Abstract:
An improved fuel injection system is provided which incorporates a common rail and a plurality of unit injectors which function together to efficiently produce various injection rate shapes and different combinations of discreet and blended pilot, main, post and after treatment fuel injection events during a single injection. The fuel injection system effectively permits flexible, responsively timed, square shaped, discreet low to mid pressure level injections utilizing the fuel pressure of the common rail while also allowing the unit injector portion to function independently to produce flexible and responsively timed and pressure controlled, triangular shaped high pressure fuel injection events and to recover excess energy. In one embodiment, a separate common rail pump is avoided by utilizing the unit injector plunger to replace fuel into the common rail which was previously injected.

Description:
TECHNICAL FIELD 
     This invention relates to a fuel system for an internal combustion engine and more particularly to a unit injector fuel system including a common rail capable of offering a variety of injection pressure and rate-shaping characteristics. 
     BACKGROUND 
     A fuel system is the component of an internal combustion engine which often has the greatest impact on performance and cost. Accordingly, fuel systems for internal combustion engines have received a significant portion of the total engineering effort expended to date on the development of the internal combustion engine. For this reason, today&#39;s engine designer has an extraordinary array of choices and possible permutations of known fuel system concepts and features. Design effort typically involves extremely complex and subtle compromises among considerations such as cost, size, reliability, performance, ease of manufacture, and retrofit capability on existing engine designs. 
     The challenge to contemporary designers has been significantly increased by the need to respond to governmentally mandated emissions abatement standards while maintaining or improving fuel efficiency. In view of the mature nature of fuel system designs, it is extremely difficult to extract both improved engine performance and emissions abatement from further innovations in the fuel system art. Commercially competitive fuel injection systems of the future will almost certainly need to not only incorporate new design features for better achieving various objectives including improved engine performance and emissions abatement but, combine the appropriate features in the most effective manner to form a system capable of most efficiently, effectively and reliably achieving the greatest number of objectives. 
     Some of the most important features for achieving objectives such as improved engine performance and emissions abatement include high injection pressure capability, improved hydraulic and mechanical efficiency, quick pressure response and effective and reliable injection rate shaping capability. Other important features include drive train noise control and packaging flexibility for enabling installation on various engine configurations including retrofitting existing engines. In recent years, in an effort to provide greater operating flexibility and performance, the fuel systems industry has focused considerable attention on developing energy accumulating, nozzle controlled concepts that provide engine speed and load independent control over fuel injection timing, pressure, quantity and multiple injection rate shaping. This attention has lead to the commercialization of fuel systems packaged in the general form of a fluid pressurizing pump connected to a hydraulic energy storage device, i.e. an high pressure accumulator or common rail, connected to one or more electrically operable injector nozzles. SAE Technical Papers 960870 and 980803 both disclose common rail fuel injection systems having a high pressure common rail containing high pressure fuel for delivery to multiple injectors. Each injector includes a servo-controlled needle valve element assembly. The assembly includes a control volume in communication with an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve. U.S. Pat. No. 5,133,645 to Crowley et al. discloses a similar system. These systems all permit speed and load independent pressure control, a broad fuel injection timing range, and fast injection timing, quantity, and multiple pulse rate shaping transient response. However, in these systems, fuel injection pressure is varied by changing the pressure in the entire common rail making fuel pressure responsiveness difficult to achieve. Moreover, the large volume of fuel at high pressure requires more expensive seals along with greater risk of leakage and repair costs. 
     Another conventional approach is disclosed in U.S. Pat. Nos. 5,535,723 and 5,551,398 and SAE Technical Paper 961285 which disclose a unit fuel injector system including a mechanically actuated plunger in each injector and a servo-controlled needle valve element assembly. Fuel is supplied to each injector at a low supply pressure and pressurized by the plunger to a very high injection pressure. Each of these systems also includes a pump control valve for controlling the flow out of a pumping chamber to control the pressurization of the fuel to a high pressure level. These systems advantageously permit very high injection pressures. U.S. Pat. No. 5,463,996 issued to Maley et al. discloses a similar system wherein the high pressure fuel for injection is generated by a hydraulically driven intensifier plunger. 
     Consequently, there is a need for a high pressure fuel system for an internal combustion engine which is capable of providing enhanced operating flexibility and performance with respect to at least injection pressure and rate shaping while protecting investments in existing engine architectures. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention, therefore, to overcome the disadvantages of the prior art and to provide a fuel injection system capable of effectively and predictably controlling fuel injection timing and metering. 
     It is another object of the present invention to provide a fuel injection system capable of controlling fuel injection pressure independent from engine speed. 
     It is yet another object of the present invention to provide a fuel injection system capable of providing greater operating flexibility and performance. 
     It is a further object of the present invention to provide a fuel injection system capable of providing a wide range of selectable injection pressure levels for creating multiple injections including pilot and/or post injections in combination with a main injection event. 
     It is a still further object of the present invention to provide a highly efficient high pressure fuel injection system capable of recuperating the pressure energy stored in the pressurized fuel in the common rail following an injection event. 
