Patent Publication Number: US-2015075251-A1

Title: Detecting pressure sensor offset in a phev fuel tank

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure generally relate to Evaporative Emission Control Systems (EVAP) for automotive vehicles, and, more specifically, to improvements in pressure sensors installed within the fuel-tanks of Plug-in Hybrid Electric Vehicles (PHEVs) that incorporate EVAP systems. 
     BACKGROUND 
     Automotive fuel, primarily gasoline, is a volatile liquid subject to potentially rapid evaporation. Thus, the fuel contained in automobile gas tanks presents a major source of potential evaporative emission of hydrocarbons into the atmosphere. Industry&#39;s response to this potential problem has been the incorporation of the evaporative emission control systems into automobiles. These systems are described in more detail below, but generally, they rely upon a canister charged with activated carbon pellets connected to the fuel tank. The pellets absorb fuel vapor, and periodically, the canister is purged by routing fresh air through the canister to the intake manifold. Fuel vapor is entrained by the airflow, and the mixture then proceeds into the engine where it is combusted. 
     Hybrid electric vehicles, including plug-in hybrid electric vehicles (HEV&#39;s or PHEV&#39;s), pose a particular problem for effectively controlling evaporative emissions with this kind of system. Although hybrid vehicles have been proposed and introduced having a number of forms, these designs share the characteristic of providing a combustion engine as backup to an electric motor. Primary power is provided by the electric motor, and careful attention to charging cycles can result in an operating profile in which the engine is only run for short periods. Systems in which the engine is only operated once or twice every few weeks are not uncommon. Purging the carbon canister can only occur when the engine is running, of course, and if the canister is not purged, the carbon pellets can become saturated, after which hydrocarbons will escape to the atmosphere, causing pollution. 
     PHEVs have sealed fuel-tanks, designed to withstand differences in pressure and vacuum within the tank resulting from ambient temperature variations. A fuel-tank pressure transducer (FTPT), which is a high-pressure sensor, is generally mounted within the tank&#39;s interior to monitor fuel system pressure. 
     Every FTPT varies slightly from perfect accuracy, and that variation is referred to as a sensor offset. Such inherent inaccuracy may be as great as 3.5%. Without correction, that inaccuracy can result in unreliable operation of the evaporative emission control system. 
     In conventional gasoline or diesel vehicles fuel-tank vents facilitate sensor offset determination, which is established usually after a prolonged vehicle halt, before an ignition. PHEV fuel-tanks, being sealed, present a greater challenge. 
     Typically, determining the FTPT offset requires either an extended period or runs the risk of venting hydrocarbons to the atmosphere. One method requires the vehicle to remain stationary for an extended period, until the system cools sufficiently for the FTPT to return a zero reading. At that point, true atmospheric pressure can be measured, the difference between true atmospheric and the FTPT reading being the FTPT offset. Not only does that process require considerable time, but a number of readings must be taken to catch the FTPT at exactly zero. Otherwise, measuring a sensor offset requires tank venting, which almost inevitably releases hydrocarbons into the atmosphere. Alternatively, sensor offsets may be learned during refueling, but that has been observed to produce erroneous results, as that operation introduces severe system pressure variations in the internal pressure. In effect, the current circumstances provide an inappropriate platform to gather accurate sensor offset information. 
     Options to determine sensor offset information more accurately thus remain open. 
     SUMMARY 
     The present disclosure provides a method for determining a pressure sensor offset value in an evaporative emission control system. 
     One aspect of the present disclosure relates to a method for determining pressure sensor offset values in an evaporative emission control system. The method is performed during initial vehicle power up, and the method begins by receiving a reference signal indicating existing atmospheric pressure, as well as a check signal from a fuel tank pressure sensor indicating pressure within the fuel tank, at a time when the fuel system is open to atmosphere. A controller then calculates a sensor offset value based upon the reference signal and the check signal, and the controller stores the sensor offset value in a system memory location. During operation of the evaporative emission control system, the controller modifies pressure values received from the fuel tank pressure sensor by applying the sensor offset value. 
     Another aspect of the present disclosure is a method for correcting a stored pressure sensor offset value in an evaporative emission control system. First, the method determines a fuel tank pressure fall off rate, based on at least two fuel tank pressure measurements, the pressure measurements being offset by the stored pressure sensor offset value. Then, the method calculates a predicted fuel tank pressure value at a selected time, based on the fuel tank pressure fall off rate. An existing fuel tank pressure value is then determined at the selected time, and an actual pressure value within the fuel tank is calculated using the stored pressure offset value. Finally, the method corrects the stored pressure sensor offset value by an amount corresponding to any difference between the predicted fuel tank pressure value an actual fuel tank pressure value. 
