Method for controlling a fuel-supply system for a gaseous-fuel engine

Abstract

A method for controlling a fuel-supply system 25 for a gaseous-fuel engine 30. The fuel-supply system comprises a reservoir portion 2 connecting a fuel reservoir 1 to a regulator device 8. An injector

Description

[0001] The invention relates to a method for controlling a fuel-supply system for a gaseous-fuel engine, and to fuel-supply system for a gaseous-fuel engine. Background Art

[0002] 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. Apart from electric vehicles that are powered by batteries, mostly Li-ion batteries, hydrogen-powered vehicles are a promising option. These vehicles fall into two major categories. The first category are hydrogen fuel cell electric (H2FCE) vehicles, which also have an electric traction motor, but are powered by fuel cells in which the energy of a chemical reaction of hydrogen and oxygen is converted into electric energy. The other category are hydrogen internal combustion engine (H2ICE) vehicles. In a H2ICE, hydrogen (H2) is used as a fuel and burned with oxygen, the reaction product being water.

[0003] The hydrogen fuel is stored in one or several hydrogen fuel tanks, which are connected to the engine by a fuel-supply system, which normally comprises a plurality of pipes, manifolds, and valves. In case of an H2ICE vehicle, the fuel-supply system may comprise a fuel rail. The pressure in the fuel rail is regulated by a regulator device which reduces the fuel pressure with respect to its upstream side. It also disconnects the fuel rail from the fuel tank(s) if no injections are required. At this time, the injectors are closed so that under ideal conditions, the hydrogen fuel should be enclosed in an isolated subsystem. However, due to a temporary or permanent malfunction, the regulator device may provide an imperfect seal and hydrogen fuel from the upstream side may leak through the regulator device. Depending on the amount of leakage, the pressure downstream of the regulator device can rise quickly to an unexpected dangerous level that could damage the injectors or other components. As a safety measure, it is possible to connect the fuel rail to the environment via a relief valve. The relief valve has an opening pressure that is selected to prevent pressure-induced damage. However, releasing hydrogen into the atmosphere is considered environmentally harmful. Technical Problem

[0004] It is thus an object of the present invention to avoid pressure-induced damage while at the same time minimizing fuel release from a fuel-supply system of a gaseous-fuel engine.

[0005] This problem is solved by a method according to claim 1 and by a fuel-supply system according to claim 15. General Description of the Invention

[0006] The invention provides a method for controlling a fuel-supply system for a gaseous-fuel engine. In this context, a gaseous-fuel engine is an internal combustion engine that runs on a gaseous fuel, such as coal gas, producer gas, biogas, landfill gas, natural gas, or hydrogen. This may specifically be a hydrogen internal combustion engine (H2ICE), which is used to burn hydrogen with oxygen, the reaction product being water. It will be understood that since air is used rather than pure oxygen, the combustion may produce minor amounts of other products. As a rule, the engine has at least one cylinder with an intake valve through which it communicates with an intake duct. A movable piston is disposed in the cylinder, which piston in turn can be connected to a crankshaft. Each cylinder may have an injector for injecting hydrogen fuel directly into the cylinder. Normally, the cylinder also has an exhaust valve through which it communicates with an exhaust duct (which term explicitly includes an exhaust manifold). Gaseous fuel, specifically hydrogen fuel, may be directly injected into the respective cylinder. It may also be injected into the intake duct (i.e. upstream of the cylinder), corresponding to an indirect injection. Both possibilities may be combined. The engine can specifically be a drive engine for a vehicle, like a road vehicle, but other applications are also possible. Here and in the following, the terms “fuel” and “gaseous fuel” are synonymous and refer to a fuel that is gaseous under normal conditions, i.e., 1 bar and 20°C. Specifically, “gaseous fuel” may refer to hydrogen fuel or hydrogen.

[0007] While some aspects of the method are implemented by physical or “hardware” components, other aspects may be software-implemented. The fuel-supply system is adapted to supply fuel to the gaseous-fuel engine, wherefore it comprises pipes or other components that are adapted to contain and guide the gaseous fuel. Although it is conceivable that some aspects (in particular control aspects) of the method could be realized by components outside the fuel-supply system, it is preferably entirely performed by using system-integrated components. Control functions of the method may be performed by a control device of the fuel-supply system. Such a control device may control various functions and may receive sensor signals.

