Patent Description:
There is an increasing drive in modern technology areas to move away from fossil fuels as a source of energy and to replace them with renewable energy sources. One notable development in recent years has been the development of electric vehicles where the fuel tank of the traditional internal combustion engine is replaced with a battery. However, current electric vehicle technologies have not achieved an energy density from the battery which is comparable with that achieved using traditional fuels (e.g. gasoline, diesel). Furthermore, such systems are limited with their range of travel which does not suit all user requirements, and for heavy duty applications where the size of the battery is impractical.

One alternative to these systems is to use a traditional internal combustion engine (ICE) but running on ecologically produced hydrogen gas. Such systems have been proposed in the art, but there are various efficiency concerns over those solutions and commercially viable options for such "hydrogen ICE" systems remain a challenge. One problem is that, for system efficiency, the hydrogen needs to be injected at pressures considerably higher than atmospheric pressure, which poses technical challenges for existing tank and injector designs.

Other gaseous fuels are also known for use in generating motive power, including compressed natural gas (CNG). <CIT> and <CIT> disclose examples of gaseous fuel systems adapted for use in vehicles.

Fuel cell technology, which relies on the ionisation of hydrogen within an electrolyte to generate electricity, is also well known for use in vehicles. Both systems require a source of gaseous fuel to generate motive power for the vehicle.

According to the present invention, there is provided a fuel system for supplying gaseous fuel to a power plant, the fuel system comprising a tank array comprising at least a first tank and a second tank configured to receive pressurised gaseous fuel for supply to the power plant, in use; a source of auxiliary control fluid for supply to the tank array when the fuel system is disconnected from the filling station, and a pump for pressurising auxiliary fluid supplied to the tank array when control fluid flows through the pump in a forward direction, and an energy store. A valve arrangement is operable to control the supply of auxiliary control fluid to the tank array so as to control the discharge of the gaseous fuel from the tank array, the valve arrangement being further operable to enable a reverse direction of flow of auxiliary control fluid from the tank array to enable a reverse direction of flow of auxiliary control fluid from the tank array through the pump when the fuel system is connected to a filling station, in use, to generate energy from the pump to store in the energy store.

The invention provides the advantage that energy harnessed from the filling phase can be recovered to an energy store (such as a battery) for later use, either in driving the pump for pressurising the auxiliary fluid or for other uses in the plant. The system therefore provides efficiency and running cost benefits.

The valve arrangement may include, for each tank of the array, an inlet one-way valve which is operable to control the supply of auxiliary control fluid to the associated tank and an outlet one-way valve which is operable to control the supply of auxiliary control fluid from the associated tank to the source of auxiliary control fluid.

The valve arrangement may further comprises an additional valve which is operable to close the source of auxiliary control fluid to allow auxiliary fluid within one of the first and second tanks to be returned in the reverse direction through the pump to the other of the first and second tanks, bypassing the source.

The valve arrangement may include, for each tank of the array, a fuel inlet one-way valve which is operable to control the supply of gaseous fuel to the tank array. The valve arrangement may be operable by means of an electronic control unit (ECU).

The invention is particularly applicable for use in engines for vehicles, so that the power plant may be an internal combustion engine of a vehicle.

According to a second aspect of the invention, there is provides a method for supplying gaseous fuel to a power plant, the fuel system comprising receiving pressurised gaseous fuel from a filling station in a tank array when the fuel system is connected with the filling station; pressurising auxiliary control fluid from a source of auxiliary control fluid by a pump when the auxiliary control fluid flows through the pump in a forward direction; supplying pressurised auxiliary control fluid to the tank array, when the fuel system is disconnected from the filling station, so as to discharge gaseous fuel from the tank array to the power plant; and enabling, under the control of a valve arrangement, a reverse direction of flow of auxiliary control fluid from the tank array through the pump when the fuel system is connected to the filling station so as to generate energy from the pump for storage within an energy store.

The fuel system may include an inlet one-way valve configured to control the flow of control fluid to an associated tank of the tank array, and an associated inlet valve configured to control the supply of gaseous fuel from the filling station to the associated tank, the method comprising operating the inlet one-way valve for control fluid prior to operating the inlet valves for gaseous fuel from the filling station when it is required to return control fluid in the reverse direction through the pump.

Operating the inlet one-way valves prior to the inlet valves from the filling station helps to reduce the effects of cavitation at the valve seats.

