Abstract:
A hydraulically-actuated unit fuel injector of the intensifier type is provided with two independently operable active control valves. A selectively actuable fuel pressure control valve is disposed on the hydraulic actuation fluid side to control the fuel pressure actuation process and provide a window of injection opportunity wherein the fuel pressure is maintained at high pressure. A selectively actuable timing control valve is disposed on the high pressure fuel side to provide precise control of injection timing events and duration, such as start of injection, end of injection, timing of interruption and duration of interruption, which all may occur during a single injection event within the window of opportunity. The timing control valve may take various forms including piezo controlled direct needle actuation valves and common rail injector needle control valves. Both control valves are independently controlled to prevent reverse motion of the intensifier piston and plunger during dwell or interruption of injection while maintaining the full injection pressure. Dwell or interruption is controlled by using the timing control valve to port fuel under pressure to a fuel injector needle valve surface to generate a force on the fuel injector needle valve surface acting to close the fuel injector needle valve. Methods of defining a fuel injection event fuel injector having a fuel pressure intensifier, includes the steps of (a) preparing fuel pressure with a fuel injection pressure control valve, and (b) controlling the timing of a fuel injection event with a fuel injection timing control valve, the fuel pressure preparation and the timing of the fuel inject event being independently controllable. Preferably, full intensified fuel pressure is made available to the injector throughout a single injection event which may include a pilot injection, a main injection, a rate-shaped injection, and dwell periods wherein no injection occurs. Various methods of operating the fuel injector to provide various functions during a single injection event are also disclosed.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. Non-Provisional application Ser. No. 09/365,965, filed Aug. 2, 1999 which claims the benefit of U.S. Provisional Application Serial No. 60/104,662, filed Oct. 16, 1998. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention is related to the fuel supply for internal combustion engines and, more particularly, to a fuel injector having two active control valves to control needle valve motion. One control valve is used to control the injection pressure process. The second control valve is used to directly control the fuel injector needle valve. Depending on the coordination between two control valves, different injection characteristics are obtained as desired.  
         THE PRIOR ART  
         [0003]    A hydraulically-actuated, electronically-controlled, unit injector (HEUI), of the type described in U.S. Pat. No. 5,181,494 and in SAE Technical Paper Series 930270,  HEUI—A New Direction for Diesel Engine Fuel Systems , S. F Glassey, at al, Mar. 1-5, 1993, which are incorporated herein by reference, is depicted in prior art FIG. 1.  
           [0004]    The prior art HEUI  200  is depicted in prior art FIG. 1. HEUI  200  consists of four main components: (1) control valve  202 ; (2) intensifier  204 ; (3) nozzle  206 ; and (4) injector housing  208 .  
           [0005]    The purpose of the control valve  202  is to initiate and end the injection process. Control valve  202  is comprised of a poppet valve  210 , and electric control  212  having an armature and solenoid. High pressure actuating oil is supplied to the lower seat  214  of the valve  210  through oil passageway  216 . To begin injection, the solenoid of electric control  212  is energized, moving the poppet valve  210  upward off the lower seat  214  to the upper seat  218 . This action admits high pressure oil to the spring cavity  220  and the passage  222  to the intensifier  204 . Injection commences and continues until the solenoid of the control  212  is de-energized and the poppet  210  moves from the upper seat  218  to lower seat  214 . Oil and fuel pressure decrease as spent actuating oil is ejected from the injector  200  through the open upper seat oil discharge  224  to the valve cover area (not shown) of the internal combustion engine.  
           [0006]    The middle segment of the injector  200  is comprised of the hydraulic intensifier piston  236 , the plunger  228 , the plunger chamber  230 , and the plunger return spring  232 .  
           [0007]    Intensification of the fuel pressure to desired injection pressure levels is accomplished by the ratio of areas between the upper surface  234  of the intensifier piston  236  and the lower surface  238  of the plunger  228 . The intensification ratio can be tailored to achieve desired injection characteristics. Injection begins as high pressure actuating oil is supplied to the upper surface  234  of the intensifier piston  236 . Fuel is admitted to the plunger chamber  230  (formed in part by lower surface  238 ) through passageway  240  past check valve  242 .  
           [0008]    As the piston  236  and plunger  228  move downward, the pressure of the fuel in plunger chamber  230  below the lower surface  238  of the plunger  228  rises. High pressure fuel flows in passageway  244  past check valve  246  to act upward on needle valve  250 . The upward force opens needle valve  250  and fuel is discharged from orifice  252 . The piston  236  continues to move downward until the solenoid of the control  212  is de-energized, causing the poppet  210  to return to the lower seat  214 , thereby blocking actuating oil flow. Oil pressure above the intensifier piston is now vented to the ambient through drain passage  224 . The plunger return spring  232  returns the piston  236  and plunger  228  to their initial positions. As the plunger  228  returns, the plunger  228  draws replenishing fuel into the plunger chamber  230  across ball check valve  242 .  
           [0009]    The nozzle  206  is typical of other diesel fuel system nozzles. The valve-closed-orifice style is shown, although a mini-sac version of the tip is also available. Fuel is supplied to the nozzle orifice  252  through internal passages. As fuel pressure increases, the nozzle needle valve  250  is lifted from the lower seat  254  (compressing spring  256 ), thereby opening the needle valve  250  and causing fuel injection to occur. As fuel pressure decreases at the end of injection, the spring  256  returns the needle valve  250  to its closed position on the lower seat  254 .  
           [0010]    The HEUI Intensifier System  
           [0011]    For all unit injectors in production today, there is only one active control valve in each injector. Fuel injectors are typically of the common rail or intensifier types. The common rail type (Lucas and Bosch type systems) has a very high pressure fuel rail that supplies fuel to the injector at a pressure ready for injection, on the order of 20,000 psi. The intensifier injector (HEUI type) includes an intensifier plunger in the injector itself to bring low supply fuel pressure to a desired injection pressure level internally. This process is as described above.  
           [0012]    One of very desired characteristics of the HEUI intensifier system is its similarity in performance to the Bosch type pump and nozzle injection system (cam system), where injection pressure is gradually built up during an injection event. This gradual building up process produces a generally triangle shaped rate-of-injection single shot injection event where the initial portion of the injection pressure rate trace rises gradually, as distinct from a sharp rising. See FIG. 3, case  4 . This type of injection rate trace provides a benefit to reduce NOx emissions at high speed engine operation. This is a very special feature of the intensifier system. Common rail systems can not produce this feature.  
           [0013]    In the HEUI injector concept shown in U.S. Pat. No. 5,460,329, pilot injection is produced through double action of a single spool digital control valve. The result is similar to the solid line injection event depicted in FIG. 3, case  1 . The entire injection event, having a pilot injection event preceding a main injection event, is considered as two independent, pulse-width-controlled, single injection events occurring in very close sequence. The pilot portion of injection is a single shot of injection but with very short pulse width. With this philosophy, the intensifier chamber pressure is vented to terminate the pilot injection at the end of the pilot injection event and recharged again to start the main injection.  
