the system flow diagram of fig1 shows the overall mechanical interaction and operation of the preferred embodiment of the present invention in a dual tank configuration . it illustrates the flow paths for both liquid and vapor phases of an lng system . this embodiment has two liquid natural gas storage tanks 10a and 10b . embodiments with one tank , or with more than two parallel connected tanks , are also within the spirit and scope of the invention . preferably , tanks 10 are highly insulated dewar type tanks . they are coupled in parallel with respect to their ports 11 and 14 , as shown with 11a and 11b directly coupled as are 14a and 14b . ports 12b and 13b of tank 10b are also coupled in parallel with ports 12a and 13a , respectively , as shown . thus , tanks 10a and 10b maintain a common internal pressure even if their liquid lng levels differ and independently of the magnitude of any such difference . the parallel coupling of ports 11a and 11b , along with ports 14a and 14b , ensures that the internal pressure is common to both tanks 10a and 10b , and remains the same even under extreme circumstances . in typical operating conditions , an abnormal imbalance should never occur , such as where one tank is nearly full of lng and the other completely empty of lng . the levels of liquid in tanks 10a and 10b will remain almost the same from initial filling of those tanks to their exhaustion of lng , unless the couplings of one of them are plugged or abnormally restricted . the vapor phase pressure in both tanks 10a and 10b is the same even if their levels are not equal , such as might result from vehicle orientation or mismatch in the fluid flow resistance of delivery lines 15a and 15b , for instance . tanks 10 are filled with liquid natural gas ( lng ) through fill valve 52 and line 15 , which terminates in open ended pipes 20 . the description later herein with respect to fig3 provides a discussion of how the pressure within the tanks is initialized . when demand is made for fuel by the engine ( not shown ), lng passes from tanks 10 through pipe 15 and through fill valve 52 , which is switched during normal operation to direct the liquid fuel into heat exchanger 61 . in the preferred embodiment , heat exchanger 61 is a conventional heat exchanger available from commercial vendors which uses a refrigerant as a thermal exchange medium . preferably , it transfers heat from the coolant for the vehicle engine into the lng from line 15 . thus , heat is extracted from the engine coolant , and the lng is vaporized into natural gas . the natural gas then passes to a common junction 30 connected to both pressure control valves 32 and 33 . the pressure within the upper portion of both tanks 10 is sensed at line 18 by pressure sensitive valves 31 , 32 , 33 , 34 and 51 , as shown . pressure regulator valve 31 responds to a low level of pressure within tanks 10 , as reflected at line 18 , by withdrawing lng from lower ports 11 through heat exchanger 61 to produce vapor at a higher pressure for introduction to ports 14 . that is , under static conditions , pressure regulator valve 31 is adequate to respond to a lower pressure level at line 18 than is desired by withdrawing lng from lower ports 11 through heat exchanger 61 to produce vapor for introduction to ports 14 . vapor thus formed raises the pressure of tanks 10 up to the normal operating level at which time valve 31 closes . valve 31 may concurrently act with valve 32 when fuel is delivered , since valves 31 and 32 are both operable in response to a lower pressure sensed in line 18 , but flow through valve 31 will remain minimal because differential pressure in the static pressurization loop is low . pressure control valves 32 and 33 determine the flow path of the fuel vapor demanded by the engine coupled to output valve 53 . if the pressure within the tanks 10 as reflected in line 18 is not high enough , pressure control valve 32 opens to allow the heated fuel vapor to return into tanks 10 through pipe 16 . the result is the fuel passes through heat exchangers 62 , and exits tanks 10 via pipes 17 through check valve 42 . it thereafter flows through heat exchanger 63 into the engine coupled output valve 53 . if the pressure within the tanks is adequate to ultimately supply the engine , valve 32 closes , while pressure control valve 33 opens to allow the fuel to pass through a final heat exchanger 63 before passing through the common output connection pipe 48 , and thence through throttle controlled output valve 53 into the engine ( not shown ). thus , the invention is able to maintain constant pressure within tanks 10 while concurrently compensating for changing demands by the engine . when a greater fuel demand occurs , the pressure within tanks 10 is kept at the optimal level by heating the lng therein . in actual operation , valves 32 and 33 do not mutually switch in a binary manner . instead , they provide modulating control of the gas vapor coupled to the utilization output connection to valve 53 . valves 32 and 33 concurrently increase and decrease the amount of flow they permit so that there is a phasing of the switch function between them over a discrete but relatively short time period . pressure sensitive valve 34 functions as a so - called economizer valve in that it monitors the pressure level in the gas phase of tanks 10 via line 18 . whenever this pressure exceeds a predetermined maximum , fuel gas is delivered directly from the vapor phase present in the upper portion of tanks 10 . this gas is passed through pipe 18 , through economizer pressure control valve 34 and check valve 43 to final heat exchanger 63 , and ultimately delivered to the engine . normally , valve 34 is set to open at a higher pressure than valve 33 which senses the pressure level on line 18 in parallel with valve 34 . if the pressure within tanks 10 should continue to increase even further , safety valve or valves 51 open to allow gas to vent from pipe 18 until the