A storage system for a gaseous fuel

Abstract

A working machine 10 (fig 1a) comprising; a gas engine 25 (fig 1a) and a gas storage system 38 comprising; at least one gaseous fuel tank 40a-c, a tank valve 42, a shut-off valve 60 (fig 4c), at least

Description

FIELD The present invention relates to a gas storage system, a working machine, in particular, though not exclusively, a backhoe loader including the gas storage system, and to a method of monitoring a gas storage system. BACKGROUND Off-highway vehicles / working machines are typically those used in construction industries (e.g. backhoe loaders, slew excavators, telescopic handlers, forklifts, skid-steer loaders, dump trucks, bulldozers, graders), agricultural industries (e.g. tractors, combine harvesters, wheeled loading shovels, telescopic handlers, self-propelled harvesters and sprayers), quarrying (e.g. excavators, wheeled loading shovels, aggregate crushing equipment), and forestry (e.g. timber harvesters, feller bunchers). Many working machines have a primary function of moving material using either a lifting arm (e.g. a pivoting boom) or a working arm (e.g. an excavator arm) and may be referred to as material handling machines. Conventionally, working machines of the type referred to above are generally powered by diesel internal combustion engines. However, there is a general need to reduce vehicle emissions in the face of global warming leading to working machine OEMs considering alternative prime movers. Proposed alternatives include battery, hydrogen fuel cells, hydrogen internal combustion engines and various hybrid options, etc. The present invention seeks to provide an improved working machine or gas storage system. SUMMARY The present invention provides a working machine according to the appended claims. According to a first aspect, there is provided a working machine comprising: a ground engaging structure for providing propulsion over a ground surface; a chassis mounted to the ground engaging structure; a gas engine configured to provide motive power to the ground engaging structure; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine; at least one pressure sensor configured to monitor pressure in the gas storage system and / or gas engine; and a controller configured to control the gas storage system and to activate a valve diagnosis mode of the gas storage system, wherein, in the valve diagnosis mode, the controller is configured to close the tank valve or the shut-off valve and to monitor pressure from the at least one pressure sensor to determine a leak condition of the respective valve. The working machine may comprise a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank. The controller may be configured, in the valve diagnosis mode, to close the tank valve, monitor the tank pressure over a predetermined period, and determine a leak condition of the tank valve in response to a reduction in tank pressure above a pre-determined threshold. The working machine may comprise a tank pressure sensor and a tank temperature sensor configured to monitor tank pressure and tank temperature in the at least one storage tank, respectively. The controller may be configured to determine a mass of gaseous fuel in the at the least one storage tank based on the tank pressure and the tank temperature, and determine a leak condition of the tank valve in response to a reduction in the mass of gaseous fuel in the at least one storage tank above a pre-determined threshold. The working machine may comprise a fuel line pressure sensor configured to monitor fuel line pressure in the fuel line. The controller may be configured, in the valve diagnosis mode, to close the tank valve, monitor fuel line pressure over a predetermined period, and determine a leak condition of tank valve in response to no reduction in fuel line pressure, in response to a reduction in fuel line pressure below a pre-determined threshold and / or in response to a rate of reduction of fuel line pressure below a predetermined threshold rate. The controller may be configured to activate the valve diagnosis mode and close the tank valve only when the gas engine is in an active state, optionally when the gas engine is in an idling state. The controller may be configured to activate the valve diagnosis mode and close the tank valve only when the gas engine is in idling state, and wherein the controller is configured to deactivate the valve diagnosis mode in response to a demand on the gas engine. The controller may be configured to deactivate the valve diagnosis mode in response to an increase in RPM of the gas engine. The working machine may comprise a plurality of storage tanks each comprising a tank valve and a tank pressure sensor. The controller may be configured to close each tank valve independently, for example sequentially. The controller may be configured to only activate the valve diagnosis mode when a fuel level of gaseous fuel in the at least one storage tank is above a predetermined threshold. The gas engine may comprise a fuel rail and a gas engine pressure sensor configured to monitor pressure within the fuel rail, and the controller may be configured, in the valve diagnosis mode, to close the shut-off valve, purge the fuel rail of the gaseous fuel, monitor pressure in the fuel rail over a predetermined period, and determine a leak condition of the shut-off valve in response to an increase in pressure in the fuel rail above a predetermined threshold. The controller may be configured, in the valve diagnosis mode, to close the shut-off valve, monitor pressure upstream and downstream of the shut-off valve over a predetermined period to determine a pressure differential, and determine a leak condition of the shut-off valve in response to the pressure differential being below a pre-determined threshold and / or in response to a rate of increase in the pressure differential below a pre-determined threshold rate. The working machine may comprise a fuel line pressure sensor configured and arranged to monitor fuel line pressure upstream of the shut-off valve and a gas engine pressure sensor configured and arranged to monitor pressure downstream of the shut-off valve within the gas engine. The gas engine may comprise a fuel rail and the gas engine pressure sensor may be configured and arranged to monitor pressure within the fuel rail. The controller may be configured to activate the valve diagnosis mode and close the shutoff valve only when the gas engine is in an inactive state. The controller may be configured to generate an output in response to a determined leak condition of the tank valve and / or shut-off valve. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. The controller may be configured to generate an alarm or alert in response to the valve diagnosis mode not being activated within a predetermined period of time. According to a second aspect, there is provided a method of detecting a leak condition of a valve of a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, a gas engine configured to provide motive power, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine, the method comprising: activating a valve diagnosis mode; closing the tank valve or the shut-off valve; and monitoring pressure within the gas storage system and / or the gas engine to determine a leak condition of the respective valve. According to a third aspect, there is provided a generator set comprising: an electrical generator for supplying electrical energy to a battery pack and / or to an electrical energy outlet; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine; at least one pressure sensor configured to monitor pressure in the gas storage system and / or gas engine; and a controller configured to control the gas storage system and to activate a valve diagnosis mode of the gas storage system, wherein, in the valve diagnosis mode, the controller is configured to close the tank valve or the shut-off valve and to monitor pressure from the at least one pressure sensor to determine a leak condition of the respective valve. The generator set may comprise a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank. The controller may be configured, in the valve diagnosis mode, to close the tank valve, monitor the tank pressure over a predetermined period, and determine a leak condition of the tank valve in response to a reduction in tank pressure above a pre-determined threshold. The generator set may comprise a tank pressure sensor and a tank temperature sensor configured to monitor tank pressure and tank temperature in the at least one storage tank, respectively. The controller may be configured to determine a mass of gaseous fuel in the at the least one storage tank based on the tank pressure and the tank temperature, and determine a leak condition of the tank valve in response to a reduction in the mass of gaseous fuel in the at least one storage tank above a pre-determined threshold. The generator set may comprise a fuel line pressure sensor configured to monitor fuel line pressure in the fuel line. The controller may be configured, in the valve diagnosis mode, to close the tank valve, monitor fuel line pressure over a predetermined period, and determine a leak condition of tank valve in response to no reduction in fuel line pressure, in response to a reduction in fuel line pressure below a pre-determined threshold and / or in response to a rate of reduction of fuel line pressure below a predetermined threshold rate. The controller may be configured to activate the valve diagnosis mode and close the tank valve only when the gas engine is in an active state, optionally when the gas engine is in an idling state. The controller may be configured to activate the valve diagnosis mode and close the tank valve only when the gas engine is in idling state, and wherein the controller is configured to deactivate the valve diagnosis mode in response to a demand on the gas engine. The controller may be configured to deactivate the valve diagnosis mode in response to an increase in RPM of the gas engine. The generator set may comprise a plurality of storage tanks each comprising a tank valve and a tank pressure sensor. The controller may be configured to close each tank valve independently, for example sequentially. The controller may be configured to only activate the valve diagnosis mode when a fuel level of gaseous fuel in the at least one storage tank is above a predetermined threshold. The gas engine may comprise a fuel rail and a gas engine pressure sensor configured to monitor pressure within the fuel rail, and the controller may be configured, in the valve diagnosis mode, to close the shut-off valve, purge the fuel rail of the gaseous fuel, monitor pressure in the fuel rail over a predetermined period, and determine a leak condition of the shut-off valve in response to an increase in pressure in the fuel rail above a predetermined threshold. The controller may be configured, in the valve diagnosis mode, to close the shut-off valve, monitor pressure upstream and downstream of the shut-off valve over a predetermined period to determine a pressure differential, and determine a leak condition of the shut-off valve in response to the pressure differential being below a pre-determined threshold and / or in response to a rate of increase in the pressure differential below a pre-determined threshold rate. The generator set may comprise a fuel line pressure sensor configured and arranged to monitor fuel line pressure upstream of the shut-off valve and a gas engine pressure sensor configured and arranged to monitor pressure downstream of the shut-off valve within the gas engine. The gas engine may comprise a fuel rail and the gas engine pressure sensor may be configured and arranged to monitor pressure within the fuel rail. The controller may be configured to activate the valve diagnosis mode and close the shutoff valve only when the gas engine is in an inactive state. The controller may be configured to generate an output in response to a determined leak condition of the tank valve and / or shut-off valve. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. The controller may be configured to generate an alarm or alert in response to the valve diagnosis mode not being activated within a predetermined period of time. According to a fourth aspect, there is provided a portable refuelling device for refuelling a vehicle, the device comprising: a refuelling outlet nozzle configured to discharge a gaseous for refuelling a vehicle; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine; at least one pressure sensor configured to monitor pressure in the gas storage system and / or gas engine; and a controller configured to control the gas storage system and to activate a valve diagnosis mode of the gas storage system, wherein, in the valve diagnosis mode, the controller is configured to close the tank valve or the shut-off valve and to monitor pressure from the at least one pressure sensor to determine a leak condition of the respective valve. The portable refuelling device may comprise a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank. The controller may be configured, in the valve diagnosis mode, to close the tank valve, monitor the tank pressure over a predetermined period, and determine a leak condition of the tank valve in response to a reduction in tank pressure above a pre-determined threshold. The portable refuelling device may comprise a tank pressure sensor and a tank temperature sensor configured to monitor tank pressure and tank temperature in the at least one storage tank, respectively. The controller may be configured to determine a mass of gaseous fuel in the at the least one storage tank based on the tank pressure and the tank temperature, and determine a leak condition of the tank valve in response to a reduction in the mass of gaseous fuel in the at least one storage tank above a predetermined threshold. The portable refuelling device may comprise a fuel line pressure sensor configured to monitor fuel line pressure in the fuel line. The controller may be configured, in the valve diagnosis mode, to close the tank valve, monitor fuel line pressure over a predetermined period, and determine a leak condition of tank valve in response to no reduction in fuel line pressure, in response to a reduction in fuel line pressure below a pre-determined threshold and / or in response to a rate of reduction of fuel line pressure below a predetermined threshold rate. The controller may be configured to activate the valve diagnosis mode and