           [0014]    The HEUI B injector, described in U.S. Pat. No. 5,682,858, improves its performance by using direct control of the needle valve. However, the intensifier is also passively controlled by the same control valve. The actuation process is not totally independent of needle timing control. This type of injector cannot have fully flexible injection timing and rate shaping control across the whole engine speed and load range. It may have difficulty producing certain dwell and certain pilot injection size when the actuation pressure is mismatched. Another desirable characteristic of the intensifier system is its product safety. High injection pressure is developed within the injector only during a short period during the engine cycle, only during the time window where injection events are going to occur, as distinct from a high pressure common rail system. The injector stays in a low pressure environment for the rest of the engine cycle. Additionally, there is no external plumbing required to transport fuel from a high pressure pump to the injector as in the common rail system. Compared to the common rail system, the intensifier system demonstrates a much superior advantage that appeals to a large number of engine manufacturers.  
           [0015]    Common Rail Systems (Lucas &amp; Bosch Type Systems)  
           [0016]    The common rail fuel system is very different from the previously described injectors that incorporate an intensifier system. In the common rail system, the injector is not responsible for the injection pressure development process. Rather, the high pressure fuel, on the order of 20,000 psi is delivered to the injector from the common rail ready for injection into the combustion chamber of an engine. The injector has direct timing control of the injector needle valve with a relatively simple timing control process to produce the desired pilot injection and injection event dwell (duration). Injection timing and duration are purely a timing issue. In any unit injection system, the speed of control valve response is considered as the most crucial element and the limiting factor for achieving small pilot and small dwell size especially under high engine speed and high injection pressure operation conditions. Using one control valve to handle both pressure and timing, as in the intensifier system, can be very challenging and limiting. Thus, decoupling the pressure development process from the timing control process becomes a necessary step to further improve injection system performance in the future. The common rail system by its nature is decoupled, being responsible only for timing. For this reason, the common rail system has much superior control of the pilot size and dwell duration due to its direct needle control and independent fuel pressure control outside of the injector as compared to the intensifier system.  
           [0017]    Both the Lucas and the Bosch type unit injectors have only one active control valve on each injector. For both of them, the single control valve is used to directly control the timing of the needle valve opening and closing. The sole function of the control valve in a common rail system is control of the timing of injection events (e.g., starting, ending and duration of the injection).  
           [0018]    Timing control of the fuel injector is highly dependent on the response time of the control valve. For this reason, the Lucas type system apparently has better response than the Bosch type system due to its faster response of the control valve.  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention injector has the advantages of both the intensifier system and the common rail system, while substantially avoiding the problems of the two systems as indicated below.  
           [0020]    Decoupling The Injection Pressure Preparation From Timing Control Without Going To A High Pressure Common Rail.  
           [0021]    This is achieved by having two active control valves in one unit injector of the intensifier type. One control valve (the pressure control valve) is on the actuation liquid side and other control valve (the timing control valve) is on the high pressure fuel side. In order to maintain the advantages of the intensifier system, the pressure control valve is used to control the pressure actuation process. The pressure control valve is responsible for opening up the window of injection opportunity. The timing control valve is responsible for controlling when and how long the injection event takes place within the window of opportunity. This two control valve system is the marriage between the intensifier system and the common rail system. The present invention keeps the advantages of both systems (intensifier and common rail) and provides the opportunity to eliminate the undesired characteristics of each of the systems alone. Since the injector of the present invention has two active control valves, coordination of the control schedule between two valves can produce markedly different and desirable injection characteristics. More particularly, the pressure control valve is used to define the window of operation during which the actuation pressure will be used. The timing control valve is responsible within the window for the precise control of injection timing events and duration, such as start of injection, end of injection, timing of interruption and duration of interruption.  
           [0022]    The Pilot Injection Process Of The Present Invention Is Accomplished By Controlled Interruption Of A Normal Injection Event.  
           [0023]    With the present invention, an injection event, including pilot injection and/or rate shaping, is considered as a single shot injection event, but with a certain duration of interruption. The duration of interruption (dwell) is effected by the timing control valve and is the consequence of dwell. When the interruption (dwell) is short, it results in a rate shaping injection. See FIG. 3, case  5  and FIG. 4, case  5 . When the interruption is long, it causes split or pilot injection. See FIG. 3, case  1  and FIG. 4, case  3 . Without any interruption, the injection is a normal single shot. See FIG. 3, case  4  and FIG. 4, case  1 . But with interruption, depending on the duration of interruption (dwell), the injection flow curve can be formed to provide rate shaping, split injection, pilot injection and more injection segments as needed. This controlled interruption to a normal injection event can happen any time during the injection event as long as actuation pressure or injection pressure exists.  
           [0024]    Independent Control Of Pilot Injection And Main Injection Within A Single Shot Injection Event.  
           [0025]    All present unit injection systems need to achieve pilot injection and main injection by generating two independent single shot injection events. For example, the injection system described in U.S. Pat. No. 5,460,329 requires the decay of actuation pressure to define between the pilot and main injection events. In the prior art, this may be accomplished by reversal of the motion of the intensifier. Such reversal has the disadvantage of diminishing the injection pressure in the fuel injector. Once the injection pressure is developed in the fuel injector during an injection event, the injection pressure should not be destroyed for the purpose of pilot injection pressure, if possible. The total time allowed for injection to occur is too short to waste in diminishing and rebuilding injection pressure. Therefore, the concept of the present invention is to emphasize no reverse motion of the intensifier piston and plunger during pilot injection, thereby maintaining injection pressure. Dwell in the pilot injection is caused by closing the needle valve rather than by reducing or eliminating the injection pressure. The timing control valve of the present invention is used to spill part of the high pressure fuel to the back of the needle valve to force needle valve closing. This closing creates the separate pilot and main injection events while maintaining injection pressure in the injector.  
           [0026]    The Present Invention Improves the Digital Control Valve HEUI Injection System (U.S. Pat. No.  5 , 460 , 329 ), Making it More Efficient in Main Injection Pressure and Shorter in Duration.  
           [0027]    This improvement is achieved in the present invention by having main injection occur under maximum injection pressure situation. Maximum injection pressure is obtained by having the full actuation pressure level acting on the intensifier piston at all times during the injection event. The intensifier chamber pressure is maintained at maximum actuation pressure, since the pressure control valve stays open all the time throughout the injection event, i.e., the plunger chamber fuel pressure then is maintained at maximum intensified level. There is no double action of the pressure control valve as in the past.  
           [0028]    Improved Response in Shaping the Injection Event as Desired.  
           [0029]    In the present invention, the pressure control valve is much larger (in terms of flow area) than the timing control valve and is therefore much less responsive than the timing control valve. This is because the flow rate of actuation liquid is about seven times more than the fuel injection flow rate. Therefore, with the concept of the present invention, the large pressure control valve is only operated once per injection event while the small timing control valve can be operated multiple times if needed during an injection event in order to effect the desired rate-of-injection shape. This is evident in reviewing the valve positions depicted in cases  1 - 5  of FIG. 4. The relatively small timing control valve has much better response than the relatively larger pressure control valve.  
           [0030]    More Varied Injection Characteristics are Achieved with the Two Active Control Valves of the Present Invention in One Unit Injector of the Intensifier Type Than can be Achieved With a Single Control Valve.  
           [0031]    No present fuel injection system is able to generate all the noted flexible injection characteristics without introducing significant variability from injection event to injection event and deterioration of performance. Most production injectors can only do some of the features listed in FIG. 3. All of the FIG. 3 features are attainable by the present invention. It is highly desirable that a unit injector be able to do all of these features in order to meet high emission standards, reduced noise, and improved drivability.  
