Method for operating a fuel injection system

Abstract

The invention relates to a method of operating fuel injection in an internal combustion engine comprising a fuel delivery system with at least one fuel solenoid actuated fuel injectors. Injection events are piloted by a drive module that generates a drive signal including an opening phase whereby a boost voltage is applied to the fuel injector to regulate an oscillating opening current, each oscillation with an increasing interval and a decreasing interval. The method implements an opening time determination routine comprising the steps of: within an analysis window comprising a plurality of current oscillations of an opening phase corresponding to an injection event operated for a respective fuel injector, evaluating for each oscillation the difference between a duration for its increasing interval and a duration for its decreasing interval, thereby obtaining a delta duration mapping; determining a minimum timepoint at which the delta duration mapping reaches a local extremum or which corresponds to an extremum of the derivative of the delta duration mapping; determining an opening time from the minimum timepoint; controlling fuel injection using the determined opening time for the respective fuel injector. Also disclosed are diagnostic strategies.

Description

[0001] P-DELPHI-446 / WO 1

[0002] METHOD FOR OPERATING A FUEL INJECTION SYSTEM

[0003] Technical field

[0004] The present invention generally relates to internal combustion engines, more specifically to an improved method for operating a fuel injection system.

[0005] Background Art

[0006] Fuel injectors are typically controlled by generating pulses which are sent to the actuators of the fuel injectors. The amount of fuel injected typically depends on the length of a pulse sent to the actuator. Typically, an Engine Control Unit (ECU) adjusts the pulse length as a result of the demand quantity of fuel to be injected. The demand quantity of fuel is conventionally stored in a map which relates it to the engine speed and the torque demand.

[0007] Characteristics of fuel injectors may vary over time, e.g. as a result of wear. It is thus important to calibrate injection systems periodically to account for variations in the lifetime of their injectors. Techniques are known which apply learning strategies, whereby injector characteristics are refreshed, and the injectors are consequently appropriately controlled.

[0008] An important such characteristic of fuel injectors is the opening time. A nominal opening delay is generally known for a given injector model / design and given working conditions. However, there may be a spread on this opening delay between injectors, and a drift may occur over time during the injector life.

[0009] Conventionally, the actual opening time of a solenoid actuated injector is determined by measuring the solenoid current and using a knee-point detection method to identify a discontinuity in the rate of change of the current during an opening phase. This approach is effective when the solenoid is driven with a constant voltage. However, when the solenoid is driven in chopped current mode using a boosted voltage, i.e. greater than battery voltage, this discontinuity may not appear in the solenoid current trace before the chopped current regulation phase. P-DELPHI-446 / WO 2

[0010] Technical problem

[0011] It is an object of the present invention to provide a method for operating a fuel injection system in an internal combustion engine, which overcomes the aforementioned drawbacks and accurately determines the opening time of injectors driven using chopped current regulation.

[0012] General Description of the Invention

[0013] This object is achieved by a method for operating fuel injection system in an internal combustion engine as claimed in claim 1.

[0014] The internal combustion engine comprises a fuel delivery system with at least one fuel injector, which comprises a solenoid actuator configured to selectively operate a valve member to control a fuel outlet orifice.

[0015] Injection events are operated by applying a drive signal to the fuel injector to inject fuel into a cylinder of the engine, whereby the drive signal is applied by a drive module and includes an opening phase whereby a boost voltage is applied to the fuel injector to regulate an opening current at a predetermined level. Opening current regulation is done by repeatedly switching the boost voltage between an ON state and an OFF state, such as to obtain an oscillating current, each oscillation with an increasing interval and a decreasing interval.

[0016] According to the invention, an opening time determination routine comprises: a) within an analysis window comprising a plurality of current oscillations of an opening phase corresponding to an injection event operated for a respective fuel injector, evaluating for each oscillation the difference between a duration for its increasing interval and a duration for its decreasing interval, thereby obtaining a delta duration mapping; b) determining a minimum timepoint at which the delta duration mapping reaches a local extremum or which corresponds to an extremum of the derivative of the delta duration mapping; c) determining an opening time from the minimum timepoint; P-DELPHI-446 / WO 3 d) controlling fuel injection using the determined opening time for the respective fuel injector.

[0017] The inventive method proposes a robust and accurate approach to determine the timing of opening of the fuel injector, i.e. the opening time. The determination routine distinguishes between increase and decrease time of the oscillations of the opening current. More specifically, the difference between increase and decrease durations is evaluated, and mapped vs. time. Experience has shown that the opening time corresponds to a local extremum that precedes a steep curve portion. The routine thus detects this local extremum.

