Fuel vapour storage canister with volume change compensator

Abstract: A bed (18) of activated carbon granules is maintained tightly packed within a fuel vapour storage canister housing (12) by a specially designed compensator. A pair of spring loaded, nested trays (22,24) move axially relative to one another and push continually into the lower face of the carbon bed to keep it packed. The trays also define a continuous, self maintaining gap that is smaller than the expected carbon granule size, preventing them from falling out in spite of the swelling of the canister housing.

Description: This invention relates to fuel vapour storage canisters in general, and specifically to such a fuel vapour storage canister with an improved means for compensating for volume expansion caused by environmental factors. Fuel vapour storage canisters for vehicles are by now a standard feature of production automobiles in the United States of America. The basic design includes an elongated canister housing with closed ends that is substantially filled with a bed of packed fuel adsorbent granules, typically activated carbon. Nylon is a material choice for the canister housing, because of its durability and light weight. However, nylon and similar materials are subject to expansion both from water absorption and heat. Fuel vapour storage canisters are typically mounted under the bonnet, an environment that is highly subject both to high heat and water splash. The carbon bed is usually retained in the canister housing by at least one plug or end plate that is pressed into the end face of the carbon bed to keep it tightly packed. The end plate is sized based on the nominal size of the perimeter of the inner surface of the canister housing. Under the bonnet, environmentally induced expansion or swelling of the canister housing can potentially cause two problems. One is that the carbon bed becomes loose, since it does not expand in volume correspondingly. This can be compensated by making the end plate movable, and biasing it continually into the face of the carbon bed so that it remains tightly packed as, for example, shown in US Patent No 4,496,379. A more serious problem is that canister housing expansion can create a significant increase in the perimeter of the inner surface, causing it to pull away from the edge of the end plate. Granules of the carbon bed can fall through, reducing the fuel vapour storage canister efficiency and capacity, and potentially interfering with the compression spring. A fuel vapour storage canister in accordance with the preamble of claim 1 is disclosed in EP-A-0355976. The invention provides an improved fuel vapour storage canister having a volume change compensator that maintains the packing of the carbon bed, and which prevents the loss of carbon granules. A fuel vapour storage canister in accordance with the present invention is characterised by the features specified in the characterising portion of claim 1. In the preferred embodiment disclosed, an elongated canister housing of generally rectangular cross section is moulded of nylon, with two closed ends. The perimeter of the inner surface of the canister housing can increase from its nominal, time of manufacture size, due to the kind of environmental expansion described above. A fuel vapour adsorbent bed of packed activated charcoal granules, which have a predetermined granule size, is retained by a lower foam screen that defines an end face of the carbon bed. To maintain the bed tightly packed, a special floating volume change compensator is located between the foam screen and the bottom end cover of the canister housing. The volume change compensator includes a pair of upper and lower trays that are biased continually apart by a pair of coil springs. The lower tray sits on the end cover, while the upper tray is continually biased into the foam screen. The lower tray has an upstanding coaming with a perimeter substantially equal to the nominal perimeter of the canister housing, while the upper tray has a depending peripheral flange that is nested inside the coaming with a continuous gap therebetween. The gap is deliberately made smaller than the carbon bed granular size. Even if the housing expands, that gap maintains itself, and the granules are prevented from falling out of the bed and between the trays. It is, therefore, a general object of the invention to maintain the carbon bed of a fuel vapour storage canister tightly packed, while preventing fallout of the carbon granules when the canister housing swells. It is another object of the invention to provide an improved fuel vapour storage canister with a special floating volume change compensator that resiliently biases the carbon bed to maintain it tightly packed, but which maintains a gap between the moving parts small enough to prevent loss of the carbon granules. The present invention may provide a floating volume change compensator that has a pair of nested trays that can be pre-assembled together to form a subassembly before the fuel vapour storage canister is built up. The present invention will now be described, by way of example, with reference to the following written description, and the accompanying drawings, in which:- Figure 1 is a side view, partly in cross-section, of a fuel vapour storage canister embodying a preferred embodiment