           [0032]    The present invention includes a needle valve controller for use in a fuel injector to control the opening and closing of a fuel injector needle valve, including a selectively actuatable timing control valve being in flow communication with a source of fuel under pressure and being in flow communication with a fuel injector needle valve surface, the valve being shiftable between an open and a closed disposition. A controller is operably coupled to the timing control valve for controlling the shifting of the timing control valve between the valve open and closed dispositions, opening of the timing control valve acting to port fuel under pressure to the fuel injector needle valve surface, the fuel generating a force on the fuel injector needle valve surface acting to close the fuel injector needle valve.  
           [0033]    The present invention is further a method of defining a fuel injection event in a fuel injector having a fuel pressure intensifier, including the steps of (a) preparing fuel pressure with a fuel injection pressure control valve, and (b) controlling the timing of a fuel injection event with a fuel injection timing control valve, the fuel pressure preparation and the timing of the fuel inject event being independently controllable. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is a sectional side view of the prior art HEUI injector;  
         [0035]    [0035]FIG. 2 is a sectional side view of a HEUI-type injector with the needle valve control of the present invention;  
         [0036]    [0036]FIG. 2 a  is an enlarged depiction of the area  2   a  of FIG. 2 in the closed disposition;  
         [0037]    [0037]FIG. 2 b  is an enlarged depiction of FIG. 2 a  in the open disposition;  
         [0038]    [0038]FIG. 3 is a series of graphic depictions of injection features attainable by the present invention;  
         [0039]    [0039]FIG. 4 is a series of graphic depictions of the effects of different coordination between the injection control valve and the timing control valve and the resulting rate of injection;  
         [0040]    [0040]FIG. 5 is a graphic depiction of pilot and dwell control parameters;  
         [0041]    [0041]FIG. 6 is a graphic depiction of the performance characteristic;  
         [0042]    [0042]FIG. 7 is a sectional side view of a further embodiment of the invention incorporating piezo controlled direct needle actuation with the needle valve in the closed position;  
         [0043]    [0043]FIG. 8 is a sectional side view of a further embodiment of the invention incorporating piezo controlled direct needle actuation shown in FIG. 7 with the needle valve in the open position;  
         [0044]    [0044]FIG. 9 is yet a further embodiment of the invention utilizing a known direct needle control system;  
         [0045]    [0045]FIG. 10 a  is yet a further embodiment of the invention utilizing yet another known direct needle control system with the needle valve in the closed position; and  
         [0046]    [0046]FIG. 10 b  is yet a further embodiment of the invention utilizing the direct needle control system shown in FIG. 10 a  with the needle valve in the open position. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0047]    [0047]FIG. 2 shows the injector  10  of the present invention. The HEUI injector  200  is used as the baseline injector, as depicted in prior art FIG. 1, and has been modified to incorporate the present invention. Other intensifier type injectors may be utilized to incorporate the present invention. The injector  10  of the present invention has two active control valves. The first control valve (the pressure control valve  12 ) is on the actuation liquid side and the second control valve (the timing control valve  14 ) is on the high pressure fuel side.  
         [0048]    The injector body  16  contains the injection pressure control valve  12 , a pressure intensifier  18 , the timing control valve  14 , and a spring loaded conventional needle valve  20  disposed in the injector tip housing  21  of the injector  10 . The timing control valve  14  and associated fluid passageways (as will be discussed below) of the present invention are included for direct hydraulic control of the needle valve  20 . As will be described in more detail below, the basic function of the timing control valve  14  is to pass high pressure fuel to the needle valve control surface  22  of the needle valve  20 . Such fuel acts on the needle valve control surface  22  to accurately, directly, and hydraulically control the opening and closing motions of the needle valve  20  as desired to effect desired injection characteristics.  
         [0049]    There are two flow passageways from the bottom of the plunger chamber  24  to needle valve  20 . High pressure fuel passageway  26  is conventionally connected to the nozzle chamber  28  where the needle front area  30 , formed by an increased diameter of the needle valve  20 , is exposed to the fuel pressure. Fuel pressure generated in the chamber  28  acts upwardly on the front area  30  to open the needle valve  20  by opposing the closing bias of the needle valve spring  32 .  
         [0050]    The first bleed off passageway  34  is fluidly coupled to the spool  36  of the timing control valve  14 . A second bleed off passageway  38  is fluidly coupled to the spool  36  and is further fluidly coupled to a chamber  40  defined in part by the needle valve control surface  22  of the needle valve  20 . In a preferred embodiment, surface  22  is a top margin at the back of the needle valve  20 .  
         [0051]    [0051]FIGS. 2 a  and  2   b  show the enlarged timing control valve  14  and the relation to the high pressure fuel passage  26 . The timing control valve  14  includes a coil spring  42 , an end cap  44 , a valve body  36 , and the valve housing  46 . Leakage between the timing valve body  36  and the housing  46  is preferably controlled to a minimum. There is a spool groove  52  on the valve body  36  which defines in part the spool chamber  53 . The spool chamber  53  provides flow communication between the intensifier chamber  54  to the chamber  40  at the needle back when the control valve  14  is in the open position. A sealing portion  41  of the valve body  36  depends from the groove  52 .  
         [0052]    The timing control valve  14  is a simple open(on)/closed(off) two position valve, FIG. 2 b  being a depiction of the open(on) configuration of the timing control valve  14  and FIG. 2 a  being a depiction of the closed(off) configuration of the timing control valve  14 .  
         [0053]    When the timing control valve  14  is at its off position (FIG. 2 a ), chamfered valve face  56  is seated on the valve seat  58  and fuel flow through the spool chamber  53  from the first bleed off passageway  34  to the second bleed off passageway  38  is blocked. The fuel flow to the chamber  40  via second bleed off passageway  38  at the back of the needle valve  20  is accordingly also blocked. The chamber  40  is vented to an external low pressure fuel reservoir  63  (depicted schematically in the figures) through the needle back drain orifice  60  and through the drain passageway  62 . Drain passageway  62  is preferably in a different plane as the section and is therefor shown in phantom in FIGS. 2 a  and  2   b . It should be emphasized that the drain passageway  62  is not fluidly coupled to the high pressure fuel passageway  26 .  
         [0054]    Drain passageway  62  is drained to the fuel reservoir  63  located external to the injector  10 . The fuel reservoir  63  is typically at the pressure (about 50 psig) generated by the engine fuel pump. Drain orifice  60  is relatively restrictive, (preferably between 0.1 and 1.0 mm and more preferably less than 0.5 mm in diameter), having a very small cross-sectional area, and is preferably allowed to flow in both directions (to and from the fuel reservoir  63 ).  
         [0055]    A one way ball check valve  66  is placed in a refill passageway  67  extending between the chamber  40  and the drain passage  62  to the fuel reservoir  63 . The check valve  66  is controlled by fuel pressure. When pressure in chamber  40  exceeds pressure in passageway  62 , check valve  66  is seated against valve seat  67 . Accordingly, fuel flow through check ball  66  is blocked when the chamber  40  is pressurized by the high pressure fuel admitted by the timing control valve  14  and is also blocked during the opening motion of the needle valve  20 . The check valve  66  permits sufficient refilling of fuel (at 50 psi) from the fuel reservoir  63  to the chamber  40  to accommodate the volume change in chamber  40  which occurs during the closing motion of the needle valve  20 .  
