Positive pressure canister purge system integrity confirmation

Abstract: The tank/canister (16,14) volume's integrity against unacceptable leakage is either confirmed or denied by a diagnostic test performed by an on-board diagnostic system (10) which includes an electrically operated air pump (22) and a pressure switch (26). At the beginning of a test, the engine management computer (18) closes the canister purge solenoid valve (12) and operates the pump to begin pressurization of the tank/canister volume. Failure to build to a predetermined pressure indicates a gross leak. Upon attainment of the predetermined pressure, the pump is shut off by the operation of the pressure switch from one state to another. The pressure switch has hysteresis so that if the pressure falls below the predetermined pressure by a certain amount during the predetermined duration of the test, the pressure switch returns to its one state thereby giving a signal denying the integrity. Integrity is confirmed by the pressure switch remaining in its another state during the test.

Description: Field of the Invention This invention relates generally to evaporative emission control systems that are used in automotive vehicles to control the emission of volatile fuel vapors. Specifically the invention relates to an on-board diagnostic system for determining if a leak is present in a portion of the system which includes the fuel tank and the canister that collects volatile fuel vapors from the tank's headspace. Background and Summary of the Invention A typical evaporative emission control system in a modern automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the fuel tank. During conditions conducive to purging, the canister is purged to the engine intake manifold by means of a canister purge system that comprises a canister purge solenoid valve that is operated by an engine management computer. The canister purge valve is opened in an amount determined by the computer to allow the intake manifold vacuum to draw vapors from the canister through the valve into the engine. U.S. governmental regulations require that certain future automobiles that are powered by volatile fuel such as gasoline have their evaporative emission control systems equipped with on-board diagnostic capability for determining if a leak is present in a portion of the system which includes the fuel tank and the canister. One proposed response to that requirement is to connect a normally open solenoid valve in the canister vent, and to energize the solenoid when a diagnostic test is to be conducted. A certain vacuum is drawn in a portion of the system which includes the tank headspace and the canister, and with the canister and the tank headspace not being vented due to the closing of the canister vent, a certain loss of vacuum over a certain time will be deemed due to a leak. Loss of vacuum is detected by a transducer mounted on the fuel tank. Because of the nature of the construction of typical fuel tanks, a limit is imposed on the magnitude of vacuum that can be drawn. Too large a vacuum will result in deformation and render the measurement meaningless. In order to avoid this problem, a relatively costly vacuum transducer is required. Since typical automotive vehicles are powered by internal combustion engines which draw intake manifold vacuum, such vacuum may be used for performance of the diagnostic test, but typically this requires that the engine be running in order to perform the test. In document WO-A-9116216 there is described an arrangement for monitoring a system for collecting fuel vapors and releasing them into the throttle body connected to an internal combustion engine of a motor vehicle. The vapors from the fuel tank are collected in a canister interposed the fuel tank and the throttle body. A vent valve from the canister to the atmosphere turns off when the engine is running to assess the system by a pressure sensor in the fuel tank or by monitoring the regulating signal of an exhaust cleaning installation with lambda control. The invention disclosed in WO-A-9217698, provides a solution to the leak detection problem which is significantly less costly. The key to that solution is a new and unique vacuum regulator/sensor which is disposed in the conduit between the canister purge solenoid and the canister. The vacuum regulator/sensor is like a vacuum regulator but with the inclusion of a switch that is used to provide a signal indicating the presence or the absence of a leak. A diagnostic test is performed by closing the tank vent and using the engine manifold vacuum to draw, via the canister purge solenoid valve and the vacuum regulator/sensor, a specified vacuum in the tank headspace and canister. Upon the requisite vacuum having been drawn, the vacuum regulator/sensor closes to trap the drawn vacuum. If unacceptable leakage is present, a certain amount of vacuum will be lost within a certain amount of time, and that occurrence causes the switch of the vacuum regulator/sensor to give a signal indicating that condition. The present invention relates to a diagnostic system and method for evaluating the integrity of a portion of the canister purge system that includes the tank and