Method for controlling fuel injection

Abstract

A method of controlling fuel injection in an internal combustion engine with fuel injectors (2, Fig 1) comprises the steps of; for a given demand fuel quantity and rail pressure, performing an injecti

Description

Technical field The present invention generally relates to a method for controlling a fuel injector. Background Art The contemporary design of internal combustion engines must cope with the increasingly stringent regulations on pollutant emissions. Accordingly, automotive engineers strive for designing engines with low fuel consumption and low emissions of pollutants, which implies including electronic devices capable of monitoring the combustion performance and emissions in the exhaust gases. A proper operation of a combustion engine requires that its fuel injectors and their controller enable timely and precise fuel injections. Indeed, it is well known that problems arise when the performance of an injector, i.e. its precision in timing and quantity of fuel delivered, diverges beyond acceptable limits. For example, inconsistent injector performance will cause different torques to be generated between cylinders due to unequal fuel quantities being injected, or improper injection timing. As it is known, fuel injectors are typically controlled by generating pulses which are sent to the actuators of the fuel injectors. The amount of fuel injected depends on the length of the pulse sent to the actuator. Typically, an Engine Control Unit computes the pulse width required for a demand quantity of fuel to be injected. The demand quantity of fuel is itself typically stored in a map against engine speed and torque demand. Characteristics of fuel injectors may differ between injectors, and may vary over their lifetime, e.g. as a result of wear. It is thus important to periodically calibrate injection systems so that these variations are taken into consideration. Techniques are known which apply learning strategies, whereby injector characteristics are determined, and the injectors are consequently appropriately controlled. A characteristic of interest in this context is the injected fuel quantity. The injected fuel quantity of an injector can be determined by performing a so-called Pressure Drop Analysis (PDA), as disclosed by GB 2533104 A and GB 2610600 A. PDA strategies typically monitor the pressure within the fuel rail (common rail / accumulator), evaluate the pressure drop caused by an injection event, and then compute the corresponding injected fuel quantity. This value will herein be referred as computed injected fuel quantity, or computed quantity for short. The computed quantity of an injector may differ from the demand quantity of fuel to be injected. To prevent this, a fuel injector compensation is computed to correct the quantities of fuel injected by said injector. This fuel injector compensation is typically calculated as the difference between the computed quantity of the injector and the average value (or mean value) of the computed quantities of all injectors. Unfortunately, the inventors have found that this method may be unreliable in some specific cases, as will be detailed below. Technical problem It is an object of the present invention to provide a method for controlling fuel injectors without the aforementioned drawbacks This object is achieved by a method for controlling fuel injectors as claimed in claim 1. General Description of the Invention In order to achieve the aforementioned object, the present invention provides a method for controlling fuel injection in an internal combustion engine comprising a plurality of fuel injectors arranged to inject fuel in respective cylinders. The method comprises the steps of, for a given demand fuel quantity and rail pressure, operating each injector to perform an injection event and determining a corresponding, actual injected fuel quantity for each injector, processing the actual injected fuel quantities to bind them to a lower bound and / or an upper bound, thereby obtaining bound quantities, computing an average injected value of the bound quantities, computing for each injector a fuel compensation based on a difference and / or a ratio between the actual injected fuel quantity and the average injected value of the bound quantities, and controlling the fuel injectors based on their respective fuel compensations. In the inventive method, the fuel compensations are thus computed from the actual injected fuel quantity and the average injected value. Advantageously, this average injected value is computed from bound quantities of injected fuel. The contributions of injectors with large error in injected fuel quantity, which require large fuel compensation values, towards this average injected value is thus limited. Therefore, the inventive method prevents injectors with large error from overly affecting the compensation values of other injectors. In embodiments, the method further comprises the step of computing an average value of the actual injected fuel quantities, and the lower bound and / or the upper bound are computed based on said average value of the actual injected fuel quantities. In embodiments, the lower bound and / or the upper bound are computed based on a product between the average value of the actual injected fuel quantities and a constant. In embodiments, said constant is comprised between 0.9 and 1.0 for the lower bound and between 1.0 and 1.10 for the upper bound. In embodiments, the actual injected fuel quantities are determined based on the pressure drop occurring in a fuel rail during the injection event. In embodiments, processing the actual injected fuel quantities comprises clamping the actual injected fuel quantities between the lower bound and the upper bound to obtain the bound quantities. In embodiments, the step of controlling the fuel injectors based on the computed fuel injector compensations is performed by an Engine Control Unit. In embodiments, the method further comprises the step mapping the fuel compensation against the given rail pressure and the demand fuel quantity, said mapping being stored in a memory of a processing unit. In embodiments, a fuel compensation is mapped against a rail pressure and a demand fuel quantity for at least two values of rail pressure. The method hence provides an inventive learning