GB2438703A

GB2438703A - Method for controlling an on-board vehicle emissions system

Info

Publication number: GB2438703A
Application number: GB0707575A
Authority: GB
Inventors: Kent Dawson; Zafar Shaikh; Michael Levin; David James Cook; Daniel Michael Kabat
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2006-05-30
Filing date: 2007-04-19
Publication date: 2007-12-05
Also published as: GB0707575D0; US20070277505A1; DE102007022783A1

Abstract

A method for controlling venting of an on-board vehicle emissions treatment substance storage and distribution system 10 is disclosed that selectively dispenses an emissions treatment substance 14 from a storage tank 12 on the vehicle to an exhaust system 30 of the vehicle. The method includes a step 208 of selectively coupling the storage tank 12 to the exhaust system 30 when pressure in the storage tank 12 exceeds a predetermined pressure associated with the exhaust system 30. The invention may also include a step 214 of selectively coupling the storage tank 12 to atmosphere when a pressure differential between the storage tank 12 and atmosphere exceeds a predetermined pressure differential.

Description

A METHOD FOR CONTROLLING AN ON-BOARD

VEHICLE EMISSIONS TREATMENT SYSTEM

The present invention relates to systems and methods for controlling venting of an on-board emissions treatment system of a vehicle.

Various types of vehicle emission control systems introduce one or more substances directly or indirectly to the engine via the fuel supply, air/fuel intake, exhaust, or directly to an engine cylinder or emissions control device, such as a catalyst. For example, substances acting as reducing agents or reductants, such as aqueous urea or hydrocarbons (other than fuel) may be used in lean air/fuel ratio engine applications including diesel engines in combination with lean NOx catalysts (or selective catalytic reduction (SCR)) to treat nitrous oxide feedgas emissions.

These substances generally require a storage and distribution system separate from the primary fuel storage and distribution system.

Liquid urea selective catalytic reduction (SOR) is a method of mobile exhaust aftertreatment particularly suited for diesel engines for treating oxides of nitrogen (NOx) emissions. In a representative liquid urea SCR application, a urea solution, typically 33% urea in water, is stored on board the vehicle and metered or dosed by a pump and injector into the exhaust where it decomposes by thermal hydrolysis to ammonia and carbon dioxide. Ammonia then reacts over the SCR catalyst to reduce NOx compounds to nitrogen, oxygen, and water before being released to atmosphere.

Urea stored on-board the vehicle poses unique technical challenges. For example, when subjected to temperatures common for underbody exhaust components, urea stored in the tank may begin to decompose and release ammonia to the air space above the liquid. In an open vented tank system, some ammonia, which is lighter than air, may travel up and out the tank vent. Because ammonia has a pungent odor that most.

humans can detect in concentrations as low as 5-2oppmv, it is desirable to minimize or eliminate ammonia escaping from the emissions treatment system.

Prior art approaches have employed a selectively

operable fan to introduce air into the reductant storage tank or to remove vapours from the tank as disclosed in, for example, tJS2003/0213234 and W02005/028826.

It is an object of the invention to provide an improved method and system for controlling the venting of a vehicle emission treatment system.

ccording to a first aspect of the invention there is provided a method for controlling an on-board vehicle emissions treatment system that selectively dispenses an emissions treatment substance from a storage tank to an exhaust system of the vehicle, the method comprising selectively coupling the storage tank to the exhaust system, when pressure in the storage tank exceeds a predetermined pressure associated with the exhaust system, and, otherwise, decoupling the storage tank from the exhaust system.

The exhaust system may include a catalyst and wherein the step of selectively coupling may comprise selectively coupling a portion of the storage tank above a maximum level of the emissions treatment substance to the exhaust system upstream of the catalyst to selectively deliver vapours from the storage tank to the exhaust system.

The step of selectively coupling may comprise positioning a passive valve between the storage tank and the exhaust system that operates solely in response to a pressure differential across the valve to open when the pressure in the storage tank exceeds the predetermined pressure and to close when the pressure in the storage tank is less than the predetermined pressure.