of the invention, showing the nominal volume of the canister housing; Figure 2 is a cross sectional view of just the bottom portion of the fuel vapour storage canister, showing its expanded condition; Figure 3 is an enlarged area of Figure 2; and Figure 4 is a perspective of some of the basic components of the volume change compensator. Referring first to Figure 1, a fuel vapour storage canister embodying the invention is indicated generally at 10. Fuel vapour storage canister 10 includes an (elongated) canister housing 12 of nylon and of generally rectangular cross section, with a closed upper end 14 that is moulded integrally with canister housing 12, and an axially opposed bottom closed end comprised of a bottom cover 16. The most significant dimension of canister housing 12 in terms of the invention is its perimeter as measured in a cross section perpendicular to its axis (which is vertical in Figure 1). Specifically the perimeter of its inner surface is important, and is a function of its width W and its thickness measured perpendicular to its width. When exposed to water, the nylon material of canister housing 12 will expand significantly, increasing its width as shown by, for example, a differential d. The thickness will grow as well, as does the whole perimeter. Temperature increase can cause a similar, if lesser, expansion. A fuel vapour adsorbent bed 18 of carbon granules is packed inside canister housing 12, sandwiched in place between closed upper end 14 and a foam screen 20. Foam screen 20 creates a bottom end face of bed 18 that has a nominal axial spacing X from bottom cover 16. Foam screen 20, in turn, is supported by structure described below. For efficient, rattle free operation of fuel vapour storage canister 10, it is important that the carbon granules of bed 18 be kept fairly tightly packed, in intimate contact with the inner surface of canister housing 12. Clearly, as canister housing 12 swells and expands, that condition can be jeopardized. Referring next to Figures 1 and 4, bed 18 is kept packed by a special floating volume compensator that includes a lower tray, indicated generally at 22, and an upper tray, indicated generally at 24, biased apart by a pair of compression coil springs 26. Lower tray 22 is integrally moulded of a plastic material, preferably a material more stable than canister housing 12. Lower tray 22 has an axially directed, continuous coaming 28 that has a perimeter just slightly smaller than the nominal perimeter of canister housing 12, and an axial height H, that is just less than X. A pair of cylindrical spring supports 30 sized to fit around the coil springs 26 surround a pair of T slots 32. Upper tray 24, also integrally moulded of the same material, has an axially directed, depending peripheral flange 34 with an axial height H₂ close to H₁ and a perimeter that is slightly less than coaming 28. Therefore, flange 34 can be nested within coaming 28, creating a lateral gap g, that is, a gap perpendicular to the axis of canister housing 12, that runs continuously all the way around the inside perimeter of canister housing 12. The relative perimeter of flange 34 is chosen so that g is less than the expected size of the individual carbon granules of bed 18. Upper tray 24 also has a pair of T slots 36 that match T slots 32 in shape and location, surrounded by an array of radiating fins 38 sized to fit inside the coil springs 26. The pairs of matched T slots 32 and 36 are axially aligned with each other and with a pair of upstanding T shaped stems 40 on bottom cover 16, for a purpose described next. Referring next to Figures 1 and 4, the structures described above cooperate to allow fuel vapour storage canister 10 to be easily assembled. To build up fuel vapour storage canister 10, canister housing 12 is supported upside down and the bed 18 poured in place. Foam screen 20 is then added, which leaves an empty volume of axial height X, described above. Next, bottom cover 16 and the two trays 22 and 24 are put together as a subassembly. This is accomplished by first setting lower tray 22 onto bottom cover 16, passing the T-shaped stems 40 axially through the wide end of the T slots 32. Then, the springs 26 are set into the spring supports 30, and upper tray 24 is added by pressing it axially toward lower tray 22. The fin arrays 38 are pushed inside the supported coil springs 26, which compress until the T-shaped stems 40 pass axially through the wide portion of the T slots 36. Then, upper tray 24 and lower tray 22 are moved laterally, hooking the T-shaped stems 40 into the narrow portion of the T slots 36, 32, and locking all the pieces together as a convenient, rattle free subassembly. Finally, the subassembly is installed by pushing it axially into the open end of canister housing 12, pressing upper tray 24 into foam screen 20 and compressing coil springs 26 until bottom cover 16 seats on the edge of the open end of canister housing 12. Bottom cover 16 is then glued or welded in place. The exclusionary lateral gap g is thus located axially between the end face of the bed 18 and the bottom cover 16. Referring next to Figures 1, 2 and 3, the operation of fuel vapour storage canister 10 is illustrated. When canister housing 12 expands, coil springs 26 expand from the Figure 1 to the Figure 2 position. Because both the coaming 28 