[0008] The fuel-supply system comprises a reservoir portion connecting a fuel reservoir to a regulator device, and an injector portion connecting the regulator device to at least one injector of the engine. While the reservoir portion and the injector portion are treated as different parts of the fuel supply system, at least one component may be shared by the reservoir portion and the injector portion. Each of the portions may comprise one or several pipes that are adapted for containing and guiding fuel. The injector portion preferably comprises a fuel rail to which a plurality of injectors are connected. The regulator device is interposed between the two portions. It can receive fuel from the reservoir through the reservoir portion and deliver the fuel through the injector portion to the at least one injector. The reservoir may comprise one or several fuel tanks in which the fuel is stored in compressed or possibly even liquid form. The regulator device integrates the functions of a shut-off valve and a pressure regulator. It is within the scope of the invention, that the shutoff valve and the pressure regulator are two distinct subunits of the regulator device, as well as a single device functions as the shut-off valve and the pressure regulator at the same time. The regulator device, as well as parts of the reservoir portion and / or the injector portion, may be part of a regulation module, specifically a hydrogen regulation module (HRM). The regulation module may also comprise a filter to remove foreign particles or droplets from the gaseous fuel. Other elements of the regulation module may include a heat exchanger or other heating or cooling device for adjusting the temperature of the fuel.

[0009] The regulator device is adapted to fluidly connect the injector portion and the reservoir portion in an open state while regulating an injector pressure in the injector portion, and to fluidly disconnect the injector portion from the reservoir portion in a closed state. In the open state, the regulator device enables fuel transfer from the reservoir portion to the injector portion but also tries to maintain a defined injector pressure in the injector portion. The injector pressure, or the target value for the injector pressure, may be e.g., between 21 bar(a) and 41 bar(a), wherein “(a)” designates absolute pressure.

[0010] It will be understood that the actual injector pressure will at least temporarily deviate from the defined value. Pressure fluctuations in the injector portion are mainly due to changing injection characteristics. When the amount of fuel injected into the engine increases temporarily, the injector pressure will briefly drop, which is then compensated by the regulator device. In a gaseous-fuel vehicle, the open state of the regulator device corresponds to a “foot-on” state, i.e., the driver depressing the accelerator pedal or the like. The closed state, on the other hand, corresponds to a “foot-off’ state. In this state, the driver does not request any acceleration or additional torque. In this state, the engine is still running, but no fuel is injected and therefore the regulator device separates the injector portion from the reservoir portion.

[0011] The method comprises at least the following steps, which may be performed in the sequence in which they are mentioned but also in any other sequence or at least partially simultaneously, unless obviously impossible.

[0012] In one step of the method, it is determined if the injector pressure is increasing during a closed period in which every injector is closed and the regulator device is closed. The closed period represents a time interval during which the injector portion should be closed for gas exchange upstream (via the regulator device) as well as downstream (via the at least one injector). In a vehicle, this may correspond to the abovementioned “foot-off’ state. This step may include, or may be preceded by, a detection that every injector is closed and the regulator device is closed. Such a detection could be performed using dedicated sensors or by detecting a switching state of the injector(s) and the regulator device, respectively. On the other hand, the same control device that determines whether the injector pressure is increasing may control the injector(s) and the regulator device and may therefore be aware of their status at any time. The injector pressure can be measured directly by a pressure sensor in the injector portion. However, it is within the scope of the invention that the pressure could be measured or determined indirectly, e.g., via a related quantity. During the closed period, the injector pressure should not change at all if there was no leakage. If there was a leakage on the injector side, this could lead to a decrease of the injector pressure. If, however, the injector pressure increases, this is due to a leakage within the regulator device.

[0013] In another step of the method, if the injector pressure is increasing, a relief-injection process is performed, which comprises at least one injection into the engine, to at least limit the pressure increase. The increasing pressure could damage the injector portion or the injectors. Potentially, this could be alleviated by releasing gaseous fuel to the environment, which is undesirable for most fuels, including hydrogen. The relief-injection process, on the other hand, releases fuel into the engine by at least one injection, normally a plurality of injections. Preferably, the step comprises burning the injected fuel inside the engine. However, it is conceivable that the injected fuel is at least partially converted in an after-treatment system. Either way, the amount of unburned fuel that is released to the outside is drastically reduced, ideally to zero. At the same time, the pressure increase is at least reduced or slowed down. As will be explained below, the pressure increase may be inverted, resulting in a decreasing injector pressure. It is preferred that the relief-injection process is performed automatically in response to the injector pressure increasing. In other words, it is preferred that no human decision or initiating action by a human being is needed for performing the process. For example, the relief-injection process may be initiated and / or controlled by the abovementioned control device.

