voltage output V 1  and the low side voltage output V 2  of the switching circuit  31 . Thus, the high side voltage output V 1  of the switching circuit  31  is also a high side voltage input to the injector bank circuit  32  and the low side voltage output V 2  of the switching circuit  31  is a low side voltage input to the bank circuit  32 . The first branch  38  of the injector bank circuit  32  contains the first injector  34  and the second branch  40  contains the second injector  36 . Each branch  38 ,  40  also includes an associated injector select switch QS 1 , QS 2  by which means the respective one of the injectors,  34  or  36 , can be selected for operation, as will be described later. The injector bank circuit  32  also includes a third branch  41  connected in parallel with the first and second branches  38 ,  40 . The third branch  41  comprises a recirculation switch RSQ connected in series with a diode RD 1 . Operation of the recirculation switch RSQ is described in more detail later. 
     The low side voltage output V 2  of the injector bank circuit  32  is connected to the mid voltage rail V MID  via a current sensing and control means  42 . The current sensing and control means  42  comprises a current comparator module  43  connected in parallel with a sense resistor  44 . The current comparator module  43  is operable to monitor the current flowing through the sense resistor  44 . The operation of the current sensing and control means  42  is not described in detail herein, but is described in more detail in applicant&#39;s co-pending application EP 06256140.2. 
     A DC-DC converter  45 , which is described in more detail later, supplies energy to the injector drive circuit  6   a . The DC-DC converter  45  is connected to a vehicle battery (not shown) and boosts the voltage of the vehicle battery (e.g. 12 Volts) to a higher voltage (e.g. 55 Volts). The DC-DC converter  45  regulates the voltage of the mid voltage rail V MID  at 55 Volts, as described in more detail later. A first energy storage capacitor C, is connected between the high and mid voltage rails V HI , V MID , and a second energy storage capacitor C 2  is connected between the mid and ground voltage rails V MID , GND. The capacitors C 1 , C 2  store energy which is used to power the charging and discharging of the piezoelectric injectors  34 ,  36  for each injection event as described in more detail below. 
     The piezoelectric injectors  34 ,  36  in this example are of a ‘discharge-to-inject’ type. This means that in order to initiate an injection event, the injector drive circuit  6   a  must cause the differential voltage between the high and low voltage terminals V 1 , V 2  of a selected injector  34  or  36  to transition from a relatively high voltage (e.g. 255 V) at which no fuel delivery occurs, to a relatively low voltage (e.g. 55 V) which causes the actuator  12  to contract, thus lifting the injector valve needle  14  ( FIG. 1 ) away from the valve needle seat  16  ( FIG. 1 ) to permit fuel delivery through the outlets  20  ( FIG. 1 ). This process is referred to hereinafter as ‘discharging’ the injector, and occurs when the injector drive circuit  6   a  is operated in a ‘discharge phase’. 
     Conversely, in order to terminate an injection event, the injector drive circuit  6   a  causes the differential voltage between the high and low voltage terminals of the injector, V 1  and V 2 , to transition from a relatively low voltage (e.g. 55 V), to a relatively high voltage (e.g. 255 V), which increases the actuator voltage, causing the actuator to expand, thus seating the injector valve needle  14  ( FIG. 1 ) back on the valve needle seat  16  ( FIG. 1 ) to terminate fuel delivery through the outlets  20  ( FIG. 1 ). This process is referred to hereinafter as ‘charging’ the injector, and occurs when the injector drive circuit  6   a  is operated in a ‘charge phase’. There now follows a brief description of the discharge and charge phases of operation of the drive circuit  6   a.    
     To initiate the discharge phase, the discharge switch Q 2  is closed and the charge switch Q 1  remains open. As described in more detail in applicant&#39;s co-pending application EP 06254039.8, under the control of the microprocessor  26  and the current sensing and control means  42 , the discharge switch Q 2  is rapidly pulsed on and off to regulate the flow of current. An injector  34  or  36  ( FIG. 1 ) is selected for injection by closing the associated injector select switch QS 1  or QS 2  respectively. For example, to inject from the first injector  34 , the first injector select switch QS 1  is closed and current flows from the positive terminal of the second capacitor C 2 , through the current sensing and control means  42 , through the terminals of the selected first injector  34  (from the low side − to the high side +), through the inductor L, through the discharge switch Q 2  and back to the negative side of the second capacitor C 2 . No current is able to flow through the unselected second injector  36  because the associated injector select switch QS 2  remains open. 
