Abstract:
A system and method comprises operating an engine during a first cycle to drive a piston in a cylinder without energizing a fuel injector of the cylinder; acquiring first pressure data of the cylinder for a predetermined crank angle window during the first cycle; energizing the fuel injector for an energizing time during a second cycle; acquiring second pressure data of the cylinder for the predetermined crank angle window during the second cycle; calculating a pressure ratio difference average (PRDA) from the first pressure data and the second pressure data; and modifying the operation of the fuel injector based on the PRDA value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/043,220, filed on Apr. 8, 2008. The disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to engine systems, and more particularly to control of fuel injection in an engine system. 
       BACKGROUND 
       [0003]    An engine control system of a vehicle controls the delivery of air and fuel to a cylinder of an engine. The mixture of air and fuel is combusted within the cylinder to generate torque. More specifically, combustion of the air/fuel mixture releases thermal energy that drives pistons within the cylinders to power the vehicle. A fuel injector associated with the cylinder provides the fuel of the air/fuel mixture. The amount of fuel provided by the fuel injector is based on an amount of air provided to the engine for a target torque. 
         [0004]    One way to reduce emissions from an engine involves recirculating exhaust gas into the combustion process. For example, exhaust gas recirculation (EGR) may be used in a diesel engine. EGR decreases exhaust emissions but tends to make combustion less stable. Providing a small pilot injection quantity (or “shot”) prior to a main fuel injection shot may help to stabilize combustion when EGR is used. The amount of fuel in the pilot injection shot is typically less than the main shot. The amount and timing of the pilot injection shot is usually based on a calibrated amount for the engine. Deviations from the calibrated amount and timing of the pilot shot may reduce its effectiveness in aiding combustion and decreasing exhaust emissions. 
         [0005]    Fuel injectors may be operated by associating a fuel injector energizing time with a fuel injection amount. The actual amount of the resulting pilot shot or main shot may be a function of the fuel injector construction and the pressure of the fuel delivered to the fuel injector from a fuel rail. A fuel injector calibration map may be created by performing a bench test. Actual injection amounts may be measured and stored for different injector energizing times at different fuel rail pressures. When a control system of the engine commands a particular fuel amount to be injected, the calibration map may be consulted to return a fuel injector energizing time for the fuel rail pressure. Any values not included in the calibration map may be interpolated from the calibration map. 
         [0006]    Fuel injectors and engine systems may have variations such that a calibration map does not precisely match fuel injection characteristics in a particular vehicle. Fuel injectors may also be faulty or may degrade over time (i.e., injector aging). Injector aging may result in injection of fuel quantities different from the expected quantity for a particular energizing time and rail pressure. Pilot injection may involve quantities of fuel at the low end of the fuel injector operating range, particularly when the fuel rail pressure is high. The fuel injector may have a minimum energizing time, and at high fuel rail pressures may not be able to deliver a small amount of fuel desired for a pilot shot. 
       SUMMARY 
       [0007]    A system and method comprises operating an engine during a first cycle to drive a piston in a cylinder without energizing a fuel injector of the cylinder; acquiring first pressure data of the cylinder for a predetermined crank angle window during the first cycle; energizing the fuel injector for an energizing time during a second cycle; acquiring second pressure data of the cylinder for the predetermined crank angle window during the second cycle; calculating a pressure ratio difference average (PRDA) from the first pressure data and the second pressure data; and modifying the operation of the fuel injector based on the PRDA value. 
         [0008]    In other features, the energizing time is associated with a desired injected fuel amount. The system and method includes indexing a calibration map to determine the energizing time based on the desired injected fuel amount and a fuel rail pressure. The modifying includes changing at least one energizing time value of the calibration map based on the PRDA value. 