[0014] Preferably, the reservoir portion comprises a pressure regulator that reduces a reservoir pressure at the fuel reservoir to an intermediate pressure upstream of the regulator device. The reservoir pressure, i.e., the pressure at which the fuel is released from the reservoir, may be several hundred bar. This reservoir pressure could potentially damage the regulator device, which of course depends on the layout of the regulator device and the specific value of the reservoir pressure. The pressure regulator reduces the reservoir pressure to an intermediate pressure, which may correspond to e.g., 5%-20% of the reservoir pressure. By way of example, the intermediate pressure may be between 40 bar(a) and 60 bar(a), e.g., 50 bar(a). Assuming that the injector pressure is e.g., between 20 bar(a) and 40 bar(a), the pressure difference to be handled by the regulator device (i.e., the difference between the injector pressure and the intermediate pressure) is rather moderate, while the pressure difference to be handled by the pressure regulator (i.e., the difference between the intermediate pressure and the reservoir pressure) may be significantly greater. It should be noted that the pressure regulator could also integrate the function of a shut-off valve. Alternatively, a dedicated shut-off valve could be disposed upstream of the pressure regulator. Such a shut-off valve could be integrated into the fuel reservoir.

[0015] According to one embodiment, the injector portion is connected to a relief valve having an opening pressure, wherein the relief-injection process is performed so that the injector pressure is kept below the opening pressure. For safety reasons, it is preferred that such a relief valve is present, which enables fuel release to the atmosphere as an emergency measure or as a last resort. For environmental reasons, such fuel release should be avoided if possible. Therefore, the relief-injection process should result in keeping the injector pressure below the opening pressure. Normally, this implies that the injector pressure is stabilized or reduced. However, in some embodiments of the invention, the injector pressure could be allowed to rise temporarily, whereafter it is reduced again. Normally, the relief-injection process should be performed to keep the injector pressure not only below the opening pressure, but below a pressure limit that is defined to be somewhat lower than the opening pressure.

[0016] Preferably, an injection schedule is determined for the relief-injection process, and the relief-injection process is performed based on the injection schedule. The injection schedule may specify various parameters for the relief-injection process. Such parameters may include the total number of injections, the frequency of the injections, the total duration of the relief-injection process, the timing of an injection within an engine cycle, the amount of fuel that is injected during a single injection, or other values. The injection schedule is determined or set up before the relief-injection process, whereafter the relief-injection process is carried out accordingly. This includes the possibility that the injection schedule could be adapted or re-determined after some time. It is preferred that the injection schedule is determined automatically, i.e. without the need for human intervention or decision. The injection schedule can be determined by the above-mentioned control device and can be stored in a volatile memory that is accessible by the control device. Here and in the following, “based on” is always to be understood as “based at least on” and not as “based exclusively on”. E.g., “A is based on B” and “A is based on C” do not exclude that “A is based on B and C”.

[0017] The relief-injection process is performed during a time when there is no mechanical power demand on the engine. Accordingly, the injection(s) of this process should not lead to any significant torque generation or transmission e.g., to the drive wheels of a vehicle. This could be handled by temporarily interrupting the mechanical connection between the engine and the wheels. Such an approach may be undesirable, though. It is therefore preferred that the relief-injection process comprises at least one injection that is followed by a late ignition. Specifically, every injection of the relief-injection process may be followed by a late ignition. The late ignition is performed during a phase of the engine cycle where it leads to no or only minimal torque generation. One could also say that the ignition is a retarded ignition in that it is retarded with respect to the ignition timing under normal operating conditions when significant engine torque is required.

[0018] According to a preferred embodiment, pressure characteristics are determined, which comprise at least one of a current injector pressure and a current pressure-change rate, and the relief-injection process is performed based on the pressure characteristics. The pressure-change rate represents the change of the injector pressure per time unit and can be measured in bar / s. One could say that this is the time derivative of the injector pressure. In practice, the pressure-change rate can be determined by measuring the injector pressure at two different points in time and dividing the difference by the length of the time interval. The pressure characteristics can be determined only once before the start of the relief-injection process, or they could be determined repeatedly, also during the reliefinjection process. Especially in the latter case, it may be sufficient to only determine the injector pressure. If the pressure characteristics are only determined once, it is advantageous or may even be necessary to also determine the pressure-change rate.

