Patent Publication Number: US-2015085894-A1

Title: Method for diagnosing fault within a fuel vapor system

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure generally relate to methods and systems for detecting leakage within EVAP systems, and, more specifically, to methods and systems for identifying the cause of leakage within 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 emission of hydrocarbons into the atmosphere. Such emissions from vehicles are termed ‘evaporative emissions’, and those vapors can be emitted even when the engine is not running 
     In response to this problem, industry has incorporated evaporative emission control systems (EVAP) into automobiles. EVAP systems include a “carbon canister” containing adsorbent carbon pellets that trap fuel vapor by adsorbing it onto the pellets. 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. 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. Careful users can achieve results in which the engine is only operated once or twice every few weeks. 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. 
     Leaks can occur in an EVAP system, however, leading to problems, problems in carrying out the functions such as purging without discharging hydrocarbons into the atmosphere. C. Vehicles are required to implement diagnostics that check for leaks of at least 0.040″, and some states require testing for leaks down to 0.020″. One method for performing leak diagnostics employs an on-board pump that evacuates the EVAP system; measuring any ensuing vacuum bleed-up identifies any possible system leaks. Knowing that a leak is present, however, does not materially assist in curing the problem. 
     Thus, the art does not provide a method that will both determine whether a leak exists and point the way to a probable cause. 
     SUMMARY 
     According to an aspect of the disclosure, the present disclosure provides a method for diagnosing a fault within an evaporative emission control system of an automotive vehicle. The method monitors the carbon canister temperature though a temperature sensor, during a system leak test. If the fuel vapor system fails to achieve a target vacuum during the leak test, the method generates a temperature response of the carbon canister. Further, the method infers a likely cause of the failure based on the temperature response of the carbon canister. If the temperature decreases, then the method concludes a fault due to an open canister vent valve or a leakage port within a first communication line. If the temperature increases, then the method concludes a fault due to a leakage port within a fuel tank, or a leakage port within a second communication line. If the temperature remains substantially constant, then the method concludes a fault sue to a closed canister purge valve or a leakage port within a third communication line. 
     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 representation of an evaporative emission control system of a vehicle, according to an aspect of the present disclosure. 
         FIG. 2  is a flowchart describing a method for diagnosing a fault within an evaporative emission control (EVAP) system. 
         FIGS. 3A and 3B  are graphs illustrating the temperature response and pressure response in case of no fault-in the EVAP system of the present disclosure. 
         FIGS. 4A and 4B  are graphs illustrating the temperature response and pressure response in case of a fault due to a leakage port in the fuel tank or a broken communication line on the fuel tank side. 
         FIGS. 5A and 5B  are graphs illustrating the temperature response and the pressure response in case of a fault due to an open canister vent valve or a broken vent line. 
         FIGS. 6A and 6B  are graphs illustrating the temperature response and the pressure response in case of a fault due to a closed CPV or a broken purge line. 
     
    
    
     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 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. 
     In general, the present disclosure capitalizes upon the fact that the adsorption of hydrocarbon vapor in the pellets of the carbon canister is an exothermic reaction. The opposite is true, of course, when a fresh air flow through the carbon canister entrains hydrocarbons from the carbon pellets, resulting in a drop in canister temperature. It has thus been discovered that one can infer the cause of a vacuum test failure by monitoring the canister temperature. 
       FIG. 1  illustrates a conventional evaporative emission control system  100  of a PHEV. 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 joined by communication lines  124   a.  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 evaporates over time, producing fuel vapor  107  in the upper portion (vapor dome  103 ) of the tank. The amount of vapor produced depends upon a number of environmental variables, such as the ambient temperature. Of these factors, temperature is probably the most important, particularly given the temperature variation produced in the typical diurnal temperature cycle. For vehicles in a sunny 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. A fuel tank pressure transducer (FTPT)  106  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 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. Vapor line  124   b  joins line  124   a  in a T intersection on the canister side of the FTIV  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 . A powertrain control module (PCM)  122  controls the operations of CPV  126  and FTIV  118 . Also, PCM  122  receives input signals from FTPT  106  and other sensors as mentioned below. PCM  122  can be a standalone element, but in the illustrated embodiment it is part of the overall vehicle control system, which performs a variety of functions for the automobile. As such, PCM  122  is capable of commanding operational signals, such as opening and closing valves, as well as calculations and data storage functions. 
