Patent Publication Number: US-7219535-B2

Title: Leakage diagnosis apparatus for fuel vapor purge system and method thereof

Description:
FIELD OF THE INVENTION 
   The present invention relates to a leakage diagnosis apparatus for a fuel vapor purge system used in a vehicle installed internal combustion engine, and a method thereof. 
   RELATED ART 
   The above described fuel vapor purge system has a configuration in which fuel vapor generated in a fuel tank is trapped in a canister, and the fuel vapor trapped in the canister is purged into an intake passage of an internal combustion engine. 
   Japanese Unexamined Patent Publication No. 11-343927 discloses an apparatus for diagnosing an occurrence of leakage in the fuel vapor purge system. 
   In this diagnosis apparatus, a zone in which a leakage occurrence is to be diagnosed is blocked, and also an intake negative pressure of an internal combustion engine is introduced into the blocked diagnosis zone, to diagnose the occurrence of leakage based on a pressure change amount in the diagnosis zone due to the introduction of the negative pressure. 
   However, if the fuel vapor is generated in the fuel tank contained in the diagnosis zone when the pressure in the diagnosis zone is reduced with the intake negative pressure of the engine, the pressure change amount is changed. Therefore, in the diagnosis based on the pressure change amount, there has been a problem in that it is impossible to diagnose with high accuracy the existence of a leak hole having a small diameter. 
   SUMMARY OF THE INVENTION 
   The present invention has an object to provide a leakage diagnosis apparatus and a method thereof, capable of diagnosing with high accuracy the existence of a leak hole having a small diameter, even if fuel vapor is generated in a fuel tank during the diagnosis. 
   In order to achieve the above object, the present invention has a configuration in which a pressure of a blocked diagnosis zone is changed using an external pressure generating source, and a curvature of pressure change curve at that time is calculated, to diagnose an occurrence of leakage in the diagnosis zone based on the comparison between the curvature and a threshold. 
   The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 

   
     BRIEF EXPLANATION OF THE DRAWINGS 
       FIG. 1  is a diagram showing a system configuration of an internal combustion engine in an embodiment. 
       FIG. 2  is a flowchart showing the leak diagnosis in a first embodiment. 
       FIG. 3  is a graph showing a correlation between a pressure change for when a fuel temperature is 25° C., and a leak hole diameter. 
       FIG. 4  is a graph showing a correlation between a pressure change for when the fuel temperature is 40° C., and the leak hole diameter. 
       FIG. 5  is a flowchart showing the leak diagnosis in a second embodiment. 
   

   DESCRIPTION OF EMBODIMENTS 
     FIG. 1  is a diagram showing a system configuration of an internal combustion engine in an embodiment. 
   In  FIG. 1 , an internal combustion engine  1  is a gasoline engine installed in a vehicle (not shown in the figure). 
   A throttle valve  2  is disposed in an intake system of internal combustion engine  1 . 
   An intake air amount of engine  1  is controlled according to an opening of throttle valve  2 . 
   For each cylinder, an electromagnetic type fuel injection valve  4  is disposed in a manifold portion of an intake passage  3  on the downstream side of throttle valve  2 . 
   Fuel injection valve  4  is opened based on an injection pulse signal which is output from a control unit  20  in synchronism with an engine rotation, to inject fuel. 
   A canister  7 , into which fuel vapor generated in a fuel tank  5  is introduced via an evaporation passage  6 , is provided as a fuel vapor purge system. 
   Canister  7  is a container filled with the adsorbent  8  such as activated carbon. 
   Further, a new air inlet  9  is formed to canister  7 , and a purge passage  10  is extended out from the canister. 
   Purge passage  10  is connected to intake passage  3  on the downstream side of throttle valve  2 . 
   A closed type purge control valve  11  is disposed in the halfway of purge passage  10 . 
   An opening of purge control valve  11  is controlled based on a purge control signal output from control unit  20 . 
   The fuel vapor generated in fuel tank  5  is introduced by evaporation passage  6  into canister  7 , to be adsorptively trapped in canister  7 . 
   When a predetermined purge permission condition is established during an operation of engine  1 , purge control valve  11  is controlled to open. 
   Then, as a result that an intake negative pressure of engine  1  acts on canister  7 , the fuel vapor adsorbed in canister  7  is purged by the fresh air introduced through new air inlet  9 . 
   Purged gas inclusive of the purged fuel vapor passes through purge passage  10  to be sucked in intake passage  3 . 
   In order to diagnose an occurrence of leakage in the fuel vapor purge system, an electric-motor driven air pump  13  is disposed on the new air inlet  9  side of canister  7 . 
   Further, there is disposed an electromagnetic type switching valve  14 , which connects new air inlet  9  selectively to an outside-air communicating aperture  12  or a discharge opening of air pump  13 . 
