Abstract:
A fuel injector including a delay device, the fuel injector having an electric controller for controlling the flow of a high pressure actuating fluid responsive to initiation and cessation of a pulse width command, the pulse width command defining the duration of an injection event, and an intensifier being in fluid communication with the controller, the intensifier being translatable to increase the pressure of a volume of fuel for injection into the combustion chamber of an engine; the delay device includes an apparatus, shiftable between a first disposition and a second disposition over a certain period of time after initiation of the pulse width command, the period of time effecting a delay in initiation of fuel injection after initiation of the pulse width command. In one embodiment, a bias against shifting the apparatus from said first disposition to the second disposition is effected by the actuating fluid to reduce variations in the delay period under variable actuating pressure conditions. A method of stabilizing fuel injection events, includes the steps of sending a pulse width command to a controller to define an injection event, the controller porting an actuating fluid to affect an intensifier responsive to reception of the pulse width command, and interposing a delay in the actuating fluid affecting the intensifier.

Description:
RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Application No. 60/130,055, filed Apr. 19, 1999, incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to fuel injectors. More particularly, the present invention relates to hydraulically-actuated, electronically--controlled unit injectors (HEUI injectors). 
     BACKGROUND OF THE INVENTION 
     The prior art injectors (baseline) used here for references are the hydraulicallyactuated, electronically-controlled unit injectors described in the following references, which are incorporated herein by reference: SAE paper 930270 and U.S. Pat. Nos. 5,720,261, 5,597,118, and 5,826,562. 
     The first three above referenced injectors (SAE paper 930270 and U.S. Pat. Nos. 5,720,261, and 5,597,118) do not have any delay device between the control valve and the intensifier. The flow of actuation liquid into the intensifier piston chamber occurs almost immediately after the control valve opens. 
     The injector of U.S. Pat. No. 5,826,562 delays and limits the initial flow to the intensifier piston by adding throttle slots or a groove on top of the intensifier piston. The opening of the flow passage to the intensifier is controlled by the intensifier piston motion. In the invention of U.S. Pat. No. 5,826,562, flow to the intensifier chamber depends on the traveling velocity of the intensifier. If intensifier can not move fast enough, the flow area then cannot open up. If the flow area cannot open Lip, the intensifier cannot travel faster. This contradiction is the source of a serious limitation of an injector made according to the &#39;562 patent. 
     Referring to the drawings, FIGS. 7 and 7 a  show a prior art fuel injector  350 . The prior art fuel injector  350  is typically mounted to an engine block and injects a controlled pressurized volume of fuel into a combustion chamber (not shown). The prior art injector  350  of the present invention is typically used to inject diesel fuel into a compression ignition engine, although it is to be understood that the injector could also be used in a spark ignition engine or any other system that requires the injection of a fluid. 
     The fuel injector  350  has an injector housing  352  that is typically constructed from a plurality of individual parts. The housing  352  includes an outer casing  354  that contains block members  356 ,  358 , and  360 (not shown). The outer casing  354  has a fuel port  364  that is coupled to a fuel pressure chamber  366  by a fuel passage  368 . A first check valve  370  is located within fuel passage  368  to prevent a reverse flow of fuel from the pressure chamber  366  to the fuel port  364 . The pressure chamber  366  is coupled to a nozzle  372  through fuel passage  374 . A second check valve  376  is located within the fuel passage  374  to prevent a reverse flow of fuel from the nozzle  372  to the pressure chamber  366 . 
     The flow of fuel through the nozzle  372  is controlled by a needle valve  378  that is biased into a closed position by spring  380  located within a spring chamber  381 . The needle valve  378  has a shoulder  382  above the location where the passage  374  enters the nozzle  378 . When fuel flows into the passage  374  the pressure of the fuel applies a force on the shoulder  382 . The shoulder force lifts the needle valve  378  away from the nozzle openings  372  and allows fuel to be discharged from the injector  350 . 
     A passage  383  may be provided between the spring chamber  381  and the fuel passage  368  to drain any fuel that leaks into the chamber  381 . The drain passage  383  prevents the build up of a hydrostatic pressure within the chamber  381  which could create a counteractive force on the needle valve  378  and degrade the performance of the injector  350 . 
     The volume of the pressure chamber  366  is varied by an intensifier piston  384 . The intensifier piston  384  extends through a bore  386  of block  360  and into a first intensifier chamber  388  located within an upper valve block  390 . The piston  384  includes a shaft member  392  which has a shoulder  394  that is attached to a head member  396 . The shoulder  394  is retained in position by clamp  398  that fits within a corresponding groove  400  in the head member  396 . The head member  396  has a cavity which defines a second intensifier chamber  402 . 
