Patent Publication Number: US-2018030916-A1

Title: System for controlling fuel rail pressure in a common rail direct fuel injection system

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to systems and methods for controlling fuel injection systems, and more specifically to systems and methods for controlling fuel rail pressure in a common rail fuel injection system. 
     BACKGROUND 
     The efficiency of internal combustion (IC) engines typically relies on the combustion quality in the engine cylinders, and this is true both in spark-ignited engines and compression ignition engines. Generally, common rail fuel injection systems and direct injection of fuel into the cylinders have improved the efficiency of the IC engines, although one ongoing challenge in such fuel injection systems is achieving and maintaining accurate control of the fuel rail pressure. 
     It is desirable to reduce the pumping losses in any high pressure fuel system, and particularly in high pressure common rail fuel systems for compression ignition engines, as excessive pressurized fuel and excessive amounts of fuel bypassing the fuel rail undesirably consumes additional crankshaft energy, thereby increasing brake specific fuel consumption. A secondary effect of excessive bypassed fuel is unnecessary heating of the fuel which can potentially cause damage to wetted components. Therefore, in addition to achieving precise pressure control for the fuel rail, it is desirable to control fuel temperature throughout the system, both in transient and steady-state operational conditions, and particularly in the return line(s) from the fuel rail back to the fuel supply. Excessively high fuel temperatures in such return line(s) can result in vaporization of the fuel and even damage to the fuel lines. 
     SUMMARY 
     The present invention may comprise one or more of the features recited in the attached claims, and/or one or more of the following features and combinations thereof. In one aspect a system for controlling fuel rail pressure may comprise a fuel rail, a fuel pump to supply pressurized fuel from a source of fuel to the fuel rail, a pressure sensor to produce a rail pressure signal corresponding to fuel pressure within the fuel rail, a plurality of additional sensors each producing a signal corresponding to a different operating parameter of an internal combustion engine to which the fuel rail and the fuel pump are operatively coupled, a pressure control valve having a fuel inlet fluidly coupled to the fuel rail, a fuel outlet and a control input responsive to a PCV control signal to establish a corresponding orifice size at the fuel outlet thereof, a processor, and a memory having instructions stored therein which, when executed by the processor, cause the processor to determine a desired rail pressure value based on signals produced by one or more of the plurality of additional sensors, to determine a feedback PCV control signal based on a difference between the desired rail pressure value and the rail pressure signal, to determine a feedforward PCV control signal based on the desired rail pressure, and to produce the PCV control signal based on a sum of the feedback PCV control signal and the feedforward PCV control signal. The feedforward PCV control signal is illustratively correlated with temperature of fuel exiting the fuel outlet of the pressure control valve such that operation of the pressure control valve in response to the PCV control signal results in fuel outlet orifice sizes which one of maintains the temperature of fuel exiting the fuel outlet of the pressure control valve within a specified temperature range and limits the temperature of fuel exiting the fuel outlet to a specified maximum temperature. 
     In another aspect, a system for controlling fuel rail pressure may comprise a fuel rail, a fuel pump to supply pressurized fuel from a source of fuel to the fuel rail, a pressure sensor to produce a rail pressure signal corresponding to fuel pressure within the fuel rail, a plurality of additional sensors each producing a signal corresponding to a different operating parameter of an internal combustion engine to which the fuel rail and the fuel pump are operatively coupled, a pressure control valve having a fuel inlet fluidly coupled to the fuel rail, a fuel outlet and a control input responsive to a PCV control signal to establish a corresponding orifice size at the fuel outlet thereof, at least a first logic module to determine a desired rail pressure value based on signals produced by one or more of the plurality of additional sensors, a first feedback controller to determine a feedback PCV control signal based on a difference between the desired rail pressure value and the rail pressure signal, a first feedforward module including a map populated with feedforward PCV control values mapped to corresponding desired rail pressure values, the first feedforward module to determine a feedforward PCV signal by mapping desired rail pressure values to corresponding feedforward PCV control values using the map, and at least a second logic module to produce the PCV control signal based on a sum of the feedback PCV control signal and the feedforward PCV control signal. The feedforward control values populating the map are illustratively correlated with temperature of fuel exiting the fuel outlet of the pressure control valve such that operation of the pressure control valve in response to the PCV control signal results in fuel outlet orifice sizes which one of maintains the temperature of fuel exiting the fuel outlet of the pressure control valve within a specified temperature range and limits the temperature of fuel exiting the fuel outlet to a specified maximum temperature 
     In a further aspect, a method is provided for controlling fuel rail pressure in a common rail fuel injection system having a common rail, a fuel pump to supply pressurized fuel from a fuel source to the fuel rail, a pressure sensor to produce a rail pressure signal corresponding to fuel pressure within the fuel rail, a plurality of additional sensors each producing a signal corresponding to a different operating parameter of an internal combustion engine to which the fuel rail and the fuel pump are operatively coupled and a pressure control valve having a fuel inlet fluidly coupled to the fuel rail, a fuel outlet and a control input responsive to a PCV control signal to establish a corresponding orifice size at the fuel outlet thereof. The method may comprise determining, with a processor, a desired rail pressure value based on signals produced by one or more of the plurality of additional sensors, determining, with the processor, a feedback PCV control signal based on a difference between the desired rail pressure value and the rail pressure signal produced by the pressure sensor, populating a map stored in a memory with pressure control valve control values mapped to corresponding desired rail pressure values, the pressure control valve control values correlated with values of temperature of fuel exiting the fuel outlet of the pressure control valve such that operation of the pressure control valve in response to a combination of the feedback PCV control signal and any of the pressure control valve control values results in a fuel outlet orifice size which one of maintains the temperature of fuel exiting the fuel outlet of the pressure control valve within a specified temperature range and limits the temperature of fuel exiting the fuel outlet to a specified maximum temperature, determining, with the processor, a feedforward PCV signal by mapping the desired rail pressure value to a corresponding one of the pressure control valve control values populating the map, producing, with the processor, the PCV control signal based on a sum of the feedback PCV control signal and the feedforward PCV control signal, and controlling the pressure control valve by applying, via the processor, the PCV control signal to the control input of the pressure control valve. 
