Abstract:
Engineers have determined that the performance of fuel injectors, including those used in common rail fuel injection systems, can be increased, and undesirable emissions reduced, by controlling the mass flow rate of fuel injected into a combustion chamber during an injection event. While a number of fuel injectors have been developed that have limited rate shaping capabilities, the ability to produce some front end rate shapes has not been possible. In an effort to increase rate shaping capabilities, the present invention includes a valve assembly having at least one valve member that is movable between a plurality of positions to control fluid communication between a number of fluid inlets and outlets.

Description:
TECHNICAL FIELD 
     This invention relates generally to valve assemblies, and more particularly to rate shaping valve assemblies for fuel injectors. 
     BACKGROUND 
     Electronically controlled fuel injection systems are becoming more widespread for use with diesel engines. One example of such a system is the amplifier piston common rail system (APCRS) illustrated in the paper “Heavy Duty Diesel Engines—The Potential of Injection Rate Shaping for Optimizing Emissions and Fuel Consumption”, presented by Messrs. Bernd Mahr, Manfred Durnholz, Wilhelm Polach, and Hermann Grieshaber; Robert Bosch GmbH, Stuttgart, Germany, at the 21 st  International Engine Symposium, May 4-5, 2000, Vienna, Austria. In the Bosch fuel injection system, a controlled leakage strategy is utilized to control opening and closing of the needle. For instance, for pre-injection and the first portion of a main injection having a boot rate trace, fuel is directed along a by-pass route. After the main injection has begun, a pressure intensifier piston is activated for the required pressure controlled injection. While this system shows promise, there is still room for improvement. 
     Recently, engineers have determined that the performance of fuel injectors, including those used in common rail fuel injection systems, can be increased, and undesirable emissions reduced, by controlling the mass flow rate of fuel injected into a combustion chamber during an injection event. It is also believed that the ability to front end rate shape an injection event can further reduce emissions while increasing fuel injector performance. While a number of fuel injectors have been developed that have limited rate shaping capabilities, the ability to produce some front end rate shapes has not been possible. Therefore, a fuel injector having a broader range of front end rate shaping capabilities would allow greater flexibility for further reduction of undesirable emissions while allowing for improved fuel injector performance. 
     The present invention is directed to overcoming one or more of the problems as set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a valve assembly includes a valve body that defines a first inlet, a second inlet, a first outlet and a second outlet. An electrical actuator is attached to the valve body. At least one valve member is at least partially positioned in the valve body and is operably coupled to the electrical actuator. The at least one valve member is movable between a first position, a second position and a third position. The first inlet is fluidly connected to the first outlet via a relatively restricted flow area when the at least one valve member is in the first position. The first inlet is fluidly connected to the first outlet via a relatively unrestricted flow area when the at least one valve member is in at least one of the second position and the third position. The second inlet is fluidly connected to the second outlet when the at least one valve member is in at least one of the first position, the second position and the third position. The second inlet is fluidly closed to the second outlet when the at least one valve member is in an other of the first position, the second position and the third position. 
     In another aspect of the present invention, a fuel injection system includes an injector body that defines at least one nozzle supply passage that is fluidly connected to a nozzle chamber, and a needle control passage that is fluidly connected to a needle control chamber. An electrical actuator is attached to the injector body. At least one valve member is positioned in the injector body and is operably coupled to the electrical actuator. The at least one valve member is movable between a first position, a second position and a third position. The at least one nozzle supply passage has a relatively restricted flow area when the at least one valve member is in the first position. The at least one nozzle supply passage has a relatively unrestricted flow area when the at least one valve member is in at least one of the second position and the third position. The needle control passage is open when the at least one valve member is in at least one of the first position, the second position and the third position. The needle control passage is closed when the at least one valve member is in an other of the first position, the second position and the third position. 