     The first intensifier chamber  388  is in fluid communication with a first intensifier passage  404  that extends through block  390 . Likewise, the second intensifier chamber  402  is in fluid communication with a second intensifier passage  406 . 
     The block  390  also has a supply working passage  408  that is in fluid communication with a supply working port  410 . The supply port  410  is typically coupled to a system that supplies a working fluid which is used to control the movement of the intensifier piston  384 . The working fluid is typically a hydraulic fluid that circulates in a closed system separate from the fuel. Alternatively the fuel could also be used as the working fluid. Both the outer body  354  and block  390  have a number of outer grooves  412  which typically retain O-rings (not shown) that seal the injector  350  against the engine block. Additionally, block  362  and outer shell  354  may be sealed to block  390  by O-ring  414 . 
     Block  360  has a passage  416  that is in fluid communication with the fuel port  364 . The passage  416  allows any fuel that leaks from the pressure chamber  366  between the block  362  and piston  384  to be drained back into the fuel port  364 . The passage  416  prevents fuel from leaking into the first intensifier chamber  388 . 
     The flow of working fluid into the intensifier chambers  388  and  402  can be controlled by a four-way solenoid control valve  418 . The control valve  418  has a spool  420  that moves within a valve housing  422 . The valve housing  422  has openings connected to the passages  404 ,  406  and  408  and a drain port  424 . The spool  420  has an inner chamber  426  and a pair of spool ports that can be coupled to the drain ports  424 . The spool  420  also has an outer groove  432 . The ends of the spool  420  have openings  434  which provide fluid communication between the inner chamber  426  and the valve chamber  434  of the housing  422 . The openings  434  maintain the hydrostatic balance of the spool  420 . 
     The valve spool  420  is moved between the first closed position shown in FIG. 7 and a second open position shown in FIG. 7 a  by a first solenoid  438  and a second solenoid  440 . The solenoids  438  and  440  are typically coupled to a controller which controls the operation of the injector. When the first solenoid  438  is energized, the spool  420  is pulled to the first position, wherein the first groove  432  allows the working fluid to flow from the supply working passage  408  into the first intensifier chamber  388  and the fluid flows from the second intensifier chamber  402  into the inner chamber  426  and out the drain port  424 . When the second solenoid  440  is energized the spool  420  is pulled to the second position, wherein the first groove  432  provides fluid communication between the supply working passage  408  and the second intensifier chamber  402  and between the first intensifier chamber  388  and the drain port  424 . 
     The groove  432  and passages  428  are preferably constructed so that the initial port is closed before the final port is opened. For example, when the spool  420  moves from the first position to the second position, the portion of the spool adjacent to the groove  432  initially blocks the first passage  404  before the passage  428  provides fluid communication between the first passage  404  and the drain port  424 . Delaying the exposure of the ports reduces the pressure surges in the system and provides an injector which has more predictable firing points on the fuel injection curve. 
     The spool  420  typically engages a pair of bearing surfaces  442  in the valve housing  422 . Both the spool  420  and the housing  422  are preferably constructed from a magnetic material such as a hardened  52100  or  440 c steel, so that the hysteresis of the material will maintain the spool  420  in either the first or second position. The hysteresis allows the solenoids to be de-energized after the spool  420  is pulled into position. In this respect the control valve operates in a digital manner, wherein the spool  420  is moved by a defined pulse that is provided to the appropriate solenoid. Operating the valve in a digital manner reduces the heat generated by the coils and increases the reliability and life of the injector. 
     In operation, the first solenoid  438  is energized and pulls the spool  420  to the first position, so that the working fluid flows from the supply port  410  into the first intensifier chamber  388  and from the second intensifier chamber  402  into drain port  424 . The flow of working fluid into the intensifier chamber  388  moves the piston  384  and increases the volume of chamber  366 . The increase in the chamber  366  volume decreases the chamber pressure and draws fuel into the chamber  366  from the fuel port  364 . Power to the first solenoid  438  is terminated when the spool  420  reaches the first position. 
     When the chamber  366  is filled with fuel, the second solenoid  440  is energized to pull the spool  420  into the second position. Power to the second solenoid  440  is terminated when the spool reaches the second position. The movement of the spool  420  allows working fluid to flow into the second intensifier chamber  402  from the supply port  410  and from the first intensifier chamber  388  into the drain port  424 . 
     The head of the intensifier piston  396  has an area much larger than the end of the piston  384 , so that the pressure of the working fluid generates a force that pushes the intensifier piston  384  and reduces the volume of the pressure chamber  366 . The stroking cycle of the intensifier piston  384  increases the pressure of the fuel within the pressure chamber  366 . The pressurized fuel is discharged from the injector through the nozzle  372 . The working fluid is typically introduced to the injector at a pressure between 300-4000 psi. In the preferred embodiment, the piston has a head to end ratio of approximately 10:1, wherein the pressure of the fuel discharged by the injector is between −3000-40,000 psi. 