     In yet a further aspect, a system for controlling fuel rail pressure may comprise a fuel rail, a fuel pump to supply pressurized fuel from a source of fuel to the fuel rail, a pressure sensor to produce a rail pressure signal corresponding to fuel pressure within the fuel rail, a plurality of additional sensors each producing a signal corresponding to a different operating parameter of an internal combustion engine to which the fuel rail and the fuel pump are operatively coupled, a pressure control valve having a fuel inlet fluidly coupled to the fuel rail, a fuel outlet and a control input responsive to a PCV control signal to establish a corresponding orifice size at the fuel outlet thereof, a volume control valve having a fuel inlet to receive fuel from the source of fuel, a fuel outlet fluidly coupled to a fuel inlet of the fuel pump and a control input responsive to a VCV control signal to establish a corresponding orifice size at the fuel outlet thereof, a processor, and a memory having instructions stored therein which, when executed by the processor, cause the processor to determine a desired rail pressure value based on signals produced by one or more of the plurality of additional sensors, to determine a feedback PCV control signal based on a difference between the desired rail pressure value and the rail pressure signal, to determine a feedforward PCV control signal based on the desired rail pressure, to produce the PCV control signal based on a sum of the feedback PCV control signal and the feedforward PCV control signal, to determine a desired pressure control valve control value and an injected fuel quantity based on signals produced by one or more of the plurality of additional sensors, to determine a feedforward VCV control signal based on the injected fuel quantity, to determine a feedback VCV control signal based on a difference between the desired pressure control valve control value and the PCV control signal and to produce the VCV control signal based on a sum of the feedforward VCV control signal and the feedback VCV control signal. 
     In still a further aspect, a method is provided for controlling fuel rail pressure in a common rail fuel injection system having a common rail, a fuel pump to supply pressurized fuel from a fuel source to the fuel rail, a pressure sensor to produce a rail pressure signal corresponding to fuel pressure within the fuel rail, a plurality of additional sensors each producing a signal corresponding to a different operating parameter of an internal combustion engine to which the fuel rail and the fuel pump are operatively coupled, a pressure control valve having a fuel inlet fluidly coupled to the fuel rail, a fuel outlet and a control input responsive to a PCV control signal to establish a corresponding orifice size at the fuel outlet thereof and a volume control valve having a fuel inlet to receive fuel from the source of fuel, a fuel outlet fluidly coupled to a fuel inlet of the fuel pump and a control input responsive to a VCV control signal to establish a corresponding orifice size at the fuel outlet thereof. The method may comprise determining, with a processor, a desired rail pressure value based on signals produced by one or more of the plurality of additional sensors, determining, with the processor, a feedback PCV control signal based on a difference between the desired rail pressure value and the rail pressure signal produced by the pressure sensor, determining, with the processor, a feedforward PCV signal based on the desired rail pressure value, producing, with the processor, the PCV control signal based on a sum of the feedback PCV control signal and the feedforward PCV control signal, controlling the pressure control valve by applying, via the processor, the PCV control signal to the control input of the pressure control valve, determining, with the processor, a desired pressure control valve control value based on signals produced by one or more of the plurality of additional sensors, determining, with the processor, a feedback PCV control signal based on a difference between the desired pressure control valve control value and the PCV control signal, determining, with the processor, a feedforward VCV signal based on an injected fuel quantity value determined based on signals produced by one or more of the plurality of sensors, and producing, with the processor, the VCV control signal based on a sum of the feedback CCV control signal and the feedforward CCV control signal, and controlling the volume control valve by applying, via the processor, the VCV control signal to the control input of the volume control valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This disclosure is illustrated by way of example and not by way of limitation in the accompanying Figures. Where considered appropriate, reference labels have been repeated among the Figures to indicate corresponding or analogous elements. 
         FIG. 1  is a simplified diagram of an embodiment of a control system for controlling fuel rail pressure in a common rail fuel injection system. 
         FIG. 2  is a simplified diagram of an embodiment of a control structure for controlling fuel rail pressure in a common rail fuel injection system. 