     To charge the injectors  34 ,  36  during the charge phase, the charge switch Q 1  is closed and the discharge switch Q 2  remains open. Also as described in EP 06254039.8, under the control of the microprocessor  26  and the current sensing and control means  42 , the charge switch Q 1  is rapidly pulsed on and off to regulate the flow of current. The first capacitor C 1 , when fully charged, has a potential difference of about 255 Volts across it, and so closing the charge switch Q 1  causes current to flow around the charge circuit, from the positive terminal of the first capacitor C 1 , through the charge switch Q 1  and the inductor L, through the injectors  34 ,  36  (from the high side terminals + to the low side terminals −), through the current sensing and control means  42 , and back to the negative terminal of the first capacitor C 1 . In the charge phase, the previously discharged injector  34  is charged, which causes the injector valve needle  14  ( FIG. 1 ) of the injector  34  to close to terminate the injection of fuel into the associated cylinder (not shown). 
     The DC-DC converter  45  maintains the voltage across the second capacitor C 2  substantially at 55 Volts so that the second capacitor is ready for use in subsequent discharge phases. In order that the first capacitor C 1  is ready for use in subsequent charge phases, energy is replenished to the first capacitor C 1  during a so-called ‘regeneration phase’ of operation of the drive circuit  6   a . To commence the regeneration phase, the regeneration switch RSQ and the discharge switch Q 2  are closed whilst the charge switch Q 1  remains open. A current flows through the diode RD 1  and the regeneration switch RSQ in the third branch  41  of the injector bank circuit  32 , through the inductor L and discharge switch Q 2  to ground GND. The discharge switch Q 2  is then opened, and because of the inductance of the inductor L, some current continues to flow for a short while after the discharge switch Q 2  is opened. This current flows through the diode DQ 1  connected across the charge switch Q 1  and into the positive terminal of the first capacitor C 1  to partially charge the first capacitor C 1 . The discharge switch Q 2  is repeatedly closed and opened to further charge the first capacitor C 1  until the potential difference across the first capacitor C 1  is increased to about 255 Volts and the potential across the second capacitor is about 55 Volts. The regeneration process is described in more detail in WO 2005/028836A1. 
     Referring again to the DC-DC converter  45 , this is a three-phase DC-DC converter  45  comprising three branches  46   a ,  46   b ,  46   c . Each branch  46   a ,  46   b ,  46   c  includes an inductor  47   a ,  47   b ,  47   c  connected in series with a respective switch  48   a ,  48   b ,  48   c . The switches  48   a ,  48   b ,  48   c  are power transistors, such as metal-oxide semiconductor field-effect transistors (MOSFET) and are controlled by an internal control circuit (not shown) of the DC-DC converter  45 . 
     Each branch  46   a ,  46   b ,  46   c  of the DC-DC converter  45  is connected to the drive circuit  6   a  at a point between the first and second storage capacitors C 1 , C 2 . A diode  50   a ,  50   b ,  50   c , is located between each branch  46   a ,  46   b ,  46   c  of the DC-DC converter  45  and the drive circuit  6   a . The diodes  50   a ,  50   b ,  50   c  are oriented to permit current to flow from the DC-DC converter  45  to the drive circuit  6   a , but to prevent current flow from the drive circuit  6   a  to the DC-DC converter  45 . 
     The DC-DC converter  45  regulates the voltage of the mid voltage rail V MID  at 55 Volts. The DC-DC converter  45  must supply sufficient power to the injector drive circuit  6   a  to regulate the potential of the mid voltage rail V MID  at 55 Volts whilst the injector drive circuit  6   a  operates the fuel injectors  34 ,  36  according to the sequence of injection events calculated by the ICU  22 . In order to supply power to the injector drive circuit  6   a , the switches  48   a ,  48   b ,  48   c  of the DC-DC converter  45  are rapidly switched on and off under the control of a drive signal generated by the internal control circuit (not shown) of the DC-DC converter  45 . The drive signal is a pulse width modulated (PWM) signal. The power output of the DC-DC converter  45  is governed by the duty-cycle and frequency of the PWM signal. The internal control circuit of the DC-DC converter  45  determines the duty-cycle and frequency of the PWM signal in dependence of the power required by the injector drive circuit  6   a  to operate the fuel injectors  34 ,  36  according to the sequence of injection events calculated by the ICU  22 . 
     As shown in  FIG. 2 , the first embodiment of the present invention includes a connection  52  between the DC-DC converter  45  and an analogue input to the microprocessor  26  of the ECU  24 . A low pass filter  53 , comprising a resistor  54  and a capacitor  56 , is provided in the connection  52 . A gate terminal  57  of the transistor  48   a  in the first branch  46   a  of the DC-DC converter  45  is connected to an input of the low pass filter  53 , and the output from the low pass filter  53  is connected to the analogue input of the microprocessor  26 . In this configuration, the PWM drive signal of the DC-DC converter  45 , which is used to control the rapid switching of the DC-DC converter  45  as described above, is provided to the low pass filter  53 . The low pass filter  53  converts the PWM signal into an analogue signal, which is provided to the analogue input of the microprocessor  26  as described in further detail below with reference to  FIG. 3 . 
     Referring to  FIG. 3 , this shows the PWM signal  58  generated by the internal control circuit (not shown) of the DC-DC converter  45 . The PWM signal  58  has an on-time (τ), a period (T), and varies between zero and five Volts. The duty-cycle (D) of the PWM signal  58  is given by equation 1 below: 
     