         [0009]    In other features, the system and method includes indexing a PRDA map to determine an actual injected fuel amount based on the PRDA value and a fuel rail pressure. The system and method includes changing at least one energizing time of the calibration map when the actual injected fuel amount deviates from the desired injected fuel amount by more than a predetermined threshold. 
         [0010]    In other features, the system and method includes accessing a PRDA map to determine an expected PRDA value based on the desired injected fuel amount and a fuel rail pressure. The system and method includes changing at least one energizing time of the calibration map when the actual PRDA value deviates from the expected PRDA value by more than a predetermined threshold. 
         [0011]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0013]      FIG. 1  is a functional block diagram of a diesel engine system according to the present disclosure; 
           [0014]      FIG. 2  is a functional block diagram of an engine control module according to the present disclosure; 
           [0015]      FIG. 3  is a depiction of exemplary pressure ratio difference average (PRDA) and fuel injector energizing time look-up tables; 
           [0016]      FIG. 4  depicts PRDA vs. indicated mean effective pressure (IMEP) for an exemplary fuel injector and operating conditions; 
           [0017]      FIG. 5  depicts PRDA vs. an injected pilot quantity for an exemplary fuel injector and operating conditions; 
           [0018]      FIG. 6  depicts PRDA vs. IMEP for an exemplary fuel injector and operating conditions; 
           [0019]      FIG. 7  depicts a fuel injector energizing time and PRDA vs. a commanded fuel injection quantity; 
           [0020]      FIG. 8  is a flow diagram depicting steps in fuel injector diagnostics; and 
           [0021]      FIG. 9  is a flow diagram depicting steps in pilot injection diagnostics. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 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 executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
         [0023]    Referring to  FIG. 1 , a diesel engine system  10  including fuel injection measurement and diagnostic system is depicted. Engine system  10  includes an engine  12  that combusts an air/fuel mixture to produce drive torque. Engine system  10  may also include fuel system  14 , intake manifold  16 , exhaust manifold  18 , EGR valve  20 , inlet  22 , EGR line  24 , diesel oxidation catalyst (DOC)  26 , diesel particulate filter (DPF)  28 , control module  30 , cylinders  40 , fuel injectors  42 , intake valve  44 , sensor  46 , and exhaust valve  48 . 
         [0024]    For exemplary purposes, a diesel engine  12  is described. Air is drawn into an intake manifold  16  through an inlet  22 . A throttle (not shown) may be included to regulate air flow into the intake manifold  16 . Air within the intake manifold  16  is distributed into cylinders  40 . Although  FIG. 1  depicts eight cylinders, it should be appreciated that the engine  12  may include additional or fewer cylinders  40 . For example, engines having 1, 2, 3, 4, 5, 6, 10, 12 and 16 cylinders are contemplated. 
         [0025]    Engine system  10  includes an engine control module  30  that communicates with components of the engine system  10 , such as the engine  12 , fuel system  14 , and associated sensors and controls as discussed herein. The engine control module  30  may include a calibration map and a PRDA map used with fuel system  14  and fuel injectors  42  to control fuel injection into cylinders  40 . 
         [0026]    Fuel system  14  may include a fuel pump (not shown) to pressurize fuel and a fuel rail (not shown) to deliver fuel to the fuel injectors  42 . Fuel injectors  42  may be operated by commanding an energizing or on time. The amount of fuel delivered may be based on fuel rail pressure, energizing time, and fuel injector  42  construction. 
         [0027]    Engine control module  30  electronically controls fuel injectors  42  to inject fuel into the cylinders  40 . An intake valve  44  may selectively open and close to enable air to enter the cylinder  40 . A camshaft (not shown) may regulate intake valve position. A piston (not shown) may compress the air/fuel mixture within the cylinder  40  to cause combustion. 
         [0028]    A sensor  46  may be situated such that the pressure in the cylinder may be measured. These measured pressure values may be used by engine control module  30  for fuel injection measurement and diagnostics. Sensor  46  may be capable of providing measurements throughout the combustion cycle. In fuel injector measurement and diagnostics, the pressure over a particular crank angle window may be measured. 