[0018] The detected opening time may then be used for fuel injection control. In particular the detected opening time is injector specific and allows computing the so-called opening delay, i.e. the time period between the start of the drive signal / pulses and the opening time. Opening time, resp. opening delay, can be advantageously used in fuel control strategies for individual injector corrections.

[0019] In step a) the delta duration mapping is built by evaluating the difference between the durations of the increasing and decreasing intervals. In principle this can be done by subtracting the increasing interval from the decreasing intervals, or inversely.

[0020] In step b) the term minimum timepoint refers that it is looked for the first timepoint in the observation window that meets the defined criteria.

[0021] In embodiments, the difference between a duration for its decreasing interval and a duration for its increasing interval is evaluated (within the observation window) by:

[0022] - Determining a duration for each of the decreasing intervals of the current signal;

[0023] - Determining a duration for each of the increasing intervals of the current signal;

[0024] - Subtracting the duration of each of the increasing interval from the duration of one of its consecutive decreasing interval, thereby obtaining a delta duration mapping. P-DELPHI-446 / WO 4

[0025] The delta duration mapping may thus be a table of values representing (decrease interval - increase interval) vs. time.

[0026] In such case, the minimum timepoint is advantageously determined as a timepoint of a local minimum that immediately precedes a steep variation in values of the delta duration mapping, specifically a steep increase.

[0027] In an alternative embodiment, the minimum timepoint may be determined as a timepoint of a local maximum that immediately precedes a steep variation in values of the delta duration mapping, specifically a steep decrease.

[0028] The data processing to identify the search minimum timepoint can be done by analyzing the rate of variation of the delta duration mapping following the local extremum; or by means of a threshold, evaluating whether the difference the local minimum and next values increase above a threshold.

[0029] As a further embodiment in step b) the minimum timepoint corresponds to an extremum of the derivative (over time) of the delta duration mapping. This is an alternative way of determining the “minimum timepoint” from the variation of the values of the delta duration mapping.

[0030] In embodiments, following step b), an assessment is performed to identify whether the minimum timepoint precedes a monotonous region of the delta duration mapping, and whereby step b) and the assessment are repeated for another local minimum if the minimum timepoint does not precede a monotonous region.

[0031] In embodiments, the assessment concludes that a minimum timepoint precedes a monotonous region if the delta duration mapping is monotonous for a predetermined duration following said minimum timepoint.

[0032] In embodiments, the assessment concludes that a minimum timepoint precedes a monotonous region if the delta duration mapping exceeds a predetermined value / threshold following said minimum timepoint.

[0033] In embodiments the observation window has a lower bound that may be calibrated to begin at a predetermined timing following the start of the opening phase. The general behavior of the valve member is typically known from experimentation (and for a given injector design), and it is then possible to define the lower bound of the P-DELPHI-446 / WO 5 observation window such that it corresponds to a moment where the valve member has left the seat and already moved over part of its stroke.

[0034] Alternatively, the lower bound of the observation window may be set to begin after the second or third oscillation of the opening current regulation (after the first current ripples).

[0035] In embodiments, the observation window has an upper bound that is calibrated to end after the expected opening time and at the latest with the end of the opening phase. It may be noted that the duration of the opening phase is typically calibrated to have a length sufficient (at least) for the valve member to reach the fully open position. This duration is determined to be applicable relevant for a population / design of injectors. The duration of the opening phase may further be calibrated in function of fuel pressure.

[0036] In embodiments, the increasing and decreasing intervals are determined based on voltage switching timings operated by the drive module.

[0037] For example, the drive module may include a state machine that generates a temporal signal indicating a current state of the boost voltage, and whereby the duration for each of the decreasing intervals of the current signal is determined as the duration of a respective OFF states.

[0038] Furthermore, the drive module state machine may generate a temporal signal indicating a current state of the boost voltage, and whereby the duration for each of the increasing intervals of the current signal is determined as the duration of the respective ON state.

[0039] In embodiments, the opening time may be determined from the minimum timepoint by adding a time offset to the minimum timepoint, preferably whereby the time offset is negative or null. This offset can be used to compensate any bias due to the determination of the timepoint. The relevance and dimensioning of the offset will be decided upon by the skilled person.

[0040] In that respect, in the approach that evaluates the difference of increase / decrease duration, it is considered that the so determined minimum timepoint can be considered to correspond to the actual moment the injector (valve member) reaches the fully open position, with negligible error. P-DELPHI-446 / WO 6

[0041] However, using the derivative approach, it is considered that the minimum timepoint will be slightly later than the expected timing. This can then be corrected by a calibrated (negative) offset.

