Method for operating an internal combustion engine

Abstract

A method of operating an internal combustion engine comprising a plurality of solenoid actuated fuel injectors which inject fuel into respective engine cylinders and are fluidly coupled to a fuel rail supplied with fuel from a fuel tank. The method comprises performing, before engine start, an unsticking procedure whereby sequences of actuating pulses are applied to the fuel injectors. The pulses may be applied to all actuators simultaneously or sequentially. The fuel rail may be isolated from the fuel tank, and / or the fuel rail pressure may be below 2.0 bar, during the unsticking procedure. The pulses may be defined by an on-phase TON, with two consecutive pulses being spaced by an off-phase TOFF. The on-phase may consist of an excitation phase Pexc at an excitation current I1 to produce an electromagnetic kick and thereby induce mechanical vibrations followed by a heating phase Pheat at a lower heating current I2 to warm-up the fuel injector. The fuel may be a gaseous fuel, such as compressed natural gas (CNG) or hydrogen, and a purge routine may be carried out to remove gaseous fuel from the fuel rail before operating the unsticking procedure.

Description

Technical field The present invention generally relates to fuel delivery systems in internal combustion engines, and more specifically to the control of fuel injectors. Background Art As is well known in the art, fuel injectors comprise a nozzle assembly that is actuated by means of a solenoid actuator, which when energized lifts a valve member off a valve seat and permits fuel injection. The injection events are performed by applying a drive pulse of predetermined duration, called pulse width, which will control the injected quantity together with fuel pressure. Such fuel injectors and their control are well known in the art. This technology is used for liquid and gaseous fuel, e.g. gasoline, CNG, hydrogen, etc. Fig.1 shows conventional flow curves for a hydrogen injector: the fuel delivery is plotted versus pulse width (PW) for different fuel supply pressures. Engine emissions and driveability depend on the accurate control of injected quantity. This requires that injector flow curves remain constant whatever the environment conditions parameters (temperature for example) or that one can model the effect of each parameter and then compensate it to finally obtain the same injected quantity as the desired one. One issue is that after a long stop some injectors may experience some sticking, in some conditions which may be due to wear, design or other. Let us illustrate this based on Fig.2, which shows a hydrogen injector designed for port fuel injection (PFI), i.e. to be arranged in the engine in the intake port (i.e. before intake valves). Although such injector design is generally reliable, it may happen that after a long stay at low temperatures, a sticking effect is observed, which makes its opening difficult or impossible at least for the beginning of the engine start sequence, resulting in a longer start time at cold conditions or even no start. The issue mechanism is the following one. Let us first consider the injector functioning principle. The solenoid 1 generates an electromagnetic force that pulls the moveable magnetic armature 2 to the right, which moves an obturating / sealing member 4 (generally a polymer seal element) from a valve seat surrounding nozzle holes 6. This opens the flow path for the fuel from the injector interior to the nozzle holes 6, and fuel is thus discharged / injected. In normal / closed state, when solenoid 1 is not energized, the spring 5 pushes the armature 2 against the valve seat and closes the nozzle holes 6. Issue may happen at cold conditions because the material properties &dimensions of the polymer seal 4 change, which tend to make armature 2 stick to the nozzle seat. There may also be some difference in friction forces between the armature and its nozzle holder 7 at the sealing zone 3. The force needed to unstick these parts is too high compared to the force generated by the actuator with a conventional drive pulse operated for a typical injection event. It may be noted that whereas such sticking issue is described in relation to the injector shown in Fig.2, it may occur with other PFI injector designs, as well as with direct injection injector designs, and with different kinds of fuels, liquid or gaseous. Technical problem It is an object of the present invention to provide an improved method of operating an internal combustion engine. General Description of the Invention The present invention proposes a method of operating an internal combustion engine as claimed in claim 1. The engine comprises a plurality of solenoid actuated fuel injectors configured to inject fuel into respective engine cylinders, the fuel injectors being fluidly coupled to a fuel rail supplied with fuel from a fuel tank. According to the present invention, before engine start, an unsticking procedure is performed, by which sequences of actuating pulses are applied to the fuel injectors, i.e. to the solenoid. As will be described in more detail below, a first goal of the unsticking procedure is mechanically soliciting the injectors by sending force trains by way of the actuating pulses applied with a certain frequency. The sequence of actuating pulses will create vibrations that help unsticking injector components. In that context it may be noted that the sequences of actuating pulses are normally configured to generate mechanical vibrations in the fuel injectors. The goal is primarily mechanical solicitation and not to trigger injector opening, although this may happen. In addition to mechanical solicitation, the actuating pulses may be configured to warm up the fuel injectors by Joule effect as current