       
         
           
             
               
                 
                   D 
                   = 
                   
                     τ 
                     T 
                   
                 
               
               
                 1 
               
             
           
         
       
     
     The PWM signal  58  is provided to the low pass filter  53 , which outputs an analogue signal  60  corresponding to the average voltage (V AV ) of the PWM signal  58 . The average voltage signal  60  (V AV ) of the PWM signal  58  is given by equation 2 below: 
     
       
         
           
             
               
                 
                   
                     V 
                     AV 
                   
                   = 
                   
                     
                       
                         
                           τ 
                           T 
                         
                         · 
                         5 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       V 
                     
                     = 
                     
                       
                         D 
                         · 
                         5 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       V 
                     
                   
                 
               
               
                 2 
               
             
           
         
       
     
     The average voltage signal  60  is sampled by the analogue input of the microprocessor  26 , and is converted to a digital voltage signal  62  by an analogue to digital converter  64 . A scaling and error checking module  66  of the microprocessor  26  performs scaling and error checking functions on the digital voltage signal  62 . A power monitor module  68  then determines the output power  72  of the DC-DC converter  45  from the digital voltage signal  62 , the voltage  74  of the vehicle battery and the voltage  76  of the DC-DC converter  45 , which is 55 Volts in this example. As described in more detail below, the power monitor module  68  uses a function map  70  to determine the output power  72  of the DC-DC converter  45 . 
     For a given voltage  76  of the power source (55 Volts in this example) the output power  72  of the DC-DC converter  45  is a function of the battery voltage  74  and the average voltage V AV  of the PWM signal  58 , as represented by equation 3 below:
 
 DC - DC _converter_power= fn ( V   AV ,battery_voltage)  3
 
     The function map  70  may be visualised as a graph in which the z-axis corresponds to the output power  72  of the DC-DC converter  45 , the x-axis corresponds to V AV , and the y-axis is the battery voltage  74 . The function map may be generated from empirical calculations, modelling or simulations. For example, the function map  70  could be created by running an engine at various speeds and loads whilst measuring the output power  72  of the DC-DC converter  45  and monitoring the average voltage V AV  of the PWM signal  58  and the battery voltage  74 . The battery voltage  74  is a parameter that is monitored by the ECU  24  as standard, and so is known by the microprocessor  26 . 
     It should be noted that during engine running, the battery voltage  74  remains at approximately 13.5 Volts, and so the function map  70  then reduces to a two-dimensional relationship between the output power  72  of the DC-DC converter  45  and the average voltage V AV  of the PWM signal  58  as represented by equation 4 below:
 