[0042] In embodiments, step a) may be repeated for a plurality of injection event for the same injector; the corresponding values of the delta duration mapping for each injection event are resampled on a fixed time-step, and a delta duration mapping is obtained as a mean delta duration mapping.

[0043] The techniques according to the present invention can be applied to various fuel injection control strategies, and in particular to liquid and gaseous fuel engines. Furthermore, the inventive techniques are compatible with strategies where the injection events are operated by applying a drive signal that comprise (or consist) of two current levels, i.e. an opening phase operated with an opening current and a hold phase operate with a hold current, but could also include other phases. In particular, the drive signal could include an intermediate phase, between the opening phase and hold phase, at an intermediate current level, lower than the opening current and higher than the hold current.

[0044] These and other embodiments of the invention are also recited in the appended dependent claims.

[0045] According to another aspect, the invention relates to a computer program product comprising instructions which, when the program is executed by a control unit comprising a processor, cause the control unit to carry out the method according to the present disclosure.

[0046] According to yet another aspect, the invention relates an internal combustion engine according to claim 21 .

[0047] In general, the method and techniques according to the present disclosure can be implemented by hardware and / or software. In practice, they it may conveniently be implemented by a control unit comprising a processor and a memory, such as e.g. the Engine Control Unit. In such case the memory may contain instructions which, when executed by the ECU / processor, cause the latter to carry out the present method. P-DELPHI-446 / WO 7

[0048] Brief Description of the Drawings

[0049] A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

[0050] Fig. 1 a is a plot of solenoid current and armature displacement against time for drive signal with constant voltage;

[0051] Fig. 1 b is a plot of solenoid current and armature displacement against time using chopped current regulation driven by a boost voltage;

[0052] Fig. 2a is a plot of the current decrease period against time;

[0053] Fig. 2b is a plot of the current increase period against time;

[0054] Fig. 2c is a plot of the delta duration mapping;

[0055] Fig. 3 is a plot of voltage and current over time;

[0056] Fig. 4 is a plot of the delta duration mapping with three methods of sampling;

[0057] Fig. 5 is a plot of a resampled difference delta duration mapping;

[0058] Fig. 6 is a plot of the delta duration mappings for an injector stuck in the open position, an injector stuck in the closed position, and a functional injector;

[0059] Fig. 7 is a plot of the switching period mapping;

[0060] Fig. 8 is a plot of the delta duration mapping and the armature displacement against time for an injector with bouncing behavior;

[0061] Fig. 9 is a plot of the solenoid current and armature displacement against time for an injector with premature closing behavior;

[0062] Fig. 10 is a plot of the delta duration mapping during the hold phase for an injector with premature closing behavior;

[0063] Fig. 11 is a plot of the switching period mapping during the hold phase for an injector with premature closing behavior;

[0064] Fig. 12 is a flow chart of an embodiment of the present method;

[0065] Fig. 13 is a diagram of a gaseous fuel delivery system implementing the present method;

[0066] Fig. 14 is a plot illustrating a typical drive signal and pintle lift for an injection event; P-DELPHI-446 / WO 8

[0067] Fig. 15 is a flowchart showing an embodiment of a routine for detecting the local minimum.

[0068] Description of Preferred Embodiments

[0069] An embodiment of the present invention will now be explained in the context of injection control in a spark ignited internal combustion engine operating with gaseous fuel, in particular hydrogen. The engine comprises a plurality of engine cylinders (combustion chambers), generally between 3 and 8, and a plurality of fuel injectors (generally one per cylinder).

[0070] Fig. 13 represents a gaseous fuel delivery system 10 comprising a pressurized fuel tank 12 for the gaseous fuel (or fuel gas), a fuel rail 14 and a plurality of fuel injectors 16 for selectively injecting controlled amounts of fuel in engine cylinders. The fuel rail 14 is a fuel accumulator that is fed with fuel gas at controlled pressure via a feed line / piping 18. The gas tank 12 may comprise one or more tanks containing pressurized fuel gas at pressures up to 700 bar. The gas tank 12 may typically include a shutoff valve and a mechanical pressure reducer (not shown) to decrease the pressure to around 40 to 50 bar. Reference signs 20 and 22 indicate respectively a shutoff valve and an electronic pressure regulator to regulate the pressure in the fuel rail 14 within a working range between 5 and 40 bar. Shutoff valve 20 and pressure regulator 22 may be integrated in a single housing unit, referred to as HRM, optionally with one or more other functions such as: gas filter, pressure relief valve, purge valve...

