Abstract:
Fuel injectors equipped with direct control needle valves can add new capabilities to a fuel injection system, but can sometimes have difficulty in achieving low hydrocarbon emissions at levels comparable to ancestor fuel injectors that utilize a simple spring biased needle. The present invention seeks lower hydrocarbon emissions by reducing fuel pressure before the direct control needle valve member has reached its closed position toward the end of an injection event. Reducing fuel pressure can be accomplished in a number of ways depending upon the particular fuel injection system, including spilling fuel pressure in a cam system or possibly relieving pressure on an intensifier piston. By employing this strategy, fuel spray from the fuel injector can effectively end before the direct control needle valve member reaches its closed position, thus avoiding hydrocarbon production that could be caused by a small amount of fuel pushed into the combustion space as the needle moves over the last portion of its movement toward its closed position.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to end of injection rate shaping for fuel injection events, and more particularly to a method of operating a fuel injection system in a way that can reduce undesirable hydrocarbon and smoke emissions from an engine and improves fuel economy.  
         BACKGROUND  
         [0002]    Engineers are constantly seeking ways to reduce undesirable engine emissions without over reliance upon exhaust after treatment techniques. One strategy is to seek ways to improve performance of fuel injection systems. Over the years, engineers have come to learn that engine emissions can be a significant function of injection timing, the number of injections, injection quantities and rate shapes. However, it is also been observed that an injection strategy at one engine operating condition may decrease emissions at that particular operating condition, but actually produce an excessive amount of undesirable emissions at a different operating condition. Thus, for a fuel injection system to effectively reduce emissions across an engine&#39;s operating range, it must have the ability to produce several different rate shapes, have the ability to produce multiple injections and produce injection timings and quantities with relatively high accuracy. Providing a fuel injection system that can perform well with regard to all of these different parameters over an entire engine&#39;s operating range has proven to be elusive.  
           [0003]    In order to reduce hydrocarbon emissions, the conventional wisdom has been to seek an abrupt end to each injection event. This strategy flows from the conventional wisdom that reducing poorly atomized fuel spray into the combustion space toward the end of an injection event can reduce the production of undesirable hydrocarbon and smoke emissions. In the case of fuel injectors equipped with direct control needle valves, an abrupt end to injection is often accomplished by applying high pressure fluid to the back side of a direct control needle valve member to quickly move it toward a closed position while fuel pressure within the injector is relatively high. Recent data from some directly controlled fuel injection systems appear to show higher hydrocarbon and smoke emissions at certain operating conditions than those typically observed in relation to older systems in which the nozzle is controlled by a simple spring biased needle. In some fuel injection systems, closing the needle valve member at high pressure can also have structural consequences. When a needle is closed at high injection pressures, pressure can spike within the injector, and especially in the relatively sensitive area of the injector tip, exacerbating the structural strength requirements in the tip region of the fuel injector. These pressure spikes can sometimes cause small uncontrolled secondary injections that increase hydrocarbon emissions. In the case of hydraulically actuated fuel injection systems, closing the needle at high pressure can also result in a reduction in efficiency. This occurs when pressurized actuation fluid continues to pour into the fuel injector briefly after the needle has moved to close the nozzle outlet. Ending injection events at high pressure can also exacerbate the already difficult problem of producing small injection quantities, such as precisely controlled small post injection quantities.  
           [0004]    One effort to deal with venting pressure at the end of an injection event in order to avoid small uncontrolled secondary injections is disclosed in U.S. Pat. No. 5,682,858 to Chen et al., and entitled Hydraulically-Actuated Fuel Injector With Pressure Spike Relief Valve. In this fuel injection system, closure of the direct control needle valve member occurs before the flow control valve can end supply of high pressure actuation fluid to act on an intensifier piston. This reference teaches the use of a separate pressure relief valve that opens to relieve actuation fluid pressure as the flow control valve is moving from its open position toward its closed position. This relief of actuation fluid pressure in turn relieves the downward force on the intensifier piston/plunger to also relieve fuel pressure to avoid a pressure spike. While this strategy may be effective in reducing undesirable and uncontrolled secondary injections, there still remains room for reducing hydrocarbon emissions from engines using this type of fuel injection system.  
           [0005]    The present invention is directed to one or more of the problems set forth above.  
         SUMMARY OF THE INVENTION  
         [0006]    In one aspect, a method of operating a fuel injection system includes a step of moving a direct control needle valve member to open a nozzle outlet. An injection event is ended at least in part by reducing fuel pressure before the direct control needle valve member has reached a closed position.  
           [0007]    In another aspect, a method of rate shaping the end portion of a fuel injection event includes a step of relieving pressure on an intensifier piston at a first timing. A needle control valve is moved at a second timing. The second timing relative to the first timing is sufficient to cause fuel pressure in the fuel injector to drop before a direct control needle valve member has reached a closed position.  
           [0008]    In still another embodiment, a fuel injector includes an injector body with a needle control chamber. A direct control needle valve member is moveably positioned in the injector body and includes a closing hydraulic surface exposed to fluid pressure in the needle control chamber. The fuel injector also includes a means for reducing fuel pressure within the injector body before the direct control needle valve member has reached its closed position.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a schematic of a fuel injection system according to an embodiment of the present invention;  
         [0010]    [0010]FIG. 2 is a sectioned side diagrammatic view of a fuel injector according to an embodiment of the present invention;  
         [0011]    [0011]FIG. 3 is the fuel injector of FIG. 2 as viewed along a different section line;  
         [0012]    [0012]FIG. 4 is a sectioned side diagrammatic view of a flow control valve for the fuel injector of FIGS. 2 and 3;  
         [0013]    [0013]FIG. 5 is a sectioned side view of the needle control valve assembly from the fuel injector of FIGS. 2 and 3;  
         [0014]    [0014]FIG. 6 is an isometric view of an electrical actuator subassembly for the needle control valve shown in FIG. 5;  
         [0015]    [0015]FIG. 7 is a partially sectioned side diagrammatic view of a fuel injector according to another embodiment of the present invention;  
         [0016]    [0016]FIG. 8 is a sectioned side diagrammatic view of a flow control valve assembly according to another aspect of the present invention;  
         [0017]    [0017]FIG. 9 is a partially sectioned side diagrammatic view of a flow control valve assembly according to still another aspect; and  
         [0018]    [0018]FIGS. 10 a - e  are graphs of first electrical actuator control signal, second electrical actuator control signal, direct control needle valve member position, pressure, and fuel injection rate, verses time for an end of injection event according to one aspect of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Referring to FIG. 1, an example diesel engine  10  includes six cylinders  11  and a common rail fuel injection system  12 . The system includes an individual fuel injector  14  for each engine cylinder  11 , a single common rail  16 , an oil sump  20  fluidly connected to the common rail  16 , and a fuel tank  18  on a separate fluid circuit. Those skilled in the art will appreciate that in other applications there may be two or more separate common rails, such as a separate rail for each side of a V8 engine. An electronic control module  22  controls the operation of fuel injection system  12 . The electronic control module  22  preferably utilizes advanced strategies to improve accuracy and consistency among the fuel injectors  14  as well as pressure control in common rail  16 . For instance, the electronic control module  22  might employ electronic trimming strategies individualized to each fuel injector  14  to perform more consistently. Consistent performance is desirable in the presence of the inevitable performance variability responses due to such causes as realistic machining tolerances associated with the various components that make up the fuel injectors  14 . In another strategy, the electronic control module  22  might employ a model based rail pressure control system that breaks up the rail pressure control issue into one of open loop flow control coupled with closed loop error and pressure control.  