 DC - DC _converter_power= fn ( V   AV )  4
 
     Hence, using the function map  70 , the output power  72  of the DC-DC converter  45  can be inferred from the average voltage V AV  of the PWM signal  58 . 
     Once the output power  72  of the DC-DC converter has been determined, the microprocessor  26  compares the calculated output power  72  to a predetermined threshold value. The predetermined threshold value may be equal to, but is preferably suitably lower than the maximum output power of the DC-DC converter  45  at the present battery voltage. The maximum output power of the DC-DC converter  45  varies according to the battery voltage. However, the maximum output power of the DC-DC converter  45  is known for a given battery voltage, which allows suitable threshold values to be chosen. 
     If the calculated output power  72  substantially equals or exceeds the predetermined threshold value, then a decision is made to reduce the load and/or speed under which the engine is currently running. In order to reduce the load and/or speed of the engine, the ICU  22  ( FIG. 1 ) calculates a reduced-load sequence of injection events, which may involve disabling some or all of the pre-, and or post-injection events, from the previously calculated required sequence of injection events, since these types of injection event are of lower priority than the main injection events. The injector drive circuit  6   a  requires less power to operate the injectors  34 ,  36  according to the reduced-load sequence of injection events. As a result, the internal control circuit (not shown) of the DC-DC converter  45  modifies the duty-cycle and/or frequency of the PWM signal  58  driving the DC-DC converter  45 , such that the power output of the DC-DC converter  45  is reduced. 
     Other embodiments of the invention are envisaged in which the PWM signal  58  from the DC-DC converter  45  is connected directly to a frequency input of the microprocessor  26  instead of via a low pass filter  53 . In such embodiments, the microprocessor  26  would be configured to detect and filter the duty-cycle of the PWM signal  58 . 
     In a second embodiment of the present invention, the injector drive circuit  6   a  does not include the connection  52  from the gate terminal  57  of the transistor  48   a  to the microprocessor  26 , which is shown in  FIG. 2 . Instead, the apparatus comprises a voltage sensing circuit for sensing the supply voltage to the DC-DC converter  45  as explained below with reference to  FIG. 4 . 
     Referring to  FIG. 4 , this shows an example of a voltage sensing circuit  78  connected between the vehicle battery and the DC-DC converter  45 . The voltage sensing circuit  78  comprises a first branch  80  extending between the positive terminal  82  of the vehicle battery and a first terminal  84  of the DC-DC converter  45 , and a second branch  86  extending between the negative terminal  88  of the vehicle battery and a second terminal  90  of the DC-DC converter  45 . A current sense resistor  92  of known resistance is connected in the first branch  80 , whilst the second branch  86  is connected to ground GND. 
     A third branch  94  of the voltage sense circuit  78  is connected between the first and second branches  80 ,  86  to one side of the current sense resistor  92 , and a fourth branch  96  is connected parallel to the third branch  94 , between the first and second branches  80 ,  86 , on the other side of the current sense resistor  92 . The third branch  94  includes a first pair of resistors  98   a ,  98   b , connected in series, and the fourth branch  96  includes a second pair of resistors  100   a ,  100   b , connected in series. 
     The first pair of resistors  98   a ,  98   b  are used to determine the voltage (Va) at a first bias point  102  between the first pair of resistors  98   a ,  98   b . Similarly, the second pair of resistors  100   a ,  100   b  are used to determine the voltage (Vb) at a second bias point  104  between the second pair of resistors  100   a ,  100   b . The voltages at the bias points  102 ,  104  substantially correspond to the respective voltages on either side of the current sense resistor  92  because the resistors  98   a  and  100   a  are each of high resistance. 
     Signals  106 ,  108  indicative of the voltages at the respective bias points  102 ,  104  are provided to respective analogue inputs of the microprocessor  26  of the ECU  24 . The microprocessor  26  is configured to calculate the current (I) being supplied to the DC-DC converter  45  from the ratio of the difference (ΔV) between the first and second voltage values Va, Vb, and the known resistance (R) of the current sense resistor  92 : i.e. using I=ΔV/R. The current sense resistor  92  has a low tolerance value to ensure that calculations are accurate. The current sense resistor  92  is also able to withstand the high powers associated with the injector drive circuit  6   a.    
     Once the current being supplied to the DC-DC converter  45  has been determined, the microprocessor  26  calculates the output power of the DC-DC converter  45 . In common with the first embodiment of the invention described above, the output power of the DC-DC converter  45  is determined using a suitable function map, for example one obtained from simulating the engine running under various conditions. The output power of the DC-DC converter  45  is a function of the battery voltage, and the current (I) supplied to the DC-DC converter  13 , as expressed in equation 5 below:
 