[0019] Preferably, the injection schedule is determined based on the pressure characteristics. The pressure characteristics can be used as starting conditions of the reliefinjection process. Therefore, it is advisable to include these starting conditions when the relief-injection process is planned, i.e., when the injection schedule is determined. This is especially true when a target for the relief-injection process is defined. However, even without a specific target, the injection schedule can be planned more efficiently if the pressure characteristics are taken into account. It will be understood that if the injection schedule is determined based on the pressure characteristics, the relief-injection process is also performed based on the pressure characteristics.

[0020] With respect to prior art, the inventive method is advantageous even if the pressure increase is only reduced or stopped. For example, by slowing down the pressure increase, the strain on the injector portion is reduced and / or release of gaseous fuel into the environment can at least be postponed, thereby reducing the total amount of released fuel. However, it is preferred that the relief-injection process is performed to decrease the injector pressure. Preferably, this means that the injector pressure is decreased by the reliefinjection process. Specifically, the injection schedule may be determined so that the injection pressure is decreased. However, it is conceivable that the relief-injection process decreases the injector pressure even without an injection schedule. For instance, the injector pressure could be monitored continuously and the injections during the reliefinjection process could be adapted dynamically so that the injector pressure decreases.

[0021] In some embodiments, the relief-injection process can be performed so that the injector pressure is decreased without any specific goal or target value. However, this could be inefficient in some cases because the injector pressure could be lowered, thereby putting unnecessary strain on the injector portion, or it could be lowered too far, which could lead to ineffective injections when the engine resumes its normal operation. Therefore, one embodiment provides that a target pressure below the current injector pressure is defined, and the relief-injection process is performed to at least decrease the injector pressure down to the target pressure. The target pressure is defined before the relief-injection process. It could be defined dynamically during operation of the fuel-supply system. Alternatively, it may be predefined and read from a - volatile or non-volatile - memory.

[0022] In certain embodiments, the current injector pressure could be checked repeatedly, and the relief-injection process is performed until the target pressure has been reached, irrespective how long this takes in the specific case. However, it is mostly desirable to reduce the injector pressure within a certain time period, especially because the elevated pressure could reduce the service life of the injector portion or the injectors. According to one embodiment, a time limit for reaching the target pressure is defined and the injection schedule is determined based on the time limit. The time limit may be several seconds but could also be several ten seconds or less than one second. Then, in order to reach a given target pressure starting from a given injector pressure, more gaseous fuel needs to be injected per time unit (e.g., per second) if the time limit is shorter. Like the target pressure, the pressure limit may be defined “dynamically” during operation of the fuel-supply system, or it may be a predefined value stored in a memory.

[0023] Preferably, an injector-portion volume of the injector portion is defined, and the injection schedule is determined based on the injector-portion volume. For a given injector pressure, the amount of gaseous fuel in the injector portion is proportional to the injectorportion volume. Qualitatively, the greater the injector-portion volume is, the less the injector pressure is influenced by any gas flow into or out of the injector portion. Thus, if the volume is greater, more gaseous fuel needs to be injected to counteract a pressure increase, or to induce a pressure decrease. In this embodiment, the injector-portion volume may be defined before the injection schedule is determined. This may be done by some calibration process, or the injector-portion volume may be predefined and read from a memory. In this context, the injector-portion volume should include the entire volume between the regulator device and the valve(s) of the at least one injector, which is the entire volume that is subjected to the injector pressure.

[0024] Under certain conditions, especially for very short time intervals, temperature-induced pressure changes in the injector portion may be neglected. However, the density of the gaseous fuel will be influenced by the temperature, and this may have to be taken into account regarding the injection schedule. It is therefore preferred that an injector-portion temperature of the injector portion is determined, and the injection schedule is determined based on the injector-portion temperature. In some cases, the injector-portion temperature can be taken into account in order to determine whether a pressure change is at least partially caused by a temperature change. Also, and possibly more importantly, the temperature-dependence of the density of the gaseous fuel at a given injector pressure can be taken into account. This influences the amount (in moles) or mass (in grams) of gaseous fuel that is injected when one injector is open for a certain time interval.