         [0029]    The piston may drive a crankshaft (not shown) to produce drive torque. The crankshaft may be interconnected with the respective pistons of cylinders  40  such that the pistons are driven in a predetermined pattern. Combustion exhaust within the cylinder  40  may be forced out through an exhaust manifold  18  when an exhaust valve  48  is in an open position. A camshaft (not shown) may regulate exhaust valve position. 
         [0030]    A DOC  26  and a diesel particulate filter (DPF)  28  may treat exhaust gas. An exhaust gas recirculation (EGR) system that includes an EGR valve  20 , EGR cooler  21 , a bypass  22 , and an EGR line  24  may introduce exhaust gas into the intake manifold  16 . The EGR valve  20  may be mounted on the intake manifold  16  and the EGR line  24  may extend from the exhaust manifold  18  to the EGR valve  20 , providing communication between the exhaust manifold  18  and the EGR valve  20 . The EGR cooler  21  cools exhaust gas provided to the intake manifold  16 . The bypass  22  allows exhaust gas to bypass the EGR cooler  21 . The engine control module  30  may electronically control a position of the EGR valve  20 . 
         [0031]    Referring now to  FIG. 2 , control module  30  is described in more detail. Control module  30  may include fuel injector diagnostic module  60 , data receiving module  62 , calibration map  64 , PRDA map  66 , engine system control module  68 , and fuel injection control module  70 . These modules of control module  30  may provide for the normal functioning of fuel injector control as well as fuel injector measurement and diagnostics. 
         [0032]    During normal operation, engine system control module  68  may include communicating with fuel injection control module  70  to command fuel injectors  42  to operate with an energizing time to provide a requested amount of fuel based on a known fuel rail pressure. Fuel injection control module  70  may communicate with calibration map  64  to determine a fuel injector  42  energizing time based on a desired fuel quantity and a known fuel rail pressure. 
         [0033]    To perform fuel injector measurement and diagnostics, fuel injector diagnostic module  60  may communicate with data receiving module  62 , calibration map  64 , PRDA map  66 , engine system control module  68 , and fuel injection control module  70 . Fuel injector diagnostic module  60  may command engine system control module  68  and fuel injection control module  70  to operate in a manner that allows measurement of fuel injection quantities and diagnostics. 
         [0034]    A first step includes driving a piston in a motored state, wherein fuel is not injected into the cylinder  40 . Pressure measurements taken during the motored state at specific engine speeds may provide a baseline for determining a fuel injection amount. A stored motored PR at a specific engine speed can also be used. After the baseline is determined, engine  12  may be operated such that the fuel injector  42  being tested is fired within an energizing time associated with a desired fuel amount as provided by calibration map  64 . The fuel injector  42  being tested may be fired at specified times, such as during deceleration overruns. The baseline is compared to pressure measurements when fuel is injected into the cylinder. In other implementations, the engine  12  may be operated in a skip firing mode where the cylinder associated with the fuel injector  42  being tested is fired during two engine cycles and motored during two engine cycles. 
         [0035]    Data receiving module  62  may receive cylinder pressure data  46  for both the motored and fired cycles. Pressure may be measured at a regular interval of crank angle degrees. An exemplary crank angle interval may be every 3°. Data from data receiving module  62  may be transmitted to fuel injector diagnostic module  60 , which may store and use the pressure data to perform fuel injection measurement and diagnostics. 
         [0036]    Fuel injector diagnostic module  60  may use the pressure data to determine a pressure ratio (PR) for each pressure measurement at given crankshaft angles. PR may be equivalent to the measured pressure divided by a calculated or theoretical pressure. PRDA may be calculated from a comparison of PR for a motored cycle and PR for a fired cycle over a particular window of engine operation, as represented by the following equation: 
         [0000]    
       
