Fuel injector with actuation pressure delay device

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.

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 '562
 patent.
 Referring to the drawings, FIGS. 7 and 7a 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. 7a 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 440c
 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.
 7a) 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. 7a) 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.

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. 2a and
 2b show the delay device 10 schematic and its operation. FIG. 2a depicts
 the disposition of the delay device 10 for injection delay. FIG. 2b
 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. 2a) to its bottom position (FIG. 2b),
 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. 2a, 2b, 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 2a), 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. 2a) 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. 3b). 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. 2a) 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. 2b to its topmost position
 as depicted in FIG. 2a.
 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. 2a). 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 2a. 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. 2a.
 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 2a. 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. 2b) 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. 2a) 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. 2a). After the intensifier piston
 236 finishes its return, all components are back to the initial positions
 as depicted in FIGS. 1 and 2a.
 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. 5a (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. 5b. Because of earlier start
 of intensifier flow in the prior art injector, the start of injection is
 also very early. See FIG. 5c. 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.
 5a) and the start of injection (FIG. 5c). The longer delay helps to yield
 the smaller fuel delivery quantity as shown in the FIG. 5c. 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 '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. 3a, 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 22a.
 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. 3b, flow to the intensifier 204 starts slowly at
 beginning when the slot 50 first intersects the passageway 222 when the
 top surface 22a 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. 3c) is very desirable
 for engine drivability and emission control. As noted in FIG. 3d, 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. 3c, 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. 3e 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. 6a-6d, 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. 6a, 6b is without rate shaping and the
 embodiment of FIGS. 6c-6d is with rate shaping.
 In operation of the embodiment of FIGS. 6a, 6b, 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. 6a) 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. 6b), 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. 6b) and the spring 14 is then free to return the delay
 piston 12 to the extended disposition of FIG. 6a.
 The embodiment of FIGS. 6c, 6d 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. 6c) 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 64a 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. 6d), 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. 6d) and the spring 14 is
 then free to return the delay piston 12 to the extended disposition of
 FIG. 6a.
 Delay Device Inside of the Intensifier With Pilot Injection
 The embodiment of FIGS. 6e, 6f 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. 6e) 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. 6f), 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. 6b.
 At the end of the injection event, the high pressure actuating fluid vents
 to the control valve (FIG. 6f) and the spring 14 is then free to return
 the delay piston 12 to the extended disposition of FIG. 6e.
 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.