Abstract:
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. A fuel injector including a delay device. A method of controlling a fuel injection event, includes the steps of flowing an actuating fluid from the controller to an intensifier responsive to a pulse width command, pressurizing a volume of fuel by means of the intensifier, flowing a high pressure fuel from the intensifier to an injector nozzle, and interposing a delay in at least a portion of the flow of fuel to the injector nozzle.

Description:
RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Application No. 60/129,999 filed Apr. 19, 1999, and incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to fuel injectors for use with internal combustion engines and particularly with diesel engines. More particularly, the present invention relates to hydraulically actuated fuel injectors. 
     BACKGROUND OF THE INVENTION 
     Referring to the drawings, FIGS. 5 and 5 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 . 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-port  364  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 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 bore  386  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 position shown in FIG. 5 and a second position shown in FIG. 5 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  350  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 4140 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  438 ,  440  to be de-energized after the spool  420  is pulled into position. In this respect the control valve  418  operates in a digital manner, wherein the spool  420  is moved by a defined pulse that is provided to the appropriate solenoid  438 ,  440 . Operating the control valve  418  in a digital manner reduces the heat generated by the solenoids  438 ,  440  and increases the reliability and life of the injector  350 . 
     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  396  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  350  through the nozzle opening  372 . The actuating 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 7:1, wherein the pressure of the fuel discharged by the injector is between 2,000-28,000 psi. The fuel is discharged from the injector nozzle openings  372  and the first solenoid  438  is again energized to pull the spool  420  to the first position and the cycle is repeated. 
     The prior art HEUI injection system  350  has a relatively quick rise of the injection pressure after initiation of the injection event. As the intensifier piston  384  travels downward under the influence of the actuating fluid, injection pressure builds up very quickly. Under higher actuation fluid pressure (oil pressure), the injection pressure build-up process is abrupt, due to high acceleration of the intensifier piston  384 . With the high initial injection pressure of the HEUI injection system  350 , the initial rate of the injection is also relatively high and hence contributes to higher NOx emission in an internal combustion engine. As is known, high NOx emission is undesirable as a pollutant. With stringent emission regulations currently being imposed, there is a need in the diesel engine industry to control the initial injection rate so that a gradual rise or rate-shaped injection rate profile can be obtained and the NOx emissions may be favorably affected. 
     U.S. Pat. No. 5,492,098 presents an invention which improves HEUI injection by adding a spill port at bottom of the plunger. With some spilling of the high pressure fuel at the beginning of the injection, initial injection pressure rises more slowly, hence producing a rate shaping feature. However, due to the spilling of high injection pressure fuel, significant energy is lost to the low pressure fuel reservoir. This loss can not be recovered during the injection event. Such high energy loss is not desirable. It would be advantageous to provide for rate shaping of the rate of fuel injection without significant loss of fuel pressure energy. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to use a delay device to postpone or slow down the initial injection pressure build up while retaining high fuel pressure energy. With slow initial pressure rising in the injection nozzle chamber, rate shaping can be obtained and controllability of small pilot injection is improved. 
     Advantages of the present invention are as follows: 
     Placing a delay device between pressure generation chamber (plunger chamber) and nozzle chamber allows delay of the initial injection pressure rise and tailoring the amount of rate shaping before the main injection event commences. A slow and controllable fuel pressure rise during the initial portion of the injection event is very critical to the precision control of the initial small quantity fuel delivery, especially during a pilot injection mode. Such control further provides repeatability between injection events. 
     This delay device can be applied to any fuel injection system and specifically is not limited to the HEUI injection system. 
     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. The present invention is further a fuel injector including a delay device. Additionally, the present invention is a method of controlling a fuel injection event, includes the steps of sending a pulse width command to a controller to define an injection event, flowing an actuating fluid from the controller to affect an intensifier responsive to reception of the pulse width command, pressurizing a volume of fuel by means of the intensifier, flowing a high pressure fuel from the intensifier to an injector nozzle, and interposing a delay in at least a portion of the flow of fuel to the injector nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of an injector incorporating the delay control means of the present invention, the control portion of the injector being shown schematically; 
     FIG. 2 is an enlarged, sectional view of the present invention as depicted in FIG. 1; 
     FIG. 2 a  is a sectional view of the present invention prior to injection commencement; 
     FIG. 2 b  is a sectional view of the present invention during pilot injection; 
     FIG. 2 c  is a sectional view of the present invention during main injection; 
     FIG. 3 a  is a sectional view of a further embodiment of the present invention during pilot injection; 
     FIG. 3 b  is a sectional view of the embodiment of FIG. 3 a  during main injection; 
     FIG. 3 c  is a sectional view of the present invention depicted in the circle  3   c  of FIG. 3 b;    
     FIG. 4 a  is a sectional view of another embodiment of the present invention prior to pilot injection; 
     FIG. 4 b  is a sectional depiction of the present invention as depicted in FIG. 4 a  during main injection; and 
     FIG. 5 is a sectional view of a prior art fuel injector; 
     FIG. 5 a  is a sectional view of a prior art fuel injector electrically actuated controller; 
     FIG. 6 is a sectional view of an injector with an embodiment of the present invention having rate shaping features; 
     FIG. 6 a  is a sectional view of the delay device of FIG. 6 taken along the circle  6   a;    
     FIG. 6 b  is a sectional view of the delay device of FIG. 6 a  during main injection. 