         [0020]    When fuel injection system  12  is in operation, oil is drawn from oil sump  20  by a low pressure oil circulation pump  24 , and the outlet flow is split between an engine lubrication passage  27  and a low pressure fuel injection supply line  28 , after passing through an oil filter  25  and a cooler  26 . The oil in engine lubrication passage  27  travels through the engine and lubricates its various components in a conventional manner. The oil in low pressure supply line  28  is raised to a medium pressure level by a high pressure pump  29 . This “medium pressure” is a relatively high pressure compared to oil drain and fuel supply pressures, but still lower than peak injection pressures. Pump  29  is preferably an electronically controlled variable delivery pump, such as a sleeve metered fixed displacement variable delivery pump of a type manufactured by Caterpillar, Inc. of Peoria, Ill. High pressure pump  29  is connected to common rail  16  via a high pressure supply line  30 . Each of the individual fuel injectors  14  have an actuation fluid inlet  60  connected to common rail  16  via a separate branch passage  31 . After being used within individual fuel injectors  14  to pressurize fuel, the oil leaves fuel injectors  14  via an actuation fluid drain  62  and returns to oil sump  20  for recirculation via a return line(s)  32 . Those skilled in the art will appreciate that any available fluid, including fuel, coolant or transmission fluid, could be utilized as actuation fluid in place of the illustrated lubricating oil.  
         [0021]    Fuel is drawn from a fuel tank  18  by a fuel transfer pump  36  and circulated among fuel injectors  14  via a fuel supply line  34  after passing through a fuel filter  37 . Fuel transfer pump  36  is preferably a constant flow electric pump with a capacity sized to meet the maximum demands for engine  10 . Also, fuel transfer pump  36  and fuel filter  37  are preferably contained in a common housing. Any fuel not used by the fuel injectors  14  is recirculated to fuel tank  18  via fuel return line  35 . Fuel in the fuel supply and return lines  34  and  35  are at a relatively low pressure relative to that in common rail  16 , which contains pressurized oil. In other words, fuel injection system  12  includes no high pressure fuel lines (i.e. lines containing fuel at injection pressure levels), and the fuel is pressurized to injection levels within each individual fuel injector  14 , and then usually for only a brief period of time during an injection sequence.  
         [0022]    Fuel injection system  12  is controlled in its operation via an electronic control module  22  via control communication lines  40  and  41 . Control communication line  40  communicates with high pressure pump  29  and controls its delivery, and hence the pressure in common rail  16 . Control communication lines  41  include four wires, one pair for each electrical actuator within each fuel injector  14 . These respective actuators within fuel injectors  14  control flow of actuation fluid to the injectors from rail  16 , and the opening and closing of the fuel injector spray nozzle. Electronic control module  22  determines its control signals based upon various sensor inputs known in the art. These include an oil pressure sensor  42  attached to rail  16  that communicates an oil pressure signal via sensor communication line  45 . In addition, an oil temperature sensor  43 , which is also attached to rail  16 , communicates an oil temperature signal to electronic control module  22  via a sensor communication line  44 . In addition, electronic control module  22  receives a variety of other sensor signals via a sensor communication line(s)  46 . These sensors could include but are not limited to, a throttle sensor  47 , a timing sensor  48 , a boost pressure sensor  49  and a speed sensor  50 .  
         [0023]    Referring in addition to FIGS. 2 and 3, each fuel injector  14  includes an injector body  61  that can be thought of as including an upper portion  66  and a lower portion  68 . Fuel injector  14  can also be thought of as being divided between fuel pressurization assembly  67  and a direct control nozzle assembly  69 . In the fuel injector  14  illustrated, fuel pressurization assembly  67  is located in upper portion  66 , whereas direct control nozzle assembly  69  is located in lower portion  68 . Although the fuel injector  14  shows the fuel pressurization assembly  67  and the direct control nozzle assembly  69  joined into a unit injector  14 , those skilled in the art will appreciate that those respective assemblies could be located in separate bodies connected to one another with appropriate plumbing. The fuel pressurization assembly  67  includes a pressure intensifier  70  and a flow control valve  74 , which is operably coupled to an electrical actuator  72 . Direct control nozzle assembly  69  includes a needle control valve assembly  76  that is operably coupled to an electrical actuator  78 , which is located in, and attached to, lower portion  68 . In addition, a direct control needle valve  79  is controlled in its opening and closing by needle control valve assembly  76 , and hence electrical actuator  78 . Pressurized oil enters injector body  61  through its top surface at actuation fluid inlet  60 , and used low pressure oil is recirculated back to the sump  24  via an actuation fluid drain  62 . Fuel is circulated among the lower portions  68  of fuel injectors  14  via fuel inlets  64 .  
         [0024]    Pressure intensifier  70  includes a stepped top intensifier piston  82  and preferably a free floating plunger  84 . Intensifier piston  82  is biased to its retracted position, as shown, by a return spring  83 . The stepped top of intensifier piston  82  allows the initial movement rate, and hence possibly the initial injection rate, to be lower than that possible when the stepped top clears a counterbore. Return spring  83  is positioned in a piston return cavity  86 , which is vented directly to the area underneath the engine&#39;s valve cover via an unobstructed vent passage  87 . Free floating plunger  84  is biased into contact with the underside of intensifier piston  82  via low pressure fuel acting on one end in fuel pressurization chamber  90 . Plunger  84  preferably has a convex end in contact with the underside of intensifier piston  82  to lessen the effects of a possible misalignment. In addition, plunger  84  is preferably symmetrical about three orthogonal axes such that fuel injector  14  can be more easily assembled by inserting either end of plunger  84  into the plunger bore located within injector body  61 . When intensifier piston  70  is undergoing its downward pumping stroke, fuel within fuel pressurization chamber  90  is raised to injection pressure levels. Any fuel that migrates up the side of plunger  84  is preferably channeled back for recirculation via a plunger vent annulus and a vent passage  92 . Pressure intensifier  70  is driven downward when flow control valve  72  connects actuation fluid passages  80 / 81  to high pressure actuation fluid inlet  60 . Between injection events, flow control valve  72  connects actuation fluid passages  80 / 81  to low pressure drain  62  allowing the intensifier  70  to retract toward its retracted position, as shown, via the action of return spring  83  and fuel pressure acting on the underside of plunger  84 . Thus, when pressure intensifier  70  is retracting, fresh fuel is pushed into fuel pressurization chamber  90  past check valve  93  via fuel inlet  64 .  