         
           
             PRDA 
             = 
             
               
                 ∑ 
                 
                   ## 
                    
                   
                       
                   
                    
                   aTDC 
                 
                 
                   ## 
                    
                   
                       
                   
                    
                   aTDC 
                 
               
                
               
                 
                   ( 
                   
                     PR_Fired 
                     - 
                     PR_Motored 
                   
                   ) 
                 
                 
                   # 
                    
                   
                       
                   
                    
                   Samples 
                 
               
             
           
         
       
     
         [0037]    Once the PRDA value is determined, PRDA map  66  may be consulted to determine an amount of fuel associated with the measured PRDA value and a known fuel rail pressure, thus yielding an actual measured fuel amount. PRDA map  66  may be created by testing an exemplary engine to establish a relationship between PRDA, injection quantity and fuel rail pressure at a specific engine speed. Once PRDA map  66  is consulted to find the actual fuel injection quantity, fuel injector diagnostic module  60  may compare the measured fuel injection quantity to the desired fuel injection quantity for diagnostics and to update values in calibration map  64 . In this manner, the fuel injector diagnostic module  60  may learn and update the calibration map  64  accordingly. 
         [0038]    Referring now to  FIG. 3 , two look-up tables are depicted. The look up table on the right may be associated with calibration map  64 . If a rail pressure and desired pilot or injection quantity are known, the energizing time can be looked up from the table. When coupled with the fuel injection measurement and diagnostic system, the energizing time values of calibration map  64  may be changed based on an actual measured fuel amount associated with a particular rail pressure. 
         [0039]    The left side of  FIG. 3  may be a look-up table for PRDA map  66 . PRDA and injection quantity may have a relationship that may be measured and stored in PRDA map  66 . When PRDA values are measured and rail pressure is known, PRDA map  66  may be used to determine an actual injected fuel amount. 
         [0040]    Referring now to  FIG. 4 , a relationship between measured PRDA values and measured indicated mean effective pressure (IMEP) values is depicted. As is known in the art, IMEP provides an indication of the useful work performed by the engine. If PRDA is providing an accurate measure of fuel injection quantity as expected, the graph of  FIG. 4  should depict a linear relationship with IMEP. For this particular measurement in an exemplary engine, the engine is operating at 1000 rpm, the fuel rail pressure is 1300 bar, the pilot shot time for the fired shot is 12° before top dead center (bTDC), and the requested pilot quantity is 4 mm 3 .  FIG. 4  confirms that PRDA increases in a linear relationship with IMEP. 
         [0041]    Referring now to  FIG. 5 , a relationship between PRDA and a commanded pilot quantity is confirmed in an exemplary engine operating at 1000 rpm, a fuel rail pressure of 600 bar, and a pilot shot at 120 bTDC. PRDA measurements have been performed as described above. With the exception of the 4 mm 3  and 5 mm 3  desired pilot injection quantities, there is little overlap between the injection quantities and the PRDA measurements. Accordingly, PRDA appears to properly distinguish between different injection quantities. With respect to the 5 mm 3  commanded pilot quantity, it was confirmed that the injector at issue was actually providing less than 5 mm 3  as properly predicted by PRDA measurement. 
         [0042]    Referring now to  FIG. 6 , a plot of the PRDA values of  FIG. 5  for the different injection amounts against IMEP confirms that greater injection amounts resulted in a greater IMEP, and vice versa. This is the relationship that is expected if PRDA is providing accurate measurements of the injection amount. It is also notable that the 5 mm 3  values overlap with the 4 mm 3  values, confirming expectations from the PRDA measurements of  FIG. 5 . 
         [0043]    Referring now to  FIG. 7 , a graph depicting minimum pilot quantity detection using PRDA measurement is depicted. The solid line depicts energizing time values from an example calibration map to give the commanded injection quantity for the operating conditions of 1000 rpm, fuel rail pressure of 600 bar and injection timing of 12° bTDC. The dashed line depicts actual measured PRDA values associated with a commanded fuel injection quantity. As can be seen from the PRDA measurements, little to no fuel is actually injected below 1 mm 3  and PRDA may have a generally linear relationship above 2 mm 3 . By using measured PRDA values, the calibration map can be adjusted for an actual minimum pilot quantity. The minimum value may be associated with the minimum value above which PRDA values maintain a linear relationship with the commanded fuel injection quantity. 
         [0044]    Referring now to  FIG. 8 , a flowchart depicting control logic  100  includes steps for performing fuel injector diagnostics. At block  102 , fuel injector diagnostic module  60  may determine whether to run injector diagnostics, which may include testing actual injection quantities versus expected injection quantities. Injector diagnostics may be run at regular intervals during vehicle operation in order to test the fuel injector with various fuel rail pressures and injection amounts. If injector diagnostics are to be run, control logic  100  may continue to block  104   