The step of selectively coupling may comprise positioning a one-way valve between the storage tank and the exhaust system and opening the one-way valve when the pressure in the storage tank exceeds the predetermined pressure.

The step of selectively coupling may comprise controlling a solenoid valve disposed between the storage tank and the exhaust system such that the solenoid valve opens when the pressure in the storage tank exceeds the predetermined pressure.

The method may further comprise selectively coupling the storage tank to atmosphere when a pressure differential between the storage tank and atmosphere exceeds a predetermined pressure differential.

The step of selectively coupling the storage tank to atmosphere may comprise positioning a one-way valve between the storage tank and atmosphere, the one-way valve opening when the pressure differential between the storage tank and atmosphere exceeds the predetermined pressure differential to provide air from atmosphere to the storage tank, and closing when the pressure differential between the storage tank and atmosphere is less than the predetermined pressure differential to prevent vapour egress from the storage tank.

The step of selectively coupling the storage tank to atmosphere may comprise controlling a solenoid valve positioned between the storage tank and atmosphere to selectively provide air to the storage tank.

According to a second aspect of the invention there is provided an on-board vehicle emissions treatment system that selectively dispenses an emissions treatment substance from a storage tank on the vehicle to an exhaust system of the vehicle, the emissions treatment system comprising a venting valve disposed between the storage tank and the exhaust system to selectively couple the storage tank to the exhaust system, when pressure in the storage tank exceeds a predetermined pressure associated with the exhaust system, and, otherwise, decouple the storage tank from the exhaust system.

The system may further comprise a vacuum release valve disposed between the storage tank and atmosphere, the vacuum release valve selectively coupling atmosphere to the storage tank when a pressure differential between atmosphere and the storage tank exceeds a predetermined level and decoupling the storage tank from atmosphere otherwise.

The vacuum release valve may operate solely in response to the pressure differential between the storage tank and atmosphere.

At least one of the venting valve and the vacuum release valve may comprise a solenoid valve and the emissions treatment system further comprises a controller in communication with the at least one valve for selectively controlling the valve in response to current operating conditions.

The venting valve may operate solely in response to a pressure differential between the storage tank and the exhaust system.

The system may further comprise a catalyst disposed downstream of an internal combustion engine and the venting valve selectively couples the storage tank to the exhaust system upstream of the catalyst.

The venting valve may open when the pressure in the storage tank exceeds a maximum exhaust pressure associated with operation of an internal combustion engine of the vehicle.

According to a third aspect of the invention there is provided a computer readable storage medium having stored data representing instructions executable by a computer to control an emissions treatment system of a vehicle, the emissions treatment system selectively dispensing an emissions treatment substance from a storage tank on the vehicle to an exhaust system of the vehicle, the emissions treatment system including a conduit with a controllable valve disposed between the storage tank and the exhaust system for selectively venting vapours from the storage tank to the exhaust system, the computer readable storage medium comprising instructions for controlling the valve to open in response to pressure in the storage tank exceeding a predetermined pressure.

The predetermined pressure may correspond to a maximum anticipated exhaust pressure to prevent exhaust from entering the storage tank when the valve is open.

The emissions treatment system may include a controllable vacuum release valve and the computer readable storage medium may comprise instructions for controlling the vacuum release valve in response to atmospheric pressure exceeding pressure of the storage tank by a predetermined amount.

The computer readable storage medium may further comprise instructions for periodically opening the valve to deliver vapour from the storage tank to the exhaust system.

The computer readable storage medium may further comprise instructions for opening the valve in response to temperature of at least one exhaust system component.

The invention will now be described by way of example with reference to the accompanying drawing of which:-Figure 1 is a block diagram illustrating one embodiment of a system for controlling venting of an on-board vehicle emissions treatment system according to the present invention; and Figure 2 is flow chart illustrating operation of one embodiment of a method for controlling venting of an on-board emissions treatment system according to the present invention.

As those of ordinary skill in the art will understand, various features of the present invention as illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce embodiments of the present invention that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present invention may be desired tor particular applications or implementations.

