Abstract:
The invention provides, particularly although not exclusively in the context of a fuel injection system of a compression-ignition internal combustion engine, a method for detecting anomalous behaviour of a dynamic system ( 40 ), the method including i) determining a system model including plurality of characteristic parameters to define the dynamic system ( 40 ), ii) calculating one or more metrics indicative of the current system performance based on the plurality of characteristic parameters, iii) comparing the one or more derived metrics with one or more predetermined metrics indicative of anomalous system behaviour and iv) identifying a predetermined system fault condition if one or more of the derived metrics corresponds to one or more of the predetermined metrics. The invention also provides an apparatus for implementing the aforesaid method.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method of detecting and identifying anomalous behaviour of a dynamic system. More particularly, although not exclusively, the invention relates to a method of detecting and identifying anomalous behaviour of a common rail fuel supply system in a compression-ignition internal combustion engine. Also, the invention relates to an apparatus for implementing the aforesaid method.  
       BACKGROUND ART  
       [0002]     Fuel injection systems based on common rail technology provide important advantages to engine and vehicle manufacturers who are under continual pressure by environmental regulatory bodies to reduce the pollution caused by the engine whilst improving the performance of the vehicle offered to the end user. Principally, common rail technology enables the amount of fuel delivered to the combustion cylinders of the engine to be controlled precisely whilst providing high pressure injection and flexible injection timing. Important advantages are thus gained in terms of fuel economy and emissions. However, in order to operate efficiently, it is important that the pressure of fuel within the common rail is controlled accurately to a desired pressure level despite any disturbances that may be caused to the system. In use, the relationship between the fuel pressure within the common rail (hereafter ‘rail pressure’) in response to the amount of fuel pumped into the common rail by a high pressure supply pump is a dynamic system. Typically, therefore, the high pressure fuel pump is controlled by a combination of open-loop and closed-loop control in order to fulfil the functional requirements of i) maintaining the desired rail pressure during changes of injection quantity, ii) varying the rail pressure in response to a change in pressure demand quickly and accurately, and iii) being resilient to system disturbances such as changes in fuel viscosity due to variations in temperature and fuel grade.  
         [0003]     Although it is possible to control the pressure of fuel within the rail accurately and robustly using a combination of open-loop and control-loop control strategies, the Applicant has identified a need to provide a means to identify and characterise possible system faults and anomalous system behaviour in a cost effective manner.  
       DISCLOSURE OF THE INVENTION  
       [0004]     It is against this background that the invention provides, from a first aspect, method for detecting anomalous behaviour of a common rail fuel supply system of an internal combustion engine including a pressurised common rail fuel volume arranged to receive fuel from a pumping arrangement and to supply pressurised fuel to a plurality of fuel injectors, the method including i) determining a system model including a plurality of characteristic parameters to define a common rail pressure value in response to the amount of fuel pumped into the common rail, ii) calculating one or more metrics indicative of the current system performance based on the plurality of characteristic parameters, iii) comparing the one or more derived metrics with one or more predetermined metrics indicative of anomalous system behaviour, and iv) identifying a predetermined system fault condition if one or more of the calculated metrics corresponds to one or more of the predetermined metrics.  
         [0005]     Since the combination of the pumping means and the common rail constitute a dynamic system, preferably the system is controlled by means of a system controller based on a plurality of predetermined control parameters. The system controller ensures that the actual pressure of fuel within the common rail is substantially equal to a value of demanded rail pressure, as determined by an engine control unit, despite system disturbances such as a change in the fuelling requirement.  
         [0006]     In order to improve the performance of the system controller, the method may include calculating new system control parameters based on the characteristic parameters of the system and updating the predetermined system control parameters with the new system control parameters. It is preferred that the step of determining a system model occurs in real time during normal operation of the system such that the system model is updated repeatedly to adapt to external influences on the system such as mechanical wear and tear of fuel injection equipment components and changes in fuel temperature.  
         [0007]     In the preferred embodiment of the invention, the system is a delayed first order system having the characteristic parameters T (time constant), K (steady state gain) and L (lag time). The invention recognises that the characteristic parameters of the system may vary over time and that, by performing online system identification, the characteristic parameters and their associated rates of change may be measured and compared with one or more expected values in order to detect any abnormal behaviour of the system.  
