Patent Publication Number: US-6220229-B1

Title: Apparatus for detecting evaporative emission control system leak

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to evaporative emission control systems for automotive vehicles and more particularly to an apparatus for determining if a leak is present in an evaporative emission control system for an automotive vehicle. 
     2. Description of the Related Art 
     An evaporative emission control system includes a canister containing activated charcoal to collect and store volatile fuel vapors from a fuel tank during the time the engine is not running. The evaporative emission control system also includes a purge line or conduit connecting between an intake pipe portion downstream of a throttle valve and the canister. The purge line opens under a predetermined condition after start of the engine to draw fresh air into the canister and purge the canister. The collected volatile fuel vapors are thus drawn from the canister into the intake pipe, for combustion within a combustion chamber of the engine. 
     In this instance, if a fuel vapor flow passage extending from the fuel tank to the intake pipe has a leak or the fuel vapor flow passage has a connecting portion of which seal is defective, the fuel vapors are released to the atmosphere. To prevent such evaporative fuel emission, a diagnostic system has been proposed to determine if a leak is present in the evaporative emission control system, as disclosed in Japanese provisional patent publication No. 7-139439. A leak of the above described fuel vapor flow passage can be checked by closing the passage so that the passage is in the form of a closed space, i.e., by closing the passage in a way as to prevent fluid communication between the inside and the outside of the passage, and observing a variation of the internal pressure of the fuel vapor flow passage after the passage is pressurized in such a way that the internal pressure of the passage and the atmospheric pressure differ relatively firm each other. The diagnostic system of the above described publication thus includes a vent control valve provided to an atmospheric vent of the canister to selectively open and close the atmospheric vent. The atmospheric vent of the canister is closed by the vent control valve when the above described passage is to be closed so as to be in the form of a closed space. The diagnostic system also includes a pressure sensor provided to the above described fuel vapor flow passage for checking a pressure variation of gas enclosed in the passage. A negative pressure produced in the intake pipe portion downstream of the throttle valve is introduced into the fuel vapor flow passage for negative pressurization thereof, whereby to check if a leak is present in the passage. 
     SUMMARY OF THE PRESENT INVENTION 
     However, if the mixture of air and fuel vapors in the above described passage is drawn by intake vacuum into the intake pipe, variations of the air-fuel ratio of the engine will result. To prevent such variations of the air-fuel ratio, it has heretofore been practiced to conduct a leak detection during a feedback control of the air-fuel ratio. A three way catalytic converter will become most effective when the air-fuel mixture has a stoichiometric ratio or a ratio adjacent thereto. For this reason, the feedback control of the air-fuel ratio is performed on the basis of the output of an oxygen sensor disposed upstream of the three way catalytic converter so that the air-fuel ratio is included within a predetermined range having a stoichiometric ratio at its center. By the feedback control of the air-fuel ratio, it is intended to prevent variations of the air-fuel ratio due to introduction of the mixture of air and fuel vapors into the intake manifold from the above described fuel vapor flow passage. 
     However, the feedback control of the air-fuel ratio has for its main purpose to eliminate a steady-state deviation due to variations of the flow rate characteristics of an injector and an air flow meter resulting from variations in the manufacture thereof. Thus, the responsive speed of the feedback control is not so high, so the three way catalytic converter cannot be most effective until the air-fuel ratio returns to a value adjacent a stoichiometric ratio after a variation of the air-fuel ratio is caused. 
     Further, the feedback control of the air-fuel ratio requires the oxygen sensor to have been in an activated condition. Thus, it has heretofore been impossible to conduct the diagnosis of leak before the feedback control of the air-fuel ratio starts (e.g., immediately after the engine starts). 
     It is accordingly an object of the present invention to provide a leak detection apparatus for an evaporative emission control system for an internal combustion engine which is capable of conducting a diagnosis of leak before the feedback control of the air-fuel ratio starts, for example, immediately after the engine starts. 
     It is a further object of the present invention to provide a leak detection apparatus of the foregoing character which utilizes consumption of fuel in a fuel tank for attaining negative pressurization of a fuel vapor flow passage extending from a fuel tank to a purge control valve. 