Referring now to Figure 1, a block diagram illustrating one embodiment of a system for controlling an on-board vehicle emissions treatment system according to the present invention is shown. The system 10 includes an emissions treatment substance storage tank 12 mounted on a vehicle (not shown) . The storage tank 12 may be used to store an emissions treatment substance 14, which may include, but is not limited to various reducing agents or reductarits such as aqueous urea, hydrocarbons (other than the primary fuel) etc. The storage tank 12 may be a single or multiple wall storage tank having a generally rigid exterior and may optionally include an interior bladder that contains emissions treatment substance 14. However, the present invention is independent of the particular type of storage tank 12.

The storage tank 12 includes a vapour/air space 16 that varies in volume relative to the quantity of emissions treatment substance 14 in tank 12. A fill pipe (not shown) extends from storage tank 12 and terminates at a filler neck adapted for receiving a filling nozzle to add emissions treatments substance 14 to storage tank 12.

Pump 20 is fluidly coupled to storage tank 12 and in communication with controller 50, which controls operation of pump 20. During operation, pump 20 pumps emissions treatment substance 14 from tank 12 to pressurize a supply line 22. A dosing or metering valve 24 is selectively controlled by the controller 50 in response to current operating conditions or parameters of exhaust system 30 and/or engine 80 to deliver emissions treatment substance 14 via a corresponding nozzle or injector 26 to exhaust system upstream of a mixing element or mixer 34, which mixes exhaust tiow from engine 80 with injected emissions treatment substance 14. The exhaust system 30 includes various emissions control/treatment devices that may include a pre-oxidation catalyst 40, a selective catalytic reduction (SCR) catalyst 42 and a diesel particulate filter (DPF) 44, for example and with limitation. Of course, the presence and/or sequence of particular emissions treatment/control devices may vary depending upon the particular type of fuel, engine control strategy, and other factors affecting a particular application or implementation and the present invention is independent of the particular types or sequence of emissions treatment/control devices.

As also i]Justrated in Figure 1, system 10 includes a venting valve 60 disposed between storage tank 12 and exhaust system 30. venting valve 60 opens to selectively fluidly couple space 16, preferably above a maximum level of emissions treatment substance 14, of storage tank 12 to exhaust system 30 when pressure in storage tank 12 exceeds a predetermined pressure, such as a pressure associated with exhaust system 30, for example. Venting valve 60 closes to decouple space 16 of storage tank 12 from exhaust system 30 when pressure in exhaust system 30 is greater than pressure within storage tank 12.

In one embodiment, the venting valve 60 is implemented by a passive mechanical valve that operates solely in response to the pressure in storage tank 12 to open when the pressure in tank 12 exceeds a predetermined pressure and to close when the pressure in tank 12 is less than the predetermined pressure. For example, a passive one-way mechanical valve may be implemented by a spring-biased ball check valve that has a selected opening or crack pressure associated with the spring constant and aperture size. The opening or crack pressure of venting valve 60 may be selected or determined based on the maximum anticipated pressure in exhaust system 30 so that valve 60 opens only when tank pressure exceeds exhaust pressure to prevent the flow of exhaust into tank 12.

In an alternative embodiment, the venting valve 60 is implemented by an active/controllable valve, such as a solenoid valve, in communication with controller 50 and may be electrically, pneumatically, or electromagnetically actuated in response to one or more vehicle and/or ambient operating conditions, modes, and/or parameters. For example, an active venting valve 60 may be selectively operated directly or indirectly in response to a temperature sensor 62 and/or pressure sensor 64, which may trigger operation of venting valve 60 directly or may communicate with a dedicated or general purpose controller 50 that s actuates venting valve 60 in response to signals received from sensors 62, 64, and/or other sensors 66 and/or other actuators 68.