         [0025]    Referring in addition to FIG. 4, flow control valve  74  includes an electrical actuator  72 , which in the illustrated embodiment is a solenoid, but could equally be any other suitable electrical actuator known in the art including, but not limited to, piezos, voice coils, etc. Flow control valve  74  includes a valve body  120  that includes separate passages connected to actuation fluid inlet  60 , actuation fluid drain  62  and actuation fluid passages  80 / 81 , respectively. Flow control valve  74  includes a spool valve member  124  biased via a biasing spring  125  to a first position that fluidly connects an actuation fluid passage  80 / 81  to actuation fluid drain  62 . When electrical actuator  72  is energized, an armature  122  moves toward coil  121 . This movement causes a pushpin  123  to push spool valve member  124  away from coil  121  to compress biasing spring  125  toward a second position. At this energized position, spool valve member  124  closes the fluid connection between actuation fluid passage  80 / 81  and drain  62 , and opens high pressure inlet  60  to actuation fluid passages  80 / 81 . These fluid connections are facilitated via respective high pressure annuluses  126  and  127  formed on the outer surface of spool valve member  124 . Control communication line  41  of FIG. 1, electronic control module  22 , and electric terminals  128  that are attached to valve body  120  are electrically connected to coil  121  in a conventional manner.  
         [0026]    When pressure intensifier  70  is driven downward, high pressure fuel in fuel pressurization chamber  90  can flow via nozzle supply passage  107  to the nozzle chamber  105 , and out of nozzle outlets  104  if direct control needle valve  79  is in an open position. When direct control needle valve  79  is in its closed position as shown, nozzle chamber  105  is blocked from fluid communication with nozzle outlets  104 . Direct control needle valve  79  includes a direct control needle valve member  113  made up of a needle portion  112  separated from a piston portion  109  by a lift spacer  106 . Thus, the needle valve member in this embodiment is made up of several components for ease of manufactureability and assembly, but could also be manufactured from a single solid piece. The direct control needle valve member  113  includes an opening hydraulic surface  103  exposed to fluid pressure in nozzle chamber  105 , and a closing hydraulic surface  101  exposed to fluid pressure in a needle control chamber  100 . The thickness of lift spacer  106  preferably determines the maximum opening travel distance of direct control needle valve  79 . The direct control needle valve  79  is biased toward its downward closed position, as shown, by a biasing spring  102  that is compressed between lift spacer  106  and a VOP (valve opening pressure) spacer  108 . Thus, the valve opening pressure of the direct control valve  79  can be trimmed at time of manufacture by choosing an appropriate thickness for VOP spacer  108 . Needle control chamber  100  is fluidly connected to either low pressure fuel inlet  64  or to nozzle supply passage  107  depending upon the positioning of needle control valve assembly  76 . When needle control chamber  100  is fluidly connected to nozzle supply passage  107 , direct control needle valve  79  will remain in or move toward its closed position, as shown, under the action of fluid pressure forces on closing hydraulic surface  101  and the spring force from biasing spring  102 . When needle control chamber  100  is fluidly connected to fuel inlet  64 , while nozzle passage  107  and hence nozzle chamber  105  are above a valve opening pressure, the fluid forces acting on opening hydraulic surface  103  are sufficient to lift the direct control needle valve member  113  upward towards its open position against the action of biasing spring  102  to open nozzle outlets  104 . Although the direct control needle valve is illustrated as being controlled by applying and relieving pressure on a closing hydraulic surface of the needle valve member, the present invention also contemplates other types of direct control needle valve members. For instance, the needle valve member might be driven to move directly by energizing and de-energizing a piezo actuator and/or an electromagnetic actuator in contact with the needle valve member.  
         [0027]    Referring in addition to FIGS. 5 and 6, the inner workings of needle control valve  76  are illustrated. Valve assembly  76  includes a valve body  138  which defines a portion of nozzle supply passage  107 , a connection passage  110 , a low pressure passage  111  and a needle control passage  99 . The valve assembly  76  is a two position three way valve that includes a needle control valve member  139  that is moveable between contact with a high pressure seat  144  and a low pressure seat  145 . Depending upon the position of valve member  139 , needle control passage  99 , which is fluidly connected to needle control chamber  100  (FIGS. 2 and 3), is fluidly connected to nozzle supply passage  107  via connection passage  110  or to fuel inlet  64  via low pressure passage  111 . Needle control valve assembly  76  includes a second electrical actuator  78  which in the illustrated embodiment is a solenoid subassembly  77 , but could also be another type of electrical actuator, such as a piezo, a voice coil, etc. The solenoid subassembly  77  includes a stator  140 , a coil  142  and a pair of female electrical socket connectors  97  that are electrically connected to coil  142 . The female electrical socket connection  97 , which could instead be male, permits an electrical extension  96  to mate with solenoid subassembly  77  within injector body  71  while providing exposed terminals for insulated conductors  95  outside of upper portion  66 . Valve member  139  is biased downward to close low pressure seat  145  by a biasing spring  141  via an armature  143  that is attached to valve member  139 . When coil  142  is energized, armature  143  is lifted upward causing valve member  139  to open low pressure seat  145  and close high pressure seat  144 . Because the flow areas past seats  144  and  145  effect the performance of the fuel injector  14 , such as by effecting the opening and/or closing rate of direct control valve  79 , flow restrictions  146  and  147  are included. In particular, flow restriction  146 , which is preferably manufactured in an orifice plate  148  as a flow area that is restrictive relative to the flow area past seat  144 . Likewise, flow restriction orifice  147  preferably has a flow area that is restricted relative to the flow past low pressure seat  145 . Because these respective orifices  146  and  147  are based upon simple bore diameters rather than a clearance area between two separate moving parts, the performance between respective fuel injectors can be made more uniform. Furthermore, because these features are machined in a single orifice plate  145 , the manufactureability and assembly of needle control valve assembly  76  can be improved.  
