The present invention relates to a canister. More particularly, but not exclusively, it relates to canister suitable for capturing evaporative emissions from a vehicle fuel system. Even more particularly, but not exclusively, it relates to an arrangement of a number of such canisters to efficiently occupy a cavity within in a vehicle.

Vehicle fuel systems often emit hydrocarbon fuel vapours which can be toxic and harmful to the environment. Typically, these emissions of hydrocarbon vapours occur during fuel storage and delivery.

Vehicle fuel systems are often equipped with evaporative emission control systems to reduce the emission of vapours from the vehicle fuel system to assist in complying with hydrocarbon emission limits. Typically, such control systems comprise a vapour entrapment canister arranged to trap hydrocarbon vapour before it can enter the environment.

Generally, a vapour entrapment canister contains an adsorbent material, such as activated carbon. The adsorbent material adsorbs the hydrocarbon molecules present in the fuel vapours entering the canister. This reduces the amount of hydrocarbon vapours released into the environment.

The adsorbent material used in current vapour entrapment canisters has a finite adsorption capacity and must be refreshed periodically. Usually, the refreshing of the adsorbent material involves purging the canister of hydrocarbon vapours. Generally, an air stream passes over the adsorbent material in order to desorb the hydrocarbon vapours and purge the canister. The air/hydrocarbon mixture is the directed to the vehicle's engine, via the engine air intake system, for combustion.

Current vapour entrapment canisters have a number of problems and disadvantages associated with them.

The distribution of the fuel vapours in the adsorbent material in current vapour entrapment canisters is often non-uniform. This results in localised saturation of the adsorbent materials leading to an increase in hydrocarbon emission levels from the vehicle.

Conversely, regions of highly restricted flow, for example regions having a high concentration of adsorbent materials, and regions remote from a hydrocarbon inlet may only be partially saturated at the time of purging the vapour entrapment canister. This results in an inefficient use of the vapour entrapment system and its control system.

Additionally, in current vapour entrapment canisters the adsorbent material is not necessarily densely packed. This reduces the efficiency of adsorption of the hydrocarbon vapours as flow paths can form outside the adsorbent material. These flow paths allow the hydrocarbon vapours to escape without being adsorbed by the adsorbent material.

Exposure to moisture, alcohol fuels or variations in temperature can cause changes in the volume of the canister, or density of packing of the adsorbent material. Changes in the canister volume, or packing density of the adsorbent material, can result in loosening and destruction of particles forming the adsorbent material. This loosening and destruction of the particles of the adsorbent materials allows flow paths to occur outside the adsorbent material.

Often, these variations in canister volume and packing density of adsorbent material are compensated for by the insertion of a volume compensator into the vapour entrapment canister.

The use of a volume compensator adds to the complexity of manufacturing such canisters and increases the cost of manufacture.

An elongate vapour entrapment canister presents particular difficulties. In this instance, it is difficult for the volume compensator to exert a suitable packing force over the total length of the canister such that the adsorbent remains adequately packed throughout the canister. In particular, a suitable packing force may not be exerted towards an end of the vapour entrapment canister remote from the volume compensator.

According to a first aspect of the present invention there is provided a vapour entrapment canister comprising a chamber, and an inlet port and an outlet port arranged such that vapour can flow therebetween, characterised in that the distance between a tip of either of the inlet port, or the outlet port, and an inner surface of a wall of the chamber is uniform over the majority of the inner surface of the wall.

Such a canister results in a substantially uniform flow of vapour in between the diffuser and the chamber's periphery, or vice versa if the direction of flow is reversed.

The chamber may be spherical. The tip may be located at the centre of the chamber.

A spherical chamber obviates the requirement for a volume compensator. Additionally, a spherical canister can fit into a space where a conventional square or rectangular vapour recovery canister cannot. Also, the provision of a "standardised" spherical canister reduces the cost of production of vapour entrapment canisters compared to the prior art.

The distance from the tip to the inner surface of the wall may be substantially equal at all points.

The chamber may contain an adsorbent material arranged to adsorb vapour, for example hydrocarbon vapour. The adsorbent material may comprise activated carbon.

The wall may be spaced apart from an inner surface of the chamber.

The outlet port may be opposed to the inlet port. Alternatively, the outlet port may be inclined with respect to the inlet port.

Such freedom in the definition of the relative alignment of the inlet and outlet ports allows production of canisters tailored for a particular space.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

• Figure 1 is a sectional view of an embodiment of a vapour entrapment canister according to an aspect of the present invention; and

Referring now to Figure 1, a vapour canister 10 comprises a chamber 12 having an inlet 14 and an outlet 16. A tip 19 of the inlet 14 is located at a central. In a preferred embodiment, shown in Figure 1, the chamber 12 is spherical.

A mesh screen 18 is mounted concentrically within the chamber 12. The mesh screen 18 is spaced apart slightly from an inner wall 20 of the chamber 12, typically by between 3 and 4 mm. The interior of the screen 18 is filled with activated carbon 22.

A tip 19 of the inlet 14 is positioned at the centre of the chamber 12. This results in the distance from the tip 19 to the periphery of the chamber 12 being substantially equal in all directions. Thus, the flow path distance through the activated carbon 22 is substantially equal in all directions.

The inlet 14 connects to a fuel tank venting system (not shown) via an inlet conduit 26. The inlet 14 also connects to a purge line 28.

In use, hydrocarbon vapour from the fuel tank enters the vapour canister 10 through the inlet conduit 26. As the hydrocarbon vapour passes radially outward towards the mesh 18 the activated carbon 22 adsorbs the hydrocarbon vapour. A concentration gradient of hydrocarbon vapour is established between the inlet 14 and the outlet 16, with the concentration of hydrocarbon vapours being higher at the inlet 14 than at the outlet 16. The spherical shape of the chamber 12 ensures that the length of the flow path from the inlet 14 to the outlet 16 through the activated carbon 22 is uniform irrespective of the direction of flow of the hydrocarbon vapour

Consequently, the levels of hydrocarbon vapour exiting the chamber 12 through the outlet 16 are reduced.

To avoid build-up of hydrocarbon molecules in the activated carbon 22, the vapours must be purged at regular intervals. The vapours are generally desorbed by means of an air stream entering the chamber 12 through the purge line 16. The desorbed vapours are then sent to the engine air intake system (not shown) for combustion.

It will be appreciated that, although shown with the inlet 14 at the centre of the chamber 12 and the outlet 16 at the periphery of the chamber 12, the respective positions of the input 14 and output 16 may be reversed without affecting the operation of the vapour entrapment canister 10.

It will be appreciated that although describe with reference to a spherical canister any convenient shape of canister, for example ovoid, may be used if an appropriate flow path can be achieved. An appropriate flow path is one where the inlet to outlet distance is approximately constant irrespective of the flow path taken between them.

Various modifications and improvements may be made to the above without departing from the scope of the present invention.