Engine system for a hydrogen-powered machine

Abstract

The invention relates to an engine system (1) for a hydrogen-powered machine, which engine system (1) is adapted for supplying hydrogen fuel from fuel reservoir (2) to a hydrogen internal combustion engine (50) when the machine is in an operating state, the engine system (1) comprising a fuel-rail assembly (10) defining a rail cavity (12) in communication with a plurality of injector ports (14), each injector port (14) being connected to a fuel injector (55) of the engine. In order to propose an improved fuel management for a non-operating period of a hydrogen-powered machine, the invention provides that the fuel-rail assembly (10) comprises a buffer (20) with a sorbent (24) that is disposed in the rail cavity (12) and that is adapted to sorb hydrogen with a sorption capacity that decreases as a function of a buffer temperature (TB) of the sorbent (24).

Description

ENGINE SYSTEM FOR A HYDROGEN-POWERED MACHINETechnical Field

[0001] The invention relates to an engine system for a hydrogen-powered machine, to a fuel-rail assembly for such an engine system and to a method for operating such an engine system.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 (HFCE) 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 (HICE) vehicles. In a HICE, 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 or the fuel cell(s) by a fuel-supply system, which normally comprises a plurality of pipes, manifolds and valves. These components must be filled with hydrogen when the vehicle is operating. When the vehicle is parked, the fuel-supply system is disconnected from the fuel tanks, but unconsumed hydrogen can reside inside the engine system itself. After some time, hydrogen can leak out, in particular since the hydrogen inside the fuel-supply system typically has a pressure considerably above atmospheric pressure (e.g., between 20 and 40 bar). If the leakage is uncontrolled, the hydrogen - or rather a hydrogen-air mixture - can accumulate in volumes and cavities inside the engine and other parts of the vehicle, which poses an explosion risk, particular at the restart of the vehicle.

[0004] It is also sometimes necessary to reduce the pressures in various parts of the engine system to a safe “near atmospheric” state when the vehicle is not used for long periods of time or before servicing. This can be done by purging, i.e., releasing hydrogen from the respective system. After purging, the pressure shouldbe sufficiently low that any further leakage would not lead to a combustible mixture. Such a target pressure is typically below 2 bar(a), wherein “(a)” indicates absolute pressure. In contrast to this, the typical operating pressures of the engine system are in the range of 5 to 50 bar(a) or more specifically, 20 to 40 bar(a). To achieve this reduction, it has been proposed to either purge hydrogen directly into the atmosphere or to purge it into a buffer tank where it is temporarily stored before it is finally released into the atmosphere. On the one hand, releasing hydrogen into the atmosphere may still lead to the formation of a combustible mixture, depending on the circumstances. On the other hand, hydrogen is considered as a greenhouse gas. Finally, a considerable amount of hydrogen may be simply wasted, especially when the vehicle is parked frequently, which makes this approach uneconomical. Similar problems can arise with other hydrogen-powered machines, e.g., stationary machines that use hydrogen fuel. However, storing the hydrogen in a buffer tank necessitates additional components, including the tank, pipes for connecting the tank to the fuel-supply system, valves to control the hydrogen transfer etc. These components increase costs and need additional installation space.Technical Problem

[0005] It is thus an object of the present invention to propose an improved fuel management for a non-operating period of a hydrogen-powered machine.

[0006] This problem is solved by an engine system according to claim 1 , by a fuel-rail assembly according to claim 14 and by a method according to claim 15.General Description of the Invention

[0007] The invention provides an engine system for a hydrogen-powered machine. The term “hydrogen-powered machine” refers to any machine that uses hydrogen as a source of energy. In particular, the hydrogen-powered machine can be a hydrogen vehicle, which is considered synonymous with “hydrogen-powered vehicle”. As a rule, this refers to a road vehicle like a passenger car, a truck, or a motorcycle. However, it is conceivable to employ the inventive engine system in other vehicles, e.g., in a boat, or in a stationary machine or a machine that is mobile but does not use hydrogen fuel to power its traction system. While the system comprises physical or “hardware” components, some of its functions may be software-implemented. Although it is conceivable that some components (inparticular control components) of the system could be located outside the hydrogen- powered machine, it is normally fully integrated into the machine. The engine system may comprise a control unit that controls various functions and may receive sensor signals.

[0008] The engine system is adapted for supplying hydrogen fuel from a fuel reservoir to a hydrogen internal combustion engine of the machine when the machine is in an operating state. Here and in the following, the terms “fuel”, “hydrogen fuel” and “hydrogen” are synonymous. In the fuel reservoir, which may comprise one or several fuel tanks, hydrogen fuel is stored either in gaseous form under high pressure or in liquid form. The engine system physically connects the fuel reservoir to the engine. Both the engine and the fuel reservoir may be considered as parts of the engine system, but the term “engine system” is not to be construed in that it necessarily has to comprise the engine. Those components that connect the fuel reservoir to the engine may collectively be referred to as a “fuelsupply system”.

[0009] The hydrogen internal combustion engine (HICE) is used to burn hydrogen with oxygen, the reaction product being water. It will be understood that since not pure oxygen but air is used, the combustion may produce minor amounts of other products. The HICE has a plurality of cylinders, each 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 a direct 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). Hydrogen fuel may be directly injected into the respective cylinder. It may also be injected into the intake duct, wherein “intake duct” refers to any duct through which the engine and / or at least one cylinder of the engine receives air and therefore explicitly includes an intake manifold. During operation of the machine, the engine system also establishes a fluid connection between the fuel reservoir and the engine, thereby enabling operation of the engine. The “operating state” is a state in which the engine is operated, hydrogen is consumed and energy is produced so that the machine can perform its functions. In case of a vehicle, this is the state in which thevehicle moves or at least is ready to move. In a HICE vehicle, this is usually equivalent to an “engine on” state.

