Patent Publication Number: US-9903306-B2

Title: System and method for acquiring pressure data from a fuel accumulator of an internal combustion engine

Description:
TECHNICAL FIELD 
     This disclosure relates to a system and method for acquiring pressure data from a fuel accumulator of an internal combustion engine. 
     BACKGROUND 
     As with all mechanical devices, fuel injectors have physical dimensions that lead to variations between fuel injectors. In addition, each fuel injector has different rates of wear and responds to temperature changes differently. Since the fuel delivered by each fuel injector during a fuel injection event varies enough to affect the performance of an associated engine, it is useful to measure or calculate the fuel delivery by each fuel injector. Current systems stop fuel flow to a fuel accumulator for a specific time, leading to performance and emission challenges when the fuel pressure in the accumulator falls to a level that affects fuel injection. 
     SUMMARY 
     This disclosure provides a system for determining a fuel quantity delivered to a plurality of combustion chambers by a fuel system of an internal combustion engine, the system comprising a fuel accumulator, a sensor, a plurality of fuel injectors, and a control system. The fuel accumulator is positioned to receive a fuel flow. The pressure sensor is adapted to detect fuel pressure in the fuel accumulator and to transmit a pressure signal indicative of the fuel pressure in the fuel accumulator. Each fuel injector is operable to deliver a quantity of fuel from the fuel accumulator to one of the plurality of combustion chambers. The control system is adapted to receive the pressure signal, to transmit a control signal to stop the fuel flow to the fuel accumulator, and to analyze the pressure signal to determine the quantity of fuel delivered by one or more of the plurality of fuel injectors. The control system is further adapted to transmit a control signal to restart the fuel flow to the fuel accumulator after the fuel pressure in the fuel accumulator has decreased by a predetermined amount. 
     This disclosure also provides a method of determining an amount of fuel injected by a fuel injector of an internal combustion engine. The method comprises providing a fuel flow to a fuel accumulator, stopping the fuel flow to the fuel accumulator to define a beginning of a termination event, and determining a fuel pressure in the fuel accumulator during the termination event. The method further comprises restarting the fuel flow to the fuel accumulator when the fuel pressure in the fuel accumulator decreases by a predetermined amount, defining an end of the termination event, and determining the amount of fuel delivered by the fuel injector during a fuel injection event from the fuel pressure. 
     Advantages and features of the embodiments of this disclosure will become more apparent from the following detailed description of exemplary embodiments when viewed in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an internal combustion engine incorporating an exemplary embodiment of the present disclosure. 
         FIG. 2  is a data acquisition, analysis and control (DAC) module of the engine of  FIG. 1  in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 3  is a process flow diagram for a data acquisition process of the DAC module of  FIG. 2  in accordance with a first exemplary embodiment of the present disclosure. 
         FIG. 4  is a process flow diagram for a data acquisition process of the DAC module of  FIG. 2  in accordance with a second exemplary embodiment of the present disclosure. 
         FIG. 5  is a process flow diagram for a data analysis process of  FIGS. 3 and 4  in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 6  is a graph showing data acquired during cessation of fuel flow to an accumulator of the internal combustion engine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a portion of a conventional internal combustion engine is shown as a simplified schematic and generally indicated at  10 . Engine  10  includes an engine body  11 , which includes an engine block  12  and a cylinder head  14  attached to engine block  12 , a fuel system  16 , and a control system  18 . Control system  18  receives signals from sensors located on engine  10  and transmits control signals to devices located on engine  10  to control the function of those devices, such as one or more fuel injectors. 
     One challenge with fuel injectors is that they have a measure of variability from injector to injector because of dimensional tolerances, assembly variations, and wear over time. These variations lead to variations in fuel quantity delivered, which cause undesirable variations in output power in engine  10  and causes undesirable variation in emissions, e.g., NOX and CO. In order to combat these undesirable effects, techniques of measuring fuel delivery by each fuel injector have been developed. However, these techniques have their own undesirable side effects. One technique that avoids the use of individual flow measurements is to measure the pressure decrease in a fuel accumulator while fuel flow to the fuel accumulator is stopped for a specific time. However, this technique can lead to an undesirable drop in fuel pressure in the fuel accumulator. The apparatus and method described hereinbelow provides measurements of fuel flow from each fuel injector during an injection event while preventing an undesirable drop in fuel pressure in the fuel accumulator. Control system  18  is able to stop the flow of fuel to a fuel accumulator or rail of engine  10 . While the fuel flow to the fuel accumulator is stopped, which forms a termination event, control system  18  receives signals from a pressure sensor associated with the fuel accumulator indicative of the fuel pressure in the fuel accumulator. By ceasing fuel flow based on a fuel pressure decrease in the accumulator rather than time, the performance and emissions of engine  10  are maintained. 