[0071] According to one conventional design, the fuel injectors 16 comprise a globally tubular body defining a gas passage extending longitudinally from an inlet portion to an outlet portion formed as a nozzle. At the outlet portion the body comprises an outlet orifice surrounded by a valve seat that cooperates with a reciprocating pintle (i.e. valve member) to control gas flow through the outlet orifice. The pintle has a shaft extending inside the body and a radially protruding pintle head that is, in closed injector configuration, resting in gas-tight manner on the valve seat, outside of the gas passage (i.e. outward opening injector). The injector 16 further includes, opposite to the valve seat, a solenoid actuator that cooperates with a magnetic armature mechanically coupled to the pintle in opening direction. To open the P-DELPHI-446 / WO 9 injector, the solenoid is energized, creating a magnetic field that attracts the armature in the direction of the inlet portion, hence moving the pintle outwardly from the gas passage (opening stroke). The pintle head is thereby raised / lifted from the valve seat allowing for the discharge of gaseous fuel through the outlet orifice. When the current is suppressed, the magnetic force disappears and the pintle returns to the closed configuration on the valve seat (closing stroke). The pintle movement is limited, during the opening stroke, by a stop surface (body portion, generally a pole piece), against which the armature comes into abutment. The pintle closing stroke is limited by the pintle head abutting the valve seat.

[0072] Hence at injector opening, the pintle leaves its rest / closed position on the valve seat, noted LCLD, to move towards the fully open position, noted LOP.

[0073] The general design of the gaseous fuel delivery system 10 and of such solenoid actuated gas injectors 16 is generally known in the art and will not be further described herein.

[0074] Injection of fuel in the associated cylinders is performed during an injection event, by applying a drive signal (or drive pulse) to the gas injector, i.e. to the solenoid coil, to cause injector / pintle to open during a predetermined time period. Much simplified, injection control strategies use mappings (known as flow curves) that relate the fuel quantity to the injector actuation time that is referred to as pulse with, PW. So, to perform an injection event, a drive signal is applied during a time period PW.

[0075] Conventionally, the injection control strategies are programmed in the Engine Control Unit, ECU, that receives various signals indicating the state of the engine from various sensors, and is, inter alia, configured to determine a fuel quantity to be injected and a corresponding timing of injection. More specifically, the ECU is configured to determine a desired fuel quantity to be injected to achieve a given torque demand, and subsequently determines the PW corresponding to the desired fuel quantity. To perform the injection event, the ECU sends to an Electronic Drive Module EDM a signal 28 that defines for the respective injector the PW length to operate the injector to effect the injection of the desired fuel quantity. The PW is a logic signal defining a duration of actuation. The EDM in turn includes electronic circuits and settings configured to apply to the injectors a drive signal 30 to operate P-DELPHI-446 / WO 10 the injection event during PW, namely by applying a control voltage to the respective injector (solenoid) and supplying an electric current to the injector.

[0076] For an injection event, the drive signal typically includes two phases that are illustrated in Fig.14, which may be characterized by the current profile.

[0077] In the first phase, the current is regulated to a value Io, which corresponds to a typical high current adapted to operate injector full opening; this is the opening current. Typically, this is achieved by the solenoid controllers setting in EDM via current regulation between two predetermined levels -peak and trough- to achieve a mean targeted current value. A first voltage level V1 is applied to the solenoid until the desired peak current level (peak) is achieved. Once this is achieved the EDM steps the voltage down to lower voltage or zero to prevent any increase in current above the demand (peak level). Again, when the value of current falls below the trough level, the voltage is stepped up to the nominal level, here V1. Thus, under such control, when the desired level of current is achieved, the currents toggle around the desired level. This is often referred to as "chopping". Current Io, as well as IH (see below), is thus considered as an average current value of the current chopping.

[0078] So, in the opening phase, current is regulated to level Io, which is a high level, by chopping modulation driven by voltage V1 . Voltage V1 is advantageously a boost voltage Vboost, i.e. it is an amplified voltage that is greater than the battery voltage Vbatt. V1 may be at least double (or triple) the voltage Vbatt. During the opening phase, the pintle is pulled by the magnetic field and moves toward the fully open position, that is reached at time top.

