Patent Publication Number: US-6658925-B2

Title: Initialization method of an automotive evaporative emission leak detection system

Description:
REFERENCE TO RELATED APPLICATION AND PRIORITY CLAIM 
     This is a divisional application of copending application Ser. No. 09/867,760, filed on May 31, 2001, which is a divisional application of application Ser. No. 09/037,784, filed on Mar. 10, 1998 (now U.S. Pat. No. 6,260,410).” 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally, to an on-board system and method for detecting fuel vapor leakage from an evaporative emission space of an automotive vehicle fuel system. 
     BACKGROUND OF THE INVENTION 
     A known on-board evaporative emission control system for an automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, comprises a solenoid actuator that is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit. 
     During conditions conducive to purging, evaporative emission space that is cooperatively defined primarily by the tank headspace and the canister is purged to the engine intake manifold through the canister purge valve. A CPS-type valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine&#39;s combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions. 
     Certain governmental regulations require that certain automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission space. It has heretofore been proposed to make such a determination by temporarily creating a pressure condition in the evaporative emission space which is substantially different from the ambient atmospheric pressure, and then watching for a change in that substantially different pressure which is indicative of a leak. 
     It is believed fair to say that there are two basic types of diagnostic systems and methods for determining integrity of an evaporative emission space against leakage. 
     Commonly owned U.S. Pat. No. 5,146,902 “Positive Pressure Canister Purge System Integrity Confirmation” discloses one type: namely, a system and method for making a leakage determination by pressurizing the evaporative emission space to a certain positive pressure therein (the word “positive” meaning relative to ambient atmospheric pressure) and then watching for a drop in positive pressure indicative of a leak. 
     Commonly owned U.S. Pat. No. 5,383,437 discloses the use of a reciprocating pump to create test pressure in the evaporative emission space. 
     A reed switch is disposed to sense reciprocation of the pump mechanism, and serves both to cause the pump mechanism to reciprocate at the end of a compression stroke and as an indication of how fast air is being pumped into the evaporative emission space. The frequency of switch operation provides a measurement of leakage that can be used to distinguish between integrity and non-integrity of the evaporative emission space. 
     Commonly owned U.S. Pat. No. 5,474,050 embodies advantages of the pump of U.S. Pat. No. 5,383,437 while providing certain improvements in the organization and arrangement of that general type of pump. More specifically, the pump of U.S. Pat. No. 5,474,050: enables integrity confirmation to be made while the engine is running; enables integrity confirmation to be made over a wide range of fuel tank fills between full and empty so that the procedure is for the most part independent of tank size and fill level; provides a procedure that is largely independent of the particular type of volatile fuel being used; provides the pump with novel internal valving for selectively communicating the air pumping chamber space, a first port leading to the evaporative emission space, and a second port leading to atmosphere; and provides a reliable, cost-effective means for compliance with on-board diagnostic requirements for assuring leakage integrity of an evaporative emission control system. 
     The other of the two general types of systems for making a leakage determination does so by creating in the evaporative emission space a certain negative pressure (the word “negative” meaning relative to ambient atmospheric pressure so as to denote vacuum) and then watching for a loss of vacuum indicative of a leak. A known procedure employed by this latter type of system in connection with a diagnostic test comprises utilizing engine manifold vacuum to create vacuum in the evaporative emission space. Because that space may, at certain non-test times, be vented through the canister to allow vapors to be efficiently purged when the CPS valve is opened for purging fuel vapors from the tank headspace and canister, it is known to communicate the canister vent port to atmosphere through a vent valve that is open when vapors are being purged to the engine, but that closes preparatory to a diagnostic test so that a desired test vacuum can be drawn in the evaporative emission space for the test. Once a desired vacuum has been drawn, the purge valve is closed, and leakage appears as a loss of vacuum during the length of the test time after the purge valve has been operated closed. 
     In order for an engine management computer to ascertain when a desired vacuum has been drawn so that it can command the purge valve to close, and for loss of vacuum to thereafter be detected, it is known to employ an electric sensor, or transducer, that measures negative pressure, i.e. vacuum, in the evaporative emission space by supplying a measurement signal to the engine management computer. It is known to mount such a sensor on the vehicle&#39;s fuel tank where it will be exposed to the tank headspace. For example, commonly owned U.S. Pat. No. 5,267,470 discloses a pressure sensor mounting in conjunction with a fuel tank roll-over valve. 
