Technical Field

[0001 ] The present disclosure relates to a method and a system for delivering a cryogenically stored fuel to the air intake system of gaseous fuelled internal combustion engine.

Background

[0002] A fuel storage and delivery system for an internal combustion engine that is fuelled with a gaseous fuel such as natural gas, is an example of an application that is particularly suited for the disclosed method and system. Natural gas has been used to fuel vehicle engines for many years. The fuel supplied to a natural gas driven vehicle is stored either in a liquefied natural gas (LNG) tank or in a compressed natural gas (CNG) cylinder.

[0003] CNG is normally stored at ambient temperatures at pressures up to 3600 pounds per square inch while LNG is normally stored in a cryogenic storage vessel at temperatures of between about -240° F and -175° F (about -150° C and -115° C) and at pressures of between 15 and 200 psig. CNG has been the more broadly adopted form of fuel storage but it has a lower energy density compared to LNG. Now that natural gas is gaining greater acceptance as a fuel for transportation, the benefit of the higher energy density for LNG is attractive for vehicles that require a longer range between re-fuelling. LNG provides an energy density of about four times that of CNG with the aforementioned storage conditions. Increased demand for natural gas engines has increased the need to develop improved on-board fuel supply systems with natural gas stored as LNG instead of CNG. [0004] Natural gas fuelled engines can operate by injecting the fuel in the engine's air intake manifold or by injecting the fuel directly into the engine's combustion chambers. In systems where fuel is injected into the engine's air intake system the required fuel supply pressure is relatively low, for example around 100 psig. In systems where the fuel is injected directly into the engine's combustion chamber and where the fuel injection pressure needs to be higher than the in-cy Under pressure the fuel supply system has to deliver natural gas to the injector at higher pressures, for example at pressures of at least 3000 psig.

[0005] LNG low pressure fuelling systems described in the prior art generally comprise a liquid conduit which supplies liquid fuel from the storage vessel to the engine through a supply line comprising a vaporizer. Fuel is stored in the storage vessel at a pressure of around 150 psi which is higher than the fuel delivery pressure to the engine which can be between 70 to 100 psi.

[0006] In these systems, heat is transferred to the LNG fuel storage vessel and a portion of the stored liquid fuel can vaporize thereby increasing the pressure inside the vessel. One method of relieving the pressure inside an LNG storage vessel is to vent the gas into the atmosphere. However this method is wasteful and can be represent a hazard. As an alternative, the existing delivery systems for low pressure engines are provided with an economizer circuit which comprises a vapor conduit through which vapor can be withdrawn from the vapor headspace in the storage vessel and can be supplied to the engine. In engine supply systems provided with an economizer circuit, most of the time, the engine operates with LNG fuel supplied from the liquid space in the storage vessel and vapor is pulled from the vapor headspace only when the pressure in the storage vessel rises above a predetermined limit set by a regulator placed in the vapour conduit. An example of such a system is described in United States Patent Number 5,421,161 which illustrates an economizer circuit including means for providing a fixed back pressure in the liquid withdrawal conduit such that when the pressure in the storage vessel increases over a predetermined amount, the path of least resistance is through the vapor conduit and vapor is preferentially withdrawn from the vapor headspace of the storage vessel to thereby lower the pressure within the LNG storage tank faster.