     FIG. 7 a  is a sectional view of an alternative embodiment of the delay device depicted in the closed disposition; and 
     FIG. 7 b  is a sectional view of the delay device of FIG. 6 a  during main injection. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary HEUI injector incorporating the present invention is shown generally at  10  in FIG.  1 . It is understood that other fuel injectors may also incorporate the present invention. The delay control device  12  of the present invention is installed between the intensifier plunger chamber  14  and the nozzle chamber  16 . In a preferred embodiment, the delay control device  12  comprises a delay cylinder  18  and a delay cylinder housing  20 , in conjunction with associated fluid passageways, as will be described. The operation of the delay control device  12  is basically such that high pressure fuel flows from the plunger chamber  14  to the nozzle chamber  16  through two different paths, the pilot path  22  and the main path  24 . The pilot path  22  is open at all times between the plunger bottom chamber  34  and the nozzle chamber  16 . However, the pilot path  22  is relatively restrictive, having a flow area that is less than about 10% of the main path  24 . The amount of high pressure fuel flow through the pilot path  22  to the nozzle chamber  16  is therefore relatively limited. The significant fuel flow to the nozzle chamber  16  occurs only when the main path  24  opens up. The main path  24  opening and closing is controlled by the position of the delay cylinder  18  of the delay device  12 . 
     The delay cylinder  18  is translatable between two positions; a closed position, as depicted in FIG. 2 a,  and an open position, as depicted in FIG. 2 c.  Interim positions of the delay cylinder  18  are depicted in FIGS. 2 and 2 b.  The main path  24  of high pressure fuel is blocked when the lower portion  27  of the delay cylinder  18  closes the fuel path between the upper main path  24   a  and the lower main path  24   b.  This occurs when the delay cylinder  18  is at its topmost position (FIG. 2 a ) and in the interim positions (FIGS. 2 and 2 b ). The main path  24  is fully open when delay cylinder  18  is at its bottom stop  28  position (FIG. 2 c ), where the groove  26  (defined in the body of the delay cylinder  18 ) fully opens the upper main path  24   a  to the lower main path  24   b.    
     The delay cylinder  18  has two opposed pressure surfaces  30 ,  32 . The top surface  30  is exposable to high pressure fuel in the control chamber  34  and the bottom surface  32  forms in part a reservoir  39  and is exposable to venting pressure in the low pressure fuel passageway  36 . The venting pressure is at the same pressure as low pressure fuel reservoir  38  pressure of FIG.  1 . As the intensifier plunger  40  moves downwards, pressure under the plunger  40  in the chamber  14  builds up and a small amount of high pressure fuel flows into the delay cylinder control chamber  34  via the control chambers orifice  52  (see FIG.  2 ). 
     The delay cylinder spring  42  acting upward on the delay cylinder  18  is relatively weak. Accordingly, the delay cylinder  18  starts to move downward virtually as soon as the pressure in the control chamber  34  rises (See FIG. 2 b ). As the delay cylinder  18  travels downward, the delay cylinder  18  gradually passes the delay overlap  44  and gradually opens up the main path  24 , connecting upper main path  24   a  to lower main path  24   b.  The delay overlap  44  is the distance from the bottom margin  46  of the groove  26  to the top  48  margin of the main path  24  prior to commencing the downward stroke of the delay cylinder  18 . See FIG. 2 a.    