close the tank valve only when the gas engine is in an active state, optionally when the gas engine is in an idling state. The controller may be configured to activate the valve diagnosis mode and close the tank valve only when the gas engine is in idling state, and wherein the controller is configured to deactivate the valve diagnosis mode in response to a demand on the gas engine. The controller may be configured to deactivate the valve diagnosis mode in response to an increase in RPM of the gas engine. The portable refuelling device may comprise a plurality of storage tanks each comprising a tank valve and a tank pressure sensor. The controller may be configured to close each tank valve independently, for example sequentially. The controller may be configured to only activate the valve diagnosis mode when a fuel level of gaseous fuel in the at least one storage tank is above a predetermined threshold. The gas engine may comprise a fuel rail and a gas engine pressure sensor configured to monitor pressure within the fuel rail, and the controller may be configured, in the valve diagnosis mode, to close the shut-off valve, purge the fuel rail of the gaseous fuel, monitor pressure in the fuel rail over a predetermined period, and determine a leak condition of the shut-off valve in response to an increase in pressure in the fuel rail above a predetermined threshold. The controller may be configured, in the valve diagnosis mode, to close the shut-off valve, monitor pressure upstream and downstream of the shut-off valve over a predetermined period to determine a pressure differential, and determine a leak condition of the shut-off valve in response to the pressure differential being below a pre-determined threshold and / or in response to a rate of increase in the pressure differential below a pre-determined threshold rate. The portable refuelling device may comprise a fuel line pressure sensor configured and arranged to monitor fuel line pressure upstream of the shut-off valve and a gas engine pressure sensor configured and arranged to monitor pressure downstream of the shut-off valve within the gas engine. The gas engine may comprise a fuel rail and the gas engine pressure sensor may be configured and arranged to monitor pressure within the fuel rail. The controller may be configured to activate the valve diagnosis mode and close the shutoff valve only when the gas engine is in an inactive state. The controller may be configured to generate an output in response to a determined leak condition of the tank valve and / or shut-off valve. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. The controller may be configured to generate an alarm or alert in response to the valve diagnosis mode not being activated within a predetermined period of time. According to a fifth aspect, there is provided a working machine comprising: a ground engaging structure for providing propulsion over a ground surface; a chassis mounted to the ground engaging structure; a gas engine configured to provide motive power to the ground engaging structure; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine; a tank sensor assembly comprising a tank temperature sensor configured to monitor a tank temperature of the at least one storage tank and a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank; and a controller configured to determine a mass of gaseous fuel in the at least one storage tank based on the tank temperature and tank pressure and to determine an actual reduction of the mass of gaseous fuel in the at least one storage tank over a pre-determined period, wherein the controller is configured to determine an expected reduction in the mass of gaseous fuel in the at least one storage tank over the pre-determined period based on one or more parameters associated with the engine, and wherein the controller is configured to compare the actual and expected reduction in the mass of gaseous fuel in the at least one storage tank over the predetermined period and to determine a leak condition of the gas engine and / or gas storage system in response to a difference above a predetermined threshold. The gas engine may comprise at least one fuel injector, and the one or more parameters associated with the engine may comprise an open time of the at least one fuel injector over the predetermined period. The one or more parameters associated with the engine may comprise an engine RPM or an engine load. The controller may be configured to generate an output in response to a difference of at least 5%, optionally at least 10%, between the actual and expected reduction in the mass of gaseous fuel in the at least one storage tank. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. According to a sixth aspect, there is provided a generator set comprising: an electrical generator for supplying electrical energy to a battery pack and / or to an electrical energy outlet; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine; a tank sensor assembly comprising a tank temperature sensor configured to monitor a tank temperature of the at least one storage tank and a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank; and a controller configured to determine a mass of gaseous fuel in the at least one storage tank based on the tank temperature and tank pressure and to determine an actual reduction in the mass of gaseous fuel in the at least one storage tank over a pre-determined period, wherein the controller is configured to determine an expected reduction in the mass of gaseous fuel in the at least one storage tank over the pre-determined period based on one or more parameters associated with the engine, and wherein the controller is configured to compare the actual and expected reduction in the mass of gaseous fuel in the at least one storage tank over the predetermined period and to determine a leak condition of the gas engine and / or gas storage system in response to a difference above a predetermined threshold. The gas engine may comprise at least one fuel injector, and the one or more parameters associated with the engine may comprise an open time of the at least one fuel injector over the predetermined period. The one or more parameters associated with the engine may comprise an engine RPM or an engine load. The controller may be configured to generate an output in response to a difference of at least 5%, optionally at least 10%, between the actual and expected reduction in the mass of gaseous fuel in the at least one storage tank. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. According to a seventh aspect, there is provided a portable refuelling device for refuelling a vehicle, the device comprising: a refuelling outlet nozzle configured to discharge a gaseous for refuelling a vehicle; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine; a tank sensor assembly comprising a tank temperature sensor configured to monitor a tank temperature of the at least one storage tank and a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank; and a controller configured to determine a mass of gaseous fuel in the at least one storage tank based on the tank temperature and tank pressure and to determine an actual reduction in the mass of gaseous fuel in the at least one storage tank over a pre-determined period, wherein the controller is configured to determine an expected reduction in the mass of gaseous fuel in the at least one storage tank over the pre-determined period based on one or more parameters associated with the engine, and wherein the controller is configured to compare the actual and expected reduction in the mass of gaseous fuel in the at least one storage tank over the predetermined period and to determine a leak condition of the gas engine and / or gas storage system in response to a difference above a predetermined threshold. The gas engine may comprise at least one fuel injector, and the one or more parameters associated with the engine may comprise an open time of the at least one fuel injector over the predetermined period. The one or more parameters associated with the engine may comprise an engine RPM or an engine load. The controller may be configured to generate an output in response to a difference of at least 5%, optionally at least 10%, between the actual and expected reduction in the mass of gaseous fuel in the at least one storage tank. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. According to an eighth aspect, there is provided a working machine comprising: a ground engaging structure for providing propulsion over a ground surface; a chassis mounted to the ground engaging structure; a gas engine configured to provide motive power to the ground engaging structure; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the tank valve to the gas engine; a plurality of pressure sensors configured to monitor pressure in the gas storage system and / or gas engine, each of the plurality of pressure sensors being separated from the other pressure sensors by at least one pressure disrupting component; and a controller configured to receive data from each of the plurality of pressure sensors to determine an actual pressure differential between at least two of the plurality of pressure sensors, wherein the controller is configured to determine an expected pressure differential between the at least two pressure sensors due to the pressure disrupting component therebetween, to compare the expected pressure differential to the actual pressure differential, and to generate an output in response to a difference between the actual and expected pressure differential above a predetermined threshold. The controller may be configured to compare the expected and actual pressure differentials under different operating conditions of the gas storage system and / or gas engine. The at least one pressure disrupting component may comprise one or more of a fuel line, a pressure regulator, a fuel filter, or a valve. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. According to a ninth aspect, there is provided a generator set comprising: an electrical generator for supplying electrical energy to a battery pack and / or to an electrical energy outlet; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the tank valve to the gas engine; a plurality of pressure sensors configured to monitor pressure in the gas storage system and / or gas engine, each of the plurality of pressure sensors being separated from the other pressure sensors by at least one pressure disrupting component; and a controller configured to receive data from each of the plurality of pressure sensors to determine an actual pressure differential between at least two of the plurality of pressure sensors, wherein the controller is configured to determine an expected pressure differential between the at least two pressure sensors due to the pressure disrupting component therebetween, to compare the expected pressure differential to the actual pressure differential, and to generate an output in response to a difference between the actual and expected pressure differential above a predetermined threshold. The controller may be configured to compare the expected and actual pressure differentials under different operating conditions of the gas storage system and / or gas engine. The at least one pressure disrupting component may comprise one or more of a fuel line, a pressure regulator, a fuel filter, or a valve. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. According to a fourth aspect, there is provided a portable refuelling device for refuelling a vehicle, the device comprising: a refuelling outlet nozzle configured to discharge a gaseous for refuelling a vehicle; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the tank valve to the gas engine; a plurality of pressure sensors configured to monitor pressure in the gas storage system and / or gas engine, each of the plurality of pressure sensors being separated from the other pressure sensors by at least one pressure disrupting component; and a controller configured to receive data from each of the plurality of pressure sensors to determine an actual pressure differential between at least two of the plurality of pressure sensors, wherein the controller is configured to determine an expected pressure differential between the at least two pressure sensors due to the pressure disrupting component therebetween, to compare the expected pressure differential to the actual pressure differential, and to generate an output in response to a difference between the actual and expected pressure differential above a predetermined threshold. The controller may be configured to compare the expected and actual pressure differentials under different operating conditions of the gas storage system and / or gas engine. The at least one pressure disrupting component may comprise one or more of a fuel line, a pressure regulator, a fuel filter, or a valve. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. According to a tenth aspect, there is provided, a working machine comprising: a ground engaging structure for providing propulsion over a ground surface; a chassis mounted to the ground engaging structure; a gas engine configured to provide motive power to the ground engaging structure; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the tank valve to the gas engine; a tank sensor assembly comprising a tank temperature sensor configured to monitor a tank temperature of the at least one storage tank and a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank; and a controller and a memory, said controller configured to determine a mass of gaseous fuel in the at least one storage tank based on the tank temperature and tank pressure, wherein the controller is configured to determine a first mass of gaseous fuel in the at least one storage tank at or prior to or at deactivation of the working machine and to store the first mass in the memory, and to determine a second mass of gaseous fuel in the at least one storage tank upon activation of the working machine subsequent to the deactivation, and wherein the controller is configured to compare the first and second masses and to generate an output in response to a difference between the first and second masses above a predetermined threshold. