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