         [0008]     It should be noted that the term ‘predetermined metric’ refers to a predetermined value relating to some aspect of engine operation which is then compared to a measured value for the metric in order to determine whether there is a fault condition. For example, if the steady state gain, K, drops from relatively high value to a relatively low value in the course of several seconds (high rate of change of K), this will indicate that a serious fuel leak has developed within the system and that the pressure of fuel in the common rail cannot be maintained at the desired level.  
         [0009]     In the context of an operational internal combustion engine, the invention provides a means to monitor the fuel injection system of the engine for any anomalous behaviour patterns which may indicate that maintenance action is required. By monitoring the way in which the characteristic parameters of the system vary over time and comparing the data with predetermined metrics that are indicative of anomalous conditions, the invention provides an elegant solution to the problem of monitoring the system behaviour and identifying potential faults. Advantageously, the invention does not require any complex hardwired sensor systems distributed throughout the engine, thus reducing the complexity and overall cost of the engine installation. As well as identifying immediate faults, the invention also provides a means by which maintenance events for engine components may be predicted.  
         [0010]     Preferably, following the identification of faults or anomalous behaviour an alerting step is triggered in which a visible and/or audible alert is provided to the operator of the vehicle such that appropriate action may be taken. In addition, or as an alternative, the alerting step may trigger a change in engine power mode (limp-home mode).  
         [0011]     The invention also provides a computer program product comprising at least one computer program software portion which, when executed in an execution environment, is operable to implement the above described method. Preferably, the computer program software portion and the execution environment are constituted by firmware, for example, an electronic engine control unit of a vehicle in which the method of the invention is implemented.  
         [0012]     The invention also resides in a data storage medium having the or each computer program software portion stored thereon.  
         [0013]     According to a second aspect, the invention provides an apparatus for detecting anomalous behaviour of a dynamic system, the apparatus including i) system identification means for determining a system model including a plurality of characteristic parameters to define the system model, ii) calculation means to calculate one or more metrics indicative of the current performance of the system based on the plurality of characteristic parameters, iii) storage means to store one or more predetermined metrics that are indicative of anomalous system behaviour, iv) comparison means for comparing the one or more predetermined metrics with the one or more calculated metrics, and v) means for identifying that one or more of the calculated metrics corresponds to one or more of the predetermined metrics.  
         [0014]     It will be appreciated that preferred and/or optional features of the method of the first aspect of the invention may be implemented by features of the second aspect of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     In order that the invention may be more readily understood, reference will now be made, by way of example only, to the accompanying drawings in which:  
         [0016]      FIG. 1  is a schematic diagram of a fuel injection system to which the invention is applied;  
         [0017]      FIG. 2  is a functional block diagram of the fuel system in  FIG. 1  and an associated rail pressure control system;  
         [0018]      FIG. 3  is a graphical representation of the output of a first order transfer function in response to a step change input; and  
         [0019]      FIG. 4  is a functional flow diagram as implemented by a failure detection module of the system in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]      FIG. 1  is a schematic diagram of a fuel injection system  2  that is simplified for the purpose of this specific description and within which the present invention may be incorporated. The fuel injection system  2  includes an accumulator volume in the form of a common rail  4  that is supplied with pressurised fuel from a high pressure rail supply pump, in the form of a unit-pump  6 , via a high pressure fuel pipe  7 . It should be noted that the unit-pump  6  is not shown in detail in  FIG. 1  since it is not essential for understanding of the invention and the configuration of such a unit-pump  6  would be well known to the skilled reader. The common rail  4  is fluidly connected to four fuel injectors  8  by respective high pressure fuel supply pipes  10 . The fuel injectors are controlled electronically to deliver fuel to an associated combustion cylinder of the engine (not shown).  