         FIG. 3  is a simplified flowchart illustrating an embodiment of a process for controlling fuel rail pressure in a common rail fuel injection system. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawing and will herein be described in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives consistent with the present disclosure and the appended claims. 
     References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases may or may not necessarily refer to the same embodiment. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. Further still, it is contemplated that any single feature, structure or characteristic disclosed herein may be combined with any one or more other disclosed feature, structure or characteristic, whether or not explicitly described, and that no limitations on the types and/or number of such combinations should therefore be inferred. 
     Referring to  FIG. 1 , an embodiment is shown of a control system  10  for controlling fuel rail pressure in a common rail fuel injection system  11 . In the illustrated embodiment, the common rail fuel injection system  11  includes a conventional low pressure fuel pump  12  having a fuel inlet fluidly coupled to a source  14  of fuel, e.g., a fuel tank or fuel reservoir, and a fuel outlet fluidly coupled to a fuel inlet of a volume control valve (VCV)  18 . In some embodiments, such as that illustrated in  FIG. 1 , a conventional fuel filter  16  may be disposed in-line between the fuel outlet of the fuel pump  12  and the fuel inlet of the valve  18 , and a fuel outlet of the valve  18  is fluidly coupled to a fuel inlet of a high pressure pump  20 . In other embodiments, the fuel filter  16  may be disposed in an alternate location between the pumps  12 ,  20  or be omitted altogether. 
     In the illustrated embodiment, fluid inlets of a conventional air bleed valve  22  and of a conventional (mechanical) pressure relief valve  24  are fluidly coupled to the junction defined between the fuel outlet of the fuel filter  16  and the fuel inlet of the valve  18 . A fluid outlet of the air bleed valve  22  opens to the fuel supply  14 , and a fluid outlet of the pressure relief valve  24  is fluidly coupled to a fuel return outlet of the high pressure fuel pump  20 . The air bleed valve  22  illustratively operates in a conventional manner to release trapped, pressurized air back to the fuel supply  12 , and the pressure relief valve  24  illustratively operates in a conventional manner to release pressurized fuel to the air bleed outlet of the high pressure fuel pump  20  under conditions in which the pressure at the junction defined between the fuel outlet of the fuel filter  16  and the fuel inlet of the valve  18  reaches a threshold pressure. 
     The fuel outlet of the high pressure fuel pump  20  is fluidly coupled to one or more fuel inlets of a conventional high pressure fuel rail  26  via one or more corresponding high pressure fuel lines  28 , and one or more fuel outlets of the fuel rail  26  are fluidly coupled to fuel inlets of one or more corresponding fuel injectors. In the example embodiment shown in  FIG. 1 , the common rail fuel injection system  11  illustratively includes four conventional fuel injectors  30   1 - 30   4 , each having a fuel inlet fluidly coupled to a corresponding fuel outlet of the fuel rail  26  via a corresponding, dedicated high pressure fuel line  32   1 - 32   4 . It will be understood, however, that in alternate embodiments the common rail fuel injection system may include more or fewer such fuel injectors. In any case, a fuel outlet of each fuel injector  30   1 - 30   4  is fluidly coupled to one or more cylinders of an internal combustion engine (not shown), and the fuel injectors  30   1 - 30   4  are illustratively controlled in a conventional manner to inject fuel into the one or more cylinders via the fuel outlets thereof. 
     Each of the fuel injectors further includes a fuel return outlet, and in the embodiment illustrated in  FIG. 1  the fuel return outlet of each of the fuel injectors  30   1 - 30   4  is fluidly coupled to a corresponding, dedicated fuel return line  34   1 - 34   4 , the fuel outlets of which are all fluidly coupled to a common injector fuel return line  36 . A pressure control valve (PCV)  38  has a fuel inlet fluidly coupled to an outlet of the fuel rail  26 , and a fuel outlet fluidly coupled to a rail fuel return line  40 . The injector fuel return line  36  and the rail fuel return line  40  are both fluidly coupled to a common fuel return line that is, in the illustrated example, fluidly coupled to a fuel inlet of a conventional heat exchanger  44  and also to a return line  46  that is fluidly coupled to the fuel return outlet of the high pressure fuel pump  20  and the fuel outlet of the pressure relief valve  24 . A fuel outlet of the heat exchanger  44  opens to the fuel supply  14 . In embodiments that include it, the heat exchanger  44  is operable in a conventional manner to draw heat away from fuel passing therethrough. In some embodiments, the heat exchanger  44  may be positioned in a different location along the fuel line  36 ,  40  and/or  42 , and in other embodiments the heat exchanger  44  may be omitted altogether. 
     In addition to the valves  18  and  38 , the control system  10  illustrated in  FIG. 1  further includes an electronic control unit (ECU)  50  operable to control the valves  18 ,  38  as will be described in detail hereinafter. The ECU is conventional and illustratively includes a conventional processor (or multiple processors)  52  electrically connected to a conventional memory  54 , wherein the memory  54  illustratively has instructions stored therein which, when executed by the processor  52 , causes the processor to control the valves  18 ,  38  in the manner to be described below. In this regard, a VCV output of the ECU  50  is electrically connected to a control input of the volume control valve (VCV)  18  via a signal path  56 , and a PCV output of the ECU  50  is electrically connected to a control input of the pressure control valve (PCV)  38  via a signal path  58 . In one embodiment, the VCV  18  and the PCV  38  are each controlled by supplying pulse-width modulated, constant frequency control signals to the control inputs thereof, wherein the duty cycles of such control signals determine the orifice size, e.g., cross-sectional opening area, of the fuel outlets thereof. Those skilled in the art will recognize other conventional control techniques for varying the orifice sizes of the VCV  18  and/or PCV  38 , and it will be understood that any such other control techniques, and corresponding valve embodiments, are contemplated by this disclosure. 