     Engine body  12  includes a crank shaft  20 , a #1 piston  22 , a #2 piston  24 , a #3 piston  26 , a #4 piston  28 , a #5 piston  30 , a #6 piston  32 , and a plurality of connecting rods  34 . Pistons  22 ,  24 ,  26 ,  28 ,  30 , and  32  are positioned for reciprocal movement in a plurality of engine cylinders  36 , with one piston positioned in each engine cylinder  36 . One connecting rod  34  connects each piston to crank shaft  20 . As will be seen, the movement of the pistons under the action of a combustion process in engine  10  causes connecting rods  34  to move crankshaft  20 . 
     A plurality of fuel injectors  38  are positioned within cylinder head  14 . Each fuel injector  38  is fluidly connected to a combustion chamber  40 , each of which is formed by one piston, cylinder head  14 , and the portion of engine cylinder  36  that extends between the piston and cylinder head  14 . 
     Fuel system  16  provides fuel to injectors  38 , which is then injected into combustion chambers  40  by the action of fuel injectors  38 , forming an injection event. Fuel system  16  includes a fuel circuit  42 , a fuel tank  44 , which contains a fuel, a high-pressure fuel pump  46  positioned along fuel circuit  42  downstream from fuel tank  44 , and a fuel accumulator or rail  48  positioned along fuel circuit  42  downstream from high-pressure fuel pump  46 . While fuel accumulator or rail  48  is shown as a single unit or element, accumulator  48  may be distributed over a plurality of elements that transmit or receive high-pressure fuel, such as fuel injector(s)  38 , high-pressure fuel pump  46 , and any lines, passages, tubes, hoses and the like that connect high-pressure fuel to the plurality of elements. Injectors  38  receive fuel from fuel accumulator  48 . Fuel system  16  also includes an inlet metering valve  52  positioned along fuel circuit  42  upstream from high-pressure fuel pump  46  and one or more outlet check valves  54  positioned along fuel circuit  42  downstream from high-pressure fuel pump  46  to permit one-way fuel flow from high-pressure fuel pump  46  to fuel accumulator  48 . Though not shown, additional elements may be positioned along fuel circuit  42 . For example, inlet check valves may be positioned downstream from inlet metering valve  52  and upstream from high-pressure fuel pump  46 , or inlet check valves may be incorporated in high-pressure fuel pump  46 . Inlet metering valve  52  has the ability to vary or shut off fuel flow to high-pressure fuel pump  46 , which thus shuts off fuel flow to fuel accumulator  48 . Fuel circuit  42  connects fuel accumulator  48  to fuel injectors  38 , which then provide controlled amounts of fuel to combustion chambers  40 . Fuel system  16  may also include a low-pressure fuel pump  50  positioned along fuel circuit  42  between fuel tank  44  and high-pressure fuel pump  46 . Low-pressure fuel pump  50  increases the fuel pressure to a first pressure level prior to fuel flowing into high-pressure fuel pump  46 , which increases the efficiency of operation of high-pressure fuel pump  46 . 
     Control system  18  may include a control module  56  and a wire harness  58 . Many aspects of the disclosure are described in terms of sequences of actions to be performed by elements of a computer system or other hardware capable of executing programmed instructions. It will be recognized that in each of the embodiments, the various actions could be performed by specialized circuits (e.g., discrete logic gates interconnected to perform a specialized function), by program instructions (software), such as program modules, being executed by one or more processors, or by a combination of both. Moreover, the disclosure can additionally be considered to be embodied within any form of computer readable carrier, such as solid-state memory, magnetic disk, and optical disk containing an appropriate set of computer instructions, such as program modules, and data structures that would cause a processor to carry out the techniques described herein. A computer-readable medium may include the following: an electrical connection having one or more wires, magnetic disk storage, magnetic cassettes, magnetic tape or other magnetic storage devices, a portable computer diskette, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), or any other medium capable of storing information. It should be noted that the system of the present disclosure is illustrated and discussed herein as having various modules and units that perform particular functions. It should be understood that these modules and units are merely schematically illustrated based on their function for clarity purposes, and do not necessarily represent specific hardware or software. In this regard, these modules, units and other components may be hardware and/or software implemented to substantially perform their particular functions explained herein. The various functions of the different components can be combined or segregated as hardware and/or software modules in any manner, and can be useful separately or in combination. Thus, the various aspects of the disclosure may be embodied in many different forms, and all such forms are contemplated to be within the scope of the disclosure. 