     Yet another object of the present invention is to provide a fuel injection system which protects investments in existing engine architectures by being easy to retrofit on existing mechanically operated unit injector-equipped engines. 
     Still another object of the present invention is to provide a fuel injection system capable of providing extremely high pressures main injections while permitting lower pressure pilot injections. 
     A still further object of the present invention is to provide a fuel injection system capable of combining the desirable features of mechanically operated unit injectors and contemporary common rail concepts in packaging that is cost effective. 
     Yet another object of the present invention is to provide a fuel injection system which provides rapid pressure response, improved small injection metering repeatability and accuracy, improved limp home and fail safe functionality and energy recovery for improved operating efficiency. 
     Another object of the present invention is to provide a fuel system capable of high pressure injections, triangular rising rate shape and high modularity and redundancy. 
     It is yet another object of the present invention to provide a high pressure fuel injection system capable of a broad fuel injection timing range supporting pilot, main, post and after treatment needs; speed and load independent pressure control; low noise, vibration and harshness (NVH); and high operating efficiency. 
     It is still another object of the present invention to provide a high pressure fuel injection system capable of fast injection timing, quantity, pressure and multiple pulse rate shaping transient response. 
     A still further object of the present invention is to provide a fuel system capable of producing a variety of different combinations of discrete and blended pilot, main, post and after treatment fuel injection events during a single combustion cycle. 
     These and other objects are achieved by providing a fuel injection system for controlling fuel injection into combustion chambers of a multi-cylinder internal combustion engine, comprising a low pressure fuel supply for supplying fuel at a low supply pressure level, a common rail fluidically connected to the low pressure fuel supply and containing injection fuel at a common rail pressure level greater than the low supply pressure level, and a plurality of fuel injectors connected to the common rail to receive fuel from the common rail and inject fuel at a high pressure level greater than the common rail pressure level. Each of the plurality of injectors includes an injector body containing an injector cavity, a fuel transfer circuit and an injection orifice formed at one end of the injector body, a plunger reciprocally mounted in the injector cavity, and a high pressure chamber formed between the plunger and the injection orifice. The plunger is movable into the high pressure chamber to increase the pressure of fuel in the high pressure chamber to a high pressure level greater than the common rail pressure level. Each injector also includes a nozzle assembly including a needle valve element reciprocally mounted for movement between a closed position blocking fuel flow through the injection orifice and an open position permitting fuel flow through the injection orifice. Each fuel injector further includes a needle valve control device adapted to move the needle valve element between the closed and open positions to initiate an injection event to inject the fuel at the common rail pressure when the injection system is operating in a first mode and at the high pressure level when the injection system is operating in a third mode. The needle valve control device includes a control volume, a drain circuit for draining fuel from the control volume to a low pressure drain and an injection control valve positioned along the drain circuit for controlling the flow fuel through the drain circuit so as to cause the movement of the needle valve element between the open and the closed positions. The fuel injection system further includes a pump control valve for controlling the flow of fuel into the high pressure chamber. 
     Preferably, the common rail pressure is greater than approximately 15 MPa and, more specifically, in the range of approximately 20-50 MPa. The system may further include a common rail pump for receiving low pressure fuel from the low pressure fuel supply and delivering the fuel to the common rail at at least the common rail pressure level. The system may further include an accumulator connected to the common rail and a pressure limiter for directing high pressure fuel to the common rail during the third mode to prevent overpressurization. The fuel injection system may further include a variable inlet metering orifice positioned upstream of the common rail pump and a low pressure regulator positioned between the low pressure fuel supply and the variable inlet metering orifice. 
     In one embodiment, the fuel injection system may include a first branch circuit extending from the low pressure fuel supply and a second branch circuit extending from the low pressure fuel supply in parallel to the first branch circuit wherein the common rail is positioned along the first branch circuit and the pump control valve is positioned to receive fuel from the second branch circuit. In this embodiment, a check valve may be positioned along the fuel transfer circuit to permit fuel flow from the common rail to the closed nozzle assembly while preventing flow from the common rail to the high pressure chamber when the injection system is operating in the first mode and permitting high pressure fuel flow from the high pressure chamber to the nozzle valve assembly while preventing fuel flow from the common rail to the nozzle valve assembly when the injection system is operating in the third mode. 