     The control system  10  further includes a number of conventional sensors each configured and positioned to produce one or more signals corresponding to one or more operating parameters of the fuel injection system  11  and/or of an internal combustion engine (not shown) to which the fuel injection system  11  is operatively mounted or otherwise operatively coupled. As one example, the control system  10  illustratively includes a conventional pressure sensor (P)  60  fluidly coupled to the fuel rail  26  and electrically connected to a rail pressure input (RP) of the ECU  50  via a signal path  62 . The pressure sensor  60  is illustratively operable to produce one or more signals corresponding to the pressure, i.e., fuel pressure, within the fuel rail  26 . As another example, the control system  10  illustratively includes an engine rotational speed sensor (ERS)  64  operatively coupled to the internal combustion engine to which the fuel system  11  is operatively coupled, and electrically connected to an engine speed input (ES) of the ECU  50  via a signal path  66 . The engine rotational speed sensor  64  is illustratively operable to produce one or more signals corresponding to the rotational speed (and, in some embodiments also the position relative to a reference position) of the crankshaft of the engine. 
     The control system  10  further illustratively includes N additional sensors  68 , where N may be any positive integer, each electrically connected to one of a corresponding number of sensor data inputs (SDI) of the ECU  50  via one of a corresponding number of signal paths  70   1 - 70   N , and each configured to produce one or more signals corresponding to an operating parameter associated with operation of the fuel injector system  11 , operation of an internal combustion engine to which the fuel injector system  11  is operatively coupled and/or operation of a stationary or movable vehicle carrying the internal combustion engine. Examples of sensors included in the additional sensors  68  may include, but are not limited to, a vehicle speed sensor, a barometric pressure sensor, an ambient temperature sensor, a cylinder pressure sensor, a cylinder temperature sensor, an exhaust gas temperature sensor, an engine temperature sensor, an engine oil pressure sensor, a key switch, an accelerator pedal sensor, a cruise control set/resume/off position sensor, a boost pressure sensor, an air intake flow sensor, an exhaust gas recirculation (EGR) flow sensor, and the like. 
     Operation of the control system  10  is controlled by actuation and control of the volume control valve (VCV)  18 , alternatively known in the art as a fuel metering valve, and the pressure control valve  38  via the ECU  50  based on sensor data received from the pressure sensor  60  and from others of the sensors  64  as well as from commands generated by the ECU based on such sensor data. In the illustrated embodiment, the low pressure fuel pump  12  draws fuel from the fuel supply  14 , and pumps the fuel through a fuel filter  16  to the fuel inlet of the high pressure fuel pump  20 . In one embodiment, the low and high pressure fuel pumps  12 ,  20  are illustratively powered from the engine crankshaft, although in alternate embodiments either or both of the pumps  12 ,  20  can be powered via an alternate power source such as an electrical or electromechanical power source. At the fuel inlet of the high pressure fuel pump  20 , the VCV  18  is actuated and controlled by the ECU  50  to controllably regulate the amount of fuel entering the high pressure fuel pump  20 . There mechanical pressure relief valve  24  relieves excess pressure that may build up back to the fuel source  14 . Some of the fuel used to lubricate the pump  20 , also exits the pump  20  via the fuel return line  46 . 
     The pressurized fuel from the high pressure pump  20  is transported to the via the one or more high pressure fuel lines  28  to the fuel rail  26 , and multiple high pressure fuel lines  32   1 - 32   4  fluidly couple the fuel rail  26  to each of the multiple fuel injectors  30   1 - 30   4 . Fuel return lines  34   1 - 34   4  fluidly couple each of the fuel injectors  30   1 - 30   4  to the fuel supply  14  via a system of fuel return lines  36 ,  40  and, in some embodiments, a heat exchanger  44 . At a fuel outlet of the fuel rail  26 , the PCV  38  is actuated and controlled by the ECU  50  to controllably regulate the amount of fuel exiting the fuel rail  26  and returning to the fuel supply  14 . 
     Referring now to  FIG. 2 , an embodiment is shown of a control structure  100  executed by the ECU  50  to control actuation and operation of the VCV  18  and the PCV  38 . In one embodiment, the control structure  100  is stored in the memory  54  in the form of instructions which, when executed by the processor  52 , cause the processor  52  to control actuation and operation of the VCV  18  and the PCV  38 , e.g., by producing VCV control signals provided to the VCV  18  via the signal path  56  and by producing PCV control signals provided to the PCV  38  via the signal path  58 . In other embodiments, the control structure  100  may be implemented in whole or in part in the form of hardware components, e.g., electrical circuit components, firmware components and/or software. In any case, the control structure  100  is illustrated in  FIG. 2  in the form of logic blocks or modules and functional blocks or modules, and will be described below in this context. It will be understood that one or more of the logic and/or functional blocks or modules may be combined to form a larger logic and/or functional blocks or modules, and vice versa, and in this regard the logic and functional blocks or modules illustrated by example in  FIG. 2  should not be considered to be limiting. 