[0010] The engine system comprises a fuel-rail assembly defining a rail cavity in communication with a plurality of injector ports, each injector port being connected to a fuel injector of the engine. The fuel rail assembly, which may at least in some embodiments also be referred to as a fuel rail, constitutes the connection of the engine system to the fuel injectors. It defines a rail cavity, which is adapted to contain hydrogen. In typical embodiments, the pressure in the rail cavity may be between 20 and 40 bar(a). However, there are also fuel injection systems with higher operating pressure (up to a maximum of 350 bar(a)) or lower operating pressure (up to a maximum of 5 bar(a)). In any case, the fuel-rail assembly needs to be adapted to withstand the elevated pressure. Also, it must be chemically resistant to hydrogen. Possible materials for the construction of the fuel-rail assembly include stainless steel. The rail cavity communicates with a plurality of injector ports through which hydrogen is supplied to the plurality of fuel injectors. Apart from the injector ports, the fuel-rail assembly comprises at least one inlet port through which it receives hydrogen, normally from an upstream part of the engine system. Specifically, the fuel-rail assembly may be connected to the fuel reservoir via a hydrogen regulation module (HRM), which may perform several functions. It may comprise an electronic pressure regulator, which reduces the pressure of the hydrogen fuel, e.g., from about 50 bar(a) at the outlet of the fuel reservoir to between 20 and 40 bar(a).

[0011] The fuel-rail assembly comprises a buffer with a sorbent that is disposed in the rail cavity and that is adapted to sorb hydrogen with a sorption capacity that decreases as a function of a buffer temperature of the sorbent (i.e. when the temperature of the buffer / sorbent increases). The sorbent is disposed in the rail cavity, which includes the possibility that the sorbent is disposed on the inside of a wall that defines the rail cavity, e.g., as a lining. Since it is disposed in the rail cavity, it is in contact with the hydrogen gas in the rail cavity. Therefore, it can sorb hydrogen gas from the rail cavity or release hydrogen gas into the rail cavity, respectively. The sorption effect relies on an interaction between the sorbent and the hydrogen. This may refer to an adsorption as well as to an absorption, which may also be combined with each other. Also, the sorption may be based on aphysical and / or chemical process. After sorption, at least a major part of the hydrogen is no longer present in gaseous form but is chemically or physically bound by the sorbent. Irrespective of whether the sorption process is based on chemical absorption, chemical adsorption, physical absorption and / or physical adsorption, hydrogen is bound to the sorbent so that its quantity in gas phase is reduced. This coincides with a significant increase of the amount of substance (in mol) that can be contained in a given volume of sorbent as compared to the gas phase. Therefore, the sorption process can significantly reduce the gas pressure if no additional hydrogen is supplied to the rail cavity.

[0012] The sorbent has a sorption capacity that decreases as a function of a buffer temperature of the sorbent (i.e. increasing temperature). The buffer temperature is, strictly speaking, a temperature of the sorbent as such and thus may also be referred to as a “sorbent temperature”. However, in a typical embodiment, even if the buffer contains other components apart from the sorbent, the temperature of the sorbent and the temperature of other parts of the buffer are roughly the same. The sorption capacity is the ability of the sorbent to sorb hydrogen. It may be defined as the mass of hydrogen that can be sorbed, divided by the mass of the sorbent. Another definition would be amount of substance (in mol) of hydrogen divided by the mass of the sorbent. This sorption capacity decreases as a function of the buffer temperature, i.e., when the buffer temperature increases, the sorption capacity decreases and vice versa. It should be noted that the sorption capacity as a rule also depends on the pressure or the partial pressure of the hydrogen, respectively. Therefore, strictly speaking, the absorption capacity at a given (partial) pressure of hydrogen gas in the rail cavity decreases as a function of the buffer temperature. In general, the sorption capacity decreases as described at least in a limited temperature range, which should be a typical range of the buffer temperature, e.g., between -31 °C and 150°C. If, for some type of sorbent, the sorption capacity does not decrease at temperatures outside of such a “realistic” range, this would be irrelevant for the invention. Due to the temperature dependency of the sorption capacity, it is possible to promote hydrogen sorption by reducing the buffer temperature and to promote hydrogen release by increasing the buffer temperature.

[0013] When the machine is in the operating state, the buffer temperature is normally comparatively high, at least partially due to the operation of the hydrogen internal combustion engine from which heat can be transferred to the buffer, e.g., by heat conduction and / or convection. Optionally, the temperature can be increased by additional measures. Anyhow, the sorption capacity of the sorbent is comparatively low at this time. When the machine enters a non-operating state, the engine is deactivated and begins to cool down. Accordingly, heat transfer to the buffer is reduced and the buffer temperature starts to decrease. Additional measures may be taken to promote the cooling of the sorbent. In any case, the sorption capacity increases. As a rule, the engine system is adapted to prevent hydrogen supply from the fuel reservoir to the fuel-rail assembly when the machine is in the non-operating state. Under these circumstances, hydrogen sorption leads to a decrease in fuel pressure. When the buffer temperature reaches ambient temperature, an equilibrium pressure is reached that is significantly lower than the pressure in the operating state. Preferably, the equilibrium pressure at ambient temperature is 5 bar(a) or less, preferably 2 bar(a) or less, more preferably 1 ,5 bar(a) or less. Also, preferably at least 50 %, at least 70 %, at least 90% or at least 95% of the hydrogen fuel initially contained in the fuel-rail assembly is sorbed into the buffer. The equilibrium pressure depends on various factors, e.g., on the pressure in the operating state, the inner volume of the fuel-rail assembly, the mass of the sorbent, the temperature difference between the operating state and the non-operating state et cetera. However, it has been found that the pressure can be reduced to a level that can be considered as safe with a relatively small volume of sorbent. By way of example, a sorbent volume may be in the range of 1 - 80 cm3, preferably 5 - 60 cm3, more preferably 8 - 40 cm3.

