Patent Publication Number: US-6666072-B2

Title: Evaporative emission control system and method for detecting leaks therein

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2001-212268 filed on Jul. 12, 2001, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an evaporative emission control system in which a canister is provided in an evaporative gas purge system connecting an air intake pipe of an engine and a fuel tank. 
     BACKGROUND OF THE INVENTION 
     In an evaporative emission control system disclosed in JP-A-2000-345934 and JP-A-11-343927, for example, a negative pressure generated in an air intake pipe during an engine operation is introduced into a fuel tank. An evaporative gas purge system connecting the air intake pipe and the fuel tank is then maintained under the negative pressure. Change of the pressure is sensed by a sensor, thereby determining leaks in the purge system. 
     In this kind of evaporative emission control system, the leak check is executed when the engine is running. While a vehicle is moving, a fuel level in the fuel tank is unstable due to vibration of the vehicle. It causes changes of the pressure during the leak check. Therefore, it is difficult to accurately determine the leak when the engine is running. Because the purge system is negatively pressurized in a short time, a large amount of fuel vapor is introduced into the engine at a time. It is likely to affect an air-fuel ratio control, thereby worsening the emission. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above problem, and it is an object of the present invention to provide an evaporative emission control system capable of determining leaks accurately. 
     It is another object of the present invention to provide an evaporative emission control system that suppresses emission from becoming worse. 
     It is further another object of the present invention to provide a method to accurately determine a leak in an evaporative emission control system. 
     In an evaporative emission control system of the present invention, a canister communicates with a fuel tank through a negative pressure intake line and an air intake pipe of an engine through a purge line. Fuel vapor generated in the fuel tank is temporarily stored in the canister, and sucked into the air intake pipe by negative pressure generated in the air intake pipe. The negative pressure in the air intake pipe is introduced into the fuel tank through the negative pressure intake line. The purge line is provided with a purge valve to open and close the purge line. 
     After the engine is switched off, the purge valve is closed and defines a closed space between the purge valve and the fuel tank. The negative pressure introduced in the fuel tank is introduced into the closed space. After the closed space is maintained under a uniform negative pressure, a leak check of the closed space is started. 
     Because the leak check is executed when the engine halts, that is, when a fuel level in the tank is stable, the leak is accurately detected. Further, because the fuel tank is maintained under the negative pressure when the engine is running, the fuel vapor is prevented from leaking outside through the fuel tank, canister and the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of embodiments will become more apparent from the following detailed description made with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram showing an evaporative emission control system according to an embodiment of the present invention; and 
     FIG. 2 is a flowchart showing a leak check process executed by an ECU, according to the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described hereinafter with reference to the drawings. 
     Referring to FIG. 1, an evaporative emission control system has an air intake pipe  2  of an engine  1 , a fuel tank  3  communicating with the intake pipe  2  through an evaporative gas purge system and a canister  4  provided in the purge system. Fuel vapor generated in the fuel tank  3  is temporarily stored in the canister  4  and then drawn into the intake pipe  2  by manifold air pressure (negative pressure) of the engine  1 . The system is controlled by an ECU (electronic control unit)  5 . 
     The fuel tank  3  has a fuel pump  6  for drawing and supplying fuel to the engine  1 , a fuel gauge unit  7  for gauging a fuel level (remaining amount of the fuel) and a roll over valve  8  for stopping an outflow of the fuel when the fuel tank  3  is inclined due to inclination of a vehicle, and the like. A pressure sensor  9  is provided outside of the fuel tank  3 . The pressure sensor  9  senses an inside pressure of the fuel tank  3 , and continuously sends outputs to the ECU  5 . 
     The canister  4  has a fuel vapor inlet  4   a  and a fuel vapor outlet  4   b.  The canister  4  stores adsorbent such as activated carbon to adsorb the fuel vapor. The canister  4  has an air intake port  10  and a pressure release port  11 . The air intake port  10  is provided with a normally-closed valve (electromagnetic valve)  12  that opens and closes to introduce the air into the canister  4  through the air intake port  10 . The pressure release port  11  is provided with a positive pressure relief valve  13  that opens and closes to release pressure in the canister  4  through the pressure release port  11 . When the pressure in the canister  4  becomes greater than the atmospheric pressure by a predetermined pressure, the positive pressure relief valve  13  opens. 
     The purge system includes a purge line  14 , a negative pressure intake line  15 , a first bypass line  16  and a second bypass line  17 . The purge line  14  connects the vapor outlet  4   b  and the intake pipe  2 . The negative pressure intake line  15  connects the vapor inlet  4   a  and the fuel tank  3 . The first bypass line  16  and the second bypass line  17  are connected to the negative pressure intake line  15 , respectively. That is, both the first and second bypass lines  16  and  17  communicate with the canister  4  and the fuel tank  3 . 
     The purge line  14  is provided with a purge valve (electromagnetic valve)  18 . The purge valve  18 , which is a normally-closed valve, opens and closes the purge line  14 . The negative pressure intake line  15  is provided with a negative pressure maintaining valve  19 . The negative pressure introduced into the fuel tank  3  through the negative pressure intake line  15  is maintained equal to or less than a predetermined negative pressure by the negative pressure maintaining valve  19 . 