     Again the fuel is discharged from the injector nozzle  372 , the first solenoid  438  is again energized to pull the spool  420  to the first position (FIG. 7) and the cycle is repeated. 
     In the prior art, the intensifier piston  384  starts to move immediately after control valve  418  starts to open. When a minimum pulse width command (the pulse width defining the time between the open and the close signals to the control valve  418  which permits the spool  420  to fully open (FIG. 7 a ) before being retracted (FIG. 7) which defines the minimum round trip time of the spool  420 ) is given to the control valve  418 , the corresponding fuel delivery amount is referred to as the minimum fuel delivery quantity. This is illustrated in FIG.  4 . If a smaller than minimum fuel quantity is desired, the controller would need to command a smaller pulse width which requires the solenoid of the control valve  418  to go through a partial motion, e.g., the spool  420  of the control valve  418  does not achieve a full open (FIG. 7 a ) disposition before it is recalled to its closed disposition (FIG.  7 ). This is less than a full round trip of the spool  420 . However, partial motion of the spool  420  results in poor injector performance due to injector-to-injector variability and injection event-to-event controllability. This causes very rough engine running and undesirable emission levels. 
     With hydraulically-actuated, electronically-controlled unit fuel injectors (HEUI injector) as described above, the initial portion of an injection event is frequently unstable due to the aforementioned partial motion of the spool  420 . Such instability is often induced by partially opening the spool  420  and then abruptly retracting the spool  420 . Such partial opening is not very repeatable in a certain injector and is typically not repeatable from injector-to-injector due to manufacturing and other variances. There is a need in the industry for injectors, particularly HEUI injectors, that avoid the noted region of instability. 
     SUMMARY OF THE INVENTION 
     The injector of the present invention substantially meets the aforementioned needs of the industry. The object of the present invention is to delay the start of the intensifier actuation process in a hydraulically-actuated, electronically-controlled unit fuel injector (HEUI injector), especially one having a spool-type control valve of the type described in U.S. Pat. No. 5,720,261, by a desired amount of time after the control valve opens. With a certain amount of delay built in between control valve initiation signal and the start of the intensifier motion, it is possible to not have to use control valve partial motion. Further, the unstable motion region that occurs with control valve partial motion is avoided. Hence injection variability is improved with the present invention, especially for very small quantities of fuel delivery. Minimum fuel delivery quantity can be advantageously and significantly reduced even at very high actuation pressure with the present invention. 
     The present invention is a delay device for use with a fuel injector, the fuel injector having an electric controller for controlling the flow of a high pressure actuating fluid responsive to initiation and cessation of a pulse width command, the pulse width command defining the duration of an injection event, and an intensifier being in fluid communication with the controller, the intensifier being translatable to increase the pressure of a volume of fuel for injection into the combustion chamber of an engine; the delay device includes an apparatus, shiftable between a first disposition and a second disposition over a certain period of time after initiation of the pulse width command, the period of time effecting a delay in initiation of fuel injection after initiation of the pulse width command. In one embodiment, a bias against shifting the apparatus from said first disposition to the second disposition is effected by the actuating fluid to reduce variations in the delay period under variable actuating pressure conditions. The present invention is further a method of stabilizing fuel injection events, includes the steps of sending a pulse width command to a controller to define an injection event, the controller porting an actuating fluid to affect an intensifier responsive to reception of the pulse width command, and interposing a delay in the actuating fluid affecting the intensifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a HEUI injector incorporating the delay device of the present invention; 
     FIG. 2 a  is a sectional view of the delay device depicted in the circle  2  of FIG. 1 during injection delay; 
     FIG. 2 b  is a sectional view of the delay device depicted in FIG. 2 a  during main injection; 
     FIG. 3 a  is a sectional view of a delay device used for rate shaping prior top injection; 
     FIG. 3 b  is a sectional view of a delay device of FIG. 3 a  during rate shaping injection; 
     FIG. 3 c  is a sectional view of a delay device of FIG. 3 a  during main injection; 
     FIG. 3 d  is a graphic representation of the delay and rate shaping effected by the embodiment of FIGS. 3 a - 3   c;    
     FIG. 3 e  is a sectional view of a delay device using rail pressure to return the delay piston. 
