Abstract:
An arrangement for determining purge valve flow tolerance for use with evaporative emissions control systems includes developing an equation based on data relating purge valve duty cycle to flow, wherein the equation describes a flow curve with reference to a first axis and a second axis. The arrangement further includes using the equation as a base equation for flow, and adapting the equation for part-to-part tolerance as a function of an intercept point of the equation with respect to the first axis, wherein the first axis relates to duty cycle.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to systems and methods of purge valve flow control for use with evaporative emissions control systems, and particularly relates to systems and methods for determining purge valve flow tolerance.  
         BACKGROUND OF THE INVENTION  
         [0002]    Variable flow rate purge valves used in evaporative emissions control systems suffer from problems relating to part-to-part tolerance. For example, flow rate at low duty cycle can vary from part-to-part by as much as three-hundred percent, resulting in serious drivability issues at low engine speeds, especially with smaller displacement engines. As a result, past evaporative emissions control systems have not been able to utilize less expensive purge valves with less demanding part-to-part tolerance. As a further result, other solutions have focused on utilizing constant flow rate purge valves to avoid the tolerance issue.  
           [0003]    What is needed is a solution that allows evaporative emissions control systems to utilize less expensive purge valves with less demanding part-to-part tolerance. Thus, the need remains for an adaptive system and method of purge valve control that is capable of determining purge valve tolerance and operating accordingly. The present invention fulfills this need.  
         SUMMARY OF THE INVENTION  
         [0004]    In one aspect of the invention, a method for determining purge valve flow tolerance for use with evaporative emissions control systems includes developing an equation based on data relating purge valve duty cycle to flow, wherein the equation describes a flow curve with reference to a first axis and a second axis. The method further includes using the equation as a base equation for flow, and adapting the equation for part-to-part tolerance as a function of an intercept point of the equation with respect to the first axis, wherein the first axis relates to duty cycle.  
           [0005]    In another aspect of the invention, a purge valve flow tolerance determination system for use with evaporative emissions control systems includes a memory storing an equation based on data relating purge valve duty cycle to flow, wherein the equation describes a flow curve with reference to a first axis and a second axis. A purge valve operation module is adapted to operate a purge valve based on the equation. An adaptation module adapts the equation for part-to-part tolerance as a function of an intercept point of the equation with respect to the first axis, wherein the first axis relates to duty cycle.  
           [0006]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0008]    [0008]FIG. 1 is a flow diagram depicting the general method of determining purge valve flow tolerance according to the present invention;  
         [0009]    [0009]FIG. 2 is a graph depicting a purge flow curve according to the present invention;  
         [0010]    [0010]FIG. 3 is a flow diagram depicting the detailed method of determining purge valve flow tolerance according to the present invention; and  
         [0011]    [0011]FIG. 4 is a block diagram depicting the purge valve flow tolerance system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0013]    The general method of the present invention is illustrated in FIG. 1. Beginning at  10 , the method includes developing an equation describing a flow curve at step  12  based on data  14  relating duty cycle to flow. For example, preexisting data  14 , such as provided by an existing lookup table, can be fitted into a point slope equation to determine a slope. Exemplary lookup table data of Table 1 is plotted in FIG. 2 as an example, wherein the X-axis relates to flow rate and the Y-axis relates to duty cycle.  
                           TABLE 1                                   X   Y                           1.0   2.0           2.5   3.0           4.0   4.0           5.5   5.0           7.0   6.0           8.6   7.0                      
 
         [0014]    The data points of Table 1 are substantially linear, as is typical of purge valve behavior, so that development of a flow curve corresponding to line  16  is appropriate. The data point  18  at the higher duty cycle/flow rate end of the curve diverges slightly, as is consistent with a purge valve that has fully opened before reaching the corresponding duty cycle. Thus, it is further appropriate to discard data point  18 , and develop an equation describing line  16  by fitting other data of Table 1 into a point-slope equation, (y 2 −y 1 )=m(x 2 −x 1 ). It follows that (6−2)=m(7−1) such that m=2/3. It is possible to additionally determine, either by solving the point-slope equation for an x value of zero, or from y-intercept equation, y=mx+b, that the y-intercept at  20  is 4/3 for the data set of Table 1. This particular y-intercept information, however, is not generally needed to develop the equation, but may be useful in some contexts relating, for example, to adaptation of an existing lookup function based on the equation.  
         [0015]    The y-intercept equation and the known value of m suffice to describe any corresponding curve simply by determining an appropriate y-intercept as at  22 A and  22 B, where y-intercepts respectively correspond, for example, to 5/2 and 0 at  24 A and  24 B. A new data set can thus be obtained, if desired, for any y-intercept. Alternatively, a particular duty cycle for a desired flow rate can be determined for any y-intercept. Notably, only two data points are needed to develop the equation where the duty cycle to flow relationship is reliably linear. Thus, if data  14  must be collected, it is conceivable that only two measurements need be taken to provide the necessary data  14 . Thus, the process of applying pulse width modulated current to a purge valve, modifying a duty cycle of the current, and measuring a plurality of rates of flow through the purge valve for a plurality of duty cycles can be simplified compared to developing a lookup table.  