     Additional aspects, advantages, features and objects of the present disclosure would be made apparent from the drawings and the detailed description of the illustrative embodiments construed in conjunction with the appended claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures described below set out and illustrate a number of exemplary embodiments of the disclosure. Throughout the drawings, like reference numerals refer to identical or functionally similar elements. The drawings are illustrative in nature and are not drawn to scale. 
         FIG. 1  is a schematic view of an exemplary EVAP system installed in a PHEV. 
         FIG. 2  is a flowchart illustrating an exemplary method for gathering a pressure sensor offset value in an evaporative emission control system, according to the present disclosure. 
         FIG. 3  is a flowchart depicting an exemplary method to correct a stored pressure sensor offset value in an evaporative emission control system. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is made with reference to the figures. Exemplary embodiments are described to illustrate the subject matter of the disclosure, not to limit its scope, which is defined by the appended claims. 
     Overview 
     In general, the present disclosure describes a method for determining pressure sensor offset values in an evaporative emission control system in a vehicle. To this end, sensor offset information is obtained during vehicle assembly, at the initial vehicle power-up and before initial refueling. At that point, a powertrain control module (PCM) gathers sensor offset information and stores that within a memory in the vehicle control system. From that point forward, the PCM can apply a sensor offset value during EVAP leak detection and refueling procedures, increasing the accuracy of those readings. 
     Exemplary Embodiments 
     The following detailed description illustrates aspects of the disclosure and its implementation. This description should not be understood as defining or limiting the scope of the present disclosure, however, such definition or limitation being solely contained in the claims appended hereto. Although the best mode of carrying out the invention has been disclosed, those in the art would recognize that other embodiments for carrying out or practicing the invention are also possible. 
     The system set out below proposes a method to determine inherent inaccuracies in PHEV fuel-tank pressure sensors before the vehicle leaves the manufacturing plant. That information is stored in system memory, and it is used during later operation. Throughout this in and in and application, sensor inaccuracies may be interchangeably referred to as sensor offsets. 
       FIG. 1  illustrates a conventional evaporative emissions control system  100 . As seen there, the system  100  is made up primarily of the fuel tank  102 , a carbon canister  110 , and the engine intake manifold  130 , all operably connected by lines and valves  105 . It will be understood that many variations on this busy design are possible, but the illustrated embodiment follows the general practice of the art. It will be further understood that the system  100  is generally sealed, with no open vent to atmosphere. 
     Fuel tank  102  is partially filled with liquid fuel  105 , but a portion of the liquid will evaporate over time, producing fuel vapor  107  in the upper portion (vapor dome  103 ) of the tank. The amount of vapor produced will depend upon a number of environmental variables, such as the ambient temperature. Of these factors, temperature is probably the most important, given the temperature variation produced in the typical diurnal temperature cycle. For vehicles in a warm climate, particularly a hot, sunny climate, the heat produced by leaving a vehicle standing in direct sunlight can produce very high pressure within the vapor dome  103  of the tank  102 . A pressure sensor  106 , known as the FTPT, monitors the pressure in the fuel tank vapor dome  103 . 
     Vapor lines  124  join the various components of the system. One portion of that line, line  124   a  runs from the fuel tank  102  to carbon canister  110 . A normally closed valve  118  regulates the flow of vapor from fuel tank  102  to the carbon canister  110 , so that vapor generated by evaporating fuel can be adsorbed by the carbon pellets under control of a PCM  122 . Vapor line  124   b  joins line  124   a  in a T intersection beyond valve  118 , connecting that line with a normally closed canister purge valve (CPV)  126 . Line  124   c  continues from CPV  126  to the engine intake manifold  130 . CPV  126  is controlled by signals from the powertrain control module (PCM)  122 , which also controls valve  118 . 
     Canister  110  is connected to ambient atmosphere at vent  115 , through a normally closed valve  114 . Vapor line  124   d  connects that  115  in canister  110 . Valve  114  is controlled by PCM  122 . 
     During normal operation, valve  118  is open, while valves  126  and  114  are closed. When pressure within vapor dome  103  rises sufficiently, under the influence, for example, of increased ambient temperature, the PCM opens valve  118 , allowing vapor to flow to the canister, where carbon pellets can adsorb fuel vapor. 
     To purge the canister  110 , valve  118  is closed, and valves  126  and  114  are opened. It should be understood that this operation is only performed when the engine is running, which produces a vacuum at intake manifold  130 . That vacuum causes an airflow from ambient atmosphere through vent  115 , canister  110 , and CPV  126 , and then onward into the intake manifold  130 . As the airflow passes through canister  110 , it entrains fuel vapor from the carbon pellets. The fuel vapor mixture then proceeds to the engine, where it is mixed with the primary fuel/air flow to the engine for combustion, and therefore, is utilized, preventing its release into the atmosphere. 