         [0045]    At block  104 , fuel injector diagnostic module  60  may check fuel injector settings for fuel injector diagnostics. For example, fuel injector diagnostic module  60  may provide the sampling resolution, injection timing, injection quantity, injection pressure, and engine speed for fuel injector diagnostics. An example sampling resolution may include the crank angle measurement window during which the PR measurements will be performed for the motored and fired cycles. A sampling resolution may also include how often measurements are made during the crank angle window, such as every 3-6 crank angle degrees. Injection timing may include the timing for providing the injection shot, an example of which may be 12° bTDC. Finally, injection quantity may be a desired injection quantity that will be used to access an energizing time from calibration map  64 . Control logic  100  may continue to block  106 . 
         [0046]    At block  106 , fuel injector diagnostic module  60  may determine PR values for a motored cycle of a cylinder  40  associated with the fuel injector  42  to be tested. Fuel injector diagnostic module  60  may communicate with engine system control module  68  and fuel injection control module  70  to operate engine  12  such that a piston is driven in the cylinder  40  associated with the fuel injector  42  to be tested and no fuel is injected from the fuel injector  42  to be tested. Data receiving module  62  may receive cylinder pressure data from sensor  46  and provide the data to fuel injector diagnostic module  60  to calculate PR values. Once fuel injector diagnostic module  60  has pressure data for the motored cycle at a specific engine speed, control logic  100  may continue to block  108 . 
         [0047]    At block  108 , fuel injector diagnostic module  60  may determine PR values for a fired cycle of the cylinder  40  associated with the fuel injector  42  to be tested. Fuel injector diagnostic module  60  may communicate with engine system control module  68  and fuel injection control module  70  to operate engine  12  such that a requested amount of fuel should be injected by the fuel injector  42  to be tested based on the energizing time values stored in calibration map  64  and the particular rail pressure. Data receiving module  62  may receive cylinder pressure data from sensor  46  and provide the data to fuel injector diagnostic module  60  to calculate PR values over the designated crank angle window. Fuel injector diagnostic module may establish the pressure data for the fired cycle and control logic  100  may then continue to block  110 . 
         [0048]    At block  110 , fuel injector diagnostic module  60  may access external parameters to determine whether the engine operated properly during the measuring process. If any of the external parameters indicate an error in engine operation, control logic  100  may return to block  104  to attempt to measure pressure data again. Otherwise, control logic  100  may continue to block  112 . 
         [0049]    At block  112 , fuel injector diagnostic module  60  may calculate PRDA based on the difference between the fired PR values and motored PR values over the crank angle measurement window. Control logic may continue to block  114 . At block  114 , fuel injector diagnostic module  60  may access PRDA map  66  to determine an actual fuel injection amount associated with the calculated PRDA value. Control logic may continue to block  116 . 
         [0050]    At block  116 , the actual PRDA value may be compared to a target PRDA value accessed from PRDA map  66  for the particular rail pressure, injection amount, and engine speed. Alternatively, the actual injection amount from PRDA map  66  may be compared to the desired injection amount. Control logic  100  may continue to block  118 . At block  118 , fuel injector diagnostic module  60  may compare the error in the injected amount of fuel or PRDA value to an error threshold. The error threshold may be an absolute value or may be a percentage of the target injection amount. If the error does not exceed the threshold, control logic  100  may end. If the error exceeds the threshold, control logic  100  may continue to block  120 . 
         [0051]    At block  120 , fuel injector diagnostic module  60  may communicate to engine system control module  68  that an error has occurred in a fuel injector  42 . Engine system control module  68  may utilize this information to provide diagnostic codes that may be accessed by a technician. Fuel injector diagnostic module  60  may also update calibration map  64  based on the error. For example, fuel injector diagnostic module  60  may use the measured fuel injection amount to compute new energizing times and/or fuel amounts associated with calibration map  64 . This may include updating the calibration map  64  with a new energizing time for the desired fuel amount and fuel rail pressure associated with the measurement. Other energizing times may also be changed based on the most recent measured fuel injection amount and other previous measured amounts. Once calibration map  64  is updated, control logic  100  may end. 
         [0052]    Referring now to  FIG. 9 , a flowchart depicting control logic  200  includes steps for performing pilot injection diagnostics. At block  202 , fuel injector diagnostic module  60  may determine whether to run pilot diagnostics, which may include starting from a predetermined pilot amount and decreasing the pilot amount until pilot injection fails to fall within a linear pattern. Pilot diagnostics may be run at regular intervals during vehicle operation in order to test the fuel injector with various fuel rail pressures. If pilot diagnostics are to be run, control logic  200  may continue to block  204 . 