     In multiple embodiments, the plunger is movable to spill fuel from the high pressure chamber through the pump control valve into one of a supply rail and a common rail. In these embodiments, the injection system is operable in a second mode to move the needle valve element between the closed and the open positions to initiate an injection event to inject fuel during spilling of fuel from the high pressure chamber. The plunger may be movable through a retraction stroke to refill the high pressure chamber with fuel. In this case, the injection system is operable in a fourth mode to move the needle valve element between the closed and the open positions to initiate an injection event to inject fuel during refill of the high pressure chamber. The low pressure fuel supply may include a low pressure supply pump and a supply rail positioned in parallel with the common rail for receiving fuel from the low pressure supply pump. In one embodiment, the high pressure chamber is fluidically connected to the supply rail so as to receive low pressure fuel directly from the supply rail and fluidically connect the common rail to receive fuel at common rail pressure directly from the common rail via the pump control valve. A check valve may be positioned between the supply rail and the high pressure chamber to permit fuel flow from the supply rail to the high pressure chamber while preventing fuel flow from the high pressure chamber to the supply rail. The injection system may be operable in a common rail recharge mode during which the plunger moves at least partially through the retraction stroke while the pump control valve is maintained in a closed position to cause fuel flow from the supply rail through the check valve into the high pressure chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a first embodiment of the fuel injection system of the present invention; 
     FIG. 2 is a cross-sectional view of a closed nozzle injector used in the fuel injection system of FIG. 1; 
     FIGS. 3 a - 3   d  are schematic views of the system of FIG. 1 showing various operating modes; 
     FIGS. 4 a  and  4   b  are graphs showing a shorter duration cam lift and a longer duration cam lift, respectively, along with exemplary fuel injection system operating sequences/strategies; 
     FIG. 5 is a schematic view of a second embodiment of the fuel injection system of the present invention; 
     FIGS. 6 a - 6   d  are schematic views of the system of FIG. 5 showing various operating modes; 
     FIG. 7 is a schematic view of a third embodiment of the fuel injection system of the present invention; 
     FIGS. 8 a - 8   d  are schematic views of the system of FIG. 7 showing various operating modes; and 
     FIG. 9 is a graph showing an exemplary fuel injection sequence for the fuel injection system of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, there is shown a needle controlled, common rail fuel injection system  10  of the present invention including a mechanically actuated unit injector  12  resulting in a flexible, efficient system capable of producing various combinations of discreet and blended pilot, main, post, and after treatment fuel injection events during a single injection period of a single combustion cycle. Fuel injection system  10  includes a low pressure fuel supply  14  for supplying fuel at a low supply pressure level, a common rail pump  16  for receiving fuel at the low supply pressure level and increasing the pressure to a common rail pressure level, and a common rail  18  containing fuel at the common rail pressure level. Unit fuel injector  12  is connected to common rail  18  for receiving fuel at the common rail pressure level for injection into the combustion chamber of an engine at the common rail pressure level or at a higher pressure level as discussed hereinbelow. Although only one unit fuel injector  12  is shown in detail, it is understood that fuel injection system  10  includes at least one fuel injector for each cylinder of a multi-cylinder internal combustion engine having any number of cylinders, such as four, six, eight, ten or twelve cylinders. 
     Low pressure fuel supply  14  includes a fuel source  20 , i.e. fuel tank, a low pressure supply rail  22 , and a low pressure fuel supply pump  24  for pumping fuel from fuel source  20  to supply rail  22 . A low pressure regulator  26  may be provided to maintain the fuel pressure in supply rail  22  at approximately a desired pressure level, e.g. 2 MPa, by returning excess fuel to the fuel tank  20  as necessary. Fuel flows from supply rail  22  to the inlet side of common rail pump  16 . An adjustable or variable inlet metering orifice  28  may be provided immediately upstream of common rail pump  16  to restrict fuel flow to common rail pump  16  to maintain the outlet side pressure from common rail pump  16  within a desired predetermined pressure range, i.e. 20-50 MPa. Common rail pump  16  supplies fuel within the predetermined pressure range to common rail  18 , thereby maintaining the fuel pressure in common rail  18  at a predetermined common rail pressure level within the desired pressure range. A discreet or distributed accumulator  30  may be connected to common rail  18  to ensure an adequate supply of pressurized fuel at the common rail pressure level throughout fuel system operation. Common rail  18  may be in the form of a drilling formed in the cylinder head of an engine or, alternatively, may be a separate section of high pressure tubing. Again, although only one fuel injector  12  is shown in FIG. 1, additional fuel injectors, although not shown, are connected to common rail  18  at, for example,  32 . 
     Referring to FIG. 1, a schematic representation of the unit fuel injector  12  is illustrated which includes an injector body, indicated generally at  34 , which includes a plunger bore  36  and a plunger  38  reciprocally mounted in plunger bore  36  to form a high pressure chamber  40 . Plunger  38  is movable through an advancement stroke under the force of a plunger actuating assembly  42  to raise the pressure of fuel in high pressure chamber  40  from the common rail pressure level to a high pressure level, i.e. 50-200 MPa. In the embodiment of FIG. 1, plunger actuating assembly  42  includes an overhead cam  44 , a coupling or tappet  46  for continuously abutting cam  44  and a coupling spring  48  for providing an upward bias against tappet  46  to force tappet  46  against cam  44 . Tappet  46  is connected to plunger  38  thereby biasing plunger  38  upwardly as shown in FIG. 1 causing plunger  38  to move through a retraction stroke as tappet  46  moves toward the inner base circle of cam  44 . As cam  44  rotates and tappet  46  moves toward the outer base circle of cam  44 , spring  48  is compressed and plunger  34  is moved through an advancement stroke expelling fuel from chamber  40 . As discussed hereinbelow, the fuel expelled from chamber  44  may be at a high pressure level or at some lower pressure level depending on the operation of the system. Fuel injector  12  also includes a fuel supply transfer circuit  50  connecting fuel injector  12  to common rail  18 . In addition, fuel injector  12  includes a pump control valve  52  mounted along fuel supply transfer circuit  50  for controlling communication between common rail  18  and high pressure chamber  40 . Fuel supply transfer circuit  50  also includes a delivery passage  53  connecting high pressure chamber  40  to a nozzle assembly  54 , preferably of the closed nozzle type. A needle valve control device  56  is provided within injector body  34  to control the operation of closed nozzle assembly  54  as described more fully hereinbelow. 