 DC - DC _converter_power= fn ( I ,battery_voltage)  5
 
     Also as described above with reference to the first embodiment of the invention, the battery voltage remains at about 13.5 Volts during engine running, and hence the output power of the DC-DC converter  45  is directly related to the current supplied to the DC-DC converter  45  by the relationship in equation 6 below:
 
 DC - DC _converter_power= fn ( I )  6
 
     In common with the first embodiment of the invention described above, the microprocessor  26  compares the calculated value of the output power of the DC-DC converter  45  with a predetermined threshold value, and if this calculated output power substantially equals, or exceeds this predetermined threshold value, then a decision is made to reduce the load and/or speed under which the engine is currently running in. In order to reduce the load and/or speed of the engine, the ICU  22  ( FIG. 1 ) calculates a reduced-load sequence of injection events, which may involve disabling some or all of the pre-, and or post-injection events, from the previously calculated required sequence of injection events, since these types of injection event are of lower priority than the main injection events. Since the injector drive circuit  6   a  requires less power to operate the injectors  34 ,  36  according to the reduced-load sequence of injection events, the internal control circuit (not shown) of the DC-DC converter  45  modifies the duty-cycle and/or frequency of the PWM signal  58  driving the DC-DC converter  45  to reduce the output power of the DC-DC converter  45 . 
     As an alternative to the voltage sensing circuit shown in  FIG. 4 , it should be appreciated that other embodiments of the invention could use a differential voltage circuit. 
     It should also be appreciated that whilst it is preferable to calculate the actual output power of the DC-DC converter  45  and compare this value to a threshold level, the step of calculating the actual power output of the DC-DC converter  45  is not essential to the present invention. Since the voltage signals that are monitored in the first and second embodiments of the invention are directly related to the output power of the DC-DC converter  45 , these signals could be compared to suitable threshold levels without first being converted into the output power of the DC-DC converter  45 . For example, an alternative embodiment of the invention is envisaged in which the duty-cycle D, or average voltage V AV  of the PWM drive signal  58  of the DC-DC converter  45  is compared to a suitable threshold level, and a reduced-load injection event sequence calculated if the duty-cycle D or average voltage V AV  equals or exceeds this threshold level. Alternatively, the input current I, which is calculated in the second embodiment of the invention, could be compared to a suitable threshold level. 
     An advantage of the techniques described above, is that they are primarily hardware-based, and as such they do not provide a significant drain on the processing power of the microprocessor  26  of the ECU  24 . This means that these techniques can be incorporated into existing ECUs without requiring additional or upgraded microprocessors, which are expensive. Implementing the technique of the first embodiment of the invention is particularly inexpensive, because the cost associated with connecting the drive signal of the DC-DC converter  45  to the microprocessor  26  is negligible. 
     Whilst injectors of the discharge-to-inject variety have been specifically described herein, the invention is equally suited to other types of fuel injectors, in particular fuel injectors of the ‘charge-to-inject’ variety, in which an injection event is initiated by increasing the voltage across the piezoelectric stack. 
     It will be appreciated that various modifications or alternations may be made to the techniques described above without departing from the scope of the invention as defined in the appended claims.