[0079] Then in the second phase, or hold phase, the current is regulated to correspond to a hold current IH. That is, the current intensity is set to be sufficient to maintain the valve member (already stabilized in fully open position) in the open position, while limiting energy consumption. This current IH is conventionally regulated by chopping using the battery voltage, i.e. V2=Vbatt. The current level IH is a low current compared to Io. For example, current Io may be double or triple the current IH. During the hold phase the pintle remains open in LOP. Cutting the hold current causes a decrease of the magnetic force, and the pintle will fall shortly after the end of the hold phase (after PW) to return to the closed position LCLD. P-DELPHI-446 / WO 11

[0080] Fig.14 shows the current regulation for a typical injection event, with corresponding pintle displacement. The drive signal as a simple profile with an opening phase followed by hold phase. In other embodiments, other profiles may be envisaged, e.g. where the opening phase comprises two current pulses at a high current sufficient to open the valve, separated by a brief current cut-off. Alternatively, there may be an intermediate phase between the opening phase and hold phase, that is carried out with an intermediate current that is greater than IH and smaller than Io.

[0081] Turning now to Fig.1 a, it shows a plot of solenoid current (solid line) and armature displacement (dashed line) against time during an opening motion of the injector, for a solenoid actuator driven by a drive signal with constant voltage. As it can be seen, the armature reaches its open position at a timing which substantially matches the timing of a discontinuity in the current trace. Conventionally, the timing of this discontinuity, and thus the opening time, may determined by knee-point detection.

[0082] Fig. 1 b shows a plot of solenoid current and armature displacement against time for a solenoid actuator driven by current chopping regulation, causing current oscillations. This chopping regulation mode has been explained above with reference to Fig.14. To recap, the current evolution in the solenoid is driven by application of voltage, here a boosted voltage level Vboost. The boost voltage results in a sharper solenoid current increase and improved injector responsiveness. However, the solenoid current must be limited to prevent overheating of the electromagnetic coil. This is thus commonly achieved by chopping regulation, namely by automatically cutting the boost voltage and applying zero voltage when the current reaches a predetermined maximum threshold (peak level), and reenabling the boost voltage when the current reaches a predetermined minimum threshold (trough level).

[0083] As it can be seen on figure 1 b, this method results in an oscillating solenoid current, with each oscillation having a decreasing interval and an increasing interval. It may be noted that the discontinuity in the rate of change of the current which enabled the determination of the opening time in the plot of Fig. 1a is not apparent. The inventive method enables to accurately determine the opening time of injectors controlled by current chopping. P-DELPHI-446 / WO 12

[0084] < METHOD >

[0085] The invention concerns a method of operating fuel injection in an internal combustion engine, for example of the type shown in Fig.13. Injection events are operated, by which respective drive signals are applied to the solenoid actuated fuel injectors to cause injector opening and fuel discharge. Injector operation is as disclosed above, the drive signal typically comprising an opening phase and hold phase, which are both advantageously operated by current chopping at an opening current level and hold current level, respectively. The discharge fuel quantity is varied by adapting the duration of the drive signal, known as pulse width PW (opening phase + hold phase).

[0086] The inventive method then provides for a routine, referred to as opening time determination routine, that is configured to determine the actual opening time of the fuel injector.

[0087] As will be clear below, the period of oscillation (also known as ripple) of the current around the opening is processed and analyzed to determine the actual opening time of a solenoid actuated injector driven by current chopping. An embodiment of this routine will now be described with reference to Fig.12.

[0088] In an initial step S1 , an analysis window is defined, that encompasses multiple opening current oscillations of the drive signal applied to operate an injection event for a given injector. The analysis window is calibrated to include the expected opening event, with large tolerances.

[0089] According to the present routine, the duration of each of the increasing and decreasing intervals of the opening current signal is determined. This may be achieved based on the switching times of the voltage Vboost applied to the solenoid, i.e. the timepoints tnat which the boost voltage is cut or re-enabled, and subtracting two consecutive timepoints. The switching times tnmay be obtained directly from the EDM, namely from the microcontroller responsible for switching the drive voltage on and off. Said timepoints are represented on figure 3, which shows a plot of voltage and current over time during the oscillating phase of the solenoid current. P-DELPHI-446 / WO 13

[0090] In this embodiment, the EDM includes a state machine with a high frequency timer (few MHz). The state machine indicates the status of the drive voltage. A temporal signal is emitted each time the voltage level is switched (on / off and off / on); this corresponds to the switching times tn. Thanks to the state machine, it is also known whether the switching times correspond to an increase or decrease period.