[0014] Due to the small sorbent volume, the buffer can be integrated into the fuel-rail assembly without the need to significantly increase the size of the fuel-rail assembly. Also, since the buffer is integrated, there is no need for any piping that connects the fuel-rail assembly to the buffer or any valves that control gas transfer between the fuel-rail assembly and the buffer. When the machine re-enters the operating state, the buffer temperature increases due to the activation of the engine, and possibly additional measures. As the sorption capacity decreases, hydrogen is released from the sorbent directly into the rail cavity. Accordingly, the releasedhydrogen can be directly transferred to the individual fuel injectors. Again, no piping is necessary to transfer hydrogen from the buffer to the fuel-rail assembly. During this phase, the hydrogen previously stored in the buffer is mostly released so that the buffer is ready to receive hydrogen when the machine enters the non-operating phase the next time. Since the sorption and release of the hydrogen is temperature- driven and may be caused by the heating and cooling of the engine, the invention may be realized without any active control.

[0015] As already described above, it is preferred that the fuel-rail assembly is adapted for passive heat transfer between the buffer and the engine, whereby the buffer temperature at least partially depends on an engine temperature. The engine temperature is a temperature of at least a portion of the engine, specifically a portion that is closest to the fuel-rail assembly. “Passive heat transfer” refers to heat transfer that occurs without any external energy, i.e. , without electrical heating or cooling, without pumping of a heat-carrying fluid etc. The heat transfer can mainly rely on heat conduction through the (usually metallic) structure of the fuel rail, which is either in direct thermal contact with the engine or in indirect contact, e.g., via the fuel injectors. It will be understood that although the buffer temperature at least partially or even entirely depends on the engine temperature, it will in general not be identical to the engine temperature. On the one hand, the buffer temperature will respond to changes in the engine temperature with a time delay. On the other hand, the fuelrail assembly and / or the buffer also transfer heat to other structures or to the surrounding air, wherefore the buffer temperature is usually between the engine temperature and ambient temperature. However, the passive heat transfer may be sufficient to induce changes of the buffer temperature of at least 50°C or at least 70°C, which is sufficient to substantially change the sorption capacity.

[0016] The invention is not limited to a specific type of sorbent or a specific sorption mechanism. For instance, the sorbent may comprise a metal-organic framework, a molecular sieve, a carbon nanotube material, or another suitable material. It is preferred, though, that the buffer comprises a metallic sorbent that is adapted to sorb hydrogen by formation of a metal hydride. A metallic sorbent is a sorbent that comprises at least one metal and that preferably consists predominantly of at least one metal, at least when it has not sorbed any hydrogen. Non-limiting examples for suitable metallic sorbents include LaNis, LaNi4,7sAlo,25 and TiFe. Suchmetallic sorbents may chemically absorb hydrogen by formation of a chemical compound, namely a metal hydride. Without limiting this embodiment to any type of sorption process, hydrogen may form covalent or ionic bonds with the metal(s) of the sorbent. Although the specific weight of the sorbent may be comparatively high, a small volume of sorbent can be enough to absorb a much higher volume of hydrogen. I.e., the effective density of the hydrogen in the sorbent can be much higher than in the gaseous phase.

[0017] A preferred embodiment provides that the fuel-rail assembly comprises a rail body defining a rail-body cavity, which is at least a portion of the rail cavity, and an installation opening communicating with the rail-body cavity, wherein the buffer is a buffer module having a module wall connected to the rail body so that it closes the installation opening in a gas-tight manner, and a sorbent portion that comprises the sorbent and is disposed inside the rail cavity. The rail body can be made of stainless steel or another suitable material. It defines the railbody cavity that corresponds to a portion of the rail cavity or is identical to the rail cavity. Thus, the rail cavity at least partially is disposed inside the rail body. The rail body also defines an installation opening, which is an opening that communicates with the rail-body cavity. In an assembled state, however, the installation opening is closed by the module wall of the buffer module. The buffer module can be manufactured separately from the rail body and can then be connected to the rail body to realize the buffer functionality. The module wall closes the installation opening in a gas-tight manner, which means that the connection between the module wall and the rail body is gas-tight and the module wall as such is gas-tight. Specifically, the connection is designed to withstand the typical gas pressure in the rail cavity. Apart from the module wall, the buffer module comprises a sorbent portion. This sorbent portion comprises or contains the sorbent. It may at least partially be connected to the module wall, either directly or indirectly.

[0018] According to one embodiment, the rail body comprises a cylindrical (or tubular) portion extending along a rail axis with the installation opening disposed at an axial end of the cylindrical portion. The cylindrical portion may have a circular cross-section, but other geometries are possible. It extends along the rail axis, and may in particular be aligned parallel to the rail axis. The above-mentioned injector ports may be oriented perpendicular to the rail axis. In this embodiment, theinstallation opening is disposed at an axial and of the cylindrical portion, i.e. , at an end in the axial direction as defined by the rail axis. The cylindrical / tubular design for fuel rails is commonly known in the art. Sometimes, an electrical connection or the like can be disposed at one end, which therefore is unsuitable for the position of the installation opening. In such a case, the opposite end can be used for the installation opening, though.