[0025] According to one embodiment, a leakage flow rate is determined based on the pressure characteristics, a required injection flow rate for the relief-injection process is determined based on the leakage flow rate, and the injection schedule is determined based on the required injection-flow rate. Each of the leakage flow rate and the injection flow rate may be a volume flow rate, measured in cm3 / s, a mass flow rate, measured in g / s, or an amount flow rate, measured in mol / s. It will be understood that a mass flow rate can be directly calculated from an amount flow rate, and vice versa. Also, a volume flow rate can be calculated from a mass flow rate, and vice versa, if the temperature and the pressure are known. The leakage flow rate represents the flow through the regulator device in the closed state, which can be calculated if, on the one hand, the injector pressure is known for at least two points in time or if the pressure-change rate is known, and on the other hand, the injector-portion volume is known. The injection flow rate is a flow rate of the at least one injection during the relief-injection process, optionally an average flow rate, which considers that the injections are not occurring continuously. Then, in order to reduce the injector pressure, the injection flow rate has to be greater than the leakage flow rate. In order to achieve a specific net flow rate, the injection flow rate has to be equal to the sum of the net flow rate and the leakage flow rate. Either way, once a desired or necessary injection flow rate has been determined, the injection schedule can be determined based on that injection flow rate. I.e., the injection schedule is determined so that this injection flow rate is achieved.

[0026] One embodiment provides that an injection quantity for each injection is determined as part of the injection schedule. In case of a plurality of injections, the injection quantity may be the same for each injection but may also be different for different injections. By way of example, the injection quantity may be determined based on the required injection-flow rate, the number of injectors and the engine speed. To calculate a constant injection quantity, the injection-flow rate can be divided by the number of injectors and the engine speed.

[0027] The invention also provides a fuel-supply system for a gaseous-fuel engine, which fuel-supply system comprises a reservoir portion connecting a fuel reservoir to a regulator device, and an injector portion connecting the regulator device to at least one injector of the engine, which regulator device is adapted to fluidly connect the injector portion and the reservoir portion in an open state while regulating an injector pressure in the injector portion, and to fluidly disconnect the injector portion from the reservoir portion in a closed state, wherein the fuel-supply system is adapted to: - determine if the injector pressure is increasing during a closed period in which every injector is closed and the regulator device is closed, and - if the injector pressure is increasing, perform a relief-injection process, which comprises at least one injection into the engine, to at least limit the pressure increase.

[0028] All these terms have been explained above with respect to the inventive method and therefore will not be explained again. Preferred embodiments of the inventive fuel-supply system correspond to those of the inventive method.

[0029] It will be understood that the fuel-supply system may comprise a control device which may at least partially perform or control one or several operations. The control device may at least partially be software-implemented. It may be connected to at least one sensor, e.g., to detect the injector pressure or other quantities. It may also be connected to the at least one injector in order to control the relief-injection process. Brief Description of the Drawings

[0030] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of components of a hydrogen vehicle with a fuel-supply system according to an embodiment of the present invention; and Fig.2 is a flowchart illustrating an inventive method for controlling the fuel-supply system of fig.1. Description of Preferred Embodiments

[0031] Figs. 1 shows a schematic view of some elements of a hydrogen vehicle, more specifically an H2ICE vehicle, with an embodiment of an inventive fuel-supply system 25. The vehicle comprises a hydrogen internal combustion engine 30, which is only shown schematically. A plurality of injectors 20 (in this case, six injectors) are disposed to inject hydrogen fuel into the engine 30. The hydrogen fuel needed to supply the engine 30 is stored in a fuel reservoir 1, which comprises at least one highly pressure-resistant fuel tank. Inside the fuel reservoir 1, a reservoir pressure may be several hundred bar(a), e.g., up to 700 bar(a). The fuel reservoir 1 is connected to the engine 30 via the fuel-supply system 25, or parts thereof. It should be noted that the fuel-supply system 25 may comprise additional components which are not shown because they are either optional or are not necessary for explaining the invention.