[0091] Alternative embodiment (not shown): it may be noted that if the switching times are not available from such state machine, an alternative may be to obtain a derivative of the current vs. time. The increase and decrease slopes will be given directly by the derivative. Practically, the current derivative can be obtained by analog electronic circuit: a shunt resistor is placed in serial with the coil current. Shunt voltage will be proportional to the coil current, and a passive RC or RL circuit can be used to get the shunt voltage derivative, which is an image of the current derivative. An active analog differentiator could also be used.

[0092] Figure 2a and 2b respectively show the duration of each of the decreasing and increasing intervals plotted against time.

[0093] In subsequent step S2, the duration of the increasing interval is removed from the duration of the decreasing interval and mapped against time, thereby generating a delta duration mapping. Figure 2c represents such a delta duration mapping, with the actual injector opening represented by a solid vertical line.

[0094] Step S1 and S2 may be combined by implementing the following formula: where Atnis the value of the delta duration mapping for the nthswitching time. Equivalent formulae may be implemented, such as

[0095] Figure 4 represents such delta duration mappings for each of these equivalent formulae, with the actual injector opening represented by a solid vertical line. The three curves vary depending on the equation to calculate Atn. P-DELPHI-446 / WO 14

[0096] Preferably, as shown in figure 5, the delta duration mapping is resampled with a fixed time step. That is, the delta duration mapping for one injection event is processed, typically using extrapolation, to have a mapping of delta duration at predetermined, fixed time intervals.

[0097] For robustness, it may be desirable to have a mean mapping of delta duration. In an embodiment of the invention, the steps S1 and S2 are repeated for a plurality of injections event with the same injector, and a mean delta duration mapping is computed from the corresponding re-sampled delta duration mappings, thereby decreasing possible noise. To further improve the accuracy of the mean delta duration mapping, injections of the plurality of injections can be performed with different switching time. This may be achieved by e.g. changing on the drive voltage level.

[0098] In subsequent step S3, a minimum timepoint at which the (mean) delta duration mapping reaches a local minimum value is determined. This can be achieved by iterating through values of the delta duration mapping in chronological order, with e.g. a fixed timestep or for each switching time.

[0099] In subsequent step S4, an assessment is performed to identify whether the minimum timepoint precedes a sharp increase. For example, the assessment may conclude that a minimum timepoint precedes a sharp increase if the delta duration mapping increases monotonously for a predetermined duration following said minimum timepoint. Alternatively, or in addition to the above, the assessment may conclude that a minimum timepoint precedes a sharp increase if the delta duration mapping exceed a given value / threshold following said minimum timepoint. Said given value may be predetermined or dependent on the value of the delta duration mapping at the minimum timepoint.

[0100] If the minimum timepoint of the delta duration mapping does not precede a sharp increase, step S3 and S4 are repeated with another local minimum.

[0101] If the minimum timepoint of the delta duration mapping does precede a sharp increase, then it is considered that this minimum timepoint is the characteristic event in the opening current ripple that corresponds to the full valve opening, i.e. injector opening. Preferably, a time offset is added to the minimum timepoint at step S5, and the resulting value is considered to represent the actual opening time. The time P-DELPHI-446 / WO 15 offset may have a positive, negative or a null value. It is used to compensate for opening time reference measurement based on external sensor which may exhibit systematic time offset compared to the described methodology.

[0102] In general, the minimum timepoint may be determined as a timepoint of a local minimum that immediately precedes a steep variation in values of the delta duration mapping. The data may be processed iteratively in chronological order, wherefore the language ‘minimum timepoint’ that indicates the first occurrence that is found.

[0103] Any appropriate method to identify a local minimum before a monotonous region describing a sharp increase may be used. They can be based on the duration of the monotonous increase, or over the amplitude of the increase (threshold).

[0104] One possible routine to detect the local minimum based on the threshold approach is disclosed in Fig. 15. The first increase and decrease periods are skipped, to avoid initialization issues. Then the algorithm detects if the delta duration mapping values increase. Where the value exceeds a threshold (or trigger), it is considered that the steep increase has been found. This means that the value before the steep increase is the local minimum. In Fig.15 opening time is indicated as value time - offset. However, taking the previous value (before the threshold) would directly give the local minimum, the timing of which can be considered as the opening time. The flowchart of Fig.15 is only an example; other approaches can be used.

[0105] In an equivalent embodiment, the duration of the decreasing interval is removed from the duration of the increasing interval and mapped against time to generate the delta duration mapping in step S2. In that case, a minimum timepoint at which the delta duration mapping reaches a local maximum value is determined at step S3, and in step S4, the assessment is performed to identify whether the maximum timepoint precedes a sharp decrease.