It will be appreciated that the various features of the first aspect of the invention are equally applicable to, alone or in appropriate combination, the second aspect of the invention also.

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:.

The present invention relates to the use of pressurised gaseous fuel to generate power within a power plant, such as an engine. One specific example of such a fuel system is shown in <FIG> which shows a fuel system for use in supplying pressurised hydrogen gas to an internal combustion engine, referred to generally as <NUM>, of a vehicle.

The fuel system includes a tank array including at least a first tank <NUM>. In the embodiment shown, the fuel system includes at least a first tank <NUM> and a second tank <NUM> configured to receive gaseous hydrogen from a supply tank <NUM> at a refuelling station. The first tank <NUM> is connected via a supply line <NUM> to the supply tank <NUM> and the second tank <NUM> is connected via the supply line <NUM> to the supply tank <NUM>. The supply tank <NUM> has a supply non-return valve <NUM> which is operable to open only when a fuel system is connected to the supply tank <NUM> for refilling. The supply line <NUM> is also provided with a supply inlet non-return valve <NUM> which ensures the system (i.e. the supply line <NUM>) remains closed when it is detached from the supply tank <NUM> at the refuelling station.

The supply line <NUM> from the supply tank <NUM> has two branches, one into the first tank <NUM> and one into the second tank <NUM>. The branch to the first tank <NUM> is provided with a first tank inlet non-return valve <NUM> which is operable to control the pressure of gas within the first tank <NUM> when the supply tank <NUM> is connected to the first tank <NUM>. When the pressure of hydrogen gas within the supply tank <NUM> exceeds that within the first tank <NUM>, the first tank inlet non-return valve <NUM> is caused to open to allow hydrogen gas to flow into the first tank <NUM>. The first tank inlet non-return valve <NUM> closes when the pressure of hydrogen gas within the first tank <NUM> equalises with that of the supply tank <NUM> and the first tank <NUM> is full. Likewise, the branch to the second tank <NUM> is provided with a second tank inlet non-return valve <NUM> which is operable to control the pressure of gas within the second tank <NUM>. When the pressure of hydrogen gas within the supply tank <NUM> exceeds that within the second tank <NUM>, the second tank inlet non-return valve <NUM> is caused to open to allow hydrogen gas to flow into the second tank <NUM>. The second tank inlet non-return valve <NUM> closes when the pressure of hydrogen gas within the second tank <NUM> equalises with that of the supply tank <NUM> and the second tank <NUM> is full.

In <FIG>, both the first tank <NUM> and the second tank <NUM> are full of hydrogen gas at the end of the system filling phase.

Each of the first and second tanks <NUM>, <NUM> is also provided with a respective outlet line, <NUM>, <NUM>, which connects the respective tank to a supply line <NUM> for a fuel rail <NUM> for receiving pressurised hydrogen gas from the tanks <NUM>, <NUM>. An outlet non-return valve <NUM>, <NUM>, respectively, is provided for each tank within the associated outlet line <NUM>, <NUM>. A first tank outlet non-return valve <NUM> is associated with the first tank <NUM> and a second tank outlet non-return <NUM> valve is associated with the second tank <NUM>. The outlet non-return valves <NUM>, <NUM> are operable to open when the pressure of hydrogen gas in the associated tank <NUM>, <NUM> exceeds the pressure of hydrogen gas within the common outlet line <NUM> (and hence the fuel rail <NUM>) but they prevent the return flow of pressurised hydrogen gas from the fuel rail <NUM> to the first and second tanks <NUM>, <NUM>.

Typically, the hydrogen gas that is supplied from the supply tank at the refuelling station is pressurised to a level of either <NUM> bar or <NUM> bar, or at a level between these two levels.

The fuel rail <NUM> is configured to deliver gaseous hydrogen to a plurality of fuel injectors <NUM> of the fuel system. In the embodiment shown the fuel system includes four injectors, each corresponding to a respective cylinder (not shown) of the engine. The injectors inject the hydrogen fuel at an injection pressure Pi, which is typically less than the storage pressure Ps.

Each of the first and second tanks <NUM>, <NUM> is identical internally and includes a separation member in the form of a movable membrane, referred to as the first and second tank membranes <NUM>, <NUM>. Considering the first tank <NUM>, a first tank membrane <NUM> is movable depending on the presence of an auxiliary control fluid that is supplied to the first tank <NUM> via an auxiliary control fluid delivery system, referred to generally as <NUM>. Likewise, a second tank membrane <NUM> is associated with the second tank <NUM> and is movable depending on the presence of an auxiliary fluid supplied to the second tank <NUM> by the auxiliary control fluid delivery system <NUM>.