[0007] The known systems using LNG storage vessels to supply fuel to a low pressure internal combustion engine fuelled with natural gas rely on a high saturation pressure in the storage vessel to push fuel out of the liquid space of the storage vessel and to the engine. When heat is transferred from the surrounding environment to the storage vessel, the saturation pressure of the LNG increases and can be sufficient for pushing fuel out of the vessel. When the engine operates at high load the saturation pressure of the LNG in the storage vessel can drop below a level that is required for pushing fuel out of the tank. In these situations, the engine becomes starved of fuel and can run in underperforming conditions. In these situations, the driver has to stop the vehicle and wait for the pressure in the storage vessel to increase until he can restart the vehicle. [0008] One known method of increasing the pressure in the LNG storage vessel is to use pressure building coils interposed between the walls of a double-walled cryogenic tank which circulate low temperature fuel from the tank. Heat transferred from to the exterior through the wall of the LNG storage vessel to the pressure building coil vaporizes the liquid fuel and the created vapor can be supplied directly into the headspace of the storage vessel through a regulator when the pressure in the headspace becomes lower than a predetermined value. Such a pressure building circuit is described in United States Patent Number 4,947,651. In other pressure building circuits used for cryogenic tanks in general, such as the one described in United States Patent Number 5,937,655, the pressure building coil is external to the cryogenic tank. In such systems cryogenic liquid from the tank is fed to a pressure builder heat exchanger where the liquid is vaporized and the produced gas is delivered to the tank to pressurize it.

[0009] The disadvantage of the methods using pressure building circuits to pressurize a cryogenic fluid storage vessel is that the heat used for generating the vapor that is supplied to the headspace of the storage vessel to pressurize it, is also transferred to the liquid contained in the vessel reducing the vessel's fluid holding time and requiring more frequent venting.

[0010] One other disadvantage of the existing low pressure fuel supply systems which rely on the fuel saturation pressure in the storage vessel to supply fuel to the engine is that they cannot adequately supply engines with fuel under transient conditions, when the required fuel supply pressure can vary dynamically between a lower pressure that is required for low load operation and a higher pressure that is required for high load operation. This issue becomes more important if such natural gas fuel systems are installed on larger vehicles, such as heavy duty trucks, which have larger engines that consume fuel at a higher rate. [001 1 ] Accordingly, there is a need for a method of reliably delivering fuel from a cryogenic storage vessel into the air intake system of a gaseous fuelled internal combustion engine at low pressures, to ensure that fuel is delivered to the engine at the required fuel injection pressure at all times and during different engine operating modes including during transients.

Summary

[0012] A method is disclosed for delivering a fuel in a gaseous state into the air intake system of a gaseous fuelled internal combustion engine. The method comprises: determining a required fuel supply pressure according to an operating condition of the engine;

measuring pressure in a vapor space of a storage vessel which stores the fuel; comparing the measured pressure to the required fuel supply pressure, and supplying fuel in gaseous state to the engine from the vapor space in the storage vessel when the measured pressure is equal to or higher than the required fuel supply pressure, or

activating a fuel pump and delivering fuel to the engine from a liquid space in the storage vessel when the measured pressure in the vapor space is lower than the required fuel supply pressure.

[0013] In this method the step of determining the required fuel supply pressure comprises first determining a required engine intake pressure as a function of engine operating conditions and then adding to it a predetermined pressure threshold. Because the required fuel supply pressure is preferably higher than the engine intake pressure the dynamic engine demands can be easily met by the system pressure. In preferred embodiments, the predetermined pressure threshold which is added to the engine intake pressure to determine the required fuel supply pressure is also determined as a function of the engine operating condition. The engine operating condition can be determined from a map stored in the controller's memory that correlates the engine speed with a parameter indicative of the engine condition, for example a parameter indicative of the fuel demand, such as the throttle position. [0014] In the present method, the step of activating the fuel pump comprises supplying hydraulic fluid from a hydraulic pump to a hydraulic drive unit that drives the fuel pump. The hydraulic pump is preferably driven by an electric motor and the electric motor is driven independently from the operation of the internal combustion engine such that the fuel pump can be activated independently of the engine condition. [0015] In some embodiments, at least two hydraulic pumps can supply hydraulic fluid to the hydraulic drive unit that the drives the fuel pump and the present method further comprises electrically activating at least two hydraulic pumps at the same time for delivering hydraulic fluid to said hydraulic drive unit when a commanded hydraulic fluid flow rate is higher than that which can be supplied by only one hydraulic pump.

