PURGING OF A VAPOUR CANISTER

Abstract: A method and engine management system are described for purging a vapour canister which stores fuel vapour evaporated from a fuel storage tank of an engine by drawing ambient air through the canister and delivering the resultant air and fuel vapour mixture to the air intake system of the engine. The flow rate of ambient air drawn through the vapour canister while the vapour canister is being purged is measured and the air and fuel vapour mixture is cooled to condense the free fuel vapour so as to leave the air fully saturated with vapour. By measuring the temperature and pressure of the vapour laden air, the fuel content of the vapour laden air supplied to the engine is estimated. This enables the engine management system to make allowance for the fuel supplied to the engine from the vapour canister.

Description: Purging of a vapour canister Field of the invention The present invention relates to the purging of a vapour canister that stores fuel vapour evaporated from the fuel storage tank of an engine. Background of the invention Hitherto, vapour produced while purging of a vapour canister could not be used in a controlled manner. When such vapour is dumped into the intake system of an engine without the engine fuel system compensating for its presence, it disturbs the fuel calibration of the engine and is not burnt efficiently. It is difficult to determine the quantity of fuel vapour purged from the vapour canister for several reasons. First, the .amount of vapour stored in the canister may vary. Second, as the canister is being purged, it is cooled and the rate of evaporation varies with the local temperature occurring inside the canister. Third, the vacuum pressure applied to purge the canister may be variable and this, in turn, will affect the rate of evaporation of the vapour and the flow rate of purge air drawn through the canister, the two effects resulting in variable concentrations of the vapour in the air. As a result of all the above variables, the content of the purge flow from the canister may range from neat fuel vapour, through mixtures of varying concentrations of vapour in air, to air containing little or no fuel vapour. Object of the invention The present invention seeks to enable the quantity of fuel contained in the gas flow drawn from a vapour canister and delivered to an engine intake system to be more accurately determined. Summary of the invention According to a first aspect of the present invention, there is provided a method of purging a vapour canister that stores fuel vapour evaporated from a fuel storage tank of an engine by drawing ambient air through the canister and delivering the resultant air and fuel vapour mixture to the air intake system of the engine, the method comprising the steps of measuring the flow rate of ambient air drawn through the vapour canister while the vapour canister is being purged, cooling the air and fuel vapour mixture to condense the free fuel vapour and leave the air fully saturated with vapour, measuring the temperature and pressure of the vapour laden air and estimating the fuel content of the vapour laden air supplied to the engine intake system based upon the temperature,- pressure and rate of flow of the vapour laden air, thereby enabling an engine management system to allow for the fuel supplied to the engine from the vapour canister. Once the air and fuel vapour mixture has been cooled to the point where some of the vapour contained in the air is condensed, the air will be fully saturated with vapour and the fuel content will be known from the measured pressure and temperature of the air. The condensed fuel can be returned to the fuel tank leaving a known amount of fuel carried by the air supplied to the engine. The rate at which this vapour laden air is supplied to the engine can then be regulated so that a predetermined rate of fuel vapour is supplied to the engine while purging the vapour canister enabling direct metering of this fuel flow by the engine management system. According to a second aspect of the present invention, there is provided an engine management system comprising means for drawing ambient air through a vapour canister that stores fuel vapour evaporated from a fuel storage tank of the engine and delivering the resultant air and fuel vapour mixture to the air intake system of the engine, means for measuring the flow rate of ambient air drawn through the vapour canister while the vapour canister is being purged, means for cooling the air and fuel vapour mixture to condense the free fuel vapour and leave the air fully saturated with vapour, means for measuring the temperature and pressure of the vapour laden air, means for estimating the fuel content of the vapour laden air supplied to the engine intake system based upon the temperature, pressure and rate of flow of the vapour laden air, and means for reducing the remainder of the fuel metered to the engine by the engine management system to compensate for the fuel supplied to the engine from the vapour canister. Preferably, means are provided for regulating the rate at which ambient air is drawn through the vapour canister into the engine intake system, as a function of the estimated concentration of the fuel vapour in the air, such that a predetermined mass flow rate of fuel is supplied to the engine during the purging of the canister regardless of the vapour content of the canister at the time of purging. The means for metering the flow rate of ambient air may be an orifice, a venturi or a hot wire flow meter located in a pipe connecting the vapour canister to the ambient atmosphere . The means for cooling the air and fuel vapour mixture may be a low temperature condenser. If the purge flow through the vapour canister contains both free fuel vapour and vapour laden air, the condenser will condense all the free vapour leaving only the air saturated with fuel vapour. If the purge flow does not contain free fuel vapour, provided that the air contains sufficient fuel vapour so that at least some of the vapour is condensed when the air is cooled, the air leaving the condenser will still be saturated with fuel vapour. The invention therefore ensures that for substantially the whole range of the state of fill of the vapour canister except when it is almost completely purged, only air saturated