     In the illustrated embodiment, the control structure  100  includes a Desired Fuel Rail Pressure (DFRP) block  102  which receives as inputs a Requested Torque (RT) value and an engine speed value, RPM, determined by the processor  52  based on the engine speed signal, ES, produced by the engine rotational speed sensor (ERS)  64 . In the illustrated embodiment, the Requested Torque value RT is illustratively determined by the processor  52  in a conventional manner as a function of RPM and accelerator pedal position (or cruise control set speed), although this disclosure contemplates other embodiments in which RT is produced by the processor  52  based on more, fewer or different operating signals. In any case, the DFRP block  102  is illustratively operable, in a conventional manner, to process RT and RPM to determine therefrom a pressure value corresponding to a desired fuel pressure within the fuel rail  26 , and to produce as an output a corresponding desired rail pressure value (DRP). 
     The output of the DFRP block  102  is provided to one additive input of a summation block  104  having another additive input receiving the output of a Pressure Correction (PC 1 ) block  106  receiving as input(s) one or more sensor value(s) (SV) corresponding to the signals produced by one or more of the sensors  68 . The PC 1  block  106  is illustratively operable, in a conventional manner, to process SV to determine therefrom a pressure correction value, CP 1 , based on the one or more sensor values SV. The output of the summation block  104  is the sum of DRP and CP 1  and is a final desired fuel rail pressure value, FDP. Those skilled in the art will recognize other sensor values that may be used alternatively to or in addition to those just described to produce CP 1 , and it will be understood that any such other sensor values are contemplated by this disclosure. 
     In the illustrated embodiment, the control strategy  100  includes a filter (F 1 ) block  108  that receives FDP as an input and produces a filtered final desired pressure value, FFDP, as an output. In one embodiment, the filter block  108  is implemented in the form of a preconditioning filter, e.g., a pre-filter, that is or includes a conventional rate limiter function configured to regulate the rate of change of FDP to ensure smooth operation of the illustrated control strategy. In some alternate embodiments, the filter block  108  may alternatively or additionally include one or more additional or different conventional filtering functions, and it will be understood that any such filter function(s) is/are contemplated by this disclosure. In other alternate embodiments, the filter block  108  may be omitted. 
     In the illustrated embodiment, FFDP is provided as an input to a first feed forward (FF 1 ) block  110  and to an additive input of another summation block  112 . The FF 1  block  110  is illustratively operable, as will be described below, to produce as an output a switching signal, e.g., in the form of a pulse-width modulated (PWM), constant frequency signal, PCVFF, having a duty cycle determined, in part, by FFDP. A subtractive input of the summation block  112  receives a measured rail pressure value, MRP, determined by the processor  52  based on the pressure signal, RP, produced by the fuel rail pressure sensor (P)  60 . The output of the summation block  112  is a difference pressure value, ΔP, corresponding to the difference between of FFDP and MRP, and ΔP is provided as an input to a conventional feedback controller (FB 1 ) block  114 . In one embodiment, the controller FB 1  is a conventional gain-scheduled proportional-integral-derivative (PID) controller, although in alternate embodiments FB 1  may be or include a conventional proportional-integral (PI) controller or other conventional controller. In any case, the F 131  controller  114  is illustratively operable in a conventional manner to process the pressure difference value ΔP to produce a switching signal, e.g., in the form of a pulse-width modulated (PWM), constant frequency signal, PCVFB having a duty cycle determined by ΔP. 
     The output signal PCVFF of the FF 1  block  110  is provided to one additive input of another summation block  116 , and the output signal PCVFB of the FB 1  controller block  114  is provided to another additive input of the summation block  116 . The output PCVS of the summation block  116  is the sum of PCVFF and PCVFB and is a pulse-width modulated (PWM), constant frequency signal that is illustratively provided to an input of a Saturation (S 1 ) block  118 . The output of the Saturation block  118  is the PCV actuation and control signal PCVCS provided by the ECU  50  to the input of the pressure control valve (PCV) via the signal path  58 . In the illustrated embodiment, the Saturation block  118  is operable in a conventional manner to limit the magnitudes of PCVCS to values between 0 and 1. The PCV  38  is illustratively configured to be responsive to the control signal PCVCS to establish a fuel outlet orifice or opening size through which fuel in the fuel rail  26  may pass to the fuel supply  14  via the fuel return lines  40 ,  42 . 