[0016] In preferred embodiments the temperature of the fuel is increased by flowing it through a heat exchanger. The method can comprise increasing the heat exchange rate in the heat exchanger when the fuel is supplied from the liquid space of the fuel storage vessel, compared to when the fuel is supplied from the vapor space of the fuel storage vessel. In other embodiments the heat exchange rate in the heat exchanger is not controlled by the system controller and is set to be constant.

[0017] If the fuel delivery system comprises a plurality of storage vessels each having a respective vapor space and a liquid space, and the present method of delivering fuel to the engine further comprises: measuring pressure in the vapor space of each one of the plurality of storage vessels; comparing the measured pressure in the vapor space of each one of the plurality of storage vessels with the required fuel supply pressure; supplying fuel from the vapor space of any one of the plurality of storage vessels in which vapor pressure is higher than the required system fuel supply pressure; or activating the fuel pump and supplying fuel from one of the plurality of storage vessels when none of the plurality of storage vessels has a vapor pressure higher than the required system fuel supply pressure. [0018] This method which involves supplying fuel from one of the plurality of storage vessels can further comprise measuring the amount of fuel in each of the plurality of storage vessels and, when activating the fuel pump, supplying fuel from one of the plurality of storage vessels that has the largest amount of fuel. [0019] If the fuel delivery system comprises a plurality of storage vessels, each having a respective vapor space and liquid space, and each having a respective fuel pump, the present method further comprises: measuring pressure in the vapor space of each one of the plurality of storage vessels; comparing the measured pressure in the vapor space of each one of the plurality of storage vessels with the required fuel supply pressure; supplying fuel from the vapor space of any one of the plurality of storage vessels in which vapor pressure is higher than the required system fuel supply pressure; or selecting and activating one of the fuel pumps and supplying fuel from an associated one of the plurality of storage vessels when none of the plurality of storage vessels has a vapor pressure higher than the required system fuel supply pressure.

[0020] In this method the step of selecting one of the fuel pumps that is to be activated is determined by operating each one of the fuel pumps in sequential order. Alternatively, the step of selecting one of the fuel pumps that is to be activated is determined by measuring the amount of fuel in each one of the plurality of storage vessels and selecting the one of the plurality of fuel pumps that is associated with the storage vessel that has the largest amount of fuel.

[0021 ] In preferred embodiments of the method where fuel is supplied from one of a plurality of fuel storage vessels, the method further comprises increasing the temperature of the fuel by flowing it through a heat exchanger associated with a respective storage vessel from which fuel is supplied to the engine.

[0022] A system is also disclosed for delivering fuel in a gaseous state into the air intake system of a gaseous fuelled internal combustion engine. The system comprises: a) a storage vessel for holding the fuel; b) a fuel pump fluidly connected to receive fuel from a liquid space of the storage vessel; c) a liquid supply line in fluid communication with a discharge outlet of the fuel pump for delivering fuel from the discharge outlet of the fuel pump to a delivery line which delivers fuel to the engine; d) a vapor supply line in fluid communication with a vapor space of the storage vessel for delivering fuel in a gaseous state from the vapor space to the delivery line; e) a pressure sensor for measuring vapor pressure in the vapor space of the storage vessel; and f) a controller which receives pressure measurements from the pressure sensor, determines a required fuel supply pressure according to an operating condition of the engine and compares the measured pressure to the required fuel supply pressure, wherein the controller commands the fuel pump to operate when the measured pressure is lower than the required fuel supply pressure.

[0023] The system preferably comprises a heat exchanger placed in the delivery line for increasing the temperature of the fuel being supplied to the engine. The system also comprises a hydraulic pump which supplies hydraulic fluid to a hydraulic drive unit that drives the fuel pump. Preferably, the hydraulic pump is driven by an electric motor which operates independently from the operation of the engine such that the operation of the fuel pump can be controlled independently of the engine operation.

[0024] In some embodiments, the system comprises at least two hydraulic pumps which can be electrically activated to deliver hydraulic fluid to the hydraulic drive unit that drives the fuel pump. In some embodiments the fuel pump is disposed within a cryogenic space of the storage vessel.

