BACKGROUND ART Automotive pollution regulations require that vehicles be equipped with a temporary accumulator or so- called"canister"for storing fuel vapours produced inside the tank when parking. Known canisters substantially comprise a chamber filled with adsorbent material, such as activated carbon in granules.

To ensure constant efficiency of the canister, the characteristics, in particular the grain size, of the activated carbon inside must remain unchanged; for which purpose, the granules must be compacted with no possibility of relative movement.

This is normally done by providing the storage chamber with a movable first wall or wall portion, which is loaded by springs or by means of spongy material to exert constant pressure on the activated carbon granules; and a second wall defined by a metal grille fitted with retaining and filtering means, or so-called retaining felts, made of polyester and which should provide for effectively filtering air flow in and out of the canister, while at the same time preventing any load

loss.

In actual practice, however, it is difficult to select materials capable of maintaining high filtration power with no load loss.

DISCLOSURE OF INVENTION It is a main object of the present invention to provide a temporary vapour accumulator comprising retaining and filtering means which provide for high filtration power with very little load loss.

According to the present invention, there is therefore provided a temporary vapour accumulator, as claimed in Claim 1.

In a preferred embodiment of the present invention, the first layer and second layer are made of polymer material.

BRIEF DESCRIPTION OF THE DRAWING A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawing, which shows a schematic axial section of a vapour accumulator in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in the accompanying drawing indicates a temporary fuel vapour accumulator or canister comprising a substantially cylindrical casing 2, in turn comprising a top end wall 3; a lateral wall 4; a bottom cover 5; and

a vertical inner partition wall 6 extending axially from end wall 3 along a diametrical plane of casing 2 down to a distance D from cover 5, and cooperating hermetically with lateral wall 4 of casing 2.

Two blanks of a retaining and filtering web material 7 are fixed parallel to and a short distance from top wall 3 against spacer pins 31 on opposite sides of partition wall 16, so as to form, with top wall 3 and partition wall 6, a first and second compartment 8 and 9 interfacing canister 1 with the outside, as explained in detail later on. Preferably, web material 7 is welded, e. g. hot-blade welded, to peripheral shoulders 18 projecting inwards from top end wall 3. Web material 7 defines, upwards, a main chamber 10 of canister 1, which is divided by partition wall 6 into two top compartments 11, 12, and has a continuous bottom portion 13 by which compartments 11,12 communicate.

Chamber 10 houses granular adsorbent material 14, such as activated carbon.

According to a feature of the present invention, retaining and filtering web material 7 is a multilayer web material including a first layer 7a, preferably of polymer material, in particular polycarbonate, for ensuring the necessary rigidity of retaining and filtering web material 7; and a second layer 7b of polymer material, preferably a nonwoven fabric of

polyester or polyamide, in particular aromatic polyamide (or so-called aramide), for ensuring the necessary filtration capacity. The total weight of web material 7 preferably ranges between 150 and 200 gr/sq. m, and is preferably 190 gr/sq. m, and the total thickness preferably ranges between 0.6 and 1.5 mm, is preferably about 0.9 mm, and is therefore advantageously thinner than the known web material 7 described previously, which on average are about 2.5 to 3.5 mm thick.

In bottom portion 13 of chamber 10, a layer 15 of elastically deformable spongy polymer material 16, in particular polyurethane foam, is placed in contact with bottom cover 5; which material 16 is precompressed to compact adsorbent material 14 between retaining and filtering web material 7 and layer 15, and exert sufficient compression to ensure maximum compaction of adsorbent material 14 and prevent any relative movement of the granules.

The height of layer 15, in the precompressed in-use condition, is less than distance D between the bottom end of partition wall 6 and bottom cover 5, so that part of bottom portion 13 of chamber 10 is filled with a continuous layer of material 14, and defines, with top compartments 11 and 12 of chamber 10, a substantially U- shaped air and fuel vapour path connecting compartments 8 and 9 through material 14.

Canister 1 comprises a first fitting 20 extending through end wall 3 and retaining and filtering web material 7, and which comes out directly inside compartment 11 of chamber 10, is connected to the vehicle tank (not shown) by a pipe (not shown), and defines the fuel vapour inlet of canister 1. In end wall 3 are also formed a second fitting 21 connecting compartment 8 to the atmosphere; and a third wash fitting 22 communicating with compartment 9, and which is connected to the intake manifold of the engine (not shown) via a control valve (not shown).

Operation of canister 1 when parking and washing is known and therefore not described in detail.

Canister 1 is assembled by placing the blanks of retaining web material 7 on spacer pins 31, welding web material 7 to shoulders 18, filling chamber 10 with adsorbent material 14, placing layer 15 of spongy material and closing chamber 10 with cover 5.

Retaining web material 7 preferably provides for a 100% filtration power of less than 25 ym, and in particular of 15 Am, as compared with 40 Am to 80 Am of known web materials of the same section and made of polyester or polyamide, in particular aromatic polyamide (or so-called aramide), felt as described above.

At the same time, the load loss, measured at 20 cm/s, of web material 7 according to the present

invention is 0.18 mbar, i. e. much less than that of known web material, which ranges roughly between 0.30 and 0.35 mbar.

The advantages of the vapour accumulator according to the present invention will be clear from the foregoing description.

In particular, the retaining and filtering web material employed provide for high air filtration capacity with no load loss, while at the same time providing the necessary rigidity to keep the granules of adsorbent material 14 compact, thus eliminating the drawbacks typically associated with known vapour accumulators.

Moreover, by virtue of the materials used, the retaining and filtering web material are thinner and have a longer working life.

Clearly, changes may be made to the vapour accumulator as described herein without, however, departing from the scope of the present invention.

In particular, different materials may be used for each of the two layers defining retaining web material 7.