SYSTEM FOR PURGING A VAPOUR CANISTER

Abstract: A system is described for purging a main vapour canister (12) arranged in an air passage (24) venting a fuel tank (10) to the atmosphere. The system comprises a purge line (26) for supplying air and fuel vapour from the vapour canister (12) to the engine intake system (20) during purge cycles of the main vapour canister (12). A buffer vapour storage canister (14) having a larger capacity than the main vapour canister (12) is arranged within the purge line (26) so that purge air drawn through the main vapour canister (12) passes through the buffer canister (14) before reaching the engine (20). A pump (16) and three-way valve (18) circulate fuel vapour from the ullage space of the tank (10) through the buffer canister (14) to ensure that it remains saturated with fuel vapour in between purge cycles of the main vapour canister (12).

Description: System for Purging a Vapour Canister Field of the invention The present invention relates to a system for purging a vapour canister arranged in an air passage venting a fuel tank to the atmosphere. Background of the invention It is nowadays necessary to provide a vapour canister in the air passage venting a motor vehicle fuel tank to atmosphere. The venting is required in order to prevent pressure build up in the ullage space of the tank and the vapour canister is needed to prevent the discharge of vapour through the venting passage. A vapour canister has only a limited capacity to store vapour and once it is fully saturated it ceases to function and allows vapour to break through to the atmosphere. It is therefore essential when the engine is operating to take steps to purge the vapour out of the canister. This is usually effected by drawing ambient air through the canister and supplying the vapour and air mixture to the engine intake system. A problem encountered in purging a vapour canister in this manner is that the quantity of vapour present in the purge air is unknown. If the vehicle had been at a standstill for a long time, then the purge air could be fully saturated with vapour while on other occasions the canister may have been purged recently and the purge air may contain practically no fuel vapour. The engine fuel metering system cannot therefore rely on any amount of fuel being present in the purge air and such fuel cannot therefore be used effectively in any open loop fuel metering system associated with the engine. Such open loop system are commonly used for cold starts and during warm up because the EGO sensors used in closed loop control do not operate reliably while they and the engine are cold. It is highly desirable to be able to supply fuel to the engine n vapour form when it is cold, as this gives better cold starts and lower emissions during warm up. However, the fuel drawn from the vapour canister cannot be used effectively under these conditions because the quantity of fuel is indeterminate. Object of the invention The invention accordingly seeks to provide a system for purging a vapour canister having an unknown fuel saturation while at the same time supplying an air and fuel mixture to the engine intake system that has a known vapour concentration. Summary of the invention According to the present invention, there is provided a system for purging a mam vapour canister arranged an air passage venting a fuel tank to the atmosphere, comprising a purge line for supplying air and fuel vapour from the vapour canister to the engine intake system during purge cycles of the mam vapour canister, a buffer vapour storage canister having a larger capacity than the mam vapour canister arranged within the purge line so that purge air drawn through the main vapour canister passes through the buffer canister before reaching the engine, and means for saturating the buffer canister with fuel vapour n between purge cycles of the mam vapour canister. Advantageously, the buffer vapour canister may be an elongate canister connected to the mam vapour canister and the ullage space of the fuel tank at one end and connected to the engine intake system at its other end. The means for saturating the buffer canister with vapour in between purge cycles of the ma vapour canister may suitably comprise an additional line leading from the ullage space of the fuel tank to the said other end of the buffer canister, a vapour recirculation pump arranged in said additional line to circulate vapour around a circuit that includes the ullage space of the fuel tank and the buffer canister, and a valve in the purge line for isolating the said other end of the buffer canister from the engine intake system. In such a system, the vapour drawn by the recirculation pump from the ullage space of the fuel tank is preferably delivered to the said other end of the buffer canister so that the end of the canister nearest the engine should always be fully saturated with fuel vapour. In the invention, fuel vapour supplied to the engine intake system is drawn from the end of the buffer canister that is always maintained fully saturated with fuel. The engine metering system therefore knows the quantity of fuel being introduced by the canister purging system and can be calibrated to supply a desired mixture strength to the engine regardless of the degree of saturation