Patent Publication Number: US-2015075267-A1

Title: Fuel tank pressure sensor rationality test for a phev

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure generally relate to Evaporative Emission Control Systems (EVAP) for automotive vehicles, and, more specifically, to pressure sensors disposed within the fuel tanks of Plug-in Hybrid Electric Vehicles (PHEVs) incorporating EVAP systems. 
     BACKGROUND 
     Gasoline, used as an automotive fuel in many automotive vehicles, is a volatile liquid subject to potentially rapid evaporation, in response to diurnal variations in the ambient temperature. Thus, the fuel contained in automobile gas tanks presents a major source of potential evaporative emission of hydrocarbons into the atmosphere. Such emissions from vehicles are termed ‘evaporative emissions’. The engine produces such vapors even if it is turned off. 
     Industry&#39;s response to this potential problem has been the incorporation of evaporative emission control systems (EVAP) into automobiles, to prevent fuel vapor from being discharged into the atmosphere. EVAP systems include a canister (the carbon canister) containing adsorbent carbon) that traps fuel vapor. Periodically, a purge cycle feeds the captured vapor to the intake manifold for combustion, thus reducing evaporative emissions. 
     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 in a number of forms, these designs all provide a combustion engine as backup to an electric motor. Primary power is provided by the electric motor, and careful attention to charging cycles can produce 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. 
     Further, PHEVs have a sealed fuel tank, designed to withstand differences in pressure and vacuum within the tank resulting from diurnal ambient temperature variations. Sealing the fuel tank in these vehicles is important; otherwise the canister may be excessively loaded with fuel vapors. A Fuel tank pressure transducer (FTPT), being a high pressure sensor, is generally disposed within the fuel tank, to measure tank pressure. As the tank is sealed, the FTPT is always at a specific pressure or vacuum condition, and therefore, it becomes difficult to rationalize the FTPT to atmospheric reference conditions. Also, performing a rationality test to ensure effective operation is difficult in PHEVs, as the tank cannot be easily vented. In any event, venting would not be desirable, as that action could emit hydrocarbons to the atmosphere, causing pollution. Further, during a refueling event, the refueling door of a PHEV is not opened until the fuel-system is depressurized. If the FTPT is not fails in a higher range, the fuel tank depressurization logic will prevent the refueling door from opening since the interior pressure is high. In such cases, the operator may need to manually override the system, requiring a T-handle to open the refueling door. 
     Considering the problems mentioned above, and other shortcomings in the art, there exists a need for a more effective and efficient method and a system for ensuring reliable operations of the fuel tank pressure transducer positioned within the fuel tank of a vehicle. 
     SUMMARY 
     The present disclosure provides a system and a method for ensuring reliable operation of a fuel tank pressure transducer (FTPT) coupled to a fuel tank of a plug-in hybrid electric vehicle. 
     One aspect of the disclosure is a method for verifying reliable operation of a fuel tank pressure transducer coupled to a fuel tank of a PHEV. The method monitors pressure and fuel level within the fuel tank, employing a fuel level sensor providing fuel level signals to a controller, as well as a pressure transducer providing pressure signals to the controller. The system identifies a sloshing event, as indicated by high amplitude, rapidly fluctuating fuel level signals, and it then analyzes the vapor dome pressure signals to determine whether vapor dome pressure signals respond to the sloshing event. Reliable operation of the fuel tank pressure transducer is indicated upon a determination that the vapor dome pressure signals appropriately respond to the sloshing event. 
     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 
         FIG. 1  is a schematic view of a conventional Evaporative Emission Control System configured to reduce vehicle evaporative emissions. 
         FIG. 2  is a schematic view of an exemplary Evaporative Emission Control system configured to ensure reliable operation of a fuel tank pressure transducer coupled to the fuel tank, according to an embodiment of the present disclosure. 
         FIG. 3  illustrates an exemplary method for ensuring reliable operation of a fuel tank pressure transducer coupled to a fuel tank of a vehicle, in accordance with the present disclosure. 
         FIG. 4  is a graph depicting response of a reliably operative fuel tank pressure transducer to fuel sloshing within the tank. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE 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 to the specification. 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. 
     Environmental regulators are steadily tightening the standards for vehicle vapor emissions. Environmental authorities in certain regions, such as California, typically require less than about 500 mg of hydrocarbons released as vehicle evaporative emissions in a standard 3 day test. Given other sources of emissions, the standard effectively limits canister emissions to less than about 200 mg. Euro 5/6 regulations enforce a limit of about 2 grams of evaporative emissions per day. Such stringent conditions demand a highly efficient and effective evaporative emission control system, which in turn should by leakage free. 
     The On-Board Diagnostic regulations mandate that the EVAP system should be regularly checked for leakage. Many conventional EVAP leakage detection monitors use an Evaporative Leak Check Module (ELCM) pump to evacuate the canister side of the system each time the monitor runs. If the fuel tank still holds pressure or vacuum, then one can infer that no leak exists, and the evacuation of the fuel tank is not initiated at all. 