     Once the main path  24  is open, fuel flow from the plunger chamber  14  to the nozzle chamber  16  will have a rate that is typical of the prior art injector  350 . The opening of the main fuel flow path  24  is delayed from the initiation of the flow of the high pressure actuating fluid to the intensifier plunger  40  as controlled by the control valve  50 . The delay is equal to the amount of time it takes the delay chamber  18  to travel from its topmost disposition to decrease the overlap amount  44  to zero where the groove  26  commence opening the main path  24 . The amount of the delay overlap  44  may be adjusted to fit specific injection system needs by adjusting the distance of the delay overlap  44  during manufacture of the injector. Such adjustment, for example, may be made by increasing the distance from the bottom  46  of the groove  26  to the top  48  (point of intersection with) of the main flow path  24 . The delay time may be further adjusted by changing the area of the top pressure surface  30 , or by changing the flow area of control chamber orifice  52 , or changing the flow area of the drain orifice  54 . 
     The control chamber orifice  52  extends between the high pressure fuel chamber  14  and delay cylinder control chamber  34 . The purpose of this orifice  52  is to control the rate of the fuel pressure rising within the control chamber  34 . The orifice  52  is used to control the speed of delay cylinder  18  motion by throttling the admission of high pressure fuel to the control chamber  34 . If the orifice  52  is relatively large, the delay cylinder  18  moves very fast and main path  24  opening delay becomes nearly negligible. A smaller orifice  52  throttles the high pressure fuel to the control chamber  34 , thereby reducing the speed of the downward motion of the delay cylinder  18 . The pressure inside of control chamber  34  is preferably lower than the fuel pressure at plunger chamber  14  due to the throttling effect of the orifice  52 . As indicated above, the throttling is effected by the relatively small flow area of orifice  52 . A lower pressure in the control chamber  34  allows the delay cylinder  18  to move downward with a slower, more controllable and more desirable velocity. 
     A drain orifice  54  is at the venting (lower) side of the delay cylinder  18  and is fluidly coupled to the bottom pressure surface  32 . The orifice  54  is used to vent fuel pressure to the low pressure fuel reservoir  38  when the delay cylinder  18  is moving downward. This orifice  54  purposely restricts the venting process so that the delay cylinder  18  downward motion is damped. Such damping slows down the delay cylinder  18  opening process (FIGS. 2 a  to  2   c ). Varying the flow area of the orifice  54  as desired varies the amount of damping of the delay cylinder  18  and has a direct effect on the duration of the delay time. 
     The delay cylinder spring  42  is primarily used to return the delay cylinder  18  to its topmost position (FIG. 2 a ) at the end of the injection event after the previously described downward motion of the delay cylinder  18 . Accordingly, the spring  42  has a relatively weak spring constant. As long as there is a higher pressure in the control chamber  34  acting downward on the delay cylinder  18  than the pressure in the low pressure fuel reservoir  38  (FIG. 1) pressure (preferably about 50 psi), the delay cylinder  18  will stay at its bottom stop position. Such downward pressure on top pressure surface  30  overcomes the upward bias of the spring  42 . Therefore, the closing of the main path  24  can occur at very end of the injection event when the pressure in the control chamber  34  drops to near the pressure in the low pressure fuel reservoir  38  (which is the pressure in reservoir  39 ). With substantially equal fuel pressure acting on both surfaces  30 ,  32 , the spring  42  is free to return the delay piston  18  to its retracted initial disposition as noted in FIG. 2 a.  The delaying effect of the delay cylinder  18  therefore only occurs at the initial portion of each injection event as described below. 
     The pilot path  22  connects intensifier plunger chamber  14  to the lower main path  24   b  and to the nozzle chamber  16 . The pilot path  22  is used to allow a limited amount of high pressure fuel flow to the nozzle chamber  16  of the needle valve  60  before the main path  24  flow path opens to admit the high pressure fuel for the main fuel injection event. This small amount of initial flow to the nozzle chamber  16  acts to open the needle valve  60  a small amount to permit a small amount of initial fuel injection to occur and provides a rate shaped feature to the injection system prior to main injection. Varying the flow area of the pilot path  22  as desired affects the volume of high pressure fuel flow through the pilot path  22  and therefore affects the rate shaping of the injection event as desired to fit particular application needs. 
     Description of the Operation 
     Operation may be appreciated with reference to FIGS.  1  and  2 - 2   c.  Before the injection event starts, the injector control valve  50  is at its closed position and the intensifier plunger  40  is at its topmost position. The fuel pressure in the passageway  36 , the chamber  14 , the control chamber  34 , the reservoir  39 , and at orifice  54  is all at the same pressure, such pressure being the pressure in the low pressure fuel reservoir  38 . This pressure is about 50 psi. The delay cylinder  18  of the delay control device  12  is at its topmost position (FIG. 2 a ) due to the upward bias of the spring  42 . Initially, the fuel pressure on both surfaces  30 ,  32  of the delay cylinder  18  is balanced so that the upward bias of the spring  42  alone is affecting the delay cylinder  18  position. The needle valve  60  is also closed under the influence of the spring  62 . 