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine or generator set. According to an eleventh aspect, there is provided a generator set comprising: an electrical generator for supplying electrical energy to a battery pack and / or to an electrical energy outlet; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the tank valve to the gas engine; a tank sensor assembly comprising a tank temperature sensor configured to monitor a tank temperature of the at least one storage tank and a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank; and a controller and a memory, said controller configured to determine a mass of gaseous fuel in the at least one storage tank based on the tank temperature and tank pressure, wherein the controller is configured to determine a first mass of gaseous fuel in the at least one storage tank at or prior to or at deactivation of the working machine and to store the first mass in the memory, and to determine a second mass of gaseous fuel in the at least one storage tank upon activation of the working machine subsequent to the deactivation, and wherein the controller is configured to compare the first and second masses and to generate an output in response to a difference between the first and second masses above a predetermined threshold. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine or generator set. According to a twelfth aspect, there is provided a portable refuelling device for refuelling a vehicle, the device comprising: a refuelling outlet nozzle configured to discharge a gaseous for refuelling a vehicle; a gas storage system comprising at least one storage tank comprising a tank valve and configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the tank valve to the gas engine; a tank sensor assembly comprising a tank temperature sensor configured to monitor a tank temperature of the at least one storage tank and a tank pressure sensor configured to monitor a tank pressure in the at least one storage tank; and a controller and a memory, said controller configured to determine a mass of gaseous fuel in the at least one storage tank based on the tank temperature and tank pressure, wherein the controller is configured to determine a first mass of gaseous fuel in the at least one storage tank at or prior to or at deactivation of the working machine and to store the first mass in the memory, and to determine a second mass of gaseous fuel in the at least one storage tank upon activation of the working machine subsequent to the deactivation, and wherein the controller is configured to compare the first and second masses and to generate an output in response to a difference between the first and second masses above a predetermined threshold. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine or generator set. According to a thirteenth aspect, there is provided a working machine comprising: a ground engaging structure for providing propulsion over a ground surface; a chassis mounted to the ground engaging structure; a gas engine configured to provide motive power to the ground engaging structure; a gas storage system comprising at least one storage tank configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the at least one storage tank to the gas engine; a gas leak sensor assembly comprising at least one gas detector configured and arranged to detect gas leaking from the gas storage system and / or gas engine, and at least one temperature sensor configured and arranged to detect an increase in temperature at or near the at least one gas detector. The working machine may comprise a controller configured to generate an output in response to a detected increase in temperature at or near the at least one gas detector. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. The temperature sensor may comprise a thermal fuse configured to rupture at a predetermined temperature. According to a fourteenth aspect, there is provided a generator set comprising: an electrical generator for supplying electrical energy to a battery pack and / or to an electrical energy outlet; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the at least one storage tank to the gas engine; a gas leak sensor assembly comprising at least one gas detector configured and arranged to detect gas leaking from the gas storage system and / or gas engine, and at least one temperature sensor configured and arranged to detect an increase in temperature at or near the at least one gas detector. The generator set may comprise a controller configured to generate an output in response to a detected increase in temperature at or near the at least one gas detector. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. The temperature sensor may comprise a thermal fuse configured to rupture at a predetermined temperature. According to a fifteenth aspect, there is provided a portable refuelling device for refuelling a vehicle, the device comprising: a refuelling outlet nozzle configured to discharge a gaseous for refuelling a vehicle; a gas engine configured to provide motive power to the electrical generator; a gas storage system comprising at least one storage tank configured to store a gaseous fuel for the gas engine, and a fuel line fluidly connecting the at least one storage tank to the gas engine; a gas leak sensor assembly comprising at least one gas detector configured and arranged to detect gas leaking from the gas storage system and / or gas engine, and at least one temperature sensor configured and arranged to detect an increase in temperature at or near the at least one gas detector. The portable refueller may comprise a controller configured to generate an output in response to a detected increase in temperature at or near the at least one gas detector. The output may comprise one or more of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine. The temperature sensor may comprise a thermal fuse configured to rupture at a predetermined temperature. The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the aspects, embodiments or examples disclosed herein may be applied to any other aspect, embodiment or example. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and / or combined with any other feature described herein. BRIEF DESCRIPTION OF DRAWINGS Embodiments will now be described by way of example only with reference to the accompanying figures, in which: Figures la to le show a working machine from the left, right, front, top and frontleft perspective; Figures 2a and 2b show a gas storage system from rear and forward perspective views with an upper housing portion removed according to the present disclosure; Figures 3a to 3c show side, rear perspective and plan views of a gas storage tank mounting frame of the gas storage system of Figures 2a and 2b; Figures 4a to 4c show a filling nozzle inlet, shut-off valve, purge valve and associated housings according to the present embodiment; Figure 5 shows a storage compartment provided in an embodiment of the working machine of shown in Figures la to le; Figures 6a and 6b show an alternative storage location for a plurality of storage tanks according to the present disclosure; Figures 7a and 7b show a yet further alternative storage location for one or more storage tanks according to the present disclosure; and, Figure 8 shows a schematic diagram of the gas storage system of the present disclosure. DETAILED DESCRIPTION In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments and the inventive concept. However, those skilled in the art will understand that: the present invention may be practiced without these specific details or with known equivalents of these specific details; that the present invention is not limited to the described embodiments; and, that the present invention may be practiced in a variety of alternative embodiments. It will also be appreciated that well known methods, procedures, components, and systems may have not been described in detail. The same reference numerals may be used for corresponding features throughout the drawings. In such cases, the description of these features may not be repeated. The present disclosure provides advantageous storage systems for a working machine. The principal storage location may comprise the roof of an operator cab which provides sufficient footprint area for a plurality of storage tanks. However, further embodiments show additional or alternative locations along the side of the machine between the front and rear wheels, or underneath the machine, optionally towards the rear of the machine. Providing multiple storage locations allows the amount of gaseous fuel to be adapted according to a desired use. The present disclosure provides multiple solutions for the storage of a gaseous fuel, each of which may be used in isolation of the other features or in combination. To simplify the disclosure, the various solutions are described with reference to a working machine in the form of a backhoe loader. However, it will be appreciated that similar storage solutions may be provided on other forms of working machine such as wheeled loading shovels, telehandlers, forklift trucks, tractors, excavators or others disclosed in the background section. Hence, although the storage solutions described herein are particularly advantageous for a backhoe loader due to the existing machine layout and architecture, references to a backhoe loader may be interchangeable with other working machines where suitable. It will also be appreciated that similar storage solutions may be provided on other forms of apparatus, such as generator sets or portable refuelling devices. Conventional backhoe loaders, as with other working machines, typically comprise prime movers in the form of diesel engines. The present disclosure is concerned with working machines which utilise gas engines which operate using a gaseous fuel. Of particular relevance in the present disclosure are gas engines in the form of hydrogen engines. Hydrogen engines are known in the art and not described in detail in the present disclosure for the sake of brevity. Suffice to say that the gas engine of the working machine is configured to receive a flow of gaseous fuel from a gas storage system according to the present disclosure. The gas engine may be configured to operate solely on gaseous fuel. Hence, the gas engine may be considered to be a single fuel engine rather than a hybrid or dual fuel engine configured to operate on, for example, compressed natural gas and hydrogen, or some other combination of gaseous or liquid fuels. Figure la to le show a working machine 10 in the form of a backhoe loader. The working machine 10 has a chassis 12 mounted to and supported by a ground engaging structure which provides propulsion over a ground surface. The ground engaging structure may be provided in the form of wheels 14. Mounted on the chassis is a lifting or loading arm 16 to which is mounted an implement. In some embodiments, the implement may be a loading shovel 18, although any suitable implement may be attached to the arm 16. The loading arm 16 and loading shovel 18 may be mounted on the front of the machine 10. The machine 10 may be described in relation to a principal axis 11 which extends fore-aft along a centreline of the machine 10. Mounted on the back of the machine 10 is a working arm 20 (which may be referred to a backhoe). The working arm 20 may include a boom 21 and a dipper arm 22. In the illustrated embodiment, a bucket 23 is mounted to the working arm 20, but any suitable implement may be attached to the working arm. The machine 10 includes a gas engine 25 which provides motive power to drive the machine 10 over the ground. Put another way, the gas engine 25 is configured to provide motive power to the ground engaging structure 4 to drive the machine 10 over the ground. The gas engine 25 provides power to operate a hydraulic pump (not shown) which can selectively provide pressurised hydraulic fluid to various rams 27 to operate, by way of example, the loading arm, loading shovel, boom, dipper, bucket etc so as to enable material to be handled. The gas engine 25 may be housed within a conventional engine housing comprising, for example, a front facing grill, sidewalls, fire wall and a bonnet (as shown but not numbered). The gas engine 25 includes a fuel rail 25-1 which receives gaseous fuel from the outlet line 44-3 and supplies the gaseous fuel to the fuel injectors 25-2 of the gas engine 25. In some embodiments, the gas engine includes a pressure sensor 25-3 configured and arranged to monitor a pressure within the fuel rail 25-1. The machine 10 includes an operator cab 30 mounted to the chassis. The operator cab 30 includes an operator seat and operator controls such as a steering wheel, a foot brake, a foot throttle, a hand throttle and backhoe control lever. The operator seat may face forwards and be rotatable such that it can face rearwards to enable operation of the working arm 20. Access to the operator cab 30 may be provided by a plurality of steps, as shown. The steps are conventional and not explained further. Steps may be provided on either or both sides of the machine 10 depending on the layout of the machine 10 and / or operator cab 30. The working arm 20 is mounted to the chassis 12 via a vertical pivot mounting, commonly referred to as a king post 24, which allows the arm to slew left and right relative to the chassis. The working arm 20 is shown as being in a pre-work position longitudinally aligned position such that an extension axis of the boom 21 and dipper arm 22 is generally aligned with the longitudinal or principal axis 11 of the machine 10. The working arm 20 would typically be moved into a transverse position so as to lie alongside and parallel to the rear of the cab 30 for transport. Retractable telescopic stabilizer legs 15 may be provided for stabilizing the machine 10 when the working arm 20 is operable. With additional reference to Figures 2a to 3c, the working machine 10 comprises a gas storage system 38. The gas storage system 38 includes a plurality of storage tanks 40a-c and associated tank valves 42. The gas storage system 38 may be provided within a housing 46 on the operator cab 30. The storage system 38 is configured to receive, store and deliver gaseous fuel to the engine 25. In the embodiment shown, there are three elongate cylindrical storage tanks 40a-c having corresponding longitudinal axes arranged in parallel in a fore-aft direction so as to be aligned with the principal axis 11. It will be appreciated that in some embodiments there may be more, or fewer storage tanks 40a-c and they may be orientated differently, e.g. transverse to the principal axis 11. The storage tanks 40a-c may be arranged in a side-by-side axially aligned relation in a common horizontal plane. However, in some embodiments of the present disclosure