Abstract:
A diagnostic system and method identifies fuel injector failure in a fuel cell system including a fuel processor and a fuel source. A fuel injector supplies fuel from the fuel source to the fuel processor. A pressure sensor generates a pressure signal based on pressure between the fuel source and the fuel injector. A fuel injector diagnostic identifies fuel injector failure based on the pressure signal. The fuel injector diagnostic includes a moving window tracker that tracks the pressure signal over a moving window. The fuel injector diagnostic further includes a standard deviation or variance calculator that generates a standard deviation or variance based on the pressure signal in the moving window.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to fuel injectors of fuel cells, and more particularly to a diagnostic system for identifying fuel injector failure in a fuel cell system.  
         BACKGROUND OF THE INVENTION  
         [0002]    Pulsed fuel injectors are used to control the delivery of fuel in internal combustion engines of vehicles. Pulsed fuel injectors are also typically used with fuel cell systems that convert liquid fuel to a hydrogen-rich gas stream. Pulsed fuel injectors are proven, robust, and cost effective due to experience gained from their use in internal combustion engines.  
           [0003]    Conventional methods for diagnosing degradation and failure of a fuel injector in internal combustion engines are inadequate for fuel cell applications. Diagnostic systems for internal combustion engines typically check for negative effects of a failed fuel injector. For example, a vehicle On Board Diagnostic (OBD) method monitors cylinder misfires. When an injector fails, the failed injector causes a momentary decrease in crankshaft speed. A position sensor detects the decrease and triggers a check engine light.  
           [0004]    In internal combustion engines, an injector typically fails in a closed position, which does not harm the engine. A short term misbalance between fuel, air and spark does not typically impact engine durability. In a fuel cell fuel processor, however, the balance of inputs is more critical. A proper balance between fuel, air, and water keeps the fuel processor catalysts in proper working condition.  
           [0005]    If the fuel injector fails while running, the fuel processor temperature begins changing. If the injector fails in the near closed position, temperature may rise due to a higher oxygen to carbon ratio (O/C). If the injector fails fully closed or open, temperature will eventually drop. Since the fuel processor has appreciable mass, the temperature drop may not occur quickly. During this time, water and air are reaching the fuel processor components without proper reactions taking place, which may harm the fuel processor components. Also, any change in temperature may be caused by other problems such as incorrect air or water. Therefore, monitoring temperature does not necessarily identify a failed injector.  
           [0006]    If the fuel injector fails during startup, the system may run for several minutes before detection. Temperatures often takes a significant amount of time to rise in a cold fuel processor, even with a working fuel injector. A failed injector cannot be diagnosed based on temperature until long after the first fuel command. By this time, a significant amount of incorrect fuel has been supplied to the fuel processor.  
           [0007]    Fuel flow sensors can be used to diagnose a failed injector. Fuel flow sensors are expensive and typically have moving parts that may fail. Fuel flow sensors add cost, size, and weight, reduce reliability and have wiring and controller I/O requirements.  
         SUMMARY OF THE INVENTION  
         [0008]    A diagnostic system and method identifies fuel injector failure in a fuel cell system including a fuel processor and a fuel source. A fuel injector supplies fuel from the fuel source to the fuel processor. A pressure sensor generates a pressure signal based on pressure between the fuel source and the fuel injector. A fuel injector diagnostic identifies fuel injector failure based on the pressure signal.  
           [0009]    In other features, a fuel pump pumps fuel from the fuel source to the fuel injector. A regulator communicates with the fuel tank and regulates pressure between the fuel pump and the fuel injector. The fuel injector diagnostic is implemented using a controller with a processor and memory.  
           [0010]    In still other features, the fuel injector diagnostic includes a moving window that tracks the pressure signal over a moving window. The fuel injector diagnostic further includes a standard deviation calculator that generates a standard deviation based on the pressure signal in the moving window. Alternately, the fuel injector diagnostic includes a variance calculator that generates a variance based on the pressure signal in the moving window.  
           [0011]    In still other features, the fuel injector diagnostic includes a diagnostic enabler that enables the fuel injector diagnostic. The diagnostic enabler includes a comparator that receives an injector command and an injector command minimum signal. A noise reducer communicates with the comparator and transitions from low to high after a first period after the comparator signal goes high and from high to low after a second period after the comparator signal goes low.  
           [0012]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0014]    [0014]FIG. 1 is a functional block diagram of a fuel cell system including a fuel injector;  
         [0015]    [0015]FIG. 2 is a graph illustrating the operation of an operating injector;  
         [0016]    [0016]FIG. 3 is a graph illustrating the operation of an injector that failed in the full on condition;  
         [0017]    [0017]FIG. 4 is a functional block diagram of a diagnostic system for a fuel injector of a fuel cell system; and  
         [0018]    [0018]FIG. 5 is a flowchart illustrating steps performed by the controller.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements.  
         [0020]    In a fuel processor of a fuel cell, the balance of inputs is critical. Inputs typically include fuel, air, and water. If fuel stops flowing to the fuel processor, air and water pass through components of the fuel processor. Air can deactivate certain catalysts. The water may damage non-combusting fuel processors. Since fuel is not entering the fuel processor due to a failed injector, the water entering the fuel processor may be liquid water instead of steam. Liquid water that remains in components of the fuel processor after shutdown causes problems with catalysts. Liquid water also promotes corrosion, decreases durability, and causes restart problems at and below freezing temperatures. If too much fuel is flowing, resulting in a very low O/C and S/C, there is a possibility of forming carbon, which will damage the fuel processor and other downstream components (e.g. the fuel cell stack).  