[0032] The fuel-supply system 25 comprises a reservoir portion 2 and an injector portion 11. A reservoir pipe 3 of the reservoir portion 2 is directly connected to the fuel reservoir 1 and leads to a pressure regulator 4. The pressure regulator 4 reduces the reservoir pressure down to an intermediate pressure of about 50 bar(a) in an intermediate pipe 5, which leads to a regulator device 8. The intermediate pipe 5 is also a part of the reservoir portion 2. Various elements can be arranged in the intermediate pipe 5, like a heat exchanger 6 to adjust the temperature of the hydrogen fuel and a filter 7 to remove foreign particles or droplets from the hydrogen fuel. The regulator device 8 is shown schematically as two components 9, 10 arranged in series, but may be a single, integrated device. In this example, it comprises a shut-off valve 9 disposed upstream of a pressure regulator 10. The pressure regulator 10 is adapted to reduce the pressure of the hydrogen fuel, from the intermediate pressure of about 50 bar(a) to an injector pressure between 20 and 40 bar(a) in the injector portion 11. In this embodiment, the injector portion comprises a fuel-rail pipe 12 and a fuel rail 13. The injectors 20 are connected to the fuel rail 13 and therefore receive hydrogen fuel having the injector pressure. The injector pressure and an injector-portion temperature of the injector portion 11 can be measured by a pressure sensor 14 and a temperature sensor 15, respectively. Operation of the regulator device 8 and the injectors 20 can be controlled by a control device 22, which may partially be software-implemented. The control device 22 is also connected to the pressure sensor 14 and the temperature sensor 15.

[0033] For safety reasons, the intermediate pipe 6 is connected to a relief pipe 21 via a first branch pipe 16 and a first relief valve 17. The first relief valve 17 may have an opening pressure somewhat above the regular intermediate pressure, e.g., 62 bar(a). Likewise, the fuel-rail pipe 12 is connected to the relief pipe 21 via a second branch pipe 18 and a second relief valve 19. The second relief valve 19 may have an opening pressure somewhat above the regular injector pressure, e.g., 47 bar(a). If the opening pressure of either relief valve 17, 19 is exceeded, hydrogen fuel can be released through the relief pipe 21 into the atmosphere 40.

[0034] Such a situation could potentially occur during a closed period of the injector portion 11, during which all injectors 20 are closed and the regulator device 8 (i.e., the shut-off valve 9) are closed. This closed period can correspond to a “foot-off’ state in which the driver does not depress an accelerator pedal and therefore there is no torque demand. If the injectors 20 are tightly closed but the closure of the regulator device 8 is imperfect, high-pressure fuel can leak from the intermediate pipe 5 into the fuel-rail pipe 12. While releasing hydrogen fuel into the atmosphere 40 is satisfactory under safety aspects, it is rather undesirable under environmental aspects. Therefore, the control device 22 is adapted to perform an inventive method, which will now be explained with reference to the flow chart of fig.2. It will be appreciated that some of the steps shown in the flow chart could be performed in a different sequence, or even simultaneously.

[0035] In a first step, at 100, an injector-portion volume, a target pressure and a time limit are defined. The injector-portion volume is the entire volume of the injector portion 11. The target pressure is a pressure well below the opening pressure of the second relief valve 19 and may correspond to 35 bar(a). The time limit is a time interval within which the target pressure has to be reached, e.g., 1,0 s. All these values may be predefined and can be read from a memory that is accessible by the control device 22. At 110, it is checked whether the fuel-supply system 25 is in a closed period, i.e., if the regulator device 8 and the injectors 20 are closed. The control device 22 controls the regulator device 8 and the injectors 20 and is therefore informed about their state at any time. If there is not closed period, the method repeats the check. In case of a closed period, pressure characteristics are determined at 120, which include a current injector pressure and a current pressure-change rate. At 130, it is checked if the injector pressure is increasing, i.e., if the pressure-change rate is positive. If not, the method returns to step 110 to check again if the fuel-supply system 25 is still in a closed period. If the injector pressure is increasing, an injection schedule is determined in a block 140. At 150, the injector-portion temperature is measured. At 160, a leakage flow rate is determined, which may be a mass flow rate through the regulator device 8. Knowing the pressure change rate, the injector pressure, the injector-portion temperature and injector-portion volume, the leakage flow rate can be determined, either by explicit calculation or using a lookup table. At 170, a required injection flow rate is determined. This takes into account the leakage flow rate, the target pressure, and the time limit. The injection flow rate is determined to over-compensate the leakage flow rate and reduce the injector pressure to the target pressure within the time limit. At 180, an engine speed (in rotations per second or the like) is determined. With knowledge of the engine speed and the number of injectors 20, a fuel quantity per injection is determined at step 190.

[0036] After the injection schedule has been determined, a relief-injection process is performed at 200. This includes injecting the previously determined amount with each of the injectors 20 until the end of the time limit. These injections are each followed by a late ignition. In other words, the ignition is performed during a phase of the engine cycle where it leads to no or only minimal torque generation. Thereafter, the method returns to step 110.