[0025] In some embodiments the system comprises more than one storage vessel. Each storage vessel in the system has a respective vapor space and a liquid space, and is fluidly connected to the fuel pump. In these systems the controller is programmed to activate the fuel pump to supply fuel from the liquid space of one of the plurality of storage vessels when none of the plurality of storage vessels has a vapor pressure higher than the required fuel supply pressure.

[0026] In other embodiments, the system comprises more than one storage vessel, each storage vessel having a respective vapor space and liquid space, and each being fluidly connected to a respective fuel pump. In these systems the controller is programmed to select and activate one of the fuel pumps and supply fuel from a storage vessel associated with the selected fuel pump when none of the plurality of storage vessels has a vapor pressure higher than the required fuel supply pressure.

[0027] In preferred embodiments the system comprises an engine that is the prime mover for a vehicle.

Brief Description of the Drawings

[0028] Figure 1 is schematic diagram of a system for delivering fuel in gaseous state into the air intake system of an internal combustion engine according to the present disclosure; [0029] Figure 2 is a schematic diagram illustrating the basic steps of a method for delivering fuel from a cryogenic storage vessel to the air intake system of a gaseous fuelled internal combustion engine according to the present disclosure;

[0030] Figure 3 is a representation of a map correlating the engine speed with the throttle position which is used by the system's controller to determine the engine operating condition;

[0031 ] Figure 4 is schematic diagram of another embodiment of a fuel delivery system comprising two hydraulic pumps for supplying hydraulic fluid to the hydraulic drive unit of the system's fuel pump;

[0032] Figure 5 is a schematic diagram of another embodiment of a fuel delivery system comprising two fuel storage vessels, each storage vessel being associated with a fuel pump, and a controller which activates each of the fuel pumps according to the method described in the present disclosure; [0033] Figure 6 is a schematic diagram of another embodiment of a fuel delivery system comprising two fuel storage vessels which are both fluidly connected to one external fuel pump and a controller which activates the fuel pump according to the method described in the present disclosure. Detailed Description

[0034] Figure 1, schematically illustrates a fuel delivery system 100 which is employed to supply a fuel in gaseous state into the air intake system of an internal combustion engine. Engine 110 is an internal combustion engine which is operated by injecting fuel into the engine's air intake manifold or into the air intake port, which is a different method than injecting fuel directly into the engine's combustion chamber. Gaseous fuel is injected into the engine's air intake manifold 111 or into the air intake ports 113 at pressures that are generally around 70 to 100 psig (pounds per square inch gauge) and can reach around 500 psig. Such pressures are much lower than the operating pressure of a direct injection internal combustion engine where gaseous fuel is injected directly into the combustion chamber at around 4000 psig.

[0035] Fuel delivery system 100 comprises fuel storage vessel 112 which stores gaseous fuel in liquefied form at cryogenic temperatures in liquid space 114 within the cryogenic space of the storage vessel. Since heat is transmitted from the surrounding environment to the walls of the storage vessel, liquid fuel stored in the vessel can vaporize and the generated vapor occupies the headspace of the storage vessel, creating a vapor space 116.

[0036] Liquid space 114 is fluidly connected to fuel pump 118 which can be placed inside the cryogenic space of the storage vessel, as illustrated in the embodiment shown in Figure 1, or can be an external pump that communicates through a supply line with the liquid space in the storage vessel. Fuel pump 118 can be activated by starting hydraulic pump 120 which supplies hydraulic fluid from storage vessel 121 through flow switching device 124 to the hydraulic drive unit of fuel pump 118. Hydraulic pump 120 is driven by an electric motor and therefore can operate independently of the operating condition of the engine. In the existent prior art fuel delivery systems, the hydraulic pump that activates the fuel pump is mechanically actuated by the engine accessory drive and therefore depends on the engine rotation (rpm).