flows through the solenoid. The unsticking procedure is normally implemented before engine start, i.e. before the injection events (where fuel is injected) are to be operated to generate torque. The present method is advantageously software implemented can hence be implemented using conventional techniques and components usually found in the engine. The unsticking procedure can be applied in parallel or sequential mode. This may generally depend on the drive electronics. In parallel mode, the respective sequences of actuating pulses are applied to all injectors simultaneously. In alternating mode, the sequences of actuating pulses are applied sequentially to the injectors or groups of injectors. For example, a first train of actuating pulses is applied to the first half of the injectors, while the second injectors half are not energized. Next a second train of actuating pulses is applied to the second injectors half, while the first second injectors half is not energized. Then a third train of actuating pulses is applied to the first injectors half, and then a forth train of actuating pulses is applied to the second injectors half, and so forth. The number of actuating pulses per sequence and their frequency will typically be calibrated based on the application. When operating in alternating mode, it is desirable that the duration of the sequence is not too long, to avoid cooling of the injectors that are in the non-energized altemance and to get balance temperature on the whole set of injectors at the end of the unsticking procedure In embodiments, the sequences are operated by applying actuating pulses with a duty cycle of at least 75%, preferably at least 80, 85 or 90%. In other words, the sequence mainly consists of active time, where current is applied to the injector actuators. In embodiments, the actuating pulse has profile that consists of an on-phase, Ton, during which the solenoid is energized; and two actuating pulse (Ton phase) are spaced by an off-phase, Toff, during which the solenoid is not energized. In other words, the sequence of actuating pulses is built as a repetition of pattern Ton+Toff. In embodiments, Ton phase consists of an excitation phase at a predetermined excitation current to produce an electromagnetic kick and thereby influence mechanical vibrations; the excitation phase being followed by a heating phase at a predetermined heating current configured to warm-up the fuel injector. The excitation current is larger than the heating current; and the duration of the excitation phase is less than 50% of the Ton phase, preferably less than 75%, 80%, 85% or 90%. In embodiments, the Ton phase may consist of a unique current phase (i.e. a single current level is applied during Ton) configured to generate heat without causing the injector to open. In embodiments, the sequences of actuating pulses are applied at a predetermined frequency that is designed to excite resonance within the injector valve assembly. The duration of the pattern Ton+Toff is thus adapted to match the resonance frequency. Hence, the sequence of actuating pulses will apply kicks at the resonant frequency, which can lead to increased amplitude of vibrations as the energy input from the kicks coincides with the natural oscillation frequency of the valve assembly. The unsticking procedure is advantageously applied before engine start, i.e. before torque generating injection events are operated by the engine control unit. Before engine start, in the case of an engine operating on gaseous fuel, the fuel rail is normally isolated from the gas tank and the fuel rail is normally at low pressure (e.g. atmospheric to 1 bar(g)). Indeed, a fuel purge has normally been performed at engine shut down. So in practice, before the unsticking procedure the rail pressure may be compared to a predetermined threshold of e.g. 2.0 or 1.0 or 0.5 bar(g). If the current rail pressure is below the threshold (enabling condition), then the unsticking procedure is started. If desirable, a fuel purge can be carried out after keying, before the present unsticking procedure is operated and before engine start, as a safety measure or after a rail pressure check indicating that the current rail pressure is above said predetermined threshold. These and other embodiments of the invention are recited in the appended dependent claims. According to another aspect, the invention also relates to an internal combustion engine as claimed in claims 14 and 15. Brief Description of the Drawings 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: Fig. 1 is a plot of various flow curves (fuel delivery vs. pulse width) for increasing fuel pressures (P1<P2<P3<P4<P5); Fig. 2 is a principle cross-section view through a conventional PFI injector; Fig. 3 is a plot of sequences of actuation pulses in parallel over the injectors; Fig. 4 is a plot of sequences of actuation pulses applied sequentially to injector groups; Fig. 5 is a detail of an actuation pulse (detail A in Fig.3); Fig. 6 is a plot illustrating current levels for the actuation pulse of Fig.5; Fig. 7 is a flowchart of an embodiment of the present method; Fig. 8 is a diagram of an internal combustion engine operating with gaseous fuel such as hydrogen. Description of Preferred Embodiments In order to address potential injector sticking issues, the present invention proposes a method of controlling an internal combustion engine with one or more fuel injector(s) arranged to inject fuel into respective engine cylinder(s). The method is applicable to direct (in-cylinder) fuel injection and indirect fuel injection (PFI), as well as for liquid (gasoline, etc.) and gaseous fuels (CNG, hydrogen,...). In the following the invention is described in the context of combustion engines operating on gaseous fuels, in particular hydrogen. The engine 