Abstract:
A supplemental fluid tank, preferably having two chambers each partially containing a fluid, fluid communicated intermediate a fuel tank and vent to reduce fuel vapor emissions, particularly for a boat. More specifically, when fuel is used or cooled, pressure or volume, respectively, of the remaining fuel in the fuel tank is reduced in prior art systems. Accordingly, air is drawn into the fuel tank through the vent line and becomes saturated with fuel (i.e., fuel vapor). Conversely, when fuel in the fuel tank is warmed it expands and fuel vapor is forced out of the vent into the environment. An exemplary embodiment reduces entry of air in through the vent and escape of fuel vapor out of the vent using two intermediate chambers in fluid communication with each other, each preferably having a non-evaporative fluid (e.g., oil), to provide volume/pressure compensation of the fuel in the fuel tank.

Description:
CROSS REFERNCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/553,039, filed Mar. 12, 2004 the contents of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to a system and method for tank pressure compensation and specifically to a system and method for fuel tank pressure compensation for an internal combustion engine and, more particularly, this invention relates to a barrier tank assembly utilized to reduce diurnal emissions from a fuel tank, particularly in a marine vessel.  
         [0003]     Vehicles powered by internal combustion engines have at least one fuel tank that generally holds a supply of liquid fuel for the engine. The tanks are typically connected to a filler tube that is used to introduce fuel into the tank. The outer opening of the filler tube is usually covered with a removable cap.  
         [0004]     When fuel is added to the tank, it displaces the air in the tank. The air, which is laden with fuel vapor, rushes out of the tank as the fuel enters. In many situations, foam is created by agitation of the fuel entering the tank. In some vehicles, the displaced air and foam rushes back to the filler tube as the tank is filled and splashes out on the person filling the tank. Other fuel systems include a vent line that extends from the interior of the tank to the atmosphere. The vent line enables air to escape from the tank as it is filled with fuel through the filler tube. The vent line also enables air to enter the tank as fuel is withdrawn for delivery to the engine.  
         [0005]     The fuel tank vent line also serves to prevent pressure from building in the tank. If the tank were un-vented, increasing temperature of the fuel would cause fuel and vapor expansion that would cause the pressure in the tank to rise. If the pressure became too high, the fuel tank could rupture, causing fire or explosion.  
         [0006]     Fuel systems used on marine crafts usually include a vent line from the fuel tank. The vent line typically opens to the atmosphere over the water. As the fuel tank is filled to near the top, the air flowing out of the vent line can carry fuel and foam overboard on to the water. Wave action that rocks a boat can also cause fuel to be discharged overboard both during fueling and when the tank is full. In addition, thermal expansion of the fuel due to an increase in fuel temperature may also cause either or both fuel and fuel vapor to be discharged overboard when the tank is full.  
         [0007]     Thermal expansion refers to the expansion of fuel when it is heated to a higher temperature. Both gasoline and diesel fuel expand when their temperature rises. For example, fifty gallons of gasoline will expand by approximately  1 . 61  gallons when the temperature of the gasoline increases by thirty-four degrees Celsius. Similarly, two hundred gallons of gasoline will expand by approximately 6.46 gallons when the temperature of the gasoline is raised by thirty-four degrees Celsius. Diesel fuel expands at a lower rate than gasoline. For example, fifty gallons of diesel fuel will expand by approximately 1.36 gallons and two hundred gallons of diesel fuel will expand by approximately 5.44 gallons when the temperature of the diesel fuel is raised by thirty-four degrees Celsius. Thermal expansion can cause fuel to expand and fuel vapor to be forcibly discharged overboard via the vent line when the fuel tank does not have the space to accommodate the excess fuel and fuel vapor. Fuel and vapor discharged overboard poses a pollution hazard and is harmful to wildlife. There is also a risk that fuel floating on the water or emitted fuel vapor may catch fire causing injury to life or property. Furthermore, when fuel in the fuel tank is consumed and/or cooled, the volume is reduced. Air is drawn into the fuel tank through the vent line and becomes saturated with fuel vapor. Conversely, when this fuel in the tank is then warmed or is filled with additional fuel, the fuel expands and fuel vapor is forced out the vent line.  