[0019] It is highly preferred that the buffer module is releasably connected to the rail body. “Releasably connected” means that the buffer module can be removed from the rail body in a non-destructive manner, i.e., without damaging the rail body or the buffer module. This is particularly advantageous in order to perform a replacement of the buffer module or a part thereof. While the rail body can be expected to have a service life similar to that of the engine or the machine as a whole, the service life of the buffer module may be shorter. Specifically, the sorbent could show some degradation after, e.g., several 10,000 cycles. In such a case, when the sorption capacity is significantly reduced, the buffer module, or at least the sorbent portion, can be replaced.

[0020] One embodiment provides that the buffer module and the rail body comprise cooperating mounting structures by which the buffer module is connected to the rail body. The mounting structures can also be described as complementary. One example would be an outer threading of the buffer module that cooperates with an inner threading of the rail body, or vice versa. Preferably, the mounting structures are adapted for a releasable connection. While the mounting structures facilitate the mechanical connection as such, they may also enable or enhance the gas-tightness of the connection. The buffer module may comprise a first mounting structure that is disposed at least at the module wall. The rail body may comprise a second mounting structure that is disposed adjacent the installation opening.

[0021] It is conceivable that the sorbent portion consists only of the sorbent, which could be connected by gluing or the like to the module wall. However, such a connection may be unreliable. Also, this configuration increases the likelihood of the sorbent being damaged or its surface being polluted before or during the installation process. According to another embodiment, the sorbent portion comprises an at least partially gas-permeable gas-exchange wall at least indirectly connected to the module wall and at least partially defining a sorbent cavity in which the sorbent isdisposed. The gas-exchange wall at least partially defines the boundary of the sorbent cavity which holds the sorbent. Even if the sorbent is not connected to any part of the buffer module, it is retained inside the sorbent cavity. However, the sorbent could be connected, e.g., bonded, to the gas-exchange wall, the module wall or some other part of the buffer module. In order to enable hydrogen sorption and release, the gas-exchange wall is at least partially gas-permeable, i.e. , it can be traversed by a gas. Specifically, hydrogen can traverse the gas-exchange wall.

[0022] One option would be that the gas-exchange wall is at least partially made of a material that is gas-permeable, like a thin membrane through which the gas can be transferred. However, such a membrane or a comparable structure may be highly susceptible to mechanical damage. Another preferred possibility is that the gas-exchange wall comprises at least one aperture, preferably a plurality of apertures, which enable gas exchange through the gas-exchange wall. In such a case, the gas-exchange wall may be made of a mechanically resistant material like stainless steel, which can be regarded as impermeable, but hydrogen gas may freely traverse the at least one aperture. Each aperture should be small enough to provide reasonable protection for the sorbent during the installation and to retain the sorbent if it is not bonded to the module wall or another part of the sorbent module, or if such a bonding connection fails. The gas-exchange wall may comprise a multitude of apertures and may, e.g., be a perforated sheet, a mesh, or the like.

[0023] It is conceivable that the sorbent portion is disposed outside of the railbody cavity, in which case the module wall may define a cup-shaped structure on the outside of the rail body. Preferably though, the buffer module is connected to the rail body so that the sorbent portion is at least partially disposed inside the rail-body cavity. One could also say that the sorbent portion protrudes into the rail-body cavity. This configuration is very space saving since only a minor part of the buffer module is disposed on the outside of the rail body. Also, the size of the module wall can be minimized and may roughly correspond to the size of the installation opening.

[0024] In some embodiments, the necessary temperature changes of the sorbent may be caused exclusively by the temperature changes of the engine. In other embodiments, as already mentioned above, additional measures may be taken. In such a case, the fuel-rail assembly may comprise at least one tempering element adapted for at least indirect heat exchange with the sorbent and beingcontrollable to influence the buffer temperature. The heat transfer between the tempering element and the sorbent may specifically be based on heat conduction, while convection or heat radiation may also contribute. The tempering element may be adapted to increase the buffer temperature, i.e., to heat the sorbent, or to decrease the buffer temperature, i.e., to cool the sorbent. In any case, the heattransfer element is controllable, either by the engine system or by an external system. Such control may in particular include activating and deactivating the heattransfer element. Specifically, the element can be controllable independently of an operating state of the engine. One option would be a heat exchanger for transferring heat between the sorbent and a fluid path of the machine. The fluid path may in particular be adapted to conduct oil, cooling fluid, or exhaust gas. In this context, the term “heat exchanger” is not to be construed in a limiting manner, but rather refers to any device that enables heat transfer without substance transfer. The engine system can be adapted to control the flow through the fluid path or through the heat exchanger, respectively.

[0025] As an alternative to a heat exchanger or in addition thereto, the engine system may comprise an electrical heater adapted to increase the buffer temperature. The electrical heater can be useful in various situations. For instance, when the machine has been non-operative for a longer time period, the temperature of a cooling fluid is low and even the exhaust system - if present - may initially have a low temperature. In situations like these, it may take several minutes before enough heat is available to effectively increase the buffer temperature, e.g., via the abovementioned heat exchanger. An electric heater can increase the buffer temperature without significant delay. The heater can be activated even before the engine is started. The electrical heater may be controlled regarding its operating times as well as regarding its power output. If an electrical heater is used, the engine could be operated during its initial operating phase only with hydrogen from the buffer. Accordingly, the engine system could be adapted to delay fuel supply from the fuel reservoir to the fuel-rail assembly for an initial start phase of the engine. Yet another option for a tempering element would be a Peltier device, which could be used for electrically cooling the sorbent.