     In yet another aspect of the present invention, a method of injecting fuel includes a step of relieving pressure on a closing hydraulic surface of a needle valve member at least in part by energizing an electrical actuator with current above a first threshold level. Fuel is then directed to a nozzle outlet via a restricted passage and an unrestricted passage at least in part by energizing the electrical actuator with a higher current that is above a second threshold level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectioned diagrammatic representation of a fuel injection system according to the present invention; 
         FIG. 2  is a front sectioned diagrammatic representation of the valve assembly of the fuel injector of  FIG. 1 ; 
         FIGS. 3   a-b  are graphical representations of current level and mass flow rate versus time for a boot injection according to the present invention; 
         FIGS. 4   a-b  are graphical representations of current level and mass flow rate versus time for a square injection according to another aspect of the present invention; 
         FIGS. 5   a-b  are graphical representations of current level and mass flow rate versus time for a ramp or triangle injection according to yet another aspect of the present invention; 
         FIGS. 6   a-b  are graphical representations of current level and mass flow rate versus time for a split injection having a boot according to still another aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , there is illustrated a fuel injection system  10  that includes a fuel pressurizer that provides a reciprocating plunger  14 . Reciprocating plunger  14  defines a portion of a fuel pressurization chamber  15 . As illustrated in  FIG. 1 , the fuel pressurizer is preferably a unit pump  12  that supplies pressurized fuel to one or more fuel injectors  20  and is separated from the one or more fuel injectors  20 . Unit pump  12  includes a plunger  14  that is moved to reciprocate between an advanced position and a retracted position by the rotation of a cam  11 . Unit pump  12  preferably includes a conventional electronically controlled spill valve  16  which typically has two positions. When spill valve  16  is in its first, open position, low pressure fuel is spilled from a fuel pressurization chamber  15  to a pump inlet/spill port  18  for recirculation. When plunger  14  is undergoing its pumping stroke and spill valve  16  is in its second, closed position, fuel in fuel pressurization chamber  15  is pressurized to injection levels and displaced toward a nozzle outlet  70  of fuel injector  20  via a pump outlet  17  and a fuel supply line  19 . While the fuel pressurizer has been illustrated as a unit pump  12  that is separate from the one or more fuel injectors  20 , it should be appreciated that it could instead be a pumping unit that is positioned inside each fuel injector. In this case, the fuel injectors could be either hydraulically or mechanically actuated. Further, while the fuel pressurizer preferably includes a plunger that can increase pressure during the injection event to allow injection events having ramp shaped front ends, the present invention could also be used with a common rail fuel injection system. However, it should be appreciated that greater rate shaping flexibility can be achieved when the present invention is utilized with a fuel pressurizer that can increase the pressure of fuel being injected during the injection event, such as a fuel pressurizer including a plunger, as illustrated. 
     Returning to  FIG. 1 , fuel injector  20  provides an injector body  21  that defines a fuel inlet  22 . In addition, injector body  21  preferably defines a plurality of fuel passages, including at least one nozzle supply passage, a fuel passage  23  and a needle control passage  28 . As illustrated, the at least one nozzle supply passage includes a restricted nozzle supply passage  24  and an unrestricted nozzle supply passage  26 . Restricted nozzle supply passage  24  includes a flow restriction orifice  25  and fluidly connects fuel inlet  22  to a nozzle chamber  68 . Unrestricted nozzle supply passage  26  has an upper portion  57  separated from a lower portion  58  by a rate shaping valve assembly  30 . Depending upon the position of rate shaping valve assembly  30 , nozzle chamber  68  is either fluidly connected to fuel inlet  22  via a relatively restricted flow path, including restricted nozzle supply passage  24  alone, or via a relatively unrestricted flow path, including both restricted nozzle supply passage  24  and unrestricted nozzle supply passage  26 . Needle control passage  28  has an upper portion  44  that is separated from a lower portion  45  by valve assembly  30  and can fluidly connect fuel inlet  22  to a needle control chamber  60  depending upon the position of valve assembly  30 . As illustrated in  FIG. 1 , a first end of restricted nozzle supply passage  24  and a first end of unrestricted nozzle supply passage  26  are both fluidly connected to fuel passage  23 . In addition, a first end of needle control passage  28  is also fluidly connected to fuel passage  23 . 