[0037] Liquid fuel from liquid space 114 of fuel storage vessel 112 can be pumped by fuel pump 118 through liquid fuel supply line 123 to delivery line 122 which supplies fuel to engine 110. Liquid fuel supply line 123 comprises a check valve 125 which prevents fuel backflow to the storage vessel.

[0038] Fuel in vapor form can be supplied from vapor space 116 to delivery line 122 through vapor supply line 126 whose one end fluidly communicates with vapor space 116 of storage vessel 112. Vapor supply line 126 is provided with a check valve 128 to prevent fuel backflow to the storage vessel.

[0039] Liquid fuel line 123 and vapor supply line 126 are each connected to delivery line 122 downstream of their respective check valve. Delivery line 122 further comprises heat exchanger 130 which transmits heat from a heat exchange fluid to the fuel as further explained below. The heat exchange fluid in heat exchanger 130 can be for example the engine coolant. Delivery line 122 further comprises module 132 for dampening the pressure fluctuations in the stream of fuel being supplied to the engine and pressure regulator 134 for adjusting the pressure of the fuel supplied to the engine to the required engine intake pressure. Automatic fuel shut-off valve 136 is provided on delivery line 122 between pressure regulator 134 and engine 110. Fuel shut-off valve 136 is used as a safety measure for stopping any fuel flow to the engine when the engine is not operating.

[0040] The system further comprises controller 140 which commands the operation of fuel pump 118 and hydraulic pump 120 and receives measurement signals from pressure sensors 150 and 152 and from temperature sensor 154. Pressure sensor 150 measures the pressure in vapor space 116 of fuel storage vessel 112 and pressure sensor 152 measures the fuel supply pressure which is the pressure in fuel delivery line 122 downstream of module 132 and upstream of pressure regulator 134. Temperature sensor 154 measures the temperature of the fuel flowing through delivery line 122 downstream of heat exchanger 130 and controller 140 can command the operation of heat exchanger 130 to ensure that the temperature of the fuel supplied to engine 110 is higher than a predetermined limit. [0041 ] Controller 140 also receives input from the engine regarding the engine speed and another parameter indicative of the engine operating condition, for example one of the parameters indicative of the fuel demand, for example the throttle position. The controller can determine the required fuel supply pressure based on the information from a map, which correlates the engine speed data with the other parameter indicative of the engine operating condition as further detailed below.

[0042] The method of delivering fuel in gaseous state into the air intake system of a gaseous fuelled internal combustion engine will now be described in relation to the embodiment of the fuel delivery system illustrated in Figure 1. The steps of the method are illustrated in Figure 2. In first step 210 of method 200, controller 140 determines the required fuel supply pressure which is the pressure of the fuel flowing through delivery line 122 upstream of pressure regulator 134. At this step, controller 140 receives information from the engine regarding its operating condition and uses the map illustrated in Figure 3 to determine a point on the map which indicates the current engine operating condition. Map 300 stored in the controller's memory is divided into two regions a low flow region 310 and a high flow region 320. Low flow region 310 is delimited by boundary line 330 set by a low speed S _L and by boundary line 340 set by a low throttle position T _L and high flow region 320 is delimited by boundary line 350 set by a high engine speed S _H and a high throttle position ¾. If the engine operates for example at a point A, which is characterized by coordinates (Si, Ti) corresponding to a value Si for the engine speed and a value Ti for the throttle position, it is determined that the engine operates in a low flow region 310. Once the point on the map where the engine operates is identified, controller 140 can determine based on predetermined algorithms, the engine intake pressure P _eng ine _A required for operating the engine efficiently. The required fuel supply pressure P _SU ppiy _A is then determined by the formula P _SU ppi _y A = Pengine A + PthreshoidA, where PthreshoidA is the pressure value that needs to be added to the engine intake pressure to obtain the fuel supply pressure. The fuel supply pressure, which is the pressure in fuel delivery line 122 upstream of pressure regulator 134 is higher than the engine intake pressure which is the pressure in fuel delivery line 122 downstream of pressure regulator 134 such that dynamic engine demands can be easily met. Similarly, if the engine operates at point B characterized by coordinates (S ₂ , T ₂ ), corresponding to a different engine speed S ₂ and a different throttle position T ₂ , located in the high flow region 320 on map 300, the controller determines that a different engine intake pressure P _en gine B is required for operating the engine efficiently. The required fuel supply pressure is then determined by the same formula: P _supp iy B = Pengine B + Pthreshoid B- The value of the pressure threshold that is added to the required engine intake pressure when the engine operates in the low flow region 310 is different than the pressure threshold that is added to the required engine intake pressure when the engine operates in the high flow region 320. Both the engine intake pressure P _eng ine and the threshold pressure Pthreshoid are determined by the controller through control algorithms based on the region on map 300 where the engine operates. [0043] In next step 220 of the present method, pressure P _v within vapor space 116 of fuel storage vessel 112 is measured by pressure sensor 150 and the measured value is communicated to controller 140.