The auxiliary control fluid delivery system includes an auxiliary supply tank (referred to as the auxiliary tank <NUM>) containing an auxiliary control fluid such as liquid oil, pressurising means in the form of a pump <NUM>, an auxiliary control fluid pipeline (comprising an auxiliary control fluid supply line <NUM> and an auxiliary control fluid return line <NUM>) and a valve arrangement for controlling the supply of auxiliary fluid to the tank array <NUM>, <NUM>. The auxiliary control fluid is considered to be a control fluid, for reasons that will become clear from the following description.

The pump <NUM> is located in the auxiliary control fluid supply line <NUM> and both the auxiliary control fluid supply <NUM> and return <NUM> lines are in fluid communication with a sole inlet/outlet port of the auxiliary tank <NUM>. The pump <NUM> is driven by a crank or shaft whose motion is coupled to that of a corresponding crank or shaft of the internal combustion engine. In other embodiments, the pump <NUM> may be electrically driven, or driven through a separate shaft which is not shared with the engine.

The valve arrangement includes four valves, two of which <NUM>, <NUM> are associated with the first tank <NUM> and two of which <NUM>, <NUM> are associated with the second tank <NUM>. For the first tank <NUM>, a first inlet one-way valve <NUM> controls the supply of auxiliary control fluid between the auxiliary tank <NUM> and the first tank <NUM> along the auxiliary control fluid supply line <NUM> and a first outlet one-way valve <NUM> controls the return flow of auxiliary fluid from the first tank <NUM> to the auxiliary tank <NUM> along the auxiliary control fluid return line <NUM>. Likewise, for the second tank <NUM>, a second inlet one-way valve <NUM> controls the supply of auxiliary control fluid between the auxiliary tank <NUM> and the second tank <NUM> along the auxiliary control fluid supply line <NUM> and a second outlet one-way valve <NUM> controls the return flow of auxiliary fluid from the second tank <NUM> to the auxiliary tank <NUM> along the auxiliary fluid return line <NUM>. By way of example, the auxiliary control fluid may take the form of oil.

The four valves <NUM>, <NUM>, <NUM>, <NUM> of the valve arrangement are controlled by means of an electronic control unit (ECU) <NUM>, as indicated by the electrical connections shown in dashed lines. Likewise, the ECU <NUM> controls the pump <NUM> which pressurises the auxiliary control fluid for supply to the first and second tanks <NUM>, <NUM>, as further illustrated by the electrical connections shown in dashed lines.

In the configuration shown in <FIG>, the first and second tanks <NUM>, <NUM> have just been re-filled at the refuelling station such that both the first and second tanks <NUM>, <NUM> are full of hydrogen gas. In each of the fuel tanks <NUM>, <NUM>, the gas is separated from the oil by the associated membrane <NUM>, <NUM> which is movable within the associated tank depending on the quantity of oil supplied to the tank from the auxiliary tank <NUM>.

The method of operation of the fuel system will now be described with reference to <FIG>.

<FIG> shows the fuel system having been detached from the re-fuelling system.

Initially, hydrogen gas can be supplied to the injectors without the intervention of the auxiliary control fluid delivery system, since the pressure of hydrogen gas in the first and second tanks and the common outlet line and fuel rail exceeds the injection pressure Pi. As fuel is supplied to the injectors from the fuel rail, the pressure of hydrogen gas in the fuel rail and common outlet line decreases. This causes the first and second tank outlet non-return valves to open to allow the pressure of hydrogen to equalise between the first and second tanks and the common outlet line and fuel rail. Eventually, as more hydrogen gas is supplied from the first and second tanks, the pressure in the first and second tanks and in the common outlet line and the fuel rail decreases to match the injection pressure Pi. At this point, the injectors cannot be supplied with hydrogen gas without the assistance of the auxiliary control fluid delivery system.

