Conductivity sensor for fuel vapor handling system

Abstract: An apparatus and method for use in diagnosing an evaporative emission canister collection and purge system. An EVAP system of a vehicle is diagnosed through the use of a mechanism wherein a change in conductivity occurs in response to a change in concentration of fuel vapor in the variably conductive mechanism. By sensing the change in conductivity, a diagnostic mechanism determines whether the vapor collection and purge systems are operating correctly.

Description: Background of the Invention This invention relates to a fuel vapor handling system. More particularly, the invention is directed to an apparatus and method for use in diagnosing the system. Automobiles conventionally include a system directed to controlling the emission of fuel vapors generated by the fuel carried in the fuel system. These evaporative emission control systems, known as "EVAP" systems, are implemented as a collateral system to the vehicle's fuel system. The diurnal and running loss vapors they collect result primarily from ambient temperature excursions and the cyclic operation and parking of the vehicle that results from the operator's use of the vehicle as transportation. An EVAP system typically includes a vapor collection system with an adsorption mechanism to collect and store vapors generated by the fuel system. The EVAP system also includes a purge system to transfer the stored fuel vapors from the adsorbent to the vehicle's engine for consumption in the normal combustion process. The purge system generally includes a normally closed purge valve that selectively opens a passage between the EVAP system and the vehicle's engine to effect a controllable rate of purge. Conventionally, diagnosis of an EVAP system is generally provided through manual inspection of the system in response to noticeable engine performance degradation or noticeable fuel or vapor leakage. Periodic manual vacuum testing for leaks and purge valve functional checking provides additional effectiveness in diagnosing system operation. Research has been conducted into developing on-board means for automatically diagnosing EVAP systems, capable of automatically detecting leaks in the system and determining whether the vapor collection and purge systems are operating correctly. Development of on-board automatic diagnostic systems has generally resulted in proposed systems related to mechanisms that close the EVAP system off from the atmosphere and then generate a positive or negative internal system pressure. By then measuring changes in the system pressure, the diagnostic mechanisms attempt to discern whether the evaporative control system is functioning correctly. Generally, sensitive diagnostic systems are proposed with precision pressure detection devices to work on small pressure differentials. To avoid unacceptable erroneous fault reporting, a pressure based diagnostic system must be able to discern that unexpected pressure gradients are a result of system malfunctions and not changing ambient conditions or other normal collateral effects. This tends to complicate and drive up the cost of a diagnostic mechanism. Accordingly, automatic EVAP diagnostic systems have proven difficult to implement. Adsorption canister collection and storage system use in on-board refueling vapor recovery (ORVR) systems is known. ORVR systems are vehicle based systems directed at capturing fuel vapors generated by the transfer of fuel from a pump to a vehicle. ORVR systems have been proposed that are configured in a manner similar to an EVAP system including a storage canister and a purge system. Therefore, an automatic system capable of diagnosing an ORVR system is also needed. US-A-5,088,466 discloses a fuel gas purging system having a chamber full of an absorbent made of activated charcoal. Two electrodes are positioned in the adsorbent and are connected to a control circuit by lead wires. The system compares the amount of adsorption prior to the start of the purging operation and the amount of adsorption after a prescribed amount of time. The system has an open outlet port for discharging to atmosphere. US-A-4,633,614 discloses a probe with a conductive polymer and a pair of electrodes placed to be contacted with a chemical for detecting leaks. Contact with a chemical causes a change in molecular structure or arrangement of the polymer. Carbon molecules in the polymer combine with molecules from the chemical changing the resistance. Change in resistivity is measured and an alarm is actuated. Summary of the Invention A fuel vapour system in accordance with the present invention is characterised over US-A-5088466 by the features specified in the characterising portion of claim 1. This invention is directed to a diagnostic mechanism for a fuel vapour handling system and generally includes a variably conductive mechanism associated with a fuel vapour storage canister. A sensor to detect the conductivity of the mechanism is provided for use in assisting automatic diagnosis of the system. A diagnostic routine is preferably provided as part of a vehicle's conventional electronic controller, capable of utilizing conductivity data generated from the variably conductive mechanism for diagnosing the system. The variably conductive mechanism and sensor are used to detect fuel vapour concentration changes caused by fuel vapour flow into the canister and canister purge vapour flow out of the canister for use in diagnosing the system. Optionally, the variably conductive mechanism is either integrated with or located remotely from the canister. Other electrical properties related to conductivity will likewise vary with the varying concentration of fuel vapour and are therefore amenable to providing data for diagnosing the system. A method of diagnosing a fuel vapour handling system in accordance with the present invention is characterised by the features specified in claim 3 in which steps (a), (c), and (e) are known from US-A-5088466. Brief Description of the Drawings Figure 1 is a schematic of an EVAP system. Figure 2 is a graph of conductivity versus time illustrating a typical canister vapor loading and purge cycle for a selected variably conductive material. Figure 3 