[0044] In the next step 230, the measured pressure P _v in vapor space 116 is compared to the required fuel supply pressure P _SU ppiy , which was determined in previous step 210. If the measured pressure in the vapor space is equal to or higher than the predetermined required fuel supply pressure, fuel is supplied in gaseous state from vapor space 116 in storage vessel 112 through vapor supply line 126 to fuel delivery line 122, which is step 240 of the present method. If the measured pressure in the vapor space is lower than the predetermined required fuel supply pressure, the controller executes step 250 of the present method and activates fuel pump 118 whereby fuel is supplied from liquid space 114 through liquid fuel supply line 123 to fuel delivery line 122.

[0045] The temperature of the fuel supplied to fuel delivery line 122 is increased by flowing the fuel through heat exchanger 130. If fuel is supplied from liquid space 114 the heat exchange rate in heat exchanger 130 is increased compared to when the fuel is supplied from vapor space 116 because the temperature of the fuel supplied from the liquid space is slightly lower than the temperature of the fuel supplied in vapor state from the vapor space.

[0046] The advantage of the present method over the known methods for delivering fuel in gaseous state to the air intake system of a gaseous fuelled engine is that fuel can be delivered to the engine without relying on the fuel saturation pressure to push the fuel out of the storage vessel.

[0047] Other embodiments of fuel delivery system are illustrated in Figures 4 to 6. These embodiments have many components that are equivalent to like components of the embodiment presented in Figure 1 and like components are identified by like reference numbers. Persons skilled in the technology will understand that in this disclosure like-numbered components function in substantially the same way in each embodiment. Accordingly, if like components have already been described with respect to one embodiment, while identified in the figures for other embodiments, the purpose and function of like components may not be repeated for each of the illustrated embodiments.

[0048] Figure 4 illustrates another embodiment of the present fuel delivery system. Fuel delivery system 400 is delivering fuel in gaseous state from cryogenic storage vessel 412 to air intake manifold 411 or to air intake ports 413 of gaseous fuelled internal combustion engine 410. Fuel can be delivered to fuel delivery line 422 either from vapor space 416 through vapor supply line 426 or it is supplied by fuel pump 418 from liquid space 414 through liquid fuel supply line 423. Liquid supply line 423 and vapor supply line 426 are each provided with a check valve 425 and respectively 428 for preventing fuel backflow. Fuel passing through fuel delivery line 422 is heated in heat exchanger 430 and the pressure fluctuations in the fuel stream are dampened in module 432. Pressure regulator 434 regulates the fuel pressure to the engine intake pressure. Automatic shut-off valve 436 is provided as a safety measure during the times when the engine is not operating. The pressure in fuel delivery line 422 is measured by pressure sensor 452. [0049] System 400 illustrated in Figure 4 is different than the system illustrated in Figure 1 in that it uses two hydraulic pumps 420 and 460 for activating fuel pump 418. Hydraulic pumps 420 and 460 supply hydraulic fluid from storage vessel 421 through flow switching unit 424 to the hydraulic drive unit that drives fuel pump 418. In this arrangement, both hydraulic fluid pumps can be used at the same time when the hydraulic fluid flow rate commanded by controller 440 is higher than that which can be supplied by only one hydraulic pump. [0050] In this embodiment heat exchanger 430 is not commanded by controller 440 and provides the same amount of heat to the fuel flowing through delivery line 422 independently of where the fuel is supplied from. The temperature of the fuel flowing through fuel delivery line 422 is measured by temperature sensor 454 and communicated to controller 440.