     The components of fuel injector  12  briefly described hereinabove relative to FIG. 1 will now be described in more detail with respect to the exemplary embodiment of FIG.  2 . The injector of FIG. 2 includes all the components described in the injector of FIG.  1  and is merely a practical embodiment of the schematic representation of the injector of FIG. 1 except that the plunger actuating assembly of FIG. 2 includes a rocker arm assembly  58  pivoted by, for example, a cam and push rod assembly. In this practical embodiment, injector body  34  includes a barrel  60  containing the plunger bore  36 , a valve housing  62 , an upper nozzle housing  64 , a lower nozzle housing  66  and a retainer  68 . The outer end of retainer  68  contains internal threads for engaging corresponding external threads on the lower end of barrel  60  to permit the entire set of injector body components to be held together by simple relative rotation of retainer  68  with respect to barrel  60 . Closed nozzle assembly  54  is of a conventional design including a needle valve element  70  biased into a closed position by spring  72  to prevent fuel flow from needle cavity  74  through injector orifices  76  into a combustion chamber. Needle valve control device  56  controls the opening of needle valve element  70  by controlling the fuel pressure forces acting on needle valve element  70 . In particular, in the present embodiment, needle valve control device  56  includes an injection control valve  78  for controlling the flow of fuel through a drain passage  80  connected to a control volume  82  positioned at the outer end of needle valve element  70  as fully described in U.S. Pat. No. 5,819,704, the entire contents of which is hereby incorporated by reference. When injection control valve  78  is in a closed position, drain flow through drain circuit  80  is blocked thereby causing the fuel pressure in control volume  82  to increase to a level sufficient to cause the pressure-induced closing bias forces on needle valve element  70  to be sufficient in combination with spring  72  to bias needle valve element  70  into the closed position. To initiate an injection event, injection control valve  78  is actuated and opened to allow high pressure fuel to drain from control volume  82  through drain circuit  80  thereby creating a pressure force imbalance on needle valve element  70  which permits the movement of needle valve element  70  into an open position to initiate an injection event. Injection control valve  78  includes an actuator assembly  84  for moving the control valve between open and closed positions. Actuator assembly  84  may be a solenoid actuator assembly such as disclosed in U.S. Pat. No. 6,056,264 or U.S. Pat. No. 6,155,503, the entire contents of both of which are incorporated herein by reference. 
     As shown in FIGS. 1 and 2, pump control valve  52  is preferably mounted on injector body  34 . Pump control valve  52  may be a pressure balanced or outwardly opening, or hybrid type two-way valve operated by, for example, a solenoid actuator. For example, pump control valve  52  may be of the type disclosed in U.S. Pat. No. 4,905,960 or U.S. Pat. No. 6,045,120, the entire contents of both of which are hereby incorporated by reference. Although in the exemplary embodiment shown in FIG. 2, pump control valve  52  is mounted on the side of injector body  34 , the present invention is intended to encompass a pump control valve positioned in other locations, such as separate from the injector or mounted along the central longitudinal axis of the injector such as suggested by U.S. Pat. No. 5,301,875, the contents of which is hereby incorporated by reference. 
     Fuel injector  12  may further include a pressure limiter valve  86  (FIG. 1) to provide passive over-pressure protection in certain operational scenarios as described hereinbelow in order to ensure spilling of high pressure fuel during the conclusion of a main injection event to prevent overpressurization by plunger  38  thereby protecting the injector. However, it should be noted that pressure limiter valve  86  may not be necessary where pump control valve  52  is of a pressure balanced design to permit the spilling of high pressure fuel at the conclusion of a main injection event. 
     Reference is now made to FIGS. 3 a - 3   d  for discussion of four different modes of operation for injecting fuel at different pressures to achieve different injection rate shapes. Referring to FIG. 3 a , fuel injection system  10  may be operated in a first mode wherein fuel from common rail  18  at the common rail pressure level is injected. As shown in FIG. 3 a , the first mode, or common rail injection mode, occurs during a dwell period of plunger  38  as it is maintained in its retracted position with tappet  46  riding on the inner base circle of cam  44 . Pump control valve  52  is maintained in its normally open position permitting fuel to flow from common rail  18  into the inner passages and cavity of fuel injector  12 . At a predetermined time, normally closed injection control valve  78  of needle valve control device  56  is opened by actuator assembly  84  (FIG.  2 ). As fuel from control volume  82  flows through drain passage  80  (FIG.  2 ), the fuel pressure forces acting on needle valve element  70  become imbalanced sufficiently so that the opening forces overcome the combined fuel pressure closing force and the closing force of spring  72  thereby causing needle valve element  70  to move into an open position to initiate injection. After a predetermined injection time period, the injection event is terminated by actuating actuator assembly  84  causing injection control valve  78  to move to a closed position stopping fuel flow through drain passage  80  thereby allowing the fuel pressure in control volume  82  to increase to common rail pressure resulting in the closing of needle valve element  70 . The available pressure drop across injection orifices  76 , the effective flow area through the injection orifices and the duration of the opening of needle valve element  70 , all determine the quantity of fuel delivered to the combustion chamber of an engine. The shape of the fuel injection rate versus time is naturally square. 