of the ma canister and the rate of purging of the mam canister. The vapour from the mam canister is not directly supplied to the engine during a purge cycle but is only used to replenish some of the fuel stored in the buffer canister as fuel is withdrawn from the opposite end of the buffer canister. As long as the buffer canister is larger than the mam canister, it can be ensured that the part of the buffer canister nearest the engine remains fully saturated right up to the time when the main canister is completely purged. Preferably, means may be provided to terminate each purge cycle once it has been determined that sufficient air has been drawn through the main vapour canister to empty the main canister. Such determination may be effected by metering the mass of air drawn into the engine intake system from the vapour canister purging system or by sensing when the fuel vapour concentration in the air entering the buffer canister drops below a predetermined level. At the end of a purge cycle, the amount of fuel purged from the main canister is likely to be less than the amount of fuel drawn by the engine from the buffer canister. The buffer canister will therefore be partially purged and only the parts of the canister nearest the engine will remain fully saturated. If several purge cycles could take place without the buffer canister being refilled, the buffer canister would be progressively emptied to the point where the quantity of fuel reaching the engine would be indeterminate. To avoid this possibility, it is desirable to commence recharging the buffer canister with fuel vapour immediately following each purge cycle in readiness for the next purge cycle by switching on the vapour recirculation pump . While it is possible to run the recirculation pump at all times to ensure that buffer canister remains fully saturated, it is possible to switch off the pump once it is known that no further vapour is being absorbed by the buffer canister from the recirculated vapour. As the absorption of fuel vapour is an exothermic reaction, saturation of the buffer canister may conveniently be determined by comparing the temperature of different regions of the buffer vapour canister during operation of the vapour recirculation pump. In normal operation of a vehicle, it is possible that the engine may be switched off immediately after a purge cycle and the vehicle then left to stand for a long period of time. To ensure that the buffer canister is fully recharged at the commencement of the next purge cycle, it is desirable to ensure that the vapour recirculation pump remains running for sufficient time to recharge the buffer canister, even if the engine should be switched off while the buffer canister is being recharged. Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawing which is a schematic representation of a system in accordance with the invention. Detailed description of the preferred embodiments The drawing shows a fuel tank 10 having an air vent 24 and a main vapour canister 12 within the air vent 24 to prevent the discharge of vapour directly into the atmosphere. The vapour canister 12 will become saturated with fuel unless steps are taken to purge it periodically and this is done by means of a purge line 26 that leads from the air vent 24 to the intake system of the engine which is generally designated 20 and represented by a venturi and a main butterfly throttle. The low pressure within the venturi draws air from the ambient atmosphere through the main vapour canister 12 and in this way succeeds in purging it. As so far described, the system would be conventional and would have the disadvantage that the quantity of vapour carried by the purge air would be indeterminable. To mitigate this problem, a buffer vapour storage canister 14 is arranged in series with the purge line 26. The buffer canister 14 is elongate and has a larger storage capacity than the mam vapour canister 12. Typically the mam storage canister may have a volume of 1 litre while the buffer canister 14 may have a volume of 2 litres. The canister 14 forms part of a closed circuit that includes the ullage space of the fuel tank 10, being connected to the fuel tank 10 at both its ends. The first end is connected to the fuel tank 10 by way of the purge line 26 and the air vent 24, while the other end is connected to the fuel tank 10 by way of a three-way valve 18, a vapour recirculation pump 16 and an additional line 28. This circuit serves as a means for rapidly recharging the buffer canister 14 with fuel vapour in between purge cycles of the mam canister 12, the canister 14 being filled with fuel vapour from its end nearer the engine 20. In operation, at times when the mam canister 12 is not being purged, the valve 18 isolates the purge system from the engine air intake system 20 but permits circulation of fuel vapour through the buffer canister 14 in a clockwise direction as viewed. Advantageously temperature sensors may be provided along the buffer canister 14 to monitor the rise in temperature as the canister fills up with vapour. When the temperature sensors cease to show a rise in temperature, it may be assume that the buffer canister 14 is saturated and the pump 16 may be switched off. At the commencement of a purge cycle, the three-way valve 