In <FIG>, the fuel system has been detached from the re-fuelling system (supply tank <NUM>) and the first tank <NUM> is filling the fuel rail <NUM> with pressurised hydrogen gas via the first tank outlet non-return valve <NUM>. The second tank <NUM> is in a "waiting phase", now full with pressurised hydrogen gas, and with the second tank outlet non-return valve <NUM> closed so that hydrogen gas cannot escape the second tank <NUM>. The supply inlet non-return valve <NUM> is closed (as the system is detached from the filling station) and the first and second tank inlet non-return valves <NUM>, <NUM> are also closed.

The first inlet one-way valve <NUM> of the first tank <NUM> is opened by the ECU <NUM> so that oil within the auxiliary tank <NUM> is able to flow, via the pump <NUM>, into the first tank <NUM>. As a result of the incoming oil flow, the first tank membrane <NUM> is displaced upwardly (in the illustration shown), reducing the volume of the available space for hydrogen gas and causing the pressure of the hydrogen gas within the tank <NUM> to increase above the pressure of the hydrogen gas in the common outlet line and the fuel rail. As a result, the first tank outlet non return valve <NUM> in the outlet line <NUM> is caused to open to discharge hydrogen gas from the first tank <NUM> into the common outlet line <NUM> to the fuel rail <NUM>. This is described as the "delivery phase" for the first tank <NUM> as hydrogen gas is delivered into the fuel rail <NUM> and enables the supply of hydrogen gas to the injectors once the pressure in the tank array, the common outlet line and the fuel rail has reached the injection pressure, Pi. In <FIG> it can be seen that the oil is starting to empty from the auxiliary tank <NUM> during this phase, displacing the hydrogen gas from the first tank <NUM> to the fuel rail <NUM>.

While the first tank is in the delivery phase, the second tank is in a "waiting phase", still full with pressurised hydrogen gas at the injection pressure Pi. The non-return aspect of the second tank outlet non-return valve prevents the hydrogen in the common outlet line entering the second tank, despite being at a higher pressure than the hydrogen in the second tank. The supply inlet non-return valve is closed (as the system is detached from the filling station) and the first and second tank inlet non-return valves are also closed.

Referring to <FIG>, the first tank <NUM> has been depleted of hydrogen gas and is near fully-filled with oil. The first tank <NUM> then enters an "oil discharge phase" during which the first inlet one-way valve <NUM> to the first tank is closed by the ECU <NUM> to prevent the further supply of oil through the auxiliary control fluid inlet line <NUM> to the first tank <NUM>, and the first outlet one-way valve <NUM> from the first tank <NUM> is opened to allow the oil within the first tank <NUM> to be discharged back to the auxiliary tank <NUM> via the auxiliary control fluid return line <NUM>. The oil, having been pressurised by the pump <NUM> before entering the first tank <NUM>, flows out of the first tank <NUM> under its own pressure as the auxiliary tank <NUM> is not pressurised and so a pressure gradient exists between the first tank <NUM> and the auxiliary tank <NUM>. Both the second inlet one-way valve <NUM> and the second outlet one-way valve <NUM> remain closed at this time so the status of the second tank <NUM> does not change during the oil discharge phase of the first tank <NUM>.

With the first tank <NUM> depleted of hydrogen gas, the first tank outlet non-return valve <NUM> closes, under the pressure of hydrogen gas within the common supply line <NUM>, to prevent any return flow of hydrogen gas into the first tank <NUM>. Hydrogen gas within the outlet line <NUM> and the fuel rail <NUM> is therefore unable to return to the first tank <NUM>. In summary, the first tank outlet non-return valve <NUM> is only open when the first tank <NUM> is being charged with auxiliary fluid.

It will be appreciated by the skilled person that, with the first tank outlet non-return valve <NUM> closed, the discharge of the auxiliary fluid back into the auxiliary tank leaves the first tank <NUM> substantially empty, save for some small amount of residual hydrogen gas. However, the residual pressure existing in the first tank <NUM> when the auxiliary fluid has been fully discharged still exceeds atmospheric pressure.