     A direct control needle valve  65  is positioned in injector body  21  and includes a piston portion  63  and a needle valve member  66 . Needle valve member  66  is movable between a biased, closed position and an open position, and is biased toward its closed position by a biasing spring  62 . Piston portion  63  includes a closing hydraulic surface  61  that is exposed to fluid pressure in needle control chamber  60  while needle valve member  66  includes an opening hydraulic surface  67  that is exposed to fluid pressure in nozzle chamber  68 . When a valve opening pressure is reached within nozzle chamber  68 , needle valve member  66  can be moved to its open position. Recall, however, that nozzle chamber  68  can be open to fuel inlet  22  via only restricted nozzle supply passage  24 . Thus, it should be appreciated that flow restriction orifice  25  should be large enough that a valve opening pressure can be attained, and maintained, in nozzle chamber  68  when nozzle chamber  68  is open to fuel inlet  22  via only restricted nozzle supply passage  24 . In other words, flow restriction orifice  25  should be sufficiently large to prevent chatter of needle valve member  66  when needle valve member  66  is in its open position and nozzle chamber  68  is open to fuel inlet  22  via only restricted nozzle supply passage  24 . However, it should be further appreciated that flow restriction orifice  25  should be sufficiently small to allow restricted nozzle supply passage  24  to function as a relatively restricted flow passage. In addition, a drain passage  64 , defined by injector body  21 , is provided to allow any fuel that has migrated past the piston or needle portions of direct control needle valve  65  to be evacuated from fuel injector  20 . 
     Referring in addition to  FIG. 2 , rate shaping valve assembly  30  is illustrated in greater detail. Valve assembly  30  provides a valve body  31 , which is a portion of injector body  21 , that preferably defines a needle control inlet  42 , a nozzle supply fuel inlet  56 , a needle control outlet  47  and a nozzle supply outlet  59 . As illustrated, needle control inlet  42  and nozzle supply fuel inlet  56  are both fluidly connected to a common fluid source, unit pump  12 , via fuel passage  23  and fuel inlet  22 . Valve assembly  30  includes at least one valve member operably coupled to an electrical actuator  34 , which preferably includes a needle control valve member  35  and a rate shaping valve member  50 , as illustrated in FIG.  2 . Electrical actuator  34  is attached to injector body  21  and is preferably a solenoid that includes a coil  33  and an armature  32  and is energizable to a first, low current position and a second, higher current position. However, it should be appreciated that another suitable actuation device could instead be utilized, such as a piezoelectric actuator. In addition to the low current and high current positions, which can be thought of as low current pull and high current pull positions, solenoid  34  can also be energized to a low current hold position and a high current hold position. Thus, the low current pull level can be thought of as the amount of current above a first threshold level that is needed to move needle control valve member  35  from its biased position to its unbiased position, while the low current hold level is a lesser amount of current that is needed to hold needle control valve member  35  in its unbiased position. Similarly, the second, high current pull level is the amount of current above a second, higher threshold level that is needed for solenoid  34  to move rate shaping valve member  50  from its biased position to its unbiased position, while the high current hold level is the amount of current needed for rate shaping valve member  50  to be maintained in its unbiased position. 
     Returning to rate shaping valve assembly  30 , needle control valve member  35  is operably coupled to move with armature  32  and is movable within injector body  21  between a biased, open position and a closed position. Needle control valve member  35  is biased toward its open position by a biasing spring  40  that is compressed between rate shaping valve member  50  and needle control valve member  35 . Rate shaping valve member  50  preferably defines a portion of needle control passage  28  and is movable between a biased, closed position and an open position. Rate shaping valve member  50  is biased toward its open position by a biasing spring  51  that is compressed between injector body  21  and rate shaping valve member  50 . It should be appreciated that if rate shaping valve assembly  30  is configured as illustrated, the force exerted by armature  32  and needle control valve member  35  when solenoid coil  33  is energized by an amount of current below the second threshold level should not be sufficient to overcome the force of biasing spring  51 , which preferably biases rate shaping valve member  50  toward its closed position. In other words, biasing spring  51  should have a higher preload to produce a stronger biasing force than biasing spring  40  when rate shaping valve assembly  30  is configured as illustrated, such that a greater force is required to move rate shaping valve member  50  toward its advanced position than is needed to move needle control valve member  35  toward its advanced position. 