     FIG. 4 is two related graphic depictions of the effects of partial valve motion in the prior art HEUI injectors; 
     FIG. 5 is three related graphic representations comparing the prior art to the present invention with FIG. 5 a  depicting control valve motion, FIG. 5 b  depicting flow at the intensifier, and FIG. 5 c  depicting injection rate; 
     FIG. 6 a  is a sectional view of a further embodiment of the delay device of the present invention prior to injection; 
     FIG. 6 b  is a sectional view of the delay device of FIG. 6 a  during injection; 
     FIG. 6 c  is a sectional view of a further embodiment of the delay device of FIG. 6 a  of the present invention with rate shaping; 
     FIG. 6 d  is a sectional view of the delay device of FIG. 6 c  during injection; 
     FIG. 6 e  is a sectional view of a further embodiment of the delay device of the present invention with pilot injection prior to main injection; 
     FIG. 6 f  is a sectional view of the delay device of FIG. 6 e  during injection; 
     FIG. 7 is a sectional view of a prior art HEUI injector; and 
     FIG. 7 a  is a sectional view of a prior art HEUI injector control valve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The delay device of the present invention is depicted generally at  10  in the figures. Referring to FIG. 1, the delay device  10  of the present invention is installed in the injector  200  between injector control valve (3-way valve)  202  and the intensifier  204  as shown in FIG.  1 . FIGS. 2 a  and  2   b  show the delay device  10  schematic and its operation. FIG. 2 a  depicts the disposition of the delay device  10  for injection delay. FIG. 2 b  depicts the disposition of the delay device  10  for main injection. 
     The delay device  10  includes delay piston  12 , return spring  14  and delay cylinder housing  16 . The delay cylinder housing  16  is a stationary piece comprising a cylinder. The delay piston  12  is free to translate up and down inside of the housing  16 . As will be described in more detail below, the general operation of the delay piston is such that as the delay piston  12  moves from its top position (FIG. 2 a ) to its bottom position (FIG. 2 b ), the delay piston  12  gradually passes through the delay phase (overlap  18  during which no actuating fluid is ported to the intensifier  204 ) and gradually opens up the flow passage  222  to the intensifier  204 . 
     As shown in FIGS. 2 a,    2   b,  the delay piston  12  has top surface  22  which faces the flow the control valve  202  in actuating fluid passageway  24 . As the control valve  202  opens, high pressure actuation fluid flows in from the high pressure rail  26  (FIG. 1) through control valve  202 . The high pressure actuation fluid flows to bear on the top  22  of the delay piston  12 . The top surface  22  of the delay piston  12  is pressurized when the control valve  202  is at its working position (open position). The pressure acting on the top surface  22  of the delay piston  12  is at substantially the same level as the pressure from the high pressure rail  26 . This pressure may range from about 500 psi to 3500 psi. In practice, the actuating fluid pressure after the control valve  202  is slightly less than pressure at the rail  26  due to flow resistance and loss at control valve  202 . 
     The bottom side  28  of the delay piston  12  is exposed to ambient pressure by drain  30 . A slot  31  defined in the bottom side  28  of the delay piston  12  extends to the drain  30  defined in the injector housing  208 . The slot  31  provides the venting of any leakage by the delay piston  12 , as well as accessing to ambient pressure. 
     The flow passageway  222  between the control valve  202  and intensifier chamber  223  is intersected by the delay piston  12  during a portion of the downward travel distance of the delay piston  12 . When the delay piston  12  is at its topmost position (FIGS. 1 and 2 a ), the passageway  222  is completely blocked. As the delay piston  12  moves downward, the delay piston  12  must pass through the overlap distance  18  (see FIG. 2 a ) before the delay piston  12  opens up the flow passage  222  to the intensifier  204 . During the overlapping phase, flow to the intensifier  204  is either completely blocked (see FIG. 2) or partially blocked (see the rate shaping embodiment of FIG. 3 b ). The blocking of significant flow into the intensifier piston chamber  223 , results in the motion of the intensifier piston  236  and plunger  228  being substantially restrained or limited. The travel time of the delay piston  12  through the overlap length  18  indicates the time delay between control valve  202  opening and the start of the intensifier piston  236  downward motion. 
     Once the delay piston  12  passes its overlapping phase  18 , the flow passageway  222  gradually opens up as the delay piston travels downward to expose an ever increasing portion of the passageway  222  and significant actuating fluid flow between the control valve  202  and the intensifier  204  occurs. The injection process after the piston  12  commences to open the passageway  222  is quite abrupt and comprises the main injection event. 
     The leakage around the delay piston  12  (between the delay piston  12  and the housing  16 ) is controlled to be at a minimum flow rate. 