Referring now to <FIG>, with the first tank <NUM> depleted of hydrogen gas, subsequent delivery of hydrogen gas to the common rail must be made by the second tank. However, the pressure of the hydrogen gas within the second tank <NUM> is still at the injection pressure Pi, as a result of the initial discharge of hydrogen gas to the common outlet line and fuel rail immediately after the tank array was refilled. In this subsequent method step, hydrogen gas from the second tank <NUM> is discharged through the common outlet line <NUM> to the fuel rail <NUM> as a result of oil being delivered through the second inlet one-way valve <NUM>, in the same way as in <FIG> for the first tank <NUM>. As a result of the incoming oil flow from the auxiliary tank <NUM>, the second membrane <NUM> within the second tank <NUM> is displaced upwardly (in the illustration shown), reducing the volume of the available space for the hydrogen gas and causing the pressure of the hydrogen gas within the second tank <NUM> to increase above the pressure of the hydrogen gas in the common outlet line and the fuel rail. This causes the pressure of the hydrogen gas within the second tank <NUM> to increase and forcing the second tank outlet non return valve <NUM> to open to discharge hydrogen gas from the second tank <NUM> into the common outlet line <NUM>. This is described as the "delivery phase" for the second tank <NUM> as hydrogen gas is delivered into the fuel rail <NUM> through the common outlet line <NUM>. As discussed above, in this delivery phase of the second tank <NUM> the first tank <NUM> is already fully discharged of hydrogen gas. Throughout the delivery phase of the second tank <NUM> the first inlet one-way valve <NUM> is maintained in the closed position. Likewise, the first outlet one-way valve <NUM> remains closed.

In an alternative step to that described above, it is possible for the discharge of oil from the first tank <NUM> to the auxiliary tank to be implemented at the same time as oil is delivered to the second tank <NUM> to displace hydrogen gas from the second tank <NUM> to the fuel rail <NUM>. For this to occur, the ECU <NUM> sends a control signal to the second inlet one-way valve <NUM> of the second tank <NUM> to cause it to open at the same time as the first outlet one-way valve <NUM> of the first tank <NUM> is opened to return oil to the auxiliary tank <NUM>. This process will be described in further detail below.

<FIG> shows the situation where the hydrogen gas within the second tank <NUM> is fully depleted, at which point the second tank outlet non-return valve <NUM> is caused to close (due to the pressure of hydrogen within the supply line <NUM> to the fuel rail <NUM>). Hydrogen gas within the fuel rail <NUM> is therefore unable to return to the second tank <NUM>. Also, the second outlet one-way valve <NUM> is operated to open so as to allow the oil within the second tank <NUM> to start to discharge back to the auxiliary tank <NUM>, in the same way as described above for the first tank <NUM> with reference to <FIG>. Both the first inlet one-way valve <NUM> and first outlet one-way valve <NUM> of the first tank <NUM> remain closed during this phase. As with the first tank <NUM>, once the oil has been fully discharged from the second tank <NUM> back to the auxiliary tank <NUM>, the second tank <NUM> is left substantially empty of hydrogen gas, with only a small residual amount of hydrogen gas left inside the tank <NUM> at a residual pressure exceeding atmospheric pressure.

The system provides an efficient way of discharging pressurised hydrogen gas, at a pressure in excess of atmospheric pressure, to the internal combustion engine, using convenient control of a valve arrangement controlling the supply of auxiliary fluid into the tanks.

Referring to <FIG>, once the first tank <NUM> and the second tank <NUM> have been fully discharged of hydrogen gas, the system requires re-filling at the filling station (as in <FIG>). The system may also be re-filled with hydrogen gas when either one or both of the tanks <NUM>, <NUM> are partially emptied, depending on the convenience of the user.

<FIG> illustrates the system when the first tank <NUM> is full of hydrogen gas but the second tank <NUM> has been depleted of hydrogen gas and requires refilling from the supply tank <NUM> at the service station. The second tank <NUM> is filled with auxiliary fluid, having been filled with auxiliary fluid so as to discharge the hydrogen gas within the second tank <NUM> to the supply line <NUM> and, hence, to the fuel rail <NUM>. The system in <FIG> differs from that in <FIG> in the following respects. Firstly, the first tank inlet non-return valve <NUM> is replaced with a first tank fuel inlet one-way valve <NUM> which is controlled by means of the ECU <NUM>, as illustrated by the dashed line electrical connection to the ECU <NUM>. Correspondingly, the second tank inlet non-return valve <NUM> is replaced with a second tank fuel inlet one-way valve <NUM> which is controlled by means of the ECU <NUM>, as illustrated by the dashed line electrical connection to the ECU <NUM>. In addition, the pump <NUM> is connected to an energy store in the form of a battery <NUM> which is chargeable by running the pump <NUM> as a motor, as described in further detail below. The battery <NUM> may be used to provide power to the pump <NUM>, when running in a pump mode, or may be used to supply power to other parts of the engine.