         [0028]    Referring now to FIG. 7, a fuel injector  214  according to another embodiment of the present invention includes an injector body  261  with a lower portion  268  that could be used in conjunction with the upper portion  61  of fuel injector  14  shown in FIGS. 2 and 3. This lower portion  268  differs from lower portion  68  in that it includes a reduced diameter portion that effects the structure of needle control valve  276 . Like the earlier embodiment, lower portion  268  includes a direct control nozzle assembly  269  which includes a direct control needle valve  279  and a needle control valve  276 . Like the earlier embodiment, direct control needle valve  279  includes a direct control needle valve member  213  that includes a needle portion  299  separated from a needle piston portion  209  by a VOP spacer  208 . Needle portion  299  includes a opening hydraulic surface exposed to fluid pressure in a nozzle chamber  205  that is fluidly connected to nozzle outlets  204  when direct control needle valve member  213  is lifted to an upward open position. When in such a position, fuel pressurization chamber  290  is fluidly connected to nozzle outlet  204  via nozzle supply passage  207  and nozzle chamber  205 . Direct control needle valve member  213  is preferably biased to a downward closed position by a biasing spring  202 . Depending upon the positioning of needle control valve  276 , needle control chamber  200  is fluidly connected via needle control passage  199  to either nozzle supply passage  207  via connection passage  210 , or to fuel inlet  264  via low pressure passage  211 . Direct control needle valve member  213  includes a closing hydraulic surface  201  exposed to fluid pressure in needle control chamber  200 . When the plunger for fuel injector  214  is undergoing its upward retracting stroke, fuel pushes open check valve  293  to refill fuel pressurization chamber  290  for a subsequent injection sequence. The needle control valve  276  includes a needle control valve member  239  that is moveable by an electrical actuator  278  between a low pressure seat  245  and a high pressure seat  244 . Electrical actuator  278  includes a coil  242 , a biasing spring  241  and an armature  243  attached to valve member  239 . Armature  243 , in this embodiment, is preferably a wagon wheel shaped armature such that a body component that includes a portion of nozzle supply passage  207  protrudes through the arms of the armature wagon wheel to provide for fluid communication and permit the reduced diameter shown.  
         [0029]    Referring now to FIG. 8, a flow control valve assembly  374  according to another embodiment of the present invention could be substituted in place of the flow control valve assembly  74  shown in FIGS.  2 - 4 . Unlike the single stage valve assembly  74  shown in FIGS. 2 and 3, flow control valve assembly  374  includes a pilot valve assembly  373  which controls flow via controlling the positioning of a spool valve member  320 . Like the earlier embodiment, flow control valve assembly  374  includes a valve body  321  that includes a top surface with an actuation fluid inlet  360 , an actuation fluid drain  362 , and an actuation fluid passage  380 . Spool valve member  320  includes a biasing hydraulic surface  322  always exposed to fluid pressure inlet  360 , and a control hydraulic surface  324  exposed to fluid pressure in a pressure control chamber  331 . Hydraulic surfaces  322  and  324  are preferably about equal in effective area such that spool valve member  320  is substantially hydraulically balanced when the fluid pressure acting on the opposite ends is equal. This is facilitated by spool valve member  320  including a pressure communication passage  327 . Spool valve member  320  also includes a low pressure annulus  326  that connects actuation fluid passage  380  to actuation fluid drain  362  when spool valve member  320  is biased to its drain position, as shown, by biasing spring  330 . When pressure in control chamber  331  is low, fluid pressure on surface  322  moves spool valve member  320  to its actuation position compressing spring  330  and moving annulus and radial passages  325  to communicate fluid from actuation fluid inlet  360  to actuation fluid passage  380 . At the same time, annulus  326  moves out of fluid communication with actuation fluid passage  380 .  
         [0030]    Pressure in control chamber  331  is controlled by pilot valve assembly  373 . Pilot valve assembly  373  includes a pilot valve member  344  that moves between a high pressure seat  340  and a low pressure seat  338 . When pilot valve member  344  is closing low pressure seat  338 , pressure control chamber  331  is fluidly connected to actuation fluid inlet  360  via pressure communication passage  332  and branch passage  334 . Pilot valve member  344  is biased to that position by a biasing spring  348 . When the electrical actuator  372  is energized, coil  342  attracts armature  346  and pilot valve member  344  to compress spring  348  and close high pressure seat  340 . This fluidly connects pressure control chamber  331  to drain passage  362  via control passage  332  and vent passage  336 .  
         [0031]    Referring now to FIG. 9, a flow control valve assembly  474  according to still another aspect of the present invention could be substituted in place of the flow control valve assembly  74  shown in FIGS. 2 and 3. This embodiment differs from the embodiment of FIG. 8 in that the spool valve member  420  is oriented vertically instead of horizontally as shown in FIG. 8. Flow control valve assembly  474  includes a pilot valve assembly  373  substantially identical to that shown in FIG. 8. Like the earlier embodiments, flow control valve assembly  474  includes a valve body  421  that includes a top surface with an actuation fluid inlet  460 , and actuation fluid drain  462  and an actuation fluid passage  480 . Spool valve member  420  includes a biasing hydraulic surface  422  always exposed to the high pressure of actuation fluid inlet  460  and a control hydraulic surface  424  exposed to fluid pressure in a pressure control chamber  431 , which is connected to pilot valve assembly  373  via a pressure communication passage  432  similar to that shown in FIG. 8. Spool valve member  420  is normally biased to its upward position, as shown by a biasing spring  430  to connect actuation fluid passage  480  to actuation fluid drain  462  via low pressure annulus  426 . When pilot valve assembly  373  connects pressure control chamber  431  to low pressure, spool valve member  420  moves downward to close the actuation fluid drain  462 , and open actuation fluid passage  480  to actuation fluid inlet  460  via vertical passages  429  and annulus  428 . When high pressure exists in pressure control passage  431 , spool valve member  420  is preferably hydraulically balanced via the respective surface areas  422  and  424  as well as the balancing effect provided by pressure communication passage  427 .  
       INDUSTRIAL APPLICABILITY  
       [0032]    Each engine cycle can be broken into an intake stroke, a compression stroke, a power stroke and an exhaust stroke. During each engine cycle, each fuel injector  14  has the ability to inject up to five or more discrete shots per engine cycle. While a majority of these injection events will take place at or near the transition from the compression to power strokes, injection events can take place at any timing during the engine cycle to produce any desirable effect. For instance, an additional small injection event elsewhere in the engine cycle might be useful in reducing undesirable emissions. During each engine cycle, a number of basic steps are performed to inject fuel, and each of those acts is performed at a timing and in a number to produce a variety of fuel injection sequences, which include one or more injection events.  