     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, valves  118 ,  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 , FTIV  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 The vacuum present in intake manifold  130  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 resulting fuel vapor/air mixture proceeds to the engine, where it is mixed with the primary fuel/air flow to the engine for combustion. 
     The canister  110  includes a temperature sensor  108 , positioned to measure the temperature within the canister  110 . Temperature sensor  108  is connected to PCM  122 . Operation of these devices will be discussed below. 
       FIG. 2  is a flowchart describing a method for diagnosing a fault within the evaporative emission control system  100 . It will be understood that device references will refer to the system depicted in  FIG. 1 . The method initiates at a time when the engine is running and the vehicle is proceeding at a steady rate of about 40 mph. It will be understood that these initiation conditions imply that this test cannot be simply conducted under complete the automated control; a degree of driver participation is required. where 
     At step  203 , the evaporative emission control system  100  is evacuated to a target vacuum. Those in the art will understand that a variety of vacuum levels can be employed, but a reasonable target vacuum can be about −8″ H 2 O. In the illustrated embodiment, the system  100  is evacuated using engine vacuum, normally present in the intake manifold. To create the target vacuum the CPV  126 , is opened, subjecting the EVAP system to the vacuum generated by the engine. To ensure the creation of a vacuum, the canister vent valve (CVV)  114 , located between the canister  110  and the vent  115 , is closed. At the same time, FTIV  118  is opened, opening a flow path between the fuel tank  102  and the canister  110 . In step  205 , PCM  122  monitors signals from the FTPT  106  and the temperature sensor  108 . It will be useful if the monitoring commences just prior to setting the valves as noted above, ensuring that the system obtains a good reading for the beginning canister temperature. The evacuation proceeds for a set amount of time, sufficient to ensure achieving the target level of vacuum, provided the system operates properly. Those of skill in the art will understand how to select the time factors for this test. 
     In step  207 , the method analyzes the results obtained from the test, after the selected time has elapsed. The basic question, set out in step  209 , is whether the evacuation step has succeeded in reaching the target vacuum. If the target vacuum is reached, as shown in step  211 , then the question is whether a temperature gain was observed. Given that the target vacuum was achieved, the only flow through the EVAP system necessarily occurred from fuel tank  102 , through FTIV  118  and onward through canister  110 , continuing through CPV  118  and onto the intake manifold  130 . Vapor flowing through canister  110  would at least in part be adsorbed by carbon pellets, resulting in an increase in temperature. Thus, an increase in temperature, coupled with achievement of the target vacuum indicates that the system is operating without fault, as reflected in step  213 . An increase in temperature corresponds to Compares the pressure response with the pre-stored pressure response to determine whether the evacuation succeeded in reaching a target vacuum level. It accurate system. 
     If the target vacuum level is not achieved, then the analysis carried out by PCM  122  can infer the likely source problem, based on the temperature monitored by temperature sensor  108 . In this situation, one would expect a flow vapor through canister  110  to produce a temperature gain, while a flow of air would produce a temperature drop, due to the fact that airflow into the canister would entrain fuel vapor from the pellets, an endothermic reaction. In general, it can be said that the system will observe a temperature gain, a temperature drop, or little to no change. The first of those conditions is set out in step  223 , which is executed if the system identifies a temperature gain during the test. Here, the fact of a temperature gain means that vapor is flowing from the fuel tank  102  through the canister  110 , in spite of the fact that the desired vacuum level has not been reached. That fact leads to an inference that the reason for the failure to achieve the target vacuum is most likely a hole in the fuel tank  102 , or an insufficient flow through CPV  126 . Both of those items should be subjected to a thorough maintenance inspection. 
     The situation of observing a temperature drop coupled with failure to achieve the target vacuum is shown in step  225 . Here one can infer that fresh air, not fuel vapor, is flowing through the canister  110 . The suspects in this case include an open CVS  114 , or some other leak between the canister and fresh air vent  115 . 