         [0008]     Accordingly, what is needed is a system and method to allow for some expansion and contraction without inducing air into the fuel tank or fuel vapor to the atmosphere.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The above drawbacks and deficiencies are overcome or alleviated by A tank pressure compensation system including a chamber having a first end and a second end, the first end connectable to a fluid repository having a first fluid for fluid communication with the first end and the second end in fluid communication with the atmosphere. The chamber is receptive to a second fluid contained in a portion of the chamber intermediate the first and second ends disposed above a level of the second fluid, wherein the chamber allows displacement of the second fluid away from the first end toward the second end of the chamber when the first fluid expands while preventing flow of the first fluid into the atmosphere.  
         [0010]     In one exemplary embodiment, a marine vessel fuel tank pressure compensation assembly for use in the hull of a marine vessel that has a fuel system including a fuel tank vented to a vent that communicates with the atmosphere is disclosed. The system includes a first chamber having a first lower portion and a first upper portion, the first upper portion configured for fluid communication with a first fluid disposed in the fuel tank via a fuel vent line; a second chamber having a second lower portion and a second upper portion, the second upper portion configured for fluid communication with the vent via a vent line, the second lower portion in fluid communication with the first lower portion of the first chamber; a barrier fluid disposed in at least a portion of the first chamber, the barrier fluid configured to allow displacement of the barrier fluid from the first chamber into the chamber when the first fluid expands while preventing flow of the first fluid into the atmosphere.  
         [0011]     In another exemplary embodiment, a method of reducing fuel vapor emitted from a vent line utilizing a pressure compensation assembly in the hull of a marine vessel that has a fuel system including a fuel tank connected to a vent via the vent line that communicates with the atmosphere, wherein the vent line includes a first vent line fitting and a second vent line fitting both adapted for direct parallel communication with the pressure compensation assembly is disclosed. The method includes attaching the pressure compensation assembly to the marine vessel, wherein the pressure compensation assembly includes: a first chamber having a first lower portion and a first upper portion, the first upper portion configured for fluid communication with a first fluid disposed in the fuel tank via a fuel vent line; a second chamber having a second lower portion and a second upper portion, the second upper portion configured for fluid communication with the vent via a vent line, the second lower portion in fluid communication with the first lower portion of the first chamber; a barrier fluid disposed in at least a portion of the first chamber, the barrier fluid configured to allow displacement of the barrier fluid from the first chamber into the chamber when the first fluid expands while preventing flow of the first fluid into the atmosphere. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Referring to the exemplary drawings wherein like elements are numbered alike in the several FIGURES:  
         [0013]      FIG. 1  is a diagrammatic view of a portion of a hull in a marine vessel, partially cut away to show an arrangement of a fuel expansion tank, a fuel tank, a pressure compensation tank assembly, a fuel filler tube and a fuel vent line in accordance with an exemplary embodiment of the present invention;  
         [0014]      FIG. 2  is an enlarged diagrammatic view of a partial portion of the hull of  FIG. 1  illustrating an alternative exemplary embodiment of a pressure compensation tank assembly;  
         [0015]      FIG. 3  is a schematic diagram of  FIG. 2  illustrating a same level of barrier fluid in each chamber of the pressure compensation tank assembly when a pressure of the fuel tank  6  is equal to an ambient pressure of ambient air  48 ;  
         [0016]      FIG. 4  is a schematic diagram illustrating a first chamber (vent side) located below a second chamber  18  (tank side) of a pressure compensation tank assembly in an alternative exemplary embodiment;  
         [0017]      FIG. 5  is diagram of  FIG. 3  illustrating a decreasing pressure of the fuel tank that has moved most of the barrier fluid from the first chamber (vent side) to the second chamber (tank side) via a cross over pipe;  
         [0018]      FIG. 6  is a schematic diagram of the application as in  FIG. 4  where the first chamber (vent side) is located below the second chamber (tank side) and illustrates movement of barrier fluid flow when there is a decreased volume (or pressure) of the fuel tank;  
         [0019]      FIG. 7  is a schematic diagram of  FIG. 5  illustrating further decreasing pressure of the fuel tank that has moved all of the barrier fluid from the first chamber (vent side) to the second chamber (tank side) via the cross over pipe, thus allowing ambient air into the fuel tank;  
         [0020]      FIG. 8  is a schematic diagram of  FIG. 3  illustrating a situation when increasing fuel tank pressure (or volume) has moved most of the barrier fluid from the second chamber to the first chamber during normal diurnal heating, for example;  
         [0021]      FIG. 9  is a schematic diagram illustrating that the increasing fuel tank pressure (or volume of fuel vapor) depicted in  FIG. 8  has reached a point where all of the barrier fluid from the second chamber has moved to the first chamber, or at least empty into a horizontal portion of the crossover pipe  44 , thus allowing fuel vapor to be drawn through the barrier fluid in the first chamber and out to the ambient; and  
         [0022]      FIG. 10  is a schematic diagram of a pressure compensation tank assembly having a barrier fluid containing chamber where the chamber is defined by a first end in fluid communication with a first fluid and a second end in fluid communication with the atmosphere in accordance with an exemplary embodiment.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Intent of the invention is to limit flow of a fluid from a tank into the atmosphere, and more particularly, limiting hydrocarbon emissions from fuel tanks. Temperature changes of fuel and fuel vapor cause a change in volume. Heating causes expansion of the fuel and fuel vapor resulting in the expulsion of fuel vapor from the fuel tank. Cooling of the fuel and fuel vapor causes a contraction of fuel and fuel vapor resulting in the induction of air into the tank. Air induction into the fuel tank creates additional fuel vapor.  