     There is a very lightly loaded spring  14  on the bottom side of the delay piston  12 . This spring  14  has a sole purpose which is to return delay piston  12  to its top position (FIG. 2 a ) after completion of the injection event as signaled by the closing of the control valve  202  close the high pressure actuating fluid from rail  26  vent the actuating fluid to ambient  27 . When pressure acting on the top  22  of the delay piston  12  is near ambient pressure level, the pressure on the bottom surface  28  and on the top surface  22  are then balanced and the spring  14  force returns the delay piston  12  upward from the disposition of FIG. 2 b  to its topmost position as depicted in FIG. 2 a.    
     The delay piston  12  does not return to its upper, closed position until the intensifier piston  236  finishes its return to its upper initial position (FIG. 2 a ). This results from the delay cylinder spring  14  being relatively weak such that even a minimal pressure on top  22  of the delay piston  12  prevents the delay piston  12  from returning to the full up disposition of FIGS. 1 and 2 a.  Due to a heavy intensifier spring  232  load and drain resistance created by the control valve  202 , the pressure acting on the top surface  22  of the delay piston  12  is greater than the ambient pressure level during the intensifier  204  return process. Therefore, the delay piston  12  cannot return when the intensifier piston  236  is returning since the drain pressure on the top surface  22  of the delay piston  12  is greater than force from delay cylinder spring  14 . The returning of the delay piston  12  occurs only after the intensifier  204  finishes its return to its upper, initial position, as depicted in FIG. 2 a.    
     The time required for the delay piston  12  to travel the overlapping distance  18  can be adjusted to a similar order as the travel time required for the control valve  202  to open. Therefore, with selected calibrated delay piston  12  dimensions, it is possible to achieve virtually any desired delay time to harmonize the control valve signal that acts to command the control valve  202  to open and the intensifier  204  response in commencing downward motion of the intensifier piston  236 . 
     Injector Operation 
     Before opening commands are given to the control valve  202 , all components are at the resting position as depicted in FIGS. 1 and 2 a.  The nozzle needle valve  250  is at its closed position due to the biasing force of the spring  256  on the needle back  257 . Accordingly, the orifices  252  are also closed. The intensifier spring  232  is forcing the intensifier piston  236  and the plunger  228  to seat at the topmost position. The plunger chamber  230  is filled with low pressure fuel from fuel rail  231 , which connects to a low pressure fuel tank. The intensifier spring cage  233  is always vented to the ambient pressure. The back side chamber  17  of delay piston  12  is also vented to ambient by drain  30 . The delay piston  12  top surface  22  is at ambient pressure due to venting of the control valve  202 . Therefore, before control valve  202  opens, the delay piston  12  rests at its topmost position due to the spring force of spring  14  and the balanced ambient actuating fluid pressure force on both the top  22  and bottom side  28  of the delay piston is  12 . The flow passage  222  between the control valve  202  and intensifier chamber  223  is blocked by the delay piston  12 . 
     A pulse width command is a signal of a certain duration. Initiation of the command opens the control valve  202 . The control valve  202  remains open for the duration of the pulse width command and is closed at the termination of the pulse width command. When a pulse width command is given to open the control valve  202 , the control valve  12  opens its working port and closes its drain (vent) port. High pressure actuation fluid starts to flow from the high pressure rail  26  through passageway  24  into the delay cylinder chamber  25 . The high pressure actuation fluid acts on surface  22 , forcing the delay piston  12  to move downward. 
     The delay piston  12  takes a certain amount of the time to travel through the overlap distance  18  before the delay piston  12  starts to open the flow passage  222 . No fuel is injected into the combustion chamber in the interval between the initiation of the pulse width command and the first intersection of the top  22  of the delay piston  12  and the passageway  222 . Once the flow passage  222  opens, flow from the control valve  202  to intensifier chamber  223  begins. The high pressure actuation fluid generates a force on the intensifier piston  236  causing the intensifier piston  236  to move downward. 
     The pressure of the fuel in chamber  230  increases very quickly after the downward motion of the intensifier piston  236  of the intensifier  204  starts to compress the fuel in chamber  230 . Injection starts once the pressure of the fuel exceeds the needle valve  250  opening pressure. Meantime, the delay piston  12  continuously travels downward to its bottom seat (FIG. 2 b ) and completely opens the flow passageway  222 . When the delay piston  12  is at its bottom seat, the pressure loss caused by the delay device  12  is negligible and the injector flows at substantially similar volume and pressure of fuel as prior art injectors without the delay device  10 . 