     As cam  44  begins to move plunger  38  through an advancement stroke as shown in FIG. 3 b , a second mode of fuel injection may be achieved. Specifically, the second mode involves injection at common rail pressure during the spilling of fuel from chamber  40  back into common rail  18  as plunger  38  moves through an advancement stroke. At a predetermined time during the advancement of plunger  38  and the displacement of fuel into common rail  18  through normally open pump control valve  52 , actuator assembly  84  is energized and injection control valve  78  moves to an open position. The pressure of the fuel in the high pressure chamber  40  in the second mode is slightly higher than in the first mode. The magnitude of this increase is limited by the ratio of the volume of the common rail  18  to the volume of the high pressure chamber  40  and the flow area of pump control valve  52 . The available pressure drop across injection orifices  76 , the effective flow area through injection orifices  76  and the duration of the opening of needle valve element  70  again determine the quantity of fuel delivered while the injection rate shape remains naturally square over time. 
     FIG. 3 c  illustrates a third mode of operation or injection wherein the normally open pump control valve  52  is closed at some point during the advancement stroke of plunger  38 . Subsequently, the normally closed injection control valve  78  is opened as plunger  38  continues to advance. Of course, this third mode of injection will occur at a high injection pressure level in the range of approximately 50-200 MPa. The particular rate shape produced during this mode of injection, although naturally triangular, is dependent upon the pumping rate, discharge rate and trapped volume within fuel injector  12 . 
     FIG. 3 d  illustrates yet a fourth mode of operation or injection wherein plunger  38  is moving through a retraction stroke with pump control valve  52  open to permit fuel from common rail  18  to flow through fuel supply transfer circuit  50  into chamber  40  thereby refilling the chamber. During this period, normally closed injection control valve  78  is opened resulting in an injection. The fourth mode permits late cycle injections to control particulates and to regenerate emissions after-treatment devices. 
     FIGS. 4 a  and  4   b  illustrate two hypothetical fuel injection sequences or strategies. In the strategy used in FIG. 4 a , the cam is designed to create a shorter duration advancement stroke or lift resulting in a steeper slope of the tappet displacement curve shown in FIG. 4 a  as compared to FIG. 4 b , the strategy of FIG. 4 b  utilizes a cam designed to create a more gradual, longer duration advancement stroke with the emphasis on storing energy prior to injection and recuperating energy after injection. Referring to FIG. 4 a , a pilot injection is achieved by utilizing the first mode of operation (FIG. 3 a ) wherein the injection control valve  78  is commanded to open resulting in the fuel injection rate curve of FIG. 4 a  prior to any substantial displacement of the tappet  46 /plunger  38 . After the first mode of injection has been completed and the pilot injection terminated, a second mode injection is performed with the system operating as shown in FIG. 3 b  to achieve the initial portion of a primary injection event as shown by the fuel injection rate curve in FIG. 4 a . During this second mode injection, the pump control valve is closed to transition into the third mode of injection (FIG. 3 c ) to increase the fuel injection rate as shown in FIG. 4 a . At a predetermined time, injection control valve  78  is closed causing the fuel injection rate to drop dramatically thereby terminating injection as needle valve element  70  closes. Pump control valve  52  is then commanded to open resulting in a high pressure spill as shown by the dramatic decrease in nozzle pressure back to the common rail pressure in FIG. 4 a . Subsequently, an early post injection quantity of fuel may be injected using again the second mode of operation as plunger  38  continues to move through the end of its advancement stroke with pump control valve  52  in the open position. Thus injection control valve  78  is again opened and then closed to create a post injection event shown in the injection rate curve of FIG. 4 a . A late post injection event may also be used by employing the fourth mode of operation of FIG. 3 d  upon movement of plunger  38  and tappet  46  through a retraction stroke. In the same manner as the early post injection event, injection control valve  78  is open and then closed to achieve the late post fuel injection rate shape shown in FIG. 4 a . Consistent with the strategy of FIG. 4 a , pump control valve  52  would be of a pressure balanced design to permit the spilling of high pressure fuel at the conclusion of a main injection event and thus avoiding overpressurization. In this case, a pressure limiter  86  would not be needed. 
