Method of controlling fuel gas injection in an internal combustion engine

Abstract

The invention relates to a method of controlling fuel injection in a fuel gas powered internal combustion engine, wherein injection event are performed by applying a drive signal to the fuel injector, which as has a profile including: a first phase, wherein a first voltage V1 is applied to the fuel injector to regulate a drive current at a first level I1 during a first period T1, wherein the first voltage V1 is a boost voltage and the first level I1 corresponds to an opening current; a second phase, wherein the current is decreased to second level I2 about zero during a time period T2, in order to decelerate the valve member in its opening stroke; a third phase, wherein a third voltage V3 is applied to the fuel injector to regulate a drive current at a third level I3 during a third period T3, said third phase being configured to deliver sufficient energy to open and stabilize the valve member in its open position, the third level I3 representing at least 80% of the first level I1; a fourth phase, wherein a fourth voltage V4 is applied to the fuel injector to regulate a drive current at a fourth level I4 during a fourth period T4, wherein the fourth level I4 corresponds to a hold current and V4 is a battery voltage. The invention is of particular application to outwardly opening fuel injectors in hydrogen internal combustion engines.

Description

[0001] METHOD OF CONTROLLING FUEL GAS INJECTION IN AN INTERNAL COMBUSTION ENGINE

[0002] Technical field

[0003] The present invention generally relates to the control of fuel injectors in internal combustion engines operating with gaseous fuel.

[0004] Background Art

[0005] With the increasing demand to reduce CO2 emissions from road vehicles, alternatives to traditional internal combustion engines like Diesel or gasoline engines have been developed. One option are electric vehicles, mostly powered by Li-ion batteries or by fuel cells, in which the chemical energy of hydrogen and oxygen is converted into electric energy. Another option is hydrogen powered internal combustion engine (H2- 1CE) vehicles. These represent, in principle, a fast route to eliminate CO2 from powertrains, since the H2-1CE is basically a modification of a conventional diesel or gasoline engine.

[0006] As for conventional engines, hydrogen injection is performed by fuel injectors that may be arranged for direct injection or indirect (port fuel) injection.

[0007] Modern direct fuel injectors for hydrogen are designed as outwardly opening injectors. Such an injector relies on a solenoid actuator where an injector solenoid coil is energized and an armature moves a pintle to release / open an outlet orifice (in flow direction) at the injector tip. Hence, it enables the discharge of gaseous fuel into the cylinder. When the solenoid is de-energized, the magnetic force on the armature is suppressed the pintle is urged back into the closed position by a return spring.

[0008] Injection events are piloted by the Engine Control Unit (ECU) based on predetermined combustion schemes. In general, an injection event is performed by applying a drive signal to the fuel injector (specifically to the solenoid) in order to open the needle for a given time period to discharge a predetermined quantity of gas, which corresponds to a given torque demand. Therefore, a drive signal basically comprises an opening phase operated with current level sufficient to open the injector and bring it into its open position, and is followed by a hold phase, under reduced current, sufficient to maintain the pintle in open position.

[0009] As will be known in the art, the drive schemes are generally adapted based on performance criteria such as minimum injection quantity (Q_min), part-to-part injection variation, robustness against operating conditions and ageing, noise and other criteria.

[0010] Furthermore, the opening and closing events of the injector pintle are responsible of a wear phenomenon, as the pintle hits an upper stop at opening and the valve seat at closing.

[0011] In order to address the wear caused by the impact force of the pintle at injector opening, it is known in conventional engines to operate injection events with a drive signal as illustrated in Fig.9. The drive signal comprises a plurality of phases: a first phase under a peak current; a second, cut-off phase (fast transition time) where the current is reduced to zero before the pintle reaches the its opening stop; followed by a current ramp (bypass time) leading to a hold phase with a hold current to keep the injector open.

[0012] It should be noted that the wear phenomenon is exacerbated in gas combustion engines due to poor lubrication provided by the fuel, in particular hydrogen. During actuation, surfaces of pintle / armature contact each other at certain speeds. Impact / contact speeds result from hydraulic forces, magnetic forces and spring forces acting on the pintle / armature assembly. Design parameters such as spring settings, sealing surface area or coil dimensions are such as to achieve primary fueling function and the consequential impact speeds are consequently dictated by the injector's primary function.

[0013] In particular, the return spring imposes a substantial biasing force to the pintle - greater than in typical gasoline injectors - to ensure a gas-tight seal, hence with virtual no hydraulic damping and leading to several rebounds.