strategy, which may be implemented in known methods for controlling fuel injection. These methods typically implement a fuel injection strategy whereby injector events are performed to inject a fuel quantity that is a compensated (i.e. compensated fuel quantity). The compensated fuel quantity is computed from the demand fuel quantity and the injector specific fuel compensation (Ci). As is known in the art, the demand fuel quantity qdis typically read from a reference mapping depending on Torque demand (based on accelerator pedal and / or other torque requests). The duration of the injector actuation (pulse width) is then determined based on demand fuel quantity qd, the current rail pressure and the injector specific compensation Ci. The invention further provides a processing unit comprising instruction which, when executed, carry out the method according to any of the preceding claims. The invention further provides a fuel delivery system comprising a plurality of injector fluidly coupled to a common fuel rail and further comprising an engine control unit, wherein the common fuel rail comprises a pressure sensor configured to monitor the pressure therein, and wherein the engine control unit is configured to carry out the method according to any of the preceding claims. Brief Description of the Drawings A preferred embodiment will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a principle diagram of a known liquid fuel delivery system; Fig. 2 is a flowchart representing the steps of the inventive method; Fig. 3 is a graphical representation of the actual injected fuel quantities for a combustion engine having 6 fuel injectors; Fig. 4 is a graphical representation of the fuel injector compensations for the combustion engine of figure 3. Description of Preferred Embodiments Figure 1 is a principle diagram of a known liquid fuel delivery system as commonly used in combustion engines. It comprises a common fuel rail 1 (or accumulator) fluidly connecting the fuel therein to a plurality of injectors 2 (e.g. of solenoid actuated fuel injectors). The circuit typically includes an in-tank electrical fuel pump 3, a fuel filter 4, and a high-pressure pump 5. A high-pressure sensor 6 is located on the common rail 1 as shown in order to measure the fuel pressure inside the common rail 1. A high-pressure valve 8 is provided on the common rail 1, which is a safety valve that opens when the pressure exceeds a preset value (typically passive in gasoline engines but can be controlled, e.g. in diesel systems). Reference sign 7 indicates an optional backleak circuit including a backleak regulator and injector return line, which is typically present in diesel fuel delivery systems. An Engine Control Unit (ECU) is connected to the plurality of injector 2 and is configured to compute the pulse width required for a demand quantity of fuel to be injected. The ECU is further configured to receive data from the high-pressure sensor 6 and control operation of the fuel injectors 2 accordingly. Gaseous fuel delivery systems (not represented) are mostly similar to liquid fuel delivery system, but components upstream of the fuel rail may differ. More specifically, gaseous fuel delivery systems may comprise a pressurized fuel tank able to endure internal pressures of up to 350 or 700 bars, as well as pressure regulating means between said fuel tank and the fuel rail to decrease the fuel flow pressure to a predetermined value, e.g. 5 to 50 bar, prior to entry in the fuel rail. The inventive method for controlling fuel injectors is applicable to both gaseous and liquid fuel delivery systems, which may operate at pressures ranging from 100 bar to a few thousand bar. Gaseous fuels include for example hydrogen and CNG. Liquid fuels include gasoline, diesel, biofuels, synthetic fuels, etc. Hence, as used herein, the term fuel refers to gaseous fuel and liquid fuel, depending on the application. The invention finds particular application in a method for controlling fuel injection, where fuel compensations q are calculated for each fuel injector. In such control strategy, a given demand fuel quantity qd is first determined for a given operating point (namely based on torque demand - as is generally known), and is subsequently adjusted by an injector specific value known as fuel compensation q. Typically, the operating point is defined at least by the demand fuel quantity qd and by the fuel rail pressure. Accordingly, a mapping is used that relates the fuel compensation q to the demand fuel quantity qd and, optionally, to the fuel rail pressure (i.e. to an operating point). It may be noted that for diesel engines the compensations will typically be learned for a several rail pressures. For gasoline engines, learning at a single rail pressure may be sufficient, but could be done for more as well. According to the invention, a learning routine is carried out to learn a fuel compensation ct for each fuel injector for a given rail pressure and a demand fuel quantity qd. Said fuel compensations ct may then be used to control the fuel injectors during operation of the vehicle, e.g. by determining a corresponding compensated fuel quantity qc i to inject, as will be explained in detail below. Figure 2 is a flowchart representing the different steps of said learning routine. In an initial step SO, an injection event is performed for each injector for a given operating point, i.e. at a given rail pressure and for a demand fuel quantity qd. This demand fuel quantity qd is typically derived from a reference mapping and represents the fuel amount normally required to achieve the desired torque without accounting for injector variability (i.e. uncompensated). An injection event is generally defined as an event during which an injector 2 of the fuel delivery system performs a fuel injection. An injection pulse width is determined based on a demand fuel quantity qd, the current rail pressure and the fuel compensation q of the corresponding injector. The injection pulse width is subsequently used to generate a pulse sent to the injector. The pulse width determines the open time of the injector and thus the actual fuel quantity qt injected. In step S1, an injected fuel quantity qt representing the actual injected fuel quantity is computed for each injector 2 of the fuel delivery system for the operating point. Said