In the representative embodiments illustrating the present invention, the space 16 of the storage tank 12 is vented to exhaust system 30 via selective fluid coupling facilitated by venting valve 60. Fluid coupling of space 16 to exhaust system 30 directs vapours away from the vehicle, particularly during filling of emissions treatment substance storage tank 12. While venting of tank 12 away from an associated filling tube (not shown) according to the present invention reduces or eliminates detection of odour during filling of tank 12 by vehicle operators/occupants, additional benefits of the invention may be provided by venting tank 12 to a device that can decompose or store the vapour for future treatment to reduce or eliminate vapour escape from system 10. In the representative embodiment illustrated in Figure 1, tank 12 is selectively coupled to exhaust system 30 upstream of a vapour storage/decomposition device implemented by SCR catalyst 42. Depending upon the current operating temperature of SCR catalyst 42, vapour from tank 12 will react with exhaust flow from engine 80 in exhaust system 30, or may be stored by SOP. catalyst 42 and subsequently reacted when a suitable operating temperature is reached. For example, if valve 60 opens while engine 80 is operating or has just been shut off and SCR catalyst 42 has reached an appropriate operating temperature, vapour from tank 12 will be reacted with exhaust constituents in SCR catalyst 42. If valve 60 opens based on pressure in tank 12 exceeding a predetermined pressure threshold due to refilling of substance 14, or due to increasing temperature of tank 12 when SCR catalyst 42 is below operating -10 -temperature, vapour is stored by SCR catalyst 42 for subsequent reaction.

As also illustrated in Figure 1, system 10 may include a vacuum release valve 70 disposed between storage tank 12 and atmosphere. The vacuum release valve 70 selectively couples atmosphere to space 16 of storage tank 12 when a pressure differential between atmosphere and storage tank 12 exceeds a predetermined level or threshold. Vacuum release valve 70 closes to decouple space 16 of storage tank 12 from atmosphere when the differential pressure is below the predetermined threshold. The vacuum release 70 may be implemented by a passive one-way mechanical device that operates solely in response to the pressure differential between storage tank 12 and atmosphere, or may be actively controlled by a sensor or controller, such as pressure sensor 64 or controller 50, for example, in response to one or more operating conditions, modes, and/or parameters.

In one embodiment, a vacuum release device 70 is implemented by a mechanical device such as a spring-biased check valve that allows air to enter space 16 while preventing vapour egress from space 16 of tank 12 to atmosphere.

Preferably, the opening or cracking pressure of vacuum release valve or device 70 is selected to allow air to enter space 16 when even a small pressure differential, such as 2300 Pa gauge (1/3 psig) is present to avoid vacuum lock of pump 20.

Those of ordinary skill in the art will recognize that various other types of valves or flow control devices may be used depending upon the particular application to provide the features and advantages of the present invention that may include but are not limited to mechanically, -11 -electrically, magnetically, electromagnetically, or pneumatically actuated valves, for example.

As those of ordinary skill in the art will appreciate, system 10 includes various conventional sensors 66 and actuators 68 in addition to those specificaLLy illustrated in igure 1 to control system 10. Various sensors and actuators may communicate with at least one dedicated or general-purpose controller 50 that includes a microprocessor 92, also called a central processing unit (CPU) , in communication with a memory management unit (MMU) 94. 4MU 94 controls movement of data and/or instructions among various computer readable storage media 96 and communicates data to and from Cpu 92. The computer readable storage media preferably include volatile and non-volatile or persistent storage in read-only memory (ROM) 98, keep-alive memory (KAM) 100, and random-access memory 102, for example.

KAM 100 may be used to store various engine and/or ambient operating variables while CPU 92 is powered down. Computer-readable storage media 96 may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM) , EEPROMs (electrically erasable PROM) , flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU 92 in controlling system 10.

Computer-readable storage media 96 may also include floppy disks, CD-ROMs, hard disks, and the like depending upon the particular application. CPU 92 communicates with the sensors and actuators via an input/output (I/O) interface 1.04. Interface 104 may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing, and/or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or firmware chips may be used to condition and process particular signals before being supplied to CPU 92. Some controller architectures do not -12 -contain an MMU 94. If no MMU 94 is employed, CPU 92 manages data and connects directly to RON 98, KAN 100, and RAM 102.

Of course, the present invention could utilize more than one controller 90 or more than one CPU 92 to provide system control and each controller 90 may contain multiple RON 98, KAM 100, and RAM 102 coupled to MMU 94 or CPU 92 depending upon the particular application.