     Control system  18  also includes an accumulator pressure sensor  60  and a crank angle sensor. While sensor  60  is described as being a pressure sensor, sensor  60  may be other devices that may be calibrated to provide a pressure signal that represents fuel pressure, such as a force transducer, strain gauge, or other device. The crank angle sensor may be a toothed wheel sensor  62 , a rotary Hall sensor  64 , or other type of device capable of measuring the rotational angle of crankshaft  20 . Control system  18  uses signals received from accumulator pressure sensor  60  and the crank angle sensor to determine the combustion chamber receiving fuel, which is then used to analyze the signals received from accumulator pressure sensor  60 , described in more detail hereinbelow. 
     Control module  56  may be an electronic control unit or electronic control module (ECM) that may monitor conditions of engine  10  or an associated vehicle in which engine  10  may be located. Control module  56  may be a single processor, a distributed processor, an electronic equivalent of a processor, or any combination of the aforementioned elements, as well as software, electronic storage, fixed lookup tables and the like. Control module  56  may include a digital or analog circuit. Control module  56  may connect to certain components of engine  10  by wire harness  58 , though such connection may be by other means, including a wireless system. For example, control module  56  may connect to and provide control signals to inlet metering valve  52  and to fuel injectors  38 . 
     When engine  10  is operating, combustion in combustion chambers  40  causes the movement of pistons  22 ,  24 ,  26 ,  28 ,  30 , and  32 . The movement of pistons  22 ,  24 ,  26 ,  28 ,  30 , and  32  causes movement of connecting rods  34 , which are drivingly connected to crankshaft  20 , and movement of connecting rods  34  causes rotary movement of crankshaft  20 . The angle of rotation of crankshaft  20  is measured by engine  10  to aid in timing of combustion events in engine  10  and for other purposes. The angle of rotation of crankshaft  20  may be measured in a plurality of locations, including a main crank pulley (not shown), an engine flywheel (not shown), an engine camshaft (not shown), or on the camshaft itself. Measurement of crankshaft  20  rotation angle may be made with toothed wheel sensor  62 , rotary Hall sensor  64 , and by other techniques. A signal representing the angle of rotation of crankshaft  20 , also called the crank angle, is transmitted from toothed wheel sensor  62 , rotary Hall sensor  64 , or other device to control system  18 . 
     Crankshaft  20  drives high-pressure fuel pump  46  and low-pressure fuel pump  50 . The action of low-pressure fuel pump  50  pulls fuel from fuel tank  44  and moves the fuel along fuel circuit  42  toward inlet metering valve  52 . From inlet metering valve  52 , fuel flows downstream along fuel circuit  42  through inlet check valves (not shown) to high-pressure fuel pump  46 . High-pressure fuel pump  46  moves the fuel downstream along fuel circuit  42  through outlet check valves  54  toward fuel accumulator or rail  48 . Inlet metering valve  52  receives control signals from control system  18  and is operable to block fuel flow to high-pressure fuel pump  46 . Inlet metering valve  52  may be a proportional valve or may be an on-off valve that is capable of being rapidly modulated between an open and a closed position to adjust the amount of fluid flowing through the valve. 
     Fuel pressure sensor  60  is connected with fuel accumulator  48  and is capable of detecting or measuring the fuel pressure in fuel accumulator  48 . Fuel pressure sensor  60  sends signals indicative of the fuel pressure in fuel accumulator  48  to control system  18 . Fuel accumulator  48  is connected to each fuel injector  38 . Control system  18  provides control signals to fuel injectors  38  that determines operating parameters for each fuel injector  38 , such as the length of time fuel injectors  38  operate and the number of fueling pulses per a firing or injection event period, which determines the amount of fuel delivered by each fuel injector  38 . 
     Control system  18  includes a process that controls the components of engine  10  to enable measurement of fuel delivery by each individual fuel injector  38 . Turning now to  FIG. 2 , a data acquisition, analysis and control (DAC) module  70  in accordance with an exemplary embodiment of the present disclosure is shown. DAC module  70  includes a timer module  72 , a fuel flow control module  74 , a data acquisition and analysis module  76 , and a fuel injector control module  78 . 