         [0024]     Daily cycles of temperature change are referred to as diurnal cycles. The invention creates a barrier between the fuel vapor in the fuel tank and the atmosphere. Two tanks, or a single compartmented tank are filled to a little less than about ½ capacity with a fluid, such as oil. The oil can move between the two chambers allowing for volume changes in the fuel tank while preventing outside air and fuel tank vapors from mixing.  
         [0025]     By displacing the fluid from one compartment to the other and back, small volumetric changes caused by temperature or atmospheric pressure can be compensated for while maintaining a barrier between fuel tank vapor and outside air.  
         [0026]      FIG. 1  is a diagrammatic view of a portion of a hull  2  on a marine vessel, partially cut away to show a tank vent system arrangement of a fuel expansion tank  5 , a fuel tank  6 , a fuel filler tube  4 , a fuel vent line  8 , and a pressure compensation tank  10  in accordance with an exemplary embodiment of the present invention. The fuel tank  6  supplies fuel to an inboard engine, not shown. A typical fuel tank  6  has a fitting thereon that receives the fuel filler tube  4  and the fuel filler tube  4  extends to a fuel deck type fuel fitting  12  mounted to the gunwale of the boat hull  2 . Another fitting on the fuel tank  6  receives the fuel vent line  8 . The fuel vent line  8  leads from the fuel tank  6  to a vent  14  that extends through the hull  2  of the marine vessel and vents the interior of the fuel tank  6  to the ambient atmosphere. The vent  14  may be located anywhere in the hull  2  of the marine vessel dependent on the choice of the boat designer and/or manufacturer.  
         [0027]     The fuel expansion tank  5  is optionally attached to the fuel vent line  8  in accordance with copending U.S. patent application Ser. No. 10/460,243, entitled, “MARINE VESSEL FUEL OVERFLOW TANK SYSTEM,” filed on Jun. 11, 2003, the contents of which are incorporated herein in their entirety. The fuel expansion tank  5  is mounted above the fuel tank  6  to allow fuel collected in therein to drain back into the fuel tank  6  when the fuel tank  6  has excess capacity.  
         [0028]     The pressure compensation tank  10  is disposed in fluid communication with and intermediate the vent  14  and fuel tank  6 . Pressure compensation tank  10  includes a first chamber  16  in fluid communication with a second chamber  18  (shown in phantom) disposed in the first chamber  16 . First chamber  16  in fluid communication with second chamber  18  via an opening  20  disposed at a bottom surface defining second chamber  18 . First and second chambers are filled with a barrier fluid, such as oil  22 , but not limited thereto, indicated below a dashed line  24 . Chambers  16  and  18  are filled with oil  22  by removing a cap  26  from a filler tube  28  extending first chamber  16 . Fluid  22 , such as oil, for example, may be drained from chambers  16  and  18  via an outlet  30  extending from first chamber  16 . In one embodiment, outlet  30  may be used to draw oil  22  therefrom for injecting oil  22  directly into the engine rather than premixing the oil  22  in the fuel for combustion in a two-stroke engine.  
         [0029]     First chamber  16  is in fluid communication with vent  14  and fuel tank  6  via a first tube  36  connected to vent line  8 . Second chamber  18  is in fluid communication with vent  14  and fuel tank  6  via a second tube  38  connected to vent line  8 . A pressure equalizing valve  40  is disposed in vent line  8  intermediate fluid communication between first and second tubes  32  and  38 . Pressure equalizing valve  40  may be opened to equalize pressure between first and second chambers  16  and  18  when filling the same with fluid  22  via filler tube  28 . It will be noted that equalizing valve  40  is normally closed during normal operation preventing fluid communication therethrough.  