     In one embodiment, the feedforward block FF 1   110  is illustratively implemented in the form of a lookup table or other suitable mapping structure that maps desired rail pressure values FFDP to PCVFF values in order to establish corresponding PCV orifice sizes or opening levels. Illustratively, the PCVFF values are correlated with values of temperature of fuel exiting the fuel outlet of the PCV  38  such that operation of the PCV  38  in response to a combination of the feedback PCV control signal and any of the PCVFF values results in a fuel outlet orifice size of the PCV  38  which maintains the temperature of fuel exiting the fuel outlet of the PCV  38  within a specified temperature range or limits the temperature of fuel exiting the fuel outlet of the PCV  38  to a specified maximum temperature. In the illustrated embodiment, such PCVFF values populating the lookup table are determined experimentally as a function of fuel temperature to ensure that the temperature of fuel exiting fuel outlet the PCV  38  to the rail fuel return line  40  does not exceed the specified temperature, and/or remains within the specified temperature range, over an entire operating range (or specified operating sub-range) of the internal combustion engine to which the fuel injection system  11  is operatively coupled. In one embodiment, the PCVFF values populating the lookup table are illustratively duty cycle values which, when combined with the PCVFB values, result in PCVCS having a duty cycle to which the PCV  38  will be responsive at the control input thereof to establish corresponding PCV orifice sizes that limit the temperature of fuel passing therethrough to the maximum fuel temperature, or that maintain the fuel temperature within the specified temperature range, over a specified operating range of the engine. 
     In the illustrated embodiment, the control structure  100  further includes a Desired PCV command (DPCV) block  120  which receives as inputs the Requested Torque value, RT, and the engine speed value, RPM. The DPCV block  120  is illustratively operable to process RT and RPM to determine therefrom a desired PCV command value that, if provided to the control input of the PCV  38 , would result in a desired PCV orifice size or opening level, e.g., based on a desired fuel pressure within the fuel rail  26  also determined as a function of RT and RPM as described above with respect to the DFRP block  102 , and to produce as an output a corresponding desired PCV command value (PCVC). 
     The output of the DPCV block  120  is provided to one additive input of another summation block  122  having another additive input receiving the output of another Pressure Correction (PC 2 ) block  124  receiving as input(s) one or more sensor value(s) (SV) corresponding to the signals produced by one or more of the sensors  68 . The PC 2  block  124  is illustratively operable, in a conventional manner, to process SV to determine therefrom a pressure correction value, CP 2 , based on the one or more sensor values SV. Those skilled in the art will recognize other sensor values that may be used alternatively to or in addition to those just described to produce CP 2 , and it will be understood that any such other sensor values are contemplated by this disclosure. In any case, the summation block  122  also has a subtractive input receiving the PCV control signal PCVCS produced by the Saturation block  118  as described above. The output of the summation block  122  is thus the value (PCVC+CP 2 )−PCVCS and is a PCV command error value, EPVC, determined as the difference between the corrected, desired PCV command (PCVC+CP 2 ) and the actual PCV command PCVCS provided to the PCV  38 . 
     In the illustrated embodiment, the control structure  100  includes another filter (F 2 ) block  126  that receives EPVC as an input and produces a filtered PCV command error value, FEPVC, as an output. In one embodiment, the filter block  126  is implemented in the form of a preconditioning filter, e.g., a pre-filter, that is or includes a conventional rate limiter function configured to regulate the rate of change of EPCV to ensure smooth operation of the illustrated control strategy. In some alternate embodiments, the filter block  126  may alternatively or additionally include one or more additional or different conventional filtering functions, and it will be understood that any such filter function(s) is/are contemplated by this disclosure. In other alternate embodiments, the filter block  126  may be omitted. 
     FEPVC is provided as an input to a second feedback (FB 2 ) controller block  128 . In one embodiment, the controller FB 2  is a conventional gain-scheduled proportional-integral-derivative (PID) controller, although in alternate embodiments FB 2  may be or include a conventional proportional-integral (PI) controller or other conventional controller. In any case, the FB 2  controller  126  is illustratively operable in a conventional manner to process the PCV command error value FEPVC to produce a switching signal, e.g., in the form of a pulse-width modulated (PWM), constant frequency signal, VCVFB having a duty cycle determined by FEPVC. 
     In the illustrated embodiment, the control structure  100  illustratively includes a second feedforward (FF 2 ) block  130  which is operable, as will be described in detail below, to produce as an output a switching signal, e.g., in the form of a pulse-width modulated (PWM), constant frequency signal, VCVFF, having a duty cycle determined by an injection quantity (IQ) input value. VCVFB and VCVFF are each provided to additive inputs of yet another summation block  132 , and the output of the summation block  132  is the sum of VCVFB and VCVFF. 
     The output signal VCVFB of the FB 2  controller block  128  is provided to one additive input of yet another summation block  132 , and the output signal VCVFF of the FB 2  block  130  is provided to another additive input of the summation block  132 . The output VCVS of the summation block  132  is the sum of VCVFB and VCVFB and is a pulse-width modulated (PWM), constant frequency signal that is illustratively provided to an input of another Saturation (S 2 ) block  134 . The output of the Saturation block  134  is the VCV actuation and control signal VCVCS provided by the ECU  50  to the input of the volume control valve (VCV) via the signal path  56 . In the illustrated embodiment, the Saturation block  134  is operable in a conventional manner to limit the magnitude of VCVCS to values between 0 and 1. The VCV  18  is illustratively configured to be responsive to the control signal VCVCS to establish a fuel outlet orifice or opening size through which fuel provided by the low pressure fuel pump  12  may pass to the high pressure fuel pump  20  for pumping by the high pressure fuel pump  20  to the fuel rail  26 . 