is a graph of an EVAP system's conductivity sensor output for an in-vehicle operating condition. Figure 4 is a graph of conductivity versus time for the experimental set-up graphed in Figure 3 with a 1.016 mm (0.04 inch) diameter opening in the system. Figure 5 is a diagram illustrating alternative steps of a digital control module in diagnosing a fuel vapor handling system. Detailed Description of the Presently Preferred Embodiment Figure 1 illustrates an EVAP system of a vehicle's fuel system employing principles of the present invention. The system includes a vapor storage canister 110. Canister 110 exhibits a chambered arrangement created by the exterior wall 112 and the interior wall 111. Chambers 114 and 115 result from this arrangement. Interior wall 111 does not completely separate chamber 114 from chamber 115, rather a passage 117 is established between the chambers. An alternative canister such as a single chambered canister can be used as canister 110. Wall 112 establishes a substantially closed container by providing a surface on all sides of the canister 110. In the preferred arrangement, wall 112 creates a canister 110, that is substantially rectangular parallelepiped in shape, although the shape is not a critical feature. Selected openings are provided in canister 110 through wall 112. Opening 121 between chamber 114 and conduit 136 functions as a threshold to conduit 136 and operates as a vent. Opening 122, between chamber 115 and conduit 134, functions as a threshold to conduit 134 permitting selected flow out of canister 110. Opening 123, between chamber 115 and conduit 135, functions as a threshold for chamber 115 to conduit 135 permitting flow therebetween. Conduit 135 establishes a closed flow path between canister 110 and the top of fuel tank 150 that carries a supply of fuel 160. Conduit 134 establishes a closed flow path between canister 110 and an engine 140. Conduit 134 includes purge valve 170 that functions to selectively control the fluid flow rate through conduit 134. Conduit 136 includes normally open solenoid operated vent valve 143 which normally allows a flow of air into and out of canister 110 through conduit 136, thereby facilitating the transfer of fluid into and out of the canister 110 through openings 123 and 122. Vent valve 143 is in communication with ECM 141. Interconnected with conductor 136 and bypassing vent valve 143 is pressure relief 144. Pressure relief 144 limits the maximum pressure permitted in canister 110 by providing a vent point when solenoid vent valve 143 is closed. Pressure relief 144 comprises a conventionally known relief such as a spring loaded diaphragm pressure relief or an umbrella valve in parallel or integrated with vent valve 143. Chamber 114 principally contains an adsorbent such as activated carbon particles 180. Activated carbon is a conventional adsorbent used in EVAP systems. Chamber 114 contains a screen 118 to prevent the passage of particles from chamber 114 to opening 121 and aid in distributing air flow passing through the vent and into chamber 114. A conventional material used as screen 118 is a polymeric foam, although alternate materials can readily be substituted. Similarly, filter 116 is positioned between chamber 115 and openings 122 and 123. Chamber 115 also contains activated carbon particles 180 that function as an adsorbent medium. A sensor 147 is disposed in chamber 115. The sensor illustrated represents a pair of electrodes 151 and 152, between which an electrical current is communicated and conductive foam 154. Other sensing devices are readily applicable to serving this function. Sensor 147 is used to detect the electrical conductive characteristic of conductive foam 154 in relation to varying concentrations of fuel vapors. The sensor 147 is positioned between openings 122 and 123 and the activated carbon 180. Conductive foam 154 is segregated from the activated carbon 180 by nonconductive foam filter element 116. Electrodes 151 and 152 are positioned in contact with or integrated into conductive foam 154. The conductive foam 154 is comprised of a polymeric foam such as polyether urethane made conductive by impregnation with carbon, preferably adsorbent type carbon. Examples of acceptable materials are FOAMEX SORBACELL and LEWCOTT ACF-F-.312-T50-150G, which are polyether urethane foams made conductive by dipping in a carbon solution. Preferably, conductive foam 154 comprises polyether urethane foam coated with coconut based activated carbon. As shown in Figure 1, the conductive foam 154 covers the entire upper cross section of chamber 115 with the electrodes 151 and 152 positioned to measure the conductivity across the top surface of the foam. Sensor 147 enables the detection of EVAP system leaks as small as 0.508 mm (.02 inches) in diameter. Leaks in the tank 150, conduit 135, canister 110 or purge line 134 are detectable. Stage I EVAP system integrity and purge tests are conducted under all conditions and are enabled after every start of engine 140. Stage II diagnostic tests are conducted under certain conditions including when the vapor generation factor (VGF), is greater than 1. Therefore, a two part test as illustrated in Figure 5 is utilized. The VGF is a means of expressing the running loss vapor generation which depends on the ambient temperature and the level of fuel 160 in fuel tank 150. VGF = 0.0014 (100-%Full)EXP(0.041MAT). Where %Full is the fuel level in fuel tank 150 as a percentage of full and MAT is the manifold air temperature. Figure 5 illustrates a diagnostic test utilized in evaluating the EVAP system of Figure 1. The diagnostic test occurs during the initial start up routine of engine 140. The test is entered at point 170 and proceeds to step 172 where, with the purge valve 170 open, the vent valve is closed for approximately 6 seconds to draw vapors into the canister 110 and then re-opened to engage vapor purge to the engine 140. From step 172 the program proceeds to step 174 where ECM 141 evaluates the vapor collection function of the EVAP system by evaluating conductivity changes perceived by sensor 147 in stage I diagnostics. If, at step 172, it is determined that the vapor collection function or purge function are not operating correctly, the