[0014] Injector control is thus more difficult than in gasoline engines. The strong spring force on the pintle may, due to part to part variations, affect the injector opening behavior. The injector needs higher energy to open and is thus qualified as slow injector compared to a nominal injector (i.e. greater opening delay than a nominal injector). In the case of such slow injector, there is a risk that the drive signal shown in Fig 8 cannot open the slow fuel injector (see displacement line 9).

[0015] Technical problem

[0016] It is an object of the present invention to provide a more robust method of controlling fuel injection in an H2-ICE, which is able to reduce injector wear.

[0017] General Description of the Invention

[0018] The present invention relates to a method of controlling fuel injection in a fuel gas powered internal combustion engine as claimed in claim 1. The engine comprises at least one engine cylinder with an associated fuel injector arranged to inject fuel gas in said cylinder, the fuel injector comprising a solenoid actuator configured to selectively operate a valve member to control an outlet orifice. The method comprises the step of performing an injection event by applying a drive signal to the fuel injector, wherein the drive signal has a profile including:

[0019] - a first phase, wherein a first voltage VI is applied to the fuel injector to regulate a drive current at a first level II during a first period Tl, wherein the first voltage VI is a boost voltage and the first level II corresponds to an opening current;

[0020] - a second phase, wherein the current is decreased to second level 12 about zero during a time period T2, in order to decelerate the valve member in its opening stroke;

[0021] - a third phase, wherein a third voltage V3 is applied to the fuel injector to regulate a drive current at a third level 13 during a third period T3, said third phase being configured to deliver sufficient energy to open and stabilize the valve member in its open position, the third current level 13 being no less than 80% of the first current level II;

[0022] - a fourth phase, wherein a fourth voltage V4 is applied to the fuel injector to regulate a drive current at a fourth level 14 during a fourth period T4, wherein the fourth level 14 corresponds to a hold current and V4 is a battery voltage. The inventive method proposes an injector drive signal where current is regulated according to predetermined phases. Injector wear is reduced through the use of a current cut-off phase (second phase) that reduces the speed of the valve member and hence its impact speed. The third phase is designed to deliver sufficient energy to open and stabilize the valve member in its open position. Hence, the third phase is a strong phase that is by design, able to open the injector, even in case of a ‘slow’ injector. Accordingly, the invention proposes a more robust method of controlling fuel injection, in particular of the injector opening phase.

[0023] The present invention has been developed to address the specific requirements of hydrogen combustion engines, where injector wear is a concern due to the particularly dry nature of hydrogen gas. Nevertheless, the invention is operable with other gaseous fuels. As used herein, the terms fuel, fuel gas and gaseous fuels are used as synonyms and designate fuel in gaseous state supplied to the fuel injectors.

[0024] In general, the various phases are determined by calibration. The calibration is preferably performed for a nominal injector. In general, the phases are operated one after another without delay or wait time, except due to electronics switching. In embodiments, the method consists of these four phases.

[0025] The term "nominal injector" refers to an injector that serves as a typical or standard representation for a group or population of injectors sharing the same design characteristics (given make / model). In other words, it is a representative sample chosen to reflect the expected behavior of the entire group. A nominal injector hence exhibits as-expected performance, particularly in terms of opening / closing behavior and flow characteristic that are related to the design parameters.

[0026] In contrast, a slow injector, despite sharing the same design as a nominal injector, deviates from nominal operation, particularly in terms of its opening behavior. Referred to as having a long opening delay in the field, a slow injector takes a longer time to open compared to the nominal injector.

[0027] The third current level 13 is no less than 80% of the first current level 11, and may be defined as 0.8*11 < 13 < 11.

[0028] 11 and 13 are high current levels that are suitable to operate injector opening. 11 may for example be at least 2 times 14, or at least 3 times 14. In embodiments, the first voltage VI is at least twice the fourth voltage V4, or VI is at least the triple of voltage Vbatt. Advantageously, the third phase is configured to allow input sufficient energy to operate full opening of a slow injector, even in case the valve member may return to the closed position due to the cut-off phase.

[0029] For improved robustness, in particular where 13 is less than II, the duration of the third period T3 may be greater than that of first period Tl. For example, T3 may represent between 1.1 and 2 times Tl, where the interval includes all intervening decimals between endpoints, in particular 1.2, 1.3, 1.4 and 1.5.

[0030] In embodiments, the currents II and 13 are regulated with a current ramp of at least 50 A / ms. This again is favorable to robustness and dynamics, and facilitates opening of slow injectors.

