Patent Publication Number: US-8122758-B2

Title: Purge valve leak diagnostic systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/030,395, filed on Feb. 21, 2008. The disclosure of the above application is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to fuel systems and more particularly to fuel vapor purge valves. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Internal combustion engines combust a mixture of air and fuel to generate torque. The fuel supplied to the engine may be liquid fuel and/or vapor fuel. Generally, liquid fuel is contained within a fuel tank. Liquid fuel is drawn from the fuel tank and provided to the engine by one or more fuel injectors. 
     Various factors, such as vibration and heat, may cause the liquid fuel to vaporize within the fuel tank. Vehicles include a purge system that traps fuel vapor and provides the fuel vapor to the engine for combustion. The purge system includes a vapor canister traps and stores fuel vapor from the fuel tank. The fuel vapor is purged from the canister and provided to the engine. 
     The purge system also includes a purge valve and a vent valve (e.g., a diurnal valve). Operation of the engine causes a vacuum (e.g., low pressure relative to barometric pressure) to form within an intake manifold of the engine. Selective actuation (i.e., opening and closing) of the purge valve and the vent valve allows the fuel vapor to be drawn from the vapor canister into the intake manifold. In this manner, fuel vapor is provided to the engine for combustion and purged from the vapor canister. 
     SUMMARY 
     A leak diagnostic system for a vehicle comprises a tank pressure module and a leak diagnostic module. The tank pressure module selectively outputs first and second fuel tank pressures when an engine is shut down and when engine vacuum is greater than a predetermined engine vacuum, respectively. The leak diagnostic module selectively diagnoses a leak in a fuel vapor purge valve based on the second fuel tank pressure when the first fuel tank pressure is less than a first predetermined pressure. 
     A plug-in hybrid vehicle system comprises the leak diagnostic system of claim  1  and the fuel vapor purge valve. 
     In other features, the leak diagnostic module diagnoses the leak when the second fuel tank pressure is greater than a second predetermined pressure that is greater than the first predetermined pressure. 
     In still other features, the leak diagnostic module selectively disables diagnosing the leak based on the second fuel tank pressure when the first fuel tank pressure is greater than the first predetermined pressure. 
     In further features, the tank pressure module determines that the fuel vapor purge valve and a vent valve are in closed positions before outputting the first and second fuel tank pressures. 
     In still further features, the second predetermined pressure is based on the predetermined engine vacuum. 
     In other features, the tank pressure module selectively determines a pressure offset based on a difference between the first fuel tank pressure and the first predetermined pressure. The tank pressure module subtracts the pressure offset from the second fuel tank pressure before outputting the second fuel tank pressure. 
     In still other features, the tank pressure module determines the pressure offset when the first fuel tank pressure is less than the first predetermined pressure. 
     In further features, the tank pressure module outputs the first fuel tank pressure a predetermined period after the engine is shut down. 
     In still further features, the predetermined period based on an expected period when a vacuum forms within a fuel tank after the engine is shut down. 
     A leak diagnostic method for a vehicle comprises selectively outputting first and second fuel tank pressures when an engine is shut down and when engine vacuum is greater than a predetermined engine vacuum, respectively, and selectively diagnosing a leak in a fuel vapor purge valve of the vehicle based on the second fuel tank pressure when the first fuel tank pressure is less than a first predetermined pressure. 
     In other features, the vehicle is a plug-in hybrid vehicle. 
     In still other features, the selectively diagnosing the leak comprises diagnosing the leak when the second fuel tank pressure is greater than a second predetermined pressure that is greater than the first predetermined pressure. 
     In further features, the leak diagnostic method further comprises selectively disabling the selectively diagnosing the leak when the first fuel tank pressure is greater than the first predetermined pressure. 
     In still further features, the leak diagnostic method further comprises determining that the fuel vapor purge valve and a vent valve are in closed positions before the outputting the first and second fuel tank pressures. 