     Initiation of the injection event is controlled by the control valve  50 . As the control valve  50  opens, high pressure actuation fluid from an engine associated high pressure actuation fluid rail  51  flows, at a pressure ranging from 500-3500 psi, into intensifier piston chamber  64  and drives the intensifier plunger  40  downwards against the bias of the return spring  66 . Fuel pressure under intensifier plunger  40  in the chamber  14  builds up due to compression of the fuel effected by the force exerted by the high pressure actuation fluid acting on the plunger  40 . 
     A small amount of the increasing pressure fuel flows through the pilot path  22  to the lower main path  24   b  and then further down to the nozzle chamber  16 . See FIG. 2 b.  Since the flow volume through the pilot path  22  is very small, the injection pressure at nozzle chamber  16  rises relatively slowly. Such pressure acts to generate an upward directed force on the needle valve  60  and the needle valve  60  is opened only a small amount to permit a small amount of fuel to be injected from orifices  61 . Such small injection may be either pilot injection or rate shaping as desired. 
     At the same time as the pilot injection or rate shaping noted above, a small amount of fuel flows into the delay cylinder control chamber  34  through the orifice  52 . The delay cylinder  18  moves downward at a controlled rate against the bias of the spring  42 . Since there is offset (delay overlap  44 ) between the delay cylinder groove edge  46  and the top  48  of main path bore  24 , the main path  24  does not start to open until the travel of the delay cylinder  18  is more than the amount of the overlap  44 . The opening of the main path is delayed by the time it takes for the travel of the delay cylinder  18  to reduce the overlap  44  amount to zero, which occurs the point where the groove  26  commences to intersect the main path  24 . 
     The main path  24  then starts to open gradually as the groove increasingly intersects the main path  24  after the delay cylinder  18  passes the overlap  44 . As soon as the main path  24  begins to open, a significant amount of high pressure fuel flows to the nozzle chamber  16  and causes the needle valve  60  to open fully, resulting in the main injection event. The delay cylinder  18  continues downward until the main path  24  is fully opened as indicated in FIG. 2 c.    
     The end of the injection event is also controlled by the control valve  50 . The control valve  50  closes to cause the end of the injection event. At such closing, the actuation fluid is vented to ambient pressure at the low pressure reservoir  66 . The intensifier plunger  40  starts to return to its top stop position and the injection pressure in the main path  24  available to the needle valve  60  decays. As injection pressure drops, the needle valve  60  is closed by the spring  62 . The refill check valve ball  68  starts to open to refill the chamber  14 . During the refilling process, the fuel pressure at top surface  30  of the delay cylinder  18  is same (balanced) as the pressure at the bottom surface  32  (about 50 psi fuel reservoir  38  pressure). The delay cylinder spring  42  now starts to push the delay cylinder  18  upward to return the delay cylinder  18  to top stop position (FIG. 2 a ) to complete the injection cycle. 
     It should be noted that the delay cylinder spring  42  has a very small initial load and spring rate. This allows the delay cylinder  18  to stay at its bottom disposition until the pressure in the control chamber  34  goes substantially low during the end of an injection event. This feature is desirable for dwell control of a split injection event when the control valve makes two round trips. Although the first injection (pilot injection) is delayed, the main injection will not be delayed which causes an increase of dwell time between the pilot injection and the main injection. 
     Alternative Preferred Embodiments 
     Push Pin Design 
     This further preferred embodiment of the delay control means  12  is used to minimize the total amount of fuel used during retraction of the delay piston  18 , as indicated in FIGS. 3 a - 3   c.  As the delay piston  18  moves downward (translating between the position of FIG. 3 a  to the position of FIG. 3 b ), the delay piston  18  creates displacement in the control chamber  34  and therefore requires some additional amount of the fuel to fill the control chamber  34 . It is very desirable that this amount of the fuel should be minimized for energy efficiency concerns. Fuel used to drive the delay piston  18  is not available for injection into the engine combustion chamber. A small pin  70  is used to push the delay cylinder  18  during the downward opening process. This pin  70  can be designed much smaller than is possible with the control chamber  34  of the above embodiment of FIGS.  2 . Accordingly, the volume of the control chamber  34  is minimized and hence the amount of fuel used to cause translation of the delay piston  18  is substantially smaller. This increases the volume of fuel available for injection by needle valve  60 . Referring to FIG. 3 c,  there is a drain hole  72  at center of the delay cylinder. Together with the transverse slot  74  at bottom of the pin  70 , the drain hole  72  balances the pressure on both sides of the delay cylinder  18 . 