     When solenoid coil  33  is de-energized, valve assembly  30  is in a first position. It should be appreciated that for the purposes of the present invention, solenoid  34  can be thought to be in its de-energized state when it is receiving no current or when it is receiving an amount of current below the first threshold level which is an insufficient amount of current to move needle control valve member  35  from its biased position. When valve assembly  30  is in the first position, needle control valve member  35  is in its biased open position away from a valve stop included on rate shaping valve member  50 . As illustrated, the valve stop is preferably a conical valve seat  37 . Needle control inlet  42  is thus fluidly connected to needle control outlet  47  to open needle control passage upper portion  44  to needle control passage lower portion  45 . Therefore, pressurized fuel entering fuel injector  20  via fuel inlet  22  can act on closing hydraulic surface  61  in needle control chamber  60 . In addition, when valve assembly  30  is in the first position, rate shaping valve member  50  is in its biased, closed position. In this position, a valve seat  53  defined by valve body  31  is closed by a valve surface  54  provided by rate shaping valve member  50 . Nozzle supply fuel inlet  56  is therefore blocked from fluid communication with nozzle supply outlet  59 , thus preventing fluid communication between unrestricted nozzle supply passage upper portion  57  and unrestricted nozzle supply passage lower portion  58 . Therefore, fuel inlet  22  is fluidly connected to nozzle chamber  68  via only restricted nozzle supply passage  24 , a relatively restricted flow area flow passage. However, it should be appreciated that because needle control chamber  60  is fluidly connected to fuel inlet  22  via needle control passage  28  when solenoid coil  33  is de-energized or receiving an amount of current below the first threshold level, nozzle outlet  70  remains closed at this time. 
     When solenoid coil  33  is energized by a current level above a first threshold value but below a second threshold value to its low current pull position, valve assembly  30  is moved to a second position. In this position, needle control valve member  35  is pushed toward its closed position by armature  32 . However, because biasing spring  51  preferably has a higher preload than biasing spring  40 , rate shaping valve member  50  will remain in its biased, closed position at this time. When needle control valve member  35  is in its closed position, a valve surface  38  provided by needle control valve member  35  contacts, and therefore closes, valve seat  37 . Thus, that portion of needle control passage  28  that is defined by rate shaping valve member  50  is closed when valve surface  38  closes valve seat  37 . Needle control inlet  42  is thus blocked from needle control outlet  47  to prevent fluid communication between needle control passage upper portion  44  and needle control passage lower portion  45 , thus blocking fluid communication between fuel inlet  22  and needle control chamber  60 . Therefore, pressure acting on closing hydraulic surface  61  is relieved when solenoid coil  33  is energized with an amount of current above the first threshold level. Further, because rate shaping valve member  50  is still in its biased, closed position, nozzle chamber  68  is still fluidly connected to fuel inlet  22  via a relatively restricted flow area including only restricted nozzle supply passage  24 . 
     When solenoid coil  33  is energized by a current level above the second threshold to a high current pull position, corresponding to a third position of valve assembly  30 , rate shaping valve member  50  is moved to its open position by armature  32  and needle control valve member  35 . In this position, valve seat  53  is opened by rate shaping valve member  50  to allow fluid communication between nozzle supply fuel inlet  56  and nozzle supply outlet  59 , thus fluidly connecting unrestricted nozzle supply passage upper portion  57  to unrestricted nozzle supply passage lower portion  58 . Therefore, when valve assembly  30  is in its third position, fuel inlet  22  is fluidly connected to nozzle chamber  68  via a relatively unrestricted flow area including both restricted nozzle supply passage  24  and unrestricted nozzle supply passage  26 . In other words, fuel is directed to nozzle outlet  70  by both a relatively restricted passage, restricted nozzle supply passage  24 , and a relatively unrestricted passage, unrestricted nozzle supply passage  26 , when solenoid coil  33  is energized with a higher current that is above the second threshold level. Thus, for certain rate shaping applications, as illustrated below, it might be preferable to direct fuel to nozzle outlet  70  via only restricted nozzle supply passage  24  for a predetermined duration of time, and then to increase the current being supplied to solenoid coil  33  to a level above the second threshold level to open nozzle outlet  70  to unrestricted nozzle supply passage  26 . 