program proceeds to 173 where a fault report is generated and the test is complete. If, at step 172, no function irregularities are perceived, the first part of the two-part diagnostic test is complete and the program proceeds to step 181 for a determination of whether to run the second part of the two-part diagnostic test. At step 181 a delay of approximately eight minutes occurs and the program then proceeds to step 182. After the eight minute delay, DMAT, the change in manifold air temperature since engine start is calculated. Of the conditions necessary for running part two of the test is the requirement that DMAT be greater than approximately 8.33 degrees Celsius (fifteen degrees fahrenheit). Additionally, the vapor generation factor must be greater than one. If these conditions are not met, the second part of the test is not conducted and the test is complete. If these conditions are met then conditions are appropriate for conducting the second part of the test and the program proceeds to step 183. The second part of the two-part diagnostic test is capable of perceiving breaches as small as 0.508 mm (0.02 inches) in diameter anywhere in the system. At step 183 the vent valve 143 and purge valve 170 are closed to enable system pressurization due to running loss vapor generation. In step 184, an increase in vapor concentration, essentially immediately detected by sensor 147 is indicative of a fault such as a breach in purge line 134 and the program proceeds to step 187 where a fault report is generated and the test is complete. No change in vapor concentration in step 186 as detected by sensor 147, after a period of time such as ten minutes, is indicative of a fault such as a breach in vapor line 135 or fuel tank 150 and the program proceeds to step 188 where a fault report is generated and the test is complete. A delayed increase in vapor concentration in step 185 as detected by sensor 147 is indicative of a correctly operating closed system with an accompanying pop opening of relief valve 144 caused by running loss vapor generation and the test is complete. From steps 185, 188 or 187 the program proceeds to reopen vent valve 143 and purge valve 170 before ending. In the foregoing manner the EVAP system is diagnosed. System function diagnosis is performed and breaches as small as 0.508 mm (0.02 inches) in diameter are detectable during appropriate ambient conditions. Alternatively, a system (not illustrated) configured like the system of Figure 1 without pressure relief 144 is operable for EVAP system diagnosis. In such a system, stage II of the two-part diagnostic test proceeds by closing vent valve 143 and purge valve 170. This is followed by a timed delay of approximately ten minutes. During the delay, pressure in tank 150 is allowed to build due to running loss vapor generation. Then vent valve 143 is opened to relieve tank pressure. During the venting process, fuel vapors should collect in adsorbent 180. By means of sensor 147, the extent of vapor collection is detected. The extent of vapor collection is indicative of system integrity and operation. Figure 2 is a graphical depiction of the variation in conductivity of a conductive foam vapor sensor 154 in response to variations in the concentration of fuel vapors in a canister. The graph segment labeled "vapor loading" correspond to a scenario where the purge system is turned off and fuel vapors are accumulating in the adsorbent 180. Notable is the fact that the sensor output voltage increases with the increasing fuel vapor concentration. This phenomenon is indicative of a decrease in conductivity between electrodes 151 and 152, through the foam layer. Therefore, the response of this carbon containing polymeric foam is opposite that of activated carbon when subjected to increases in fuel vapor concentration. This characteristic results in a mechanism that can be used to identify the functional performance of the vapor collection system of an EVAP system and can be utilized in diagnosing the system. The segment of the graph of Figure 2 labeled "purging" coincides with a scenario where the purge function is occurring and fuel vapors are being removed from the adsorbent 180. As the concentration of fuel vapors in the canister 110 decreases, the sensor output voltage drops and therefore, conductivity of the conductive foam layer 154 increases. This characteristic results in a mechanism that can be used to identify the functional performance of the purge system of an EVAP system and can be utilized in diagnosing the system. Figure 3 illustrates the continued repeatability of the variably conductive sensing mechanism during the operation of a vehicle containing an EVAP system according to this invention. For the duration of the graph the in-tank fuel temperature (not illustrated), demonstrates a continuously increasing trend as warmed fuel 160 is returned to the fuel tank 150 from the engine 140. Therefore, the quantity of fuel vapor being generated in the system is increasing with time, which effects the sensor output as illustrated by a comparison between the first loading-purge cycle between approximately 10 and 15 minutes, driving time and the second loading-purge cycle between approximately 16 and 21 minutes, driving time. This incremental increase can be predicted to correspond with the increasing tank temperature and is taken into consideration in analyzing conductivity data obtained from the sensor signal when diagnosing a system. Figure 4 illustrates the effect a 1.016 mm (0.040 inch) diameter opening in an EVAP system has on the sensor signal. Although the associated engine 140 is running, returning warmed fuel 160 to the tank 150 and the purge system is cycled on an off, a response showing a marked variation in the conductivity of the mechanism is absent. Certain criteria, necessary for vapor generation exist for the duration of the graph. Therefore, a variation in conductivity indicative of a correctly functioning vapor loading-purge cycle scenario is predicted to occur and should be shown in Figure 4. Since the conductivity is not varying as predicted, the conductivity signal data is evaluated and it is determined that a malfunction in the vapor collection or purge system exists.