canister by means of positive pressurization rather than negative pressurization (i.e., rather than by drawing vacuum). In certain canister purge systems, such a diagnostic system and method may afford certain advantages over the system and method described in the aforementioned commonly assigned patent application. For example, it may be possible to omit the normally open solenoid operated vent valve that must be operated closed when the diagnostic test is to be performed. Certain types of leaks, for example cracked hoses and faulty gas caps, may be more susceptible to successful detection. Moreover, the evaporative emission control system may be diagnosed either with or without the automobile's engine running. One means to perform positive pressurization of the fuel tank's headspace and the canister is a devoted electric-operated air pump, which can be of quite simple construction, and therefore relatively inexpensive. If the vehicle already contains a source of suitably pressurized air, that could constitute another means, thereby eliminating the need for a separate devoted pump. A further benefit of positive pressurization over negative pressurization is that the increased pressure suppresses the rate of fuel vapor generation in the tank, and such attenuation of fuel vapor generation during a diagnostic test reduces the likelihood that the test will give, under hot weather conditions which promote fuel vapor generation, a false signal that would erroneously confirm the integrity of the canister and tank whereas the same test during cold weather would indicate a leak. Further specific details of the construction and arrangements of the inventive system, and of the method of operation thereof, along with additional features and benefits, will be presented in the ensuing description. A drawing accompanies this disclosure and portrays a presently preferred embodiment of the invention according to the best mode presently contemplated for carrying out the invention. Brief Description of the Drawing Fig. 1 is a schematic diagram of a representative canister purge system, including a diagnostic system embodying principles of the present invention. Description of the Preferred Embodiment Fig. 1 shows a representative canister purge system 10 embodying principles of the invention. System 10 comprises a canister purge solenoid valve 12 (CPS valve 12), and a carbon canister 14, associated with the intake manifold 15 of an automotive vehicle internal combustion engine and with a fuel tank 16 of the automotive vehicle which holds a supply of volatile liquid fuel for powering the engine. CPS valve 12 is under the control of an engine management computer 18 for the engine. A pressure/vacuum relief valve 19 is associated with canister 14. It is normally closed, but opens at a predetermined positive pressure, such as +10 inches of water for example, to prevent excessive positive pressure in the canister, and it also opens at a predetermined negative pressure, such as -2 inches of water for example, to prevent excessive negative pressure in the canister. The canister purge system operates in conventional manner, and may be briefly described as follows. Under conditions conducive to purging, computer 18 causes the normally closed CPS valve 12 to open in a controlled manner. The result is that a certain amount of the engine manifold vacuum is delivered to canister 14 causing collected vapors to flow from the canister through the CPS valve to the engine manifold where they entrain with the induction fluid entering the engine's combustion chamber space to be ultimately combusted. To the extent that the pressure might seek to fall below -2 inches of water, relief valve 19 opens to allow the purge flow to continue without further pressure reduction in the tank/canister. Alternatively, relief valve 19 could be replaced by another device, such as a normally open solenoid operated vent valve which is operated closed for the diagnostic test. In accordance with the invention, a pressure/sensing module 20 is associated with the system. It comprises an electric operated pump (blower motor) 22, a check valve 24, and a pressure sensing switch 26 having a set of contacts 28, which in the exemplary embodiment of Fig. 1 are normally open. Pump 22 has an air inlet 30 that is communicated to ambient air and an air outlet 32 that is communicated to an inlet of check valve 24. An outlet of the check valve is communicated to the headspace of tank 16. Pressure sensing switch 26 has a pressure sensing port 34 that is communicated to the tank headspace. When the pressure sensed by switch 26 at port 34 is below a set point, contacts 28 are open; when the pressure sensed by switch 26 at port 34 is above the set point, contacts 28 are closed. However, the switch is intentionally designed and calibrated to have a certain hysteresis at its set point. For example, the switch may close contacts 28 at a certain high positive pressure, say +5 inches of water, which is below the positive pressure at which relief valve 19 opens, but re-open them only after the pressure has fallen a predetermined amount below the pressure that closed them, for example re-opening the contacts at +2 inches of