     INDUSTRIAL APPLICABILITY 
     Referring now to  FIGS. 1 and 2 , just prior to the desired start of an injection event, needle valve member  66  is in its downward, closed position blocking nozzle outlet  70  from nozzle chamber  68 . Valve assembly  30  is in its first position such that needle control valve member  35  is in its biased, open position fluidly connecting needle control chamber  60  to fuel inlet  22  and rate shaping valve member  50  is in its biased, closed position blocking fluid communication between nozzle chamber  68  and fuel inlet  22  via unrestricted nozzle supply passage  26 . Fuel inlet  22  is open to nozzle chamber  68  via restricted nozzle supply passage  24 . Just prior to the desired start of an injection event, solenoid coil  33  is energized. The amount of current supplied to solenoid coil  33 , and the timing of energization of solenoid coil  33 , are dependent upon the desired rate trace of the injection event. Thus, the present invention will be described creating injection events having a number of different rate shapes. However, it should be appreciated that the following description is not intended to limit the present invention to only those rate shapes disclosed. Injection events having the disclosed rate shapes have been selected for illustrative purposes only. 
     Referring in addition to  FIGS. 3   a-b , the present invention will be described for creation of an injection event having a boot shaped rate trace. Prior to the desired start of an injection event, pump inlet/spill port  18  closes and rotation of cam  11  causes plunger  14  to be moved toward its advanced position, thus beginning the pressurization of fuel within fuel pressurization chamber  15 . Just prior to the desired start of the injection event, solenoid coil  33  is supplied with an amount of current above the first threshold level (Time I,  FIG. 3   a ). Needle control valve member  35  is then pushed to its closed position by armature  32 . With high pressure no longer acting on closing hydraulic surface  61  in needle control chamber  60 , the high pressure acting on opening hydraulic surface  67  is sufficient to move needle valve member  66  toward its open position (Time A,  FIG. 3   b ). Once needle control valve member  35  is moved to its closed position, current to solenoid coil  33  is reduced to a low current hold level that is sufficient to maintain valve member  35  in this position (Time  2 ,  FIG. 3   a ). It should be appreciated that because pump  12  has not yet raised the pressure of fuel being supplied to injector  20  to maximum injection levels prior to the start of the injection event, pressure of fuel being supplied to injector  20 , and therefore the pressure of fuel being injected by injector  20 , will continue to rise. 
     When it is desired to ramp up to higher injection levels, solenoid coil  33  is supplied with a higher amount of current above the second threshold level (Time  3 ,  FIG. 3   a ). Rate shaping valve member  50  is thus moved toward its open position (Time B,  FIG. 3   b ) and pressurized fuel can now flow through unrestricted nozzle supply passage  26  into nozzle chamber  68 . After rate shaping valve member  50  is moved to its open position, current to solenoid  32  is reduced to a level below the second threshold, but above the first threshold, to a high current hold level that is sufficient to maintain valve members  35  and  50  in their respective closed and opened positions (Time  4 ,  FIG. 3   a ). It should be appreciated that the pressure of fuel being injected by injector  20  could continue to rise once rate shaping valve member  50  is moved to its open position if pump  12  had not raised the pressure of fuel being supplied to fuel injector  20  to maximum injection levels prior to the increase in current to solenoid coil  33 . 
     When the desired amount of fuel has been injected, current supplied to solenoid coil  33  is ended (Time  5 ,  FIG. 3   a ). Needle control valve member  35  is returned to its biased, open position under the influence of biasing spring  40  while rate shaping valve member  50  is returned to its biased, closed position under the influence of biasing spring  51 . Needle control chamber  60  is once again fluidly connected to fuel inlet  22  via needle control passage  28 , thus re-exposing closing hydraulic surface  61  to high pressure. With pressurized fuel acting on closing hydraulic surface  61 , needle valve member  66  is returned to its downward, closed position to end the injection event (Time C,  FIG. 3   b ). At the same time, with rate shaping valve member  50  once again in its closed position, unrestricted nozzle supply passage  26  is once again blocked such that nozzle chamber  68  is fluidly connected to fuel inlet  22  via only restricted nozzle supply passage  24 . As the various components of fuel injector  20  reset themselves for the next injection event, cam  11  continues to rotate, and plunger  14  is returned to its downward position. As plunger  14  retracts, fuel is drawn into fuel pressurization chamber  15  via pump inlet/spill port  18 . 