it may be preferable to have the tanks 40a-c vertically or axially offset with respect to one another, depending on the desired external shape or other packaging considerations. The storage tanks 40a-c are enclosed in the housing 46 which is attached to the top of the operator cab 30. The housing 46 may be attached to or provide the operator cab roof 30-1. Put another way, the plurality of storage tanks 40a-c are arranged on the roof 30-1 of the operator cab 30. In embodiments, the housing 46 may be provided by a unitary structure which extends over the storage tanks 40a-c etc, or, as shown, may comprise a multi-part structure which is assembled around the storage tanks 40a-c. Thus, as can be seen, there the housing 46 may comprise a lower part 46-1, which is located on the top of the operator cab 30, and an upper part 46-2 which is received by and attached to the lower part via a plurality of suitable fixings. The roof of the cab 30 may be understood to be an upper member of the cab 30 which provides a canopy under which an operator is located when operating the machine 10. The mating of the upper and lower parts 46-1, 46-2 may be provided by any suitable attachment such as a plurality of circumferentially distributed fixings 46f, such as bolts. A compressible seal (not shown) may additionally be provided between the lower and upper structures 46-1, 46-2 to prevent ingress of water. The lower part 46-1 may comprise an annular member which is located around the plurality of storage tanks 40a-c and has a sidewall 46-4 which extends upwardly to partially surround the storage tanks 40a-c. An external surface of the lower part 46-1 may be angled relative to horizontal for aesthetic purposes and to provide adequate run off. The lower part 46-1 may additionally provide support for one or more lights or other ancillary components as shown in Figure le for example, but not labelled. The lower part 46-1 may comprise a base wall (not shown) which extends underneath the storage tanks 40a-c and which attaches to the operator cab 30. However, a base wall may be omitted in some embodiments with the sidewall 46-4 being attached to the top of the operator cab 30 around the storage tanks 40a-c or to the mounting frame 48 which carries the storage tanks 40a-c. Regardless of whether the lower part 46-1 comprises a base wall, the top of the operator cab 30 may comprise an internal liner or structural panel to separate the interior of the operator cab 30 from the storage system 38. Generally, the operator cab 30 may be fluidly isolated from the storage system 38 so that storage system to prevent ingress of the gaseous fuel into the cab 30. A drainage channel 46-3 may be provided around the periphery of the housing 46, for example around the base of the upper part 46-2 where it mates with the lower part 46-1. The drainage channel 46-3 may comprise one or more outlets 46-8 located at either or both the front and rear of the operator cab 30 so that water can exit at a desired location. As shown, the upper part 46-2 may comprise a plurality of elevated portions 46-5 which extend longitudinally along the housing 46 in alignment with the storage tanks 40a-c. Each elevated portion 46-5 may correspond to and partially receive a portion of a respective storage tank 40a-c. As such the number of elevated portions 46-5 may correspond to the number of storage tanks 40a-c. The elevated portions 46-5 are defined in part by elongate channels 46-6 which extend front to back between adjacent tanks 40a-c to provide a trough or gutter. Each channel 46-6 may be formed by a base wall 46-6b and opposing side walls 46-6sw which extend between the base 46-6b to the upper surfaces of the elevated portions 46-5. It will be appreciated that the side walls 46-6sw may equally be thought of as part of the elevated portions 46-5. The side walls 46-6sw are angled relative to vertical so as to be inclined at approximately 40 degrees to the base wall 46-6b which extends horizontally therebetween. It will be appreciated that the form of the elevated portions 46-5 and channels 46-6 may vary according to different embodiments and may include rounded surfaces as well as or instead of the facetted surfaces shown in the drawings. In order to provide suitable ventilation for the housing 46 some embodiments may include ventilation apertures positioned to allow an escape of any gaseous fuel which has escaped from the storage tanks 40a-c, valves 42 or gas lines 44 provided within the housing 46. Ideally, for hydrogen and other gaseous fuels which are lighter than air, it is preferable to provide ventilation at the highest location in the housing 46 thereby removing the need for forced ventilation. In the embodiment shown, the side walls 46-6sw includes a plurality of apertures 46-7 suitable for venting any escaped gas from the storage tanks 40a-c. The venting apertures 46-7 may be provided in any desirable suitable location but are advantageously distributed along the length of the channel side walls in the described embodiment so as to prevent any local build-up of escaped gas. Each side wall 46-6sw comprises three apertures equally distributed along the length of the housing 46 but more or fewer may be provided. The outermost elevated portions 46-5 may include downwardly inclined flanks located along an outboard edge thereof which may also include suitable venting apertures 46-7, as described. Providing the apertures 46-7 on an elevated and inclined surface can help reduce ingress of water, e.g. rain water, into the interior of the housing 46. As such, the apertures 46-7 may be open at all times and have a relatively small diameter to prevent ingress of water or other foreign matter into the housing 46 in normal use. However, in some embodiments, it may be preferable to provide the apertures with hoods which overhang the aperture 46-7 to help deflect water away. In some embodiments, the present disclosure may comprise a housing 46 with forced ventilation in which a blower unit, e.g. a fan, is arranged to provide a flow of air to expel any leaked gas. The sizes of the apertures 46-7 may be between 3mm and 15mm, optionally between 5mm and 10mm in diameter. The gas storage system 38 may comprise one or more compressible elements 41 located between an upper surface of one or more of the storage tanks 40a-c and an underside of the upper part 46-2 to provide some cushioning therebetween and, optionally, provide some force distribution from foreign object strikes. The storage tanks 40a-c may be carried by a mounting frame 48. The mounting frame 48 may comprise one or more mounting members for receiving the tanks 40a-c and any suitable arrangement of structural members for distributing the weight of the tanks 40a-c across the structural frame of the operator cab 30. Thus, the mounting frame 48 may comprise a framework of cross-members 48-1, 48-2 which extend transverse to the longitudinal axes of the tanks 40a-c and a plurality of interconnected longitudinal members 48-4 to provide sufficient strength and rigidity. The mounting frame 38 may be mounted to the cab 30 via one or more attachment members which are attached to one or more structural members of the operator cab 30. The mounting frame 48 and the storage tanks 40a-c and associated valves 42 and gas lines 44 may be provided as a sub-assembly for loading on the cab 30 as a single unit. In doing so, there is provided a convenient way of assembling and testing a gas storage system 38 for a working machine 10 prior to mounting it the working machine 10. In the embodiment shown, the mounting frame 48 comprises fore and aft cross members 48-1, 48-2 which extend laterally in front of and to the rear of the tanks 40a-c. The cross members 48-1, 48-2 may be spanned by one or more longitudinal members 48-4 which provide rigidity and stability to the cross members 48-1, 48-2 and help prevent stress in the storage tanks 40a-c which span therebetween. The cross members 48-1, 48-2 and / or longitudinal members 48-4 may be attached to the operator cab 30 by a plurality of attachment members in the form of legs 48-3. The legs 48-3 may be attached via any suitable fixings e.g. threaded bolts, or via welding, for example. In the embodiment shown, the legs 48-3 extend from the cross-members 48-1, 48-2 or longitudinal members 48-4 to a height shortly below the underside of the tanks where they terminate in rectangular pads which are bolted to the cab. The mounting frame 48 may be attached to the structural framework of the operator cab 30 so to provide sufficient support. The structural framework of the operator cab 30 may vary between embodiments, but in the machine shown comprises a plurality of posts 30-2 having cross members 30-3 extending therebetween. More specifically, there is provided fore, aft and mid cab posts 30-2 which extend upwardly from the operator cab floor with cross members 30-3 connecting the terminal ends thereof so as to from a ring beam. Fore and aft transverse cross members 30-3 which respectively extend between the fore and aft cab posts 30-2 are configured to have the mounting frame 48 mounted thereto such that the weight of the storage tanks 40a-c can be reacted by the cross members 30-3 and cab posts 30-2 to the chassis 12. Although the mounting frame 38 of the embodiment shown extends between transverse cross members, additional or alternative attachments may be made to the side cross members which extend fore aft between the fore, mid and aft posts. It will be appreciated that the lower part 46-1 and / or upper part 46-2 of the housing 46 may be attached to the mounting frame 48 or direct to the structural framework of the cab 30. The mounting of the storage tanks 40a-c to the mounting frame 38 may be by any suitable means. In the embodiment shown, the storage tanks 40a-c are neck mounted. As such, each storage tank 40a-c comprises neck portions 41 at either end which are received by suitable mounting features provided on the mounting frame 48. The tank neck portions 41 may comprise relatively short cylindrical bosses which extend axially and concentrically with the longitudinal axis of the respective tank 40a-c. Thus, the mounting frame 48 may comprise first and second neck attachments 48-5 for receiving the respective neck portions 41 at either end of the storage tanks 40a-c. The neck attachments 48-5 may comprise any suitable attachment mechanism. In the described embodiment, the neck attachments 48-4, 48-5 comprise annular collars comprising a cuboidal block having a circular aperture therethrough for snugly receiving the neck portions 41. The neck attachments 48-5 and corresponding neck portions 41 may comprise a close tolerance fit which allows for axial and rotational movement to allow for suitable positioning once the mounted. As such, tank valves 42 which are provided on an end of each tank 40a-c, may be axially and rotationally aligned for connection to the gas lines 44. Either or both of the collars 48-5 provided at the opposing ends may comprise a clamp which is operable to fixedly attached the respective storage tank 40a-c to prevent axial or rotational movement in the correct position. Thus, either or both of the neck attachments 48-5 may comprise an open configuration in which the tank position is adjustable and a closed configuration in which the tank position is fixed in relation to the mounting frame 48. The clamp may be provided by closing the diameter of the annular collar about the neck portion 41. In the embodiment shown, neck attachment 48-4 comprises a closed, full annular collar which is not adjustable, whilst neck attachment 48-5 comprises an open annulus having radially separated ends which are closed via a tangential clamping bolt. Alternatively, the clamp may be provided by a two part fixture comprising a semi-circular saddle mounted to the mounting frame 48 and an opposing semi-circular clasp which sits over the neck and bolts to the saddle. As shown, the neck attachments 48-5 may be fixed to the cross members via bolts, the heads of which can be best seen in Figure 2b. The fixing bolts may also act as clamping bolts. In some embodiments either or both of the neck attachments 48-5 may comprise a liner to act as a bearing element to assist with positioning and / or to help prevent local stress concentrations on the respective neck portion 41. An alternative method of attaching the tanks 40a-c to the operator cab 30 may be with the use of tank straps which extend circumferentially around the tanks. However, tank straps typically require a saddle on to which the storage tanks 40a-c can be mounted adding height to the overall structure. The provision of neck attachments 48-4, 48-5 is favourable in that it allows the height of the tanks 40a-c to be lowered with respect to the top of the operator cab 30. Hence, the underside of the storage tanks 40a-c may be provided proximate to the operator cab roof lining which provides the interior of the cab 30. As can be best seen in the side view of Figure 3a, the tanks 40a-c may be configured to overhang the front and / or rear of the cab 30 to usefully increase the capacity of the tanks 40a-c or to reduce the height of the tank storage system 38 and height of the machine 10. In the embodiment shown, the front of the tanks 40a-c seen on the right hand side of the drawing are provided approximately in-line with the operator cab fore cross member, with an overhang provided at the rear. Providing no or limited overhang at the front of the operator cab 30 may be beneficial for improving visibility, for example, when using the loading arm 16 which is often operated at height. However, the visibility requirements are generally less at the rear of a backhoe loader due to the work undertaken with the working arm 20, hence the overhang may be larger on the rear. In the present embodiment, the overhang at the rear is limited to prevent any possible contact with the working arm during normal operating activities or when the arm 20 is in a transverse transport position. In machines without such a working arm or which have reduced visibility requirements the tanks 40a-c may be configured to overhang further. The extent of the overhang in the embodiment shown is less than 10% of the length of the tanks 40a-c from the terminal end of tank valve 42 to a tank cap inserted in the neck portion 41 at the opposing end. However, the overlap may be greater in some embodiments. In order to provide the necessary support to the storage tanks 40a-c when overhanging, the rear supporting legs 48-3 may be provided on the