Emission control standards for vehicle fuel systems have been in force for some time, and a typical production motor vehicle now has a fuel vapor recovery system that includes a vapor storage canister. The canister receives and stores fuel vapors that would otherwise escape to atmosphere. The greatest volume of these vapors are produced from the carburetor fuel bowl, the so called "hot soak" losses, and also from the fuel tank as it sits, often referred to as the tank diurnal losses. Vapors are also lost during the fuel fill operation, due to the fact that the fuel flowing down from the nozzle must displace the air-fuel vapor mixture that is already in the tank, or else a positive pressure would develop in the tank that would soon prevent the fuel from entering the tank at all. These may be referred to as fuel fill losses. Clearly, with a nearly empty tank, the volume of fuel fill loss is not insignificant. In a conventional fuel system, a simple vent line running from the tank interior to the top of the filler neck just vents the displaced vapors out of the top of the filler neck to atmosphere. With emission standards becoming more rigorous, the future will require that some or most of the vapors previously allowed to escape vent to atmosphere be recovered as well.
Many designs for fuel vapor recovery systems are disclosed in the issued U.S. patents. These are mostly concerned, however, with the recovery of the hot soak vapors and tank diurnal losses described above, rather than with fuel fill losses. Even those systems that incorporate a vapor passage near the area at the top of the filler neck do not generally use that passage to recover fuel fill losses, or do not recover a significant portion of the fuel fill losses. For example, the U.S. Pat. No. 3,728,846 to Nilsson, while it discloses a lower vent line 4 that runs from the tank interior through the top of the filler neck 2, and an upper vent line 5 that runs from the top of the filler neck 2 to a storage canister, does not attempt to recover fuel fill losses. Instead, the lower vent line 4 is actually closed off during fuel fill, and displaced tank vapors are just vented out the filler neck to atmosphere by a conventionally acting vent pipe 8. The system disclosed in U.S. Pat. No. 4,572,394 to Tanahashi et al. does seek to recover some fuel fill losses, but it is concerned primarily with the smaller volume of pressurized vapors that escapes from the filler neck when the filler neck cap is first removed, referred to generally as the "puff loss." A separate vapor storage canister 5 surrounds a flared upper portion 9 of the filler neck 2, and the interior of the tank is vented through a vent tube 19 into the canister 5. A bellows type valve 12 is released when the filler neck cap 3 is removed, so that the pressurized vapors in the tank vent through tube 19 into the interior of the canister 5, rather than to atmosphere. However, the act of inserting the fuel nozzle pushes in the valve 12, and blocks tank vapors from going from the tank interior through tube 19 to the canister. While this would prevent the fuel fill loss vapors from escaping to atmosphere out the filler neck, those vapors still must go somewhere, or tank pressure will build up until fuel cannot enter. No structure is disclosed to deal with that problem. The system disclosed in U.S. Pat. No. 4,625,777 to Schmidt, assigned to the assignee of the subject invention, speaks to fuel fill losses to an extent, but is primarily concerned with regulating the pressure within the tank when the tank is closed, and also with handling tank diurnal losses. The system disclosed there has a fuel tank 10 with a filler neck 11 that opens into the tank 10 at a fairly high point in the tank. Thus, during most of the filling of the tank, displaced vapors can simply exit to atmosphere through the neck 11, and a conventional vent line is not needed. When the fuel rises high enough to cover the opening of the filler neck 11 into the tank 10, a vent line 14 that runs from a high point in the interior of the tank 10 to a port 29 near the top of the neck 11 takes over and provides a path for the fuel fill loss vapors, as seen in FIG. 4. From port 29, the fuel fill loss vapors can enter an opening in another valve 30 that is diametrically opposed to the port 29, and from there be routed to a canister. However, in the absence of a tight seal between the fuel nozzle and the neck 11, those fuel fill loss vapors entering the filler neck from port 29 could as easily exit to atmosphere as enter valve 30. Again, as with most patents, the system disclosed is not primarily concerned with fuel fill losses.
Those systems that are concerned with fuel fill losses generally use a tight seal between the nozzle and the filler neck, which will block fuel fill loss vapors from exiting to atmosphere. Of course, if the air-fuel vapors displaced from the tank during fuel fill cannot go to atmosphere, they must have an alternate path out of the tank, or, again, tank pressure will build up. The U.S. Pat. No. 3,907,153 to Mutty, assigned to the assignee of the subject invention, uses a bellows seal 92 on the fuel nozzle to seal the top of the filler neck 14, thereby blocking vapors from escaping to atmosphere. However, the act of removing the filler neck cap 18 also uncovers a port 52 at the top of the filler neck 14, which provides an unrestricted exit path out of the filler neck 14 and through a conduit 55 to the canister, so that there is no pressure build up in the tank. Likewise, the U.S. Pat. No. 4,630,749 to Armstrong et al., also assigned to the assignee of the subject invention, uses a tight seal 14 around the nozzle 54 to block fuel fill losses to the atmosphere, and a vent valve 28 that opens when the nozzle 54 is inserted provides a path for fuel vapors to the canister. Although not specifically discussed in the patent, it is contemplated that either the filler neck 26 would empty into the fuel tank at a high point within the tank, so that fuel vapors displaced from the tank during fill could go up the filler neck 26 and out to the canister, or a separate vent line would be provided to take displaced tank vapors to the filler neck, and then out to the canister.
While these last two systems work, the first requires that a bellows seal be available on the nozzle, which will not always be the case. Furthermore, it is desirable to eliminate a tight seal altogether in cases where the size of the nozzle is not standard, or where seal wear may be a problem. An alternative considered was the use of a liquid seal, instead of a tight seal around the nozzle. By liquid seal, what is meant is that the filler neck enters the tank at a fairly low point within the tank, with a configuration that assures that liquid fuel always blocks the neck to prevent vapors displaced from the tank from exiting up the neck and out to atmosphere. Then, some alternate and unrestricted path out of the tank interior to the canister would be provided from the tank for the displaced air-fuel vapors, since they can't go up the filler neck. This alternate path would be open only during fuel fill. However, research on such a liquid seal system uncovered a phenomenon that had apparently not been well appreciated in the art before. It was discovered that, with a liquid seal system, the volume of air-fuel vapors displaced from the tank was actually greater than the amount of empty tank volume being displaced by the entering fuel. Further research discovered that the rapid flow of fuel was pulling atmospheric air down the filler neck. While pulling atmospheric air down the filler neck certainly helped in preventing fuel vapors from exiting to atmosphere out the filler neck, it also was acting to entrain that drawn in air with the fuel. This increased the volume of air-fuel vapors within the tank interior that had to be blown through the canister in order for the fuel vapor part of the fuel-air mixture to be adsorbed and stored. That increase in the volume of vapor, and the consequent extra working of the granules in the canister was not a problem with the tight seal type of system, of course, since the entry of atmospheric air was blocked by the tight seal around the nozzle.