     SUMMARY OF THE INVENTION 
     One generic aspect of the present invention relates to a method of initializing an evaporative emission space of a fuel storage system of an automotive vehicle preparatory to performing a leak detection test on the evaporative emission space, the vehicle being powered by a fuel-consuming engine and including an evaporative emission control system for purging fuel vapor from the evaporative emission space to the engine for combustion therein during conditions conducive to purging, the method comprising: creating a differential between pressure in the evaporative emission space and atmospheric pressure sufficient to perform a leak detection test; varying, over time, the created pressure differential within a range of differential pressures sufficient to perform a leak detection test; and then isolating the evaporative emission space from communication with both the engine and atmosphere, and performing a leak detection test. 
     Another generic aspect of the present invention relates to an engine-powered automotive vehicle evaporative emission control having a fuel storage system comprising an evaporative emission space for containing volatile fuel vapors generated by the volatilization of fuel in the storage system and a purge valve for purging fuel vapors from the evaporative emission space to an engine for combustion therein during conditions conducive to purging, including a leak detection system for detecting leakage from the evaporative emission space which comprises: a selectively operable prime mover for pumping gaseous fluid with respect to the evaporative emission space; a selectively operable valve which operates to a first condition for allowing the prime mover to move gaseous fluid with respect to the evaporative emission space, and to a second condition disallowing the prime mover from moving gaseous fluid with respect to the evaporative emission space; and a sensor providing an electric signal related to pressure in the evaporative emission space for controlling operation of the prime mover and of the valve; the leak detection system initializing the evaporative emission space preparatory to performing a leak detection test by causing the pump to create a differential between pressure in the evaporative emission space and atmospheric pressure sufficient to perform a leak detection test, including varying, over time, the created pressure differential within a range of differential pressures sufficient to perform a leak detection test, and then isolating the evaporative emission space from communication with both the engine and atmosphere and performing a leak detection test. 
     The foregoing, and other features, along with various advantages and benefits of the invention, will be seen in the ensuing description and claims which are accompanied by drawings. The drawings, which are incorporated herein and constitute part of this specification, disclose a preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general schematic diagram of an automotive vehicle evaporative emission control system including a leak detection system embodying principles of the invention. 
     FIG. 2 is a more detailed schematic diagram of a portion of the system. 
     FIG. 3 is a representative graph plot useful in explaining certain principles of the invention. 
     FIG. 4 is a flow diagram illustrating steps according to principles of the invention. 
     FIG. 5 is a flow diagram like that of FIG. 4, illustrating expanded detailed of a portion of that flow diagram. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an automotive vehicle evaporative emission control (EEC) system  10  in association with an internal combustion engine  12  that powers the vehicle, a fuel tank  14  that holds a supply of volatile liquid fuel for the engine, and an engine management computer (EMC)  16  that exercises certain controls over operation of engine  12 . EEC system  10  comprises a vapor collection canister (charcoal canister)  18 , a proportional purge solenoid (PPS) valve  20 , a leak detection module (LDM)  22 , and a particulate filter  24 . In the illustrated embodiment, LDM  22  and canister  18  are shown as an integrated assembly, or module,  25 . 
     Headspace of fuel tank  14 , a port  25   a  of module  25 , and an inlet port  20   a  of PPS valve  20  are placed in common fluid communication by a conduit  26  so that the tank headspace and the canister cooperatively define evaporative emission space within which fuel vapors generated by volatilization of fuel in tank  14  are temporarily confined and collected until purged to an intake manifold  28  of engine  12 . Another conduit  30  fluid-connects an outlet port  20   b  of PPS valve  20  with intake manifold  28 . Another conduit  34  fluid-connects a port  25   b  of module  25  to atmosphere via filter  24 . 
     EMC  16  receives a number of inputs (engine-related parameters for example) relevant to control of certain operations of engine  12  and its associated systems, including EEC system  10 . One electrical output port of EMC  16  controls PPS valve  20  via an electrical connection  42 ; other ports of EMC  16  are coupled with module  25  via electrical connections, depicted generally by the reference numeral  44  in FIG.  1 . 
     From time to time, EMC  16  commands LDM  22  to an active state as part of an occasional leak detection test procedure for ascertaining the integrity of EEC system  10 , particularly the evaporative emission space that contains volatile fuel vapors, against leakage. During occurrences of such a diagnostic procedure, EMC  16  commands PPS valve  20  to close. At times of engine running other than during such leak detection procedures, LDM  22  reposes in an inactive state, and in doing so provides an open vent path from the evaporative emission space, through module  25  and filter  24 , to atmosphere. A vapor adsorptive medium within canister  18  prevents escape of fuel vapor to atmosphere during such venting. 