         [0021]     The unit-pump  6  includes a pumping module  12  defining a pumping chamber (not shown) within which fuel is pressurised by an associated pumping plunger  14 . The pumping plunger  14  is driven in a reciprocating motion to perform an inward, pumping stroke, and an outward, return stroke, by a cam-drive arrangement  16 . In a known manner, the cam-drive arrangement  16  includes a driven cam shaft  17  having a cam surface that acts upon a roller/shoe arrangement  19  associated with the pumping plunger  14 . Although only one unit-pump  6  is shown in  FIG. 1  for simplicity, it should be noted that one or more of such pumps may be provided depending on the requirements of the engine installation.  
         [0022]     The pumping chamber of the unit-pump  6  is supplied with relatively low pressure fuel from a transfer pump  18  via a low pressure supply pipe  20  and non-return valve  21 . Low pressure fuel is therefore able to fill the pumping chamber when the pumping plunger  14  performs a return stroke ready for fuel pressurisation. As the cam-drive arrangement  16  drives the pumping plunger  14  on a pumping stroke, the pumping plunger  14  reduces the volume of the pumping chamber and so the fuel trapped therein is pressurised.  
         [0023]     The pumping module  12  is provided with a rail control valve  23  which controls whether or not the pumping chamber communicates with the common rail  4  and thus controls the flow of pressurised fuel thereto. In order to control the volume of fuel that is supplied to the common rail  4 , control means in the form of a unit-pump controller  22  (hereinafter ‘the controller’) is provided, the functionality of which forms part of an engine control unit  24  (hereinafter ‘the ECU’). The controller  22  is electrically connected to the unit-pump  6  and supplies electronic signals to the rail control valve  23 .  
         [0024]     In order to supply pressurised fuel to the common rail  4 , the controller  22  causes the rail control valve  23  to transition from an open state to a closed state during the return stroke of the plunger thus breaking communication between the pumping chamber and the common rail  4 . A relative vacuum will therefore be drawn in the pumping chamber which will cause the non-return valve  21  to open so as to permit fuel at transfer pressure to fill the pumping chamber. At the end of a plunger return stroke, the non-return valve  21  will close thus preventing fuel from flowing back to low pressure from the pumping chamber. During the pumping phase of the pumping plunger  14 , fuel trapped within the pumping chamber is pressurised until such time as the controller  22  causes the rail control valve  23  to open so as to permit pressurised fuel to flow from the pumping chamber to the common rail  4 . By controlling the point at which the rail control valve opens during the pumping stroke of the pumping plunger  14 , the controller  22  determines the effective stoke of the pumping plunger  14  for which pressurised fuel is supplied to the common rail  4  from the unit-pump  6 . The electronic signal necessary to control the rail control valve  23  is known as the ‘filling pulse’ and is measured as degrees of rotation of the cam-shaft  17  that drives the unit-pump  6 .  
         [0025]     During operation of the fuel injection system, it is important that the pressure of fuel within the common rail  4  remains as close as possible to a specific demanded rail pressure that is set by the ECU  24 . To achieve this, the controller  22  utilises negative feedback control to modulate the filling pulse appropriately so as to ensure the actual rail pressure equals the demanded rail pressure despite disturbances that may affect the system. The process by which the controller  22  maintains the fuel pressure within the common rail  4  at the demanded rail pressure will now be described with reference to  FIG. 2 .  
         [0026]     In  FIG. 2 , the ECU  24  outputs a rail pressure demand signal  30 , that is determined based upon the prevailing operating conditions of the engine, to the controller  22  via a summing junction  36 . For example, the ECU  24  will output a comparatively high rail pressure demand signal  30  when the engine is operating under a high engine load/speed condition as compared to a relatively low rail pressure demand signal  30  when the engine is at an idle operating condition.  
         [0027]     A pressure sensor  32  mounted to the common rail  4  measures the actual pressure of fuel in the common rail  4  and outputs a feedback signal  34  that is subtracted from the rail pressure demand signal  30  at the summing junction  36 . The output signal of the summing junction  36  is provided as an input to the controller  22  and represents the difference between the demanded common rail pressure and the actual common rail pressure. The output of the summing junction  36  shall hereinafter be referred to as ‘the pressure error signal’  38 . The function of the controller  22  is to calculate a filling pulse signal to control the rail control valve  23  of the unit-pump  6  so as to cause the pressure of fuel within the common rail  4  to substantially correspond to the demanded rail pressure, so that the pressure error signal  38  is substantially equal to zero.  