         [0056]    The injector  10  acts just like the prior art HEUI injector  200  when the timing control valve  14  is in the closed configuration as described in FIG. 2 a . Such action is noted above in the background section.  
         [0057]    Opening of the timing control valve  14  is effected by a solenoid  64 . When the current is supplied to the solenoid  64 , the timing control valve  14  moves upward against the spring load of the timing valve spring  42  to the full open position of the timing control valve  14 . See FIG. 2 b . In this open position, the high pressure fuel passage  26  is fluidly connected to the second bleed off passageway  38  through the spool chamber  53  defined by the spool groove  52 . High pressure fuel is bled off from plunger bottom chamber  54  to the chamber  40  at the back of the needle valve  20 . In this open position, bleed passageways  34 ,  38  are fully open and the chamber  40  is pressurized. The pressure acts on the surface  22  in conjunction with spring  32  to prevent upward, opening motion of the needle valve  20  or to close the needle valve  20  if the needle valve  20  is open at the time that the timing control valve  14  is opened. Therefore, the needle valve  20  is in the closed position when the timing control valve  14  is in the open position. If the timing control valve  14  stays in the open position for some period of time during an injection event, a measurable duration of the needle valve  20  being closed after injection event initiation is obtained. The needle valve  20  closing duration may be equal to the dwell of the pilot injection event.  
         [0058]    The drain orifice  60  is open all the time, but the drain orifice  60  has a very small flow area in order to throttle down fuel flow through the drain orifice  60 . Therefore, when high pressure fuel flows into the chamber  40 , sufficient pressure is trapped in the chamber  40  to cause needle valve  20  closing by the fuel pressure generating a force acting on surface  22  of the needle valve  20  (in conjunction with spring  32 ). A constant through-flow occurs at the orifice  60  when timing control valve  14  is in the open position (FIG. 2 b ). (This is very similar to the common rail type system, in which constant leakage of high pressure fuel occurs during the whole injection process.) During a regular single shot injection, the timing control valve  14  is never used and the drain orifice  60  slows down lifting of the needle valve  20  slightly due to the restriction of the drain orifice  60  in permitting fuel to escape from the chamber  40  to the fuel reservoir.  
         [0059]    Bleeding off high pressure fuel to the chamber  40  by opening timing control valve  14  causes the needle valve  20  to close if the needle valve  20  is in an open condition. If the timing control valve  14  is open at the very beginning of the injection event (the condition where the intensifier plunger  18  is just about to move downward to increase the fuel pressure), the needle valve  20  will stay in a closed position regardless of what happens to the injection pressure due to the fuel pressure generating the force acting on the surface  22  of the needle valve  20 . This can cause a delayed start of injection into the combustion chamber, as desired.  
         [0060]    With this strategy, the user can selectively choose the starting condition of each injection event since needle valve  20  opening pressure is controlled by the timing control valve  14 . If the timing control valve  14  is opened after injection has already started, an interrupted injection event occurs due to a sudden closing of the needle valve  20 . The sudden closing of the needle valve  20  is effected by the opening of the timing control valve  14  to port high pressure fuel to chamber  40 . This is pilot injection and results in dwell (a definitive elapsed time occurring) between the pilot injection and the main injection during which no fuel injection is occurring. If the timing control valve  14  is opened at end of the injection event, the timing control valve  14  will cause the needle valve  20  to close even before the pressure control valve  12  is turned off. This produces a sharp end of the injection event, as desired.  
         [0061]    The opening/closing of the needle valve  20  is directly controlled by the timing control valve  14 . Therefore, this concept is called direct-controlled needle valve and is similar in this regard to a common rail system, having needle valve  20  closing to shape and control the rate of injection, to end pilot injection and form dwell although injection pressure.  
         [0062]    Referring to FIGS. 5 and 6, during pilot injection, if the timing control valve  14  stays in the open position for a relatively long duration, it produces longer dwell as described above. If the timing control valve  14  stays in the open position for a relatively short duration, a closed pilot injection (no dwell) or rate shaping of the injection event occurs, affecting the shape of the ascending portion of the rate of injection of the injection event.  
         [0063]    During the period when the timing control valve  14  is open, the needle valve  20  is closed and the intensifier plunger  18  may continue to move downward due to leakage at the drain orifice  60  from chamber  40  at needle valve  20 . The drain orifice  60  is open to the fuel reservoir (approximately 50 psi). Since the drain orifice  60  is very small, the leakage flow from chamber  40  is relatively small. Injection pressure is maintained and the downward compressive motion of the intensifier  18  continues even during temporary shut off of nozzle fuel flow to the combustion chamber from the needle valve  20 . This is as a result of the timing control valve  14  being open to exert pressure on surface  22  of needle valve  20 . The injection process efficiency is improved by such method of producing dwell by maintaining the injection fuel pressure at a high level throughout the full injection event, instead of decreasing the pressure as a result of reversing the motion of the intensifier  18  in order to shape the rate-of-injection, as in some prior art injectors.  
         [0064]    Sizing of the needle drain orifice  60  is very important. The needle drain orifice  60  is open to low fuel pressure (approximately 50 psi) through passageway  62  to the fuel reservoir  63  all the time. With the right size orifice  60 , sufficient fuel pressure can be trapped in the chamber  40  to act on surface  22  of the needle valve  20  when high pressure fuel flows from plunger chamber  54  to the chamber  40  as a result of opening the timing control valve  40 . The drain orifice  60  allows back pressure in chamber  40  to release slowly when bleed flow into the chamber  40  is stopped. Slow bleed flow at the drain orifice  60  helps to adjust and control the lifting velocity of the needle valve  20  to meet preselected requirements. The size of the drain orifice  60  is very critical to keep the needle valve  20  closed when the timing valve  14  is open, to prevent an excess amount of high pressure fuel from leaking through the drain orifice  60 , and to have a slow drain flow at the orifice  60  when the needle valve  20  lifts up again (after fuel pressure bleed off from chamber  40  through orifice  60 ). The size of the drain orifice  60  is optimized to the needs of the particular injector  10  and the diameter is preferably about 0.1 mm-1.0 mm. In a preferred embodiment, the drain orifice  60  is about 0.5 mm or less. The volume of fuel acting on the surface  22  of the needle valve  14  is partially trapped in the chamber  40  having a volume defined by the needle back  22 , the needle housing  24 , and check ball plate  68 . The needle back surface area  22  is sized properly so that force generated by fuel pressure on the back of the needle valve  20  plus needle spring force exerted by spring  32  is greater than the countering force generated by the high pressure fuel acting on needle front  30 . Such force on needle front  30  acts counter to the force of the fuel pressure acting on surface  22  in conjunction with the bias of spring  32 . Proper sizing of surface  22  with regard to the surface of needle front  30  and the bias exerted by spring  32  ensures proper closing of the needle valve  20  when the timing control valve  14  is open. This sizing is important since the high pressure fuel is simultaneously to both open and close needle valve  20 .  
         [0065]    Since the total flow required to the chamber  40  at the needle back is very small, the necessary size of the timing control valve  14  is much smaller than the pressure control valve  12 . Further, the travel distance of the timing valve  14  (valve total opening) is also much smaller than the travel (valve total opening) distance of the pressure control valve  12 . Therefore, the response of the timing control valve  14  is much faster than the response of the pressure control valve  12 .  