Referring now to Figure 2 there is shown a flow chart illustrating a method for controlling venting of an on-board vehicle emissions treatment substance storage and distribution system according to one embodiment of the present invention.

As those of ordinary skill in the art will appreciate, the diagram of Figure 2 generally represents a control process or logic, some of which may be implemented by any one or more of a number of known processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted.

Likewise, the order of processing is not necessarily required to achieve the features and advantages of the invention, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending upon the particular processing strategy or implementation.

Steps of the process performed by a controller may be implemented primarily in software executed by a microprocessor-based controller that may be dedicated to controlling the emissions treatment system, or may also be used to control the engine and/or vehicle. Of course, these steps may be implemented in software, hardware, or a -13 -combination of software and hardware depending upon the particular application. When implemented in software, the control logic is preferably provided in a computer-readable storage medium having stored data representing instructions executed by a computer or controller to control the system.

The computer-readable storage medium or media may be any of a number of known physical devices which utilize electric, magnetic, and/or optical devices to temporarily or persistently store executable instructions and associated calibration information, operating variables, and the like.

Block 200 of Figure 2 represents optionally determining operating and/or ambient parameters associated with the engine, vehicle, and/or emissions treatment system and may include determining pressure(s) 202 and/or temperature(s) 204 associated with an emissions treatment system including an emissions treatment substance storage tank and/or one or more exhaust system components, such as catalysts.

Temperature(s) and/or pressure(s) may be measured by corresponding sensors and/or inferred based on one or more operating modes, conditions, or parameters. Of course, for implementations using a passive flow control device for selective coupling of the emissions treatment storage tank, this step is unnecessary.

Block 206 generally represents a determination of whether to vent the emissions treatment substance storage tank in response to current operating conditions or modes.

If block 206 determines that venting of the tank is desired, block 208 actuates one or more flow control devices to selectively couple the on-board vehicle emissions treatruent.

substance storage tank to the exhaust system of the vehicle in response to at least one operating condition and/or parameter. For example, the selective coupling may be performed by actuating a solenoid valve in response to operating and/or ambient conditions as determined by -14 -corresponding sensors and/or actuators. Alternatively, selective coupling represented by block 208 may be performed by a mechanical device operating solely in response to one or more operating conditions, such as temperature and/or pressure within the emissions treatment substance storage tank and/or exhaust system. Depending upon the particular application and implementation, the selective coupling may be performed in response to a predetermined pressure or pressure differential exceeding a predetermined threshold with the threshold based on an absolute or differential pressure associated with the exhaust system of the vehicle to reduce or eliminate exhaust flow into the storage tank.

As also generally represented by block 208, a space above the maximum fluid fill level of the storage tank is preferably selectively connected to the exhaust system upstream of a treatment device capable of storing and/or treating vapours exiting the emissions treatment substance storage tank. In one embodiment, vapours from the emissions treatment substance storage tank are selectively introduced to the exhaust system upstream of an SCR catalyst and downstream of the emissions treatment substance injector and mixing device.

If block 206 determines that current operating conditions and/or mode are not favourable or do not require tank venting, the method decouples the storage tank from the exhaust system in response as represented by the block 210.

This step or function may include controlling one or more devices to decouple or block flow between the storage tank and exhaust system. For example, the system or method may decouple the storage tank space from the exhaust system when the storage tank pressure is below a predetermined pressure or pressure differential that may be selected to include hysteresis for actively controlled valve applications.

As also illustrated in Figure 2, the present invention may incJude a determination for selectively coupling the -15 emissions treatment substance storage tank to atmosphere as represented by block 212. If current operating conditions and/or operating mode are suitable for vacuum release, then the emissions treatment substance storage tank is coupled to atmosphere as represented by block 214. Operating conditions may include ambient and/or system temperature(s) and/or pressure(s), operating state of pump 20, engine 80, and the like. Preferably, a vacuum release device operates in response to a pressure differential between the storage tank and atmosphere to couple the storage tank to atmosphere and allow air to enter the storage tank and to subsequently decouple the storage tank from atmosphere as represented by block 216 to stop vapour egress from the storage tank to atmosphere.