     Referring now to  FIGS. 4   a-b , the present invention will be described for an injection event having a square rate trace. Pump inlet/spill port  18  is closed and plunger  14  begins to advance with rotation of cam  11  to pressurize fuel within fuel pressurization chamber  15 . It should be appreciated that for the front end of the injection event to have a square rate shape, as opposed to a ramp rate shape, fuel being supplied to fuel inlet  22  should be at or near the maximum desired injection pressures for the injection event before nozzle outlet  70  is opened. Thus, solenoid coil  33  is preferably not energized until pump  12  has pressurized the fuel to about the desired injection pressure. Just prior to the desired start of the injection event, solenoid coil  33  is supplied with a high pull current amount, which is above the second threshold current level (Time  10 ,  FIG. 4   a ). Needle control valve member  35  is moved to its closed position by armature  32  to close needle control passage upper portion  44  from needle control passage lower portion  45 , ending fuel flow from fuel inlet  22  to needle control chamber  60 . At the same time, rate shaping valve member  50  is moved to its open position by needle control valve member  35  to open unrestricted nozzle supply passage upper portion  57  to unrestricted nozzle supply passage lower portion  58  thus opening nozzle chamber  68  to fuel inlet  22  via unrestricted nozzle supply passage  26 . With closing hydraulic surface  61  no longer exposed to high pressure within needle control chamber  60 , needle valve member  66  can be lifted to its upward, open position as a result of the high pressure fuel acting on opening hydraulic surface  67  in nozzle chamber  68  (Time G,  FIG. 4   b ). In addition, once valve members  35  and  50  have been moved to their respective closed and open positions, current to solenoid coil  33  is reduced to a high current hold level that is sufficient to maintain the valve members in their respective positions (Time  11 ,  FIG. 4   a ) 
     When the desired amount of fuel has been injected by fuel injector  20 , solenoid coil  33  is de-energized (Time  12 ,  FIG. 4   a ). Needle control valve member  35  is returned to its biased, open position under the influence of biasing spring  40  while rate shaping valve member  50  is returned to its biased, closed position under the influence of biasing spring  51 . Needle control chamber  60  is once again fluidly connected to fuel inlet  22  via needle control passage  28 , thus re-exposing closing hydraulic surface  61  to high pressure. With pressurized fuel acting on closing hydraulic surface  61 , needle valve member  66  is returned to its downward, closed position to end the injection event (Time H,  FIG. 4   b ). At the same time, with rate shaping valve member  50  once again in its closed position, unrestricted nozzle supply passage  26  is once again blocked such that nozzle chamber  68  is fluidly connected to fuel inlet  22  via only restricted nozzle supply passage  24 . As the various components of fuel injector  20  reset themselves for a subsequent injection event, pump inlet/spill port  18  is opened. As cam  11  continues rotating, plunger  14  ends its advancing movement and begins to retract. As plunger  14  retracts, fresh fuel is drawn into fuel pressurization chamber  15  via pump inlet/spill port  18  for the next injection event. 
     Referring now to  FIGS. 5   a-b , the present invention will be described for an injection event having a triangle rate trace. Whereas the previous injection rate trace was generated by energizing solenoid coil  33  once unit pump  12  built up fuel pressure to about the maximum desired injection level, for a triangle rate trace, solenoid coil  33  is energized before pump  12  has raised fuel in fuel pressurization chamber  15  to the maximum desired injection pressure. Thus, just prior to the desired start of an injection event, and prior to maximum injection pressure being reached by pump  12 , solenoid coil  33  is supplied with an amount of current above the second threshold level (Time  13 ,  FIG. 5   a ). Needle control valve member  35  is thus moved to its closed position blocking fluid communication between needle control chamber  60  and fuel inlet  22 , while rate shaping valve member  50  is moved to its open position fluidly connecting nozzle chamber  68  to fuel inlet  22  via unrestricted nozzle supply passage  26 . With high pressure no longer acting on closing hydraulic surface  61  in needle control chamber  60 , needle valve member  66  is moved away from its closed position once a valve opening pressure is reached in nozzle chamber  68  to allow the commencement of fuel spray from nozzle outlet  70  (Time I,  FIG. 5   b ). As pump  12  increases the pressure of fuel being supplied to injector  20 , the mass flow rate of fuel being injected via nozzle outlet  70  continues to rise. Further, once valve members  35  and  50  have been moved to their respective closed and opened positions, current to solenoid coil  33  is reduced to a high current hold level that is sufficient to maintain valve members  35  and  50  in their respective opened and closed positions (Time  14 ,  FIG. 5   a ). 