         [0030]      FIG. 2  illustrates an alternative pressure compensation tank assembly  10  of  FIG. 1  generally indicated at  42 . In this embodiment, pressure compensation tank assembly  42  includes the first chamber  16  in fluid communication with the second chamber  18  disposed next to or in series with the first chamber  16 . First chamber  16  is in fluid communication with second chamber  18  via one end of a crossover pipe  44  extending from the opening  20  disposed at the bottom surface defining the second chamber  18 . An opposite end of crossover pipe  44  extends to an opening  46  disposed in a bottom surface defining the first chamber  16 . First and second chambers are filled with a barrier fluid, such as oil  22 , but not limited thereto, indicated below line  24 .  
         [0031]     Minimal internal pressure differences, changes, daily temperature swings, known as diurnal cycles cause fuel in rigid fuel tanks to expand and contract causing the release of hydrocarbons into the atmosphere. Continuous diurnal cycles cause daily fluctuations in fuel and fuel vapor volume. Without a way to compensate for this daily volume change, gasoline vapors (hydrocarbons) are emitted daily into the atmosphere. Air that is induced into the fuel tank mixes with the fuel creating more fuel vapor.  
         [0032]     At 40% saturation in air, 520 gallons of hydrocarbon vapors equate to approximately 1 gallon or 3622 grams of liquid fuel. One gallon of fuel vapor contains approximately 6.97 grams of liquid fuel.  
         [0033]     The EPA has expressed concern about the amount of hydrocarbons emitted into the atmosphere and have proposed limiting diurnal emissions to 1.1 grams/gal./day from the estimate of approximately 1.39 grams/gal./day, and estimate that would result in a 25% reduction of evaporative emissions from spark ignition marine vessels. One aspect of the present invention is to reduce diurnal emissions as well as stop loss due to diffusion of vapor out the vent line  8  by effectively sealing the vent line  8  with a the barrier fluid  22 .  
         [0034]     An internal fuel tank temperature rise from 20° C. to 30° C. will cause an increase in volume of approximately 2.2% if the pressure of tank  6  remains the same. A barrier oil  22  height differential of 12 inches between first and second chambers  16  and  18  results in approximately 0.37 pounds per square inch (PSI) pressure differential resulting in a volume increase of approximately 0.91%.  
         [0035]     520 gallons of gasoline vapor at 40% saturation equate to approximately 1 gallon of gasoline, while 1 gallon of gasoline vapor approximately 6.966 grams of gasoline.  
         [0036]     A 100 gallon fuel tank ¾ full of fuel, heated from 26° C. to 38° C., and no tank pressure change, will emit approximately 2.3 Gal. of fuel vapor equating to approximately 16 grams of fuel.  
         [0037]     Still referring to  FIGS. 1 and 2 , first chamber  16  and second chamber  18  installed in the tank vent system arrangement cause oil  22  to be pushed or drawn from one chamber  16 ,  18  to the other until all of the oil has moved to one from the other, at which point, in the case of decreasing volume of fuel tank  6 , such as from cooling, air is drawn through the oil into the fuel tank  6  or as in the case of increasing volume, such as from heating, fuel vapor is expelled through the oil  22  into the atmosphere via vent  14 .  
         [0038]     More specifically, with specific reference to  FIG. 3 , the embodiment of  FIG. 2  is schematically illustrated.  FIG. 3  illustrates that a level of barrier fluid  22  in first chamber  16  is at the same level of barrier fluid  22  in second chamber  18  when a pressure of the fuel tank  6  is equal to an ambient pressure of ambient air  48 . Barrier fluid  22  is shown to move from one chamber to another via cross over pipe  44  in both directions  49 . Barrier fluid  22  separates ambient air  48  and fuel vapor  50  above liquid fuel  52  in tank  6 , thereby preventing mixing of ambient air and fuel vapor  50 .  