[0051 ] The method of delivering fuel from storage vessel 412 to engine 410 is the same as the method described in relation to Figure 2 and therefore it will not be described here in great detail. The pressure in vapor space 416 is measured by pressure sensor 450 and it is compared to the required fuel supply pressure. If the pressure measured in vapor space 416 is equal to or higher than the required fuel supply pressure fuel is supplied in gaseous state from vapor space 416 to the engine, and if the pressure measured in vapor space 416 is lower than the required fuel supply pressure, fuel pump 418 is activated and fuel is delivered by the pump from liquid space 414 to the engine. The required fuel supply pressure is determined following the same algorithm as the one applied for the system illustrated in Figure 1.

[0052] Figure 5 illustrates another embodiment of the present fuel delivery system. Fuel delivery system 500 is delivering fuel in gaseous state to air intake manifold 511 or to air intake ports 513 of gaseous fuelled internal combustion engine 510. Fuel system 500 is different than the previous embodiments because it comprises two cryogenic storage vessels 512A and 512B, each storage vessel having a respective vapor space 516A and 516B, and a respective liquid space 514A and 514B, and being fluidly connected to a respective fuel pump 518A and 518B.

[0053] Each of the two fuel pumps 518A or 518B is activated by supplying hydraulic fluid from hydraulic storage vessel 521 through one of the two hydraulic pumps 520 or 560 or through both of them, through flow switching units 562, and further through one of the flow switching units 524A or 524B, to the hydraulic drive unit of the respective fuel pump to be activated. Hydraulic fluid pumps 520 and 560 are driven by an electric motor and therefore can be operated independently from the operation of the internal combustion engine. [0054] Similar to the other embodiments described here, pressure in fuel delivery line 522 is measured by pressure sensor 552 upstream of pressure regulator 534 and downstream of module 532. Pressure fluctuations in fuel delivery line 522 are dampened in module 532. Temperature in fuel delivery line 522 is measured by temperature sensor 554. Automatic shut-off valve 536 is provided as a safety measure on fuel delivery line 522 during the times when the engine is not operating. [0055] A similar method of delivering fuel to the engine as described in relation to Figures 1 and 4 is applied here. The pressure in the vapor space of each tank 512A and 512B is measured by a respective pressure sensor 550A and 550B and communicated to controller 540. Controller 540 compares the pressure measured in each vapor space with the required fuel supply pressure which is determined in a similar way as in the methods described in relation to Figures 1 and 4 and fuel is supplied from a vapor space of one of storage vessels 512A or 512B in which vapor pressure is higher than the required fuel supply pressure. Fuel is supplied from the respective vapor space through vapor liquid line 526A or 526B and through heat exchanger 530A or 530B to delivery line 522. When none of two storage vessels has a vapor pressure higher than the required fuel supply pressure, controller 540 selects and activates one of the two fuel pumps 518A and 518B to supply fuel from the liquid space of the storage vessel associated with the activated pump. Liquid fuel is supplied through a respective liquid supply line 523A or 523B and through heat exchanger 530A or respectively 530B to delivery line 522. Liquid fuel supply lines 523A and 523B and vapor supply lines 526A and 526B are each provided with a check valve 525 A, 525B and respectively 528A, 528B to prevent backflow.

[0056] In the embodiment illustrated in Figure 5 controller 540 selects which one of the two fuel pumps should be activated such that the two fuel pumps are operated in sequential order. Alternatively, the amount of fuel remaining in each storage vessel 512A and 512B is measured and controller 540 selects one of fuel pumps 518A or 518B based on which storage vessel has the most fuel.