     The first bypass line  16  bypasses the negative pressure maintaining valve  19 . The first bypass line  16  is provided with a normally-open valve (electromagnetic valve)  20 . The normally-open valve  20  opens and closes the first bypass line  16 . The second bypass line  17  is provided in parallel with the first bypass line  16 . The second bypass line  17  is provided with a pressure relief valve  21 . When the pressure in the canister  4  becomes greater than the pressure in the fuel tank  3  by a predetermined pressure, the pressure relief valve  21  opens. 
     A leak check, which determines whether a leak exists in the evaporative gas purge system or not, is executed by the ECU  5 . This ECU  5  is programmed to execute a process of the leak check shown in FIG.  2 . The ECU  5  is held operative with electric power supply while the engine  1  is running but also for a predetermined period after the engine  1  is stopped. 
     First, at step S 10 , it is determined whether the engine  1  is running (ON) or not. When the engine  1  is running, the process proceeds to step S 11 . At step S 11 , it is determined whether purge condition is established or not. When the purge condition is established, the process proceeds to step S 12 . The purge condition includes the following items. 
     1. It is not in refueling. 
     2. It is not in vehicle decelerating. (It is not in fuel-cutting.) 
     When all items are satisfied, it is determined that the purge condition is established. 
     At step S 12 , it starts purging, that is, the negative pressure is introduced into the fuel tank  3 . Specifically, the purge valve  18  is opened, and the normally-closed valve  12  and the normally-open valve  20  are closed. Thus, the negative pressure in the engine  1  is introduced into the fuel tank  3  through the negative pressure maintaining valve  19 . 
     Next, at step S 13 , it is determined whether the inside pressure (negative pressure) of the fuel tank  3  is normal or abnormal. When the pressure sensed by the pressure sensor  9  is within the range of a setting pressure of the negative pressure maintaining valve  19  and a setting pressure of the pressure relief valve  21 , the inside pressure is determined normal and the process proceeds to step S 14 . 
     At step S 14 , it is determined whether the engine  1  is switched to a halt (OFF) or not. When the engine  1  halts, the process proceeds to step S 15 . At step S 15 , the negative pressure in the fuel tank  3  is introduced into the purge system. Specifically, the purge valve  18  and the normally-closed valve  12  are closed and the normally-open valve  20  is opened. Thus, the purge system from the fuel tank  3  to the purge valve  18  becomes a closed space. The negative pressure in the fuel tank  3  is introduced into the closed space in the purge system and the closed space is maintained under a continuous and uniform negative pressure. 
     At step S 16 , it is determined whether it is in refueling or not. When a fuel tank cap is removed for refueling, the inside pressure of the fuel tank  3  suddenly rises. Therefore, whether it is in refueling or not is determined by monitoring the inside pressure of the fuel tank  3  with the pressure sensor  9 . When the refueling is detected, the process proceeds to step S 17 . When the refueling is not detected, the process proceeds to step S 18 . At step S 17 , the leak check process is discontinued because it is difficult to maintain the closed space in the purge system under the negative pressure. 
     At step S 18 , the leak check is started. Here, a pressure change in the fuel tank  3  or the pressure change per unit time is calculated based on the pressures detected by the pressure sensor  9 . At step S 19 , it is determined whether a leak exists in the purge system or not. When the leak is determined, the process proceeds to step S 20 . When no leak is determined, the process returns to step S 10 . At step S 20 , MIL (Malfunction Indication Lamp) turns on, thereby warning the leak to a driver. 
     In the system of the present embodiment, since the negative pressure intake line  15  is provided with the negative pressure maintaining valve  19 , it is possible to maintain the purge system, which is the closed space defined between the purge valve  18  and the fuel tank  3 , under the negative pressure for a predetermined period after the engine  1  is switched off. Therefore, the leak check is executed without using power when the engine  1  halts. Since the leak check is executed when the fuel level in the fuel tank  3  is stable, the leak can be accurately determined. 
     Further, since the leak check is executed when the engine  1  halts, a fuel ratio control during the engine running is not affected, thereby preventing the emission from becoming worse. Since the fuel tank  3  can be maintained under the negative pressure by the negative pressure maintaining valve  19  when the engine  1  is running, the fuel vapor generated in the fuel tank  3  is restricted from leaking outside through the fuel tank  3 , canister  4  and pipes of the purge system. 
     The first and second bypass lines  16  and  17 , which are connected to the negative pressure intake line  15 , bypass the negative pressure maintaining valve  19 . Further, the first bypass line  16  is provided with the normally-open valve  20  and the second bypass pipe  17  is provided with the pressure relief valve  21 . Even when the negative pressure maintaining valve  19  closes during the leak check, the normally-open valve  20  can open and allow the negative pressure in the fuel tank  3  through the first bypass pipe  16 . Therefore, the closed space defined between the purge valve  18  and fuel tank  3  can be maintained under the negative pressure and the leak check is executed. 
     When the engine  1  is running, the fuel tank  3  is maintained under the negative pressure. If the negative pressure increases, the fuel tank  3  is likely to be deformed. However, when the engine  1  is running, that is, when the normal open valve  20  is closed, if the negative pressure in the fuel tank  3  exceeds the predetermined pressure, the pressure relief valve  21  opens. Therefore, the negative pressure in the fuel tank  3  can be released to the canister  4  through the second bypass pipe  17 . Accordingly, the deformation of the fuel tank  3  due to the excessive negative pressure can be restricted. 
     The present invention should not be limited to the disclosed embodiments, but may be implemented in other ways without departing from the spirit of the invention.