         [0033]    Among the steps performed at least once each engine cycle in each portion of the illustrated injection system (e.g., fuel injector) for each engine cylinder is the step of positioning a needle control valve  76 ,  276  in a position that raises pressure in the needle control chamber  100 ,  200  by connecting the same to the fuel pressurization chamber  90 ,  290 , and fluidly blocking the needle control chamber  100 ,  200  to the low pressure passage  111 ,  211 . In the illustrated embodiment, that is accomplished by biasing the needle control valve member  139 ,  239  into contact to close a low pressure seat  145 ,  245  by a spring  141 ,  241 . The valve  139 ,  239  could be biased in the other direction and operate in a manner opposite to that described with regard to the illustrated embodiments. In all cases, that act is performed by a three way valve. With this configuration, the pressurization chamber  90  is only briefly connected to the fuel inlet  64  when the needle control valve member  139 ,  239  is moving between low pressure seat  145 ,  245  and the high pressure seat  144 ,  244 . Between injection events when pressure in fuel pressurization chamber  90 ,  290  is relatively low, very little leakage occurs past needle control valve assembly  76 ,  276 . In addition, little leakage occurs during each injection event since the respective high pressure seats  144 ,  244  are closed. When the needle control chamber  100 ,  200  is fluidly connected to the fuel pressurization chamber  90 ,  290  and blocked from the low pressure passage  111 ,  211 , no fuel injection takes place. In other words, when that occurs, direct control needle valve  79 ,  279  is preferably held in or moved toward its downward closed position, as shown.  
         [0034]    Those skilled in the art will appreciate that applying high pressure to the closing hydraulic surface of a direct control needle valve member can be accomplished in other ways without departing from the present invention. For instance, a two way valve in the low pressure passage (see Bosch APCRS system) could be substituted in place of the three way valve illustrated. In such an example, the needle control chamber is always connected to the nozzle supply passage, but via a flow restriction. Thus, when the two way valve is open, pressure drops in the needle control chamber due to the fact that the flow through the low pressure passage is less restricted than flow coming into the needle control chamber from the nozzle supply passage. When the two way valve is closed, the needle control chamber is only connected to the source of high pressure fuel. In still another alternative, the direct control needle valve member may be controlled in its movement by applying actuation fluid pressure to the closing hydraulic surface instead of fuel as in the illustrated embodiment. This alternative could use either a three way valve similar to that illustrated, or a two way valve in the low pressure passage, as previously described. In most instances, the step of increasing pressure on the closing hydraulic surface of the direct control needle valve member is accomplished by either energizing or deenergizing an electrical actuator. In the present case, electrical actuator  78 ,  278  is deenergized. In other words, energy to an electrical actuator is either increased or decreased in order to apply high pressure to the closing hydraulic surface of the direct control needle valve member.  
         [0035]    In still another possible alternative, the nozzle outlet is held closed by energizing or de-energizing an actuator in contact with the needle valve member. For instance, a piezo actuator and/or an electromagnetic actuator may be in contact to directly control movement of the needle valve member. In such a case, the nozzle outlet is held closed by either de-energizing or energizing the actuator to move the needle toward, or hold it in, its downward closed position.  
         [0036]    Another act that is performed at least once during each engine cycle includes increasing fuel pressure within the fuel pressurization chamber  90 ,  290  at least in part by moving the flow control valve  74 ,  274 ,  374 ,  474  to a first position. The first position described is preferably the position at which valve  74 ,  274 ,  374 ,  474  opens actuation fluid inlet  60 ,  260 ,  360 ,  460  to actuation fluid passage  80 ,  280 ,  380 ,  480 . In the case of the embodiments shown in FIGS. 8 and 9, energization of pilot valve assembly  373 ,  472  causes the spool valve member  320 ,  420  to connect actuation fluid inlet  360 ,  460  to actuation fluid  380 ,  480 . When this step is performed, high pressure actuation fluid bears down onto the intensifier piston  82 , which compresses fuel in fuel pressurization chamber  90 ,  290  to injection levels. Thus, in all of the illustrated embodiments, increasing fuel pressure in the fuel injector is accomplished by energizing an electrical actuator  72 ,  272 . Nevertheless, those skilled in the art will appreciate that this step will be accomplished by deenergizing an electrical actuator if the valve is biased in an opposite direction. In addition, those skilled in the art will appreciate that in other fuel injection systems that fall within the present invention, the fuel pressure can be increased within the fuel injector in a number of different ways, including but not limited to rotating a cam to move a plunger within the fuel injector, or a pump, or by connecting the fuel injector to a common rail of pressurized fuel. In another possibility, a mechanically or electronically controlled flow distributor could connect a hydraulically actuated fuel injector to a source of high pressure actuation fluid. In any event, any suitable manner of increasing fuel pressure within a fuel injector is compatible with the end of injection rate shaping of the present invention.  
         [0037]    Another act that is performed at least once each engine cycle in the illustrated embodiment, and in some cases many times per engine cycle, includes moving the needle control valve  76 ,  276  to a second position that fluidly connects the needle control chamber  100 ,  200  to the low pressure passage  111 ,  211 , and fluidly blocks the needle control chamber  100 ,  200  to the fuel pressurization chamber  90 ,  290 . This act is accomplished at least in part by increasing electrical energy to an electrical actuator  78  associated with a direct control nozzle assembly  69 . In the illustrated example, that includes supplying electrical energy to terminals  95  located outside the upper portion of fuel injector  14  and channeling that electricity via electrical socket connection  97  to electrical actuator  72 ,  272  located in the lower portion  68 ,  268  of the injector body  61 ,  161 . When this occurs, needle control valve  39 ,  239  is lifted to close high pressure seat  144 ,  244  such that needle control chamber  100 ,  200  is fluidly connected to low pressure passage  111 ,  211 . If fuel pressure in nozzle chamber  105 ,  205  is above a valve opening pressure, the direct control needle valve  79 ,  279  will move to, or stay in, an open position that fluidly connects fuel pressurization chamber  90 ,  290  to nozzle outlet  104 ,  204  via nozzle supply passage  107 ,  207 . If fuel pressure is below a valve opening pressure, the direct control needle valve  79 ,  279  will move toward, or stay in, its biased closed position due to the action of biasing spring  102 ,  202  being the dominant force. Thus, each injection event is initiated by relieving pressure on the closing hydraulic surface of a direct control needle valve member. In the illustrated embodiment this is accomplished by energizing the electrical actuator associated with a three way needle control valve. Those skilled in the art will appreciate that if the valve were biased in an opposite direction, this same act of relieving pressure could be accomplished by deenergizing an electrical actuator. In addition, in the case of a two way needle control valve positioned in the low pressure passage, (see Bosch APCRS system) this is accomplished by energizing an electrical actuator to open the low pressure passage connected to the needle control chamber. In still other versions of the present invention, the direct control needle valve member is moved to an open position by energizing or de-energizing either a piezo actuator and/or an electromagnetic actuator in contact with the needle valve member. Thus, in all cases of the present invention, an injection event is initiated by moving a direct control needle valve member to a position that opens the nozzle outlet.  