     The operation sequence of FIG. 4 b  preferably utilizes an outwardly opening design or other suitable design to eliminate parasitic leakage and to simplify the control over the reopening of pump control valve  52 . In this case, the pressure limiter  86  would be desirable. Specifically, as shown in FIG. 4 b , a first pilot injection is achieved by utilizing the second mode of operation during the advancement stroke of plunger  38 . Subsequently, pump control valve  52  is closed causing the inner volume of fuel injector  12  to become pressurized by the compressive force applied by the advancing movement of plunger  38 . At a predetermined time, the primary injection event is initiated by opening injection control valve  78  and terminated by closing injection control valve  78  while pump control valve  52  remains in the closed position. Again although pump control valve  52  has been deenergized, since it is of the outwardly opening design, when pump control valve  52  is closed, the high pressure fuel maintains the valve element of pump control valve  52  in a closed position. At the end of the primary injection event, a post injection event may be achieved by again opening and closing injection control valve  78 . Unlike the first strategy, this post injection event can be obtained at a high pressure as shown by the injection control valve command and the fuel injection rate curves occurring at the high pressure of the nozzle pressure curve of FIG. 4 b . Thus, two injections may occur while operating in the third mode of operation. Once plunger  38  retracts to decrease the pressure on the valve element of pump control valve  52  sufficiently, pump control valve  52  will again open as shown by the pumping control valve position curve in FIG. 4 b . At this point, a second post injection event may be achieved by operating the injection control valve to achieve injection at the common rail pressure level while plunger  38  moves through a retraction stroke. 
     Reference is now made to FIG. 5 disclosing a second embodiment of the fuel injection system of the present invention, indicated generally at  100 , which is similar to the previous embodiment in many respects and thus like reference numerals will be used to indicate the components and features which are the same or similar to the previous embodiment of FIG.  1 . The embodiment of FIG. 5 differs from the first embodiment in that pumping control valve  52  is supplied with fuel directly from supply rail  22  as opposed to being supplied with fuel from common rail  18  as in the first embodiment. It should be noted that the pump control valve  52  may still be mounted on the fuel injector  102  in the manner shown in FIG. 2 or any other variations discussed hereinabove relative to the embodiment of FIG. 1 except that the upstream side of pump control valve  52  would connect to a drilling positioned in communication with supply rail  22 . In addition, the present embodiment of FIG. 5 differs from the previous embodiment in that a check valve  104  is utilized to control communication between common rail  18  and the closed nozzle assembly. Specifically, supply rail  22  is divided into a first branch circuit  106  supplying low pressure fuel to common rail pump  16  and a second branch circuit  108  provided in parallel to common rail pump  16  and common rail  18 . Pump control valve  52  is positioned along second branch circuit  108  which ultimately connects to chamber  40 . 
     Four distinct injection modes of the second embodiment will now be discussed with reference to FIGS. 6 a - 6   d . It should be noted that the fundamental operation and injection pressures for each mode may be similar to the respective modes of operation for the first embodiment as shown in FIGS. 3 a - 3   d  in that the first mode is a common rail injection during a fuel injector dwell; the second mode is a common rail injection during fuel injector spill; the third mode is a high pressure injection during plunger advancement and the fourth mode is a common rail injection during fuel injector refill. More specifically, during the first mode, supply rail fuel is supplied through the second branch  108  and through normally open pump control valve  52  into chamber  40  while plunger  38  is maintained in a retracted position. Meanwhile, common rail fuel is supplied to closed nozzle assembly  54  via check valve  104 . Check valve  104  however blocks flow of common rail fuel into chamber  40 . Injection control valve  78  is opened and then closed to initiate and terminate, respectively, fuel injection of fuel at common rail pressure. The rate shape of injection is naturally square. During a second mode of injection as shown in FIG. 6 b , cam  44  bears against tappet  46  causing compression of spring  48  thereby advancing plunger  38  and displacing fuel from chamber  40  through pump control valve  52  into second branch passage  108 . During this spill process, injection control valve  78  is operated to initiate and terminate an injection event as described hereinabove. During this spill injection process, check valve  104  isolates chamber  40  from common rail  18  while allowing common rail fuel to flow to the nozzle assembly  54 . The available pressure drop across the injection orifices, the effective flow area of the orifices and the open duration of the needle valve element of the nozzle assembly, again determine the quantity of fuel delivered. High pressure injection is achieved using the third mode of operation with plunger  38  moving through an advancement stroke caused by cam  44 . Normally open pump control valve  52  is closed to prevent the further spilling of fuel back into second branch passage  108  and cause an increase in fuel pressure within the injector internal trapped volumes. The injection control valve  78  is then opened and then closed to initiate and then terminate, respectively, the injection of high pressure fuel. Once pump control valve  52  is closed, check valve  104  moves to isolate common rail  18  from closed nozzle assembly  54  while permitting communication between chamber  40  and closed nozzle assembly  54 . The rate shape of injection during this high pressure mode is naturally triangular. For the fourth mode of injection as shown in FIG. 6 d , fuel at common rail pressure is injected during unit pump refill with pump control valve  52  in the open position supplying refill fuel from second branch circuit  108  into chamber  40  while check valve  104  permits the common rail fuel to flow to nozzle assembly  54 . The injection control valve  78  is opened and closed to define the injection event while the injector is being refilled. The rate shape of injection is again naturally square. 