     Timer module  72  receives a signal indicative of the operating condition of engine  10  and a process complete signal from fuel flow control module  74 . The function of timer module  72  is to initiate the data acquisition process of DAC module  70  when the operating condition of engine  10  permits and at a specific or predetermined interval. Timer module  72  also monitors the engine operating condition and may adjust the timing interval to include measurements under a variety of engine conditions, such as a variety of fueling quantities and accumulator pressure levels. Timer module  72  may also inhibit a new measurement if accumulator  48  remains at a constant pressure level or if fuel injectors  38  are commanded at the same fueling level, though such inhibitions may have a maximum length of time. Timer module  72  may also monitor the convergence of each fuel injector  38 . A fuel injector  38  is converged when new measurements from the process described hereinbelow match the adapted or adjusted fueling characteristics, which means that the measurement interval may be increased to avoid unnecessary fuel flow stoppages. If convergence never occurs, the processes described below may indicate a system malfunction requiring operator intervention. Timer module may also limit the number of times fuel flow is stopped to avoid excessive fuel flow stoppages, which may be accomplished by overriding inlet metering valve  52 . In order to initiate the data acquisition process, timer module  72  initiates or starts a timing process using either the operating condition of engine  10  or the completion of a previous data acquisition process. When engine  10  initially starts, timer module  72  receives an engine operating signal from control system  18  that indicates engine  10  is operating, which initiates a timer in timer module  72 . When the timer reaches a specified or predetermined interval, which may be in the range of one to four hours and may be described as a drive cycle or an OBD (on-board diagnostics) cycle, timer module  72  transmits a process initiation signal to flow control module  74 . Subsequent timing processes are initiated from the process complete signal received from flow control module  74 . 
     Fuel flow control module  74  receives the process initiation signal from timer module  72 , a data acquisition complete signal from data acquisition and analysis module  76 , and a crankshaft angle signal from control system  18 . Flow control module  74  provides the process complete signal to timer module  72 , a data acquisition initiation signal to data acquisition and analysis module  76  and a flow control signal to fuel system  16 . The process initiation signal from timer module  72  causes flow control module  74  to wait for a predetermined crankshaft angle and, once the predetermined angle is reached, to send a fuel flow control signal to fuel system  16  that stops the fuel flow to accumulator  48 , forming the start of a termination event. After transmitting the signal to stop fuel flow, flow control module  74  then sends the data acquisition initiation signal to data acquisition and analysis module  76 . The data acquisition complete signal from data acquisition and analysis module  76  causes flow control module  74  to send the fuel flow control signal to fuel system  16  that re-starts the fuel flow to accumulator  48 , ending the termination event. After transmitting the signal to re-start fuel flow, flow control module  74  transmits the process complete signal to timer module  72 . 
     Data acquisition and analysis module  76  receives the data acquisition initiation signal from flow control module  76  and a fuel pressure data signal from fuel rail or accumulator pressure sensor  60 , and provides one or more injector operating parameter signals to fuel injector control module  78  and the data acquisition complete signal to flow control module  74 . When data acquisition and analysis module  76  receives the data acquisition initiation signal from flow control module  76 , module  76  begins to store fuel pressure data signals from accumulator pressure sensor  60 . Module  76  will acquire the fuel pressure data signals and analyze the fuel pressure data signals to determine when a predetermined fuel pressure decrease has been reached. Once the predetermined fuel pressure decrease has been reached, module  76  will complete the analysis of the fuel pressure data signals to determine whether the operating parameters for one or more fuel injectors  38  needs to be modified, described further hereinbelow. If one or more operating parameters for any fuel injector  38  require adjustment, module  76  will transmit the modified fuel injector operating parameters to fuel injector control module  78  for use in subsequent fuel injection events. Data acquisition and analysis module  76  also sends the data acquisition complete signal to flow control module  74 . 
     Fuel injector control module  78  receives fuel injector operating parameters from data acquisition and analysis module  76  and provides signals to each fuel injector  38  that control the operation of each fuel injector  38 . For example, the operating parameters may include the time of operation for each fuel injector  38 , the number of fueling pulses from a fuel injector  38 , and placement of a fuel injection event with respect to the crank angle or crankshaft angle. Though not shown, fuel injection control module  78  also receives information regarding a desired fuel quantity, desired start-of-injection timing, and other information that may be needed to control the operation of each fuel injector  38  properly. 
     Turning now to  FIG. 3 , a flow diagram describing a data acquisition process  100  of control system  18  in accordance with a first exemplary embodiment of the present disclosure is shown. Data acquisition process  100  may be distributed in one or more modules of control system  18 , such as timer module  72 , flow control module  74 , and data acquisition and analysis module  76 . Data acquisition process  100  is likely to be part of a larger process incorporated in control module  56  that controls some or all of the functions of engine  10 . Thus, while  FIG. 3  shows data acquisition process  100  as a self-contained process, it is likely that data acquisition process  100  is “called” by a larger process, and at the completion of data acquisition process  100  control is handed back to the calling process. 
     Data acquisition process  100  initiates with a process  102 . Process  102  may include setting variables within data acquisition process  100  to an initial value, clearing registers, and other functions necessary for the proper functioning of data acquisition process  100 . From process  102 , control passes to a process  104 . At process  104 , a timer is initiated and a time T 0  is set. Data acquisition process  100  may use another timing function of engine  10  to establish an initial time T 0  for the requirements of data acquisition process  100 . For convenience of explanation, the timing function is described as part of data acquisition process  100 . 