     FTPT  106  is operably connected to the PCM  122  and is mounted suitably within the interior of the fuel tank  102 . FTPT  106  may be chosen from among the devices widely known and available to the art. As noted, the fuel tank  102  is generally under pressure or vacuum owing to the drive/diurnal cycle. As a result, the sensor readings fluctuate around the atmospheric value. As will be clear from the discussion below, determining the FTPT offset requires a device in addition to the components incorporated in the vehicle evaporative emission control system. Here, an external controller  150  is provided, with connections to FTPT  106  and PCM  122 . Controller  150  includes a reference pressure sensor  152  and control unit  154 . Reference pressure sensor  152  can be a highly accurate pressure sensor, and control unit  154  can be any computing device capable of receiving pressure inputs, calculating the difference between the inputs, and outputting that difference as an offset value. In one embodiment, control unit  154  can be a laptop computer, appropriately programmed, and controller  150  can include appropriate interconnections. 
     Controller  150  receives two inputs. A first input is provided directly from FTPT  106 , indicating that device&#39;s reading of the pressure within vapor dome  103 . A second input originates at reference sensor  152 , indicating atmospheric pressure. Control unit  154  calculates the difference between the FTPT reading and true atmospheric pressure, and it then stores that difference as the sensor offset. The storage location can be any convenient memory location in the automobile system, accessible by the PCM  122 . Appropriate interconnections can be provided to connect controller  150  to PCM  122  and FTPT  106 . 
     In some embodiments, PCM  122  may include a controller (not shown) of a known type connected to the FTPT  106  and the reference sensor  152 . Other sensors may be connected as well. The controller may be of a known type, forming one part of the system hardware, and it may be a microprocessor-based device that includes a central processing unit (CPU) for processing incoming signals from known source. The controller may be provided with volatile memory units, such as a RAM and/or ROM that function along with associated input and output buses. Further, the controller may also be optionally configured as an application specific integrated circuit, or may be formed through other logic devices that are well known to the skilled in the art. More particularly, the controller may be formed either as a portion of an existing electronic controller, or may be configured as a stand-alone entity. 
     In an alternative embodiment, PCM  122  may be programmed to perform the calculations required to determine the sensor offset value. In that embodiment, PCM  122  is operatively connected to reference sensor  152 , in addition to the already existing connection to FTPT  106 . The PCM can then operate as discussed below to determine and stored the sensor offset. 
     The controller may include a memory where information can be stored for periodic retrieval. There, a calculating module (not shown) may be configured, and alongside, algorithm(s) may be installed to carry out specific functions. More particularly, the installed algorithm(s) may be configured to predict a pressure reading inside the fuel tank  102 , at a selected time, based on an initial pressure determined by the pressure sensor  106 . Additionally, a timer (not shown) may operate along with the PCM  122  to facilitate the algorithmic pressure output predictions. 
     The one time and location where the operation set out above can be undertaken without danger of venting hydrocarbons to the atmosphere occurs at the end of the vehicle assembly, before initial refueling. At that point, all components of the evaporative emission control system are installed, yet the fuel tank is empty, preventing any fuel vapor discharge. The process set out below takes advantage of that opportunity. 
     In light of the previous discussion, the disclosure set out below describes an exemplary method of operation of the system  100  for determining and storing a sensor offset value.  FIG. 2  is a flowchart  200  setting out that process. Understandably, the method is discussed in connection with  FIG. 1 . 
     One embodiment of the present method takes advantage of the opportunity described above. Thus, step  201  is performed during automotive assembly, at a point where the components of the evaporative emission control system, as well as the fuel tank have been assembled, yet before fuel is added to the fuel tank. A convenient point for this operation during the manufacturing process occurs toward the conclusion of the vehicle assembly, at the initial power up of the vehicle, just before fuel is added to the tank. 
     At that point, the vehicle under test is connected to controller  150 . As described above, one connection is made to PCM  122  and another to FTPT  106 . 
     Then, at step  202 , the reference sensor  152 , connected externally to the system  100 , senses and indicates an atmospheric pressure value. Step  204  involves receiving a similar signal from FTPT  106 . 
     In some embodiments, the atmospheric pressure and FTPT signals are both provided directly to PCM  122 , while other embodiments route those signals to control unit  152 . Those inputs are employed either by the control unit  152  or PCM  122  to calculate the sensor offset value, as a difference between the two signals, at step  206 . That offset value is stored for future use in system memory, at step  208 . 
     In some embodiments, the present disclosure directs procedures, such as refueling and EVAP leak detection, to exploit provisions of a stored sensor offset value. More often than not, that avoids forcible system venting to correct the offset values, consequently preventing the release of hydrocarbon into the atmosphere. 