with fuel vapour is delivered to the engine . The condenser may be a Peltier effect thermoelectric element which transfers heat from a set of cooling fins exposed to the vapour laden air to a set of fins acting as a heat sink exposed to the ambient atmosphere. The condenser may alternatively be a cooling element integrated with the air conditioning system of a vehicle. As the temperature and pressure of the air leaving the condenser are known, these values will confirm that all the free vapour has condensed, having been cooled to a temperature lower than the boiling point of the lightest fractions of the fuel at the prevailing pressure. As previously stated, the condensed fuel produced by the condenser may be drained back to the fuel storage tank of the engine. It is alternatively possible, however, to store this condensed fuel in an auxiliary fuel tank and to use it subsequently for special purposes, such as cold starting of the engine. This takes advantage of the fact that the condensed fuel will contain a higher proportion of the volatile lighter fraction of the fuel than fuel drawn directly from the fuel storage tank. It is preferred for a substantial proportion of the purge vapour that is condensed to be returned to the fuel storage tank. This prevents the fuel in the storage tank from losing a large proportion of its lighter fraction to the vapour canister and its purge system. The quality of the fuel is not therefore degraded with repeated purging of the vapour canister, which could otherwise adversely affect the cold starting of the engine. Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawing which is a schematic diagram of an engine management system. Detailed description of the preferred embodiment The drawing showing an engine 10 having an intake manifold 16 to which air is supplied by way of a venturi 12 disposed upstream of a main butterfly throttle 14. Fuel injectors 18 inject fuel into the intake charge from a pressurised fuel rail 34, the pressure in the rail 34 being maintained by a fuel pump 32 that draws fuel from the main fuel storage tank 30 and a pressure relief valve 36 that returns surplus fuel to the storage tank 30 by way of a return line 38. The storage tank 30 is vented to atmosphere through a pipe 40 that includes a vapour canister 20 that stores fuel vapour from the ullage space of the storage tank 30 to prevent it from being discharged into the atmosphere. The canister 20 needs to be purged from time to time to prevent it from being saturated, this being effected by drawing air through the canister 20 and returning the vapour laden air to the engine intake system. As so far described the engine and the fuelling system are conventional and the problem that arises is that it is not readily possible for the engine management system to take into account the fuel vapours derived from the vapour canister 20. This is because the amount of fuel returned to the intake system varies with several parameters of which some, such as the extent of saturation of the vapour canister, are not quantifiable. In the preferred embodiment of the invention the amount of air drawn through the vapour canister 20 to purge it is measured by an air flow meter 56 that may be of conventional design. The flow meter for example comprise a hot wire, a venturi or an orifice across which the pressure drop is measured. The purge air flows along a pipe 46 to a condensing chamber 22 that contains a Peltier effect cooler 24 with cooling fins 26 in the chamber 22 and heat radiating fins 28 exposed to the atmosphere. The purge air is cooled in the condensing chamber 22 to the point where some of the fuel vapour is condensed back to liquid leaving a fully saturated vapour for return to the engine intake system. The condensed fuel passes down a pipe 48 to the return pipe 38 of the fuelling system and is from there returned to the fuel storage tank 30. The vapour laden air from the condensing chamber 22 is returned by a first pipe 44 containing a regulating valve 54 to a point in the intake manifold downstream of the main butterfly throttle 14, and by a second pipe 42 containing a regulating valve 52 to the venturi 12 disposed upstream of the butterfly throttle 14. The engine management system 100 receives as input data the purge air flow rate from the meter 56 over a line 106 and the temperature and pressure of the vapour laden air in the condensing chamber 22 over a line 108. The management system 100 also has output lines 102, 104 that are connected to the regulating valves 52, 54 to regulate the flow rate of the purge air. The concentration of the fuel vapour in the purge air is uniquely defined if the temperature and pressure are known, provided only that the fuel vapour is known to be saturated. In the present invention this is achieved by cooling the purge air to the point where condensation of some of the vapour occurs by means of a condenser. It is possible to use a Peltier effect device as described above or an element cooled by the climate control system of the vehicle. Such a cooling system can condense a major proportion of the evaporated fuel and return it to the fuel tank so that it is not necessary to dump an excessive amount of fuel into the engine intake system merely to comply with legal requirements on evaporative emissions control. A further advantage of such recycling of the fuel vapour is that the fuel that tends to evaporate into the ullage space of the fuel storage tank 30 is the lighter fraction essential for cold starting of the engine and its recycling ensures that the quality of the fuel in the storage tank is not degraded with time and repeated purging. Knowing the concentration of the fuel and the flow rate of the purge air allows the engine management system to determine the quantity of fuel entering the engine from the canister purging system thereby allowing the management system 100 to take this amount into account when controlling the fuel injectors 18. The reason for two return lines 42, 44 is that at high speed the venturi 12 produces the required vacuum to draw purge air through the vapour canister 20 whereas at low load and speed the manifold vacuum downstream of the butterfly throttle 14 is used for this purpose. Neither of these sources of vacuum on its own can provide the necessary suction over the entire engine operating range.