and flange 34 are axially disposed and do not touch, they can move axially freely past one another, allowing the upper tray 24 to move axially up into the end face of the bed 18 under the force of coil springs 26. This maintains the carbon granules firmly packed and in intimate contact with the inner surface of canister housing 12. The inner surface of the canister housing 12 may enlarge enough to part from the edge of foam screen 20, allowing individual grains of bed 18 past. However, the lateral gap g between the shorter and stiffer coaming 28 and flange 34 will maintain itself. Therefore, granules of carbon will be prevented from falling into the relatively large volume between the lower and upper trays 22 and 24, which could reduce the efficiency of fuel vapour storage canister 10 and interfere with the operation of the coil springs 26. If the inner surface of canister housing 12 moves away from coaming 28, carbon granules may fall between, but this will not present a problem, as there is very little volume there to fill. H₁ and H₂ are sufficient to continually maintain an axial overlap between flange 34 and coaming 28, and thus maintain the exclusionary lateral gap g, throughout the expected volume increase of canister housing 12 and attendant rise of upper tray 24. The lateral gap g acts like a labyrinth seal, in that there is no rubbing between the coaming 28 and the flange 34, but particulates are still effectively excluded. Variations in the disclosed embodiment could be made. Most fundamentally, a floating volume compensator could be comprised of any two members that had a pair of surfaces that were disposed generally parallel to the axis of the fuel vapour storage canister and which ran continuously around the perimeter of the inner surface of the canister housing. When such surfaces are nested or axially overlapped one within the other, they can move axially freely past one another under the force of the coil springs, keeping the carbon granules packed while maintaining the gap that prevents carbon fall out. Some means other than the T-shaped stems 40 and the aligned T slots 32 and 36 could be used to retain the lower and upper trays 22 and 24 together as a subassembly, even apart from the bottom cover 16, so long as enough free axial motion was available to let the coil springs 26 compress and expand as needed. It is particularly convenient if the bottom cover 16 is included in the subassembly, however. The total subassembly is also an advantage in that it would allow similar subassemblies of any shape and thickness to be installed later. Therefore, it will be understood that it is not intended to limit the invention to just the embodiment disclosed.

Claims: A fuel vapour storage canister (10) comprising a canister housing (12) having closed ends (14,16) and formed of a material the inner surface of which has a perimeter that is subject to expansion beyond a nominal value due to environmental conditions; a bed (18) of fuel vapour adsorbent granules of predetermined size packed in intimate contact with the inner surface and having one end face (20) respective to one of the closed ends; and a volume change compensator comprising a compression spring means (26), a first tray (22) abutted with the one closed end (16), and a second tray (24) abutted with an end face of the bed (18) whereby the second tray is maintained tight against the end face of the bed to maintain the granules tightly packed; characterised in that the first tray (22) has a coaming (28) conforming to the inner surface of the canister housing; the second tray (24) is abutted with said one end face of the bed (18) and has a continuous peripheral flange (34) nested inside of the coaming so as to create a continuous gap (g) therebetween smaller than the predetermined size of the granules; and the compression spring means (26) biases the trays apart, the gap retaining the granules against fall out regardless of the expansion of the canister housing. A fuel vapour storage canister as claimed in claim 1, wherein the compression spring means comprises a pair of coil springs (26); the first tray (22) comprises a pair of cylindrical spring supports (30) sized to fit around the coil springs; and the second tray (24) comprises a pair of arrays of radiating fins (38) sized to fit inside the coil springs. A fuel vapour storage canister as claimed in claim 1 or claim 2, wherein the one closed end (16) of the canister housing (12) is separately formed from the other parts of the canister housing, the one closed end and the volume change compensator being assembled as a sub-assembly prior to positioning in the canister housing. A fuel vapour storage canister as claimed in claim 3, wherein the one closed end (16) has a least one upstanding T-shaped stem (40), and the first and second trays (22,24) have corresponding T-shaped slots (32,36) through which the T-shaped stem can pass to secure the trays to the one closed end to form the sub-assembly. A fuel vapour storage canister as claimed in any one of claims 1 to 4, wherein the end face of the bed (18) is defined by a foam screen (20).

Inventors: DUBOIS PATTI SMITH, DUNHAM BRIAN JEFFERY, TURNER KENNETH WILLIAM, DUBOIS, PATTI SMITH, DUNHAM, BRIAN JEFFERY, TURNER, KENNETH WILLIAM
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: F02M25
IPC codes: F02M25