         [0028]     It should be mentioned at this point that  FIG. 2  represents a simplified system and that, in a practical embodiment, the controller  22  provides a contributory filling pulse input to the unit-pump  6 . Further filling pulse signal components would also be provided, for example via open loop or feed forward control functions, to compensate for fuel system losses such as the amount of fuel that is currently being injected.  
         [0029]     The unit-pump  6  and the common rail  4  together constitute a dynamic pump/rail system  40  which is initially modelled prior to engine installation in order to derive a mathematical model defining the variables that describe the state of the system as a function of time. Such a mathematical model is referred to as a ‘transfer function’ and would be well known to the skilled reader. The transfer function is used to calculate the P, I and D controller parameters prior to engine installation such that the pump/rail system  40  is controlled acceptably when the engine is operated for the first time.  
         [0030]     In the embodiment described, the controller  22  is a three-term controller having a proportional gain value ‘P’, an integral gain value ‘I’ and a derivative gain value ‘D’. Such a three-term controller is typically referred to as a ‘PID’ controller and its functionality would be familiar to the skilled reader.  
         [0031]     In this embodiment, the transfer function of the pump/rail system is a delayed first order function having three specific parameters that define the characteristic response of the system to an input: a steady state gain value ‘K’; a time constant value ‘T’; and a lag time value ‘L’. By way of explanation, a characteristic first order system is shown in  FIG. 3 , which illustrates the actual rail pressure ‘A’ in response to a step-change in filling pulse ‘B’. The steady state gain K is the ratio of the actual rail pressure A at steady state conditions to the filling pulse input B. The time constant T is the time taken for the actual rail pressure to reach 63% of the demanded rail pressure following a step change in demanded rail pressure. The lag time L is the time period between the start of the step change input and the start of the rise in common rail pressure.  
         [0032]     Referring once again to  FIG. 2 , although the characteristic parameters of the pump/rail system  40  are initially modelled prior to engine installation, the invention provides an online system identification means in the form of a system identification module  42  and a controller parameter calculation means in the form of a controller parameter calculation module  44  (hereinafter ‘calculation module’) for modifying the parameters of the controller  22  online in order to compensate for changes in the response characteristics of the pump/rail system  40 .  
         [0033]     The system identification module  42  is implemented online, that is to say during normal operation of the engine, continuously at predetermined periods in synchronisation with a pseudo random binary input sequence of filling pulses (hereafter ‘PRBS’) that is input to the pump/rail system  40  by the controller  24 . The skilled reader will be familiar with the principles of applying a pseudo random binary sequence as an input signal to a system so further explanation is omitted here.  
         [0034]     In order to calculate the characteristic parameters of the pump/rail system  40 , the system identification module  42  monitors the PRBS signal that is input to the unit-pump  6  and the actual rail pressure that is measured by the rail pressure sensor  32 . Since the PRBS input signal comprises a set of known input stimuli, the system identification module  42  compares the response of the actual common rail fuel pressure to the known stimuli and calculates revised characteristic parameters of K, T and L for the pump/rail system  40 .  
         [0035]     The system identification module  42  communicates electronically with the controller parameter calculation module  44  which, in turn, communicates with the controller  22 . The calculation module  44  receives the revised system parameter values K, T and L from the system identification module  42  and calculates new P, I and D values for the controller  22 .  
         [0036]     In addition to communicating with the calculation module  44 , the system identification module  42  also transmits the new characteristic parameter values K, T and L to a failure detection module  50 , the functionality of which will now be described.  
         [0037]     The failure detection module  50  monitors the incoming flow of data from the system identification module  42 , namely the characteristic parameters K, T, and L, and performs calculations in order to identify certain phenomena associated with the system. The embodiment of the invention described herein is particularly concerned with the identification of two types of phenomenon: quantification of mechanical wear and the occurrence of ruptured high pressure pipe work. Referring to  FIG. 1 , for example, the high pressure fuel supply pipe  5  connecting the unit-pump  6  and the common rail  4  or the high pressure supply pipes  10  connecting the common rail  4  to the injectors  8  may crack or burst such that fuel leaks from the common rail at an uncontrolled rate which compromises the ability of the controller  22  to maintain the fuel pressure in the common rail  4  at a desired level. The detection of the aforesaid phenomenon is discussed below with reference to  FIG. 4 .  