Claims: CLAIMS 1. A method of purging a vapour canister that stores fuel vapour evaporated from a fuel storage tank of an engine by drawing ambient air through the canister and delivering the resultant air and fuel vapour mixture to the air intake system of the engine, the method comprising the steps of measuring the flow rate of ambient air drawn through the vapour canister while the vapour canister is being purged, cooling the air and fuel vapour mixture to condense the free fuel vapour and leave the air fully saturated with vapour, measuring the temperature and pressure of the vapour laden air and estimating the fuel content of the vapour laden air supplied to the engine intake system based upon the temperature, pressure and rate of flow of the vapour laden air, thereby enabling an engine management system to allow for the fuel supplied to the engine from the vapour canister . 2. A method of purging a vapour canister as claimed in claim 1, wherein the rate at which the vapour laden air is supplied to the engine is regulated so that a predetermined rate of fuel vapour is supplied to the engine while purging the vapour canister, so as to enable direct metering of this fuel flow by the engine management system. 3. An engine management system comprising means for drawing ambient air through a vapour canister (20) that stores fuel vapour evaporated from a fuel storage tank (30) of the engine and delivering the resultant air and fuel vapour mixture to the air intake system of the engine, means (56) for measuring the flow rate of ambient air drawn through the vapour canister (20) while the vapour canister is being purged, means (26) for cooling the air and fuel vapour mixture to condense the free fuel vapour and leave the air fully saturated with vapour, means (108) for measuring the temperature and pressure of the vapour laden air, means (100) for estimating the fuel content of the vapour laden air supplied to the engine intake system based upon the temperature, pressure and rate of flow of the vapour laden air, and means (100) for reducing the remainder of the fuel metered to the engine by the engine management system to compensate for the fuel supplied to the engine from the vapour canister. 4. An engine management system as claimed in claim 3, wherein means (56) are provided for regulating the rate at which ambient air is drawn through the vapour canister into the engine intake system, as a function of the estimated concentration of the fuel vapour in the air, such that a predetermined mass flow rate of the fuel is supplied to the engine during the purging of the canister regardless of the vapour content of the canister at the time of purging. 5. An engine management system as claimed in claim 3 or 4, wherein the means (56) for metering the flow rate of ambient air comprise an orifice, a venturi or a hot wire flow meter located in a pipe connecting the vapour canister to the ambient atmosphere. 6. An engine management system as claimed in any of claims 3 to 5, wherein the means (24) for cooling the air and fuel vapour mixture comprise a low temperature condenser . 7. An engine management system as claimed in claim 6, wherein the condenser comprises a Peltier effect thermoelectric element (24) serving to transfer heat from a set of cooling fins (26) exposed to the vapour laden air to a set of fins (28) acting as a heat sink exposed to the ambient atmosphere. 8. An engine management system as claimed in claim 6, wherein the condenser comprises a cooling element integrated with an air conditioning system of a vehicle to which the engine is fitted. 9. An engine management system as claimed in any of claims 3 to 8, wherein means (48) are provided for returning the condensed fuel to the fuel storage tank of the engine. 10. An engine management system as claimed in any of claims 3 to 8, wherein an auxiliary fuel tank is provided for storing the condensed fuel for subsequent use in cold starting of the engine.

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: 1998
Main classification: F02M25
IPC codes: F02D35, F02D41, F02D45, F02M25, F02M33