     In one embodiment, the feedforward block FF 2   130  is illustratively implemented in the form of a lookup table or other suitable mapping structure that maps injected fuel quantity (IQ) to VCFF values in order to establish corresponding VCV orifice sizes or opening levels that will provide for the supply of fuel to the fuel rail  26  based on fuel drawn out of the fuel rail  26  via actuation of the fuel injectors  30   1 - 30   4 . In the illustrated embodiment, the injected fuel quantity value IQ is illustratively determined by the processor  52  in a conventional manner, e.g., as a function of RT and RPM, although this disclosure contemplates other embodiments in which IQ is produced by the processor  52  based on more, fewer or different operating signals. In one embodiment, the VCVFF values populating the lookup table are illustratively duty cycle values which, when combined with the VCVFF values, result in VCVCS having a duty cycle to which the VCV  18  will be responsive at the control input thereof to establish corresponding VCV orifice sizes that provide for passage to the high pressure fuel pump  20  sufficient amounts of fuel to maintain a desired fuel pressure within the fuel rail  26  over all or a specified operating range of the engine. 
     Generally, high pressure ratios across the PCV  38  result in increased temperatures at the outlet of the PCV  38  due to valve contraction effects, and this effect becomes amplified as the flow of fuel through the PCV  38  increases. Accordingly, it is desirable to control the amount of fuel entering the fuel rail  26  so as to control both the pressure ratio across the PCV  38  and the amount of fuel flow therethrough. This is illustratively accomplished in the control structure  100  via control of the VCV  18  based on the error between the desired opening level for the PCV  38  and the actual opening level for the PCV  38 . This control strategy illustratively maintains an optimal fuel pressure within the fuel rail  26  while also minimizing both the amount of fuel in the fuel return lines  36 ,  40 ,  42  and the temperature of fuel in the fuel return lines  40 ,  42 . 
     In some embodiments, it may not be desirable to control VCV  18  and/or PCV  38  as just described over all operating conditions of the engine, and in such embodiments one or more override blocks may be inserted into the control structure  100 . In the embodiment illustrated in  FIG. 2 , for example, an override block (OR 1 )  136  may optionally be inserted between the summation block  116  and the Saturation block  118  as illustrated by dashed-line representation. Alternatively or additionally, an override block (OR 2 )  138  may be optionally inserted between the summation block  132  and the Saturation block  134  as illustrated by dashed-line representation in  FIG. 2 . In either case, the override block  136 ,  138  receives as inputs an engine running mode value, RM, and, in some embodiments, an emergency stop signal (EST). 
     The running mode value, RM, is illustratively a value indicative of the operating state of the internal combustion engine to which the fuel injection system  11  is operatively coupled, and by way of example the running mode value may be one of engine stopped, engine running, engine cranking, or engine shutting down. The running mode value is illustratively determined by the ECU as a function of one or more signals produced by one or more of the sensors  64 ,  68 , e.g., the engine rotational speed sensor  56 , the key switch, the accelerator pedal position sensor, etc. In some embodiments, the emergency stop signal, EST, is likewise produced by the ECU as a function of one or more signals produced by one or more of the sensors  64 ,  68 , although in alternate embodiments EST may be alternatively or additionally produced via manual actuation of a button or lever. 
     In any case, in embodiments that include either or both of the override blocks  136 ,  138 , each is operable to pass the respective PCVS or VCVS value to the corresponding Saturation block  118  or  134  under certain operating conditions, e.g., RM=engine running or engine cranking and EST=OFF, and to override operation of the control structure  100  by blocking passage of PCVS or VCVS respectively under other operating conditions, e.g., RM=engine stopped or engine shutting down or EST=ON. When operating in override, the override block  136  may illustratively control PCVS with a default signal to a default state, e.g., ON or OFF, and the override block  138  may likewise illustratively control VCVS to a default state, e.g., ON or OFF. In alternate embodiments, the override block  136  may be operable in override to control PCVS to a default state corresponding to an intermediate static value or a dynamic value e.g. the output of either FF 1  or FB 1 , and/or the override block  138  may be operable in override to control VCVS to an intermediate static value or a dynamic value, e.g., the output of either FF 2  or FB 2 . Those skilled in the art will recognize other override control strategies in which the override block  136  may be operable to control PCVS to an alternate default state and/or the override block  138  may be operable to control VCVS to an alternate default state, and it will be understood that any such other override control strategies are contemplated by this disclosure. 
     Referring now to  FIG. 3 , a flowchart is shown of an embodiment of a process  200  for controlling PCV  38  and VCV  18  as just described with respect to the control structure of  FIG. 2 . In one embodiment, the control process  200  is illustratively stored in the memory  54  in the form of instructions which, when executed by the processor  52 , cause the processor  52  to control PCV  38  and VCV  18  as described. Alternatively or additionally, one or more aspects of the process  200  may be implemented in the form of one or more hardware components, e.g., electrical circuit components, firmware or software. In any case, it will be understood that the illustrated steps of the process  200  need not be carried out by the control structure  100  illustrated in  FIG. 2  and may instead by carried out in accordance with one or more alternative control structures. 