[0037] Legend of reference numbers: 1 fuel reservoir 13 fuel rail 2 reservoir portion 14 pressure sensor 3 reservoir pipe 15 temperature sensor 4 pressure regulator 16, 18 branch pipe 5 intermediate pipe 17, 19 relief valve 6 heat exchanger 20 injector 7 filter 21 relief pipe 8 regulator device 22 control device 9 shut-off valve 25 fuel-supply system 10 pressure regulator 30 engine 11 injector portion 40 atmosphere 12 fuel-rail line

Claims

1. A method for controlling a fuel-supply system (25) for a gaseous-fuel engine (30), which fuel-supply system (25) comprises a reservoir portion (2) connecting a fuel reservoir (1) to a regulator device (8), and an injector portion (11) connecting the regulator device (8) to at least one injector (20) of the engine (30), which regulator device (8) is adapted to fluidly connect the injector portion (11) and the reservoir portion (2) in an open state while regulating an injector pressure in the injector portion (11), and to fluidly disconnect the injector portion (11) from the reservoir portion (2) in a closed state, the method comprising at least the following steps:- determining (130) if the injector pressure is increasing during a closed period in which every injector (20) is closed and the regulator device (8) is closed, and- if the injector pressure is increasing, performing (200) a relief-injection process, which comprises at least one injection into the engine (30), to at least limit the pressure increase.

2. The method according to claim 1, wherein the reservoir portion (2) comprises a pressure regulator (4) that reduces a reservoir pressure at the fuel reservoir (1) to an intermediate pressure upstream of the regulator device (8).

3. The method according to any of the preceding claims, wherein the injector portion (11) is connected to a relief valve (19) having an opening pressure, wherein the relief-injection process is performed (200) so that the injector pressure is kept below the opening pressure.

4. The method according to any of the preceding claims, wherein an injection schedule is determined (140) for the relief-injection process, and the relief-injection process is performed (200) based on the injection schedule.

5. The method according to any of the preceding claims, wherein the relief-injection process comprises at least one injection that is followed by a late ignition.

6. The method according to any of the preceding claims, wherein pressure characteristics are determined (120), which comprise at least one of a current injector pressure and acurrent pressure-change rate, and the relief-injection process is performed (200) based on the pressure characteristics.

7. The method according to any of the preceding claims, wherein the injection schedule is determined (140) based on the pressure characteristics.

8. The method according to any of the preceding claims, wherein the relief-injection process is performed (200) to decrease the injector pressure.

9. The method according to any of the preceding claims, wherein a target pressure below the current injector pressure is defined (100) and the relief-injection process is performed (200) to at least decrease the injector pressure down to the target pressure.

10. The method according to any of the preceding claims, wherein a time limit for reaching the target pressure is defined (100) and the injection schedule is determined (140) based on the time limit.

11. The method according to any of the preceding claims, wherein an injector-portion volume of the injector portion is defined (100), and the injection schedule is determined (140) based on the injector-portion volume.

12. The method according to any of the preceding claims, wherein an injector-portion temperature of the injector portion (11) is determined (150), and the injection schedule is determined (140) based on the injector-portion temperature.

13. The method according to any of the preceding claims, wherein a leakage flow rate is determined (160) based on the pressure characteristics, a required injection flow rate for the relief-injection process is determined (170) based on the leakage flow rate, and the injection schedule is determined (140) based on the required injection-flow rate.

14. The method according to any of the preceding claims, wherein an injection quantity for each injection is determined (190) as part of the injection schedule.

15. A fuel-supply system (25) for a gaseous-fuel engine (30), which fuel-supply system (25) comprises a reservoir portion (2) connecting a fuel reservoir (1) to a regulator device (8), and an injector portion (11) connecting the regulator device (8) to at least one injector (20)of the engine (30), which regulator device (8) is adapted to fluidly connect the injector portion (11) and the reservoir portion (2) in an open state while regulating an injector pressure in the injector portion (11), and to fluidly disconnect the injector portion (11) from the reservoir portion (2) in a closed state, wherein the fuel-supply system (2) is adapted to:- determine (130) if the injector pressure is increasing during a closed period in which every injector (20) is closed and the regulator device (8) is closed, and- if the injector pressure is increasing, perform (200) a relief-injection process, which comprises at least one injection into the engine (30), to at least limit the pressure increase.16

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