     The end of the pulse width command signals the end of the injection event. The control valve  202  returns from its open disposition to its closed disposition, closing its working port and opening its drain port to vent actuating fluid to ambient pressure  27 . Actuating fluid in the chamber  223  above the intensifier piston  236  begins the draining process. As actuating fluid pressure in chamber  223  diminishes, the intensifier spring  232  forces the intensifier piston  236  to move back upward. The injection pressure of the fuel drops quickly, resulting in closure of the needle valve  250  and the orifices  252 , thereby ending the injection event. During the actuating fluid venting process, the intensifier piston  236  returns to its topmost position (FIG. 2 a ) and refilling of the fuel takes place in chamber  230  beneath the plunger  228 . The delay piston  12  then also starts to return to its topmost position (FIG. 2 a ). After the intensifier piston  236  finishes its return, all components are back to the initial positions as depicted in FIGS. 1 and 2 a.    
     Advantages of the Present Invention 
     It is advantageous to interpose a certain delay between an excitation signal to the control valve  202  and a reaction signal to the intensifier  204  in order to obtain better overall system controllability, smoothness of operation and harmonization between components. The delay device of the present invention effects such a delay. 
     With the delay device  10  of the present invention, it is possible to build in virtually any desired amount of the delay between the control valve  202  initiation signal and the start of the reaction time from intensifier  204 . Overlap  18  is an adjustable and calibratable parameter for any given injector. The delay device  10  permits controllably injecting less fuel than the minimum controllable fuel delivery quantity of a prior art injector as depicted in FIG.  7 . 
     The motion of control valve  202  is relatively independent of rest of the system. With the delay device  10 , one can achieve much smaller controllable fuel injection quantity with smaller variability from injection event to injection event. This is illustrated by the figures of FIG.  5 . Control valve  202  motion is the same for the prior art injector and the present invention injector, the solid line and the dashed line being in fact coincident in FIG. 5 a  (but being slightly separated in the depiction for clarity of understanding). As the control valve  202  opens, flow to the intensifier  204  starts much earlier in the prior art case compared to the present invention. See FIG. 5 b.  Because of earlier start of intensifier flow in the prior art injector, the start of injection is also very early. See FIG. 5 c.  This results in a relatively larger quantity of fuel delivery at minimum pulse width command. With the present invention, delaying flow to the intensifier produces a relatively longer hydraulic delay between the start of the control valve  202  motion (FIG. 5 a ) and the start of injection (FIG. 5 c ). The longer delay helps to yield the smaller fuel delivery quantity as shown in the FIG. 5 c.  Compared to prior art perfornance, the injector with the present invention produces much smaller fuel delivery for the same given control valve command. This is very desirable for noise emission control and for drivability. 
     Further, by increasing the amount of the overlap  18 , the minimum fuel delivery quantity can be reduced to zero, if desired. With the delay device  10 , the control valve  202  does not need to work in the partial undesirable motion region at all. See FIG.  4 . The control valve  202  may be fully opened for each injection event for the minimum time necessary to fully open and then return to the closed disposition. This is true even for an injection event in which no fuel is injected. All variability and uncontrollability caused by the partial control valve motion of the prior art is eliminated by a suitably calibrated delay device  10  of the present invention. 
     Another significant advantage of the delay device  10  is that the delay device  10  always opens up the flow passage  222  to the intensifier chamber  223  regardless of the rail pressure in the high pressure rail  26 . Since the load of spring  14  is so small, virtually any positive pressure differential acting on surface  22  will force the delay piston  12  to move downward. Whether there is relatively lower or relatively higher rail pressure, delay piston  12  always moves down to open the flow passage  222  for the intensifier  204 . This is advantageous under engine operating conditions with relative low actuating fluid pressure. 
     Yet another advantage of the delay device  10  compared to the stepped intensifier piston shown in U.S. Pat. No. 5,826,562 is that under cold temperature conditions, for example, during a cold start-up, the delay device  10  of the present invention always opens up the flow passage  222  to the intensifier chamber  222  regardless of low pressures at the top surface of the delay piston  12  because there is very low resistance to movement of the delay piston  12  due to the weak spring  14  and further because it is a separate component and thus does not have to also move the intensifier piston  236  against both the force of its spring  232  and the hydraulic resistance in the plunger chamber  230 . With a stepped intensifier piston, as described in the &#39;562 patent, the passage to the main intensifier piston surface is more difficult to open especially with highly viscous lubricating oil as the actuating fluid. The present invention is thus very different from the prior art injector of U.S. Pat. No. 5,826,562. In the present invention, the delay piston  12  motion is totally independent of intensifier  204  motion. 
     OTHER PREFERRED EMBODIMENTS 
     Rate Shaping Feature 
     As shown in FIG. 3 a,  the top portion of the delay piston  12  has a slightly smaller diameter to form a cylindrical peripheral slot  50  defined between the cylinder  51  and the cylinder wall  16 . Incoming high pressure actuating fluid bears first on the top surface  22  and, after slight downward translation of the delay piston  12 , additionally on the top surface  22   a.  With this slot  50 , delay initially occurs during the time it takes for the delay piston  12  to travel the distance of the overlap  18 . 