     EMC  16  selectively operates PPS valve  20  such that the valve opens under conditions conducive to purging and closes under conditions not conducive to purging. Thus, during times of operation of the automotive vehicle, the canister purge function is performed in a known manner for the particular vehicle and engine so long as the leak detection test procedure is not being performed. When the leak detection test procedure is being performed, the canister purge function is not performed. During a leak detection test, the evaporative emission space is isolated from both atmosphere and the engine intake manifold so that it can be initially pressurized by LDM  22 , and the pressure thereafter allowed to decay if leakage is present. 
     LDM  22  comprises a solenoid-operated valve  78 ; an electric motor  80 , a D.C. motor in the disclosed embodiment for use with an automotive vehicle D.C. electric system; and an electric sensor  82  for supplying an electric signal related to a fuel vapor parameter, the, disclosed embodiment being a pressure switch that supplies a signal related to vapor pressure to EMC  16 . 
     A pumping mechanism that comprises an impeller is operated by motor  80 . This construction forms a variable displacement pump for pumping gaseous fluid, i.e. a blower, that is designated by reference numeral  89  in FIG.  2 . 
     A pressure switch that has a certain pre-defined hysteresis in its switching characteristic is particularly well-suited for use as sensor  82 . Such a sensor comprises a first pressure sensing zone communicated to port  25   b , and a second pressure sensing zone communicated to the same portion of canister  18  to which port  25   a  is communicated. Sensor  82  assumes a first switch state (open for example) so long as the pressure difference between its two sensing zones is less than a certain magnitude. When that magnitude is exceeded, the sensor operates to a second switch state (closed for example). The sensor possesses a certain hysteresis in its switching characteristic whereby it will switch back to its first state only when the magnitude of the pressure difference between its two sensing zones returns to a certain magnitude that is smaller by a predetermined amount than the magnitude at which it switched from its first state to its second state. 
     The “dirty air” side of the vapor adsorbent medium within canister  18  is in continuous communication with port  25   a . Hence, when valve  78  is not being energized, the earlier-mentioned vent path to atmosphere through module  25  is open because there is no significant flow restriction between ports  25   a  and  25   b . FIG. 2 schematically depicts the organization and arrangement of blower  89 , valve  78 , canister  18 , and sensor  82  in the flow path. It can be seen that blower  89  and valve  78  are on the clean air side while sensor  82  is on the dirty air side. 
     When valve  78  is energized, the vent path to atmosphere is closed. Energization of its solenoid closes valve  78 , blocking communication between port  25   a  and the pump. 
     When no leak detection test is being performed, PPS valve  20  is operated by EMC  16  to periodically purge vapors from canister  18  and the tank headspace to engine  12 . The exact scheduling of such purging is controlled by the vehicle manufacturer&#39;s requirements. During non-test times the vent path to atmosphere is open through module  25  and filter  24  so that the evaporative emission space is communicated to atmosphere, keeping the evaporative emission space generally at atmospheric pressure. 
     Preparatory to performing a leak detection test on EEC system  10 , PPS valve  20  is operated closed by EMC  16 . EMC  16  also commands operation of motor  80  to rotate impeller  88 . Valve  78  remains de-energized, causing the internal flow path between ports  25   a  and  25   b  to be open. The operation of the pump by motor  80  begins building pressure in the evaporative emission space comprising headspace of tank  14 , canister  18 , and any spaces, such as associated conduits, that are in communication therewith. Naturally all closures, such as the vehicle tank filler cap, must be in place to close the evaporative emission space under test except for the air being pumped into it via module  25 . By being exposed to port  25   a , the second sensing zone of sensor  82  is exposed to a pressure representative of the pressure in the evaporative emission space under test. 
     If there are no conditions, such as a “pinched line” or a “gross leak” for example, that prevent a pre-defined test pressure programmed into EMC  16  from being created in the evaporative emission space within a pre-defined window of time along a time line commenced by internal counting within EMC  16  at the beginning of the initialization of the evaporative emission space preparatory to performing a leak detection test, sensor  82  will eventually switch from its first state to its second state to signal that the pre-defined initial test pressure has been reached At that time, EMC  16  throttles down motor  80 , either partially or entirely. 
     The graph plot  200  of FIG. 3 shows a representative plot of evaporative emission space pressure referenced to atmosphere as a function of time. The initialization phase comprises the time intervals marked A and B. Time interval A shows the increasing positive pressurization of the evaporative emission space from the beginning of the initialization until sensor  82  switches to its second state at the predefined initial test pressure. The beginning of time interval B depicts a condition where motor  80  is operated so as to throttle down blower  89 , causing the pressure in the evaporative emission space to begin to decrease. 