   Switching valve  14  connects new air inlet  9  to outside-air communicating aperture  12  in an OFF condition thereof, and to the discharge opening of air pump  13  in an ON condition thereof. 
   Further, there is disposed an air filter  17 , which is common to outside-air communicating aperture  12  and a suction opening of air pump  13 . 
   Control unit  20  incorporating therein a microcomputer, receives signals from various sensors. 
   As the various sensors, there are provided a crank angle sensor  21  outputting a crank angle signal, an air flow meter  22  detecting an intake air amount of engine  1 , a vehicle speed sensor  23  detecting a running speed of the vehicle in which engine  1  is installed, a pressure sensor  24  detecting a gas pressure inside fuel tank  5 , a fuel gauge  25  detecting a remaining fuel quantity in fuel tank  5 , and a current sensor  26  detecting a current of air pump  13 . 
   Control unit  20  controls fuel injection valve  4  and purge control valve  11  based on engine operating conditions detected by the various sensors. 
   Further, control unit  20  controls air pump  13  and switching valve  14 , to diagnose an occurrence of leakage in the fuel vapor purge system. 
   A flowchart of  FIG. 2  shows the detail of leakage diagnosis. 
   In step S 1 , the remaining fuel quantity in fuel tank  5 , which is detected by fuel gauge  25 , is read. 
   In step S 2 , a diagnosis zone, which contains purge passage  10  on the downstream side of purge control valve  11 , canister  7 , evaporation passage  6  and fuel tank  5 , is blocked, and the blocked diagnosis zone is pressurized by air pump  13 . 
   Namely, after purge control valve  11  is controlled to close, and switching valve  14  is turned to the ON condition, air pump  13  is driven, so that the air discharged from air pump  13  is fed into the blocked diagnosis zone. 
   In step S 3 , the pressure in fuel tank  5  detected by pressure sensor  24  is read to be stored sequentially. 
   In step S 4 , it is judged whether or not the pressure in fuel tank  5  reaches a standard pressure or above. 
   If the pressure in fuel tank  5  (the diagnosis zone) does not reach the standard pressure or above, control proceeds to step S 5 . 
   In step S 5 , it is judged whether or not a pressurization time being an elapsed time after the pressurization of diagnosis zone has been started by air pump  13 , is equal to or less than a previously stored upper limit time. 
   If it is judged in step S 5  that the pressurization time is less than or equal to the upper limit time, control returns to step S 2 . In contrast, if it is judged in step S 5  that the pressurization time exceeds the upper limit time, it is judged that the pressure in the diagnosis zone did not reach the standard pressure within the upper limit time, since gas of a predetermined amount or above is leaked out from the diagnosis zone. 
   When it is judged in step S 5  that the pressurization time exceeds the upper limit time, control proceeds to step S 6 . In step S 6 , a diagnosis signal indicating that a large leak hole having a diameter of reference diameter (for example, 0.04 inches in diameter) or above, exists in the diagnosis zone. 
   On the other hand, if, in step S 4 , the pressure in the diagnosis zone reaches the standard pressure or above within the upper limit time, control proceeds to step S 7 . 
   In step S 7 , an inclination of a pressure change curve in the diagnosis zone (pressure change speed) is calculated. 
   The inclination of the pressure change curve (pressure change speed) is obtained as a pressure change amount in a fixed short time. 
   In step S 8 , a curvature of the pressure change curve in the diagnosis zone (curvature=absolute value of pressure change acceleration) is calculated. 
   In step S 9 , a threshold to be compared with the curvature is set based on the remaining fuel quantity in fuel tank  5 . 
   This is because space volume of the pressurized diagnosis zone is changed depending on the remaining fuel quantity, and the curvature is changed due to the change in the space volume. 
   The more the remaining fuel quantity is, in other words, the smaller the space volume of the diagnosis zone is, the threshold is set to a larger value. 
   In step S 10 , the curvature obtained in step S 8  and the threshold set in step S 9  are compared with each other. 
   Then, when it is judged in step S 10  that the curvature is equal to or less than the threshold, in other words, in the case where the pressure in fuel tank  5  is increasingly changed at a substantially fixed speed, control proceeds to step S 11 . 
   In step S 11 , the diagnosis signal indicating that there is no leak hole, is output. 
   A diagnosis result of no leak hole shows a diagnosis result indicating that there is no leak hole, or, even if there is a leak hole, such a leak hole has a diameter smaller than a permissible reference minor diameter (0.02 inches in diameter). 
   On the other hand, when it is judged, in step S 10 , that the curvature exceeds the threshold, that is, in the case where a speed of pressure rise by the pressurization shows a tendency to be decreased gradually, control proceeds to step S 12 . 
   In step S 12 , the diagnosis signal indicating that there exists a leak hole of 0.02 inches or above in diameter, is output. 