         [0039]      FIG. 4  illustrates an application where partial vacuum in the fuel tank  6  is acceptable but pressure is not, wherein first chamber  16  (vent side) is located below second chamber  18  (tank side). Second chamber  18  is in fluid communication with first chamber  16  via standpipe  54  extending from opening  20  of chamber  18  and into chamber  16 . A pressure relief valve  56  is in fluid communication with second chamber  18  and vent  14  via vent line  8  preventing pressure build up while allowing a partial vacuum. The arrangement depicted in  FIG. 4  is fitted with a one way pressure relief valve  56  to prevent positive pressure in the fuel tank  6  indicated with arrow  58 , while still allowing displacement of barrier oil  22  with a decrease in volume, or lower pressure, in fuel tank  6 . In such a case, it will be recognized by one skilled in the pertinent art that capacity of chambers  16  and  18  will have to be increased to compensate for increased volume.  
         [0040]      FIG. 5  illustrates that a decreasing pressure of fuel tank  6  has moved most of the barrier fluid  22  from first chamber  16  to second chamber  18  via cross over pipe  44  in a direction indicated by arrow  60 . Such a decreased pressure differential is due to normal diurnal cooling. In this manner ambient air  48  is prevented from entering fuel tank  6  and only fuel vapor  50  disposed at a top portion of second chamber  18  is forced back into fuel tank  6  by movement of barrier fluid in direction  60 .  
         [0041]     If the barrier fluid  22  is only allowed to rise  12  inches before either ambient air  48  or fuel vapor  50  can pass through cross over pipe  44 , for example, a pressure differential between the fuel tank  6  and ambient air  48  would not exceed 0.5 PSI. In one embodiment, for example, each chamber  16  and  18  is configured as a rectangular chamber as indicated in  FIGS. 3 and 5  having dimensions of 12×6×6 inches. The two chambers  16  and  18  will prevent hydrocarbon emissions from a half full 100 gallon fuel tank  6  that is subjected to a 10° C. (18° F.) diurnal cycle temperature swing. It will be noted, however, that a 20° C. temperature swing is also contemplated with the chambers  16 ,  18  and tank  6  having the same dimensions.  
         [0042]      FIG. 6  is an application as in  FIG. 4  where partial vacuum in the fuel tank  6  is acceptable but pressure is not, and wherein first chamber  16  (vent side) is located below second chamber  18  (tank side). This arrangement, like  FIG. 5 , illustrates a result of barrier fluid  22  flow when there is a decreased volume (or pressure) of fuel tank  6 . Barrier fluid  22  is shown to be drawn into second chamber  18  without allowing air  48  to enter the fuel tank  6 . One way pressure relief valve  56  prevents positive pressure in the fuel tank  6 , while still allowing displacement of barrier fluid  22  with such a decrease in volume (or pressure) in fuel tank  6 .  
         [0043]      FIG. 7  illustrates a situation when decreasing fuel tank volume (or pressure) causes all of the barrier fluid from first chamber  16  to second chamber  18 , or at least empty into a horizontal potion of crossover pipe  44 . At this point air  48  is drawn through the barrier fluid  22  disposed in second chamber  18  and into fuel tank  6 . As discussed above, if the barrier fluid in second chamber  18  is only allowed to rise twelve inches in chamber  18 , for example, the pressure differential between the fuel tank  6  and ambient air  48  would not exceed 0.5 PSI.  
         [0044]      FIG. 8  illustrates a situation when increasing fuel tank pressure (or volume) has moved most of the barrier fluid  22  from second chamber  18  to first chamber  16  during normal diurnal heating, for example. As pressure or (or volume) of fuel vapor  50  increases, barrier fluid moves through cross over pipe  44  in a direction indicated with arrow  64 .  
         [0045]      FIG. 9  illustrates that the increasing fuel tank pressure (or volume of fuel vapor  50 ) depicted in  FIG. 8  has reached a point where all of the barrier fluid  22  from second chamber  18  has moved to first chamber  16 , or at least empty into a horizontal portion of crossover pipe  44 . At this point fuel vapor  50  is drawn through the barrier fluid  22  disposed in first chamber  16  and out vent  14 . Again, as discussed above, if the barrier fluid in first chamber  16  is only allowed to rise twelve inches in chamber  18 , for example, the pressure differential between the fuel tank  6  and ambient air  48  would not exceed 0.5 PSI.  