Although described with reference to coupling and decoupling the storage tank space to the exhaust system and/or atmosphere, those of ordinary skill in the art will recognize that these features of the invention may be stated differently as controlling egress of vapours from the storage tank while allowing ingress of air to prevent vacuum lock of an associated emissions treatment system pump.

As such, the present invention provides a passive or active control for venting a vehicle emissions treatment system to provide storage and/or treatment of any vapours before being released to atmosphere. The present invention reduces or eliminates detectable odour associated with the release of untreated vapours from a vehicle emissions treatment system. In addition, the invention provides an alternative to open venting without inducing vacuum lock in the emission treatment system.

Therefore in summary, the invention provides a system and method for controlling venting of an on-board vehicle emiss Eons treatment substance storage and distribution system that selectively dispenses an emissions treatment -16 -substance from a storage tank on the vehicle to an exhaust system of the vehicle include selectively coupling the storage tank to the exhaust system when pressure in the storage tank exceeds a predetermined pressure associated with the exhaust system. The invention may also include selectively coupling the storage tank to atmosphere when a pressure differential between the storage tank and atmosphere exceeds a corresponding predetermined pressure differential. The emissions treatment substance storage tank may be coupled to the exhaust system upstream of a catalyst that may store amxnona or facilitate its decomposition to reduce or eliminate ammonia escaping from the vehicle. Embodiments of the present invention include an on-board emissions

treatment system having a first valve for selectively coupling a vehicle emissions treatment substance storage tank to a vehicle exhaust system when pressure in the storage tank exceeds a predetermined threshold. A passive one-way check valve may be used with a predetermined crack or opening pressure selected to be above the highest anticipated exhaust system pressure to allow selective venting of the storage tank while reducing or eliminating exhaust flow into the storage tank. Alternatively, a computer controlled solenoid valve may be used to selectively couple the storage tank to the vehicle exhaust system based on current engine, vehicle, and/or ambient operating conditions. A second check valve or solenoid valve may be used to selectively couple the storage tank to atmosphere to reduce or eliminate vacuum and associated vacuum lock in the emissions treatment storage and distribution system.

The present invention provides a number of advantages.

For example, the present invention provides a passive control for venting a vehicle emissions treatment system to -17 -provide storage and/or treatment of any vapours before being released to atmosphere.

The present invention also reduces or eliminates detectable odour associated with the release of untreated vapours from a vehicle emissions treatment system.

In addition, the invention provides an alternative to open venting without inducing vacuum lock in the emission treatment system.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is riot limited to the disclosed embodiments and that one or more modifications to the disclosed embodiments or alternative embodiments could be constructed without departing from the scope of the invention.

Claims

-18 -

CLAIMS

1. A method for controlling an on-board vehicle emissions treatment system that selectively dispenses an emissions treatment substance from a storage tank to an exhaust system of the vehicle, the method comprising selectively coupling the storage tank to the exhaust system, when pressure in the storage tank exceeds a predetermined pressure associated with the exhaust system, and, otherwise, decoupling the storage tank from the exhaust system.

2. A method as claimed in claim 1 wherein the exhaust system includes a catalyst and wherein the step of selectively coupling comprises selectively coupling a portion of the storage tank above a maximum level of the emissions treatment substance to the exhaust system upstream of the catalyst to selectively deliver vapours from the storage tank to the exhaust system.

3. A method as claimed in claim 1 or in claim 2 wherein the step of selectively coupling comprises positioning a passive valve between the storage tank and the exhaust system that operates solely in response to a pressure differential across the valve to open when the pressure in the storage tank exceeds the predetermined pressure and to close when the pressure in the storage tank is less than the predetermined pressure.

4. A method as claimed in claim 1 or in claim 2 wherein the step of selectively coupling comprises positioning a one-way valve between the storage tank and the exhaust system and opening the one-way valve when the pressure in the storage tank exceeds the predetermined pressure.

5. A method as claimed in claim 1 or in claim 2 wherein the step of selectively coupling comprises -19 -controlling a solenoid valve disposed between the storage tank and the exhaust system such that the solenoid valve opens when the pressure in the storage tank exceeds the predetermined pressure.