     When the pressure has decreased sufficiently to cause sensor  82  to revert to its first state, EMC  16  causes motor  80  to throttle up, causing the pressure in the evaporative emission space to again increase. Time interval B is characterized by several of these cycles of throttling the blower up and down, as portrayed in FIG.  3 . Such cycling has been found beneficial in achieving improved stability of the pressure in the evaporative emission space at the commencement of a leak detection test. However, by programming EMC  16  with different algorithms, improved stability may be obtained is various ways to be described later. 
     The end of time interval B marks the end of the initialization phase. The leak detection test commences at the beginning of time interval C at which time valve  78  is operated closed and blower  89  throttled down to the extent of complete shut off. Because PPS valve  20  has been closed during time intervals A and B, the closure of valve  78  results in isolation of the evaporative emission space from both atmosphere and the engine. 
     Had a pressure within a range of pressures suitable for performing a leak detection test not been attained in the evaporative emission space by the end of a certain amount of time after commencement of the initialization phase at time 0.0 seconds, a “gross leak” would have been indicated and the ensuing leak detection test aborted. Had a pre-defined pressure been attained before a pre-defined minimum time after commencement of the initialization phase at time 0.0 seconds, a “pinched line” would have been indicated and the ensuing leak detection test aborted. 
     On the other hand, if there is neither a gross leak nor a pinched line, and if some leakage from the evaporative emission space actually exists, that leakage will cause the pressure to begin dropping, as represented by the portion of graph plot  200  during time interval C. At the beginning of time interval C, EMC  16  commences timing a pre-defined test time duration. If the evaporative emission space pressure has not decayed sufficiently to cause sensor  82  to revert to its first state by the end of this pre-defined time duration, the evaporative emission space is deemed to have successfully passed the test. Should the pressure decay to a level causing sensor  82  to revert to its first state, such reversion is detected by EMC  16  and the EMC logs the time at which this occurred. EMC  16  can then calculate the extent of leakage and indicate whether the test has been passed or failed. 
     In the example illustrated by graph plot  200 , it is shown that time interval B comprises cycling of the pressure within a range between 3.2 millibars and 3.0 millibars. Hence 3.1 millibars may be considered a nominal pressure above and below which the created pressure repetitively alternately increases and decreases. This varying of pressure occurs over a range of positive pressures that are sufficiently high to enable a leak detection test to proceed, and has been found to promote repeatable accuracy of test results. 
     Because the amount of liquid fuel in the fuel tank influences the volume of the tank headspace, and hence evaporative emission space volume, a tank with less liquid fuel will take longer both to pressurize and to de-pressurize than one with more liquid fuel. Therefore, in order to obtain a proper measurement of effective leak size, compensation for the amount of liquid fuel in the tank is part of the disclosed procedure. 
     Variation in tank fuel vapor pressure may also affect test results. Incorporation of the inventive principles into a negative pressurizing system would tend to promote fuel volatilization when vacuum is drawn, and if volatilization were significant, correction for it might be appropriate. On the other hand, a system like the one described, that positively pressurizes the evaporative emission space for a test, tends to inhibit fuel volatilization. For practical purposes, such a positive pressure system is believed not to require fuel volatilization correction in light of the expectation that testing will be repeatedly periodically conducted over times that include times of non-volatilization of fuel when a test will give a true leak measurement, free of influence by volatilizing fuel. During times of fuel volatilization however, a test will give a measurement which, although affected by volatilizing fuel, will be smaller than the true leak size, and therefore will not cause a fault to be flagged. In the case of a negative pressure system, a fault would be flagged and might result in an unnecessary and wasteful visit to a service facility. Overall considerations therefore suggest that positive pressurization may be more robust and is to be preferred. 
     Altitude variations can be corrected in vehicles that have MAP sensors because such sensors have the capability of approximating altitude. Correction is made by a suitable algorithm. 
     FIG. 4 discloses a flow diagram representing another algorithm for conducting a leak detection test. Steps  300 ,  302  correspond to the stabilization procedure that is performed during time intervals A and B in FIG. 3 while PPS valve  20  is closed. The stabilization time may be 120 seconds by way of example. Once pressure stability has been achieved, step  304  executes to assure that the vehicle is static, i.e. at a complete stop with the engine confirmed at idle. Step  306  provides that the vehicle should be static for a certain amount of time before the test proceeds. Should the vehicle start to move at any time during a test, that test will be aborted and the algorithm will revert back to the stabilization, or initialization, phase. 