     Delayed Pilot Hole Design 
     Referring to FIGS. 4 a  and  4   b,  the pilot hole  80  of the pilot path  22  draws fuel from the delay cylinder control chamber  34 . The pilot hole  80  is covered by the delay cylinder  18  when delay cylinder  18  is at topmost position. See FIG. 4 a.  As the delay cylinder  18  travels downward, the pilot hole  80  is uncovered and exposed to the fuel under pressure in the chamber  34 . The uncovering occurs prior to the opening of the main path  24 . This is evident in FIG. 4 b.  The distance between pilot hole  80  and main path  24  defines the amount of rate shaping that will occur before the main injection event occurs. Rate shaping occurs during the time that the pilot path  22  alone is supplying fuel to the needle valve. Such fuel flow in the pilot path  22  commences only after the pilot hole  80  is uncovered and continues as the only source of fuel to the needle valve  60  until the groove  26  of the delay cylinder  18  intersects the main path  24 , at which time the main injection event commences. 
     Spool Cylinder Design 
     A further embodiment of the present invention is depicted in FIGS. 6,  6   a,  and  6   b.  The injector of FIG. 6 is a HEUI type injector substantially as described with respect to the prior art injector  350  of FIGS. 5 and 5 a.    
     Ignoring the delay device  10  of the present invention, the injector  200  has four main components: control valve  202 , intensifier  204 , nozzle  206 , and injector housing  208 . The injector housing  208  may be formed of several components such as housing  208   a,  housing  208   b,  or be made as a unitary housing. 
     The control valve  202  initiates and ends an injection event. The control valve  202  has a spool valve  210  and an electric control  212  for shifting the spool valve  210  from a right closed disposition to a left open disposition and return to the right closed seat. The spool valve  210 , responsive to electric inputs, ports high pressure actuating fluid to and from the intensifier  204 . 
     To begin injection, a solenoid of the electric control  212  is energized, moving the spool valve  210  from its right closed seat to its left open seat. This action admits high pressure actuating fluid via internal passages (not shown) to the piston chamber  223  of the intensifier  204 . As will be seen, absent the delay device  10 , fuel injection commences substantially simultaneously with the porting of the high pressure actuating fluid to the intensifier  204  and continues until a solenoid of the electric control  212  is energized and the spool valve  210  is shifted rightward to its right closed seat. Actuating fluid and fuel pressure within the injector  200  then decrease as spent actuating fluid is discharged from injector  200  by the spool valve  210 . Such discharge is typically to the valve cover area of the engine, which is at ambient pressure. 
     The center segment of the injector  200  includes the intensifier  204 . The intensifier  204  includes a preferably unitary device comprising the hydraulic intensifier piston  236  and plunger  228 , in addition to the fuel chamber  230  and the plunger return spring  232 . 
     Intensification of the fuel pressure to a desired injection pressure level is accomplished by the ratio of areas between the upper surface  234  of the intensifier piston  236 , acted on by the high pressure actuating fluid, and the lower surface  238  of the plunger  228 , acting on the fuel in the chamber  230 . The intensification ratio can be tailored to achieve desired injection characteristics. Fuel is admitted to chamber  230  through the passageway  240  past check valve  242 . Injection begins as the high pressure actuating fluid is supplied to the upper surface  234  of the intensifier piston  236 , driving the intensifier piston  236  downward to compress the fuel in chamber  230 . 
     As the intensifier piston  236  and plunger  228  move downward responsive to the force exerted by the high pressure actuating fluid, the pressure of the fuel in chamber  230  below the plunger  228  rises dramatically. Absent the delay device  10  of the present invention, the chamber  230  is directly fluidly coupled to the passageway  244 . High pressure fuel from the chamber  230  flows through the passageway  244  to act upwardly on the needle valve surface  248 . The upward force on the surface  248  overcomes the bias of the needle valve spring  256  and opens the needle valve  250 . Fuel is then discharged from the orifices  252  into the combustion chamber of the engine. The intensifier piston  236  continues to move downward and compressing the fuel in chamber  230  until a solenoid of the electric control  212  is energized causing the spool valve  210  to shift rightward to its closed right seat. In such disposition, the high pressure actuating fluid bearing on the surface  234  is discharged from the injector  200  to ambient pressure. At this point, the plunger return spring  232  returns the piston  236  and plunger  228  to their initial upward seated position. As the plunger  228  returns upward, the plunger  228  draws replenishing fuel into the plunger chamber  230  across the ball check valve  242 . 