         [0038]    Another step that occurs at least once each engine cycle includes decreasing fuel pressure in the fuel pressurization chamber  90 ,  290  at least in part by moving a flow control valve  74 ,  274 ,  374 ,  474  to a position that fluidly connects the actuation fluid passage  80 ,  280 ,  380 ,  480  to the actuation fluid drain  62 ,  262 ,  362 ,  462 . In the illustrated embodiments, this is the act that allows the fuel injector  14 ,  214  to reset itself for a subsequent injection sequence. When this step occurs, intensifier piston  82  and plunger  84  will stop moving downward and will begin to retract upward toward their retracted positions as shown, under the respective actions of return spring  83  and fuel pressure in fuel pressurization chamber  90 ,  290 . In all of the illustrated embodiments, this act is accomplished by ending or reducing electrical energy to actuator  72 ,  372  in order to allow flow control valve  74 ,  274 ,  374 ,  474  to return to its biased position that opens actuation fluid drain  62 ,  262 ,  362 ,  462 . In other types of fuel injection systems that fall within the scope of the present invention, fuel pressure is reduced in the fuel injector in different ways. For instance, a cam actuated fuel injection system might include a spill valve that is operated by an appropriate electrical actuator to spill fuel at an appropriate timing to relieve fuel pressure within the fuel injector. Reducing fuel pressure could also be accomplished in the illustrated embodiment by including either a fuel spill valve to spill pressurized fuel back to the low pressure supply, or possibly even an actuation spill valve that would relieve pressure on the top surface of the intensifier piston.  
         [0039]    Each of these steps is performed a number of times and at particular timings to produce a wide variety of injection event profiles. Whether the front of injection takes on the shape of a boot, ramp or a square is related in the illustrated embodiment with the relative timing of opening the actuation fluid passage  80  to high pressure flow from the rail, and the step of relieving pressure in needle control chamber  100 ,  200 . Although the illustrated embodiments show fuel injectors having separate actuation fluid inlets from fuel inlets, some aspects of the present invention are directly applicable to systems, such as Bosch APCRS, in which the fuel and actuation fluid inlets are one in the same. Because fuel pressure between injection events is usually low and because the fuel pressurization chamber  90 ,  290  is blocked from the actuation fluid inlet  64  while injecting, the illustrated system can achieve low leakage rates. This leakage occurs over that brief instant when the fuel pressurization chamber  90 ,  290  is directly connected to the low pressure passage  111 ,  211  as the valve member  139 ,  239  moves between seats. Because of the quick action of needle control valve  76  with direct control needle valve  79 , the system can achieve short dwell times between a pilot and/or post with a main injection event. In addition, these small injection events, including small splitting injection events at idle can be produced reliably and consistently with relatively low volumes on the order of about ten cubic millimeters. For instance, a combined total split injection in about equal shots with combined volume of about 25 cubic millimeters at idle are achievable.  
         [0040]    The system produces various front rate shapes including square, ramp, a boot or even an electronic rate shape that lies somewhere between a boot and a ramp, via the timing in actuating flow control valve  74 ,  374 ,  474  relative to needle control valve  76 ,  276 . The relative timing of the actuators associated with these two valves, along with the fact that the intensifier piston  82  may include a stepped top, allows for a variety of front end rate shapes. In order to produce a boot shaped front end, needle control valve  76 ,  276  is actuated before or at about the same time as flow control valve  74 ,  374 ,  474 . By doing so, the closing hydraulic surface  101 ,  201  of direct control needle valve  79 ,  279  is exposed to low pressure passage  111 ,  211  before the fuel pressure in fuel pressurization chamber  90 ,  290  is above valve opening pressures. Thus, in order to maximize a boot front end, the needle control valve  76 ,  276  should be actuated before the fuel pressure in fuel pressurization chamber  90 ,  290  is above valve opening pressures. When this occurs, the full affect of the top hat of intensifier piston  82  is exploited. In other words, the intensifier piston&#39;s  82  initial downward movement is relatively slow since high pressure is mostly acting only via actuation fluid passage  80  on the central small area portion of intensifier piston  82 . The flow of fluid to the annular shoulder portion of intensifier piston through passage  81  is relatively restricted so that the hydraulic force on the annular shoulder is lower than the hydraulic pressure force acting on the central top hat portion of intensifier piston  82 . The length of the toe of the boot shape is determined by the height of the central top hat portion of intensifier piston  82 . In other words, when the central top hat portion clears its counter bore in passage  80 , high pressure can act over the entire top surface of intensifier piston  82  causing its movement to accelerate and injection pressures to go up (the instep of the boot). Thus, when producing a boot shaped front end, direct control needle valve  79 ,  279  is set to behave like an ordinary spring biased check valve, and the rate shape is influenced by the top hat geometry of the intensifier piston along with the relative flow areas of actuation fluid passages  80  and  81 .  
         [0041]    When a square shaped front end is desired, the actuation of needle control valve  76 ,  276  is delayed relative to that of flow control valve  74 ,  374 ,  474 . In other words, the flow control valve opens, and high pressure acts on the top of intensifier piston  82  causing it to move slightly downward to compress fuel in fuel pressurization chamber  90 , but direct control needle valve  79 ,  279  remains in its downward closed position due to the force of high pressure fuel acting on closing hydraulic surface  101 ,  201 . The slight movement of intensifier piston  82  and plunger  84  downward reflects the compressibility of the fuel in fuel pressurization chamber  90  and nozzle supply passage  107 . Because direct control needle valve  79 ,  279  is held closed, oil pressure acting on the top of intensifier piston  82  is relatively high in the central portion exposed to actuation fluid passage  80 , as well as the annular should or portion, which is supplied by relatively restricted passage  81 . When needle control valve  76 ,  276  is finally actuated, high oil pressure is pushing on the entire top surface of intensifier piston  82 , and fuel in fuel pressurization chamber  90  is already at pressures that are well above the valve opening pressure of direct control needle valve  79 ,  279 . As a result, when direct control needle valve  79 ,  279  moves to its open position, the injection rate goes from zero to near its maximum rate in a very short amount of time. Thus, the effect of the piston&#39;s top hat can be virtually negated to produce a square front end rate shape by delaying the activation of needle control valve  76 ,  276  until after fuel pressure within the injector is well above valve opening pressure, and approaching its maximum injection pressure level at that rail pressure.  