         [0021]    Referring now to FIG. 1, a fuel system  20  for a fuel processor  22  of a fuel cell includes a fuel pump  24 , a fuel regulator  26 , and a fuel injector  28 . A fuel pressure sensor  30  generates pressure signals that are used by a controller  34  to diagnose a failed pump or regulator. If the pump  24  stops working or the regulator  26  fails in an open position, the controller  34  senses a pressure drop and takes appropriate action(s). The action(s) may include shutdown, turning on an indicator or other actions.  
         [0022]    If the regulator  26  fails in a closed position, the pressure rises above a normal pressure and the controller  34  takes appropriate action. The fuel injectors  28  usually fail in an off or closed position. If this happens, the fuel pressure remains at the appropriate level. Prior methods to detect the failed injector  28  include sensing a temperature change in the fuel processor  22 . If the fuel injector  28  fails during startup, prior methods detect a failure by waiting for light off of the fuel processor  22 . In both of these cases, there is a significant amount of time when air and water enter the fuel processor  22  without fuel, which damages the fuel processor  22 .  
         [0023]    The present invention takes advantage of existing hardware to provide a timely and accurate diagnosis of fuel injector problems. The on/off control of the fuel injector  28  creates a pressure wave in the fuel system  20 . The pressure wave occurs when the fuel injector  28  is opening and closing to inject fuel. The controller  34  that monitors the pressure sensor  30  (for fuel pump  24  or regulator  26  problems) can also monitor the pressure sensor for the pressure wave that is associated with an operating fuel injector. In one implementation, the controller  34  generates and monitors a standard deviation of the pressure signal over a small moving window of time. Other implementations monitor pressure variance. The controller  34  diagnoses the operation of the fuel injector  28  based on the standard deviation.  
         [0024]    Referring now to FIG. 2, graphs illustrating an operating fuel injector are shown. The fuel line pressure is shown before and after the fuel injector  28  is operating. Note a significant increase in standard deviation when the fuel injector  28  is on. A pump “on” command leads a fuel injector “on” command by several seconds to build fuel pressure in the fuel system  20 . A large standard deviation occurs when the pump is first turned on. The fuel injector diagnostic is preferably enabled after the fuel injector  28  is commanded on. Since the fuel injector diagnostic is not enabled when the pump and fuel injector are off, the low standard deviation is ignored.  
         [0025]    Referring now to FIG. 3, graphs illustrating a failed fuel injector are shown. When the standard deviation falls below a first threshold such as  15 , a warning light is turned on. When the standard deviation falls below a second threshold such as  10 , the fuel cell is shut down.  
         [0026]    The fuel system  20  is preferably calibrated to determine the pressure characteristics of an operating fuel injector. The calibration is specific to the system hardware such as pump dynamics, regulator dynamics, injection rate frequencies, and operating pressures. Once the fuel system  20  is well understood, the pressure measurement and statistical analysis determine degradation of the fuel injector. This allows advanced warning of a failing fuel injector before the problem forces a shutdown. Therefore, the first and second standard deviation thresholds that identify warning and shutdown will vary. It is also possible, with other control and mathematical techniques, to dynamically determine the characteristics of the fuel system so that changes in the system over time, as well as variations due to production tolerances, and other effects can be accounted for.  
         [0027]    Referring now to FIG. 4, an exemplary implementation of the fuel injector diagnostic is shown generally at  100 . The fuel pressure signal from the pressure sensor is input to a fuel pressure window  102 . The fuel pressure window can be implemented using a circuit, an algorithm executed by a controller or in any other suitable manner. An output of the fuel pressure window  102  is input to a standard deviation or variance calculator  104 , which outputs a fuel pressure standard deviation or variance to a fuel injector diagnostic  106 . An injector command is input to a comparator  110 . Another input of the comparator  110  is coupled to an injector command minimum, which is preferably set equal to 0.  
         [0028]    An output of the comparator  110  is input to a noise reducer  114 . The noise reducer can be implemented using a double debounce circuit or algorithm that changes state from low to high after the signal is high for a first period. The noise reducer  114  changes from high to low after the signal is low for a second period. The noise reducer  114  reduces effects of noise and outputs a fuel injector diagnostic enable signal to the fuel injector diagnostic  106 . Based on the input signals, the fuel injector diagnostic  106  sets a fuel injector flag or takes other action if a fault is detected. For example, the fuel injector diagnostic  106  turns on the warning light when the standard deviation is below  15  and shuts down the fuel cell when the standard deviation is below  10 . As can be appreciated, other mathematical methods of determining injector performance, such as variance, may be used. The present invention also works with water injection systems as well.  
         [0029]    Referring now to FIG. 5, exemplary steps for operating the controller  34  are shown generally at  130 . Control begins with step  134 . In step  138 , the controller  34  determines whether the pump  24  is on. If not, control loops back to step  138 . Otherwise, control continues with step  142  where the controller  34  determines whether the fuel injector  28  is turned on. If not, control loops back to step  142 . Otherwise, control continues with step  146  and starts a timer.  
         [0030]    In step  150 , the controller  34  determines whether the timer is up. If not, control loops back to step  150 . Otherwise, control continues with step  154  and calculates pressure standard deviation or variance over a time window. In step  158 , control determines whether the calculated measurement is less than a first threshold SD 1  (such as a first standard deviation or variance). If not, control loops back to step  154 . Otherwise, control flags a malfunction in step  162 . In step  164 , the controller  34  determines whether the measurement is less than a second threshold SD 2  (such as a second standard deviation or variance). If true, the controller  34  shuts down the fuel cell in step  166  and control ends in step  168 . If false, the controller  34  loops back to step  154 .  
         [0031]    The fuel injector diagnostic system according to the present invention identifies a failed injector before the injector damages the fuel processor. Injector degradation is detected and the controller warns the operator before forcing a shutdown. When the fuel injector fails, the root cause is diagnosed, which reduces the cost of repairs.  
         [0032]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention 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.