with reference to fig1 an on - board hydrogen production system in accordance with this invention is shown and is generally designated by reference number 10 . production system 10 is shown coupled with a representative stirling engine 12 . stirling engine 12 may be any one of numerous designs for stirling engines including those described by the previously mentioned patents assigned to the assignee of this application . stirling engine 12 would be of a type which employs hydrogen as the working gas and would be subject to the periodic need to recharge its supply of working gas via the mechanisms of static and / or dynamic loss . stirling engine 12 features internal working gas pressure sensor 14 . pressure sensor 14 would be placed in a portion of the engine 12 subject to either a cyclically pressure wave associated with the working gas volumes as they are cycled between other areas of the engine or to some other place where a less time varying pressure variation would be detected . for example , through the use of an orifice and a small reservoir , a mean pressure signal could be provided . while internal pressures of various stirling engines differ , in one representative design , a mean pressure of 15 mpa is used , with cyclical variations producing a minimum pressure of 12 mpa and a maximum pressure of 18 mpa . the output signal from pressure sensor 14 , termed a charge state signal , is sent to controller 16 which controls operation of production system 10 in a manner which is described more fully in the following sections of this description . with continued reference to fig1 production system 10 principally comprises water storage vessel 18 , filter 20 , electrolyzer 22 , check valve 24 , and storage reservoir 26 . storage vessel 18 is filled with an initial charge of a hydrogen containing liquid , preferably highly purified water . this water , which may be stored in a liquid form at various pressures and temperatures , is directed to pass through filter 20 via conduit 21 where any particulates , minerals , and other contaminants may be removed . next , the water passes into hydrolyser 22 which acts as a reformer , liberating pure hydrogen from the water . it is fundamental principal of electrochemistry that water may be separated into its constituent elements ; namely , gaseous oxygen and hydrogen through electrolysis . in electrolysis , two electrodes are maintained at opposite potentials . gas bubbles of pure oxygen form at one electrode whereas gas bubbles of pure hydrogen occur at the other . a more sophisticated form of this technology employs a so - called proton exchange membrane , which is preferred for this application . from electrolyzer 22 where electrolysis occurs , two pathways are shown . conduit 28 allows pure oxygen gas to be either vented to atmosphere or sent to an onboard combustion device . this would be useful for applications in which a hydrocarbon fuel is combusted to produce heat for stirling engine 12 . absent such a use of the oxygen gas , it can be harmlessly vented to atmosphere . hydrogen gas which is produced passes through conduit 30 through one - way check valve 24 . check valve 24 ensures that hydrogen gas which is produced may only travel in the direction from electrolyzer 22 into storage reservoir 26 . storage reservoir 26 includes an internal storage material , such as a metal hydride material 32 . storing of hydrogen in a hydride form involves the use of an alloy which can absorb and hold large amounts of hydrogen by bonding with hydrogen and forming hydrides . in the absorption process , hydrogen gas molecules stick to the metal surface and break down into hydrogen atoms . the hydrogen atoms penetrate into the interior of the metal crystal to form a new solid substance , referred to as a metal hydride . this process is an exothermic reaction . in the converse process , in which it is desired to desorb the hydrogen through an endothermic reaction , hydrogen atoms migrate to the surface of the metal hydride and flow away as hydrogen gas . various metal materials may be used to form the metal hydride including metal alloys containing titanium , zirconium , chromium , and manganese . since the absorption and desorption processes are exothermic and endothermic respectively , the processes can be controlled by temperature . thus , the mass of hydrogen gas which may be stored in an absorbed form with metal hydride 32 decreases with increasing temperature . using this principle , storage reservoir 26 may be maintained at an ambient or relatively low temperature in its normal condition . in this state , a significant quantity of hydrogen gas may be absorbed by the metal hydride 32 . when it is desired to desorb the hydrogen , for example to charge stirling engine 12 , the temperature of the storage reservoir 26 and / or metal hydride 32 , is elevated . this is achieved through the use of heater power circuit 34 and reservoir resistance heater 36 . thus , upon an appropriate charge command control signal from controller 16 , heater power circuit 34 applies an electrical potential to reservoir heater 36 . current passing through the reservoir heater rapidly increases the temperature of reservoir 26 , causing desorption of the stored hydrogen gas . this hydrogen gas is transmitted into engine 12 via conduit 38 . by choosing an appropriate temperature for elevation of reservoir 26 , an appropriate pressure of supplied hydrogen gas may be generated to force this gas into stirling engine 12 . the process of charging engine 12 with working gas may be supplemented if conduit 38 communicates with a portion of stirling engine 12 undergoing a cyclical working gas pressure variation . through the use of another check valve ( not shown ), gas may be supplied to engine 12 whenever the supply gas is greater than the gas pressure at any point in its cyclical pressure variation cycle . as mentioned previously , a representative stirling engine design has a minimum working gas pressure of 12 mpa . accordingly , it is necessary to produce hydrogen at a pressure no less than 12 mpa in order to charge the previously described engine . additional variations of the design of production system 10 may also be implemented within the scope of the present invention . for example , in some instances , the quantity of stored hydrogen may be sufficient if storage reservoir 26 is initially charged with hydrogen gas . if sufficient metal hydride or other absorber material is available , a sufficient mass of hydrogen gas may be storable in such a reservoir to enable hydrogen gas supplementation to be provided over an acceptable operating life span without the need of generating additional hydrogen gas with the system . such a system is identical to that of fig1 with elements 18 , 20 , 21 , 22 , and 24 eliminated . other variations include the use of other types of absorbers or other storage systems . moreover , multi - stage type systems might also be implemented with different types of absorber material being provided to sequentially elevate the pressure of the hydrogen gas as it moves from being generated to its point of injection into stirling engine 12 . while the above description constitutes the preferred embodiment of the present invention , it will be appreciated that the invention is susceptible to modification , variation and change without departing from the proper scope and fair meaning of the accompanying claims .