         [0042]    A ramp shaped front end and a electronic rate shaping (ERS) front end illustrated, respectively, are accomplished by activating needle control valve  76 ,  276  at a location in between that which would produce a boot shaped front end and that which would produce a square shaped front end. In other words, direct control needle valve  76 ,  276  is activated at a timing that will take some advantage of the piston&#39;s top hat but not the entire potential effect of the same. Thus, with appropriate timing of the activation of needle control valve  76 ,  276  relative to that of flow control valve  74 ,  374 ,  474  a continuity of different front end rate shapes ranging from a boot to a square can be accomplished through electronic control independent of engine speed and load.  
         [0043]    The present invention also affords the possibility of performing end of injection rate shaping in a manner similar to the front end rate shaping. The present system allows the idea that main injection events should terminate as abruptly as possible to be revisited. It might be desirable in some instances, to produce a more gradually decreasing flow rate at the end of an injection event in contrast to a relatively abrupt ending. Again, like front end rate shaping, this is accomplished by the relative timing in the deactivation of needle control valve  76 ,  276  relative to that of flow control valve  74 ,  374 ,  474 . At one extreme of this procedure, needle control valve  76 ,  276  is deactivated before, or at about the same time as, flow control valve  74 ,  374 ,  474 . By doing so, direct control needle valve  79 ,  279  is abruptly shut, even though fuel pressurization chamber  90 ,  290  is at a relatively high pressure level. At another extreme, needle control valve  76 ,  276  is deactivated well after that of flow control valve  74 ,  374 ,  474  such that direct control needle valve  79 ,  279  is closed under the action of its biasing spring,  102 ,  202  without any substantial hydraulic assistance acting on closing hydraulic surface,  101 ,  201 . Thus, in this extreme, the closing procedure of direct control needle valves  79 ,  279  is much like that of a conventional spring biased check, in that the needle closes when fuel pressure drops below a valve closing pressure which is determined by the pre-load of biasing spring  102 ,  202 . Between these two extremes a variety of different end of injection rate shapes can be produced. For instance, the needle control valve  76 ,  276  can be deactivated after deactivation of flow control valve  74 ,  374 ,  474  such that fuel pressure levels have dropped within the fuel injector, but the deactivation occurs before fuel pressure has dropped below valve closing pressure. In such a case, there would be some gradual reduction in injection flow rate at the end of the injection event followed by an abrupt closure. Thus, those skilled in the art will recognize that some substantial amount of rate shaping flexibility is available by controlling the relative timing of the deactivation of flow control valve  74 ,  374 ,  474  relative to the deactivation of needle control valve  76 ,  276 . In all cases of the present invention, fuel pressure is reduced before the direct control needle valve member reaches its closed position, regardless of how pressure is reduced or the needle valve member is moved.  
         [0044]    Referring now to FIGS. 10 a - e , one example strategy for employing end of injection rate shaping according to the present invention is graphically illustrated. These graphs show only the end portion of an injection event, which spans a relatively brief instant in time. FIG. 10 a  shows the energization state of the electrical actuator  78 ,  278  associated with the direct control needle valve, with one representing an energized state and zero representing a deenergized state. FIG. 10 a  shows electrical actuator  78 ,  278  being deenergized at a time T 2 . FIG. 10 b  shows the energization state of the electrical actuator  72 ,  372  associated with the flow control valve, with one representing an energized state and zero representing a deenergized state. Note that electrical actuator,  72 ,  372  is deenergized at a time T 1  that is at some predetermined timing before timing T 2 . By deenergizing electrical actuators  72 ,  372  before deenergizing electrical actuator  78 ,  278 , fuel pressure within the nozzle chamber  105 ,  205  begins dropping at some delay time period after time T 1  as illustrated in FIG. 10 d . For simplicity sake, cylinder pressure  11  is illustrated in FIG. 10 d  as remaining relatively constant over the brief period of time represented by the graphs of FIGS. 10 a - e . Nevertheless, cylinder pressure in a particular application may either be increasing or decreasing over the time period represented in these Figures. FIG. 10 c  shows that the direct control needle valve member  113 ,  213  remains in its open position ( 1 ) through and after the time period T 2 . After some brief delay time period after T 2 , the direct control needle valve member  113 ,  213  begins moving from its open position ( 1 ) toward its closed position ( 0 ), which occurs at a time T 4 . In one embodiment of the present invention, the relative timings of T 1  with respect to T 2  is such that fuel pressure in nozzle chamber  105 ,  205  drops to cylinder pressure  11  (FIG. 10 d ) at a time T 3  that is after the direct control needle valve member has begun moving toward its closed position but before it has reached its seat at time T 4 . Preferably, this pressure in the fuel injector drops to equal cylinder pressure when the direct control needle valve member  113 ,  213  has completed about 80-90% of its travel toward its closed position. Those skilled in the art will appreciate that the actual injection of fuel as shown in FIG. 10 e  stops when the fuel pressure within the injector equals cylinder pressure, rather than when the direct control needle valve member  113 ,  213  arrives at its seat. However, the present invention does include seating the needle valve member before fuel pressure has dropped to cylinder pressure.  
         [0045]    By ending the injection event before the nozzle outlet is blocked by the direct control needle valve member  113 ,  213  arriving at its seat, the dribbling of a small amount of fuel toward the end of an injection event can be reduced. By eliminating these potentially small amounts of fuel dribble into the engine cylinder  11 , hydrocarbon and smoke emissions from the engine can be drastically reduced. This end of injection rate shaping strategy of the present invention can be employed in virtually any sized injection event, including pilot, main and post injection events. In addition, other types of fuel injection systems can also employ this strategy to produce similar results. For instance, in the case of a cam actuated fuel injection system with a fuel pressure spill valve, the spill valve would be opened at some timing T 1  before the needle control valve is activated to increase high pressure on the closing hydraulic surface of its direct control needle valve member. Thus, those skilled in the art will appreciate that the end of injection rate shaping strategy of the present invention extends to virtually any type of fuel injection system that includes a direct control needle valve member and a means of changing fuel pressure within the fuel injector.  