longitudinal members 48-4 inboard of the cross members 48-1, 48-2. The width of the mounting frame 48 and collective width of the storage tanks 40a-c may be constrained to be within the width of the operator cab 30 to help prevent impacts from foreign objects which do not contact the cab side walls. The height of the storage system 38 including the tanks 40a-c and housing 46 may be any desirable. In embodiments where the working machine 10 is a backhoe loader, the height of the housing 46 may be advantageously below the upper extent of the working arm 20 when in a transport position thereby helping to reduce the likelihood of accidental collision with an overhead obstacle when manoeuvring the machine 10. Each tank 40a-c may be provided with a tank valve 42. The tank valve 42 may be mounted to the respective neck potion 41. Each tank 40a-c may comprise one or more of the group comprising: a shut-off valve, a pressure sensor, a temperature sensor and a thermally-activated pressure relief device, TPRD, an inlet port and an outlet port. The working machine 10 may be provided with a controller 64 configured to control the gas storage system 38 The tank valves 42 may be communicably connected to the controller 64. In use, the controller 64 controls opening and closing of the valves 42 and receives operating data therefrom. In some embodiments, each tank valve 42 may include a manually operable bleed valve or auxiliary purge valve. The bleed valve may be the valve 91. The gas storage system 38 includes a fuel line or gas flow path fluidly connecting the tank valve 42 of each gas storage tank 40a-c to the gas engine 25. The fuel line includes a plurality of gas lines, inlet lines, outlet lines etc. as is discussed in more detail below. As best seen in Figure 3b, the tank valves 42 may be connected in flow communication via a single gas line 44-1 which connects the plurality of tanks 40a-c together. Put another way, the gas fuel line includes a connecting gas line 44-1 which extends between the first tank 40a and second tank 40b and the second tank 40b and third tank 40c to connect the plurality of tanks 40a-c together. An inlet line 44-2 may be connected to gas line 44-1 between the first and second tanks 40a, b via a T-piece, with an outlet line 44-3 connected to the connecting line between the second and third tanks 40-b, c via a second T-piece. A regulator 45 may be provided along the gas fuel line to reduce the pressure from the storage tanks 40a-c to a useable pressure. The regulator 45 may be arranged local to the tank valves 42 to reduce the pressure in outlet line 44-3 to a useable pressure. Providing the regulator 45 local to tank valves 42, that is, within the housing 46, allows the pressure to be reduced prior to being transported to the engine 25, thereby reducing the extent of the high-pressure line. Further, providing the regulator 45 as part of the storage system 38 sub-assembly allows it to be tested prior to being mounted to the machine 10. The inlet line 44-2 may comprise a check valve 44-4 which prevents a return flow of gas from the tanks 40a-c back to the filling nozzle inlet 50 (described below). The check valve 44-4 may be provided anywhere along the inlet line 44-2, however, providing it local to the storage tanks 40a-c and within the housing 46 is advantageous as it prevents a reverse flow of high-pressure gas from the storage tanks 40a-c should the high-pressure inlet line 44-1 become damaged between the filling nozzle inlet 50. In alternative embodiments, the check valve 44-4 may be provided in the first tank valve 42. In such an embodiment the high-pressure inlet line 44-1 would need to be connected to a separate port of the valve 42 rather than T-ing into the connecting line 44-1. The storage tanks 40a-c may be configured to carry up to 20kg, optionally 15kg, optionally 9kg of hydrogen when fully loaded. The storage tanks 40a-c may be configured to have a rated operating pressure of up to 35MPa (350bar / 5076psi), 50Mpa (500bar / 7251psi) or 70Mpa (700bar / 10kpsi), depending on the application. Higher or lower pressures may be possible in some embodiments. It will be appreciated that the inlet line 44-1 will be configured to carry at least the same pressure as the pressure rating of the tanks 40a-c. The working pressure of the gas delivered to the engine 25 may be between 5 and 15bar (0.5Mpa and 1.5Mpa / 72psi and 218psi). Each storage tank 40a-c, e.g. each tank valve 42 may comprise a thermally activated pressure relief device, TPRD, 85. The TPRD may be configured to act as a temperature responsive valve which fully opens to vent the content of the storage tanks 40a-c in the event of an ambient temperature which exceeds a predetermined threshold. TPRDs are known and generally required to ensure hydrogen is vented in the event of a fire. In other words, TPRDs provide a thermal fuse which blows when a fire is present to prevent pressure build up in the tanks 40a-c. Each TPRD 85 is connected to a vent line 52. The TPRD vents the content of the storage tanks 40a-c via the vent line 52. The regulator 45 is provided with a pressure relief valve 94. The pressure relief valve 94 associated with the regulator 45 is connected to the vent line 52. The vent line 52 comprises an open passageway which extends from a TPRD port on the tank valve 42 to a vent line outlet in the housing 46. The vent line outlet 54 may be provided in any suitable form and location on the housing. In the embodiment shown, the vent line outlet 54 is provided by a capped aperture in the upper part 46-2 of the tank housing 46. More specifically, in an upper surface of the elevated portions 46-5, as best seen in Figures Id and 2b. Each vent line outlet 54 comprises a pressure responsive removable vent cap 56 which is retained within an aperture in the housing 46 via a suitable interference fit. The fit between the cap 56 and the aperture is such that it will be overcome by the exiting gas in the event of a sudden discharge event. As can be seen, the removable cap 56 may comprise a tether 56-1 which is attached to the housing 46 or elsewhere such that the cap 56 is retained if removed during an evacuation event. This not only prevents the cap 56 being lost or causing possible injury or damage within the vicinity of the machine 10 but may also provide a clear indication that there has been an evacuation from a quick visual inspection by an operator. As can be seen, the vent line outlets 54 are provided at the rear of the tank housing 46 such that the exhausting gas may be expelled further from the engine bay and any potential sources of ignition. The uppermost location of the vent line outlets 54 also aid the safe evacuation of escaped gas. In the embodiment shown there are four vent line outlets 54, one for each of the tanks 40a-c within the corresponding elevated portions 46-5 and a further one of the regulator 45. The distal ends of the vent lines 52 may be supported with an elongate cross member which extends between each vent line 52. In order to reduce the weight of the housing 46, the construction may comprise relatively thin sheet material. As such, there may be some movement between the housing 46 and the storage tanks 40a-c or TPRD vent lines 52 in use. In order to allow for some relative movement and a lighter weight housing, flexible connectors 58 may be provided between the vent lines 52 and the aperture in the housing 46. The flexible connector 58 may comprise a flexible portion of the vent line 52 or may comprise a flexible collar which is configured to axially flex to accommodate in service misalignments between the housing 46 and vent line 52. The flexible collar may be mated with the aperture and receive the vent line 52 within a central aperture. In the embodiment shown, the flexible connector 58 has a form similar to a gearstick or CV joint boot as known in the art. Thus there are a base which is attached to the aperture in the housing 46 and mates with the cap 56, and a concertinaed conical body which can axially flex in any direction and includes a central aperture which sealably receives the vent line 52. As will be appreciated, the removable vent cap 56 may be sealably received within the housing aperture or flexible connector 58 so as to help prevent ingress of water into the vent line 52. To help further reduce water ingress the vent outlets 54 may be provided on the elevated portions of the housing 46, are previously noted, where they are exposed to less surface water vis-a-vis the channels. In the event water enters the vent lines 52 it is important to allow it to drain to prevent a potential obstruction from the water, particularly where freezing may occur. As such, the vent lines 52 may comprise a u-bend below the tank valve 42 and / or TPRD which includes a small drainage hole 52-1 (Figure 3b) to allow any water to be drained. The drainage hole 52-1 may be located in the lowest part of the vent line 52. Inlet line 44-1 provides fluid communication between the filling nozzle inlet 50 and the storage tanks 40a-c. The filling inlet nozzle 50 may be provided at any convenient location an operator can readily access for the purposes of connecting a refuelling filling nozzle. In the present disclosure, the filling nozzle may be provided in a location which has a fixed relation to the storage tanks 40a-c such that there is minimal relative movement between the filling nozzle inlet 50 and the storage tanks 40a-c. In doing so, the high-pressure inlet line 44-1 can be provided with a rigid pipeline as there is no requirement to allow for differential movement. The alternative might be to use a flexible connection in the high-pressure line, however, this is generally more troublesome and prone to a failure. In the present disclosure the working machine 10 may comprise an operator cab 30 which is configured to move relative to the chassis 12 to increase operator comfort. The relative movement may be provided using antivibration mounts as known in the art. In such an arrangement, the filling nozzle inlet 50 may be attached to the structural framework of the operator cab 30, such as a post 30-2. In the embodiment shown, the filling nozzle inlet 50 is provided on a fore post 30-2 adjacent the cab access steps at a conveniently accessible height. The location is thus between the front and rear wheels and inboard thereof to help prevent accidental damage to the inlet 50 from foreign object strikes in use of the machine 10. The height may be that determined by the top of the cab access steps 68 and / or local to or below a cab door handle. Providing the filling nozzle inlet 50 in this location is advantageous on the backhoe loader as the height is generally convenient for most users and the fore cab post provides suitable support for the gas inlet line 44-2 extending up to the tanks. In some embodiments, the filling nozzle inlet 50 may be provided adjacent one or more other regularly accessed service points, such as a hydraulic oil tank 66 or screen wash bottle, for example. This provides convenience for an operator. The inlet line 44-2 may be surface run on the fore post 30-2 with appropriate spaced fixings. Surface running of the inlet line 44-2 ensures any escaped gas is vented. However, one or more aerated housing element (not shown) may be provided around the inlet line 44-2 to provide a mechanical shield. The housing element may comprise a cage or perforated sheet which is located around the inlet line 44-2, or simply an open channel or side wall which extends alongside and projects outwardly from the post 30-2 further than the inlet line 44-2. In some embodiments of the present disclosure, the inlet line 44-2 may be internal to a cab post 30-2 provided suitable ventilation is provided. The filling nozzle inlet 50 may be located within a housing 50-1 having a hinged door 50-2 to provide access. The door 50-2 may be horizontally hinged along the lower edge such that it readily remains in the open position under the force of gravity once unlocked and may provide a rest for the filling nozzle once attached. The lower pressure outlet line 44-3 extends between the outlet regulator 45 and engine 25. The lower pressure outlet line 44-3 may be surface run on the same fore post 30-2 as the high-pressure inlet line 44-2. The outlet line 44-3 may pass through a shut-off valve 60 which is operable to prevent flow from the outlet regulator 45 to the gas engine 25 when the engine is not in use. Put another way, the shut-off valve 60 is configured and arranged to fluidically isolate the or each storage tank 40a-c from the gas engine 25. The valve 60 may an electrically operated solenoid valve which is operatively connected to a controller 64. The controller 64 may be or form part of a conventional engine control unit (ECU) of the engine 100, or may be a standalone controller. In some embodiments, the shut-off valve 60 includes a check valve configured to prevent downstream flow of the gaseous fluid. The controller 64 may comprise any suitable circuitry to achieve control of valve 60 as required for the operation of the engine 25. As such, the valve 60 may be opened and closed as part of a start-up and shut down procedure, for example. The controller 64 may comprise: control circuitry; and / or processor circuitry; and / or at least one application specific integrated circuit (ASIC); and / or at least one field programmable gate array (FPGA); and / or single or multi-processor architectures; and / or sequential / parallel architectures; and / or at least one programmable logic controllers (PLCs); and / or at least one microprocessor; and / or at least one microcontroller; and / or a central processing unit (CPU), to perform the described methods. The controller 64 may include an associated memory or the memory may be located locally to the controller or remotely. The memory may be a non-volatile memory, for example, read only memory, erasable programmable read only memory, flash memory, solid state drives or magnetic memory. It will be appreciated that the gas storage system 38 may require emptying for maintenance or transporting purposes amongst others. As such, the gas storage system 38 may comprise one or more purge valves which are operable to remove the gas within the storage system 38. The one or more purge valves may be provided anywhere in the storage system 38. Put another way, the one or more purge valves may be provided anywhere along the gas fuel line between the tank valve 42 and the gas engine 25. In the present disclosure, the purge valve 62 may be provided