     In other features, the second predetermined pressure is based on the predetermined engine vacuum. 
     In still other features, the leak diagnostic method further comprises selectively determining a pressure offset based on a difference between the first fuel tank pressure and the first predetermined pressure and subtracting the pressure offset from the second fuel tank pressure before the selectively outputting the second fuel tank pressure. 
     In other features, the selectively determining comprises determining the pressure offset when the first fuel tank pressure is less than the first predetermined pressure. 
     In still other features, the selectively outputting the first fuel tank pressure comprises outputting the first fuel tank pressure a predetermined period after the engine is shut down. 
     In further features, the predetermined period based on an expected period when a vacuum forms within a fuel tank after the engine is shut down. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of an exemplary fuel system according to the principles of the present disclosure; 
         FIG. 2  is a functional block diagram of an exemplary implementation of a purge valve leak detection module according to the principles of the present disclosure; and 
         FIG. 3  is a flowchart depicting exemplary steps performed by the purge valve leak detection module according to the principles of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     As used herein, the term module refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     A leak diagnostic system and method according to the present disclosure selectively diagnoses a leak in a fuel vapor purge valve based on one or more fuel tank pressures measured when the purge valve is in a closed position. More specifically, the present disclosure relates to diagnosing a leak in the purge valve based on a first tank pressure and/or a second tank pressure. 
     The first tank pressure is measured while the engine is shut down, such as a predetermined period of time after the engine is shut down. As the purge valve is maintained in the closed position while the engine is shut down, a vacuum should naturally form within the fuel tank. Accordingly, a purge valve leak may be present when the first tank pressure does not reflect the presence of such a vacuum. 
     The second tank pressure is measured while the engine is running. More specifically, the second tank pressure is measured while the engine vacuum is greater than a predetermined engine vacuum. As the purge valve is also maintained in the closed position when the second tank pressure is measured, the engine vacuum should not be reflected in the second tank pressure. Accordingly, a leak may also be present in the purge valve when the second tank pressure reflects the engine vacuum. 
     Referring now to  FIG. 1 , a functional block diagram of an exemplary fuel system  100  is presented. Generally, a vehicle includes an internal combustion engine (not shown) that generates torque. For example only, the engine may be a gasoline-type engine, a diesel-type engine, and/or any other suitable type of engine. The engine combusts a mixture of air and fuel within one or more cylinders of the engine to generate torque. 
     In some vehicles, torque generated by the engine may be used to propel the vehicle. In such vehicles, torque output by the engine is transferred to a transmission, which may then transfer torque to one or more wheels of the vehicle. In other vehicles, such as plug-in hybrid vehicles, torque output by the engine is not transferred to the transmission. Instead, torque output by the engine is converted into electrical energy by, for example, a generator or a belt alternator starter (BAS). The electrical energy may be then provided to an electric motor and/or an energy storage device. The plug-in hybrid vehicle may also receive electrical energy from an alternating current (AC) power source, such as a standard wall outlet. The electric motor uses electrical energy to generate torque to propel the vehicle. 
     The fuel system  100  supplies fuel to an engine, such as an engine of a plug-in hybrid vehicle or any other suitable vehicle. More specifically, the fuel system  100  supplies liquid fuel and fuel vapor to the engine. While the operation of the fuel system  100  will be discussed as it relates to plug-in hybrid vehicles, the principles of the present disclosure are applicable to other vehicles having an internal combustion engine. 
     The fuel system  100  includes a fuel tank  102  that contains liquid fuel. Some conditions, such as heat, vibration, and/or radiation, may cause liquid fuel contained within the fuel tank  102  to vaporize. A canister  104  traps and stores vaporized fuel (i.e., fuel vapor). For example only, the canister  104  may include one or more substances, such as a charcoal substance, which store fuel vapor. 