     As indicated by FIG. 3 b,  flow to the intensifier  204  starts slowly at beginning when the slot  50  first intersects the passageway  222  when the top surface  22   a  first intersects the passageway  222 . Flow is minimized due to the relatively limited flow area presented by the constriction of the slot  50 . Such restriction decreases the slope of the leading edge of the delivered fuel curve on a graph of injection rate versus time. A more gradual building of the rate of injection as compared to the nearly square shape (as depicted in the prior art curve of FIG. 3 c ) is very desirable for engine drivability and emission control. As noted in FIG. 3 d,  the rate shaping embodiment of the present invention includes a delay between the time of the pulse width command initiation and the commencement of injection. The initial portion of injection (after the delay) is rate shaped as the actuating fluid is flowing through the constriction formed by the slot  50 . In this region, the rate of injection builds gradually before, as indicated in FIG. 3 c,  the flow passageway  222  is fully opened by further downward motion of delay piston  12  when the top surface  22  passes the leading edge of the passageway  222 . This creates a relatively slow rising of injection pressure to provide a rate shaping of the rate of injection of the injected fuel before the sharp rising of main injection event. Slow rising of initial injection pressure is very desirable for NOx emission control. This embodiment allows both delay and rate shaping to occur on one injector. 
     FIG. 3 e  illustrates yet another embodiment of the delay device  10  wherein the return spring  14  of the previous embodiments is eliminated. The high pressure actuation fluid from rail  26  is used to return the delay piston  12  to its uppermost position after each injection event. The delay device  10  consist three pieces, the delay piston  12 , a return pin  23  and the housing  16 . The return pin  23  is slidably disposed in a relatively close fit in a passage  29  disposed in the housing  16  between back side chamber  17  and fluidly connected to the high pressure rail  26 , the fit being sufficiently close to prevent significant leakage from the high pressure rail to the back side chamber  17 . The pin  23  extends into the backside chamber  17  to be in constant contact with the lower surface  28  of the delay piston  12  as long as rail pressure is available. The back side chamber  17  is vented to ambient pressure at all times. The cross-sectional area of the return pin  23  is relatively small compared to that of the piston  12 . Therefore, the return of the delay piston  12  to its uppermost position occurs only when the pressure on the top side  22  of the piston  12  is near ambient pressure. 
     The advantage of using rail pressure is to provide the delay piston  12  with a variable hydraulic return spring force. As discussed above, the rail pressure  26  may be varied by the engine control microprocessor within a range of 500-4000 psi depending on load and speed conditions. With a spring, the delay piston will travel faster under higher rail pressure conditions. However, the overlap  18  then needs to be relatively large for a given time delay requirement. When rail pressure is used in accordance with this embodiment, the biasing force on the return pin  23  is also increased; hence the delay piston motion at higher rail pressure is slower than with a spring case and the overlap length can be designed to be relatively short and significantly reduce the size of the injector. 
     Delay Device Inside of the Intensifier 
     As shown in FIGS. 6 a - 6   d,  the delay device  10  includes a cylinder  58  that is defined inside of the intensifier piston  236  of the intensifier  204 . The delay piston  12  is translatably disposed in the cylinder  58 , thereby providing a packaging advantage for the parts comprising the delay device  10 . The embodiment of FIGS. 6 a,    6   b  is without rate shaping and the embodiment of FIGS. 6 c - 6   d  is with rate shaping. 
     In operation of the embodiment of FIGS. 6 a,    6   b,  the delay piston  12  starts to travel into the cylinder  58  of the intensifier piston  236  when the control valve  202  opens. The bottom  60  of the delay piston  12  is properly vented to ambient pressure by passageway  62 . The intensifier piston  236  stays at its top position (see FIG. 6 a ) in a waiting mode before the delay piston  12  travels the delay overlap distance  64  and the top surface of the delay piston  12  is flush with the surface  234  of the intensifier piston  236 . The delay piston  12  travels at a relatively high speed and quickly reaches its bottom seating position inside of intensifier piston  236 . Once the delay piston  12  is seated within the cylinder portion  58  of the intensifier piston  236  (FIG. 6 b ), the high pressure actuating fluid acts on both the surface  22  of the delay piston  12  and the surface  234  of the intensifier piston  236  to drive the intensifier piston  236  downward, compressing the fuel in chamber  230 . High intensifier actuating fluid pressure forces the delay cylinder to stay inside of the cylinder portion  58  of the piston  236  for the rest of the injection event. At the end of the injection event, the high pressure actuating fluid vents to the control valve (FIG. 6 b ) and the spring  14  is then free to return the delay piston  12  to the extended disposition of FIG. 6 a.    