         [0046]     It will be recognized with respect to  FIGS. 7 and 9  that once all of the barrier fluid  22  is displaced from either chamber into the other, air is allowed to enter or fuel vapor is allowed to escape from assembly  10 . In this manner, this process naturally allow pressure relief at maximum and minimum pressures automatically without the use of a mechanical pressure relief valve. Furthermore, it will be recognized by one skilled in the pertinent at that displacement of the barrier fluid from one chamber to the other is a result of a pressure differential between the fuel tank and the ambient air. The maximum pressure differentials, both positive and negative, can be set by vertical position of the chambers relative to one another including the addition of a one way pressure relief valve. Lastly, it will be noted that compensation volume of barrier fluid may be controlled by a volume of barrier fluid that may move between the chambers.  
         [0047]      FIG. 10  illustrates a pressure compensation tank assembly  100  in fluid communication with a fluid repository  106  having a first fluid  110  disposed therein. Assembly  100  is configured to limit emission of first fluid  110  into the atmosphere. More specifically, assembly  100  includes a chamber  200  defined by a first chamber  116  in fluid communication with the atmosphere via at a first end  202  defining one end of chamber  200  and a second chamber  118  in fluid communication with first fluid  110  in fluid repository  106  at a second end  204  defining an opposite end of chamber  200  via a vent line  108 . In an exemplary embodiment and still referring to  FIG. 10 , vent line  108  extending from fluid repository includes a vent line  138  in fluid communication with the second chamber  118  above a barrier fluid level  124  therein. Vent line  108  is in further fluid communication with the first chamber  116  above a barrier fluid level  124  therein via a vent line  136  extending to first end  202  having a pressure relief valve  140  therebetween. Vent line  136  is in further communication with a vent  114  exposed to the atmosphere. Pressure relief valve  140 , vent line  136 , an vent  114  are shown with phantom lines to illustrate that they may be eliminated, while maintaining a primary function of assembly  100 . It will be recognized that below each barrier fluid level  124  in each chamber  116  and  118  is a barrier fluid  122  that limits emission of first fluid  110  from fluid repository  106  out to the atmosphere due to expansion of the first fluid  110 .  
         [0048]     Barrier fluid  22  and  122  as used in the exemplary embodiments described above referred to by the applicant as “barrier oil” can be any of many readily available fluids. Such fluids include, but are not limited to, fluids already stored in tanks that are part of the internal combustion engine, vehicle or vessel system that may be suitable for use as “barrier oil” in the invention. It is envisioned that any liquid with a low vapor pressure will work, but some are less troublesome and more cost effective than others. The following are examples, but are not limited to, which may be suitable, as well as cost effective, including engine injection oil, as described with reference to the embodiment depicted and described in  FIG. 1 . Engine cooling system fluid is also contemplated. Most cooling systems on modern engines utilize a ‘closed’ cooling system, which uses a separate tank containing engine coolant. When the cooling system heats up the excess coolant is stored in the coolant reservoir tank so that it can be returned to the system when the cooling system cools. As in the drawing of the invention which is using injection oil, engine coolant in place of the “barrier oil” can be drawn or returned to the bottom cross pipe as can the following). Further, hydraulic fluid is contemplated, thus eliminating a need for a hydraulic fluid reservoir. Lastly, engine crankcase oil and transmission oil are also contemplated for use for the barrier fluid.  
         [0049]     The amount of volume increase caused by a temperature increase in the fuel tank is reduced by allowing a partial pressure to build when displacing the barrier fluid, e.g., oil. Displacing the barrier oil to a height of twelve inches causes a pressure increase of 0.37 PSI (varying slightly with the specific gravity of the “barrier oil”) reducing the amount of volume increase with no pressure change, by more than half. It will be noted that 0.37 PSI was determined by using an estimated specific gravity for a light grade oil such as engine oil, which is lighter than water.  
         [0050]     For example, given a 100 gallon fuel tank filled three-quarters full with gasoline, if internal fuel tank pressure is allowed to vary from ambient by about 0.37 PSI positive and 0.37 PSI negative (i.e., ±0.37 PSI) with a fuel temperature variance from about 28° C. to about 38° C. and about 28° C. to about 18° C. The difference in volume of the fuel and vapor from about 18° C. to about 38° C. is approximately 1.8 gallons compared to approximately 3.3 gallons difference in volume with no pressure change.  
         [0000]     Information About Tank Emissions  
         [0051]     A pair of cylindrical barrier tanks each having dimensions of twelve inches high and a six inch diameter (cylindrical tanks) each hold 1.47 gallons. When each barrier tank is ½ full with barrier fluid, each barrier tank thus allows a 1.47 gallon volume swing. Rectangular barrier tanks dimensioned with a twelve inch height and a six inch square base hold 1.87 gallons each, while barrier tanks twelve inches high having a four inch square base hold 0.83 gallons each.  