     Prior to the desired end of the injection event, pump  12  ceases pressurizing fuel and dumps excess fuel via pump inlet/spill port  18 . As pump  12  ends pressurization, fuel supplied to fuel inlet  22  will begin to decrease in pressure, resulting in a decrease in the pressure within nozzle chamber  68 . Thus, with fuel pressure acting on opening hydraulic surface  67  decreasing, needle valve member  66  will be moved toward its closed position by biasing spring  62  (Time J,  FIG. 5   b ). However, it should be appreciated that needle valve member  66  will not be returned to its closed position so long as fuel pressure within nozzle chamber  68  remains above the valve closing pressure. After needle valve member has closed, solenoid coil  33  is de-energized (Time  15 ,  FIG. 5   a ) and needle control valve member  35  is returned to its biased, open position by biasing spring  40 . Needle control chamber  60  is once again fluidly connected to fuel inlet  22  via needle control passage  28 . With high pressure now acting on closing hydraulic surface  61  in needle control chamber  60 , fuel pressure acting on opening hydraulic surface  67  is no longer sufficient to maintain needle valve member  66  in an open position. Thus, needle valve member  66  is returned to its closed position to end the injection event (Time K,  FIG. 5   b ). 
     Referring now to  FIGS. 6   a-b , the present invention will be described for a split injection event. As illustrated, the injection event will include a small, square pilot injection with a boot shaped main injection. Prior to an injection event, pump inlet/spill port  18  is closed. Rotation of cam  11  causes plunger  14  to advance and begin pressurization of fuel within fuel pressurization chamber  15 . Just prior to the desired start of the pilot injection event, solenoid coil  33  is supplied with an amount of current above the first threshold level (Time  16 ,  FIG. 6   a ). Needle control valve member  35  is then pushed to its closed position by armature  32 . With high pressure no longer acting on closing hydraulic surface  61  in needle control chamber  60 , the high pressure acting on opening hydraulic surface  67  is sufficient to move needle valve member  66  toward its open position (Time L,  FIG. 6   b ). In addition, once needle control valve member  35  has been moved to its closed position, current to solenoid coil  33  is reduced to a low current hold level (Time  17 ,  FIG. 6   a ). When the desired amount of fuel has been injected for the pilot injection, solenoid coil  33  is de-energized (Time  18 ,  FIG. 6   a ) and needle control valve member  35  is returned to its open position by biasing spring  40 . With needle control chamber  60  re-connected to fuel inlet  22 , the high pressure acting on closing hydraulic surface  61  is sufficient to move needle valve member  66  to its downward position to end the pilot injection (Time M,  FIG. 6   b ). 
     When it is desired to begin the main injection event, solenoid coil  33  is re-energized with an amount of current above the first threshold level but below the second threshold level (Time  19 ,  FIG. 6   a ) and needle control valve member  35  is returned to its closed position by armature  32 . Needle control passage  28  is once again closed by needle control valve member  35  and needle control chamber  60  is again blocked from fuel inlet  22 . With high pressure no longer acting on closing hydraulic surface  61 , the fuel pressure acting on opening hydraulic surface  67  is sufficient to lift needle valve member  66  toward its open position (Time N,  FIG. 6   b ). Thus, it should be appreciated that a split injection can be produced by re-energizing solenoid coil  33  with a current above the first threshold level after nozzle outlet  70  has been closed for the pilot injection event. Once needle control valve member  35  has been moved to its closed position, current to solenoid coil  33  is reduced to the first hold level (Time  20 ,  FIG. 6   a ). When it is desired to increase to higher injection levels, current to solenoid coil  33  is increased above the second threshold level (Time  21 ,  FIG. 6   a ). Rate shaping valve member  50  is then moved toward its open position (Time O,  FIG. 6   b ). Pressurized fuel can now flow through unrestricted nozzle supply passage  26  into nozzle chamber  68 . Once again, when rate shaping valve member  50  is moved to its open position, current to solenoid coil  33  is reduced to a high current hold level (Time  22 ,  FIG. 6   a ). 