in the outlet line 44-3 which connects the outlet regulator 45 and the engine 25. Thus, the purge valve 62 is arranged downstream of the outlet regulator 45. The purge valve 62 is operable to remove the content of the tanks 40a-c and gas lines. Providing a purge valve 62 in the lower pressure outlet line 44-3 allows the valve to be simpler and minimises the risk of a high-pressure leak. The purge valve 62 may be located within 1.8m of the ground surface, optionally 1.5m, optionally 1.2m, to enable an operator to manually open and close the purge valve from the ground. In some embodiments, the purge valve 62 may be located adjacent an engine housing in which the gas engine 25 is located. In some embodiments, the purge valve 62 is located proximate an entrance to the operator cab 30, and may be connected to the operator cab 30. In some embodiments, the purge valve 62 may be attached to one of the posts 30-2 of the cab 30. In the example shown in Figure 4c, the purge valve 62 is collocated with filling nozzle inlet 50. Put another way, the purge valve 62 is located adjacent to the filling nozzle 50. Optionally, the purge valve 62 may be included in the same valve block as the shut-off valve 60 for convenience. The purge valve 62 may be located fluidically upstream of the shut-off valve 60 so that the system can be purged if the system is powered down or the valve 62 is inoperable. The purge valve 62 may comprise a manually removable plug to which a venting tool (not shown) may be attached to capture the purged gas. Once the venting tool is connected the purge valve 62 may be opened and the gas removed. The purge valve 62 may be a service valve which is operable under the control of the venting tool such that it is only operable by trained and authorised personnel. In some embodiments the venting tool may capture the vented gas in a suitable receptacle. Doing so not only conserves the fuel for use elsewhere but also prevents escape to the atmosphere which may be deleterious to the environment. In other embodiments, the venting tool may act as a discharge duct configured and arranged to discharge the gaseous fuel at a predetermined height relative to the purge valve 62. The purge valve 62 may be manually operable to move the purge valve 62 between open and closed configurations. The purge valve 62 may be manually operable and requiring a tool (not shown), for example an Allen key or any other suitable tool, to move the purge valve 62 between open and closed configurations. The purge valve 62 includes a recess or opening 62-1 configured to receive tool to enable the purge valve 62 to be moved between the open and closed configurations. In other embodiments, the purge valve 62 may comprise a manually operable switch or lever to move the purge valve 62 between open and closed configurations. In further alternative embodiments, the purge valve 62 may be controlled (i.e. opened and closed) by the controller 64. As discussed above, the shut-off valve 60 may include a check valve configured to prevent downstream flow of the gaseous fluid. During purging of the gas storage system 38, the provision of the check valve 60 can enable the gas engine 25 or fuel rail 25-1 to be purged in the event that the shut-off valve 60 fails to open. Once the gaseous fuel has been purged from the part of the gas storage system 38 upstream of the shut-off valve 60, the higher pressure present downstream of the shut-off valve 60 enables upstream flow of the gaseous fluid to the purge valve. In some embodiments, the pressure required to operate the check valve 60 may be approximately 1 bar gauge, 2 bar gauge, or any other suitable predetermined pressure. Gauge pressure, noted as barg or bar gauge, is commonly used as a monitored pressure as it uses atmospheric pressure as a base value. The at storage tanks will equalise with atmospheric pressure when fully purged or vented and left open to the atmosphere. This means that it is not necessary to subtract the atmospheric pressure to measure the change in pressure. Absolute pressure, noted as bara or bar absolute, is the pressure in a perfect vacuum (i.e. zero pressure). The difference between bar gauge and bar absolute is always equal to the atmospheric pressure. As noted above, the operator cab 30 and storage system 38 may move relative to the chassis 12 during normal use. In order to provide a connection between the storage system 38 and engine 25, the outlet line 44-3 may include a flexible portion 44-5, which may be referred to as engine line. Providing a flexible portion 44-5 in the outline line 44-3 versus the inlet line 44-2 is preferable due to the lower pressure. The flexible portion 44-5 of the outline line 44-3 may include one more flexible hoses as known in the art. The flexible hoses may comprise a laminated construction of one or more suitable materials to provide a mechanically suitable hose which is sufficiently resilient to hydrogen embrittlement. The flexible portions and rigid portions of either or both the inlet and outline lines 44-2, 44-3 may be made from one or more of the group comprising: stainless steel, e.g. 316L, nylon, PTFE, polyamide, steel braid etc. In some embodiments, the flexible portions may comprise a laminated polymer hose comprising one or more braids. The working machine may comprise one or more hydraulic actuators for operating various hydraulic services such as the rams 27 on the working and lifting arms. As such, the working machine may comprise a hydraulic oil tank 66 on a first side of the operator cab 30 in which hydraulic oil can be stored. In the present disclosure, the hydraulic tank 66 is located adjacent the filling nozzle inlet 50 to provide a convenient maintenance location for an operator. The storage system 38 described herein previously includes a plurality of storage tanks 40a-c located on a roof of the operator cab 30. The location of the storage tanks 40a-c provides potential for a large amount of storage thereby extending the duration of use between refuelling. In some embodiments of the present disclosure, the storage tanks 40a-c may be distributed about the working machine in alternative locations. For example, the one or more storage tanks 40' may be located remote from the operator cab 30, for example, on the side of the chassis 12 or on an underside of the chassis 12. Figures 6a and 6b provide an example of a plurality of storage tanks 40' being located on the side of the working machine 10 longitudinally between the front and rear wheels 14 and below the operator cab 30. The storage tanks 40' are provided within a housing 46' and supported by a structural mounting frame 48' which is attached to the chassis 12. In the embodiment shown there are provided four cylindrical tanks however there may be greater or fewer tanks in alternative embodiments. In a conventional machine comprising a diesel engine, a fuel tank may be provided on a side of the machine 10 in the location where the storage tanks 40' are provided. Thus, the presence of the storage tanks 40' in this location may be represent the swapping of one storage system for another. However, providing the tanks 40' in this location may interfere with cab access steps 68 (shown in Figure 5) and may not be of sufficient volume to accommodate enough gaseous fuel. Hence, in some embodiments, the tanks 40' may supplement a primary storage system 38 provided on the roof of the cab 30 (as described herein) rather than being the sole storage location. Although shown on the right hand side of the machine 10, the present disclosure contemplates providing tanks 40' on either side of the machine centreline. Hence, there be tanks provided on either or both of the left and right hand sides of the machine 10. The side storage tanks 40' may each comprise a volume which is smaller than the roof tanks and have normal operating pressures similar to those disclosed above for tanks 40a-c. The weight of the hydrogen stored in the side tanks 40' may be between 2kg and 7kg when fully loaded. In embodiments where the side storage tanks 40' are not implemented, the space may be usefully replaced with one or more tool storage compartments 70. Hence, as can be seen in Figure 5, there is provided an upper and lower tool storage compartment 70a, b, each having a hinged access door 70a-l, 70b-l. The took storage compartments 70 may be provided behind cab access steps 68 with the doors 70a-l, 70b-l provided in suitable locations between the steps 68 or above. The tool storage compartments 70 may comprise an external housing which defines an enclosed space in which one or more items may be stored. The housing may comprise a at least one outboard facing lateral wall in which a hinged access door 70b-l (or other form of removable panel) may be provided. The housing 70 may further comprise at least one upwardly facing wall. The upwardly facing wall may comprise a hinged access door 70a-l (or other removable panel) to provide access from above. The tool storage compartment 70 may be a single compartment having a side access and top access, or compartments 70a, 70b may be segregated with separate access doors, as shown. The upper access door 70a-l is generally L-shaped when viewed side on and hinged on the outboard wall. However, the door 70a-l may be hinged to the rear of the opening in some embodiments and / or on the upper surface. Figures 7a and 7b show a yet further alternative storage system with a tank 40" located beneath the chassis 12. Storage tank 40" is located towards the rear of the machine between the rear wheels 14. Storage tank 40" may comprise an elongate cylindrical body which is transversely orientated. The tank 40"may be provided to the rear of the rear axle 14-1 behind the rear differential 14-2 and in front of the working arm 20 and / or rear stabilizer legs 15. The tank 40" may comprise mechanical protection in the form of housing 46" which envelopes at least an underside of the tank to prevent foreign object strikes from below. The housing 46" may comprise one or more drainage holes 46" and may be generally perforated. Figure 8 shows a schematic diagram of the gas storage system 38 of the present disclosure. The gas storage system 38 may be implemented in any working machine having a gas engine 25 but may be particularly applicable to the backhoe loader 10 disclosed herein. The same reference numerals as used previously may be used for corresponding features. In such cases, the description of these features may not be repeated. The gas storage system 38 comprises a plurality of storage tanks 40a-c each having a tank valve 42. The tank valves 42 include ports to which gas lines 44 are connected to provide series flow communication between the valves 42. Thus, there can be seen connecting lines 44-1 extending between the first and second tanks 40a, b and second and third tanks 40b, c. An inlet line 44-2 is T'ed into the connecting line 44-1 which extends between the first and second tanks 40a, b. An outlet line 44-3 extends between the final tank 40c and the outlet regulator 45. The tanks 40a-c, tank valves 42 and outlet regulator 45 are provided within the housing 46 which is provided on the operator cab 30 or elsewhere on the machine. As noted above, in some embodiments of the present disclosure there may be more or fewer tanks 40a-c, for example, where the tanks 40a-c are provided on the side of the machine 10 or underneath. Upstream of the storage tanks 40a-c and housing 46, there is provided a filling nozzle inlet 50 connected by the inlet line 44-2. The filling nozzle inlet 50 comprises a filter 50-3 to help remove particulate matter from the flow of gaseous fuel as it enters. The inlet 50 may also comprise a check valve 50-4 which prevents a reverse flow of gas from the tanks 40a-c when the filling nozzle (not shown) is removed. In some embodiments, the check valve may be selectively openable to allow gas to be purged from the system 38 and captured in a reverse refuelling process. Removing fuel may be done for transport or storage purposes where the tanks 40a-c are emptied of gaseous fuel and optionally filled with an inert gas such as nitrogen. In some embodiments, the check valve 50-4 may be arranged downstream of the filter 50-3. A check valve 44-4 may be provided local to the tanks 40a-c, for example in the housing 46. The check valve 44-4 may be arranged upstream of the tank valve 42. Check valve 44-4 may be provided as part of the T-piece which connects the connecting line 44-1 and the inlet line 44-2 or where the inlet line 44-2 enters a tank valve 42 which may be the preferred connection in some embodiments. In Figure 8, the check valve 44-4 is shown as being separate from the T-piece, tank valves 42 and connecting line 44-1, but within the housing 46. The purpose of check valve 44-4 is to prevent a mass evacuation of gas in the event of an exposed portion of inlet line 44-2 being severed or otherwise damaged in use. Each tank valve 42 may comprise one or more of: a tank shut-off valve, a pressure sensor 82, filter 83, a temperature sensor 84, a thermally-activated pressure relief device, TPRD 85; a first port 86; a second port 87; excess flow valve 88; and, one or more tank valves 89, 91, for allowing flow into and out of the tanks. As can be seen, the ports may be interconnected by a flow passage. The tank valves 42 are connected to the internal volume of the tanks 40a-c via two pathways. The first pathway includes a manual normally open valve 89, a solenoid operated check valve 90, an excess flow valve 88, and a filter 83. The solenoid operated check valve is configured to allow flow into the respective tank 40a-c when gaseous fuel is provided by the inlet line 44-2 from the filling nozzle inlet 50 whilst preventing reverse flow. Activation of the solenoid may be achieved by the controller 64 and removes the check valve from the fuel line to allow reverse flow for the purpose of emptying the tanks 40a-c. The second path comprises a normally closed manually operated valve 91. This valve 91 may be operated by hand to empty the tanks manually should the machine be powered off or the solenoid valve become inoperable. The TPRD connects the internal volume of the respective tank 40a-c to air via a suitable vent line outlet 54, via vent lines 52. Each tank 40a-c has an associated tank sensor assembly configured to monitor at least one tank parameter associated with the at least one storage tank. The tank sensor assembly may include the temperature sensor 84 to monitor the gas temperature of gaseous fuel within the tank 42. The tank