         [0000]     Detection of a Burst Fuel Supply Pipe  
         [0038]     A fuel supply pipe typically fails in one of two ways. Firstly a crack may develop in a fuel supply pipe such that high pressure fuel leaks from the crack gradually. Alternatively, the crack may burst suddenly such that fuel escapes from the common rail at a high rate. In either case, it is important to identify such a failure promptly so that appropriate action may be taken, for example shutting down the engine, to avoid irreparable damage to engine components.  
         [0039]     In order to detect such failures, the raw data from the system identification module  42  is separated into two data streams, namely raw values of the steady state gain K (hereinafter ‘K_raw’) and the time constant T (hereinafter ‘T_raw’). A signal corresponding to K_raw is passed through a low pass filter at step  402  which removes insignificant high frequency variations in the signal, noise for example, so as to provide an initial filtered value of steady state gain K, hereinafter ‘K_filtered’.  
         [0040]     During the first few minutes of engine operation, the temperature of the fuel will rise to a working fuel temperature which will reduce the viscosity of the fuel. This will have the effect of increasing the level of leakage in the system which will have a corresponding effect of reducing the steady state gain value K. In order to compensate for the change in K due to fuel temperature changes, K_filtered is input into a temperature compensation unit  404 .  
         [0041]     Once the effects of increased fuel temperature have been removed, the K_filtered signal is passed through a high-pass filter  406  having a filter time constant in the order of minutes to provide a second filtered value of K, hereinafter ‘K_high_filtered’. In turn, K_high_filtered is input into a rate of change calculation unit  408  which calculates the derivative of K_high_filtered with respect to time (hereinafter ‘K′_high filtered’). Finally, the rate of change calculation unit  408  outputs the K′_high_filtered signal to a knowledge module  410 .  
         [0042]     The knowledge module  410  monitors the received value of K′_high_filtered and compares it with predetermined values of ‘K′_high_filtered’ that are indicative of a burst fuel supply pipe and which are stored by the knowledge module  410 . For example, if a fuel supply pipe bursts, the value of K_high_filtered would drop to a low value in a matter of seconds and so the derivative value, K′_high_filtered, would be comparatively high.  
         [0043]     If the knowledge module  410  detects a match between the received values of K′_high_filtered and the stored values of K′_high_filtered, it will send a signal to the ECU  24  so that appropriate action may be taken: for example, the ECU  24  may determine that the engine should be shut down immediately or merely that a warning signal should be issued to the vehicle operator in order to avoid damaging the engine. It should be mentioned at this point that a match between a received data values and stored data values in practice will include a degree of tolerance and a precise match is not essential. Furthermore, the knowledge module  410  may be configured to identify a match when the received data values are within predetermined limits for a period of time.  
         [0044]     The predetermined values of K′_high_filtered stored by the knowledge module  410  are known values, or ‘metrics’, that are determined theoretically or, alternatively, empirically by way of engine testing. It should be appreciated that the knowledge module  410  is capable of storing a plurality of metrics for comparison such that appropriate action may be taken depending on the severity of the pipe failure. For example, if a relatively minor crack is detected by the knowledge module  410  and reported to the ECU  24 , the ECU  24  may merely indicate a warning to the vehicle operator that maintenance is required. Alternatively, if a major pipe rupture is detected, the ECU  24  may cause the engine to run in a restricted power mode (limp-home mode) to enable the vehicle operator time to manoeuvre the vehicle to safety prior to shutting down the engine.  