With the supply tank <NUM> connected to the supply line <NUM>, the pressure of hydrogen gas within the supply tank <NUM> causes the supply inlet non-return valve <NUM> of the fuel system to open because the pressure of hydrogen gas in the supply tank <NUM> exceeds that within the supply line <NUM>. At the same time, the ECU <NUM> sends a signal to the second tank fuel inlet one-way valve <NUM> to cause the valve <NUM> to open, allowing pressurised hydrogen gas from the supply tank <NUM> to fill the second tank <NUM>. The first tank fuel inlet one-way valve <NUM> remains closed at this time. A sensor (not shown) may be provided on the fuel system to detect when the system is connected to the tank <NUM> at the filling station, and a signal is provided to the ECU <NUM> to indicate the connection status, so that the ECU <NUM> knows when to initiate opening of the second tank fuel inlet one-way valve <NUM>.

In other embodiments, the ECU <NUM> may be programmed with an algorithm to determine that the vehicle is filling, based on one or more other vehicle sensors which may be fitted on the vehicle for other purposes.

The first and second tank outlet valves <NUM>, <NUM> remain closed at this time because, for the first tank <NUM>, the pressure of hydrogen gas in the supply line <NUM> exceeds that within the first tank <NUM> (until such time as auxiliary fluid is supplied to the first tank <NUM>) to keep the first tank outlet valve <NUM> closed, and because the hydrogen gas is still filling the second tank <NUM> at this time and pressure within the second tank <NUM> is not high enough to cause the second tank outlet valve <NUM> to open.

The control valve arrangement is then operated so as to open the second inlet one-way valve <NUM> (as can be seen in <FIG>). The second outlet one-way valve <NUM> remains closed, as do the first inlet one-way valve <NUM> and the first outlet one-way valve <NUM>. In normal operation, the second inlet one-way valve <NUM> is used to allow auxiliary fluid to fill the second tank <NUM> during the filling phase. However, by opening the second inlet one-way valve <NUM> whilst the second outlet one-way valve <NUM> remains closed, and with the hydrogen gas filling the second tank <NUM> from the supply tank <NUM>, auxiliary fluid within the second tank <NUM> is forced to return through the open valve <NUM> into the inlet line <NUM>, back through the pump <NUM>, to return to the reservoir <NUM>. In practice, the second inlet one-way valve <NUM> is opened slightly before the valve <NUM>. During this phase, the pressure generated by hydrogen gas from the filling station is therefore used to generate energy via the reverse flow of auxiliary fluid through the pump <NUM> which acts as a motor. The energy generated by the motor can be stored in the battery <NUM> for later use in driving the pump <NUM> (or for other purposes). This phase of operation may be described as the 'energy harnessing' or 'recuperating' phase of operation as it enables energy to be harnessed from the filling station, and stored in the battery <NUM>, to reduce the demand on the pump <NUM> in later cycles. Having passed through the pump <NUM>, auxiliary fluid is returned to the supply tank <NUM> which remains charged with auxiliary fluid until such time as the inlet valve <NUM> to the first tank <NUM> is opened.

Eventually, at the end of the filling phase when the supply tank <NUM> is disconnected from the supply line <NUM>, the second tank <NUM> is fully charged with pressurised hydrogen gas once more and the supply tank <NUM> is filled with auxiliary fluid.

<FIG> shows a modification to the system shown in <FIG> in which an additional two-way valve <NUM> is provided, operable under the control of the ECU <NUM> (as indicated by the dashed control line) to control the communication between the supply tank <NUM> and the auxiliary fluid inlet and return lines <NUM>, <NUM>. The additional two-way valve <NUM> has two operating positions. In a first (closed) position, the additional valve <NUM> closes the inlet port on the supply tank <NUM> and allows the auxiliary control fluid supply line <NUM> to communicate directly with the auxiliary control fluid return line <NUM>, without the need to pass through the supply tank <NUM>. In the second (open) position, the additional two-way valve <NUM> opens the inlet port on the supply tank <NUM> to allow the auxiliary fluid return line <NUM> and the auxiliary fluid supply line <NUM> to communicate with the supply tank <NUM>.