[0031] In embodiments, in phase 1 and / or in phase 3 the drive current is applied with an increase rate configured to generate an opening force on the valve member with a force increase rate of at least 0.1 N / ps.

[0032] For improved dynamics, in the second phase, a second voltage V2 is applied to the injector, which is the inverse of the first voltage VI.

[0033] Referring more particularly to current cut off phase, it is advantageously designed to cause a reduction of speed of the valve member in its opening stroke by 40 to 70%. The speed reduction is to be understood with respect to the maximum speed reached by the valve member, in a nominal injector, during an opening phase operated at current Il during the full opening stroke, i.e. without current cut off phase.

[0034] The current cut off phase is preferably configured to end at a timing that substantially matches the moment the valve member reaches the fully open position. Stated otherwise, the current cut off phase is configured to end at a timing that substantially coincides with the fully open injector configuration. In practice, it is acceptable that the current cut off phase ends within ± 10 ps, preferably within ± 5 ps of the moment when the valve member reaches its fully open position.

[0035] In embodiments, an electronic drive module is configured to selectively control the voltage and current levels applied to the fuel injector, wherein the electronic drive module comprises a boost voltage circuit associated with a capacitor used to apply voltage VI and voltage V3, wherein during phase P2 the injector is connected to said capacitor to be filled with energy stored in the injector solenoid.

[0036] In embodiments, the fuel injector is configured such that, when the solenoid actuator is energized, it moves the valve member - in fuel flow direction - in the opening position, while a return spring urges the valve member back into the closed position.

[0037] According to another aspect, the invention concerns a gaseous fuel delivery system of an internal combustion engine comprising a pressurized fuel tank connected to a fuel rail, and a plurality of fuel injectors coupled to the fuel rail, and an electronic drive module. The electronic drive module comprises drive circuits to apply voltage and current to the fuel gas injectors configured to implement the method herein disclosed.

[0038] Brief Description of the Drawings

[0039] Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings, wherein:

[0040] Fig. 1 shows a) a graph illustrating injector current and valve member displacement vs. time; and b) the corresponding injector voltage vs. time, in a first embodiment of the present method;

[0041] Fig. 2 to 4 are graphs illustrating injector current and valve member displacement vs. time, in further embodiments;

[0042] Fig. 5 is a diagram of a gaseous fuel delivery system implementing the present method;

[0043] Figs. 6 and 7 are graphs showing the impact speed and fuel delivery plotted in function of T1 (duration of phase Pl), for several values T2 (duration of cut-off phase P2);

[0044] Fig. 8 is a graph illustrating injector current and valve member displacement vs. time according to a prior art injector control strategy.

[0045] Description of Preferred Embodiments

[0046] The present invention relates to injection control in spark ignited internal combustion engines operating with gaseous fuel (or fuel gas), 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). Fig. 5 represents a gaseous fuel delivery system 10 comprising a pressurized fuel tank 12 for the gaseous fuel, 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 gas 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 gas at pressures up to 700 bar. The gas tank may typically include a shut off 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 shut off valve and an electronic pressure regulator to regulate the pressure in the fuel rail 14 within a working range between 5 and 40 bar. Shut-off valve 20 and a 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...

[0047] 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 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 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 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.

[0048] 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. During its opening stroke, the pintle reaches a certain speed and hence contacts its stop surface (armature hitting pole piece) with a certain impact speed.

[0049] 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.

[0050] Injection of fuel in the associated cylinders is performed during an injection event, by applying a drive signal 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 referred to as pulse with, PW. To perform an injection event, a drive pulse is applied during a time period PW.

[0051] 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 ECM in turn includes electronic circuits and settings configured to apply to the injectors a drive signal 30 to operate the injection event during PW, namely by applying a control voltage to the respective injector (solenoid) and supplying an electric current to the injector.

[0052] In the method according to the present invention, the drive signal consists of a sequence of phases illustrated in Fig.l, which corresponds to a first embodiment. Graph a) in the upper part shows the solenoid current and pintle displacement, whereas graph b) illustrate the corresponding drive voltage applied to the solenoid.

[0053] As it will be understood by those skilled in the art, the current through the solenoid actuator is controlled, i.e. regulated by applying voltage modulation to the solenoid actuator. Referring to Fig. 1 a), the drive signal is characterized by a predetermined current profile in a plurality of phases:

[0054] - a firstphase Pl, wherein a first voltage VI is applied to the fuel injector to regulate a drive current at a first level 11 during a first period Tl, wherein the first voltage VI is a boost voltage and the first level 11 corresponds to an opening current. Current 11 has an intensity that is typically, by itself, sufficient to enable valve member lifting towards its fully open position.