     Data acquisition process  100  continues with a decision process  106 . At process  106 , data acquisition process  100  determines whether the current time T is equal to or greater than T 0  plus a predetermined or specific change in time ΔT since the timer initiated. In an exemplary embodiment of the disclosure, ΔT may be one hour. The time period may be greater or less than one hour, depending on measured changes in fuel delivered or on other conditions. While ΔT is described in this disclosure as a fixed or predetermined value, ΔT may be varied based on actual data. For example, if no adjustments to fuel injector  38  parameters are required for a lengthy period, such as one hour or more, ΔT may be incremented to a higher value, such as 30 minutes, by the action of one of the modules described herein. If ΔT is less than T 0  plus ΔT, data acquisition process  100  waits at decision process  106  until the present time is greater than or equal to T 0  plus ΔT. As with initial time T 0 , this timing function may be performed elsewhere in engine  10  and is included in this process for convenience of explanation. Once the condition of decision process  106  has been met, the process moves to a decision process  108 . 
     At decision process  108 , data acquisition process  100  determines whether the fuel pressure P in fuel accumulator  48  is greater than minimum fuel pressure P MIN . The purpose of process  108  is to verify that there is sufficient fuel pressure in fuel accumulator  48  to guarantee collection of valid data for at least one piston. Thus, if the fuel pressure in fuel accumulator  48  is near a pressure level that will be insufficient for proper operation of fuel injectors  38 , data acquisition process  100  will wait until high-pressure fuel pump  46  has increased the fuel pressure in fuel accumulator  48  to a suitable fuel pressure level. The minimum fuel pressure will depend on many factors, particularly the type of engine, the amount of fuel each fuel injector  38  typically delivers, and the capacity of high-pressure fuel pump  46 . If fuel injectors  38  operate most efficiently with accumulator fuel pressure at 1,500 bar, then P MIN  may be set at a normal operating fuel pressure of 1,600 bar or higher to assure accumulator  48  contains a normal operating fuel pressure even under high load conditions. In an exemplary embodiment, P MIN  is 500 bar. Data acquisition process  100  moves to a process  110  once the fuel pressure in fuel accumulator  48  has reached P MIN . 
     At process  110 , data acquisition process  100  sets fuel pressure P 0  to the current fuel pressure P C  in fuel accumulator  48 . Data acquisition process  100  then moves to a process  112 . At process  112 , control system  18  sends a control signal to inlet metering valve  52  to close, stopping fuel flow to high-pressure fuel pump  46 , forming the start of a termination event. Control system  18  begins storing signals from accumulator pressure sensor  60  at a process  114 , beginning with crank angle 0 degrees plus an offset, which may be 20 degrees. The purpose of the offset is to accommodate the length of time it takes for inlet metering valve  52  to respond, and may also accommodate timing of fuel injection events. Data acquisition will proceed through the firing sequence, which may be piston  22 , piston  30 , piston  26 , piston  32 , piston  24 , and piston  28 , or piston #1, piston #5, piston #3, piston #6, piston #2, and piston #4. At a decision process  116 , data acquisition process  100  determines whether the fuel pressure in fuel accumulator  48  is less than or equal to P 0  minus ΔP Limit , where ΔP Limit  is the maximum total fuel pressure decrease permissible in fuel accumulator  48 . Once the condition of decision process  116  has been met, data acquisition process  100  moves to a process  118 , where data acquisition from accumulator pressure sensor  60  is stopped, and the signals or data acquired is analyzed by control system  18 , described in more detail hereinbelow. Though not shown in data acquisition process  100 , process  100  may include an additional process during the data acquisition process that aborts the cutout event if the accumulator pressure drops below a preset level, regardless of any other condition. Data acquisition process  100  may also include a process that provides for multiple fuel cutout events, with each cutout event separated by an adjustable or calibratible interval, e.g., 15 seconds. 
     At a process  120 , control system  18  sends a signal to inlet metering valve  52  to open, restore, enable, re-enable, start, or re-start fuel flow to high-pressure fuel pump  46  and fuel accumulator  48  and ending the termination event. While process  120  is shown as occurring after analysis of data in process  118 , process  120  may be implemented first and then analysis of the data if the fuel flow to accumulator needs re-enabled quickly for operational reasons. At a decision process  122 , data acquisition process  100  determines whether engine  10  is in a shutdown mode. If engine  10  is shutting down, then measurement of fuel delivery by fuel injectors  38  is no longer desirable and may lead to invalid data, so data acquisition process  100  ends at a process  124 . If engine  10  is continuing to operate, data acquisition process  100  returns to process  104 , where the timer is restarted and data acquisition process  100  continues as previously described. 