         [0046]    Although a primary benefit of the present invention includes lowering hydrocarbon and smoke emissions, the end of injection rate shaping strategy of the present invention also can produce other beneficial affects. For instance, another benefit includes a reduction in injection pressure overshoot in the tip/sleeve of the fuel injector. This phenomenon relates to the fact that if you close the needle while fuel injection pressure is high and the high pressure oil is still pushing the intensifier piston/plunger downward, fuel pressure can spike within the injector as the needle closes. These pressure spikes can be relatively high and influence how robust the structural aspects in the tip region of the injector must be in order to withstand these high pressures. By reducing fuel pressure to cylinder pressure as the needle closes, there will no longer be these high pressure overshoots, and the tip/sleeve structure can be made less robust or less strong and still be able to perform with the expected pressure levels. Another advantage of the end of injection rate shaping strategy relates to efficiency. If the needle valve member is forced shut while the flow control valve remains open, some amount of high pressure fluid is wasted as it continues to flow into the fuel injector when the needle valve member is closing, and for a brief period of time after it closes. By closing the flow control valve before closing the needle, fluid pressure on the intensifier piston can be relieved, and the piston/plunger can come to a stop before the needle closes and without wasting any excess high pressure oil. Those skilled in the art will appreciate that an amount of engine horsepower is wasted whenever the engine pressurizes oil that is not utilized to perform useful work. Thus, the end result is a small savings in energy by not wasting an amount of pressurized oil at the end of an injection event. Still another advantage relates to the ability to make small post injection quantities available due to lower gain factors as pressure is reduced. This aspect of the invention relates to the fact that if you are able to lower fuel pressure, you can expand the duration of a post injection event. It is known that it is far easier to control the quantity delivered if the duration of the injection event is longer. When injection pressure is very high throughout an injection event, it is often difficult to inject very small quantities with reliable accuracy. The strategy of the present invention allows for lower injection pressures at least over a portion of the injection event, which can result in some improvement in the ability to reliably inject ever smaller quantities of fuel at a given rail pressure.  
         [0047]    With regard to pilot injections, the present invention has the capability of reliably and consistently producing relatively small injection amounts. In addition, the fuel injection system has the ability to control whether those pilot injections occur at higher or lower pressures. This again is accomplished by the relative timing of the activation of flow control valve  74 ,  374 ,  474  relative to the activation of needle control valve  76 ,  276 . In other words, if the pilot injection is desired to occur at a relatively lower injection pressure, flow control valve  74 ,  374 ,  474  and needle control valve  76 ,  276  are actuated close in time to take advantage of the lower initial injection pressures afforded by the slower initial movement of intensifier piston  82  due to its top hat design. In such a case, the pilot injection amount is often so small that needle control valve  76 ,  276  is deactuated well before the top hat of intensifier piston  82  clears its counter bore. Thus, the pressure at which the pilot injection occurs is influenced by the relative timing of actuation of the flow control valve relative to the needle control valve, but the quantity of fuel injected is still tightly controlled by the actuation duration of needle control valve  76 ,  276 . In the event that the pilot injection is desired to occur at relatively higher injection pressures, the actuation of needle control valve  76 ,  276  is delayed relative to that of flow control valve  74 ,  374 ,  474  in a manner similar to that described with respect to producing a square front end rate shape. In other words, fuel pressure is allowed to rise to levels well above valve opening pressure before needle control valve  76 ,  276  is actuated.  
         [0048]    The fuel injection system of the present invention also has the ability to combine pilot injections with a variety of front end rate shapes. This again is accomplished by the relative timing in the actuation and deactuation of needle control valve  76  relative to the actuation, and possible deactuation, of flow control valve  74 ,  374 ,  474 . The closer in time that the pilot injection event occurs to the starting of the main injection event, the less flexibility the fuel injection system has in controlling both the injection pressure of the pilot and the front end rate shape of the main injection event independent of one another. On the other hand, if the dwell between the pilot injection event and the main injection event is sufficiently long in duration, the fuel injector may actually have sufficient time to deactivate flow control valve  74 ,  374 ,  474  between the pilot and main injection events in order to allow for more independent control of the pilot injection pressure relative to the front end rate shape of the main injection event. When the pilot injection quantities are relatively small, the injection event can occur so quickly that direct control needle valve  79 ,  279  only has time to partially open before it again is hydraulically pushed shut. The ability to consistently produce small injection quantities, even when the direct control needle valve  79 ,  279  does not go completely open, is accomplished by the relatively fast moving needle control valve  76 ,  276  that does move completely between its upper and lower seats, even during a relatively small quantity pilot injection event.  
         [0049]    The fuel injection system of the present invention also has the capability of producing relatively small post injection events with dwell times from the end of the main injection event under 500 microseconds and often on the order of about 350 microseconds. Like front end rate shaping, the fuel injector also has the ability to do some end of injection rate shaping and control whether the post injection is done at a relatively high or low injection pressure level. This again is controlled by the relative timing of the activation and deactivation of needle control valve  76 ,  276  relative to the deactuation timing of flow control valve  74 ,  374 ,  474 . For instance, if a close in time post injection is desired, the needle control valve  76  is deactuated to end the main injection event, and then a short time later is actuated and then deactuated again to produce the post injection event. The flow control valve  74 ,  374 ,  474  is deactuated at around the time that the needle control valve  76 ,  276  is deactuated to end the post injection event. If the post injection event is desired to occur at a relatively lower injection pressure, the flow control valve  74 ,  374 ,  474  is deactuated at some timing before needle control valve  76 ,  276  is actuated to begin the post injection event. In other words, the fuel pressure is allowed to drop in the injector before the post injection event is initiated. This permits a main injection event at a relatively high injection pressure followed by a post injection event at a lower injection pressure level. In addition, the relative timings of actuation and deactuation of flow control valve  74 ,  374 ,  474  relative to needle control valve  76 ,  276  can allow for some end of injection rate shaping in tandem with some independent control over the injection timing and pressure of a post injection event.  
         [0050]    All of these proceeding front end rate shaping, end of injection rate shaping strategies, post injections, pilot injections can all be combined in different combinations to produce a very wide variety of injection sequences that include one or more injection events with a variety of rate shapes, quantities, and dwells. In addition, these injection characteristics can be controlled with some substantial independence from one injection to another within a given injection sequence. This capability allows the fuel injection strategy at each engine speed and load to be tailored to produce some particular effect, such as reduced emissions.  
         [0051]    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 the invention can be obtained from a study of the drawings, the disclosure and the appended claims.