[0055] - a second phase P2, wherein the current is decreased to a second level 12 about zero during a time period T2, in order to decelerate the valve member in its opening stroke. About zero here means zero or close to zero (e.g. no more than 0.5 or 0.3 A).

[0056] - a third phase P3, wherein a third voltage V3 is applied to the fuel injector to regulate a drive current at a third level 13 during a third period T3, said third phase being configured to deliver sufficient energy to open and stabilize the valve member in its open position.

[0057] - a fourth phase P4, wherein a fourth voltage V4 is applied to the fuel injector to regulate a drive current at a fourth level 14 during a fourth period T4, wherein the fourth level 14 corresponds to a hold current and V4 is a battery voltage.

[0058] In the first phase, the current is regulated to a value 11, which corresponds to a typical high current adapted to operate injector full opening. Typically, this is achieved by the solenoid controllers setting in ECM via current regulation between two predetermined levels -peak and trough- to achieve a mean targeted current value. The voltage, here VI during phase Pl, is applied to the solenoid until the desired peak current level (peak) is achieved. Once this is achieved the ECM 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 VI. 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 11, as well as 13 and 14, is thus considered as an average current value of the current chopping.

[0059] So, in phase Pl, current is regulated to level 11, which is a high level, by chopping modulation driven by voltage VI. Voltage VI is a boost voltage Vboost, i.e. it is an amplified voltage that is greater than the battery voltage Vbatt. VI may be at least double (or triple) the voltage Vbatt.

[0060] In phase P2, the current is decreased to a second level 12 by applying a second voltage V2, which is the inverse of VI. Hence, we have V2 = -VI = -Vboost. The current 12 is zero or close to zero, i.e. a current that does not permit to operate pintle opening nor maintain the pintle open.

[0061] Cutting off the current will suppress the magnetic force and hence decelerate moving pintle in its opening stroke, to reduce the valve member (pintle / armature) impact at the end of opening stroke. Phase P2 is configured to reduce the speed of the valve member during opening, and hence reduce its impact speed. Phase P2 is discussed in detail below.

[0062] In phase P3, the current is regulated to a high level again, by applying a third voltage V3 to the fuel injector to regulate a drive current at a third level 13 during a third period T3. This third phase is configured to deliver sufficient energy to ensure full opening of the valve and stabilize the valve member in its open position.

[0063] For this purpose, it is preferred that the current level 13 is close to the peak current used for phase Pl. In particular, 13 represents no less than 80% of 11. More particularly, it is preferred that 13 is defined as: 0.8*11 < 13 < 11. For example, 13 may represent at least 85%, 90% or 95% of 11.

[0064] From the practical point of view, it is convenient to operate the chopping regulation in phase Pl and P3 with the same voltage Vboost, which simplifies the design of the control circuitry.

[0065] To ensure sufficient energy input, the duration of phase P3 may be greater than that of phase Tl, i.e. T3 > Tl.

[0066] Then in phase P4 the current is regulated to correspond to a hold current. 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 14 is conventionally regulated by chopping using the battery voltage, i.e. V4=Vbatt. The current level 14 is a low current compared to 11 and 13. For example, current 11 may be double or triple the current 14. As will be understood, phases Pl to P3 are designed to control the injector opening phase in an improved manner, whereas phase P4 is rather conventional, maintaining the injector open until the desired fuel quantity is delivered. As visible in the figure, the duration of the phases Pl to P4 corresponds to the pulse with, i.e. PW=T1 + T2 + T3 + T4.

[0067] It may be noted here that the duration T1+T2+T3 is rather constant for an injector, and is not dependent on the fuel amount, resp. PW. However, the duration T4 of the last phase P4 is adapted to reach the desired PW. Hence, it is mainly T4 that will vary with the various injection lengths PW that may be performed in operation.

[0068] As it will be appreciated, the present method proposes a robust control of the opening phase that ensures that so-called “slow” injectors are properly opened. Let us refer to Fig. 8 to illustrate the deficiency of conventional approaches with cut-off phase. Dashed line 8 in Fig 8 represents the valve displacement of a ‘nominal’ injector, whereas dashed line 9 correspond to a slow injector. The term nominal refers to an injector with a nominal opening behavior, i.e. according to the design. Due to conventional part to part variations and considering injector wear, the injector opening behavior may diverge from the nominal behavior. In particular the valve member may have a longer opening delay, and may thus be qualified as ‘slow’.