sensor assembly may include the pressure sensor 82 to monitor the pressure inside the tanks 40a-c. The controller 64 may be configured to determine a fuel level of each tank 40a-c based on the pressure and temperature detected by the pressure sensor 82 and temperature sensor 84, respectively. It will be understood that the volume of the tanks is substantially constant, and the volume of each tank may be stored in the memory associated with the controller 64 and accessed by the controller to determine the mass of gaseous fuel in each tank 40a-c. In some embodiments, the working machine 10 may be provided with an ambient temperature sensor (not shown). The ambient temperature sensor may be arranged to be positioned away from (i.e. it may be remote from) the at least one storage tank, the gas engine 25, or any other component that might otherwise act as a heat source to alter the detected ambient temperature. In alternative embodiments, the ambient temperature sensor may be omitted and the controller 64 may be configured to determine or approximate the ambient temperature by any suitable means. One such example would be to approximate the ambient temperature from an engine temperature sensor and a temperature of engine coolant, although any suitable arrangement may be used. In embodiments where the ambient temperature is monitored or determined, the controller 64 may be configured to determine the mass of gaseous fuel based on one or both of the tank temperature detected by the tank temperature sensor 84 and the ambient temperature. As discussed above, the controller 64 calculates the fuel level of each storage tank 40a-c based on the determined mass of gaseous fuel in each tank 40a-c, and the calculated mass of gaseous fuel in each storage tank 40a-c is dependent on temperature. During refuelling of each tank 40a-c, the tanks 40a-c may increase in temperature. In embodiments where the gaseous fuel is hydrogen, the heating is caused by expansion of the gaseous fuel as it passes into the storage tank 40a-c (e.g. when the gaseous fuel is hydrogen). This increased temperature of each tank 40-a-c would result in an artificially increased mass of gaseous fuel being calculated by the controller 64. Once refuelling of each tank 40a-c has been completed, the tanks 40a-c would begin to cool. The determined mass of gaseous fuel in each tank 40a-c would then reduce as the temperature drops. In order to overcome, mitigate, or reduce this reduction in the determined mass of gaseous fuel in each tank 40a-c after refuelling, the controller may be configured to select either the ambient air temperature or the tank temperature to determine the mass of gaseous fuel in each tank 40a-c. In some embodiments, the controller 64 may select the lower temperature of the ambient air temperature and the tank temperature in order to calculate or determine the mass of gaseous fuel in each tank 40a-c. This may enable an effective full refuel of each tank 40a-c to occur. The controller 64 may determine a minimum and a maximum fuel mass or fuel level of each tank, and compare these to the determined mass of gaseous fuel in each tank 40a-c in order to determine a fuel level of each tank 40a-c. The minimum fuel mass or minimum fuel level of each tank 40a-c may be determined based on a predetermined minimum tank pressure. The maximum fuel mass or maximum fuel level of each tank 40a-c may be determined based on a predetermined maximum tank pressure. In some embodiments, the minimum and maximum fuel masses or levels may be based on the tank temperature and or the ambient temperature. The controller may be configured to select either the ambient air temperature or the tank temperature to determine the minimum and maximum fuel mass or fuel level of each tank 40a-c. In some embodiments, the controller 64 may select the lower temperature of the ambient air temperature and the tank temperature in order to calculate or determine the minimum and maximum fuel mass or fuel level of each tank 40a-. This may enable a more accurate determination of the current fuel level of each tank 40a-c. Once the minimum and maximum fuel masses or fuel levels have been determined, the controller then may be placed on a scale, for example a linear scale, between the minimum and maximum fuel masses or levels in order to determine the fuel level of each tank 40a-c. The controller 64 may be configured to determine a mass of gaseous fuel in each tank 40a-c, as discussed above. In some embodiments, the controller 64 monitors the mass of gaseous fuel in each tank 40a-c over a predetermined period to determine an actual reduction in the mass of gaseous fuel in each tank 40a-c over said period. The controller 64 may then compare this to an expected reduction in the mass of gaseous fuel in each tank 40a-c. The expected reduction in the mass of gaseous fuel in each tank 40a-c may be calculated by the controller 64 based on one or more parameters associated with the gas engine 25. The parameters associated with the gas engine 25 may be one or more of an engine RPM, an engine load, or an open time of fuel injectors of the gas engine over the predetermined period. The controller 64 compares the actual reduction in the mass of gaseous fuel in each tank 40a-c to the expected reduction over the predetermined period to determine a leak condition of the gas engine 25 and / or gas storage system 38. The controller 64 is configured to determine a leak condition in response to a difference between the actual and expected reduction in mass of gaseous fuel in each tank40a-c above a predetermined threshold. In some embodiments, the predetermined threshold may be a difference of at least 5%, at least 10%, at least 15% or any other suitable threshold. In some embodiments, the controller 64 may be configured to generate an output in response to a detected leak condition. The output may include an audio alarm (e.g. within the cab of the working machine 10) or a visual alarm or alert. The visual alarm or alert may be a light or may be shown on a display within the cab of the working machine 10. In some embodiments, the controller may be configured to control or restrict operation of the working machine 10 in response to a detected increase in temperature at or near the at least one gas detector. This may include controlling or restricting one or more of: a speed of movement of the working machine; movement of the or each arm of the working machine 10; and a rotational speed of the gas engine. In some embodiments, the controller 64 may be configured to determine a first mass of gaseous fuel in each tank 40a-c at or prior to or at deactivation of the working machine 10 (i.e. turning off the working machine). This determined first mass of gaseous fuel may then be stored in the memory associated with the controller 64. Upon activation of (i.e. turning on) the working machine 10, subsequent to the deactivation, the controller 64 may determine a second mass of gaseous fuel in each tank 40a-c. The first and second masses of gaseous fuel are compared, and the controller 64 is configured to generate an output in response to a difference between the first and second masses above a predetermined threshold. In some embodiments, the predetermined threshold may be a difference of at least 5%, at least 10%, at least 15% or any other suitable threshold. The output may include an audio alarm (e.g. within the cab of the working machine 10) or a visual alarm or alert. The visual alarm or alert may be a light or may be shown on a display within the cab of the working machine 10. In some embodiments, the controller may be configured to control or restrict operation of the working machine 10 in response to a detected increase in temperature at or near the at least one gas detector. This may include controlling or restricting one or more of: a speed of movement of the working machine; movement of the or each arm of the working machine 10; and a rotational speed of the gas engine. The outlet regulator 45 may be a two-stage regulator which reduces the pressure down from the high-pressure storage to the pressure required for the engine. The purge valve 62 and shut-off valve 60 may be those described above. In some embodiments, the gas storage system 38 may be provided with one or more pressure sensors 92-1, 92-2 to monitor pressure in the fuel line. In the illustrated embodiments, the gas storage system 38 is provided with first and second pressure sensors 92-1, 92-2 arranged upstream and downstream of the outlet regulator 45 to monitor pressure on either side of the outlet regulator 45. In some embodiments, the gas storage system 38 may be provided with a gas leak sensor assembly 80 configured and arranged to detect gas leaks at one or more location of the gas storage system 38. The gas leak sensor assembly 80 include a gas detector configured and arranged to detect gas leaking from the gas storage system 38 and / or the gas engine 25. In some embodiments, the gas leak sensor assembly 80 may also include a temperature sensor configured and arranged to detect an increase in temperature at or near the gas detector. It will be appreciated that one or more gas leak sensor assemblies may be provided at one or more different locations around the gas storage system 38. In some embodiments, the controller 64 may be configured to generate an output in response to a detected increase in temperature at or near the at least one gas detector (i.e. by the gas leak sensor assembly 80). The output may include an audio alarm (e.g. within the cab of the working machine 10) or a visual alarm or alert. The visual alarm or alert may be a light or may be shown on a display within the cab of the working machine 10. In some embodiments, the controller may be configured to control or restrict operation of the working machine 10 in response to a detected increase in temperature at or near the at least one gas detector. This may include controlling or restricting one or more of: a speed of movement of the working machine; movement of the or each arm of the working machine 10; and a rotational speed of the gas engine. The use of the combined gas detector and temperature sensor helps to provide protection against gas leaks that are combusted, thereby leaving no gas for the gas detector to detect. In some embodiments, the temperature sensor may be a thermal fuse configured to rupture at a predetermined temperature. The thermal fuse may be a TPRD in the fuel line that is configured to rupture at a predetermined temperature to purge the gaseous fuel from the gas storage system. However, it will be appreciated that any suitable arrangement may be used in other embodiments. In further embodiments, the temperature sensor of the gas leak sensor assembly may be omitted. In some embodiments, the controller 64 may be configured to activate a valve diagnosis mode to determine a leak condition of one or more valve of the gas storage system. A leak condition may be where one or more valve fails to move from an open condition to a closed condition, or where one or more valve fails to fully close. In the valve diagnosis mode, the controller 64 is configured to close the tank valve 42 and / or the shut-off valve 81 and to monitor pressure within the gas storage system 38 and / or gas engine 25 to determine a leak condition of the respective valve. In some embodiments, the controller 64 may determine the leak condition of the respective valve independent of temperature in the fuel tank or fuel line. In alternative embodiments, the temperature of the storage tank 40a-c and / or fuel line may be used to determine a leak condition of the respective valve. It will be understood that in some embodiments, the controller 64 may only activate the valve diagnosis mode when a fuel level of gaseous fuel in the at least one storage tank 40a-c is above a predetermined threshold. The controller 64 may generate an alarm or alert when the valve diagnosis mode has not been activated within a predetermined period of time, for example 1 day, 2 days or any suitable length of time. In some embodiments, the controller 64 closes the tank valve 42 when in the valve diagnosis mode to determine a leak condition of the respective tank valve 42. The controller 64 may only activate the valve diagnosis mode to close the tank valve when the gas engine 25 is in an active state. An active state of the engine may be an idling state. In such embodiments, the controller may deactivate the valve diagnosis mode and reopen the tank valve 42 in response to a demand on the gas engine 25, for example an in response to an increase in RPM of the gas engine 25. In embodiments containing multiple gas storage tanks 40a-c, the controller 64 may close each tank valve 42 independently, for example sequentially, to monitor pressure within each storage tank 40a-c. As discussed above, each tank valve 42 may comprise a pressure sensor 82 configured to monitor pressure in the at least one storage tank 40a-c. In some embodiments, the controller 64 closes the tank valve 42 when in the valve diagnosis mode and monitors pressure within the at least one storage tank 40a-c over a predetermined period. If the controller 64 determines that the pressure within the storage tank 40a-c has reduced while the tank valve 42 is closed, the controller 64 determines a leak condition of the respective tank valve 42. The reduction of tank pressure while the tank valve 42 is closed indicates that some gaseous fuel is passing through the respective tank valve 42 and that the tank valve 42 has failed to close. In some embodiments, the controller 64 may only determine a leak condition of the respective tank valve 42 in response to a reduction in pressure in the storage tank 40a-c above a pre-determined threshold, for example a reduction above 0%, 1%, 5%, 10% or any other suitable threshold. As discussed above, the gas storage system 38 may be provided with one or more pressure sensors 92-1, 92-2 to monitor pressure in the fuel line. In some embodiments, the controller 64 closes the tank valve 42 when in the valve diagnosis mode and monitors pressure within the fuel line over a predetermined period. If the controller 64 determines that the pressure within the fuel line has failed to reduce, has reduced below a predetermined threshold, or has reduced below a predetermined threshold rate, when the tank valve 42 is closed, the controller 64 determines a leak condition of the respective tank valve 42. The limited reduction of fuel line pressure while the tank valve 42 is closed indicates that some