         [0000]     Determination of Mechanical Wear  
         [0045]     Mechanical wear on the injection equipment components will increase the amount of fuel leakage from the pump/rail system  40 . For instance, the clearances between surfaces exposed to high pressure fuel increase over time periods of months or years which allows the volume of fuel that leaks past such clearances to increase. This increased leakage manifests itself as a gradual decrease in the steady state gain K of the system transfer function such that, over time, it is necessary to increase the filling pulse supplied to the unit-pump  6  in order for the pressure of fuel in the common rail  4  to reach a desired level. Similarly, the unit-pump  6  will take longer to increase the pressure of fuel within the common rail  4  by a given pressure such that the time constant, T, of the pump/rail system  40  will increase. The knowledge module  410  is able to identify the effects of mechanical wear in the pump/rail system  40  by monitoring the system time constant T, the system steady state gain K and their respective rates of change, as will be described in further detail below.  
         [0046]     The raw time constant value T_raw which is output from the system identification module  42  is input to a low pass filter  412  to remove insignificant high frequency components from the signal resulting in a filtered time constant signal (hereinafter ‘T_filtered’). Thereafter, T_filtered is input directly to the knowledge module  410 . In addition, T_filtered is also input into a second rate of change calculation unit  414  for calculating the derivative of T_filtered, hereafter ‘T′_filtered’. The ‘T′_filtered’ signal is also input into the knowledge module  410 .  
         [0047]     The knowledge module  410  receives a second filtered value of K, hereinafter ‘K_low_filtered’, which is obtained by passing K_raw through the low pass filter  402 , the temperature compensation unit  404  and a second low pass filter  416  having a filter time constant in the order of months. The output of the second low pass filter  416  is also passed though a rate of change calculation unit  418  that determines the rate of change of K_low_filtered, hereinafter K′_low_filtered, which is input into the knowledge module  410 .  
         [0048]     The knowledge module  410  monitors the values of T_filtered, T′_filtered, K_filtered and K′_filtered and compares the aforesaid values with stored values (i.e. predetermined metrics) of T_filtered, T′_filtered, K_filtered and K′_filtered that are indicative of a predetermined level of mechanical wear. Such predetermined metrics are obtained through engine proving testing. As a result of the above comparison, if the knowledge module determines a correspondence between the current and stored values of T_filtered, T′_filtered, K_filtered and K′_filtered, it transmits a signal to the ECU  24  indicating that a match has been found. The ECU  24  then takes appropriate action such as notifying the vehicle operator that a component requires replacement.  
         [0049]     It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the scope of the invention, as defined by the claims. For example, the detection of mechanical wear in the system and burst fuel supply pipes are two types of system abnormalities that the invention is particularly suited to recognising due to their direct influence on the characteristic parameters of the transfer function of the pump/rail system. However, it should be appreciated that the inventive concept is not limited solely to the detection of the phenomenon described. Rather, the invention is applicable to any behavioural abnormalities that may manifest themselves by variations in the characteristic parameters of the system model. For example, different grades of fuel used in an engine will have respective viscosities with which those fuels may be characterised. If a vehicle fuel tank is re-filled with a fuel having a different grade, the steady state gain value K will either increase or decrease over a few minutes (depending on whether the new fuel has a greater or lesser viscosity than the old fuel). Thus, the failure detection module  50  may be programmed with predetermined metrics to identify the derivative of K that would be expected to occur with certain fuel grades. The ECU  24  then utilises the identification of a change in fuel grade by modifying the filling pulse appropriately to account for the change in viscosity, for example.  
         [0050]     It should be appreciated that the fuel injection system  2  provides a context for the operation of the invention but is not intended to limit the scope of the claims. Alternatively, for example, the common rail  4  may be supplied with high pressure fuel by an equivalent pumping means, a radial high pressure fuel pump for instance.  
         [0051]     It should also be appreciated that although the common rail  4  is described as supplying high pressure fuel to four fuel injectors  8 , typically such an engine may include six, eight or ten fuel injectors.  
         [0052]     As an alternative to monitoring the way in which the characteristics of the system model change over time in order to infer failure of system components and/or abnormal system operation, it should be appreciated that the failure detection module  50  could be modified appropriately so as to monitor the way in which the proportional, integral and derivative gain values of the controller  22  change over time in order to achieve the same advantages provided by the invention. Furthermore, the failure detection module  50  could alternatively be configured to monitor the output of the controller  22  and the way in which the output changes value over time.