[0106] Finally, at step S6, the operation of the fuel injector is controlled using the effective opening time determined in the previous step.

[0107] For the sake of exemplification, the observation window, ObW is represented in Fig.2 and in Fig.14. The observation window ObW is positioned within the opening phase. It may have a lower bound twi that is calibrated to begin at a predetermined P-DELPHI-446 / WO 16 timing following the start of the opening phase, or set to begin after the second or third oscillation of the opening current. The lower bound twi may further be calibrated based on rail pressure. The upper bound tw2 is calibrated to end after the expected opening time top, with a certain margin, and at the latest with the end of opening phase.

[0108] < DIAGNOSTIC >

[0109] Advantageously, the delta duration mapping may be further analyzed to identify possible abnormal behavior for the fuel injector.

[0110] For example, steps S1 to S5 may be repeated to identify the presence of multiple “opening times” for a given injection. A plurality of such opening times would indicate the occurrence of bouncing during the injector opening, as shown on figure 8.

[0111] Alternatively, if the delta duration mapping does not no describe a local minimum / extremum as defined above, a flag signaling that the injector is malfunctioning may be raised. Figure 6 shows plots of the delta duration mapping for an injector stuck in the open position, an injector stuck in the closed position, and a functional injector. As it can be seen, the delta duration mappings of the injector stuck in the open position and the injector stuck in the closed position describe an increase that rapidly reaches a flat portion. Considering a possible position of observation window, the delta duration mappings values would mainly correspond to the flat portion. Accordingly, such malfunction can be detected based on the slope of the delta duration mapping. If the time derivative of the delta duration mapping is null during most (at least 50 to 80%) of the observation window, then it can be concluded that the injector is deficient (raise flag).

[0112] The inventive method may further include an injector diagnostic subroutine, which evaluates the sum of the duration for its increasing interval and the duration for its decreasing interval, thereby obtaining a switching period mapping. This switching period mapping may then be compared to predetermined thresholds to diagnose P-DELPHI-446 / WO 17 abnormal behavior for the injector. This switching period mapping is represented on the plot of figure 7.

[0113] For instance, the switching period mapping may be compared to an opening failure threshold, and a flag indicating that the injector failed to open may be raised if the switching period mapping is greater than said opening failure threshold.

[0114] Likewise, the switching period mapping may be compared to a closure failure threshold, and a flag indicating that the injector failed to close may be raised if the switching period mapping is lower than said opening failure threshold.

[0115] Said opening and closure failure thresholds may be calibrated based on the current level, the drive voltage level, or the injector electrical resistance.

[0116] It may be advantageous for the drive signal to comprise an opening phase and a hold phase. The opening phase of such a drive signal typically has a higher current than the hold phase for a given injection to increase the opening speed. However, such a drive signal may cause the injector to close prematurely, as shown on figure 9.

[0117] Advantageously, early closure of the injector may be detected by comparing the switching period mapping and / or the delta duration mapping to an early closure threshold. As shown on figure 10 and 11 , early closure of the injector during the hold phase causes a noticeable spike in the switching period mapping and the delta duration mapping. Hence, if the switching period mapping and / or the delta duration mapping is greater than the early closure threshold during the hold phase, a flag indicating that the injector closed prematurely may be raised.

Claims

P-DELPHI-446 / WO 18Claims1 . A method of operating fuel injection in an internal combustion engine comprising a fuel delivery system with at least one fuel injector, said fuel injector comprising a solenoid actuator configured to selectively operate a valve member to control a fuel outlet orifice, wherein injection events are operated by applying a drive signal to the fuel injector to inject fuel into a cylinder of the engine, whereby the drive signal is applied by a drive module and includes an opening phase whereby a boost voltage is applied to the fuel injector to regulate an opening current at a predetermined level, and regulating said opening current is done by repeatedly switching said boost voltage between an ON state and an OFF state, such as to obtain an oscillating current, each oscillation with an increasing interval and a decreasing interval, the method implementing an opening time determination routine comprising the steps of: a) within an analysis window comprising a plurality of current oscillations of an opening phase corresponding to an injection event operated for a respective fuel injector, evaluating for each oscillation the difference between a duration for its increasing interval and a duration for its decreasing interval, thereby obtaining a delta duration mapping; b) determining a minimum timepoint at which the delta duration mapping reaches a local extremum or which corresponds to an extremum of the derivative of the delta duration mapping; c) determining an opening time from the minimum timepoint; d) controlling fuel injection using the determined opening time for the respective fuel injector.