     To achieve the foregoing objects, the present invention is an apparatus for detecting a leak in an evaporative emission control system for an internal combustion engine including a fuel tank, a canister for collecting fuel vapors from the fuel tank, a purge control valve disposed between the canister and the intake pipe for controlling flow of the fuel vapors from the canister to the intake pipe such that a fuel vapor flow passage is provided which extends from the fuel tank to the purge control valve by way of the canister. The apparatus includes a vent control valve for selectively opening and closing an atmospheric vent of the canister, an actuating device for actuating the purge control valve and the vent control valve to fully close immediately after the engine starts and thereby closing the fuel vapor flow passage in such a way as to prevent communication between an inside and outside of the fuel vapor flow passage, a pressure sensor for detecting a pressure in the fuel vapor flow passage, and a diagnostic device for detecting a leak on the basis of the pressure in the fuel vapor flow passage which reduces with increase of consumption of fuel in the fuel tank after the fuel vapor flow passage is closed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an evaporative emission control system utilizing a leak detection apparatus according to an embodiment of the present invention; 
     FIG. 2 is a graph of passage pressure against flow rate illustrating operation of a vacuum cut valve of the system of FIG. 1; 
     FIG. 3 is a graph of passage pressure against output voltage illustrating operation of a pressure sensor of the apparatus of FIG. 1; 
     FIG. 4 is a timing chart showing a variation of passage pressure when it is judged that a leak is present in the evaporative emission control system, together with operations of valves and a variation of fuel consumption; 
     FIG. 5 is a flowchart illustrating a routine for detecting a leak, executed by the apparatus of FIG. 1; and 
     FIG. 6 is a flowchart illustrating a modification of the routine of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring fist to FIG. 1, there is illustrated the whole arrangement of an evaporative emission control system for an internal combustion engine and a leak detecting apparatus therefor. Indicated by  1  is a fuel tank and by  4  a canister. Fuel vapors and air in the upper part of the fuel tank  1  are drawn through a line (first conduit)  2  into the canister  4 . Only the fuel vapors are adsorbed by an activated charcoal  4   a  in the canister  4 . The remaining air is discharged to the outside through an atmospheric vent  5  disposed at the lower end portion of the canister  4  (though shown at the upper end portion in the drawing). 
     A mechanical vacuum cut valve  3  is provided which opens when the pressure in the fuel tank  3  is lower than the atmospheric pressure. FIG. 2 shows the operation of the vacuum cut valve  3 . As show in FIG. 2, the vacuum cut valve  3  is also open when the pressure in the fuel tank  1  assumes a predetermined pressure (e.g., +10 mmHg) due to generation of fuel vapors therein. In FIG. 2, on the basis of the atmospheric pressure (i.e., assuming that the atmospheric pressure is 0 mmHg), the pressure higher than the atmospheric pressure is added with “+” and the pressure lower than the atmospheric pressure is added with “−”. 
     The canister  4  is communicated with an air intake pipe  8  portion downstream of a throttle valve  7  through a purge line (second conduit)  6 . A normally closed purge control valve  11  driven by a step motor (not shown) is disposed in the purge line  6 . Under a predetermined condition (e.g., under a low load condition after warp-up of the engine), the purge control valve  11  opens in response to a signal from an ECU (electronic control unit)  21 . Whereupon, fresh air is drawn into the canister  4  through the atmospheric vent  5  by an intake vacuum prevailing in the intake pipe  8  portion downstream of the throttle valve  7 . The fuel vapors are thus drawn from the activated charcoal  4   a  together with the fresh air into the intake pipe  8  for combustion in a combustion chamber of the engine. 
     In the evaporative emission control system, a fuel vapor flow passage  10  is thus provided which extends from the fuel tank  1  to the purge control valve  11  by way of the canister  4 , i.e., which is comprised of an inner space of the fuel tank  1 , an inner space of the first conduit  2 , an inner space of the canister  4 , and an inner space of the second conduit  6 . 
     If the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  has a leak or the passage  10  has a connecting portion of which seal is defective, fuel vapors leak out to the atmosphere. To prevent such leakage, it has been proposed to conduct a leak detection by developing a negative pressure in the above described passage by utilizing a vacuum generated in the intake pipe  8  portion downstream of the throttle valve  7 . 
     In this instance, if the mixture of air and fuel vapors in the fuel vapor flow passage  10  is drawn by the intake vacuum into the intake pipe  8 , variations of the air-fuel ratio of the engine result. To prevent such variations, it has heretofore been practiced to conduct the leak detection during a feedback control of the air-fuel ratio. A three way catalytic converter is most effective when the air-fuel mixture has a stoichiometric ratio or a ratio adjacent thereto. For this reason, the feedback control of the air-fuel ratio is performed on the basis of the output of an oxygen sensor disposed upstream of the three way catalytic converter so that the air-fuel ratio is included within a predetermined range having a stoichiometric ratio at its center. By the feedback control of the air-fuel ratio, it is intended to prevent variations of the air-fuel ratio due to introduction of the mixture of air and fuel vapors into the intake pipe  8  from the fuel vapor flow passage  10 . 
     However, the responsive speed of the feedback control is not so high, so the three way catalytic converter cannot be most effective until the air-fuel ratio returns to a value adjacent a stoichiometric ratio after a variation of the air-fuel ratio is caused. Further, it is necessary for the oxygen sensor to have been activated before start the feedback control of the air-fuel ratio. Thus, it has been impossible to attain a leak detection before the feedback control of the air-fuel ratio starts (e.g., immediately after the engine starts). 