     FIG. 7 illustrates a third embodiment of the fuel injection system of the present invention, indicated generally at  200 . This embodiment includes many features which are the same and similar to the features disclosed in the first two embodiments and therefore like reference numerals will be used to indicate the same or similar components or features. As in the previous embodiments, low pressure supply pump  24  transfers fuel from the tank  20  to the supply rail  22 . The low pressure regulator  26  may be used to maintain pressure in supply rail  22  at the low pressure level, i.e. approximately 2 MPa, by returning excess fuel to tank  20  as necessary. Fuel flows from supply rail  22  to one or more unit injectors  202 . Each injector  202  includes a check valve  204  positioned to control flow from supply rail  22  to the injector and to isolate the injector internal volumes from supply rail  202  during all but a refilling phase of operation. The internal volumes of injector  202  communicate with a C 4  common rail  206  and an accumulator  208  through the normally open pump control valve  52 . CD 5  Again, the optional pressure limiter  86  may be provided as an over-pressure bypass circuit around pump control valve  52  should it be convenient or necessary to do so. As described more fully hereinbelow with the description of the operation of system  200 , plunger  38  of injector  202  serves two functions in the present embodiment: first, plunger  38  drives the main fuel injection event by pressurizing the fuel to a high pressure level and, second, functions as a common rail pump to pressurize common rail  206  and accumulator  208 . Thus, a major distinction between the present embodiment and the two previous embodiments is that the present embodiment does not include a separate common rail pump but relies on the injector plunger  38 . Pump control valve  52  functions to allow fuel flow between common rail  206  and chamber  40  alternately serving as inlet metering and pressure control valves. In the former case, pump control valve  52  is maintained in the open position during retraction of plunger  38 , fuel from common rail  206  is used to refill chamber  40  and no new fuel from supply rail  22  is supplied through check valve  204 . However, pump control valve  52  may be closed to allow the retraction of plunger  38  to draw in fuel from supply rail  22  through check valve  204  thereby adding fuel to the injector and thus allowing plunger  38  to add new fuel to common rail  206  upon the advancement stroke of plunger  38  once pump control valve  52  is open thereby allowing one or more injectors to maintain the pressure in common rail  206  within a desired range throughout engine operation. In the latter case, pump control valve  52  is closed during the advancing stroke of injector plunger  38  to rapidly pressurize fuel in anticipation of or during a main injection event. FIGS. 8 a - 8   e  illustrate four modes of fuel injection ( 8   a - 8   c  and  8   e ) and one mode of refilling chamber  40  and replacing fuel previously removed from common rail  206  ( 8   d ). The first fuel injection mode, as shown in FIG. 8 a , injects fuel at common rail pressure during the dwell period of injector  202  with plunger  38  neither advancing nor retracting. During this time, pump control valve  52  is in its normally open position to provide fuel at common rail pressure to the internal volumes of injector  202 . As discussed hereinabove relative to the previous embodiments, injection control valve  78  is operated to create an injection event at the common rail pressure level. The available pressure drop across the injection orifices, the effective flow area of the injection orifices and the duration of the opening of injection control valve  78 , determine the quantity of fuel delivered while the rate shape is naturally square. The second mode of fuel injection is an injection at common rail pressure during fuel spilling from chamber  40  through pump control valve  52  into common rail  206 . During the advancement stroke of plunger  38 , fuel is displaced from chamber  40  into common rail  206  through the normally open pump control valve  52 . It should be noted that common rail  206  and accumulator  30  are sufficiently large to maintain pressure fluctuations during spilling within acceptable limits. The available pressure drop across the injection orifices, the effective flow area of the orifices and the open duration again determine the quantity of fuel delivered. 
     The third mode of fuel injection of the embodiment of FIG. 7 is illustrated in FIG. 8 c  wherein normally open pump control valve  52  is closed during the advancement stroke of plunger  38 . Injection control valve  78  is then opened initiating an injection event of high pressure fuel into the combustion chamber of an engine. The injection event is terminated by reclosing injection control valve  78  and soon after either reopening pump control valve  52  or, optionally, allowing pressure limiter  86  to intervene to limit the maximum internal volume pressure. The natural rate shape produced during this mode of injection is dependent upon the pumping rate, discharge rate and trapped volume. The rate shape is naturally triangular. 
     Referring to FIG. 8 d , the retraction of plunger  38  presents opportunities to replace fuel volumes previously removed from common rail  206  by the injection process and to perform additional common rail fuel injection events. Refilling occurs when pump control valve  52  is closed and plunger  38  retracts thereby reducing the internal volume pressure below that of the pressure in supply rail  22 . Under these conditions, supply fuel passes through check valve  204  and into chamber  40 . The refill volume depends upon internal volume size and initial pressure, the rate of plunger retraction, supply rail pressure, the effective flow area through check valve  204  and the timed period that pump control valve  52  remains closed. Opening pump control valve  52  equalizes internal volume and common rail pressures at a level sufficiently above the pressure in supply rail  22  to prevent a further refilling and to close check valve  204  thereby preventing backflow to supply rail  22 . Once additional fuel from supply rail  22  is drawn into chamber  40  as shown in FIG. 8 d , during the next advancement stroke of plunger  38 , this additional fuel can be used to recharge common rail  206  by opening pump control valve  52  as plunger  38  moves through an advancement stroke as shown in FIG. 8 b  whether or not an injection event occurs during the advancement stroke. The fourth fuel injection mode as shown in FIG. 8 e  injects fuel at common rail pressure during the retraction stroke of plunger  38 . In this mode, pump control valve  52  is opened to allow a flow of fuel from common rail  206 . 