     Operation of the engine creates a vacuum within an intake manifold of the engine. A purge valve  106  and a vent valve  108  may be selectively operated to draw fuel vapor from the canister  104  to the intake manifold for combustion. Operation of the purge valve  106  and the vent valve  108  may be coordinated to purge fuel vapor from the canister  104 . An engine control module (ECM)  110  controls the operation of the purge valve  106  and the vent valve  108 . 
     At a given time, the purge valve  106  and the vent valve  108  may each be in one of two positions: an open position and a closed position. For example, the ECM  110  may allow ambient air into the canister  104  by commanding the vent valve  108  to the open position. When the vent valve  108  is in the open position, the ECM  110  may command the purge valve  106  to the open position to purge fuel vapor from the canister  104  to the intake manifold. The ECM  110  also controls the rate at which fuel vapor is purged from the canister  104  (i.e., a purge rate) by adjusting how long the purge valve  106  is in the open position during a given period of time (i.e., a purge valve duty cycle). 
     The vacuum within the intake manifold draws fuel vapor from the canister  104  to the intake manifold via the purge valve  106 . The purge rate may be determined based on the duty cycle of the purge valve  106  and the amount of fuel vapor within the canister  104 . Coincidentally, air at ambient (i.e., barometric) pressure is drawn into the canister  104  via the vent valve  108 . 
     A driver input module  120  receives various commands from a driver, such as commands regarding the operational status of the engine. For example, the driver input module  120  may receive an engine startup command and an engine shutdown command. The driver may command engine startup or shutdown by, for example, turning a key or pressing a button. The driver input module  120  transmits the driver&#39;s commands to a fuel system control module  122 , which may then transmit the driver&#39;s commands to the ECM  110 . 
     The ECM  110  starts the engine when the engine startup command is received. For example, the ECM  110  may activate a starter or other device to start the engine. The ECM  110  commands the vent valve  108  to the open position and controls the duty cycle of the purge valve  106  after the engine is started (i.e., when the engine is ON). 
     The ECM  110  also shuts down the engine when the engine shutdown command is received. For example, the ECM  110  eliminates combustion to shut down the engine. When the engine is shut down, the ECM  110  commands both the purge valve  106  and the vent valve  108  to their respective closed positions. Accordingly, both the purge valve  106  and the vent valve  108  are maintained in their respective closed positions when the engine is not operational (i.e., OFF). 
     A vacuum naturally forms within the fuel tank  102  after the engine is shut down. This vacuum may be attributable to heating and subsequent cooling of gas (e.g., air and/or fuel vapor) present in the fuel tank  102  after the engine is shut down. 
     The ECM  110  may receive other signals and may perform various functions based on the received signals. For example only, the ECM  110  may receive a tank pressure signal and an engine vacuum signal. A tank pressure sensor  126  measures gas pressure within the fuel tank  102  (i.e., a tank pressure) and generates the tank pressure signal accordingly. While the tank pressure sensor  126  is shown as being located within the canister  104 , the tank pressure sensor  126  may be located in any suitable location, such as within the fuel tank  102 . The engine vacuum signal may be generated based on, for example, a manifold absolute pressure (MAP) measured by a MAP sensor (not shown). For example, the engine vacuum may be the difference between the barometric pressure and the MAP. 
     The ECM  110  includes a purge valve leak detection module  200  (as shown in  FIG. 2 ) that selectively diagnoses a leak in the purge valve  106 . The purge valve leak detection module  200  according to the present application selectively diagnoses a leak in the purge valve  106  based on a first tank pressure that is measured while the engine is shut down and/or a second tank pressure that is measured while the engine is operational. Leak detection may be used, for example, to ensure that fuel vapor does not escape when the purge valve  106  is closed, such as when the engine is shut down. 
     While the purge valve leak detection module  200  is discussed as being located within the ECM  110 , the purge valve leak detection module  200  may be located in any suitable location. For example only, the purge valve leak detection module  200  may be located within the fuel system control module  122 , another module within a plug-in hybrid vehicle system, and/or any other module in any other type of vehicle system. 