gaseous fuel is passing through the respective tank valve 42 into the fuel line and that the tank valve 42 has failed to close. In some embodiments, the controller 64 may be configured to compare pressure in the at least one storage tank 40a-c and the fuel line while the tank valve 42 is closed and to determine a leak condition of the tank valve 42 based on this comparison. In some embodiments, the controller 64 closes the shut-off valve 81 when in the valve diagnosis mode to determine a leak condition of the shut-off valve 81. The controller 64 may only activate the valve diagnosis mode to close the shut-off valve 81 when the gas engine 25 is in an inactive state. In some embodiments, the controller 64 closes the shutoff valve 81 when in the valve diagnosis mode and monitors pressure downstream of the shut-off valve 81 over a predetermined period to determine a leak condition of the shutoff valve 81. If the controller 64 determines that the pressure downstream of the shut-off valve 81 has failed to reduce, has reduced below a predetermined threshold, or has reduced below a predetermined threshold rate, when the shut-off valve 81 closed, the controller 64 determines a leak condition of the shut-off valve 81. The limited reduction of pressure downstream of the shut-off valve 81 while the shut-off valve 81 is closed indicates that some gaseous fuel is passing through the shut-off valve 81 and that the shut-off valve 81 has failed to close. The controller 64 may be configured to monitor and compare pressure upstream and downstream of the shut-off valve 81 over a predetermined period with the shut-off valve 81 closed to determine a leak condition of the shut-off valve. In such embodiments, the controller 64 may be configured to determine a leak condition of the shut-off valve 81 in response to no pressure differential upstream and downstream of the shut-off valve 81, a pressure differential below a pre-determined threshold, and / or a rate of increase in the pressure differential upstream and downstream of the shut-off valve 81 below a predetermined threshold rate. As discussed above, the gas engine 25 includes a pressure sensor 25-3 configured and arranged to monitor a pressure within the gas engine 25. It will be appreciated that the pressure sensor 25-3 of the gas engine 25 monitors pressure downstream of the shut-off valve 81. In alternative embodiments, the gas storage system 38 may include a further pressure sensor configured to monitor the pressure in the fuel line downstream of the shut-off valve 81. As discussed above, the gas storage system 38 may be provided with one or more pressure sensors 92-1, 92-2 to monitor pressure in the fuel line. It will be appreciated that the pressure sensors 92-1, 92-2 monitor pressure in the fuel line upstream of the shut-off valve 81. In alternative embodiments, a further fuel line pressure sensor may be provided to monitor pressure upstream of the shut-off valve 81. In some embodiments, when in the valve diagnosis mode with the shut-off valve 81 closed, the controller 64 may purge the fuel rail 25-1 of gaseous fuel and to monitor pressure in the fuel rail 25-1 over a predetermined period. Any increase in pressure in the fuel rail 25-1 while the shut-off valve 81 is closed indicates that some gaseous fuel is passing through the shut-off valve 81 and that the shut-off valve 81 has failed to close. In such embodiments, the controller 64 may determine a leak condition of the shut-off valve 81 in response to an increase in pressure in the fuel rail 25-1 above a pre-determined threshold, for example a reduction above 0%, 1%, 5%, 10% or any other suitable threshold. In the valve diagnosis mode described above, the controller 64 may generate an output in response to a determined leak condition of the tank valve 42 and / or shut-off valve 81. The output may include an audio alarm, a visual alarm or alert such as a warning on a display or a light. In some embodiments, the output of the controller 64 may include controlling or restricting operation of the working machine 10. This may include controlling or restricting one or more of: a speed of movement of the working machine; movement of the or each arm of the working machine 10; and a rotational speed of the gas engine. It will be understood that each of the pressure sensors described above are separated from the other pressure sensors by at least one pressure disrupting component. The pressure disrupting component may be the fuel line, the pressure regulator 45, the fuel filter 83, or one of more of the valves 42, 81, although it could be any component that would result in a change in the pressure of gaseous fuel in the gas storage system 38. It will also be appreciated that additional pressure sensors may be provided in the gas storage system 38 that are separated from another pressure sensor by one or more pressure disrupting components. The output may include an audio alarm (e.g. within the cab of the working machine 10) or a visual alarm or alert. The visual alarm or alert may be a light or may be shown on a display within the cab of the working machine 10. In some embodiments, the controller may be configured to control or restrict operation of the working machine 10 in response to a detected increase in temperature at or near the at least one gas detector. This may include controlling or restricting one or more of: a speed of movement of the working machine; movement of the or each arm of the working machine 10; and a rotational speed of the gas engine. Although the gas storage system has been described above in relation to a working machine in the form of a backhoe loader, it will be understood that the gas storage system and associated controller may be applied to any working machine including such a gas storage system. Construction sites, especially during the foundation preparation stage, are generally without access to mains electricity (i.e. are "off-grid" locations). As a result, power systems such as generator sets (often referred to as "gensets"), are utilised on construction sites to provide power to one or more devices / equipment on the construction site, for example a tower crane, flood lights, site cabin power sockets etc. Gensets are also used in other analogous situations for oil and gas exploration, mining, disaster recovery and the like. In some embodiments, the gas engine and gas storage system may be used in a gas-powered generator set (or genset). Such a generator set may include a gas engine 25 and a gas storage system 38 as illustrated in Figure 8, a controller 64 configured to control the gas storage system as has been described with reference to Figures 1 to 8, and an electrical generator for supplying electrical energy to a battery pack and / or to an electrical energy outlet. In further embodiments, the gas storage system may be used in a portable refuelling device for providing fuel at the point of use such that off-highway / working machines can easily refuel with minimum disruption to their working schedule. Such a portable refuelling device may include a gas storage system 38 as illustrated in Figure 8, a controller 64 configured to control the gas storage system as has been described with reference to Figures 1 to 8, and a refuelling outlet nozzle configured to discharge a gaseous from the gas storage system for refuelling a vehicle, such as an off-highway / working machine. The one or more embodiments are described above by way of example only and it will be appreciated that there may be variations to those described above. The variations are possible without departing from the scope of protection afforded by the appended claims. 10 15 08 10 25 20 25 30 35 1. 2. 3. A working machine comprising:

Claims

ground engaging structure for providing propulsion over a ground surface; chassis mounted to the ground engaging structure;gas engine configured to provide motive power to the ground engagingstructure;a gas storage system comprising a plurality of storage tanks each comprising a tank valve and a tank pressure sensor configured to monitor a tank pressure in the respective storage tank, said storage tanks configured to store a gaseous fuel for the gas engine, a fuel line fluidly connecting the tank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine;at least one pressure sensor configured to monitor pressure in the gas storage system and / or gas engine; anda controller configured to control the gas storage system and to activate a valve diagnosis mode of the gas storage system,wherein the controller is configured, in the valve diagnosis mode, to close one of the plurality of tank valves, to monitor the tank pressure of the respective storage tank over a predetermined period, and to determine a leak condition of the tank valve in response to a reduction in tank pressure above a pre-determined threshold, andwherein the controller is configured to close each tank valve independently.The working machine according to claim 1, comprising a tank pressure sensor and a tank temperature sensor configured to monitor tank pressure and tank temperature in the at least one storage tank, respectively, wherein the controller is configured to determine a mass of gaseous fuel in the at the least one storage tank based on the tank pressure and the tank temperature, and determine a leak condition of the tank valve in response to a reduction in the mass of gaseous fuel in the at least one storage tank above a pre-determined threshold.The working machine according to any preceding claim, comprising a fuel line pressure sensor configured to monitor fuel line pressure in the fuel line, wherein the controller is configured, in the valve diagnosis mode, to close the tank valve, monitor fuel line pressure over a predetermined period, and determine a leak condition of tank valve in response to no reduction in fuel line pressure, in response to a reduction in fuel line pressure below a pre-determined threshold and / or in response to a rate of reduction of fuel line pressure below a predetermined threshold rate.08 10 254. The working machine according to any preceding claim, wherein the controller is configured to activate the valve diagnosis mode and close the tank valve only when the gas engine is in an active state.

5. The working machine according to claim 4, wherein the controller is configured to 5 activate the valve diagnosis mode and close the tank valve only when the gasengine is in idling state, and wherein the controller is configured to deactivate the valve diagnosis mode in response to a demand on the gas engine.

6. The working machine according to claim 5, wherein the controller is configured to deactivate the valve diagnosis mode in response to an increase in RPM of the gas 10 engine.

7. The working machine according to any preceding claim, wherein the controller is configured to close each tank valve sequentially.

8. The working machine according to any preceding claim, wherein the controller is configured to only activate the valve diagnosis mode when a fuel level of gaseous 15 fuel in the at least one storage tank is above a predetermined threshold.

9. The working machine according to any preceding claim, wherein the gas engine comprises a fuel rail and a gas engine pressure sensor configured to monitor pressure within the fuel rail, and wherein the controller is configured, in the valve diagnosis mode, to close the shut-off valve, purge the fuel rail of the gaseous fuel, 20 monitor pressure in the fuel rail over a predetermined period, and determine a leakcondition of the shut-off valve in response to an increase in pressure in the fuel rail above a pre-determined threshold.10.The working machine according to any preceding claim, wherein the controller is configured, in the valve diagnosis mode, to close the shut-off valve, monitor 25 pressure upstream and downstream of the shut-off valve over a predeterminedperiod to determine a pressure differential, and determine a leak condition of the shut-off valve in response to the pressure differential being below a pre-determined threshold and / or in response to a rate of increase in the pressure differential below a pre-determined threshold rate.30 11.The working machine according to claim 10, comprising a fuel line pressure sensorconfigured and arranged to monitor fuel line pressure upstream of the shut-off valve and a gas engine pressure sensor configured and arranged to monitor pressure downstream of the shut-off valve within the gas engine.08 10 2512.The working machine according to claim 11, wherein the gas engine comprises a fuel rail and the gas engine pressure sensor is configured and arranged to monitor pressure within the fuel rail.

13. The working machine according to any one of claims 9 to 12, wherein the controller 5 is configured to activate the valve diagnosis mode and close the shut-off valve onlywhen the gas engine is in an inactive state.14.The working machine according to any preceding claim, wherein the controller is configured to generate an output in response to a determined leak condition of the tank valve and / or shut-off valve.10 15.The working machine according to claim 14, wherein the output comprises one ormore of: an audio alarm; a visual alarm or alert; and controlling or restricting operation of the working machine.16.The working machine according to any preceding claim, wherein the controller is configured to generate an alarm or alert in response to the valve diagnosis mode15 not being activated within a predetermined period of time.17.A method of detecting a leak condition of a valve of a gas storage system comprising a plurality of storage tanks each comprising a tank valve and a tank pressure sensor configured to monitor a tank pressure in the respective storage tank, said storage tanks configured to store a gaseous fuel for the gas engine, a20 gas engine configured to provide motive power, a fuel line fluidly connecting thetank valve to the gas engine, and a shut-off valve arranged on the fuel line for fluidically isolating the at least one storage tank from the gas engine, the method comprising:activating a valve diagnosis mode;25 closing the tank valve of one of the plurality of storage tankstank valvesorthe shut-off valve; andmonitoring pressure within the respective storage tank over a predetermined period; anddetermining a leak condition of the tank valve in response to a reduction in 30 tank pressure above a pre-determined threshold,wherein the method comprises closing each tank valve independently.

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