injected quantities qt are typically computed using pressure drop analysis, as described in patent applications GB 2533104 A and GB 2610600 A. Such pressure drop analyses are not the focus of the present invention and with therefore not be further described. The actual injected fuel quantity qt for the given operating point is thus learned for each fuel injector. This learning can be done in a same engine cycle or different ones. The average value a± of the actual injected fuel quantities is subsequently computed (step S2) using the following formula: Where qt is the actual injected fuel quantity for the injector of index i, and k is the maximum injector index (here 5). k + 1 thus corresponds to the total number of injectors. In prior art methods, the fuel injector compensation of a given injector is typically calculated as the difference between its computed injected fuel quantity qt and the average value ai of the actual injected fuel quantities. However, the inventors have found that if an injector diverges more than the others, its contribution towards the average value ai may be too important, thereby excessively affecting the fuel compensations of the other injectors. The inventive method however does not present this drawback, as will be clear below. In the following step S3, a lower bound and an upper bound t2 are then computed from the average value ai using the following relationship: ti = ar * (1 - A); t2 = ar * (1 + / 2) Where / i and / 2 are positive constants. In preferred, non-limitative embodiments, and f2 are equal. The actual injected fuel quantities qt are then bound / clamped (step S4) between ti and t2. This operation may be expressed by the following formula: qt = max(min(Qp The average value a2 of the bound quantities q^' is then computed (step s5) using the relationship: Fuel injector compensations ct are then computed (step s6) and updated for each injector by adding the difference between the actual injected fuel quantities qt and the average value a2 of the bound quantity q^ to the previous value of the compensation q: Ci = Ci + (¾ _ Qi) where the default value of ct is 0. The fuel injectors may finally be controlled (step s7) by the ECU using the computed fuel injector compensations q. More specifically, for a demand fuel quantity qd, a compensated fuel quantity qci may be computed for each injector from the sum qd + q. Corresponding pulse widths for each injector may then be computed from the compensated fuel quantity qc i and the pressure in the fuel rail. Alternatively, the fuel injector compensations ct may be computed from a ratio between the actual injected fuel quantities qt and the average value a2 of the bound quantity q^', i.e. from c; = c;* a^lq^ where the default value of q is 1. In this case the compensated fuel quantity qci may be computed for each injector from the product qd * c^. Once the fuel injector compensation has been computed, it may be stored in a mapping of fuel injector compensation against rail pressure and demand fuel quantity. If a fuel injector compensation value is unknown for a given rail pressure and demand injection quantity, said unknown fuel injector compensation may be determined from known fuel injector compensation values for other rail pressures and demand injection quantities, e.g. by interpolation or extrapolation. Hence, in the method according to the invention, the fuel injector compensation of a given injector is calculated from the difference or the ratio between its computed injected fuel quantity qt and the average value a2 of the bound quantities qt'. Using bound quantities q^' as opposed to the actual injected fuel quantities qt limits the contribution of injectors diverging beyond the upper / lower to the average value a2 of the bound quantities qt'. The inventive method therefore ensures that no fuel injector is able to excessively affect the fuel compensations of the other injectors, thereby resulting in more accurate injection quantities. It is noted that whilst the above steps were described separately, they may be simultaneously performed as a single equivalent mathematical operation. For example, steps 2 to 6 may be simultaneously performed using the following operation: A max(min (q^ (1 - A) * ^=0 + &* ^=° k~^ ^n^n) — — Qn T / , k + 1. i=0 It is also noted that the scope of the invention is not strictly restricted to the formulae described above, and that mathematically equivalent operations may be used instead. Numerical example: An application of the inventive method will be described below using purely exemplary, non-limitative values. Figures 3 and 4 show graphical representations of the different quantities obtained at different stages of the inventive method. Considering a combustion engine with 6 injectors, for a demand injection of 70 mg and a rail pressure of 2000 bars: - The actual injected fuel quantities are first computed / determined using PDA analysis. The actual injected fuel quantities are represented on figure 3 and 4 (reference signs are omitted on figure 4 for the sake of clarity), with q3=87.1, qi=72.8, qs=69.5, q2=66.2, q4=70.1, and qo=69.8 (all units are in mg). - The average value for the above injected fuel quantities is computed: k v 87.1 + 72.8 + 69.5 + 66.2 + 70.1 + 69.8 Qi = >=---------------------------------= 72.58 1 Z-ik + 1 6 i=0 - The lower bound ti and the upper bound t2 are computed: A = * (1 - A) = 72.58 * (1 - 0.05) = 68.95 t2 = * (1 + A) = 72.58 * (1 + 0.05) = 76.20 With A = A = 0.05 - The actual injected fuel quantities qi are bound / clamped between the lower ti and the upper t2bound. Note that in this example, only qsand q2are outside of the interval [ti, t2]. Hence q3,=76.20, qi’=72.8, q5,=69.5, q2,=68.95, q4,=70.1, and qo’=69.8 - The average value a2 of the bound quantity qi’ is computed: k Qi' 76.20 + 72.8 + 69.5 + 68.95 + 70.1 + 69.8 - The fuel injector compensation is computed using cn = a2 - qn, resulting in ¢3=-15.88, Ci=-1 .58, ¢5=1.72, ¢2=2.27, ¢4=1.12, and ¢0=1.42. - Ea¢h fuel injeetor may then be eontrolled using its respeetive eompensation: For example, if an injeetion event for a demand injeetion of 70 mg and a rail pressure of 2000 bars is requested, oompensated fuel quantity qc i may be oomputed for eaoh injector using qci = qd + ct, resulting in qc3 = 54.12, qcl = 68.42, qc5 = 71.72, qc2 = 72.27, qc4 = 71.12, and qcQ = 71.42.