         [0066]    During the dwell period of a pilot injection event, there is a constant bleeding of high pressure fuel through the needle drain orifice  60 . Thus, the intensifier plunger  18  may drift down slowly replenishing fuel in chamber  40  that has been bled from the chamber  40  whenever the timing control valve  14  is in the open configuration. If the timing control valve  14  was open for a duration that is very long, the intensifier plunger  18  could bottom out. This risk is avoided by sizing the stroke of plunger  18  properly, and also by coordinating both the timing control valve  14  on and off schedules properly to avoid an overly long dwell.  
       Operation  
       [0067]    A flexible injection system should have the capability to do single shot injection mode, detached pilot injection mode, attached pilot injection mode, and rate shaped injection mode. The following section describes the operation procedure of the present invention for each different operation modes.  
         [0068]    Single Shot Injection With Triangle or Ramp Shaped Injection (FIG. 4, Case  1 : FIG. 3, Case  4 )  
         [0069]    During single shot ramp injection, the timing control valve  14  stays in the closed position and is never used throughout the injection process. Therefore, high pressure fuel flows only to the front or lower side of the needle valve  20  while the chamber  40  is never pressurized and is vented through drain orifice  60  and passageway  62  to the low fuel pressure reservoir  63 . Both timing and injection duration are controlled by the actuating pressure control valve  12 . When the pressure control valve  12  is opened, injection pressure builds up gradually in the high pressure fuel passageway  26 . The high pressure fuel acts on needle front  30 , overcoming the bias of spring  32  and lifting (opening) the needle valve  20 . When needle valve  20  opens, injection starts. The resulting single shot injection is substantially the same as a normal prior art HEUI injector  200  injection event as described above in relation to prior art FIG. 1.  
         [0070]    Single Shot Injection With Square Fuel Pressure Shape (FIG. 4, Case  2 ; FIG. 3, Case  3 )  
         [0071]    Operation of both the control valves  12 ,  14  is required to achieve a square rate of injection characteristic. The timing control valve  14  is opened ahead of or at the same time that the actuating fluid pressure control valve  12  is opened. A spill and bypass concept is used in this instance to bleed off the initial portion of the fuel pressure buildup resulting from actuation of the actuating pressure control valve  12  to thereby delay the injection starting. Opening the timing control valve  14  results in a spill and bypass through chamber  40 , drain orifice  60  and passageway  62  to the low pressure fuel reservoir  63 . The initial portion of the injection pressure is relatively low, so injection occurring under this initial portion would cause ramp shaped injection (like single shot ramp injection) if the timing control valve  14  were closed. However, the timing control valve  14  is opened here to bypass these undesired initial pressure conditions and to allow the needle valve  20  to wait to open until the more desirable higher pressure level is attained.  
         [0072]    The initial portion of the pressurized fuel is bled off to chamber  40 . Because the pressure of the fuel in chamber  40  acts on the surface  22 , the force exerted by the fuel pressure in conjunction with the bias exerted by the valve spring  32  acts to keep the needle valve  20  closed. Therefore, the needle valve  20  will stay closed until the timing control valve  14  is returned to the closed position by spring  42  after deactivation of solenoid  64 . After a desired period, deactivation of solenoid  64  occurs and valve  14  returns to the closed position. At this time, the injection fuel pressure will have already developed to a very high level. Since the pressure control valve  12  is at fully open position and the intensifier  18  downward velocity has developed, injection occurring under this condition is eruptive and has a very fast rate of injection at the beginning of the injection event. Meanwhile a constant injection pressure is maintained at the plunger chamber  24  by the intensifier  18 . This pressure equals the rail pressure of the actuating fluid times the intensification ratio of the intensifier  18 . The rail pressure of the actuating fluid may be approximately 3000 psi. The intensification ratio may be seven, resulting in fuel pressure of approximately 21,000 psi.  
         [0073]    At the end of injection, the timing control valve  14  is cycled to the open position again by activating solenoid  64  to overcome the closing bias of timing valve spring  42  before the actuating fluid pressure control valve  12  is closed. After opening of timing control valve  14 , the fuel pressure of the fuel in chamber  40  again acts on the surface  22 . The force exerted by the fuel pressure on the surface  22  in conjunction with the bias exerted by the valve spring  32  acts to forcibly, abruptly close the needle valve  20 . Injection flow is nearly instantaneously cut off to zero by this forced closing of the needle valve  20 , rather than the more gradual needle valve  20  closing caused by actuation fluid injection pressure decay, as in the prior art. Therefore, the end of injection is also very sharp, resulting in the desired, generally square fuel pressure shape.  
         [0074]    Pilot Injection With Reasonable Dwell Duration (FIG. 4, Case  3 , FIG. 3, Case  1  (Solid Line))  
         [0075]    With the present invention, pilot injection is considered as a single shot injection fully interrupted for a certain duration prior to the main injection, which is also a single shot injection separate from the pilot injection. This interruption is caused by a sudden closing of the needle valve  20  by the timing control valve  14  some time after commencement of the injection event as initiated by the pressure control valve  12 . If needle valve  20  closing duration is relatively long, the dwell between pilot injection and main injection will be long. Since both control valves  12 ,  14  are independently controlled, the on/off schedules of both valves  12 ,  14  are totally flexible and do not have any interaction and interference with each other. Just as in the case of single shot injection event, in this case the pressure control valve  12  is actuated only once to open the pressure window to the intensifier system  18 . The timing control valve  14  is initially closed when the pressure control valve  12  is opened. After the pressure control valve  12  is open, the needle valve  20  opens by lifting upward and injection will start as indicated above in relation to the single shot injection case. The timing valve  14  is then moved to the open position soon after the pressure valve  12  is opened by activation of the solenoid  64 . The needle valve  20  then closes again responsive to the timing valve  14  being open, resulting in cessation of the injection. Prior to the closing of the needle valve  20 , a small amount of fuel has escaped to the combustion chamber of the cylinder from nozzle hole  66 . This produces pilot injection, a very small quantity of injected fuel over a short duration separate in time from the main injection event. The independent pressure control valve  12  remains open and fuel pressure is maintained in a high state.  
         [0076]    The size of the pilot injection is clearly the function of the timing lag between the opening of two valves  12 ,  14 . The longer the lag is, the larger the pilot injection volume will be. Since both valves  12 ,  14  are independently controlled, the pilot injection volume is controlled in a very simple and flexible way. The timing valve  14  may stay open for a while corresponding to the size of the pilot injection dwell duration. At the end of the dwell, the timing valve  14  is turned off again. This results in the opening of the needle valve  20  and the injection event is resumed, providing the main injection event spaced in time from the pilot injection event. The intensifier  18  continues to travel downward in order to provide a continual quantity of high pressure fuel to finish the main injection. The end of injection is accomplished by turning off the pressure control valve  12 .  
         [0077]    The end of injection can also be achieved by opening the timing control valve  14  to have a forced closing of the needle valve  20  before the pressure control valve  12  turns off. This produces a sharp end of injection as described above in the case of single shot injection with square fuel pressure shape. Thus, the needle valve  20  closes before the decay of injection pressure resulting from closing the pressure control valve  12 .  