2. The method according to claim 1 , wherein the difference between a duration for its decreasing interval and a duration for its increasing interval is evaluated by:P-DELPHI-446 / WO 19 determining a duration for each of the decreasing intervals of the current signal;• determining a duration for each of the increasing intervals of the current signal;• subtracting the duration of each of the increasing interval from the duration of one of its consecutive decreasing interval, thereby obtaining a delta duration mapping.

3. The method according to any of the preceding claims, wherein said minimum timepoint is determined as a timepoint of a local minimum that immediately precedes a steep variation in values of the delta duration mapping.

4. The method according to claim 3, wherein following step b), an assessment is performed to identify whether the minimum timepoint precedes a monotonous region of the delta duration mapping, and whereby step b) and the assessment are repeated for another local minimum if the minimum timepoint does not precede a monotonous region.

5. The method according to the preceding claim, wherein the assessment concludes that a minimum timepoint precedes a monotonous region if the delta duration mapping is monotonous for a predetermined duration following said minimum timepoint.

6. The method according to any of claims 1 to 4, wherein the assessment concludes that a minimum timepoint precedes a monotonous region if the delta duration mapping exceeds a predetermined value / threshold following said minimum timepoint.

7. The method according to any of the preceding claims, wherein the observation window has a lower bound that is calibrated to begin at a predetermined timing following the start of the opening phase, or set to begin after the second or third oscillation of the opening current.P-DELPHI-446 / WO 208. The method according to any of the preceding claims, wherein the observation window has an upper bound that is calibrated to end at the latest with the opening phase.

9. The method according to any of the preceding claims, wherein increasing and decreasing intervals are determined based on voltage switching timings operated by the drive module.

10. The method according to claim 9, wherein the drive module includes a state machine that generates a temporal signal indicating a current state of the boost voltage, and whereby the duration for each of the decreasing intervals of the current signal is determined as the duration of a respective OFF states.11 . The method according to any of the claims 9 and 10, wherein the drive module includes a state machine that generates a temporal signal indicating a current state of the boost voltage, and whereby the duration for each of the increasing intervals of the current signal is determined as the duration of the respective ON state.

12. The method according to any of the preceding claims, wherein the opening time is determined from the minimum timepoint by adding a time offset to the minimum timepoint, preferably whereby the time offset is negative or null.

13. The method according to any of the preceding claims, wherein step a) is repeated for a plurality of injection event for the same injector; the corresponding values of the delta duration mapping for each injection event are resampled on a fixed time-step, and a delta duration mapping is obtained as a mean delta duration mapping.

14. The method according to any of the preceding claims, wherein a flag indicating that the injector failed to open where no minimum timepoint can be identified, respectively if the derivative of the delta duration mapping is null over most of the observation window.

15. The method according to any of the preceding claims, further comprising an injector diagnostic routine comprising the steps of:P-DELPHI-446 / WO 21 i) for each current oscillation within a further analysis window, evaluating the sum of the duration for its increasing interval and the duration for its decreasing interval, thereby obtaining a switching period mapping; ii) comparing the switching period mapping to at least one predetermined diagnostic threshold; iii) identifying and registering abnormal behavior based on the result of the comparison.

16. The method according to claim 15, wherein the at least one predetermined diagnostic threshold includes an opening failure threshold, and whereby a flag indicating that the injector failed to open is raised if the switching period mapping is greater than the opening failure threshold.

17. The method according to claim 15 or 16, whereby the at least one predetermined diagnostic threshold includes a closure failure threshold, and whereby a flag indicating that the injector failed to close is raised if the switching period mapping is lower than the closure failure threshold.

18. The method according to any of claims 15 to 17, whereby the at least one predetermined diagnostic threshold is calibrated based on at least one of current level, drive voltage level, and injector electrical resistance.

19. The method according to any of claims 15 to 18, whereby the drive signal comprises an opening phase and a hold phase, wherein the hold phase occurs after the opening phase and is operated at a lower current than the opening phase for a given injection, whereby the at least one predetermined diagnostic threshold includes an early closure threshold, and whereby a flag indicating that the injector closed early is raised if the switching period mapping is greater than the early closure threshold during the hold phase.P-DELPHI-446 / WO 2220. A computer program product comprising instructions which, when the program is executed by a control unit comprising a processor, cause the control unit to carry out the method according to any of the preceding claims.

21. An internal combustion engine comprising a fuel delivery system with at least one fuel injector, said fuel injector comprising a solenoid actuator configured to selectively operate a valve member to control a fuel outlet orifice; and a control unit configured to implement a method according to any one of claims 1 to 19.

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