Claims: A fuel vapour handling system comprising: a vapour storage canister (110); a repeatedly enabled variable conductivity mechanism (154) having an electrical conductivity proportional to a concentration of fuel vapour in the vapour storage canister; a sensor including electrodes (151, 152) contacting the variable conductivity mechanism; a vent conduit (136) exiting the vapour storage canister and opening to atmosphere; and means for monitoring the concentration of fuel vapour in the vapour storage canister with the sensor; characterised in that the conductivity mechanism is of polymeric foam impregnated with carbon; the vent conduit includes a normally open vent valve (143); by means for closing the normally open vent valve during a test conducted on the fuel vapour handling system; and by means for detecting a breach in the fuel vapour handling system by monitoring the concentration of fuel vapour in the vapour storage canister with the vent valve closed. A fuel vapour handling system according to claim 1 further comprising an engine (140), a fuel tank (150), and a diagnostic system that communicates with the sensor to diagnose the fuel vapour handling system, the diagnostic system including means for conducting and enabling a first stage (174) under all ambient conditions and after every start of the engine, means for determining during the first stage whether the fuel vapour handling system is collecting fuel vapour from the fuel tank and purging the fuel vapour to the engine; and means for closing the normally open vent valve during a second stage (183-186), the second stage being selectively conducted after the first stage, and the means for detecting a breach operating in the second stage. A method of diagnosing a fuel vapour handling system having a fuel tank (150) carrying fuel (160) and an engine (140) operating under variable ambient conditions and including a vapour storage canister (210), a repeatedly enabled variable conductivity mechanism (154) of polymeric foam impregnated with carbon and positioned in the vapour storage canister, a sensor including electrodes (151, 152) contacting the variable conductivity mechanism, a conduit (135) extending between the fuel tank and the vapour storage canister, a conduit (134) extending between the engine and the vapour storage canister and including a purge valve (170), and a conduit (136) exiting the vapour storage canister and opening to atmosphere including a normally open vent valve (143) comprising the steps of: a. starting the engine (140); b. closing the normally open vent valve (143) and opening the purge valve (170); c. waiting a period of time sufficient to draw fuel vapour from the fuel tank (150) through the conduit (135) to the vapour storage canister (110); d. opening the normally open vent valve; e. determining the conductivity of the variable conductivity mechanism by reading the sensor to test fuel vapour collection in the vapour storage canister and fuel vapour purge from the vapour storage canister; f. waiting a time sufficient for the engine to warm up; g. determining the level of fuel (160) in the fuel tank; h. determining whether the manifold air temperature has changed more than 8.33 degrees Celsius since engine start and the running loss vapour generation factor (VGF) is greater than 1, VGF being defined as VGF = 0.0014(100-%FULL)EXP(0.041MAT) with %FULL being the fuel level in the tank as a percentage of full, and MAT being the manifold air temperature; i. when the second stage test is enabled, proceeding by closing the normally open vent valve. j. reading the sensor to determine whether the fuel vapour handling system is securely closed based on sensed conductivity of the variable conductivity mechanism to test for a breach in the fuel vapour handling system; and k. opening the normally closed vent valve.

Inventors: GOKHALE KALYAN PANDURANG, REDDY SAM RAGHUMA, SCHUSTER MICHAEL STEPHEN, GOKHALE, KALYAN PANDURANG, REDDY, SAM RAGHUMA, SCHUSTER, MICHAEL STEPHEN
Applicants: GEN MOTORS CORP, SATURN CORP, GENERAL MOTORS CORPORATION, SATURN CORPORATION
Application year: 1995
Main classification: F02M25
IPC codes: F02B77, G01N27, F02M25