         [0078]    Pilot Injection With Very Long Dwell Duration (FIG. 4, Case  4 )  
         [0079]    When the dwell duration is extremely long, then pilot injection can be considered as two individual single shots effected by cycling the pressure control valve  12  through two open/close cycles. The pressure control valve  12  is turned on first to start the injection. Since pilot portion has very small total delivery, the timing valve  14  may be used to interrupt the injection commenced by the pressure control valve  12  and to prevent the needle valve  20  from being open too long. After the pilot injection is stopped, the pressure control valve  12  may be turned off to finish the first single shot event. Pressure on top of the intensifier  18  is vented to ambient and the intensifier  18  returns to the top closed position waiting for next injection event. The venting passage (not shown) is conventionally located at top of the poppet valve immediately above the poppet valve spring. To commence main injection, the pressure control valve  12  is opened again and a second injection event starts. Depending on the engine needs, either ramp, single shot, or squared single shot strategy can be used to produce a single shot as the main injection event by appropriate interaction of the timing valve  14  with the pressure valve  12 .  
         [0080]    Rate-Shaped Injection (FIG. 4, Case  5 , FIG. 3, Case  5 )  
         [0081]    The operation strategy for rate-shaped injection is almost the same as for pilot operation (reasonable dwell case), FIG. 4, case  3 . In rate shaped injection events, the timing control valve  14  “on” time is very short, for example, the minimum controllable pulse width of the timing control valve  14 . With a very short interruption from the timing control valve  14 , the needle valve  20  may not fully return to the closed position during the on time of the timing control valve  14 . Injection pressure is only interrupted for a very short period in such case. Therefore, the rate of injection trace will not be split into segments as in FIG. 4, case  3  but will not decay to a zero rate of injection condition. This results in a classic dipped rate-shaped trace.  
         [0082]    Depending on the timing control valve  14  schedule, a different rate-shaping trace can be obtained. See FIG. 3, case  5 . The rate-shaping injection is considered to be a single shot injection with a very small interruption at an early stage of the injection.  
         [0083]    Some of the novel features of the present invention are categorized into two areas: (1) design configuration and (2) injection operation.  
         [0084]    (1) Design Configuration  
         [0085]    Two active, independently controlled, control valves  12 ,  14  are used in one unit injector  10 . The pressure control valve  12  is on the actuation fluid side to open the pressure window for injection events. Without turning on the pressure control valve  12 , there will be no injection pressure, hence no injection, regardless of what happens to the timing control valve  14 . The timing control valve  14  is placed on the high pressure fuel side (as distinct from the actuation fluid side) to achieve direct control of the needle valve  20  substantially independent of the pressure control valve  12 . Thus, an injection event is stopped or interrupted when the timing control valve  14  is turned on, the timing control valve  14  being on acting to close the needle valve  20 . Additionally, because the timing control valve  14  is on the fuel side, continued operation of the intensifier plunger  18  occurs under control of the pressure control valve  12  to ensure a continuous source of high pressure fuel.  
         [0086]    (2) Injection Operation  
         [0087]    A unit injector  10  with two active control valves  12 ,  14  does not exist in production today. Therefore, the strategy based on a coordinated schedule of operation of the two control valves  12 ,  14  is new to the industry.  
         [0088]    It is very difficult for a unit injector  10  with a single control valve  12  to produce a variety of injection characteristics (such as those shown in FIG. 3) while still maintaining sufficient controllability, flexibility and simplicity. The control strategy of the present invention presented in the operation procedure section illustrates how two control valves  12 ,  14  can be coordinated to each other&#39;s on/off timing and duration to obtain the varieties of injection characteristics depicted in FIG. 3.  
         [0089]    As fuel injection systems are getting more and more sophisticated in terms of operation and control, it becomes more important to design an injector that not only provides excellent performance but also has user friendliness, simplicity and robustness in control strategy. FIGS. 5 and 6 illustrate the relationship between control parameters and performance parameters of the present invention. The injection system of the present invention has two active control valves  12 ,  14 . The valves  12 ,  14  do not interfere with each other and each valve  12 ,  14  has very clear responsibility.  
         [0090]    [0090]FIG. 5 shows the definition of timing lag and timing valve pulse width (PW). Timing lag is the time duration between the start of the pressure control valve pulse width to open the valve and the start of the opening of the timing control valve. Timing lag is an indication of how much later the timing control valve  14  may be actuated on to interrupt the injection event initiated by the pressure control valve  12 . Timing lag is also a indication of the pilot injection quantity which will escape from the nozzle before the needle valve is forced to close. Therefore, the pilot injection quantity is linearly related to the timing lag parameter as shown in FIG. 6. The timing control valve  14  pulse width duration is the indication of how long the timing control valve  14  would stay in the open position. Since the timing control valve  14  opening directly causes needle valve  20  closing, the timing control valve  14  pulse width is linearly proportional to the amount of time the needle valve  20  will stay closed. Therefore during pilot injection, dwell is linearly related to the timing control valve  14  pulse width as shown in FIG. 6.  
         [0091]    A major advantage of the fuel system of the present invention is that it incorporates the advantage of both the intensifier injection system and the common rail injection system. It is a marriage of the two systems, while avoiding some of the disadvantages of each of the two systems.  
         [0092]    (1) The injector  10  advantageously does not require high pressure fuel transporting as does the common rail system. High injection pressure is contained within the unit injector. The unit injector  10  is exposed to high pressure operation only during injection event. This is the advantage of the intensifier system.  
         [0093]    (2) The injector  10  has direct control of the needle valve  20 . This feature is very critical to pilot injection operation. Without direct needle valve  20  control, a small pilot and a small dwell can not be achieved. Direct needle valve  20  control is the advantage of the common rail system as distinct from the intensifier system. This advantage is also kept with the present invention.  
         [0094]    (3) Decoupling the actuating fluid pressure control event from the needle timing event as provided for with the present invention makes the whole injection operation much simpler, more flexible and more controllable. Each control valve  12 ,  14  has its own substantially independent responsibility. The two control valves  12 ,  14  do not interact and can be controlled independently. This indicates the simplicity of the control strategy. Results can be easily interpolated and extrapolated.  
         [0095]    (4) With the present invention, a wide variety of all desired injection characteristics can be readily achieved. No injector in production today is able to achieve all the features. The common rail system cannot achieve ramp injection and rate shaping. The HEUI intensifier system cannot achieve square injection. Pilot size and dwell range are also limited in the prior art.  
         [0096]    (5) The philosophy behind this invention is very different from the conventional approach. In this concept, the pilot and rate shaping injections are considered as a single injection interrupted for a short period. Based on this philosophy, each control valve  12 ,  14  is assigned a sole responsibility coordinated with the other control valve  12 ,  14 . The larger pressure control valve  12  only operates once to perform the single shot injection. The smaller and faster timing control valve  14  can be used many times to control the needle opening and closing during a single open cycle of the pressure control valve  12 .  
         [0097]    (6) This injector  10  has an intensifier. However, the injector  10  does not require reversal of the intensifier  18  motion to stop pilot injection. This is different from the HEUI-B and digital valve HEUI injection concepts. By avoiding reversal of the intensifier  18  motion, the hydraulic efficiency of the injection is significantly improved, by maintaining high fuel pressure throughout an injection event, even during an injection event having a pilot injection spaced in time from the main injection.  