50 comprises a plurality of engine cylinders 52 and an associated fuel delivery system 100 comprising a fuel tank 102 and a fuel rail 106 connected via a supply pipe 108, and a plurality of fuel injectors 110 coupled to the fuel rail 106 and arranged to deliver fuel to respective engine cylinders 52. The fuel tank comprises pressurized fuel in gaseous state and is configured to discharge gaseous fuel into the supply pipe, generally via a mechanical regulator. Reference sign 104 designates a hydrogen regulator module that may include a shut off valve and a controllable (electronic) pressure regulator serially mounted in the supply pipe 108 to regulate the downstream pressure within a predetermined working range. The hydrogen regulator module may further include one or more of a filter, a pressure relief valve and a remotely actuatable purge valve. A controller (control unit, possibly part of Engine control unit ECU) is configured to operate the fuel delivery system and in particular to operate predetermined fuel injection strategies by which fuel injection events are performed, whereby predetermined gas quantities are introduced into the cylinders to meet corresponding torque requests. The present method can be programmed in such controller for its implementation. The inventive method provides for a special drive sequence, referred to as unsticking procedure, that is performed before engine start. The unsticking procedure involves applying sequences of actuating pulses to the respective fuel injectors. That is, a plurality of actuating pulses are applied to each injector. Depending on the drive electronics, the actuating pulses are applied to all injectors simultaneously, or in an offset manner (e.g. individually one after another, or in groups, sequentially). The unsticking procedure is performed “before engine start” means that the unsticking procedure is performed before torque is desired, hence before actual injection events (i.e. fuel discharging injections) are operated. In practice, the unsticking procedure may be performed before cranking, or at the early stage of cranking (before engine torque is demanded / requested) before the fuel rail is pressurized. It also means that, for engine operating on the Otto-cycle, there is no spark-plug firing during the sequence of actuating pulses. As will be described in more detail below, a first goal of the unsticking procedure is mechanically soliciting the injectors by sending force trains by way of the actuating pulses applied with a certain frequency. The sequence of actuating pulses will create vibrations that help unsticking injector components. In addition to mechanical solicitation, the actuating pulses may be configured to warm up the fuel injectors by Joule effect as current flows through the solenoid. Figs. 3 and 4 show two implementations of the unsticking procedure in a 4-cylinder Otto-cycle engine with four fuel injectors. Current is plotted vs. time for each injector (inj1 to inj4). In the implementation of Fig.3, the unsticking procedure is performed by applying the sequence of actuating pulses to all injectors simultaneously (i.e. at the same time I in parallel). This is an optimal operating mode by which the current is set on all injectors. Such operating mode depends on the ECU drive configuration. If there is one specific drive bank per injector, then all injectors can be driven at the same time, leading to the scheme illustrated in Fig.3. The number of activation pulses in the sequence is calibrated depending on the application case. By contrast, in the implementation of Fig.4, the sequences of actuating pulses are applied alternately / sequentially to the injectors. This is the case e.g. where one bank drives several injectors and only one of injector per bank can be driven at a time. Let us suppose that the engine comprises two injector drive banks: injectors 1 and 3 are on bank 1 and injectors 2 and 4 on bank 2. As can be seen, a first set of actuating pulses (here 5 actuating pulses, but may be different) is applied simultaneously to injectors 1 and 2, then the same sequence is applied to injectors 3 and 4 alternately, and so on. The number of sequences, and duration of each sequence, is calibrated depending on the application case. In such case of altemating / sequential mode as shown in Fig.4, it is desirable that the time between alternations is not too long in order to ensure that temperature difference between the injectors groups is negligible. It is also preferred that the total number of injector actuations is similar between the injector groups, to target similar opening performances for all injectors. Indeed, it has been observed that if the time between alternations is too high the injectors that were not being actuated during the last sequence of the unsticking procedure would have time to cool down, possibly resulting in lower overall temperature for these injectors, being more difficult to open. So, in the mode of Fig.3, the sequence comprises a multiplicity of actuating pulses. The number of pulses and their design depends on the application. In the mode of Fig.4, the sequences are configured with a plurality of actuating pulses, here 5, which are applied sequentially to injector groups. The actuating pulses may have the same configuration (length, pulse profile, current level(s)) between the two operating modes, or may differ. However, within a sequence of pulses, the actuation pulses preferably have the same configuration. In general, an actuation pulse is designed for a given application and hence the actuating pulses of same configuration (length, pulse profile, current level(s)) are applied during the unsticking sequence. As may be noted from the figures, the sequences of actuating pulses have a rather high duty cycle. Fig. 4 shows one (single) actuating