     The process  200  illustratively begins at step  202 , where the ECU  50  receives engine speed input, ES, rail pressure input, RP and other sensor inputs from one or more of the sensors  68 , and determines the running mode (RM) of the engine as described above, the requested torque, RT and the fuel injection quantity, IQ. Based on the engine running mode, RM, the engine speed, ES, the requested torque, RT, and the injection quantity, IQ, the ECU  50  determines at step  204  a desired rail pressure value, P DES  and a desired opening level or orifice size PVC DES  for the PCV  38 . In one embodiment, the desired opening level PVC DES  for the PCV  38  is illustratively determined from an empirical map that ensures an acceptable fuel temperature in the fuel rail return line  40 . 
     At step  206 , the rail pressure signal, RP, is used to find the error, ERR P  between the desired rail pressure, P DES  and the actual rail pressure RP, i.e., ERR P =P DES −RP. Based on the pressure error, ERR P , the PCV is commanded (PVC C ) at step  208  so as to compensate for the error, i.e., in a manner that minimizes the error by driving the error to or toward zero. Also at step  206 , the error, ERR PVC , between the desired PCV opening level PVC DES  and the commanded PCV opening level, PVC C , is determined by the ECU  50 . Based on the error ERR PVC , the VCV is commanded (VCV C ) at step  210 , although with a slower time constant than used at step  208 , so as to compensate for the error, i.e., in a manner that minimizes the error by driving the error to or toward zero. In this manner, the PCV  38  is always commanded to regulate pressure fluctuations while the VCV  18  is commanded so to ensure that the orifice size of the PCV  38  is regulated to a predetermined desired value. 
     The desired opening level, PCV DES  is illustratively determined experimentally to ensure that the temperature in the fuel return line  40  leaving the PCV  38  remains within an acceptable range, or is limited to a maximum temperature value, over the entire engine operating range. The process also controls the amount of fuel entering the fuel rail  26  by controlling the VCV  18  as a function of the difference between PCV DES  and the commanded PCV value, PCV C , so as to maintain the temperature in the fuel return line  40  within the acceptable range or limited to a maximum temperature value. 
     Alternate Embodiments 
     In one alternate embodiment of the control system  10  illustrated in  FIG. 1 , a temperature sensor (T)  72  may be positioned in communication with the rail fuel return line  40 , e.g., at the outlet of the PCV  38 , and electrically connected to a Fuel Temperature (FT) input of the ECU  50  via a signal path  74  as illustrated in dashed-line representation in  FIG. 1 . In this alternate embodiment, the temperature sensor  72  is illustratively configured to produce a temperature signal (TS) on the signal path  74  corresponding to the temperature of fuel within the rail fuel return line  40  and/or corresponding to the temperature of the fuel outlet of the PCV  38 . In a corresponding alternate embodiment of the control structure  100  illustrated in  FIG. 2 , the PCV  38  can be controlled as described above, with or without FF 1  including temperature compensation. Additionally, the block  120  can be replaced in this embodiment with a desired fuel temperature block, which can be configured, e.g., as described above with respect to the FF 1  block  110 , to determine desired PCV  38  outlet temperature (T DES ) values under various fuel rail pressures, RP, and block  124  can be used to correct the T DES  values. Instead of the PCVCS signal applied to the subtractive input of the summation block  122  as illustrated in  FIG. 2 , the measured temperature signal, TS, provided to the subtractive input of the summation block  122  such that the error value, ERR, produced at the output of the summation block  122  is ERR=(T DES +PC 2 )−TS. 
     In another alternate embodiment, the fuel injection system  11  illustrated in  FIG. 1  may have no PCV  38  or fuel rail return line  40 . In this embodiment, the VCV  18  is the sole actuator of the system, and the measure rail pressure signal, RP, may be used in this embodiment for closed-loop control the VCV  18 . For example, in this embodiment, the desired rail pressure can be determined, corrected and filtered as described above with reference to  FIG. 2  to yield a final reference pressure value or signal. This reference pressure value or signal may then serve as an input for a conventional feed forward controller, and will also be subtracted from RP to yield an error signal fed to a feedback controller. The output of the feed forward and feedback controllers may then be added together, possibly overridden by pre-specified values due to the engine running mode and emergency stop signals, and possibly saturated and sent as a command or control signal to control the VCV  18 . 
     In still another embodiment, the fuel injection system  11  illustrated in  FIG. 1  may have no VCV  18 . In this embodiment, the PCV  38  is the sole actuator in the system, and control of the PCV  38  in such a system may be as described above for regulating the fuel rail pressure. In such a system, additional passive control may be desirable to maintain the temperature at the outlet of the PCV  38  in an acceptable range, such as by designing or using existing fuel pumps with limited pumping capacity, implementing one or more regulated pressure relief valves, and/or by implementing one or more fuel heat exchangers in one or more of the fuel return lines  34   1 - 34   4 ,  36 ,  40  and/or  42 . 
     While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications consistent with the disclosure and recited claims are desired to be protected.