[0026] It is possible to integrate a tempering element into the rail body. Another option is that at least one tempering element is integrated into the buffermodule. Thus, the tempering element is in close proximity of the sorbent, which enhances the heat transfer. Also, the tempering element can be removed from the rail body together with the buffer module, which can be useful for a possible replacement procedure.

[0027] On the one hand, the tempering element should have a good thermal connection with the sorbent, which can be achieved, among others, by a small distance between these elements. On the other hand, the tempering element may lack sufficient chemical resistance against hydrogen, while the sorbent needs to be in contact with the hydrogen in the rail cavity. For these and other reasons, one embodiment provides that the buffer module comprises a gas-tight separating wall separating the at least one tempering element from the sorbent portion. Accordingly, the tempering element is separated or isolated from the sorbent and the hydrogen inside the rail cavity. In some embodiments, the separating wall is at least partially identical to the module wall. In other embodiments, these may be two distinct walls, wherein the tempering element may at least partially be disposed between the two walls.According to one embodiment, the engine system is adapted to use at least one tempering element to reduce the buffer temperature in response to the machine entering a non-operating state, and / or to increase the buffer temperature in response to the machine re-entering the operating state. The engine system may comprise a control unit that is adapted to control the performance of the tempering element. E.g., it could control the electrical current in an electrical heater or a Peltier device or it could control the flow rate of a fluid in a heat exchanger. The buffer temperature may be reduced in response to the machine entering a non-operating state, which includes the possibility that the buffer temperature starts to be reduced before the machine enters the non-operating state. The non-operating state is a state in which the engine is not operating, corresponding to an “ignition off’ state in a vehicle, but not necessarily an “key off" state.

[0028] The invention further provides a fuel-rail assembly for an engine system of a hydrogen-powered machine, which engine system is adapted for supplying hydrogen fuel from fuel reservoir to a hydrogen internal combustion engine when the machine is in an operating state, the fuel-rail assembly defining a rail cavity in communication with a plurality of injector ports, each injector port beingadapted for connection to a fuel injector of the engine, wherein the fuel-rail assembly comprises a buffer with a sorbent that is disposed in the rail cavity and that is adapted to sorb hydrogen with a sorption capacity that decreases as a function of a buffer temperature of the sorbent.

[0029] All these terms have been described above with reference to the inventive engine system and therefore will not be explained again. Preferred embodiments of the inventive fuel-rail assembly correspond to those of the inventive engine system.

[0030] The invention further provides a method for operating an engine system for a hydrogen-powered machine, which engine system is adapted for supplying hydrogen fuel from fuel reservoir to a hydrogen internal combustion engine when the machine is in an operating state, the engine system comprising a fuel-rail assembly defining a rail cavity in communication with a plurality of injector ports, each injector port being connected to a fuel injector of the engine, wherein the fuel-rail assembly comprises a buffer with a sorbent that is disposed in the rail cavity and that is adapted to sorb hydrogen with a sorption capacity that decreases as a function of a buffer temperature of the sorbent, the method comprising:- the machine entering a non-operating state and the buffer temperature decreasing in response thereto, whereby hydrogen from the rail cavity is sorbed into the sorbent, and- the machine re-entering the operating state and the buffer temperature increasing in response thereto, whereby hydrogen is released from the sorbent into the rail cavity.

[0031] All these terms have been explained above with respect to the inventive engine system and therefore will not be explained again. The method may in particular include the buffer temperature increasing and / or decreasing at least partially due to heat transfer between the buffer and the engine through the fuel-rail assembly. Alternatively or additionally, it may comprise the engine system using at least one tempering element to reduce the buffer temperature in response to the machine entering a non-operating state, and / or to increase the buffer temperature in response to the machine re-entering the operating state. Other preferredembodiments of the inventive method correspond to those of the inventive engine system.Brief Description of the Drawings

[0032] 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 an engine system according to a first embodiment of the present invention;Fig.2 is a side view of a first embodiment of a fuel-rail assembly for the engine system from fig.1 ;Fig.3 is a half sectional view of a rail body and a buffer module of the fuel-rail assembly from fig.2;Fig.4 is a sectional view of another embodiment of a buffer module;Fig.5 is a schematic view of components of a hydrogen vehicle with an engine system according to a second embodiment of the present inventionFig.6 is a diagram showing the relationship between temperature and hydrogen gas pressure for different types of sorbents; andFig.7 is a diagram showing the relationship between hydrogen concentration in a sorbent and hydrogen gas pressure for different temperatures.Description of Preferred Embodiments

[0033] Figs.1 shows a schematic view of some elements of a hydrogen- powered machine, in this case a hydrogen vehicle, more specifically an HICE vehicle, with a first embodiment of an inventive engine system 1. The vehicle comprises a hydrogen internal combustion engine 50, which is shown in a highly simplified form with only a single cylinder 51 and a single piston 52 visible. The engine 50 is connected to an intake duct 53 (normally an intake manifold) and an exhaust duct 54 (normally an exhaust manifold). For each cylinder 51 , a fuel injector 55 is arranged to inject hydrogen fuel directly into a cylinder head. The engine 50 represents a power source of the vehicle, in which hydrogen is burned with oxygento convert chemical energy into mechanical energy. The mechanical energy is used to drive the vehicle and may also be converted through a generator (not shown) into electrical energy.