     As an example, a failure to accurately learn the sensor offset value can result in refueling difficulties. Typically, refueling requests in PHEVs are initiated by a request interface found on the vehicle&#39;s dashboard. Upon its activation, a fuel tank isolation valve (FTIV) opens to atmosphere, allowing the tank to equilibrate to atmospheric level. Ideally, the pressure sensor  106  signals when atmospheric pressure is reached, but given the inherent offset, pressure sensors are observed to communicate pressure values inaccurately. Only when pressure sensor  106  detects atmospheric conditions within the tank  102  does the system  100  allow a refueling door to be unlocked. An uncorrected sensor shift will not allow the FTPT to reach atmospheric pressure, causing the refueling door to remain undesirably unlocked. A negative sensor offset, on the other hand, may unlock the refueling door, but may cause the existing fuel within the fuel tank to dangerously surge out. 
     Likewise, a failure to correct sensor offsets may increase false flag generation (commonly referred to as alpha/beta error) during EVAP leak detection and monitoring procedures. In response, during operation of the evaporative emission control system (EVAP), pressure values received from the fuel tank pressure sensor may be modified by applying the acquired sensor offset value. 
     An exemplary method to correct offset values, therefore, is set out through a flowchart  300  in  FIG. 3 , and is described below. As noted above, the method described bears close resemblance to a preferred working embodiment of the system  100 . 
     Accordingly, at a first stage  302 , the PCM  122  determines the fuel tank&#39;s pressure fall off rate. Those rates may be understood as the value by which the pressure drips, subject to a vehicular halt, relative to time. To gather that data, exemplarily, at least two fuel tank pressure measurements are noted, one after the other, establishing a time gap between them. Suitable algorithms installed within the PCM  122  can acquire such pressure fall rates. At a next stage  304 , the calculating module may calculate and predict a selected time by when the pressure within the fuel tank must subside to match the atmospheric pressure conditions. Subsequently, at stage  306 , the pressure sensor  106  determines an existing pressure within the fuel tank at the selected time. That determination is however subject to correction given the sensor&#39;s inherent offset. To avoid an erroneous reading, therefore, the calculating module utilizes the stored sensor offset information in the system memory to calculate actual pressure conditions within the fuel tank  102 . That occurs and constitutes stage  308 . 
     In one example, if the stored pressure offset value is 0.5 InH 2 O, and the read pressure value is 1.5 InH 2 O, the calculating module subtracts the offset value from the existing value, and delivers a 1 InH 2 O reading as the corrected and actual pressure value existing within the fuel tank  102 . 
     Based on that output, and the stored pressure offset value, the system corrects the stored pressure sensor offset value by an amount that corresponds to a difference between the predicted fuel tank pressure value and an actual fuel tank pressure value, at a final stage  310 . Operations such as a fuel lid opening request can then be initiated. 
     As an example to the above discussion, if the PCM  122  first reads the tank pressure as 10 InH 2 O, and 30 minute later, as 5 In H 2 O, the PCM  122  will reschedule another reading after another 30 minutes, anticipating a reduction and/or equalization of the fuel tank&#39;s internal pressure to the atmospheric pressure, at that rescheduled time. Having an initial pressure determined, therefore, an approximate pressure sensor reading of a later period is established, and the offset value is suitably corrected then. Advantageously, once stored in the PCM memory, the offset value can be corrected periodically based on a timing strategy configured within the PCM  122  for the EVAP leak detection procedures. 
     Differing configurations of the system  100  may not restrict the PCM&#39;s usability as through known mechanisms someone skilled in the art may form embodiments apart from those described. In effect, despite the system&#39;s customization and/or variation to any known extent, those skilled in art can ascertain ways to incorporate the PCM  122 . 
     The discussed system  100  may be applied to a variety of other applications as well. For example, any similar application, requiring the adherence to stringent emission norms may make use of the disclosed subject matter. Accordingly, it may be well known to those in the art that the description of the present disclosure may be applicable to a variety of other environments as well, and thus, the environment disclosed here must be viewed as being purely exemplary in nature. 
     Further, the system  100  discussed so far is not limited to the disclosed embodiments alone, as those skilled in the art may envision multiple embodiments, variations, and alterations, to what has been described. Accordingly, none of the embodiments disclosed herein need to be viewed as being strictly restricted to the structure, configuration, and arrangement alone. Moreover, certain components described in the application may function independently of each other as well, and thus none of the implementations needs to be seen as limiting in any way. 
     Accordingly, those skilled in the art will understand that variations in these embodiments will naturally occur in the course of embodying the subject matter of the disclosure in specific implementations and environments. It will further be understood that such variations will fall within the scope of the disclosure. Neither those possible variations nor the specific examples disclosed above are set out to limit the scope of the disclosure. Rather, the scope of claimed subject matter is defined solely by the claims set out below.