[0057] Even if only two storage vessels are shown in the embodiment illustrated in Figure 5, a person skilled in this technology would easily recognize that the system can comprise more than two cryogenic storage vessels and that the method of selecting and activating one fuel pump for supplying fuel from a liquid space of one storage vessel will be similar to the method described above. [0058] Figure 6 illustrates another fuel delivery system 600 for delivering fuel in gaseous state to air intake manifold 611 or to air intake ports 613 of gaseous fuelled internal combustion engine 610. Fuel system 600 is different than the previous embodiments because it comprises one external fuel pump 618 and two cryogenic storage vessels 612A and 612B whereby fuel pump 618 can supply fuel from liquid space 614A or 614B of the respective storage vessels 612A and 612B. Fuel pump 618 is activated when hydraulic pump 620 supplies hydraulic fluid from storage vessel 621 to the hydraulic drive unit of the fuel pump 618 through flow switching unit 624.

[0059] Similar to the other embodiments described here, pressure in fuel delivery line 622 is measured by pressure sensor 652 upstream of pressure regulator 634 and downstream of module 632. Pressure fluctuations in fuel delivery line 622 are dampened in module 632. Automatic shut-off valve 636 is provided as a safety measure on fuel delivery line 622 during the times when the engine is not operating. Vapor supply lines 626A and 626B are each provided with a check valve 628A and respectively 628B and check valve 625 is provided on delivery line 622 downstream of fuel pump 618 to prevent fluid backflow.

[0060] A similar method of delivering fuel to the engine as described in relation to Figures 1, 4 and 5 is applied here. Pressure in vapor spaces 616A and 616B of each storage vessel 612A and respectively 612B is measured by a respective pressure sensor 650A and 650B and communicated to controller 640. Controller 640 compares the pressure measured in each vapor space with the required fuel supply pressure which is calculated in a similar way as in the methods described in relation to Figures 1, 4 and 5 and fuel is supplied from a vapor space of one of storage vessels 612A or 612B in which vapor pressure is higher than the required fuel supply pressure. Fuel is supplied from the respective vapor space through vapor liquid line 626A or 626B and through heat exchanger 630 to delivery line 622. When none of two storage vessels has a vapor pressure higher than the required fuel supply pressure, controller 640 activates fuel pump 618 to supply fuel from the liquid space of one of the two storage vessels 612A or 612B. Fuel is supplied from liquid space 614A or 614B through respective liquid supply line 623A or 623B to delivery line 622.

[0061 ] In this embodiment each liquid supply line 623A and 623B is provided with a shut-off valve 670A and respectively 670B which can be commanded by controller 640. When none of the two storage vessels has a vapor pressure higher than the required fuel supply pressure, the amount of fuel in each storage vessel is measured and fuel is supplied from the storage vessel which has the most fuel. For example, when it is determined that storage vessel 612A has the most fuel, controller 640 commands shut-off valve 670B to close and fuel pump 618 supplies fuel from liquid space 614A to delivery line 622. Alternatively if it is determined that storage vessel 612B has the most fuel controller 640 commands shut-off valve 670A to close and fuel pump 618 supplies fuel from liquid space 614B to delivery line 622.

[0062] The pressure measurements in this embodiment are communicated by pressure sensors 652, 650A and 650B to controller 640 and the temperature of the fuel flowing through delivery line 622 is measured by temperature sensor 654.

[0063] In this embodiment, the required fuel supply pressure is determined following the same logic as the one applied for the systems illustrated in Figures 1, 4 and 5.

[0064] Even if only two storage vessels are shown in the embodiment illustrated in Figure 6, a person skilled in this technology would easily recognize that the system can comprise more than two cryogenic storage vessels and that the method of selecting the liquid space from which fuel is supplied when none of the storage vessels has a vapor space higher than the required fuel supply system is similar to the one described above. [0065] The present invention has been described with regard to a plurality of illustrative embodiments. However, it will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.