18 is set m a position to connect the purge line 26 to the engine intake system 20 by way of a metering valve 22 while closing off the additional line 28. The pressure drop in the venturi will now draw ambient air through both the ma canister 12 and the buffer canister 14. The air entering the engine will be fully saturated with fuel vapour having just past tnrough a fully saturated buffer canister 14. At the same time, the air passing through the main canister 12 will purge any vapour that is stored in that canister 12 and use it to replenish the vapour that has been lost to the engine from the buffer canister 14. The purge cycle may continue for as long as necessary to fully purge the main canister 12 without affecting the air to fuel ratio that is received by the engine from the buffer canister 14. It is for this reason that the buffer canister 14 is required to have a larger volume than the main canister 12. A purge cycle may be terminated either by sensing that the main canister 12 is fully purged (for example by sensing that its temperature has stabilised or that the hydrocarbon content of the purge air has dropped) , or after a predetermined purge air mass, as measured by the metering valve 22, has passed through the main canister 12. Immediately after a purge cycle has been terminated, the pump 16 is switched on and the valve 18 is returned to its recirculation position so that the buffer canister 14 can again be saturated quickly with vapour commencing from its end near the engine. To speed up the rate at which the buffer canister 14 is recharged, a perforated emulsion tube 30 is immersed in the fuel stored in the tank 10 and its upper end is connected to the line 28. The pump 16 will reduce the pressure within the emulsion tube 30 and promote vaporisation of the fuel. Additionally, a small column of fuel may be partially drawn up the emulsion tube 30 by the reduced pressure and vapour from the surrounding ullage space may be bubbled through it. Once a purge cycle of the main canister 12 has ended, it is preferred to commence recharging the buffer canister 14 immediately by switching on the pump 16. A hold-on relay may be used to ensure that recharging of the buffer canister 14 is completed even if the engine is switched off.

Claims: CLAIMS 1. A system for purging a mam vapour canister (12) arranged in an air passage venting a fuel tank (10) to the atmosphere, comprising a purge line (26) for supplying air and fuel vapour from the vapour canister to the engine intake system (20) during purge cycles of the mam vapour canister, characterised by a buffer vapour storage canister (14) having a larger capacity than the mam vapour canister arranged within the purge line (26) so that purge air drawn through the mam vapour canister passes through -the buffer canister (14) before reaching the engine, and means (16,18) for saturating the buffer canister (14) with fuel vapour in between purge cycles of the mam vapour canister (12) . 2. A system as claimed m claim 1, wherein the buffer vapour canister (14) is an elongate canister connected to the mam vapour canister (12) and the ullage space of the fuel tank (10) at one end and connected to the engine intake system (20) at its other end. 3. A system as claimed in claim 2, wherein the means for saturating the buffer canister (14) with vapour in between purge cycles of the mam vapour canister comprises an additional line (28) leading from the ullage space of the fuel tank to the said other end of the buffer canister (14), a vapour recirculation pump (16) arranged m said additional line (28) to circulate vapour around a circuit that includes the ullage space of the fuel tank and the buffer canister (14) and a valve (18) in the purge line (26) for isolating the said other end of the buffer canister from the engine intake system. 4. A system as claimed in claim 3, wherein the vapour drawn by the recirculation pump (16) from the ullage space of the fuel tank is delivered to the said other end of the buffer canister. 5. A system as claimed claim 3 or 4 , wherein means are provided for comparing the temperature of different regions of the buffer vapour canister during operation of the vapour recirculation pump to determine when the buffer vapour canister is fully saturated. 6. A system as claimed in any preceding claim, further comprising means for terminating each purge cycle of the ma vapour canister once it has been determined that sufficient air has been drawn through the mam vapour canister to empty the mam canister. 7. A system as claimed n claim 6, wherein the means for determining when sufficient air has been drawn through the mam vapour canister comprises means (22) for metering the mass of air drawn into the engine intake system from the vapour canister purging system. 8. A system as claimed m claim 6, wherein the means for determining when sufficient air has been drawn through the mam vapour canister comprises means for sensing when the fuel vapour concentration m the air entering the buffer canister drops below a predetermined level.

Inventors: MA THOMAS TSOI-HEI, MA, THOMAS, TSOI-HEI
Applicants: FORD GLOBAL TECH INC, MA THOMAS TSOI HEI, FORD GLOBAL TECHNOLOGIES, INC., MA, THOMAS, TSOI-HEI
Application year: 2000
Main classification: F02M25
IPC codes: F02M25, F02M37