     While data acquisition process  100  is described in the context of six pistons, data acquisition process  100  may be used for any number of pistons. The only adjustment required for the process to function properly is to provide the crank angles for firing of the pistons, and the firing order. 
     While data acquisition process  100  works well, because the total fuel pressure decrease in fuel accumulator  48  caused by injection events is restricted to ΔP Limit , data may not be acquired from certain pistons because flow will be restarted before acceptable data is received from at least six pistons. A data acquisition process  200  shown in  FIG. 4  in accordance with a second exemplary embodiment of the present disclosure addresses the risk that data from certain pistons may be limited by stopping fuel flow from high-pressure pump  46  at varying positions of crankshaft  20 . As with data acquisition process  100 , data acquisition process  200  is likely to be part of a larger process incorporated in control module  56  that controls all the functions of engine  10 . Thus, while  FIG. 4  shows data acquisition process  200  as a self-contained module, it is likely that data acquisition process  200  is “called” by a larger process and at the completion of data acquisition process  200  control is handed back to the calling process. 
     Data acquisition process  200  initiates with a process  202 . Process  202  may include setting variables within data acquisition process  200  to an initial value, clearing registers, and other functions necessary for the proper functioning of data acquisition process  200 . From process  202 , control passes to a process  204 . At process  204 , a timer is initiated and a time T 0  is set. Data acquisition process  200  may use another timing function of engine  10  to establish an initial time T 0  for the requirements of data acquisition process  200 . For convenience of explanation, the timing function is described as part of data acquisition process  200 . 
     A decision process  206  is next in the process. At process  206 , data acquisition process  200  determines whether the current time T is equal to or greater than T 0  plus a specified or predetermined change in time ΔT since the timer initiated. In an exemplary embodiment of the disclosure, ΔT may be one hour. The time period may be greater or less than one hour, depending on measured changes in fuel delivered or on other conditions. If ΔT is less than T 0  plus ΔT, data acquisition process  200  waits until the present time is greater than or equal to T 0  plus ΔT. While ΔT is described in this disclosure as a fixed or predetermined value, ΔT may be varied based on actual data. For example, if no adjustments to fuel injector  38  parameters are required for a lengthy period, such as one hour or more, ΔT may be incremented to a higher value, such as 30 minutes, by the action of one of the modules described herein. As with initial time, T 0 , this timing function may be performed elsewhere in engine  10  and is included in data acquisition process  200  for convenience of explanation. Once the condition of decision process  206  has been met, data acquisition process  200  moves to a process  208 , where a selector value is set to 1. Data acquisition process  200  then moves to a decision process  210 . 
     At decision process  210 , data acquisition process  200  determines whether the fuel pressure P in fuel accumulator  48  is greater than minimum fuel pressure P MIN . The purpose of process  210  is to verify that there is sufficient fuel pressure in fuel accumulator  48  to guarantee collection of valid data for at least one piston. Thus, if the fuel pressure in fuel accumulator  48  is near a pressure level that will be insufficient for proper operation of fuel injectors  38 , data acquisition process  200  will wait until high-pressure fuel pump  46  has increased the fuel pressure in fuel accumulator  48  to a suitable pressure level. The minimum fuel pressure will depend on many factors, particularly the type of engine, the amount of fuel each fuel injector  38  typically delivers, and the capacity of high-pressure fuel pump  46 . If fuel injectors  38  operate most efficiently with accumulator fuel pressure at 1,500 bar, then P MIN  may be set at a normal operating fuel pressure of 1,600 bar or higher to assure accumulator  48  contains a normal operating fuel pressure even under high load conditions. Data acquisition process  200  moves to a process  212  once the fuel pressure in fuel accumulator  48  has reached P MIN . 
     At process  212 , data acquisition process  200  sets fuel pressure P 0  to the current fuel pressure P C  in fuel accumulator  48 . Data acquisition process  200  then moves to a process  214 . At process  214 , control system  18  sends a control signal to inlet metering valve  52  to close, stopping fuel flow to high-pressure fuel pump  46 , which is the start of a termination event. Control system  18  begins storing signals from accumulator pressure sensor  60  at a process  216 , beginning with the crank angle set by the selector value. For a selector value of 1, data collection begins with a crank angle of 0 degrees plus an offset, which may be 20 degrees, as in the example of data acquisition process  100 . Data acquisition will then proceed through the firing sequence, which may be piston  22 , piston  30 , piston  26 , piston  32 , piston  24 , and piston  28 , or piston #1, piston #5, piston #3, piston #6, piston #2 and piston #4. At a decision process  218 , data acquisition process  200  determines whether the fuel pressure in fuel accumulator  48  is less than or equal to P 0  minus ΔP Limit , where ΔP Limit  is the maximum total fuel pressure decrease permissible in fuel accumulator  48 . Once the condition of decision process  218  has been met, data acquisition process  200  moves to a process  220 , where data acquisition from accumulator pressure sensor  60  is stopped, and the signals or data acquired is analyzed by control system  18 , described in more detail hereinbelow. 