pulse of a sequence, corresponding to detail A in Fig.3. The pulse shown in Fig.5 is the logic command pulse (or drive signal) that controls the current flow through the solenoid actuator. The pulse is defined by an on-phase, noted Ton, during which current is applied through the solenoid coil. This activation phase Ton is operated by applying an injector drive signal having a predetermined duration referred to as pulse width, PW. Hence, we have Ton=PW. Two consecutive pulses Ton are space by an off-phase Toff during which no current flows through the coil. Stated otherwise, each sequence is built as a repetition of the pattern Ton+Toff. The Toff phase represents a waiting period between two consecutive Ton phases. Fig. 5 shows the electric current pulse (or pulse profile / drive signal) applied to the solenoid during the actuation pulse of Fig.4. As can be seen, during Ton current flows through the solenoid at one ore more predetermined levels and during the Toff phase there is no current. In the shown example, Ton is composed of an excitation phase Pexc followed by a heating phase Pheat. The current level h in the excitation phase Pexc is designed to generate an electromagnetic force sufficiently high to put a kick in the needle / valve member and unstick it when repeated with a predetermined frequency. The excitation phase Pexc is typically driven by a boost voltage (i.e. greater than battery voltage) to ensure a rapid current slope. The heating phase Pheat is designed with a predetermined length to put maximum energy in the solenoid to get the desired warming of the injector. The heating phase Pheat may be carried out under battery voltage, or under boost voltage if driving circuitry allows. As alluded above, the sequences of actuating pulses are carried out with a high duty cycle. In embodiments the duty cycle is least 75%, preferably at least 80, 85, 90 or 95%. For the sake of exemplification, exemplary values for the phases are given: TOFF = 10 ms TON = 65 ms Hence the duty cycle is 87%. The length of a sequence of activation of pulses in the case of Fig.4 is 75 ms * 5 = 375 ms. And supposing that the unsticking procedure comprises 4 sequences as illustrated in Fig.4, the duration of the unsticking procedure is 4* 375 ms = 1500 ms. The present method does not need any extra component and can be carried out using usual techniques already present in the engine / vehicle. The unsticking procedure may typically be implemented by software, in particular by providing computer code / instructions in a memory of an engine control unit with processor, which code / instructions cause the operation of the unsticking procedure. Turning now to Fig. 7, an exemplary embodiment of the present method for operating an internal combustion engine is shown, e.g. an H2 combustion engine with 4 cylinders. At step S10, the driver signal his / her intention to start the engine. Keying is hence to be understood as ‘key on’ or ‘ignition’. In the first case, key on, the driver simply turns the key or presses the start button to power up the electric systems and in particular the ECU. From there, the engine control method operates a number of system checks before allowing actual ignition, including the unsticking procedure S14. In case the driver directly requests ignition, by keying or by pressing pedal together with start button, then actual engine start is delayed to allow for the checks and unsticking procedure S14 to be performed. In Fig.7, a rail purge routine S12, is represented, designed to to empty the fuel rail from any remaining hydrogen. This is only an option and is therefore put between brackets. In practice, the rail purge routine is operated at engine shut-down. This being said, as a possible safety measure to avoid accidental hydrogen injection into the engine during the unsticking procedure, a purge routine S14 may be performed between S10 and S14. One possibility is to measure the fuel rail pressure at S10, and if the fuel rail pressure exceeds a predetermined threshold (e.g. 1 bar(g)), then the purge routine S12 is performed. Next, the ECU starts the unsticking procedure S14. As discloses herein, such unsticking procedure involves applying sequences of actuating pulses to the fuel injectors. These are non-com bustion injection events which are not designed to inject fuel nor generate torque. In any case, until the end of the unsticking procedure S14 the shut off valve is normally closed so that the fuel rail is isolated from the gas tank and there is no, or only a low gas pressure in the fuel rail. Furthermore, until the end of the unsticking procedure S14, the shut off valve is not opened and the ECU does not implement any injection event based on torque requests and does not trigger spark events. So, during the unsticking procedure S14, sequences of actuation pulses are applied, simultaneously to all injectors or sequentially by injector groups, to achieve mechanical vibrations and injector heating, as described above. The number of actuation pulses and their properties (duty ratio, current level(s)) are to be adapted in function of the particular application, in particular injector design. Once the unstick procedure S14, the ECU authorizes the conventional engine start procedure. In particular, the shut-off valve in HRM 104 is opened to allow hydrogen to flow into the fuel rail at the desired / controlled rail pressure, the engine is cranked, and the control unit will implement the injection strategy by performing 5 injection events to meet torque demand. In some embodiments, the unsticking procedure S14 may (slightly) overlap with the start of the engine start 16. Specifically, the unsticking procedure 14 may end at the beginning of cranking, before opening the shut-off valve and starting torque generating combustion events. 10