[0034] The hydrogen fuel needed to supply the engine 50 is stored in a fuel reservoir, in this case a single fuel tank 2, which is highly pressure-resistant. In the fuel tank 2, the pressure may be several hundred bar(a), e.g., up to 700 bar(a). The fuel tank 2 typically includes a regulator stage configured to deliver hydrogen fuel at a pressure of about 50 bar(a). The fuel tank 2 is connected to the engine 50 via a fuel-supply system 3, which is only shown schematically. A shut-off valve, which is not shown here, may be integrated into the fuel tank 2 or may be interposed between the fuel tank 2 and the fuel-supply system 3. The engine system 1 , specifically the fuel-supply system 3, comprises a fuel-rail assembly 10, which is connected to the fuel injector 55, and a hydrogen regulation module (HRM) 4, which may comprise various elements, like a pressure regulator that reduces the pressure of the hydrogen fuel, e.g., from about 50 bar(a) downstream of the regulator stage to between 20 and 40 bar(a), a heating or cooling device for adjusting the temperature of the hydrogen fuel etc. The fuel-supply system 3 comprises a shut-off valve 5, by which the fuel-rail assembly 10 can be isolated from HRM 4 and the fuel tank 2 (i.e. , flow of fuel downstream of the shut-off valve 5 is prevented when the valve is closed). The shut-off valve 5 may be independent or integrated in the HRM 4. Its state is controlled by a control unit 40, which may also control other components. While the vehicle is in an operating state, hydrogen fuel is supplied to the fuel injector 55 and the shut-off valve 2 is open. When the vehicle enters a non-operating state in which the engine 50 is turned off, the shut-off valve 2 is closed so that the fuel-rail assembly 10 is isolated from the HRM 4 and the fuel tank 2. However, a considerable amount of hydrogen fuel still resides in the fuel-rail assembly 10 and is still under considerable pressure, e.g., between 20 and 40 bar(a).

[0035] In order to mitigate any explosion risk originating from hydrogen fuel leaking out of the fuel-supply system 3, specifically out of the fuel-rail assembly 10, while avoiding any unnecessary waste of hydrogen, a buffer 20 is integrated into the fuel-rail assembly 10. Fig.2 shows the fuel-rail assembly 10 individually. It comprises a rail body 11 made of stainless steel. The rail body 11 defines a rail-body cavity 18, which constitutes a major portion of a rail cavity 12 (shown in fig.3). The rail body11 is typically the fuel rail per se; in communion language, the expressions fuel-rail assembly and fuel rail are considered synonym - but to some extent the fuel-rail assembly can be understood as encompassing the components added to the rail / body. The rail cavity 12 communicates with an inlet port 13 through which the fuel-rail assembly 10 receives hydrogen fuel, and a plurality of injector ports 14 which are connected to the fuel injectors 55. The rail body 11 comprises a cylindrical I tubular portion 15 that is aligned to a rail axis A. At one end 16 along an axial direction of the rail axis A, the rail cavity 12 communicates with an installation opening 17. An inner threading 19 is disposed circumferentially around the installation opening 17. As shown in fig.2, the buffer 20, which in this case is a buffer module 21 , is partially inserted into the rail-body cavity 18.

[0036] The configuration of the buffer module 21 can best be seen in fig.3. It comprises a module wall 22 which is connected to an outer threading 29. When the buffer module 21 is installed, the outer threading 29 cooperates with the inner threading 19 and the module wall 22 closes the installation opening 19 in a gas-tight manner. A screw drive 28, e.g., a hex drive, is disposed on the outside of the module wall 22 to facilitate the screwing process during installation of the buffer module 21 . The buffer module 21 also comprises a sorbent portion 23 with a gas-exchange wall 25 that is indirectly connected to the module wall 22. The gas-exchange wall 25 may be made of the same material as the module wall 22, e.g., stainless steel. It defines a sorbent cavity 27 in which a sorbent 24 is disposed. Since the gas-exchange wall 25 comprises a plurality of apertures 26, gas can freely traverse it. Therefore, when the buffer module 21 is installed to the rail body 11 , hydrogen fuel from the rail cavity12 can enter the sorbent cavity 27 and reach the surface of the sorbent 24. The sorbent is an alloy, e.g., LaNis, which can absorb hydrogen by formation of a metal hydride. A sorption capacity of the sorbent 24 decreases as a function of a buffer temperature TB. The buffer module 21 can be removed from the rail body 11 by unscrewing the threadings 19, 29. Such removal may be necessary when the sorbent 24 has deteriorated significantly, i.e. , the sorption capacity is reduced so much that the functionality of the buffer 20 is impaired.

[0037] When the vehicle is in an operating state, an engine temperature TE of the engine 50 is high. Due to heat transfer through the fuel-rail assembly 10, the buffer temperature TB of the sorbent 24 increases and reaches a high level at whichonly a small amount of hydrogen can be absorbed, even though the pressure in the fuel-rail assembly 10 is high, as mentioned above. In the operating state, the control unit 40 keeps the buffer valve shut-off valve 5 open. When the vehicle enters a nonoperating state, the engine 50 is stopped and the shut-off valve 5 is closed. Initially, the pressure in the fuel-rail assembly 10 remains high. However, as the engine 50 starts to cool down, the buffer temperature TB also starts to decline. Correspondingly, the sorbent capacity of the sorbent 24 increases and hydrogen is absorbed. This also leads to a decreasing pressure in the fuel-rail assembly 10. When the buffer temperature TB approaches ambient temperature, the pressure in the fuel-rail assembly 10 has reached a target pressure of e.g., 1 ,5 bar(a). A major part of the hydrogen previously present in the fuel-rail assembly 10 is now stored in the buffer 20, where it remains while the vehicle is in the non-operating state.