     At a process  222 , control system  18  sends a signal to inlet metering valve  52  to open, restoring or re-enabling fuel flow to high-pressure fuel pump  46  and fuel accumulator  48  and ending the termination event. At a decision process  224 , data acquisition process  200  determines whether the selector value is 6, which would indicate that timing of the data acquisition process has started at least once with each of the six pistons of engine  10 . If the selector value is 6, data acquisition process  200  moves to a decision process  226 , where data acquisition process  200  determines whether engine  10  is in a shutdown mode. If engine  10  is shutting down, then measurement of fuel delivery by fuel injectors  38  is no longer desirable and may lead to invalid data, so data acquisition process  200  ends at a process  256 . If engine  10  is continuing to operate, data acquisition process  200  returns to process  204 , where the timer is restarted and data acquisition process  200  continues as previously described. 
     Returning to decision process  224 , if the selector value is not equal to 6, then control passes to a decision process  228 , a decision process  230 , a decision process  232 , and a decision process  234 . In the present example, the selector value was last set to 1, so control will pass from decision process  234  to a decision process  236 . At decision process  236 , data acquisition process  200  waits for a crank angle of 120 degrees plus an offset to accommodate timing of injector firing. Once the proper crank angle is achieved, data acquisition process  200  moves to a process  238 , where the selector value is set to 2. 
     Data acquisition process  200  continues with decision process  210 , as previously described. The only difference is that with a selector value of 2, data acquisition at process  216  will begin at a crank angle of approximately 120 degrees plus the offset, which corresponds with piston  30 , which is also piston #5 in a six-cylinder engine. Data acquisition process  200  will then proceed through the previously described decision processes to decision process  234 , where data acquisition process  200  will move to a decision process  240  because the selector value is now 2. At decision process  240 , data acquisition process  200  waits until a crank angle of 240 degrees plus the previously described offset is achieved. Once the proper crank angle is reached, data acquisition process  200  moves to a process  242 , where the selector value is set to 3. Data acquisition process  200  then follows the previously described processes, with data acquisition beginning at a crank angle of 240 degrees plus the previously described offset. 
     Data acquisition process  200  will continue in this manner, reaching a decision process  244  and setting the selector value to 4 at a process  246 , reaching a decision process  248  and setting the selector value to 5 at a process  250 , and finally reaching a decision process  252  and setting the selector value to 6 at a process  254 . With a selector value of 6, when data acquisition process  200  reaches decision process  224 , control will be passed to decision process  226  and then to process  204 , if engine  10  is continuing to operate. Once at process  204 , data acquisition process  200  will continue to operate as previously described. 
     As with data acquisition process  100 , data acquisition process  200  is adjustable to accommodate more or less pistons by increasing or decreasing the number of processes associated with different crank angles, by changing the crank angles associated with fuel injection, and by changing the final selector value in decision process  224 . In this manner, data acquisition may begin with a different piston each time, assuring adequate data collection from all pistons, particularly in a high load condition where data from only one or two pistons may be acquired during a period where fuel flow from high-pressure fuel pump  46  is stopped. 
     While there are differences between data acquisition process  100  and  200 , the actual process of analyzing data may be the same between the two processes. A data analysis process  300  shown in  FIG. 5  is a representative data analysis process performed in process  118  of data acquisition process  100  and process  220  of data acquisition process  200 . 
     In a process  302 , data analysis process  300  identifies the available fuel pressure decreases acquired during the data acquisition process, described further hereinbelow, and associates those fuel pressure decreases with particular pistons. At a process  304 , data analysis process  300  discards any fuel pressure decreases that may be influenced by pumping of fuel from high-pressure fuel pump  46 . After inlet metering valve  52  is closed, there may be residual fuel in high-pressure fuel pump  46  that will flow to fuel accumulator  48 , affecting the fuel pressure in fuel accumulator  48 . Because the fuel flow affects the calculation of fuel pressure decrease due to an injection event, any such fuel pressure decrease is discarded when it is calculated to have happened. 
     At a process  306 , all data acquired is grouped by piston. Note that while the focus is on piston numbers for data collection, organization and analysis, organization could also be by fuel injectors, combustion chambers, etc., as long as the firing order is clearly defined and associated with crank angle. Also, note that the fuel pressure decrease data is used to calculate the quantity of fuel delivered by a fuel injector in a known manner. In any set of fuel pressure decrease data acquired, there may be no data for a particular piston, and there may be multiple sets of data from a particular piston, which will be explained in more detail hereinbelow. Data analysis process  300  may perform additional processes with fuel pressure decrease data, such as averaging all available data for a piston over a plurality of predetermined intervals, such as data collected over the last hour. Such averaging might be performed to reduce noise that occurs in such data. 