     Once the required static time has elapsed, step  308  executes. This step comprises operating valve  78  closed, followed by shutting down blower  89 . Leakage will cause the pressure in the space under test to decrease. When the pressure trips the upper switch set point, a leak-down timer is started. When the pressure trips the lower switch set point, the time that has been counted by the leak-down timer represents a leak-down time measurement. That measurement is a measure of system leakage and is dependent on system volume. 
     Step  312  compares the leak-down time measurement obtained from step  308  with a preset time, ten seconds in this example. If the leak-down time measurement is less than that preset time, the test proceeds directly to step  316 . 
     On the other hand, if the timer times to that preset time without the pressure having tripped the lower switch set point, then an extrapolation technique is employed to predict an extrapolated leak-down time. That technique comprises pulsing PPS valve  20  (step  314 ) until the pressure in the system under test trips the lower switch set point. The number of pulses needed to cause tripping of the lower switch set point constitutes a pulse count PC 1 , which is a measure both of remaining pressure and tank volume. After pulse count PC 1  has been obtained, the algorithm proceeds to step  316 . 
     Step  316  comprises re-pressurizing the tank to the upper switch set point for several seconds. Step  318  follows, and comprises pulsing PPS valve  20  until the pressure trips the lower switch set point. The number of pulses needed to cause such tripping constitutes a pulse count PC 2  which is primarily a measure of system volume. After pulse count PC 2  has been obtained, the algorithm proceeds to step  320 . 
     Step  320  comprises a calculation step that executes the formula shown therein to calculate an extrapolated leak-down time. 
     Step  322  applies volume compensation either to the actual leak-down time measurement obtained from step  308  or to the extrapolated leak-down time obtained from step  320 , as the case may be depending on the result of step  312 . Step  322  performs the calculation indicated therein. The numbers 2.15 and 9.6 are values of respective parameters k 1 , k 2  that are specific to the particular vehicle system. The final result of step  322  is a value that is compared to a value representing a demarcation between acceptable (pass) and unacceptable (fail) leakage. 
     FIG. 5 discloses further detail of a portion of the algorithm of FIG. 4, and like reference numerals are used to identify like steps. Steps  308  and  312  have been expanded in FIG. 4 to expressly illustrate the monitoring of certain conditions. Decision block  308 A denotes monitoring the status of sensor  82  for the purpose of ascertaining whether the lower switch set point has been tripped. Decision block  308 B denotes monitoring for continued vehicle stability. Decision block  312  denotes monitoring the time being counted by the leak-down timer and includes a showing of the iterative return that occurs when the time has not yet elapsed to the 10 second time limit. 
     When decision block  308 A is encountered, sensor  82  is monitored to ascertain whether or not the lower switch set point has been tripped. If not, step  308  continues by monitoring for continued vehicle stability (decision block  308 B). Continued stability causes decision block  312  to be encountered. As long as the 10 second time limit has not elapsed, step  308  reiterates. Should the lower switch set point be tripped before the 10 second time limit has been reached, the elapsed time is recorded (block  308 C of FIG.  5 ), and the algorithm advances to step  316 . If, at any time during the iterations of step  308 , stability of the vehicle is not confirmed, the test is aborted (block  308 D of FIG.  5 ), and the algorithm reverts to the initialization phase. If the 10 second time limit is reached, without the lower switch set point having been tripped, the algorithm advances to step  314 . 
     The pulsing that occurs during step  314  comprises one or more cycles of opening and then re-closing valve  20 , the open and closed time intervals being precisely defined so that the ensuing flow through the valve is similarly defined. The number of such cycles required to reduce the pressure in the evaporative emission space to a pressure that trips the lower switch set point forms pulse count PC 1 . An example of a sensor  82  that is suitable for the implementation of the algorithm is an MPL 9300 Series switch. Such a switch may be set to correlate the upper switch set point to 10 inches H 2 O pressure, by way of example, and the lower switch set point to 9.5 inches H 2 O pressure, by way of example. 
     The algorithm of FIGS. 4 and 5 is advantageous because it uses extrapolation and volume compensation techniques. It can reduce the overall amount of time required for performance of a leak detection test irrespective of effective leak area, evaporative emission space volume, and relative proportions of liquid and vapor in the fuel tank. 
     While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles are applicable to other embodiments that fall within the scope of the following claims.