         [0016]    Returning to FIG. 1, the method includes operating the purge valve in step  26  based on the equation describing the curve. During operation at step  26 , the method includes adapting the equation at step  28  as a function of an intercept point on the axis relating to duty cycle. Thus, a subroutine is invoked to determine the intercept point on the Y-axis when the valve initially opens by sensing an initial opening of the valve based on rate of flow, and by equating a duty cycle producing the initial opening with the intercept point on the Y-axis. The invoked subroutine closes the vent valve of the purge canister, drives the closed purge valve to a point of initial opening, saves the final duty cycle as the value for the Y-intercept b, opens the vent valve, and then returns control to step  26 . Therein, the purge valve is opened to a desired flow rate by determining the corresponding duty cycle using the Y-intercept form of the equation adapted by incorporation of the new value for b. The method ends at  30 .  
         [0017]    Further discussion relating to methodology employed in steps  26  and  28  occurs with reference to FIG. 3, wherein the detailed method of the present invention is illustrated. This detailed method begins at  32  and proceeds with closing of the vent valve at step  34  to produce a sealed environment under pressure regulated solely by operation of the purge valve. Then, with the vent valve closed, an initial duty cycle is applied to the purge valve at step  36 . The method includes waiting a predetermined amount of time at step  38  to allow the flow rate to respond to the applied duty cycle. Then, at step  40 , a flow rate is sensed that results from application of the duty cycle in step  36 , preferably by measuring changes in fuel vapor through O2 feedback at the exhaust.  Model Based Purge System,  U.S. Pat. No. 6,253,750, issued to Duty et al. contains details relating to measuring changes in fuel vapor through O2 feedback at the exhaust, and is incorporated by reference herein. If the fuel vapor level is not determined at  42  to be higher than a predetermined value corresponding to a minimum detectable fuel vapor level, then the duty cycle is incremented by a predetermined amount at step  44 , and processing continues at step  36 . If, however, the fuel vapor level is determined at  42  to be higher than the predetermined value, then the y-intercept point, b, is set equal to the duty cycle applied at step  36 . The vent valve is then opened at step  48 , and the detailed method ends at  50 .  
         [0018]    The following pseudo code serves to particularly describe the method of the present invention. Therein, PRG_DC refers to duty cycle applied to the purge valve, PRG_VLV_FRAC refers to fuel vapor level delivered to the engine by the purge valve, and PRG_ACT_FLOW_INTCPT refers to the point on the y axis relating to a duty cycle beginning to produce a detectable flow rate:  
                                                                           static void adapt ( ){         close (NVLD_VENT_VALVE); //close vent valve on purge canister         while ((PRG_VLV_FRAC &lt; PRG_MIN_VLV_DETECT)==TRUE)              {    //perform the following until an increase in flow rate is reliably           //detected           PRG_DC = (PRG_DC + PRG_FLOW_INC_AMT); //increment duty                                  //cycle           sleep(PRG_FLOW_INC_TIME); //wait for flow rate to respond to                          //new duty cycle           update(PRG_VLV_FRAC)//use O2 feedback to measure flow           if ((PRG_VLV_FRAC &gt; PRG_MIN_VLV_FRAC_DETECT)==TRUE)                {   //if an increase in flow rate is reliably detected               PRG_ACT_FLOW_INTCPT = PRG_DC; //save duty cycle as                                  //y-intercept                }           else{}              }         open (NVLD_VENT_VALVE); //open vent valve on purge canister       }                  
 
         [0019]    The system of the present invention is illustrated in FIG. 4. The purge valve flow tolerance system  52  has an equation  54  stored in memory that describes a flow curve for the purge valve. The slope, m, is particularly stored in memory to describe this curve, but the Y-intercept, b, is determined as needed to adjust for purge valve variability. Purge valve operation module  56  is adapted to determine a duty cycle for operating purge valve  60  via pulse wave modulated power source  62  by using equation  54 , a stored value of m (not shown), and an adaptively computed value for b  64 . Whenever purge valve operation module  56  needs to determine the tolerance of purge valve  60  (e.g., once per trip), it closes purge valve  60 , if required, and passes operation of purge valve  60  to adaptation module  68 . In turn, adaptation module  68  closes a vent valve (not shown), and drives the duty cycle  58  incrementally higher until flow detection returns a sensed fuel vapor level  74  that indicates purge valve  60  has begun to open according to adaptation module  68  being operable to detect flow through the purge valve based on detection of fuel vapor. The corresponding duty cycle is then passed to equation  54  as the new value of b  64 . Adaptation module  68  then opens the vent valve as at  76  and returns operation of purge valve  60  to purge valve operation module  56 .  
         [0020]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, while variable flow rate purge valves generally produce linear flow curves, it is foreseeable that non-linear flow curves may also be developed and used in accordance with the present invention. In particular, a flow curve corresponding to a parabola, y=x 2 +x+b, also corresponds to y=(x+1)x+b, so that m=(x+1). Thus, a non-linear data set can be used, and existing software tools can be implemented, if necessary, to convert a set of data points into an appropriate curve. If required, the slope can be defined as a step function. Also, the system and method of the present invention can employ more than one axis if the flow rate at a particular duty cycle is additionally related, for example, to temperature. Thus, three or more axes can be used to derive an equation of three or more variables from data controlling for those variables. Further, the system and method of the present invention can still use an existing map, but adjust lookup functions based on the developed equation. In particular, the lookup method, when passed a desired flow rate, can subtract the original y-intercept from the returned duty cycle and add the new y-intercept to arrive at the appropriate value. Such variations are not to be regarded as a departure from the spirit and scope of the invention.