     To solve such a problem, the fuel vapor flow passage  10  is negatively pressurized immediately after start of the engine by utilizing a consumption of fuel in the fuel tank according to an embodiment of the present invention. 
     Firstly, a normally open vent control valve  12  is provided to the atmospheric vent  5  of the canister  4  to close the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  for thereby closing the fuel vapor flow passage  10  so that the passage  10  is in the form of a closed space. Further, the above described vacuum cut valve  3  is provided with a normally closed bypass valve  14  in parallel relation thereto. Accordingly, when the vent control valve  12  and the purge control valve  11  are closed while the bypass valve  14  is opened in response to signals from the control unit  21 , the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  establishes communication throughout thereof while being closed so as to be in the form of a closed space, i.e., closed in a way as to prevent communication between the inside and outside of the passage  10 . 
     In the purge line  6  between the canister  4  and the purge control valve  11  is disposed a pressure sensor  13 . As shown in FIG. 3, the pressure sensor  13  produces as an output a voltage proportional to the pressure (i.e., pressure with respect to the atmospheric pressure) in the fuel vapor flow passage  10  which is closed so as to be in the form of a closed space at the time of leak detection. In the meantime, the pressure sensor  13  can be disposed at any portion of the passage between the fuel tank  1  and the purge control valve  11  or can be disposed at the fuel tank  1  as indicated by the two-dot chain lines. 
     The control unit  21  is comprised of a microcomputer and controls opening and closing of the above described three valves, i.e., the purge control valve  11 , vent control valve  12  and bypass valve  14 , whereby to detect if the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  has a leak. 
     Referring to FIG. 4, the process for leak detection executed under control of the control unit  21  will be described. 
     (1) The purge control valve  11  is held fully closed immediately after the engine starts. At the time t 1  immediately after the engine starts, the pressure in the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  1  is sampled as P 1 . Thereafter, the vent control valve  12  is closed and the bypass valve  14  is opened. By these operations, the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  establishes communication throughout thereof while being closed so as to be in the form of a closed space. 
     (2) From the time t 1 , calculation of the sum of fuel in the fuel tank  1  starts. By the consumption of fuel, there is developed in the fuel vapor flow passage  10  in the form of a closed space a pressure lower than the atmospheric pressure (i.e., a negative pressure). This will be described in detail hereinafter. 
     Under ordinary conditions in which the vent control valve  12  is fully open (i.e., at the time other than the time when the leak detection is carried out), it never occurs that the pressure in the fuel tank  1  becomes negative. This is because the vacuum cut valve  3  opens to draw the atmospheric air into the fuel tank  1  as soon as the pressure in the fuel tank becomes negative. 
     On the other hand, with an electronic fuel-injection system, a fuel pump (not shown) delivers fuel from the fuel tank  1  into a fuel supply passage  31  to supply an injector  32  with fuel which is pressurized so as to have a constant pressure. The injector  32  provided to each cylinder receives an instruction from the control unit  21  and injects a predetermined amount of fuel intermittently so that an engine torque in accordance with a driving condition is obtained. The consumption of the fuel in the fuel tank  1  thus starts with start of the engine. 
     Accordingly, when the fuel in the fuel tank  1  is consumed under the condition in which the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  is closed so as to be in the form of a closed space, the pressure in the closed fuel vapor flow passage  10  is lowered in accordance with the fuel consumption as shown in the lowest part of FIG.  4 . In the meantime, though a negative pressure is developed in the inside of the fuel tank  1 , it is only smaller than the atmospheric pressure by several millimeters of mercury. 
     (3) At the time t 2  at which the calculated sum of consumption of fuel in the fuel tank  1  exceeds a reference value, the pressure in the fuel vapor flow passage  10  is sampled as P 2  (P 2 &lt;P 1 ). Then, the variation ΔP (=P 1 −P 2 ) is calculated. 
     In this connection, in comparison between the case where there is a leak in the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  and the case where there is not any leak in the fuel vapor flow passage  10 , a smaller reduction ΔP results in the case where there is a leak. 
     Accordingly, by comparing the reduction with the reference value for judgment, it is possible to judge that there is a leak in the passage when ΔP is smaller than the reference value (refer to the lowest part of FIG. 4) and there is not any leak when ΔP is equal to or larger than the reference value. 
     (4) The vent control valve  12  is opened and the bypass valve  14  is closed, whereby to complete the leak detection of the evaporative emission control system. 
     The flowchart of FIG. 5 is a routine for executing the above described process for leak detection or diagnosis. 