     The nozzle  206  is typical of other diesel fuel system nozzles. Fuel is supplied to the nozzle orifices  252  through internal passages  244 . As indicated above, the dramatic rise in fuel pressure to the nozzle needle  250  acts to lift to the needle  250  to the open position, thereby allowing fuel injection to occur through orifices  252 . As fuel pressure decays at the end of the injection event, responsive to the rightward shift of the spool valve  210 , the spring  256  returns the nozzle needle  250  to its upward closed disposition. 
     The imposition of the delay device  10  in the injector  200  has a dramatic effect on the aforementioned injection process as will be described in greater detail below. As best shown in FIG. 6 a  and  6   b,  the delay device  10  includes the following components: piston assembly  300  and flow passage assembly  302 . The flow passage assembly  302  includes a cylinder  304  defined in the housing  306 . Cylinder  304  has a drain passage  308  defined proximate the lower margin of the cylinder  304 . The drain passage  308  is typically vented exterior of the injector  200  to fuel supply pressure (50 psi). The drain passage  308  is preferably defined between the housing  306  and the delay cylinder stop  310 . The delay cylinder stop  310  has a generally circular spring retainer groove  312  defined therein. 
     The delay piston assembly  300  includes a delay piston  314  translatably disposed within the cylinder  304 . The delay piston  314  is biased to the upward disposition as depicted in FIG. 6 a  by a return spring  316 . The return spring  316  resides in an axial chamber  318  defined within the delay piston  314 . A distal end of the return spring  316  is captured within the spring retainer groove  312 . 
     The delay piston  314  has a top surface  320  that is exposable to high pressure fuel. The top surface  320  has a centrally disposed return orifice  322  defined therein. The return orifice  322  extends between top surface  320  and the axial chamber  318 . A circumferential groove  324  is defined around the body of the delay piston  314 . The groove  324  is spaced apart from the top surface  320 . The delay piston  314  further has a lower margin  312 . As depicted in FIG. 6 b,  the lower margin  312  is in contact with the delay cylinder stop  310  in the fully open disposition of the delay piston  314 . 
     The flow passage assembly  302  further includes a plurality of flow passages as will be described. The first such flow passage is the control chamber orifice  328 . The control chamber orifice extends between the plunger chamber  230  and the cylinder  304 . High pressure fuel flowing from the plunger chamber  230  through the control chamber orifice  328  bears on the top surface  320  of the delay piston  314 . 
     The main path  330  has a substantially larger flow passageway than the control chamber orifice  328 . The main path  330  is also fluidly connected to the plunger chamber  230  and is defined at least in part in the housing  306  alongside the delay piston  314 . The main path  330  is defined in part through the delay cylinder stop  310  and in part in the housing  306 . The main path  330  is fluidly coupled to an upper groove  332  that is also defined in the housing  306 . The upper groove  332  is circumferential about the center axis of the delay piston  314 . The upper groove  332  intersects and is fluidly coupled to the cylinder  304 . A second groove, the lower groove  334  is spaced apart from and immediately beneath the upper groove  332 . Like the upper groove  332 , the lower groove  334  is defined in the housing  306  circumferential to the delay piston  314 . The lower groove  334  intersects the cylinder  304 . 
     Where rate shaping is desired, a relatively small area pilot path  336  is defined in the housing  306  extending between and fluidly coupling the upper groove  332  and the lower groove  334 . It is understood that where delay alone is desired, the pilot path  336  would not be included. As will be seen, the delay overlap  338  is defined between the lower margin of the groove  324  and the upper margin of the lower groove  334 . 
     Operation of the delay device  10  may be appreciated with reference to FIGS. 6 a  and  6   b.  FIG. 6 a  shows the delay piston  314  at its uppermost disposition within the cylinder  304 . This position is the position and defines the status prior to initiation of the injection event. The lower groove  334  is substantially sealed by the wall of the delay piston  314 . Accordingly, fuel may flow from the upper groove  332  to the lower groove  334  only through the pilot path  336 . The drain passage  308  is fully open. 
     Upon initiation of the injection event by the control valve  202 , high pressure actuating fluid is ported to the intensifier  204 . The plunger  228  starts downward dramatically compressing the fuel in the plunger chamber  230 . The high pressure fuel flows through the control chamber orifice  328  to bear upon the top surface  320  of the delay piston  314  and thereby to commence downward translation of the delay piston  314 . 