       The Embodiments of Figs.  7 - 10   b    
       [0098]    The principles put forth in the present application apply more generically to the concept of mating direct needle valve control with an intensifier type of pressure fuel generation of an appropriate level (approximately 1,500-1,600 bar) for injection into the combustion chamber of a diesel engine. It has been noted that there are certain advantages to what is termed a common rail fuel injection system. Such a system is produced by Robert Bosch GmbH and was recently chosen to provide the injection for the General Motors Duramax 6600 V8 diesel engine. It has been stated that the common rail system was chosen for this engine to meet the trend in evermore stringent exhaust emission regulations and for the following advantages:  
         [0099]    High pressure injection capabilities (it should be noted that, while the authors of the paper profess this as a reason for selecting the system that they did, in fact, the intensifier system described with reference to the above embodiment of the present invention gives higher injection pressure than the common rail system); and  
         [0100]    Flexibility of injection parameters (including variable injection timing, pilot injection, main injection, post injection, and variable injection pressure).  
         [0101]    See Society of Automotive Engineers Paper, 2000-01-3512, The New Common Rail Fuel System for the Duramax6600 V8 Diesel Engine, authored by Ohishi et al, and incorporated herein by reference. The main characteristic of the Bosch common rail system is its constant injection pressure of 1600 bar. While those that selected the common rail system for application in this engine appreciated the flexibility of injection parameters afforded by the common rail system, this flexibility is not without penalty. As stated in the above SAE paper, “ . . . durability is required of the common rail system. To achieve 1600 bar compatible with an extended life, numerous modifications and new technologies were applied.” An approach to avoiding the complexity implicitly encountered by the designers of the common rail system for the Duramax 6600 diesel engine is to avoid the high pressure common rail altogether by generating the injectable fuel pressures within the injector itself using the intensifier system of the HEUI type injector coupled with a suitable direct needle valve control as put forth above.  
         [0102]    Accordingly, a first further embodiment of the present invention is then coupling the HEUI intensifier of FIG. 2 with a piezo controlled direct needle actuation. Piezo controlled direct needle actuation is depicted in FIGS. 7 and 8 and is put forth in greater detail in U.S. Pat. Nos. 5,875,764 to Kappel et al., and 6,062,533 to Kappel et al., both patents being incorporated herein by reference. In this embodiment, the pressure control valve  12  and intensifier  18  of FIG. 2 are mated to the piezo controlled direct needle actuation of FIGS. 7 and 8. Such mating is effected by coupling the high pressure fuel passageway  26  to the piezo controlled direct needle actuation system. Such coupling is noted in FIGS. 7 and 8 as being “flow from intensifier plunger”.  
         [0103]    Piezo controlled direct needle actuation has the characteristic of relatively faster acting, hence better response time as compared to the solenoid pressure control valve  12  of the HEUI injector acting alone. The motivation behind the present invention is to use this piezo high speed response to directly control the needle valve to use the intensifier pressure generation device of the HEUI injector to develop high injection fuel pressure to supply needle valve flow demand. Combining the piezo controlled direct needle actuation with the HEUI intensifier type pressure generation is an enhancement of the prior art HEUI injector as depicted in FIG. 1. The piezo controlled direct needle actuation is shown generally at  400  in FIGS. 7 and 8.  
         [0104]    In operation, the piezo controlled direct needle actuation  400  includes a piezo solenoid  402 , a control valve actuator piston  404  bearing directly on the needle back  22  of the needle valve  20 . A feed orifice  406  fluidly couples the high pressure fuel passageway  26  to the actuator chamber  408 . The area of the actuation piston head  410  is greater than the front area  30  of the needle valve  20 . This relationship is important to operation of the piezo control direct needle actuation  400 .  
         [0105]    The feed orifice  406  has a relatively small area such that the feed orifice  406  has a throttling function. Accordingly, pressure loss occurs when high pressure fuel flows from the high pressure fuel passageway  26  through the feed orifice  406 .  
         [0106]    The piezo solenoid  402  translates between a closed position depicted in FIG. 7 and an open position depicted in FIG. 8. The piezo control direct needle actuation  400  includes a piston  412  coupled to the piezo solenoid  402 . The piston  412  is operably coupled to a ball control valve  414 . The ball control valve  414  is seatable in a seat  416 . The seat  416  is operably coupled to a vent orifice  418 . The vent orifice  418  is in fluid communication with the chamber  408 .  
         [0107]    In the closed position as depicted in FIG. 7, the ball control valve  414  is seated on the seat  416 , thereby sealing off the vent orifice  418 . High pressure fuel admitted through the feed orifice  406  is then present in the actuation chamber  408 . Pressure in the actuation chamber  408  is substantially balanced with the pressure in the high pressure fuel passageway  26  and in the plunger chamber  24  (see FIG. 2) of the intensifier  18 . The high pressure fuel is also acting on the front area  30  of the needle valve  20 . In this condition, fuel pressure acting on the head  410  of the actuator piston  404  in combination with the bias exerted by the spring  420  is greater than the opposing force generated on the front area  30  (the front area  30  being less than the area of the head  410 ). The needle valve  20  is held in the closed disposition and, accordingly, the injection orifice  422  is sealed off the by the closed needle valve  20 . No fuel injection occurs.  
         [0108]    The position of the needle valve  20  is controlled by the pressure differential between the pressure exerted on the needle front area  30  and the pressure exerted on the head  410  of the actuator piston  404  (in combination with the bias exerted by the spring  420 ). Referring to FIG. 8, the ball control valve  414  is raised off the seat  416  by action of the piezo solenoid  402 . The unseating of the ball control valve  414  opens the vent orifice  418  allowing the discharge of high pressure fuel from the actuation chamber  408  to a relatively low pressure environment, preferably ambient or near ambient (approximately  50  psi). When the ball control valve  414  is in the open position, fuel pressure in the actuation chamber  408  is much lower than the pressure of the fuel in the high pressure fuel passageway  26  due to leakage flow through the vent orifice  418  and the throttle effect of the feed orifice  406 . In this condition, the fuel pressure acting on the needle front area  30  is substantially greater than the fuel pressure acting on the head  410  of the actuator piston  404 , in combination with the bias exerted by the spring  420 . The very high pressure fuel acting on the needle valve front area  30  generates a force in opposition to the force exerted on the head  410  in combination with the bias exerted by the spring  420  to cause the needle valve  20  to translate upward, thereby opening the orifice  422  and causing injection of the high pressure fuel into the combustion chamber.  
         [0109]    The vent to ambient at the vent  422  plays an important role in the functioning of the present invention. The vent  422  acts to maintain the desired pressure differential, as described above, by venting fuel pressure from a volume, the volume including the underside of the piston  404  and in the needle back chamber  424 , to ambient or near ambient (in the area of approximately 50 psi.). Fuel is discharged from the volume in fluid communication with the vent  422  as the needle valve  20  and the actuator piston  404  translate in their respective cylinder bores such that pressure in this volume is at all times negligible.  
         [0110]    The end of the injection event is achieved in either of two ways. The first such end involves closing the piezo solenoid  402  first. In such ending, the piezo solenoid  402  is closed before the return of the actuator piston  404  to its retracted disposition. High pressure fuel in passageway  26  is still available and being generated by the intensifier plunger  18 . The needle valve  20  closes as a result of the area differential of the head  410  with respect to the front area  30 . An advantage of this type of end to the fuel injection is the very sharp (rapid termination) end of fuel injection, since the needle valve  20  closes when the fuel pressure is still very high.  