     At the step S 1 , a diagnosis experience flag is looked. As will be described hereinlater, the flag is set to “1” when leak detection at this time of operation is completed. Since the flag is set to “0” when the leak detection is not executed immediately after the engine starts, the program proceeds to the steps S 2  and S 3  to look an ignition switch (abbreviated as ING SW) and a starter switch (abbreviated as ST SW). If the ignition switch is ON and the starter switch is at the transition from ON to OFF (i.e., if it is immediately after the engine starts), the program proceeds to steps S 4 , S 5  and S 6 . At the step S 4 , the detected value by the pressure sensor  13  is transferred to P 1 . At the step S 5  the calculated sum of consumption of fuel in the fuel tank  1  is cleared. Thereafter, at the step S 6 , the vent control valve  12  is closed and the bypass valve  14  is opened. At this time, the purge control valve  11  is in a fully closed condition. 
     From the next execution of the program onward, the program proceeds from the step S 3  to the step S 7 . At the step S 7 , the starter switch and the engine speed are looked. When the starter switch is OFF and the engine speed is equal to or higher than a predetermined value, it is judged that engine is running and the program proceeds to the step S 8 . At the step S 8 , the sum of consumption of fuel in the fuel tank  1  is compared with the reference value. So long as the sum of fuel consumption is equal to or smaller than the reference value, the program proceeds to the step S 9  to calculate the sum of consumption of fuel in the fuel tank  1  and then to the step S 6  to continue the operation thereat. Repeated calculation of the sum of fuel consumption at the step S 9  soon causes the sum of consumption of fuel in the fuel tank  1  to exceed the reference value, at which timing the program proceeds from the step S 8  to the step S 10 . 
     At the step S 10 , the detected value by the pressure sensor  13  is transferred to P 2 . Then, at the step S 11 , the amount of pressure reduction ΔP (=P 1 −P 2 ) is calculated. The amount of pressure reduction ΔP is compared with the reference value (which differs from the reference value at the step S 8 ). When ΔP is larger than the reference value, the program proceeds to the step S 14  and it is judged that there is no leak in the passage. On the other hand, when ΔP is equal to or smaller than the reference value, the program proceeds to the step S 13  and it is judged that there is a leak in the passage. 
     At the step S 15 , the vent control valve  12  is opened and the bypass valve  14  is closed. At the step S 16 , diagnosis experience flag is set to “1”. By the diagnosis experience flag which is set to “1”, the program never proceeds to the step S 2 . 
     From the foregoing, it will be understood that according to the embodiment of the present invention the passage extending from the fuel tank  1  to the purge control valve  11  is closed so as to be in the form of a closed space immediately after the engine starts, the above described closed space is negatively pressurized so that the pressure therewithin is negative, by consumption of fuel in the fuel tank  1 , the variation ΔP of the pressure in the closed space with respect to the pressure in the passage before closed so as to be in the form of the closed space is calculated, and the detection of leak is carried out on the basis of the variation ΔP. Since the purge control valve  11  is not opened during the detection of leak, it never occurs that the mixture of air and fuel vapors which exists in the fuel vapor flow passage  10  extending from the fuel tank  1  to the purge control valve  11  flows into the intake pipe, whereby it becomes possible to prevent a variation of the air-fuel ratio which is otherwise caused by a prior art leak detection. 
     It will be further understood that according to the present invention it becomes possible to carry out a leak detection immediately after the engine starts, i.e., before the feedback control of the air-fuel ratio starts. 
     The entire content of Japanese Patent Application P10-109395 (filed on Apr. 20, 1998) is incorporated herein by reference. 
     Although the invention has been described above by reference to a certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art, in light of the above teachings. For example, while in the above described embodiment of the present invention the leak detection is executed at the time the sum of consumption of fuel in the fuel tank, of which calculation starts immediately after the engine starts, exceeds a reference value, it can be executed at the time a predetermined period elapses from the time immediately after the engine starts as shown in the flowchart in FIG.  6 . In the modified routine in FIG. 6, the steps S 5 ′, S 8 ′ and S 9 ′ are executed in place of the steps S 5 , S 8  and S 9  in the routine in FIG. 5, respectively. 
     However, the sum of fuel consumption, of which calculation is executed from the time immediately after the engine starts, varies depending upon a variation of the coolant temperature at the time the engine starts. Thus, in case the elapsed time starting from the time immediately after the engine starts is used, the above described pressure variation ΔP varies depending upon a variation of the coolant temperature at the time the engine starts, thus deteriorating the accuracy of leak detection by the corresponding degree. 
     In contrast to this, in case the sum of consumption of fuel in the fuel tank is used for the leak detection, it becomes easier to set the reference value which is used for comparison with the sum of the fuel consumption and furthermore the accuracy of leak detection can be improved. This is because there is a constant relation between the sum of consumption of fuel in the fuel tank and the negative pressurization of the passage.