Claims

1. A method of operating an internal combustion engine comprising a plurality of solenoid actuated fuel injectors configured to inject fuel into respective engine cylinders, the fuel injectors being fluidly coupled to a fuel rail supplied with fuel from a fuel tank, said method being characterized by performing, before engine start, an unsticking procedure, whereby sequences of actuating pulses are applied to the fuel injectors.

2. The method according to claim 1, wherein the sequences of actuating pulses are applied to all actuators simultaneously, or applied sequentially.

3. The method according to any of the preceding claims, wherein the sequences of actuating pulses are configured to generate mechanical vibrations in the fuel injectors.

4. The method according to any of the preceding claims, wherein during the unsticking procedure the fuel rail is isolated from the fuel tank.

5. The method according to any of the preceding claims, wherein the unsticking procedure is performed if the fuel rail pressure is below 2.0 or 1.0 or 0.5 bar(g).

6. The method according to any of the preceding claims, wherein the sequences of actuating pulses are operated with a duty cycle of at least 75%.

7. The method according to any of the preceding claims, wherein the sequences of actuating pulses are operated with a duty cycle of at least 80, 85, 90 or 95%.

8. The method according to any of the preceding claims, wherein the fuel injector comprises a solenoid actuator and wherein the actuating pulses are defined by an on-phase, Ton, during which the solenoid is energized,two consecutive actuating pulses being spaced by off-phase, Toff, during which the solenoid is not energized.

9. The method according to claim 8, wherein the Ton phase consists ofan excitation phase at an excitation current to produce an electromagnetic kick and thereby induce mechanical vibrations;the excitation phase being followed by a heating phase at a heating current configured to warm-up the fuel injector,wherein said excitation current is larger than said heating current; andwherein the duration of said excitation phase is less than 50% of said Ton phase, or less than 75%, 80%, 85% or 90%.

10. The method according to claim 8, wherein the Ton phase consists of a unique current phase configured to generate heat without causing the injector to open.

11. The method according to any of the preceding claims, wherein the sequences of actuating pulses are applied at a predetermined frequency that is designed to excite resonance within the injector valve assembly.

12. The method according to any of the preceding claims, wherein the fuel is a gaseous fuel and a purge routine is carried out to remove gaseous fuel from a fuel rail before operating the unsticking procedure.

13. The method according to any of the preceding claims, wherein said unsticking procedure is followed by an engine start procedure, wherein injection events are performed during cranking, whereby first injection events per injector are carried out with increased current levels.

14. An internal combustion engine comprising a plurality of engine cylinders and an associated fuel delivery system comprising a fuel tank and a fuel rail connected via a supply pipe, and a plurality of fuel injectors coupled to the fuel rail and arranged to deliver fuel to respective engine cylinders, and a controller configured to perform the method according to any of the preceding claims.

15. The internal combustion engine according to claim 14, wherein the fuel tank comprises pressurized fuel in liquid or gaseous state and is configured to discharge gaseous fuel into the supply pipe, a controllable pressure regulator being serially mounted in the supply pipe to regulate the downstream pressure within a predetermined working range.

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