     Referring now to  FIG. 2 , a functional block diagram of an exemplary implementation of the purge valve leak detection module  200  is presented. The purge valve leak detection module  200  includes an engine status module  202 , an engine load module  204 , a tank pressure module  206 , and a leak diagnostic module  208 . The engine status module  202  determines the operational status of the engine and generates an engine status indicator accordingly. More specifically, the engine status module  202  may determine whether the engine is operational or not operational. 
     The engine status module  202  may determine the operational status of the engine based on the driver&#39;s commands and/or other engine parameters, such as the output speed of the engine (RPM) and/or the engine vacuum. The engine status module  202  may determine that the engine is operational after the engine startup command is received and/or the engine vacuum is greater than a predetermined pressure, such as 0.0 inches water. Similarly, the engine status module  202  may determine that the engine is not operational after the engine shutdown command is received and/or the engine vacuum is approximately equal to 0.0 inches water. 
     The engine load module  204  generates an engine load indicator (signal) based on the engine vacuum. More specifically, the engine load module  204  generates the engine load indicator based on a comparison of the engine vacuum with a maximum possible engine vacuum (EV MAX ). The engine load indicator indicates whether specified engine load conditions are satisfied. For example only, the engine load conditions may be satisfied when:
 
Engine Vacuum&gt;80% (EV MAX ),
 
where EV MAX  is the maximum possible engine vacuum. For example only, the EV MAX  may be 100.0 kPa. In other implementations, the engine load conditions may be satisfied when the engine vacuum is greater than 50% of an EV MAX  of 80.0 kPa.
 
     The engine load module  204  may also require that the engine load conditions be satisfied for a predetermined period of time. Accordingly, the engine load conditions may be satisfied when the engine vacuum is greater than a predetermined percentage of the EV MAX  for at least the predetermined period of time. For example only, the predetermined period of time may be 10.0 seconds. In other implementations, the predetermined period of time may be 60.0 seconds. 
     As stated above, the ECM  110  commands the vent valve  108  to its open position and selectively actuates the purge valve  106  when the engine is operational. When the engine load conditions have been satisfied, as indicated by the engine load indicator, the ECM  110  commands both the purge valve  106  and the vent valve  108  to their respective closed positions. 
     The tank pressure module  206  receives the tank pressure signal from the tank pressure sensor  126  and selectively outputs tank pressures. More specifically, the tank pressure module  206  selectively outputs tank pressures based on the operational status of the engine and the engine load conditions. 
     The tank pressure module  206  outputs a first tank pressure (i.e., vacuum) while the engine is not operational. The tank pressure module  206  may output the first tank pressure, for example, at a time when the engine is shut down, a predetermined period after engine shutdown, or before engine startup when a key is inserted into an ignition. For example only, the tank pressure module  206  may output the first tank pressure 20.0-30.0 minutes after the engine is shut down. 
     The closing of the vent valve  108  and the purge valve  106  after engine shutdown coupled with heating and then cooling of the gas within the fuel tank  102  causes a natural vacuum to form within the fuel tank  102 . In various implementations, the output of the first tank pressure may be timed relative to the time at which the natural vacuum is likely the greatest. 
     The tank pressure module  206  also outputs a second tank pressure (i.e., vacuum). The second tank pressure, unlike the first tank pressure, is output at a time when the engine is operational. More specifically, the tank pressure module  206  may output the second tank pressure when the engine load conditions are satisfied. The tank pressure module  206  may also ensure that both the vent valve  108  and the purge valve  106  are in their respective closed positions before outputting the second tank pressure. 
     The leak diagnostic module  208  selectively diagnoses a leak in the purge valve  106  based on the first tank pressure and/or the second tank pressure. More specifically, the leak diagnostic module  208  selectively diagnoses the presence of a leak in the purge valve  106  based on a comparison of the first tank pressure with a first predetermined pressure (i.e., vacuum). For example only, the first predetermined pressure may be determined based on the natural vacuum and may be equal to 2.5 inches water. 