     FIG. 3  is a graph showing the pressure changes for when the pressurization by air pump  13  is performed under a condition that the fuel temperature is 25° C. and the remaining fuel quantity is 10 liters, for the cases of no leak hole, the leak hole of 0.37 mm in diameter, the leak hole of 0.54 mm in diameter, and the leak hole of 1.0 mm in diameter. 
   As shown in  FIG. 3 , in the case of no leak hole, the pressure in the diagnosis zone rises at the substantially fixed speed. However, the larger the leak hole diameter becomes, the pressure rise speed in the diagnosis zone is decreased, and in the case of the leak hole of 1.0 mm in diameter, the pressure in the diagnosis zone slightly rises, and thereafter, becomes substantially fixed. 
   Accordingly, it is possible to judge whether or not there exists a large leak hole having a diameter exceeding 1.0 mm, based on whether or not the pressure in the diagnosis zone exceeds the standard pressure (for example, 2 kPa in the example of  FIG. 3 ). 
   On the other hand, in the case where the leak hole diameter is small, and accordingly, the pressure in the diagnosis zone exceeds the standard pressure, an occurrence of leakage is judged using the curvature of the pressure change curve. 
   Namely, the pressure in the diagnosis zone rises at the substantially fixed speed in the case of no leak hole, while the speed of pressure rise in the diagnosis zone is gradually decreased in the case where there exists a leak hole, and furthermore, the drop of pressure rise speed becomes larger, as the leak hole diameter becomes larger. 
   Therefore, in the present embodiment, the existence of leak hole having a diameter smaller than 1.0 mm (0.4 inches) is judged based on the curvature of the pressure change curve (acceleration of the pressure change). 
   Here, the pressure change in the diagnosis zone is influenced by the fuel vapor. However, as is apparent from the comparison between the pressure change at the fuel temperature of 25° C. shown in  FIG. 3  and the pressure change at the fuel temperature of 40° C. shown in  FIG. 4 , the curvature of the pressure change curve is hardly to be influenced by the fuel vapor in comparison with the pressure change amount. 
   Accordingly, in the leakage diagnosis based on the curvature of the pressure change curve, the diagnosis accuracy is not significantly lowered due to the generation of fuel vapor, thereby enabling the accurate diagnosis of the existence of leak hole having a small diameter. 
   Incidentally, since a load of air pump  13  is changed according to the pressure in the diagnosis zone, by performing the leakage diagnosis based on a curvature of change curve of the load of air pump  13 , it is possible to perform the diagnosis same as the diagnosis based on the curvature of the pressure change curve. 
   A flowchart of  FIG. 5  shows a second embodiment in which the leakage diagnosis is performed based on the load of air pump  13 . 
   The flowchart of  FIG. 5  differs from the flowchart of  FIG. 2  only in step S 3 A, step S 4 A, step S 7 A, and step S 8 A. 
   In the flowchart of  FIG. 5 , the load of air pump  13  is used as data equivalent to the pressure in fuel tank  5  (pressure in the diagnosis zone). 
   In step S 3 A, the current detected by current sensor  26 , being data indicating the load of air pump  13 , is read to be stored. 
   In step S 4 A, it is judged whether or not the current (load) reaches a reference value or above. 
   Then, if the current (load) of air pump  13  reaches the reference value or above in the pressurization time within the upper limit time, control proceeds to step S 7 A, where an inclination of the current (load) change curve (current change speed) is calculated. 
   Further, in step S 8 A, the curvature of the current (load) change curve (acceleration of the current change) is calculated. 
   Then, in step S 10 , the threshold according to the remaining fuel quantity and the curvature obtained in step S 8 A (absolute value of the acceleration of the current change) are compared with each other, to diagnose whether or not there exists a leak hole having a diameter of 0.02 inches or above. 
   According to the second embodiment, the leakage diagnosis can be performed without using pressure sensor  24 . 
   In the above embodiment, the current of air pump  13  has been used as the data indicating the load of air pump  13 . However, the configuration may be such that a control signal for air pump  13 , for when air pump  13  is feedback controlled based on the pressure in the diagnosis zone, is used as the data indicating the load of air pump  13 . 
   Further, in the above embodiment, the diagnosis zone has been pressurized by air pump  13 . However, the configuration may be such that the pressure in the diagnosis zone is reduced by air pump  13 . 
   Furthermore, the configuration may be such that the outside-air communicating aperture of canister  7  is blocked during the operation of engine  1 , and also purge control valve  11  is opened, to introduce the intake negative pressure of the engine into the diagnosis zone, thereby reducing the pressure in the diagnosis zone. 
   In the case where the pressure in the diagnosis zone is reduced, the leakage diagnosis is performed based on the curvature of the pressure change curve for when the pressure in the diagnosis zone is decreasingly changed. 
   The entire contents of Japanese Patent Application No. 2003-152293 filed on May 29, 2003, a priority of which is claimed, are incorporated herein by reference. 
   While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. 
   Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for the purpose of limiting the invention as defined in the appended claims and their equivalents.