[0038] When the vehicle re-enters the operating state, the shut-off valve 5 is reopened and the pressure in the engine system again reaches a high value of e.g., 40 bar(a). As the engine temperature TE rises, the buffer temperature TB also increases, wherefore the sorption capacity decreases. Hydrogen fuel is released from the sorbent 24 and mixes with other hydrogen fuel in the rail-body cavity 18, from where it is injected through one of the fuel injectors 55 into the engine 50. After some time, the amount of hydrogen absorbed in the buffer 20 is sufficiently reduced so that the buffer 20 can be considered “empty”. When the vehicle re-enters the non-operating state, hydrogen can be absorbed again as described.

[0039] While the buffer temperature TB in the first embodiment is only influenced by passive heat transfer, figs. 4 and 5 illustrate an example in which this is augmented with active heat transfer. As can be seen in fig.4, the buffer module 21 comprises two tempering elements 30, 31 , namely an electrical heater 30 and a heat-exchanger 31 . Both tempering elements 30, 31 are separated from the sorbent portion 23 by a separating wall 26, which in this case is a portion of the module wall. As can be seen in fig.5, the heater 30 is connected to a power supply 33 which is controlled by the control unit 40. The heat exchanger 31 is connected to a coolant circuit 33, while coolant flow through the heat exchanger can be adapted through a flow valve 34 that is also controlled by the control unit 40. The control unit 40 can also monitor the buffer temperature TB through a temperature sensor 35.

[0040] When the vehicle has been in the operating state for some time, the control unit 40 keeps both tempering elements deactivated, i.e. , the power source 32 is deactivated and the valve 34 is closed. When the vehicle enters the nonoperating state, the control unit 40 may open the valve 34 so that the cooling process of the sorbent 24 is accelerated. By monitoring the buffer temperature T B, the control unit 40 can determine if and when the heat exchanger 31 can be deactivated. When the vehicle re-enters the operating state, the heating of the sorbent 24 can be accelerated by activating the heater 30. This can be particularly advantageous during a cold start of the engine 50. It is also possible to delay the opening of the shut-off valve 5 for some time during which the engine is only supplied with hydrogen that is released from the buffer 20.

[0041] Figs. 6 and 7 illustrate the characteristics of the sorption process that can be utilized by the inventive engine system 1. Fig.6 is a diagram showing the relationship between temperature, which may be equated with the buffer temperature TB and hydrogen gas pressure, which may be equated with the pressure in the fuel-rail assembly 10, for different types of sorbents. The abscissa represents the temperature in °C, or the inverted temperature in 1000 K’1, respectively. The ordinate represents the pressure in atm using a logarithmic scale. The solid line represents the graph for TiFeo,sNio,2, the long-dashed line represents the graph for LaNis, the dash-dotted line represents the graph for TiMni s, and the short-dashed line represents the graph for MmNis, where Mm represents one or several transition metals. The grey-shaded area represents a realistic operation range for the buffer module 20 in the engine system 1 , namely a pressure range between 1 and 20 atm and a temperature range between -40°C and 100°C. Although the exact dependency is different for each sorbent, all show a characteristic pressure increase with rising temperature, i.e., a pressure decrease with falling temperature. As can be seen, a typical temperature swing between 100 and 25 °C can reduce the pressure by one order in magnitude. Fig.7 is a diagram showing the relationship between hydrogen concentration in a sorbent and hydrogen gas pressure for different temperatures. Again, the temperature may be equated with the buffer temperature TB, and the hydrogen gas pressure may be equated with the pressure in the fuel-rail assembly 10. The diagram represents values for LaNi4,7sAlo,25, as an example, but other sorbents have similarcharacteristics. The abscissa represents the hydrogen concentration in the sorbent, i.e. , the mole ratio. The ordinate represents the pressure in kPa using a logarithmic scale. The solid line represents the graph for 313K, the long-dashed line represents the graph for 333K, the dash-dotted line represents the graph for 353K, and the short-dashed line represents the graph for 373K. What can be seen in particular is that for each temperature, the graph shows a plateau in which a large change in concentration leads only to a small pressure change. One could therefore say that for a certain temperature, the pressure will be in a relatively narrow range and be largely independent of the amount of hydrogen that is sorbed. On the other hand, the position of the plateau is very different for different temperatures, i.e., the pressure is highly dependent on the temperature. Points P1 and P2 in Fig.7 also illustrate an exemplary working cycle for this sorbent material. P1 is the hot situation: the rail and buffer / sorbent are hot (at 100°C), the pressure is comparatively high. As the engine cools down, the sorbent is adsorbing hydrogen, hence the pressure decreases. At point P2, which is about 40°C, the pressure has decreased by about 1 order of magnitude. When the engine will heat up again, the adsorbed amount of hydrogen will be released. On the horizontal axis, the difference between P1 and P2 represents the adsorbed, respectively desorbed, quantity of hydrogen.