     PHEVs have their fuel tank sealed through a Fuel tank Isolation Valve (FTIV). Sealing the fuel tank is necessary, since the engine running time in such vehicles is limited, and therefore, vaporization of the fuel due to rise in ambient temperature may saturate the carbon pellets in the canister with hydrocarbon vapors, which vapors may eventually escape the atmosphere. A fuel tank pressure transducer (FTPT) is disposed within the fuel tank, to measure pressure within the vapor dome. Because the fuel tank is sealed, its internal pressure is almost always above or below atmospheric pressure, making it difficult to rationalize or calibrate the sensor. 
       FIG. 1  illustrates a conventional evaporative emissions control system  100 . The system is made up primarily of a fuel tank  102 , a carbon canister  110 , and the engine intake manifold  130 , all joined by lines and valves. It will be understood that many variations on this 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 dome portion of the tank. The amount of vapor produced will depend upon a number of environmental variables. Of these factors, ambient temperature is probably the most important, particularly 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 of the tank. A fuel tank pressure sensor (FTPT)  106  monitors the pressure in the fuel tank vapor dome  107 . 
     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 Fuel tank isolation valve (FTIV)  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 the 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 . Both CPV  126  and FTIV  118  are controlled by signals from the powertrain control module (PCM)  122 . 
     Canister  110  is connected to ambient atmosphere at vent  115 , through a normally closed canister vent valve (CVV)  114 . Vapor line  124   d  connects that vent  115  to the canister  110 . Valve  114  is also controlled by PCM  118 . 
     During normal operation, valves  118 ,  126 , and  114  are closed. When pressure within vapor dome rises sufficiently, under the influence, for example, of increased ambient temperature, the PCM opens valve  118 , allowing vapor to flow to the canister  110 , where carbon pellets 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 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. 
       FIG. 2  is a schematic view of an Evaporative Emission Control System  200  of the present disclosure. The structural and functional difference between the illustrated embodiment and the conventional system of  FIG. 1  is as follows. 
     First, the FTPT  118  is coupled to an electronically operated controller  174 , which continuously analyzes the response of the FTPT  118  to fuel sloshing within the fuel tank  102 . Basically, the controller  174  checks whether FTPT  118  detects changes in pressure within the fuel tank  102  during fuel sloshing. 
     To perform EVAP leakage detection, the system  200  includes an Evaporative Leakage Check Module (ELCM). ELCM includes a pump  154 , which may be a vacuum pump of the type commonly employed by the art to evacuate EVAP systems. 
     An additional pressure sensor  158  is also positioned at the flow line  124   d,  to measure fuel-system pressure at the outlet vent. 
     A temperature sensor  166  is positioned within fuel tank  102 , to measure its interior temperature. Though only one temperature sensor  166  is shown, multiple sensors may be employed. An average of the temperature values detected by those sensors can be taken to obtain a more precise measure of the temperature within the interior of the fuel tank  102 . 
     A fuel level indication sensor  178  is provided within the fuel tank  102 , to monitor the level of fuel contained within the tank. 
       FIG. 2  illustrates an EVAP system  200 , according to the present disclosure. This system ensures reliable operation of FTPT  106 . It should be noted that the present disclosure presumes that FTPT  106  has been subjected to, and has passed, the basic operation test. That is, upon startup, or periodically, the system has ensured that a signal is being received from FTPT  106 , and that the signal has been subjected to a basic determination that the reported pressure value lies within an expected range. Thus, if FTPT  106  signals a pressure value outside a predetermined range of potentially valid readings, then the system flags an error for immediate attention. 
     Generally, system  200  identifies a situation in which all the fuel tank  102  experiences a “sloshing” event, and it employs the natural results of sloshing to perform a rationality test on FTPT  106 . As used here, “sloshing” refers to back and forth movement of liquid fuel within the tank, in response to the vehicle&#39;s acceleration or. Sloshing may occur, for example, when the vehicle accelerates or decelerates suddenly on a flat road surface, or takes a sharp turn, or encounters a speed bumps or potholes. Cumulatively, all such events are termed ‘acceleration/deceleration events’ in the discussion that follows. In some embodiments, a controller  174  may be coupled to an acceleration/deceleration sensor positioned within the vehicle for identifying acceleration/deceleration events. In addition, as noted above, controller  174  also continuously receives pressure signals from FTPT  106 . 
     Fuel sloshing is produced by an acceleration or deceleration of the vehicle, which can occur in any direction. A speed bump or pothole, for example, will impose sudden acceleration with a strong vertical component. Turns, particularly sudden or sharp turns, similarly produce acceleration in a horizontal plane. Acceleration in the real world will often have both vertical and horizontal components, all of which will produce wave action within the fuel tank, resulting in sloshing. 
     Sloshing can be detected indirectly, by sensing an acceleration/deceleration event, or directly by monitoring the fuel level. In the illustrated embodiment, the direct method is employed, using a fuel level sensor, such as float sensor  178 . The sensor can directly monitor changes in fuel level that result from an acceleration/deceleration event. It will be understood that float sensor  178  must be sufficiently sensitive to respond to and signal all relatively rapid changes in fuel level that occur during wave action within the fuel tank. Alternatively, the system can employ an acceleration sensor  172 , whose output is coupled to the controller  174 . By arranging several sensors oriented in different planes, any acceleration can be detected and reported. 