[0069] As can be seen, with such slow injector there is a risk that after the current cut-off, even by raising the current above the hold current level in the bypass phase, the energy is insufficient to actually open the injector.

[0070] To address this situation, the present method includes the third phase P3, i.e. phase after cut-off, where the current level 13 is designed to ensure that even in case of slow injector behavior the valve member can be opened. This is illustrated in Fig. 2. The design of the drive pulse is the same as in Fig.l, where the second current 13 is regulated to the same current level 11. The valve lift displacement 2 is characteristic of a slow injector, the valve displacement at the moment of the start of phase P2 is shorter than for a nominal injector (compare to Fig.l). Nevertheless, as can be observed, the third phase P3 has been designed to ensure full and stable opening of the valve member, even in case of slow injector. Hence, despite a drop of the valve element in the ascending phase, the drive pulse design is adapted to bring the valve element in the open position (LOP) and hence properly open the injector.

[0071] Figs. 3 and 4 shows two other embodiments, where the third Phase P3 is implemented with a high current 13 smaller than level 13 in Fig.l, respectively on a nominal injector (Fig.3) and a slow injector (Fig.4). The current level 13 is however no less than 80% of 11, as prescribed above. Advantageously, phase P3 is designed such as to be able to fully open the injector, even in case the pintle would collapse (return to the closed position LCLD) due to cut off phase P2. Therefore, T3 may be greater than Tl.

[0072] As can be observed, the proposed drive signal is adapted to operate a proper injector opening both in case of a nominal injector and also of a slow injector.

[0073] In embodiments, the EDM uses two voltage sources to implement the drive signal: a high voltage source for VI and V3 (where V1=V3) and the battery (Vbatt).

[0074] Typically, the ECM comprises a DC-DC boost converter to generate high voltage greater than Vbatt. In embodiments, this DC-DC boost converter may be sized for a lower current than the desired current level for 11, whereby the boost converter is equipped with an additional capacitor to handle the current peak. As a result, due to capacitor discharge, the effective voltage after current cut, P2, is lower than the effective voltage before P2. This can be seen from the voltage trace in Fig.l b).

[0075] In embodiments, during current cut phase P2, the ECM is switched to collect the energy from the solenoid coil in the boost converter capacitor. Indeed, negative voltage V2 during current cut means that coil energy stored during the first phase Pl is given back in the electrical circuit.

[0076] Advantageously, the EDM is designed to deliver rapid current ramps, which will improve injector dynamics. As will be understood, for injector drive, force increase rate at opening and closing is a key parameter. As will be understood, a faster increase force rate on the injector pintle allows reducing the spread on the injector opening time. This has a direct impact on injector delivery spread. This is particularly critical for direct injectors where the fuel is delivered directly in the combustion chamber. Therefore, the ECM is advantageously configured to generate a magnetic force that can achieve a fast force increase rate on the pintle, typically above O.lN / ps when injector opens, to allow more than + / -1 N on injector build tolerances.

[0077] For a given magnetic circuit, force increase rate is correlated to current increase rate, driven by well-known formula U=Ldl / dt. This can be adjusted by injector design and EDM design. As will be known to those skilled in the art, a high current increase rate dl / dt can be obtained by decreasing the coil inductance L (lower coil turn number) or increasing the drive voltage U Also, the actuator force depends on coil turn number x coil current.

[0078] In embodiments, this can be achieved by regulating currents 11 and 13 with current ramps of at least 50 A / ms. So, the rise time of 11 and 13 is very short, improving control dynamics.

[0079] Fast actuator dynamic is also required during the current cut phase P2 to slow down the actuator and maintain it open, when necessary, just before and after it reached the lift stop. Therefore, a high voltage source is applied before and after the current cut phase P2. Also, a high reverse voltage V2 is applied during current cut phase P2 for a fast current decay.

[0080] As understood, the purpose of the second phase P2 is to cut off the current to reduce the opening speed of the valve member in its opening stroke. This second phase P2 is advantageously designed to slow down the valve opening just before it reaches the end of stroke, and is achieved by setting the right timing (in terms of duration and position) for the current cut phase P2. More specifically, phase P2 is preferably configured to end at the moment that the pintle reaches the fully open position (LOP) for a nominal injector.

[0081] The optimization of phase P2 is conveniently done by calibration, primarily for nominal injectors. Governing factors are the following. The duration of the current cutis related to the speed reduction: the larger the cut, the larger the speed reduction. However, one doesn’t want to reach a zero speed to avoid stopping the valve member before it is completely opened. The optimization of phase P2 is additionally further performed based on data reflecting the effect of the duration of phase P2 and its timing.