         [0053]    At block  204 , fuel injector diagnostic module  60  may check fuel injector settings for pilot diagnostics. For example, fuel injector diagnostic module  60  may provide the sampling resolution, injection timing, injection quantity, rail pressure, and engine speed for pilot injection diagnostics. For example, the sampling resolution may include the crank angle measurement window during which the pressure measurements will be performed for the motored and fired cycles. A sampling resolution may also include how often measurements are made during the crank angle measurement window, such as every 3-6 crank angle degrees. Injection timing may include the timing to provide the pilot shot such as 12° bTDC. Finally, injection quantity may be an injection quantity that will be used to access an energizing time from calibration map  64 . Initially for pilot diagnostics, the pilot amount may be set at a relatively high amount for the engine configuration, such as 3 mm 3 . Control logic  200  may continue to block  206 . 
         [0054]    At block  206 , fuel injector diagnostic module  60  may determine a motored PR for the cylinder  40  associated with the fuel injector  42  to be tested. Fuel injector diagnostic module  60  may communicate with engine system control module  68  and fuel injection control module  70  to operate engine  12  such that a piston is driven in the cylinder  40  associated with the fuel injector  42  to be tested and no fuel is injected from the fuel injector  42  to be tested. Data receiving module  62  may receive cylinder pressure data from sensor  46  over the designated crank angle window and provide the data to fuel injector diagnostic module  60  to determine the motored PR. Once fuel injector diagnostic module  60  has PR data for the motored cycle, control logic  200  may continue to block  208 . 
         [0055]    At block  208 , fuel injector diagnostic module  60  may determine a fired PR for the cylinder  40  associated with the fuel injector  42  to be tested. Fuel injector diagnostic module  60  may communicate with engine system control module  68  and fuel injection control module  70  to operate engine  12  such that the pilot amount of fuel should be injected by the fuel injector  42  to be tested based on the energizing time values stored in calibration map  64  and the particular rail pressure. Data receiving module  62  may receive cylinder pressure data from sensor  46  over the designated crank angle window and provide the data to fuel injector diagnostic module  60  to determine the fired PR. Once fuel injector diagnostic module  60  establishes the PR data for the fired cycle, control logic  200  may then continue to block  210 . 
         [0056]    At block  210 , fuel injector diagnostic module  60  may access external parameters to determine whether the engine operated properly during the measuring process. If any of the external parameters indicate an error in engine operation, control logic  200  will return to block  204  to attempt to measure pressure data again. Otherwise, control logic  200  may continue to block  212 . 
         [0057]    At block  212 , fuel injector diagnostic module  60  may calculate PRDA based on the difference between the fired PR and motored PR over the crank angle measurement window. Control logic may continue to block  214 . At block  214 , fuel injector diagnostic module  60  may access PRDA map to determine a fuel injection amount associated with the calculated PRDA value. Control logic may continue to block  216 . 
         [0058]    At block  216 , the actual PRDA value may be compared to other measured PRDA values for the particular rail pressure and engine speed at different injection amounts. As is depicted in  FIG. 7 , PRDA may have a linear relationship with injection amount when the fuel injector is above the minimum pilot amount. Fuel injector diagnostic module  60  may determine the amount that the measured PRDA values deviates from the expected linear value. Control logic  200  may continue to block  218 . At block  218 , fuel injector diagnostic module  60  may compare the deviation from the expected linear relationship to a maximum deviation. If the error does not exceed the maximum deviation, control logic  200  may continue to block  222 . If the error exceeds the maximum deviation, control logic  200  may continue to block  220 . 
         [0059]    At block  220 , fuel injector diagnostic module  60  may update calibration map  64  based on the minimum pilot quantity. For example, fuel injector diagnostic module  60  may set a minimum pilot amount in calibration map  64  as an energizing time greater than the energizing time associated with the deviated pilot amount by a threshold. Once calibration map  64  is updated, control logic  200  may end. 
         [0060]    At block  222 , fuel injector diagnostic module  60  may reduce the pilot amount to be tested and continue to test the pilot amount until a minimum is found. In this manner, pilot injection diagnostics will continue to reduce the amount tested until the minimum pilot amount is determined. 
         [0061]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present disclosure can be implemented in a variety of forms. Therefore, while this disclosure has been described in connection with particular examples thereof, 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, specification, and the following claims.