water. Module 20 is also electrically connected with computer 18. One electric circuit connection 36 coupling module 20 with computer 18 provides for the computer to control the operation of pump 22; another connection 38 provides for switch 26 to signal the computer. The system functions in the following manner to perform a diagnostic test on the integrity against unacceptable leakage of that portion of the CPS system that is upstream of CPS valve 12. First, computer 18 commands CPS valve 12 to be closed and detects whether contacts 28 are open or closed. If contacts 28 are closed, a pre-existing positive pressure condition in the tank/canister exists that will preclude the performance of the diagnostic test at that time. Accordingly, the test is deferred to a later time, and in this regard it should be mentioned that the timing at which tests are attempted is determined by various other inputs to or programs of computer 18 that need not be mentioned here. On the other hand, if computer 18 detects contacts 28 to be open, then the pre-existing pressure in the tank/canister is deemed suitable for the test to proceed. That being the case, computer 18 commands pump 22 to operate and thus increasingly positively pressurize the tank/canister. As the pump operates, the tank/canister positive pressure should build. However, the presence of a grossly unacceptable leak in the tank/canister could prevent the pressure from building to a predetermined positive pressure within a predetermined time. Thus, if contacts 28 remain open for at least a certain amount of time after the computer has issued its command to operate pump 22, a fault is indicated. Such fault may be attributed to any one or more of: a gross leak in the tank/canister, a faulty connection between module 20 and computer 18, a faulty pump 22, a faulty check valve 24, or a faulty pressure switch 26. In such an event the test is terminated and a fault indication given. However, if the pressure in the tank/canister builds within a predetermined time to the setting that will cause pressure switch 26 to close contacts 28, then the test proceeds. Once closure of switch contacts 28 is detected by computer 18, the computer immediately shuts off pump 22. Check valve 24 functions to prevent loss of pressure back through the pump. This traps the pressure in the tank/canister, and the trapped pressure is initially essentially equal to that at which contacts 28 closed, i.e. +5 inches water in the example. If a leak is present in the tank/canister, positive pressure will begin to decrease. The rate at which the positive pressure decreases is a function of the severity of the leak. An unacceptable leak will cause the positive pressure to drop to at least a certain preselected level within a given time; the absence of a leak or the presence of a leak that is so small as to not be deemed unacceptable will not cause the pressure to drop below that preselected level within that given time. Associated with computer 18 is a timer which begins counting time upon detection of closure of contacts 28. If, after a certain preselected amount of time has been counted, contacts 28 remain closed, the integrity of the test-ensealed tank/canister volume is deemed to have been confirmed, and computer 18 may so indicate in any appropriate manner such by an internal flag or an external signal. On the other hand, the re-opening of the contacts during the testing time is deemed to indicate an unacceptable leak, and such occurrence will be flagged by the computer as a fault signal or called to the attention of the vehicle operator by any suitable means such as a warning lamp on the instrument panel. It may be mentioned at this point that the invention can enable a test to be performed at relatively small positive pressure levels in the canister and fuel tank so that the pressure will not cause deformation of properly designed canisters and tanks. At the completion of a test the canister purge valve is once again operated by computer 18 in the usual way for conducting canister purging. If a diagnostic test is conducted above a certain temperature, it is possible that fuel vapors may be generated in the tank at a rate that is sufficiently fast that the increase in vapor pressure will mask at least to some extent the existence of a leak. This tendency is somewhat better countered by the present invention in comparison to that of the referenced document WO-A-9217698 because the increased positive pressurization tends to attenuate the vapor generation rate. Hence, the present invention may allow testing to proceed under higher ambient temperatures than in the case of the prior system. However, since ambient temperature or engine temperature may still influence the test to some extent, one may choose to employ a temperature sensor mounted on the fuel tank to provide a fuel temperature measurement to the computer and/or the engine coolant sensor to provide a temperature measurement to the computer. If the temperature is not below a predetermined temperature above which the generation of vapor could affect the validity of the test, the test would be deemed invalid. Valid testing would therefore occur only below the predetermined temperature.