     Finally, if one observes little or no temperature change, shown at step  227 , one can conclude that little or no flow is occurring through canister  110 , most likely owing to a fault with CPV  126  or a block in purge line  124   c.    
     The advantage of the present disclosure is immediately apparent, in that the system not only can identify the presence of a leak, but it can make an informed inference of the likely cause. As a result, a maintenance investigation can be considerably shortened, because the technician can start from a position of knowledge, rather than working from a blank slate. 
       FIG. 3  includes two graphs  300   a  and  300   b.  The graph  300   a  illustrates the temperature response  301   a  of a temperature sensor  108 , and the graph  300   b  illustrates the pressure response  301   b  of a FTPT  106 , in case of no fault in the EVAP system  100 . The EVAP system  100  is evacuated to a target vacuum (−81nH 2 O). The system  100  is evacuated by closing the CVV  114  and opening the CPV  126  and the valve  118 , and running the vehicle at at a minimum steady state speed of 40 mph. In case of no fault in the EVAP system  100 , the pressure response  301   b  of the FTPT  106  decreases to the target vacuum. As the vehicle is running, the fuel evaporates from the fuel tank  102 . The fuel vapors are routed to the canister  110  through the FTIV  118 . The carbon pellets present in the canister  110  adsorb the fuel vapors. The adsorption of the fuel vapors in the canister  110  results in an increase in temperature within the canister  110 . The temperature graph  300   a  shows an increase in the temperature with time. The temperature response  301   a  and the pressure response  301   b  are pre-stored in the control module, for comparison with other responses for the diagnosis of faults. 
       FIG. 4  includes graphs  400   a  and  400   b.  The graph  400   a  illustrates the temperature response  401   a  of the temperature sensor  108 , and the graph  400   b  illustrates the pressure response  401   b  of the FTPT  106 , in case of a fault due to a leakage port in the fuel tank or a leakage port in the vapor line  124   a.  A leakage port in the fuel tank  103  or in the communication line  124   a  results in a failure to pull down the system  100  to the target vacuum of −81nH 2 O. Therefore, the pressure response  401   b  of the FTPT  106  is a substantially constant curve, as shown. The fuel vapor  107  in the fuel tank  102  flows into the canister  110  through the valve  118 . The carbon pellets in the canister  110  adsorb the fuel vapor  107 . The adsorption of fuel vapor results in a heat gain within the canister. This heat gain results in a temperature rise, as illustrated by the graph  400   a.    
       FIG. 5  includes two graphs  500   a  and  500   b.  The graph  400   a  illustrates the temperature response  401   a  of the temperature sensor  108 , and the graph  400   b  illustrates the pressure response  401   b  of the FTPT  106 , in case of a fault due to an open CVV  114  or a leakage port in the communication line  124   d.  An open CVV  114  or a leakage port in the communication line  124   d  results in a failure to pull down the system  100  to the target vacuum, as fresh air flows into the system  100  through the leakage ports. This results in a cooling effect within the canister  110 , as fresh air flows in from the vent  115 , through CVV  114 , and into the canister  110 . Therefore, there is a temperature drop, as shown in the graph  500   a.    
       FIG. 6  includes two graphs  600   a  and  600   b.  The graph  600   a  illustrates the temperature response  601   a  of the temperature sensor  108 , and the graph  600   b  illustrates the pressure response  601   b  of the FTPT  106 , in case of a fault due to a closed CPV  126  or a leakage port in the communication line  124   b.  A closed CPV  126  or a leakage port in the communication line  124   b  results in a failure to pull down the system to the target vacuum −81nH 2 O. Therefore, the pressure response  601   b  is a substantially constant curve, as shown. This type of fault results in the canister  110  not being able to purge the vapor into the engine  130 . Therefore, the fuel vapor from the fuel tank  103  is not adsorbed into the canister  110 . As a result, there is little or no temperature rise within the canister  110 . Therefore, the temperature response  601   a  is a substantially constant curve, as shown.