     Simultaneously, high pressure fuel flows through the main path  330 , the upper groove  332 , and the pilot path  336 . The limited amount of high pressure fuel passing through the pilot path  336  flows through the lower groove  334  to the passageway  244 . This limited amount of high pressure fuel acts to open the needle valve  250  to slightly open the orifices  252 , resulting in the injection of a very limited amount of fuel into the compression chamber. The limited amount of fuel injected results in a gradual ramping of the rate of injection into the combustion chamber, comprising the desired rate shaping of the leading edge of the main injection event. 
     It should be understood that by not including the optional pilot path  336 , no injection occurs during the aforementioned described period of delay. In such event, no high pressure fuel is admitted to the flow passageway  244  until the delay cylinder  314  completes the transition through the delay overlap  338 . 
     When the delay piston  314  translates downward enough to complete the translation through the region of the delay overlap  338 , the groove  324  defined in the delay piston  314  intersects both the upper groove  332  and the lower groove  334  permitting full flow of high pressure fuel from the plunger chamber  230  to the fuel passage  244  to fully open the needle valve  250 , resulting in the main injection portion of the injection event. The delay piston  314  continues downward under the influence of the force generated on the top surface  320  by the high pressure fuel until the lower margin  326  comes into contact with the delay cylinder stop  310  as depicted in FIG. 6 b  At this lower disposition, drain passage  308  is completely blocked by the delay piston body  314 . 
     Termination of the injection event is commanded by the control valve  202 . An electric signal to the control valve  202  shifts the spool valve  210  from the left open seat to the right closed seat. Such shifting vents the high pressure actuating fluid from the injector  200 . The intensifier  204  ceases to pressurize fuel in the plunger chamber  230 . The plunger  228  commences its upward travel. At this point, the delay piston  314  commences its upward travel from the lower open seat of FIG. 6 b  to the upper closed seat of FIG. 6 a.  Such translation is effected by the bias generated on the delay piston  314  by the return spring  316 . As the delay piston  314  translates upward, fuel captured within the cylinder  304  above the delay piston  314  passes through the return orifice  322  and out the drain passage  308 . The delay piston  314  continues upward until the top surface  320  is seated on the underside of the spacer  313  as depicted in FIG. 6 a.    
     The control chamber orifice  328  has a significant effect on the motion of the delay piston. If the control chamber orifice  328  is extremely small, the motion of the delay piston  314  will be very slow resulting in a longer delay time. The delay piston return spring  316  is relatively weak So that return of the delay piston occurs only when the pressure in the plunger chamber  230  decays nearly to the fuel supply pressure level (50 psi). 
     A further embodiment of the present invention is depicted in FIGS. 7 a  and  7   b.  The concept of the delay device of FIGS. 7 a  and  7   b  is similar to the embodiment described above with respect to FIGS. 6 a  and  6   b  and may be readily installed in the injector  200  of FIG.  6 . Accordingly, like numbers in the FIGS. 7 a  and  7   b  denote like components in FIGS. 6 a  and  6   b.  The delay device  10  includes components piston assembly  300  and flow passage assembly  302 . 
     The flow passage assembly  302  includes a cylinder  304  defined in the housing  306 . Cylinder  304  has a drain passage  308  defined proximate the lower margin of the cylinder  304 . The drain passage  308  is typically vented exterior to the injector  200  to fuel supply pressure. The drain passage  308  is preferably defined between the housing  306  and the delay cylinder stop  310 . The delay piston stop  310  has a generally circular spring retainer groove  312  defined therein. 
     The piston assembly  300  includes a delay piston  314  translatably disposed within the cylinder  304 . The delay piston  314  is biased in the upward disposition as depicted in FIG. 7 a  by a return spring  316 . The return spring  316  is concentrically disposed with respect to a depending cylinder  318  of the delay piston  314 . 
     The delay piston  314  has a top surface  320  that is exposable to high pressure fuel. The top surface  320  has a centrally disposed inlet orifice  321  defined therein. The inlet orifice  321  extends between top surface  320  and a circumferential groove  324  that is defined around the body of the delay piston  314 . The groove  324  is spaced apart from the top surface  320 . The delay piston  314  further has a lower margin  312 . As depicted in FIG. 7 b,  the lower margin  312  is in contact with the delay cylinder stop  310  in the fully open disposition of the delay piston  314 . 