     FIG. 9 illustrates a hypothetical fuel injection sequence/strategy for creating multiple injections during a single combustion cycle. As can be seen by the injection control valve command curve, a pilot injection is created prior to tappet displacement and thus during plunger dwell. This pilot injection is actually the first mode of injection shown in FIG. 8 a  at the common rail pressure level. Then, as shown by the fuel injection rate curve, a main injection event is initiated at common rail pressure prior to the closing of pump control valve  52 . Once the pump control valve  52  is subsequently closed, the fuel injection rate increases until injection control valve  78  is closed to terminate injection. A post injection may then be initiated with residual high pressure fuel remaining in the internal volumes of the injector, pump control valve  52  closed and plunger  38  in its most advanced position. In this example, this post injection event continues slightly into the retraction stroke of plunger  38 . A second post injection may then be achieved in a similar manner to the pilot injection. 
     The embodiment of FIG. 7 demonstrates that only the common rail portion of conventional high pressure common rail fuel systems need be uniquely combined with a unit injector fuel system to create a unique hybrid system to provide synergistic flexibility and performance. The present hybrid system relieves some difficulties relating to design constraints encountered with conventional fuel system concepts. For example, conventional standalone high pressure common rail fuel systems must be designed to meet maximum desired injection pressure and quantity requirements while, at the same time, trapped volumes are typically minimized to improve pressure response. The high pressure common rail portion of the fuel injection system of the present invention supports small quantity pilot, main and post fuel injection events at relatively low and slowly changing, or substantially constant, injection pressures. Also, the unit injector portion of the fuel injection system of the present invention supports large quantity, moderate to high and rapidly responding injection pressures. Consequently, substantial reductions can be made in common rail flow area and maximum pressure capability to realize manufacturing cost savings and package size reductions. Similarly, common rail trapped volume can be increased to reduce pressure fluctuations and injector interactions and to accommodate longer common rail distances on large “V” engines. All embodiments of the present fuel injection system, and particularly the embodiment of FIG. 7, may be able to utilize a low pressure, e.g., 20-50 MPa, common rail formed from a network of cylinder head drillings. 
     The present invention also presents an opportunity to gain relief from difficult design constraints in the area of cam Hertz stress. The added rate shaping flexibility of the present common rail subsystem and needle valve control device  56  presents an opportunity to utilize less aggressive (i.e. larger minimum radius of curvature and lower acceleration ramp) cam profiles. The Hertz stress reducing benefit of less aggressive cam profiles can be realized in terms of increased load carrying capability at previously acceptable Hertz stress limits or greater durability at lower limits or a combination of the two. 
     Overall, the present invention addresses the need for a fuel injection system that combines desirable features of traditional mechanically operated unit injector and contemporary high pressure common rail concepts in packaging that is cost effective and directly applicable to current unit injector equipped engines. The fuel injection system of the present invention has the advantages of providing very high injection pressure capability; triangular rising rate shape; high modularity and redundancy; compatibility with existing engine architectures and combustion, performance and emissions (CPE) strategies; while also providing a broad fuel injection timing range supporting pilot, main, post and after treatment needs; ultimately fast injection timing, quantity and multiple pulse rate shaping transient response; speed and load independent pressure control; high mean effective pressure fuel injection; low noise, vibration and harshness (NVH); high operating efficiency; and a closed nozzle design. Moreover, the fuel injection of the present invention desirably achieves rapid (next cylinder to fire) pressure response; improved small injection metering repeatability and accuracy; reduced drain (or return) flow to tank and associated heat rejection; energy recovery for improved operating efficiency and NVH; improved limp home and fail safe functionality; self prime with internal bleed capability; and compatibility with advanced virtual sensor-based injected quantity control, diagnostics and prognostics. 
     It should be noted that the present invention is compatible with and provides enhanced operating flexibility for incorporating electronically controlled dual needle nozzle assembly technology that utilizes fuel pressure level to select between single needle and dual needle operating modes, such as disclosed in U.S. patent application Ser. No. 09/907,814 filed Jul. 19, 2001, and entitled “Fuel Injector with Injection Rate Control.” Enhanced operating flexibility is realized by the present invention because it provides two distinctly different injection pressures (one constant and one variable) during a single multi-pulse injection cycle. The fuel injection system of the present invention is also compatible with leakage control sleeve technology such as disclosed in U.S. Pat. No. 5,899,136, which may be applied to reduce leakage and to minimize trapped volume within the injector. The fuel injection system of the present invention is also compatible with fuel system control technologies that can be used to control fuel injection pressure and quantity and to estimate effective bolt modulus, sensor bandwidth and fuel system static timing. 
     INDUSTRIAL APPLICABILITY 
     While the needle controlled fuel system of the present invention is most useful in a compression ignition internal combustion engine, it can be used in any combustion engine of any vehicle or industrial equipment in which accurate, efficient and reliable pressure generation, injection timing and injection metering are essential.