[0042] Legend of Reference Numbers:1 engine system 18 rail-body cavity2 fuel tank 19 installation opening3 fuel-supply system 20 buffer module4 hydrogen regulation module 22 module wall5 shut-off valve 23 sorbent portion10 fuel-rail assembly 24 sorbent11 rail body 25 gas-exchange wall12 rail cavity 26 aperture13 inlet port 27 sorbent cavity14 injector port 28 screw drive15 cylindrical portion 29 outer threading16 axial end 30 electrical heater17 inner threading 31 heat exchangerpower source 52 piston coolant circuit 53 intake port flow valve 54 outlet port control unit 55 fuel injector engine cylinder

Claims

Claims1 . An engine system (1 ) for a hydrogen-powered machine, which engine system (1 ) is adapted for supplying hydrogen fuel from a fuel reservoir (2) to a hydrogen internal combustion engine (50) when the machine is in an operating state, the engine system (1 ) comprising a fuel-rail assembly (10) defining a rail cavity (12) in communication with a plurality of injector ports (14), each injector port (14) being connected to a fuel injector (55) of the engine, wherein the fuel-rail assembly (10) comprises a buffer (20) with a sorbent (24) that is disposed in the rail cavity (12) and that is adapted to sorb hydrogen with a sorption capacity that decreases as a function of a buffer temperature (TB) of the sorbent (24).

2. The engine system according to claim 1 , wherein the fuel-rail assembly (10) is adapted for passive heat transfer between the buffer (20) and the engine (50), whereby the buffer temperature (TB) at least partially depends on an engine temperature (TE).

3. The engine system according to any one of the preceding claims, wherein the buffer (20) comprises a metallic sorbent (24) that is adapted to sorb hydrogen by formation of a metal hydride.

4. The engine system according to any one of the preceding claims, wherein the fuelrail assembly (10) comprises a rail body (11 ) defining a rail-body cavity (18), which is at least a portion of the rail cavity (12), and an installation opening (19) communicating with the rail-body cavity (12), and wherein the buffer (20) is a buffer module (21 ) having a module wall (22) connected to the rail body (11 ) so that it closes the installation opening (19) in a gas-tight manner, and a sorbent portion (23) that comprises the sorbent (24) and is disposed inside the rail cavity (12).

5. The engine system according to any one of the preceding claims, wherein the rail body (11 ) comprises a cylindrical portion (15) extending along a rail axis (A), wherein the installation opening (19) is disposed at an axial end 16 of the cylindrical portion (15).

6. The engine system according to any one of the preceding claims, wherein the buffer module (21 ) is releasably connected to the rail body (11 ).

7. The engine system according to any one of the preceding claims, wherein the buffer module (21 ) and the rail body (11 ) comprise cooperating mounting structures (19, 29) by which the buffer module (21 ) is connected to the rail body (11 ).

8. The engine system according to any one of the preceding claims, wherein the sorbent portion (23) comprises an at least partially gas-permeable gas-exchange wall (25) at least indirectly connected to the module wall (22) and at least partially defining a sorbent cavity (27) in which the sorbent (24) is disposed.

9. The engine system according to any one of the preceding claims, wherein the buffer module (21 ) is connected to the rail body (11 ) so that the sorbent portion (23) is at least partially disposed inside the rail-body cavity (18).

10. The engine system according to any one of the preceding claims, wherein the fuelrail assembly (10) comprises at least one tempering element (30, 31 ) adapted for at least indirect heat exchange with the sorbent (24) and being controllable to influence the buffer temperature (TB).11 . The engine system according to any one of the preceding claims, wherein at least one tempering element (30, 31 ) is integrated into the buffer module (21 ).

12. The engine system according to any one of the preceding claims, wherein the buffer module (21 ) comprises a gas-tight separating wall (16) separating the at least one tempering element (30, 31 ) from the sorbent portion (27).

13. The engine system according to any one of the preceding claims, being adapted to use at least one tempering element (30, 31 ) to reduce the buffer temperature (TB) in response to the machine entering a non-operating state, and / or to increase the buffer temperature (TB) in response to the machine re-entering the operating state.

14. A fuel-rail assembly (10) for an engine system (1 ) of a hydrogen-powered machine, which engine system (1 ) is adapted for supplying hydrogen fuel from fuel reservoir (2) to a hydrogen internal combustion engine (50) when the machine is in an operating state, the fuel-rail assembly (10) defining a rail cavity (12) in communication with a plurality of injector ports (14), each injector port (14) being adapted for connection to a fuel injector (55) of the engine (50), wherein the fuelrail assembly (10) comprises a buffer (20) with a sorbent (24) that is disposed in the rail cavity (12) and that is adapted to sorb hydrogen with a sorption capacity that decreases as a function of a buffer temperature (TB) of the sorbent 24().

15. A method for operating an engine system (1 ) for a hydrogen-powered machine, which engine system (1 ) is adapted for supplying hydrogen fuel from fuel reservoir (2) to a hydrogen internal combustion engine (50) when the machine is in an operating state, the engine system (1 ) comprising a fuel-rail assembly (10) defining a rail cavity (12) in communication with a plurality of injector ports (14), each injector port (14) being connected to a fuel injector (55) of the engine (50), wherein the fuel-rail assembly (10) comprises a buffer (20) with a sorbent (24) that is disposed in the rail cavity (12) and that is adapted to sorb hydrogen with a sorption capacity that decreases as a function of a buffer temperature (TB) of the sorbent (24), the method comprising:- the machine entering a non-operating state and the buffer temperature (TB) decreasing in response thereto, whereby hydrogen from the rail cavity (12) is sorbed into the sorbent (24), and- the machine re-entering the operating state and the buffer temperature (TB) increasing in response thereto, whereby hydrogen is released from the sorbent (24) into the rail cavity.

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