     The flow passage assembly  302  further includes a plurality of flow passages as will be described. The first such flow passage is the main path  330 . The upper main path  330   a  is fluidly connected to the plunger chamber  230  and the lower main path  334  is fluidly connected to the passage  244  to the nozzle orifices  252 . The upper main path  330   a  is fluidly coupled to an upper path extension  332  that is also defined in the housing  306 . The upper path extension  332  is intersects and is fluidly coupled to the groove  324  in the piston  314  and thence through an inlet orifice  350  to the inlet  321 . The size of inlet orifice  350  can be varied to adjust the velocity of the delay piston  314 . A second lower path extension  334  is spaced apart from and immediately beneath the upper path extension  332 . The lower path extension  334  intersects the cylinder  304 . An axially symmetric drilled passage  334   a  is placed on the other side from extension  334  to reduce the hydraulic side loading on the delay piston since the hydraulic pressure in passages  334  and  334   a  are always the same. 
     Where rate shaping is desired, a relatively small flow area pilot path  336  is defined in the housing  306  extending between and fluidly coupling the upper main path  330   a  and the lower path extension  334 . It is understood that where delay alone is desired, the pilot path  336  would not be included. As will be seen, the delay overlap  338  is defined by the width of a land  337  of the delay piston  314  that, in FIG. 7 a,  spans the gap between intersections with the cylinder  304  respectively of the upper path extension  324  and the lower path extension  334 . 
     Operation of the delay device  10  may be appreciated with reference to FIGS. 7 a  and  7   b.  FIG. 7 a  shows the delay piston  314  at its uppermost disposition within the cylinder  304 . This position is the position and defines the status prior to initiation of the injection event. The lower path extension  334  is substantially sealed from the upper path extension by the land defining the delay overlap  338 . Accordingly, fuel may flow from the chamber  230  in the injector  200  (see FIG. 6) through the upper main path  330   a,  the upper path extension  332  and to the inlet  321  to bear on the surface  320 . Simultaneously, high pressure fuel may flow from the upper main path  330   a  through the pilot path  336  to the lower main path  330   b  and thence to the orifices  252  for pilot injection. The drain passage  308  is fully open. 
     Upon initiation of the injection event by the control valve  202 , high pressure actuating fluid is ported to the intensifier  204 . The plunger  228  starts downward dramatically compressing the fuel in the plunger chamber  230  and providing high pressure fuel to the upper main path  330   a.  The high pressure fuel flows through the inlet  321  to bear upon the top surface  320  of the delay piston  314  and thereby to commence downward translation of the delay piston  314 . 
     Simultaneously, high pressure fuel flows through the main path  330   a  and the pilot path  336 . The limited amount of high pressure fuel passing through the restricted flow area of the pilot path  336  flows through the lower path extension  334  and the lower main path  330   b  to the passageway  244 . This limited amount of high pressure fuel acts to open the needle valve  250  to slightly open the orifices  252 , resulting in the injection of a very limited amount of fuel into the compression chamber. The limited amount of fuel injected results in a gradual ramping of the rate of injection into the combustion chamber, comprising the desired rate shaping of the leading edge of the main injection event. 
     It should be understood that by not including the optional pilot path  336 , no injection occurs during the aforementioned described period of delay. In such event, no high pressure fuel is admitted to the flow passageway  244  until the delay cylinder  314  completes the transition through the delay overlap  338 . 
     When the delay piston  314  translates downward enough to complete the translation through the region of the delay overlap  338 , the groove  324  defined in the delay piston  314  intersects both the upper path extension  332  and the lower path extension  334  permitting full flow of high pressure fuel from the plunger chamber  230  to the fuel passage  244  to fully open the needle valve  250 , resulting in the main injection portion of the injection event. The delay piston  314  continues downward under the influence of the force generated on the top surface  320  by the high pressure fuel until the lower margin  312  comes into contact with the piston stop  310  as depicted in FIG. 7 b.    
     It should be understood that by adjusting the length of the overlap  338 , the size of the inlet orifice  350 , and/or the size of the pilot passage  336 , different rate shaping effects can be obtained. The optimum combination will be determined empirically from engine performance testing. 
     Termination of the injection event is commanded by the control valve  202 . An electric signal to the control valve  202  shifts the spool valve  210  from the left open seat to the right closed seat. Such shifting vents the high pressure actuating fluid from the injector  200 . The intensifier  204  ceases to pressurize fuel in the plunger chamber  230 . The plunger  228  commences its upward travel. At this point, the delay piston  314  commences its upward travel from the lower open seat of FIG. 7 b  to the upper closed seat of FIG. 7 a.  Such translation is effected by the bias generated on the delay piston  314  by the return spring  316 . As the delay piston  314  translates upward, fuel captured within the cylinder  304  above the delay piston  314  passes through the inlet orifice  321  and out the drain passage  308 . The delay piston  314  continues upward until the top surface  320  is seated on the underside of the spacer  313  as depicted in FIG. 7 a.    
     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.