         [0052]     A height of the barrier tank controls and limits a maximum pressure differential between the fuel tank it is fluidly communicated with and the ambient. A specific gravity of the barrier fluid used also effects the maximum pressure differential.  
         [0053]     For example, when water is used as a barrier fluid, the specific gravity of water is one (1.0). A tank having a twelve inch height would limit pressure differential to about 0.434 PSI. A tank having a 27.7 inch tank height would limit pressure differential to about 1.0 PSI.  
         [0054]     It is well recognized by one skilled in the pertinent art that changes in temperature causes corresponding changes in pressure and volume under the ideal gas equation, PV=nRT. For example, in 10° C. diurnal cycle temperature increase of 20° C. (68° F.) to 30° C. (86° F), volume change within a half filled 100 gallon tank is inversely proportional to a pressure of the tank. In Example A, with no pressure change, there is a 2.18 gallon increase in volume. In Example B, with a 0.20 PSI increase, thee is a 1.48 gallon increase in volume. In Example C, with a 0.40 PSI increase, there is a 0.811 gallon increase in volume. Therefore, it can be seen that the volume increase decreases with increasing pressure.  
         [0055]     In Example A, 2.18 gallons of hydrocarbons (e.g., fuel vapor) would escape into the atmosphere with such a 10° diurnal cycle. In addition, when the tank cools to the original temperature, fresh unsaturated air is drawn into the tank causing additional vapor emissions as that air becomes saturated with fuel and expands.  
         [0056]     As seen above in the exemplary embodiments of the invention, we can control emissions in a 20° C. diurnal cycle on a 100 gallon/2 full tank with two rectangular barrier tanks (e.g., 12 inch height×6 inch base) half full of barrier fluid. If a third tank is added, the barrier tanks can be protected from contamination with fuel. In example, if the 100 gallon fuel tank is filled to the top and then warms up to a 20° C. differential, expansion of the fuel will cause an increase of about 1.9 gallons). Other arrangements to prevent contamination of the barrier tanks are envisioned including using a float valve and pressure relief valve. However, in any case, lack of a containment tank will result in excess fuel being lost.  
         [0057]     As discussed above, an internal fuel tank positive pressure differential can be limited to zero while still allowing internal negative differentials, or conversely, internal fuel tank negative pressure differential can be limited to zero while allowing internal positive pressure differentials by locating the barrier tanks at different heights in relation to each other and with the use of pressure valves.  
         [0058]     In the Example A above, a 10° C. diurnal cycle results in 2.18 gallons of vapor being expelled, which equates to 15.22 grams of fuel, given one gallon of liquid equals about 520 gallons of vapor. This figure is appears to be negligible until it is associated with the millions of boats and 365 days of a year in which these boats are operated. For example, assuming 5,000,000 inboard tanks each having a 50 gallon average capacity, a 10° C. diurnal cycle results in emissions of about 10,482 gallons of fuel/day, which equates to about 3,825,964 gallons/year.  
         [0059]     The EPA estimates that in the year 2000, diurnal evaporative losses from non-road S/I (spark ignition) fuel tanks were about 22,700 tons of hydrocarbons and about 67,760,000 gallons.  
         [0060]     Another consideration for such evaporative losses includes a loss from diffusion of vapor out of the fuel tank vents. EPA tests estimate that this amount to be about 0.07 to about 0.24 grams/gallon/day, given 4.5 feet of ⅝″ vent line and an ambient temperature of about 22° C. to about 36° C. Therefore, with an average of about 0.15 grams/gallon/day results in 5,000,000 boats each having a 30 gallon tank emitting about 2,700,000 gallons per year.  
         [0061]     Although the above described embodiments have been described with reference to a fuel tank for a marine vessel configured to limit emission of a fuel vapor therefrom into the atmosphere, it will be noted that the above disclosure is intended for use with a fluid in any tank where flow of the fluid from the tank into the atmosphere may be limited using a barrier fluid chamber as disclosed. In any case, the above exemplary embodiments disclose a method and apparatus that allows for some expansion and contraction of a fluid in a tank without inducing ambient air into the tank or fluid into the atmosphere. Furthermore, the above described exemplary embodiments disclose a method and apparatus to reduce diurnal emissions.  
         [0062]     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.