pressure within tanks 10 at least returns to the preselected maximum level . during periods of inactivity with no fluid withdrawn , heat leakage into the lng contents of tank 10 can cause a gradual increase in pressure . when this pressure exceeds the normal operating pressure by some predetermined amount , such as 20 %, economizer pressure valve 34 opens . at this point when the engine is operating , vapor is preferentially drawn from the tank through the low pressure drop route consisting of line 18 , valve 34 and check valve 43 . flow through this alternate route continues until pressure in the tank falls enough to close economizer valve 34 , at which time flow to the engine is again supplied by valves 32 and 33 . in the event of an extended period of inactivity , pressure in lng tank 10 will rise to its design pressure . at this point , safety valve ( s ) 51 will open and vent vapor , possibly to the atmosphere , thereby reducing the tank pressure . should the period of inactivity continue , safety valves 51 will periodically open to maintain tank pressure within a range from the design pressure down to a setting about 10 % lower . fig2 shows the configuration and operation of heat exchangers 62 located within tanks 10 . pipe 15 passes through lower port 11 , and terminates in pipe end 20 which is open to the interior of tank 10 . when introducing liquid fuel into tank 10 , the fuel exits from open pipe end 20 into the lower portion of the tank , below the level of the residual lng ( if any ). similarly , pipe 18 passes through port 14 , and opens at end 21 into the upper portion of the tank above the level of the lng . pipes 16 and 17 are coupled in fluidic communication with the input and output ends of a single tortuous pipe 19 . pipe 19 forms a gas flow path which passes through the tank , in thermal communication with the contents of tank 10 , so as to inject heat into the lng . mounting plate 25 is a thermal conductor in thermal contact with pipe 19 , as well as the liquid and vapor phases within the interior of tank 10 , to enhance the heat exchange operation . plate 25 , as seen in fig1 and 3 , extends for the entire height of the interior of tank 10 but can contain openings therethrough and / or openings around the ends to allow liquid lng to freely flow around or through it . fig3 illustrates the additional structure used to initialize the pressure level within the tanks 10 . the pressurization process is initiated when tanks 10 are filled via appropriately switched valve 52 . natural gas at ambient temperature but relatively high pressure is then introduced with pressurization to a suitable level through valve 54 into pipe 16 to cause the gas to pass through port 12 into heat exchanger 62 . the gas is discharged through port 13 into pipe 17 . it is coupled into pipe 15 via bypass 70 , including unidirectional check valve 71 . the gas then passes through port 11 , and exhausts from pipe end 20 so as to bubble through the lng in the tank 10 . this raises the pressure within tank 10 to the point that pressure control valve 33 and / or 34 can begin delivering fuel through final heat exchanger 63 into the engine . in operation , tank 10 is filled , and the initial pressurization established by pressurized natural gas through valve 54 and heat exchanger 62 . the gas exits tank 10 into pipes 17 and 70 for ultimate release from the bottom of the inner chamber of tank 10 at port 20 . until such time as the pressure in the upper portion of the chambers of tanks 10 reaches a first level , valve 32 remains open , while valves 31 , 33 , 34 and 51 all remain closed . a demand for fuel vapor during this time is met by lng from tank 10 vaporizing in heat exchanger 61 , followed by traversing of the heat exchanger 62 within tanks 10 thereby injecting heat into those tanks to accelerate the increase of pressure within tanks 10 . the fuel vapor exits tank 10 , and passes through heat exchanger 63 before delivery to the output valve 53 for introduction to the engine . eventually , the pressure within tanks 10 will exceed a first predetermined level at which point valve 32 closes , while valve 33 opens . fuel demand at this time is met by lng vaporized in heat exchanger 61 and coupling through valve 33 into node connection 48 , heat exchanger 63 and output valve 53 . should the pressure in tanks 10 exceed the first level and reach a somewhat higher second level , economizer valve 34 opens so as to directly vent gas vapor from the upper portion of the internal chamber of tanks 10 into the engine via output valve 53 . while valve 33 is open concurrently with valve 34 , most of the fuel vapor delivered to the output will come from the lower resistance path of valve 34 until the tank 10 pressure drops below the second level . further increase of the pressure above a third and even higher level will cause the safety valve 51 to open . note that at this time valves 31 , 33 and 34 are all open along with valve 51 . they sequentially close as the pressure levels drop until they all are closed and valve 32 , which has remained closed all this time opens . the actual pressure levels at which the various valves operate , as described above , depends upon the demand pressure of the utilization device coupled to the output valve 53 . this is mostly a function of the type of engine fueled by the gas . typically , engine demand pressure can range anywhere from 100 psi to 300 psi . the valves are thus designed to perform their prescribed function , as described above , with the actuating pressure level of each valve thus predetermined so as to appropriately correlate with the engine demand pressure . while the exemplary preferred embodiments of the present invention are described herein with particularity , those having normal skill in the art will recognize various changes , modifications , additions and applications other than those specifically mentioned herein without departing from the spirit of this invention .