Claims

1. A method for controlling fuel injection in an internal combustion engine comprising a plurality of fuel injectors (2) arranged to inject fuel in respective cylinders, the method comprising the steps of:for a given demand fuel quantity (qd) and rail pressure, operating each injector to perform an injection event and determining a corresponding, actual injected fuel quantity (qi) for each injector;processing the actual injected fuel quantities (qi) to bind them to a lower bound (ti) and / or an upper bound (t2), thereby obtaining bound quantities (qi’);computing an average injected value (a2) of the bound quantities (qi’);computing for each injector a fuel compensation (Cj) based on a difference and / or a ratio between the actual injected fuel quantity (qi) and the average injected value (a2) of the bound quantities (qi’);controlling the fuel injectors based on their respective fuel compensations (ci).

2. Method according to claim 1, further comprising the step of computing an average value (ai) of the actual injected fuel quantities (qi), and wherein the lower bound (ti) and / or the upper bound (t2) are computed based on said average value (ai) of the actual injected fuel quantities (qi).

3. Method according to claim 2, wherein the lower bound (ti) and / or the upper bound (t2) are computed based on a product between the average value (ai) of the actual injected fuel quantities (qi) and a constant.

4. Method according to claim 3, wherein said constant is comprised between 0.9 and 1.0 for the lower bound (ti) and between 1.0 and 1.10 for the upper bound (t2).

5. Method according to any of the preceding claims, wherein the actual injected fuel quantities (qi) are determined based on the pressure drop occurring in a fuel rail during the injection event.

6. Method according to any of the preceding claims, wherein processing the actual injected fuel quantities (qi) comprises clamping the actual injected fuel quantities(qi) between the lower bound (ti) and the upper bound (t2) to obtain the bound quantities (qi’).

7. Method according to any of the preceding claims, wherein the step of controlling the fuel injectors based on the computed fuel injector compensations (ci) is performed by an Engine Control Unit.

8. Method according to any of the preceding claims, the method further comprising the step of mapping the fuel compensation (ci) against the given rail pressure and demand fuel quantity (qu), said mapping being stored in a memory of a processing unit.

9. Method according to claim 8, wherein a fuel compensation (q) is mapped against a rail pressure and a demand fuel quantity (qd) for at least two values of rail pressure.

10. Method according to any of the preceding claims, wherein controlling the fuel injectors based on their respective fuel compensations (q) comprises performing injection events to inject a compensated fuel quantity computed from the demand fuel quantity and the injector specific fuel compensation (q).

11. Method according to claim 10, wherein the compensated fuel quantity is computed based on a sum and / or a product of the demand fuel quantity (qd) and associated fuel compensations (q / 12. Processing unit comprising instruction which, when executed, carry out the method according to any of the preceding claims.

13. Fuel delivery system comprising a plurality of injector fluidly coupled to a common fuel rail and further comprising an engine control unit, wherein the common fuel rail comprises a pressure sensor configured to monitor the pressure therein, and wherein the engine control unit is configured to carry out the method according to any of the preceding claims.

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