6. A method as claimed in any of claims 1 to 5 wherein the method further comprises selectively coupling the storage tank to atmosphere when a pressure differential between the storage tank and atmosphere exceeds a predetermined pressure differential.

7. A method as claimed in claim 6 wherein the step of selectively coupling the storage tank to atmosphere comprises positioning a one-way valve between the storage tank and atmosphere, the one-way valve opening when the pressure differential between the storage tank and atmosphere exceeds the predetermined pressure differential to provide air from atmosphere to the storage tank, and closing when the pressure differential between the storage tank and atmosphere is less than the predetermined pressure differential to prevent vapour egress from the storage tank.

8. A method as claimed in claim 6 wherein the step of selectively coupling the storage tank to atmosphere comprises controlling a solenoid valve positioned between the storage tank and atmosphere to selectively provide air to the storage tank.

9. An on-board vehicle emissions treatment system that selectively dispenses an emissions treatment substance from a storage tank on the vehicle to an exhaust system of the vehicle, the emissions treatment system comprising a venting valve disposed between the storage tank and the exhaust system to selectively couple the storage tank to the exhaust system, when pressure in the storage tank exceeds a predetermined pressure associated with the exhaust system, -20 -and, otherwise, decouple the storage tank from the exhaust system.

10. A system as claimed in claim 9 wherein the system further comprises a vacuum release valve disposed between the storage tank and atmosphere, the vacuum release valve selectively coupling atmosphere to the storage tank when a pressure differential between atmosphere and the storage tank exceeds a predetermined level and decoupling the storage tank from atmosphere otherwise.

11. A system as claimed in claim 10 wherein the vacuum release valve operates solely in response to the pressure differential between the storage tank and atmosphere.

12. A system as claimed in claim 10 or in claim 11 wherein at least one of the venting valve and the vacuum release valve comprises a solenoid valve and the emissions treatment system further comprises a controller in communication with the at least one valve for selectively controlling the valve in response to current operating conditions.

13. A system as claimed in claim 9 wherein the venting valve operates solely in response to a pressure differential between the storage tank and the exhaust system.

14. A system as claimed in any of claims 9 to 13 wherein the system further comprises a catalyst disposed downstream of an internal combustion engine and the venting valve selectively couples the storage tank to the exhaust system upstream of the catalyst.

15. A system as claimed in any of claims 9 to 14 wherein the venting valve opens when the pressure in the storage tank exceeds a maximum exhaust pressure associated -21 -with operation of an internal combustion engine of the vehicle.

16. A computer readable storage medium having stored data representing instructions executable by a computer to control an emissions treatment system of a vehicle, the emissions treatment system selectively dispensing an emissions treatment substance from a storage tank on the vehicle to an exhaust system of the vehicle, the emissions treatment system including a conduit with a controllable valve disposed between the storage tank and the exhaust system for selectively venting vapours from the storage tank to the exhaust system, the computer readable storage medium comprising instructions for controlling the valve to open in response to pressure in the storage tank exceeding a predetermined pressure.

17. A computer readable storage medium as claimed in claim 16 wherein the predetermined pressure corresponds to a maximum anticipated exhaust pressure to prevent exhaust from entering the storage tank when the valve is open.

18. A computer readable storage medium as claimed in claim 16 or in claim 17 wherein the emissions treatment system includes a controllable vacuum release valve and the computer readable storage medium comprises instructions for controlling the vacuum release valve in response to atmospheric pressure exceeding pressure of the storage tank by a predetermined amount.

19. A computer readable storage medium as claimed in any of claims 16 to 18 further comprising instructions for periodically opening the valve to deliver vapour from the storage tank to the exhaust system.

20. A computer readable storage medium as claimed in any of claims 16 to 19 further comprising instructions for -22 -opening the valve in response to temperature of at least one exhaust system component.

21. A method for controlling an on-board vehicle emissions treatment system substantially as described herein with reference to the accompanying drawing.

22. An emissions treatment system substantially as described herein with reference to the accompanying drawing.

23. A computer readable storage medium substantially as described herein with reference to the accompanying drawing.