     As the purge valve  106  and the vent valve  108  are in their respective closed positions when the engine is shut down, vacuum that forms should be retained, and the first tank pressure should reflect this vacuum. Accordingly, a leak is not likely present when the first tank pressure is greater than the first predetermined pressure. 
     The leak diagnostic module  208  may set an offset value equal to the first tank pressure if the first tank pressure is less than the first predetermined pressure. The offset value may represent an amount of measurement error that may be attributable to the tank pressure sensor  126 . This offset value may be used in conjunction with leak diagnostics involving the second tank pressure, as discussed further below. 
     If the first tank pressure is less than the first predetermined pressure, the leak diagnostic module  208  also diagnoses whether a leak is present in the purge valve  106  based on a comparison of the second tank pressure with a second predetermined pressure (i.e., vacuum). For example only, the second predetermined pressure may be determined based on the engine vacuum when the engine load conditions are satisfied. In various implementations, the second predetermined pressure may be equal to 12.0 inches water. 
     As the purge valve  106  and the vent valve  108  are closed when the second tank pressure is output, vacuum that is present in the intake manifold should be isolated. Therefore, the second tank pressure should not reflect the manifold vacuum. Accordingly, the leak diagnostic module  208  may diagnose a leak in the purge valve  106  when the second tank pressure is greater than the second predetermined pressure. The leak diagnostic module  208  generates a purge valve leak indicator (signal) based on the diagnosis. 
     The leak diagnostic module  208  may also subtract the offset value from the second tank pressure before comparing the second tank pressure with the second predetermined pressure. Such a subtraction may be implemented to prevent the leak diagnostic module  208  from incorrectly diagnosing a leak in the purge valve  106  that may instead be attributable to measurement error of the tank pressure sensor  126 . 
     The leak diagnostic module  208  may transmit the purge valve leak indicator to the ECM  110 , which may take remedial action when a leak has been diagnosed in the purge valve  106 . For example only, the ECM  110  may illuminate a “check engine light” and/or set a flag in memory when a leak has been diagnosed. 
     Referring now to  FIG. 3 , a flowchart depicting exemplary steps performed by the purge valve leak detection module  200  is presented. Control begins in step  300  where control measures the first tank pressure. The engine is not operational and both the purge valve  106  and the vent valve  108  are in their respective closed positions when the first tank pressure is measured. 
     Control continues in step  302  where control determines whether the first tank pressure is greater than the first predetermined pressure. If true, control transfers to step  304 ; otherwise, control continues to step  306 . For example only, the first predetermined pressure may be equal to 2.5 inches water. In step  304 , control indicates that the purge valve  106  does not have a leak, and control ends. 
     In step  306  (if the first tank pressure is less than the first predetermined pressure), control commands startup of the engine. The vent valve  108  is then opened, and the purge valve  106  is selectively actuated. Control continues in step  308  where control determines whether the engine load conditions are satisfied. If true, control continues to step  310 ; otherwise, control remains in step  308 . The engine load conditions may be satisfied when the engine vacuum is greater than a predetermined percentage of the EV MAX  for a predetermined period of time. For example only, the predetermined percentage may be 80%, and the predetermined period of time may be 10.0 seconds. In other implementations, the predetermined percentage may be 50% and the predetermined period may be 60.0 seconds. 
     In step  310 , control commands (i.e., maintains) both the purge valve  106  and the vent valve  108  to their respective closed positions. Control then continues in step  312  where control measures the second tank pressure. In step  314 , control determines whether the second tank pressure is greater than the second predetermined pressure. If true, control continues to step  316 ; otherwise, control returns to step  304 . For example only, the second predetermined pressure may be equal to 12.0 inches water. In step  316 , control diagnoses a leak in the purge valve  106  and control indicates a leak is present in the purge valve  106 . Control then ends. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.