     When the desired amount of fuel has been injected, current is ended to solenoid coil  33  (Time  23 ,  FIG. 6   a ). Needle control valve member  35  is returned to its biased, open position under the influence of biasing spring  40  while rate shaping valve member  50  is returned to its biased, closed position under the influence of biasing spring  51 . Needle control chamber  60  is once again fluidly connected to fuel inlet  22  via needle control passage  28 , thus re-exposing closing hydraulic surface  61  to high pressure. With pressurized fuel acting on closing hydraulic surface  61 , needle valve member  66  is returned to its downward, closed position to end the injection event (Time P,  FIG. 6   b ). At the same time, with rate shaping valve member  50  once again in its closed position, unrestricted nozzle supply passage  26  is once again blocked such that nozzle chamber  68  is fluidly connected to fuel inlet  22  via only restricted nozzle supply passage  24 . As the various components of fuel injector  20  reset themselves for the next injection event, pump inlet/spill port  18  is reopened. Plunger  14  ends its advancing movement and begins to retract with the continued rotation of cam  11 . The retracting movement of plunger  14  draws fresh fuel into fuel pressurization chamber  15  for the next injection event. 
     It should be appreciated that various modifications could be made to the embodiment of the present invention disclosed herein. For instance, while valve assembly  30  has been illustrated including a needle control valve member  35  and a rate shaping valve member  50 , both of which are poppet valves, it should be appreciated that various other valve members could be substituted. For example, one or both of these valve members could be replaced by a spool valve member or a ball valve member. Alternatively, valve assembly  30  could include a single valve member in place of valve members  35  and  50 . In addition, while needle control valve member  35  has been illustrated separating needle control chamber  60  from a source of high pressure, it should be appreciated that needle control valve member  35  could instead separate needle control chamber  60  from a source of low pressure. In this instance, needle control valve member  35  would move to a position opening needle control chamber  60  to the low pressure source just prior to the desired start of an injection event. Additionally, while the present invention has been illustrated utilizing pressurized fuel to actuate needle valve member  66 , it should be appreciated that other suitable actuation fluids, such as engine lubricating oil, could be utilized. Thus, it should be appreciated that for this alternative restricted nozzle supply passage  24  and unrestricted nozzle supply passage  26  could be fluidly connected to fuel inlet  22 , while needle control passage  28  is fluidly connected to a separate fluid inlet. 
     Further, while restricted nozzle supply passage  24  has been illustrated as being defined by injector body  21 , it should be appreciated that the flow restriction could instead be defined by rate shaping valve member  50  and injector body  21 . For instance, rate shaping valve member  50  could define restricted nozzle supply passage  24  to open at valve seat  53 . In a first position, valve surface  54  could be positioned slightly away from valve seat  53 , such that a restricted flow path would exist around valve seat  53 . In a second position, valve surface  54  could be further from valve seat  53  such that no flow restriction exists in the flow passage. In addition, restricted nozzle supply passage  24  could be defined completely by rate shaping valve member  50 . In this instance, restricted nozzle supply passage  24  would be defined by rate shaping valve member  50  to be open to nozzle supply passage lower portion  58  regardless of whether rate shaping valve member  50  is in its open or closed position. 
     Although this invention is illustrated in the context of a hydraulically actuated unit pump fuel injection system, one skilled in the art will recognize that this invention is equally applicable to other fuel systems such as the amplifier piston common rail system (APCRS) illustrated in the paper “Heavy Duty Diesel Engines—The Potential of Injection Rate Shaping for Optimizing Emissions and Fuel Consumption”, presented by Messrs. Bernd Mahr, Manfred Durnholz, Wilhelm Polach, and Hermann Grieshaber; Robert Bosch GmbH, Stuttgart, Germany, at the 21 st  International Engine Symposium, May 4-5, 2000, Vienna, Austria. 
     It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. Thus, those skilled in the art will appreciate that other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.