     Sloshing involves rapid, turbulent motion, which includes waves impacting the walls of fuel tank  102 . This motion naturally produces fuel vapor, which of course increases pressure within the tank. That increase should be reported by the FTPT  106 . If controller  174  does not receive such an indication, the system concludes that the FTPT  106  is stuck in range, either at a high or a low pressure value, and is unreliable. If the controller does receive a pressure change indication, however, then the system  200  concludes that the FTPT  106  passes the rationality test. 
       FIG. 3  is a flowchart depicting the different steps involved in a method for ensuring reliable operation of a Fuel tank pressure transducer coupled to a fuel tank of a PHEV, according to the present disclosure. 
     As shown, at step  302 , the method detects an acceleration/deceleration event for a vehicle. In certain embodiments, the method detects acceleration/deceleration values only beyond a minimum threshold, so that lesser degrees of fuel sloshing may occur within the fuel tank without triggering the method described here. The method can employ either an acceleration sensor, such as acceleration sensor  172 , or a fuel sensor, such as float sensor  178 , for this purpose. Both of these sensors are well known by an available to the art, adaptable to a variety of operating environments. Also, both sensors are coupled to controller  174  to provide continuous signaling. It will be understood that both acceleration sensor  172  and float sensor  178  are depicted here for completeness, but inoperable installation will most likely contain only one such sensor. 
     At step  306 , the method monitors the level of fuel contained with the fuel tank  102 . As noted above, the fuel level indicator  178  disposed within the fuel tank  102  monitors the fuel level and signals the results. Continuing to step  310 , controller  174  monitors the response of FTPT  106  to fuel sloshing. Controller  174  monitors signals received from FTPT  106 , and it proceeds to analyze those signals to identify significant change in those values to determine the FTPT response. Those of skill in the art will be able to define what constitutes a “slosh” for purposes of the present method. For example, some embodiments may choose to key on the amplitude of a fuel level perturbation to identify slosh, while others may look for a rapid series of up-and-down motions. A combination of such events may be chosen as the most accurate indicator of sloshing. In any event, those in the art will also understand that oversampling, which simply results in additional testing, is a lesser danger than experiencing the failure of FTPT  106 . 
     At step  314 , the system determines whether the FTPT  106  responds in line with the fuel sloshing during the acceleration/deceleration event. Some embodiments may choose a rather a relatively simple comparison, such as looking to the percentage variation of fuel level compared with the percentage variation of pressure. Other systems may use pattern recognition techniques to compare the two responses. Again, it should be emphasized that the objective of the present method is to determine simply that the FTPT  106  is not stuck in range. Relatively sophisticated analysis techniques may not be required here. 
     If FTPT  106  is responding in an acceptable manner, then at step  318 , then the system can infer that the FTPT  106  is operating reliably. If FTPT  106  does not respond acceptably, then the method concludes at step  322  that the FTPT most likely is stuck in range, and is therefore unreliable. The system may then provide an indication to the operator, either visually, audibly or otherwise, that a maintenance technician needs to examine the sensor. 
       FIG. 4  is a graph depicting the response of both FTPT  106  and float sensor to a sloshing event. Here, the upper trace, Graph — 4 response to float sensor  178 , and the lower trace, Graph — 6 response to FTPT  106 . The left half of the chart shows normal operation, with minor variations in the fuel level, and similarly minor variations in the vapor dome pressure. Essentially, the signals reflect the noise of normal operation. Then, Graph — 4 shows high-amplitude, rapidly spaced level changes. The fact that the changes swing to both very high and very low levels reflects the physical fact that an acceleration/deceleration event as generated wave motion inside the fuel tank. That motion rises to extreme levels very quickly, and equally quickly damps out in the absence of further acceleration/deceleration stimulus. Graph — 6, on the other hand, likewise reflects the physical events inside the tank as well. Here, pressure only begins rising after the sloshing event is well underway, because the additional vapor is only released as a result of the sloshing, and thus its accumulation lags the sloshing event itself. While the sloshing action gradually subsides, the pressure level continues to build, because condensation requires some time. Well after the sloshing has ceased, vapor dome pressure levels out. If the chart were to be extended for additional time, one would expect the pressure level to return to the baseline, but only relatively slowly. 
     Given an understanding of the physical phenomena involved, those of skill in the art can readily construct pattern recognition algorithms to first recognize a sloshing event and then to correlate the sloshing event with expected responses from FTPT  106 . With that understanding, the system can reliably conduct a rationality test to determine whether FTPT  106  is stuck in range. 
     The method and the system of the present disclosure is highly effective in ensuring reliable operation of the Fuel tank pressure transducers coupled to the fuel tanks in PHEVs, and easily overcomes problems faced by conventional rationality tests for such vehicles, which majorly rely on venting the fuel tank, or on the engine running time. 
     Although the current invention has been described comprehensively, in considerable details to cover the possible aspects and embodiments, those skilled in the art would recognize that other versions of the invention are also possible.