[0082] Fig.6 shows a plot of impact speed vs. T1 (duration of first pulse Pl), for four values of duration T2 of cut-off phase P2 (T2-a > T2-b > T2-c > T2d). As can be observed, a small cut duration does not significantly reduce the impact speed, whereas large cut exhibits a V shape with a strong speed reduction.

[0083] Fig. 7 shows a plot of fuel delivery vs impact speed vs. T1 (duration of first pulse Pl), for four values of duration T2 of cut-off phase P2 (T2a > T2d). This shows that a large cut off phase T2 / P2 is more likely to impact fuel delivery in case the first pulse duration T1 is not set at optimum value.

[0084] In practice, phase P2 may be advantageously designed to reduce the impact speed by 40% to 70% to keep robustness and not impact too much delivery in case of wrong positioning. The speed reduction is to be understood with respect to the maximum speed reached by the valve member (for a nominal injector) during an opening phase operated at current 11 during the full opening stroke (valve member reaches the fully open position LOP), i.e. without current cut off phase P2.

Claims

Claims1. A method of controlling fuel injection in a fuel gas powered internal combustion engine comprising at least one engine cylinder with an associated fuel injector arranged to inject fuel gas in said cylinder, said fuel injector comprising solenoid actuator configured to selectively operate a valve member to control an outlet orifice, the method comprising: performing an injection event by applying a drive signal to the fuel injector, wherein the drive signal has a profile including: a first phase, wherein a first voltage VI is applied to the fuel injector to regulate a drive current at a first level 11 during a first period Tl, wherein the first voltage VI is a boost voltage and the first level 11 corresponds to an opening current; a second phase, wherein the current is decreased to a second level 12 about zero during a time period T2, in order to decelerate the valve member in its opening stroke; a third phase, wherein a third voltage V3 is applied to the fuel injector to regulate a drive current at a third level 13 during a third period T3, said third phase being configured to deliver sufficient energy to open and stabilize the valve member in its open position, wherein the third current level 13 is no less than 80% of the first current level 11; a fourth phase, wherein a fourth voltage V4 is applied to the fuel injector to regulate a drive current at a fourth level 14 during a fourth period T4, wherein the fourth level 14 corresponds to a hold current and V4 is a battery voltage.

2. The method according to claim 1, wherein the third current level 13 represents at least 85%, 90 or 95% of the first current level 11.

3. The method according to claim 1 or 2, wherein the second phase is configured to cause a reduction of speed of the valve member in its opening stroke by 40 to 70%.

4. The method according to claim 1, 2 or 3, wherein the first voltage VI is at least twice the fourth voltage V4.

5. The method according to any of the preceding claims, wherein in the second phase, a second voltage V2 is applied to the injector, which is the inverse of the first voltage VI.

6. The method according to claim 5, wherein in the second phase the current 12 is decreased down to zero.

7. The method according to any of the preceding claims, wherein the duration of the third period T3 is greater than that of first period Tl.

8. The method according to any of the preceding claims, wherein the second phase is configured to end at a timing that substantially matches the moment the valve member reaches the fully open position, in particular is configured to end at that timing that coincides with the moment the valve member reaches the fully open position by ± 10 ps.

9. The method according to any of the preceding claims, wherein currents 11 and 13 are regulated with a current ramp of at least 50 A / ms.

10. The method according to any of the preceding claims, wherein in phase 1 and / or in phase 3 the drive current is applied with an increase rate configured to generate an opening force on the valve member with a force increase rate of at least 0.1 N / ps.

11. The method according to any of the preceding claims, wherein an electronic drive module is configured to selectively control the voltage and current levels applied to the fuel injector, wherein the electronic drive module comprises a boost voltage circuit associated with a capacitor used to apply voltage VI and voltage V3, wherein during phase P2 the injector is connected to said capacitor to be filled with energy stored in the injector solenoid.

12. The method according to any of the preceding claims, wherein said fuel injector is configured such that, when said solenoid actuator is energized, it moves said valve member, in fuel flow direction, in the opening position, while a return spring urges said valve member back into the closed position.

13. A gaseous fuel delivery system of an internal combustion engine comprising a pressurized fuel tank connected to a fuel rail, and a plurality of fuel injectors coupled to the fuel rail, and an electronic drive module configured to implement the method according to any one of the preceding claims.

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