         [0111]    The second way of ending injection is intensifier controlled end of injection. This is similar to the end of injection achieved by the prior art HEUI type injector. In this type of end of injection, the piezo solenoid  402  is open. Return of the intensifier piston  404  to its retracted disposition results in the decay of fuel pressure in the passageway  26 . As the fuel pressure in the high pressure fuel passageway  26  drops off, the spring  420  acts on the actuator piston  404  to again close the needle valve  30  as depicted in FIG. 7. It should be noted that in most modes of operation of the present invention, the first method of terminating injection is preferred as the fast end of injection results in HC reduction and reduced smoke generation.  
         [0112]    It should be noted that operation of the piezo controlled needle actuation  400  is totally independent of the operation of the intensifier  18 . The following are examples of such independent operation.  
         [0113]    (1) No Injection.  
         [0114]    The piezo controlled direct needle actuation  400  remains closed, as depicted in FIG. 7, all the time resulting in no injection. This is irrespective of operation of the intensifier  18  under control of the pressure control valve  12 . Generation of high pressure fuel in the high pressure fuel passageway  26  by the intensifier  18  does not trigger any fuel injection due to the inability of the needle valve  20  to open against the countering force developed by the high pressure fuel in the actuator chamber  408 . In this condition, the high pressure fuel generated by the intensifier  18  may be used outside the fuel injector for other engine functions such as, for example, engine valve actuation in a camless engine without interference from fuel injection.  
         [0115]    (2) Single Shot Operation.  
         [0116]    Single shot operation is essentially a single fuel injection occurrence taking place during an injection event. Prior to initiation of the injection event (t=0), the piezo control direct needle actuation  400  is in the closed position as depicted in FIG. 7. The pressure control valve  12  initiates the injection event by porting actuating fluid to the intensifier  18  at t=0. The intensifier  18  generates the high pressure fuel in the high pressure fuel passageway  26  and in the actuation chamber  408  and at this point the injector is ready for fuel injection. Injection is prevented by the pressure differential caused by the high pressure fuel in the actuation chamber  408 , the pressure being substantially equal to that acting on the front area  30 ,but the force on the piston head  410  being greater due to the greater area of the piston head  410  as compared to the front area  30 . The piezo solenoid  402  is then commanded to move to the open position in which fuel pressure is discharged from the actuation chamber  408 , shifting the pressure differential in favor of the front area  30 . Fuel at high pressure acting on the front area  30  then causes the needle valve  20  to open against the bias force acting on the piston head  410 , pressure in the actuation chamber  408  having been bled off. Fuel injection then occurs eruptively, rising almost instantaneously to the maximum rate of injection. It should be noted that a small amount of leakage occurs under this condition through the feed orifice  406 . In order to account for this leakage, the intensifier plunger chamber  24  is sized such that there is sufficient high pressure fuel to compensate for the leakage that occurs through the feed orifice  406 .  
         [0117]    The present invention provides a selection of control strategies. If at t=0, the piezo solenoid  402  is open, the intensifier  18  commences its compressive stroke and generates the high pressure fuel in the high pressure fuel passageway  26  at a relatively slow rate of pressure build up. Fuel pressure acting on the front area  30  then causes the needle valve  20  to open relatively slowly, giving a ramped shape to the rate of injection over time. On the other hand, if at t=0, the piezo solenoid  402  is closed, the intensifier  18  has sufficient time to generate maximum pressure fuel in the high pressure fuel passageway  26 . This pressure is also acting on front area  30 , but because the piezo solenoid  402  is closed, the needle valve  20  is prevented from opening. When the piezo solenoid  402  is then opened, the needle valve  20  opens virtually instantly and injection occurs eruptively, resulting in a nearly vertical trace of rate of injection over time.  
         [0118]    (3) Multiple Injection During a Single Injection Event.  
         [0119]    The injection event is initiated and terminated by the pressure control valve  12  acting to port actuation fluid intensifier  18 . During the injection event, the pressure control valve  12  need open a single time at the initiation of the injection event and close a single time at the end of the injection event. In this mode of operation, actuating fluid is ported to the intensifier  18  continually during the injection event and the intensifier  18  provides constant high pressure fuel in the high pressure fuel passageway  26  that is available for injection by the needle valve  20 . Multiple injection events are controlled directly by the piezo solenoid  402  cycling between the open and closed positioned as desired. Direct actuation of the needle valve  20  under the control of the piezo control direct needle valve actuation  400  is much faster than any other method of actuation, including multiple cycling the relatively large pressure control valve  12 , which necessarily involves the relatively inefficient stopping and starting of the intensifier  18 . The direct control of the opening of the needle valve  20  as effected by the piezo control direct needle actuation  400  results in a greatly reduced quantity of pilot injection. Further, direct control of the opening of the needle valve  20  as effected by the piezo control direct needle actuation  400  results the in the capability of reducing the dwell time of an injection occurrence for more precise control of the multiple injection occurrences taking place during a single injection event simply by cycling the piezo solenoid  402  independent of the compressive stroke of the intensifier  18 .  
         [0120]    A further embodiment of the present invention is the use of the pressure control valve  12  and intensifier  18  as depicted in FIG. 2 in conjunction with the direct needle control  112  of FIG. 9. FIG. 9 is partially excerpted from U.S. Pat. No. 5,913,300 to Drummond, incorporated herein by reference. In combining the pressure control valve  12  and intensifier  18  of FIG. 2 with the device of FIG. 9, the high pressure fuel passage  26  interfaces with the passageway  118  of FIG. 9, thereby eliminating the pump shown in U.S. Pat. No. 5,913,300. No injection, single shot operation, and multiple injection as described above with reference to the piezo control direct needle actuation  400  is readily achievable by combining the pressure control valve  12  and intensifier  18  with the direct needle actuation device  112  of FIG. 9. Consistent with the description above, pressure control valve  12  is cycled only a single time during an injection event, thereby porting actuation fluid to the intensifier  18  from initiation of the injection event through termination of the injection event as determined by the pressure control valve  12 . In this manner, continuous high pressure fuel is available to the passageway  118  of FIG. 9.  
         [0121]    A still further embodiment of the present invention is the combination of the pressure control valve  12  and intensifier  18  of FIG. 2 with the direct needle valve control system depicted in FIGS. 10 a  and  10   b.  It should be noted that FIGS. 10 a  and  10   b  are excerpted from the previously mentioned SAE paper relating to the common rail fuel system. As with the immediately aforementioned embodiment of the present invention, the pressure control valve  12  need only open to commence the injection event and close to terminate the injection event. During such cycle, actuating fluid is continuously ported to the intensifier  18  such that high pressure fuel is generated continually by the intensifier  18  during the injection event and made available to the high pressure fuel passageway  26  depicted in FIGS. 2 and 10 a ,  10   b . As with previous embodiments of the present invention, the direct needle valve control system depicted in FIGS. 10 a  and  10   b  is available to exert its control over the needle valve totally independent of operation of the pressure control valve  12  and intensifier  18 . Such independent control permits at least the aforementioned operating conditions of no injection, single shot operation, and multiple injection.