Claims: A positive pressure canister purge system (10) associated with an internal combustion engine (15) for confirming the integrity of the system having an engine management computer (18) for controlling certain functions associated with the operation of the engine, a fuel tank (16) for containing a supply of a volatile liquid fuel for the engine, a collection canister (14) for collecting volatile fuel vapors from the fuel tank, purging means (12) operatively controlled by the engine management computer for selectively purging collected fuel vapors from the canister to entrain the fuel vapors with a combustible mixture that passes into combustion chamber space of the engine for combustion therein, the method for detecting unacceptable leakage the tank/canister portion characterized by: electric pump means (22, 30, 32)positively pressurizing the tank/canister portion to a predetermined positive pressure; check valve means (24) for preventing loss of positive pressure from the tank/canister portion back through said pump means (22, 30, 32); sensing means (20) to sense a predetermined positive pressure in the tank/canister portion, and to sense if said pressure in the tank/canister portion decreases from said predetermined positive pressure by a certain amount; wherein the engine management computer (18) further comprises: control means for closing the purging means (12) and for activating said pump means (22,30,32), and for shutting off said pump means (22,30,32) when said sensing means (20) has sensed said predetermined positive pressure; timing means for generating a predetermined time period when said sensing means (20) has sensed said predetermined positive pressure; and further means to generate a signal if said sensing means (20) has sensed a decrease in the pressure of the tank/canister portion from said predetermined positive tank/canister certain amount within said time period for indicating an unacceptable leakage from the tank/canister portion. The system set forth in claim 1 in which said sensing means comprises a set of switch contacts (28) that operate from one state to another state in response to said pressure in the tank/canister portion having reaching said predetermined positive pressure and that operate from said another state to said one state when the pressure in the tank/canister portion decreases from said predetermined positive pressure by a certain amount within a certain amount of time indicative of unacceptable leakage from said portion, said signal to the computer to indicate such unacceptable leakage being generated by the operation of said set of switch contacts from said another state back to said one state. The system set forth in claim 2 in which said set of switch contacts, when operating from said one state to said another state, causes the computer to cause said pump to turn off. A method in a positive pressure canister purge system associated with an internal combustion (15) engine for confirming the integrity of the system having an engine management computer (18) for controlling certain functions associated with the operation of the engine, a fuel tank (16) for containing a supply of a volatile liquid fuel for the engine, a collection canister (14) for collecting volatile fuel vapors from the fuel tank, purging means (12) operatively controlled by the engine management computer for selectively purging collected fuel vapors from the canister to entrain the fuel vapors with a combustible mixture that passes into combustion chamber space of the engine for combustion therein, the method for detecting unacceptable leakage from the tank/canister comprising the steps of: positively pressurizing the tank/canister portion to a predetermined positive pressure using an electrically operated fuel pump (22); preventing loss of positive pressure from the tank/canister portion back through said pump; detecting the pressure in the tank/canister portion; closing the purging means (12); operating the electric pump (22) under the control of the computer (18) for building a positive pressure in the tank/canister portion; sensing a predetermined positive pressure in the tank/canister portion; stopping, under control of the computer (18) in response to said sensing, the pump from building positive pressure in the tank/canister portion; initiating a predetermined time period for determining if said pressure in the tank/canister portion decreases from said predetermined positive pressure by a certain amount; and then generating a signal to the computer (18) within said time period if said pressure decreases to indicate an unacceptable leakage from the tank/canister portion.

Inventors: BUSATO F MURRAY, COOK JOHN-EDWARD, BUSATO, F. MURRAY
Applicants: SIEMENS ELECTRIC LTD, SIEMENS ELECTRIC LIMITED
Application year: 1992
Main classification: F02M25
IPC codes: F02B77, G01M15, F02M25