     At a process  308 , the current and/or recently collected data for each piston is compared with historical data for that piston to determine any difference with current and/or recently collected data. From process  308 , data analysis process  300  moves to a process  310 , where control parameters for each fuel injector  38  associated with the one or more pistons for which data was collected and analyzed are adjusted for future injection events. Such control parameters may include an injector on-time, number of firing pulses, and/or placement of a fuel injection event with respect to the crank angle. 
     From process  310 , data analysis process  300  moves to a decision process  312 . At decision process  312 , data analysis process  300  compares the parameters of each fuel injector, which may include a fueling characteristic, with predetermined upper limits (UL) and lower limits (LL), which thus forms a range of operation for each fuel injector  38 . The fueling characteristic may be defined as a quantity of fuel delivered versus an actuation duration. The fueling characteristic may take the form of one or more equations and/or an adaptive look-up table. If any parameter of any fuel injector  38  falls outside the predetermined limits or range, which may include a trim limit, data analysis process  300  moves to a process  314 . At process  314 , data analysis process  300  may set an operator indicator, such as a “CHECK ENGINE,” “SERVICE ENGINE SOON,” or other indicator visible to an operator of engine  10 . Data analysis process  300  may also set a maintenance code in a memory of control system  18 , indicating that a particular fuel injector&#39;s operating parameters have exceeded a predetermined range. After process  314  or after process  312 , the data analysis process performed in process  118  of data acquisition process  100  and process  220  of data acquisition process  200  is complete, and the associated processes continue as previously described. 
       FIG. 6  shows representative data acquired during the operation of the previously described processes. The horizontal axis of  FIG. 6  shows the crank angle of engine  10 . The vertical axis shows relative fuel pressures of fuel accumulator  48 . The value P Min , which is used in process  108  of data acquisition process  100  and process  210  of data acquisition process  200 , is shown on the vertical axis. The value ΔP Limit , which sets the maximum total fuel pressure decrease permissible in fuel accumulator  48 , is shown on the right hand side of the graph in  FIG. 6 . 
     Two representative sets of data are shown in  FIG. 6 . Data curve  400  is data that may be collected when engine  10  is under a high load condition and the amount of fuel injected per injection event is high. Slope  402  is an injection event for fuel injector  38  associated with piston  22 . Slope  404  is an injection event for fuel injector  38  associated with piston  30 . Slope  406  is an injection event for fuel injector  38  associated with piston  26 . Note that because the cessation of fuel delivery to fuel accumulator  48  is based on the total fuel pressure decrease, i.e., ΔP Limit , data curve  400  contains fuel pressure decreases from only three pistons. Fuel flow to high-pressure fuel pump  46  is stopped at point  408 . Fuel flow to high-pressure fuel pump  46  is restored at point  410 . Process  304  of data analysis process  300  may determine that slope  402  is affected by pumping from high-pressure fuel pump  46  and may discard the fuel pressure decrease that slope  402  represents. Thus, in this example only two useful data points are available. 
     Data curve  420  is data that may be collected when engine  10  is under a lower load condition than data curve  400  and the amount of fuel injected per injection event is low. Slopes  422  and  434  are injection events for fuel injector  38  associated with piston  22 . Slopes  424  and  436  are injection events for fuel injector  38  associated with piston  30 . Slopes  426  and  438  are injection events for fuel injector  38  associated with piston  26 . Slopes  428  and  440  are injection events for fuel injector  38  associated with piston  32 . Slopes  430  and  442  are injection events for fuel injector  38  associated with piston  24 . Slopes  432  and  44  are injection events for fuel injector  38  associated with piston  28 . Because the amount of fuel, which directly correlates to fuel pressure, is less per injection event under this lower load condition, data curve  420  contains twelve data points that were collected during the total fuel pressure decrease ΔP Limit . As before, the fuel flow to high-pressure fuel pump  46  is stopped at point  408 . Fuel flow to high-pressure fuel pump  46  is restored at point  446  on data curve  420 . Process  304  of data analysis process  300  may determine that slope  422  is affected by pumping from high-pressure fuel pump  46  and may discard the fuel pressure decrease that slope  402  represents. Thus, in this example, while twelve fuel pressure decreases were collected, only eleven may be useful. 
     While various embodiments of the disclosure have been shown and described, it is understood that these embodiments are not limited thereto. The embodiments may be changed, modified and further applied by those skilled in the art. Therefore, these embodiments are not limited to the detail shown and described previously, but also include all such changes and modifications.