The method of operation of the system in <FIG> is similar to that shown in <FIG>, in that energy derived from the filling station is harnessed by reversing the flow through the pump <NUM> to act like a motor. However, in this case the returning flow of auxiliary fluid which is displaced from the second tank <NUM> is used to directly fill the first tank <NUM>, to displace hydrogen gas from the first tank <NUM>, whilst the second tank <NUM> is filled with hydrogen gas. The method of operation of the system in <FIG> will now be described in more detail.

As for <FIG>, the supply tank <NUM> is connected to the supply line <NUM> and the system inlet <NUM> is caused to open due to pressure in the supply tank <NUM> exceeding that within the supply line <NUM>. In addition, the ECU <NUM> controls the second tank fuel inlet one-way valve <NUM> to open, so as to enable hydrogen gas to fill the second tank <NUM>. In practice, the second inlet one-way valve <NUM> is opened slightly before the valve <NUM>. The first tank fuel inlet one-way valve <NUM> remains closed at this time. A sensor (not shown) may be provided on the fuel system to detect when the system is connected to the filling station and the tank <NUM>, and a signal is provided to the ECU <NUM>, so that the ECU <NUM> knows when to initiate opening of the second tank fuel inlet one-way valve <NUM>.

In addition to opening the second inlet one-way valve <NUM> to allow auxiliary fluid to return to the supply tank <NUM> via the pump <NUM>, the ECU <NUM> controls the first outlet one-way valve <NUM> so that it is also opened at the same time (as seen in <FIG>). In addition, the additional valve <NUM> is controlled by the ECU <NUM> to move to the closed position in which the inlet port to the supply tank <NUM> is closed. As a result, the supply of pressurised hydrogen gas to the second tank <NUM> displaces auxiliary fluid from the second tank <NUM> back through the inlet line <NUM> to the pump <NUM>. With the reverse flow of fluid through the pump <NUM>, the pump acts as a motor and generates energy which can be stored in the battery <NUM>. With the additional valve <NUM> closing the supply tank <NUM>, fuel in the auxiliary control fluid inlet line <NUM> is thus able to flow through the additional valve <NUM> directly into the auxiliary control fluid return line <NUM> (and into the first tank <NUM>), rather than returning to the supply tank <NUM>. In other words, the supply tank <NUM> is bypassed by the return flow through the lines <NUM>, <NUM> because the additional valve <NUM> is closed. In this system, filling of the second tank <NUM> with hydrogen gas occurs simultaneously with the filling of the first tank <NUM> to discharge hydrogen gas from the first tank <NUM> into the supply line <NUM>.

In practice, pressure sensors (not shown) may be provided for the first and second tanks, <NUM>, <NUM>, for the fuel-rail <NUM> and also in the auxiliary control fluid feed and return lines <NUM>, <NUM>. The pressure sensors would be used to optimise any opening/closing and any phasing between valve operation for any cylinder to reduce the risk of cavitation damage to the seats. For example, it is beneficial if the inlet one-way valves for control fluid (valves <NUM>, <NUM>) are opened before the inlet valves <NUM>, <NUM> from the filling station, as this would reduce the risk of cavitation.

In <FIG>, as for <FIG>, the energy that is harnessed from the hydrogen gas filling the second tank <NUM> benefits the overall system efficiency because, ultimately, the demand on the pump <NUM> can be reduced through the storage of energy in the battery <NUM> which is recovered from pressurised hydrogen supply from the filling station.

Claim 1:
A fuel system for supplying gaseous fuel to a power plant (<NUM>), the fuel system comprising:
a tank array comprising at least a first tank (<NUM>) and a second tank (<NUM>) configured to receive pressurised gaseous fuel for supply to the power plant (<NUM>), in use, from a filling station (<NUM>) when the tank array is connected to the filling station (<NUM>);
a source of auxiliary control fluid (<NUM>) for supplying auxiliary control fluid to the tank array when the fuel system is disconnected from the filling station (<NUM>);
a pump (<NUM>) for pressurising auxiliary control fluid supplied to the tank array when auxiliary control fluid flows through the pump (<NUM>) in a forward direction;
an energy store (<NUM>); and
a valve arrangement which is operable to control the supply of auxiliary control fluid to the tank array so as to control the discharge of the gaseous fuel from the tank array,
characterized in that
the valve arrangement is further operable to enable a reverse direction of flow of auxiliary control fluid from the tank array through the pump (<NUM>), when the fuel system is connected to the filling station (<NUM>), in use, to generate energy from the pump (<NUM>) to store in the energy store (<NUM>).