     The embodiment of FIGS. 6 c,    6   d  includes both the delay feature of the delay device  10  and the rate shaping feature of the delay device  10 . The intensifier piston  236  stays at its top position (see FIG. 6 c ) in a waiting mode while the delay piston  12  travels the delay overlap distance  64 . During the time it takes for the delay piston  12  to travel the delay distance, no fuel injection is occurring. Further translation of the delay piston  12  acts to open the rate shaping passage  66 . As the delay piston  12  gradually opens the rate shaping flow passage  66 , very restricted flow to the intensifier  204  occurs as a result of the relatively small flow area of the rate shaping passage  66 . The limited flow of high pressure actuating fluid causes the intensifier piston  236  to start to move downward. The rate of travel of the intensifier is limited so that the rate of pressure increase of the fuel in the chamber  230  is also limited. Fuel injection commences, but the above noted limitations minimize the rate of increase of the rate of injection as compared to the prior art, thereby effecting rate shaping. Rate shaping occurs during the rate shaping overlap  64   a  until the delay piston  12  is seated downward in the cylinder  58 . 
     Once the delay piston  12  is seated within the cylinder portion  58  of the intensifier piston  236  (FIG. 6 d ), the high pressure actuating fluid acts on both the surface  22  of the delay piston  12  and the surface  234  of the intensifier piston  236  to drive the intensifier piston  236  downward, compressing the fuel in chamber  230 . High intensifier actuating fluid pressure forces the delay cylinder to stay inside of the cylinder portion  58  of the piston  236  for the main injection portion of the of the injection event. At the end of the injection event, the high pressure actuating fluid vents to the control valve (FIG. 6 d ) and the spring  14  is then free to return the delay piston  12  to the extended disposition of FIG. 6 a.    
     Delay Device Inside of the Intensifier With Pilot Injection 
     The embodiment of FIGS. 6 e,    6   f  includes both the delay feature of the delay device  10  and a pilot injection feature of the delay device  10 . The delay device  10  includes a piston  12  having a circumferential groove  70  defined in the piston, spaced apart form the top surface  22  by a land  71 . A flow passage  72  extends form the top surface  22  through the land  71  to the groove  70 . 
     In operation, high pressure actuating fluid in the passage  24  from the control valve  212  bears on the top surface  22  and flows through the flow passage  72  to pressurize the groove  70 . The piston  12  commences downward translation. The intensifier piston  236  stays at its top position (see FIG. 6 e ) in a waiting mode while the delay piston  12  travels the delay overlap distance  64 . During the time it takes for the delay piston  12  to travel the delay distance  64 , no fuel injection is occurring. Further translation of the delay piston  12  acts to open the passage  66  to the groove  70 . As the delay piston  12  gradually opens the flow passage  66 , very restricted flow to the intensifier  204  occurs as a result of the relatively small flow area of the rate shaping passage  66 . The limited flow of high pressure actuating fluid causes the intensifier piston  236  to start to move downward. The rate of travel of the intensifier is limited so that the rate of pressure increase of the fuel in the chamber  230  is also limited. Fuel injection commences, but the above noted limitations minimize the rate of increase of the rate of injection as compared to the prior art. Pilot injection commences at this point in the injection event. 
     Further downward translation of the piston causes the land  71  to seal the passage  66 . This momentarily terminates actuating pressure to the intensifier piston  236 , causing a pause in the translation of the intensifier passage  236 . Such pause substantially terminates injection until the land  71  passes the flow passage  66 . Pilot injection terminates with the aforementioned end of injection. 
     Once the delay piston  12  is seated within the cylinder portion  58  of the intensifier piston  236  (FIG. 6 f ), the high pressure actuating fluid acts on both the surface  22  of the delay piston  12  and the surface  234  of the intensifier piston  236  to drive the intensifier piston  236  downward, greatly compressing the fuel in chamber  230 , thereby commencing the main injection portion of the injection event. High intensifier actuating fluid pressure forces the delay cylinder  12  to stay inside of the cylinder portion  58  of the piston  236  for the main injection portion of the of the injection event. It should be noted that the skirt of the piston  12  may be trimmed so that the piston top surface  22  is flush with the surface  236 , as in FIG. 6 b.    
     At the end of the injection event, the high pressure actuating fluid vents to the control valve (FIG. 6 f ) and the spring  14  is then free to return the delay piston  12  to the extended disposition of FIG. 6 e.    
     While a number of presently preferred embodiments of the invention have been illustrated and described, it should be appreciated that the inventive principles can be applied to other embodiments falling within the scope of the following claims.