Patent Publication Number: US-6705543-B2

Title: Variable pressure fuel injection system with dual flow rate injector

Description:
TECHNICAL FIELD 
     This invention relates to an improved fuel injection system and fuel injector which effectively controls the flow rate of fuel injected into the combustion chamber of an engine. 
     BACKGROUND OF THE INVENTION 
     In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-needle injector which includes a needle assembly having a spring-biased needle valve element positioned adjacent the needle orifices for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The needle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The needle valve is positioned in a needle cavity and biased by a needle spring to block fuel flow through the needle orifices. In many fuel systems, when the pressure of the fuel within the needle cavity exceeds the biasing force of the needle spring, the needle valve element moves outwardly to allow fuel to pass through the needle orifices, thus marking the beginning of injection. In another type of system, such as disclosed in U.S. Pat. No. 5,676,114 to Tarr et al., the beginning of injection is controlled by a servo-controlled needle valve element. The assembly includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve. U.S. Pat. No. 5,463,996 issued to Maley et al. discloses a similar servo-controlled needle valve injector. 
     Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. As a result, many proposals have been made to provide injection rate control devices in closed needle fuel injector systems. One method of controlling the initial rate of fuel injection is to spill a portion of the fuel to be injected during the injection event. For example, U.S. Pat. No. 5,647,536 to Yen et al. discloses a closed needle injector which includes a spill circuit formed in the needle valve element for spilling injection fuel during the initial portion of an injection event to decrease the quantity of fuel injected during this initial period thus controlling the rate of fuel injection. A subsequent unrestricted injection flow rate is achieved when the needle valve moves into a position blocking the spill flow causing a dramatic increase in the fuel pressure in the needle cavity. However, the needle valve is not servo-controlled and, thus, this needle assembly does not include a control volume for controlling the opening and closing of the needle valve and the timing of injection at least primarily fuel pressure dependent. 
     Another manner of optimizing combustion is to create pilot and/or post injection events. Most current diesel injectors include fixed needle orifice areas sized to provide optimum injection duration at rated speed and load with the highest allowable injection pressure. However, in order to optimize combustion, pilot and post injection events must include extremely small quantities of fuel at high injection pressures. With a fixed spray orifice size, this results in an extremely short event that is difficult to control. To compensate, the needle opening velocity may be reduced so that the fuel flow is throttled before the spray orifices during the pilot and post injection events. However, needle velocity is not easily controllable from injector to injector, while throttling wastes fuel energy and does not provide optimum combustion performance. At low speed and light load, it is also desirable to have small spray orifices to increase injection duration without lowering injection pressure. 
     Another fuel injector design providing some limited control over fuel injection rate and quantity includes two needle valve elements for controlling the flow of fuel through respective sets of injection orifices. For example, U.S. Pat. No. 5,458,292 to Hapeman discloses a fuel injector with inner and outer injector needle valves biased to close respective sets of spray holes and operable to open at different fuel pressures. The inner needle valve is reciprocally mounted in a central bore formed in the outer needle valve. However, the opening of each needle valve is controlled solely by injection fuel pressure acting on the needle valve in the opening direction such that the valves necessarily open when the injection fuel pressure reaches a predetermined level. Consequently, the overall and relative timing of opening of the valves, and the rate of opening of the valves, cannot be controlled independently. Moreover, the valve opening timing and rate is dependent on the injection fuel pressure. 
     U.K. Patent Application No. 2266559 to Hlousek discloses a closed needle injector assembly including a hollow needle valve for cooperating with one valve seat formed on an injector body to provide a main injection through all the injector orifices and an inner valve needle reciprocally mounted in the hollow needle for creating a pre-injection through a few of the injector orifices. However, the valve seat allowing the inner valve needle to block the pre-injection flow is formed on the hollow valve member and the inner valve needle is biased outwardly away from the injector orifices. This arrangement requires a third valve seat for cooperation with the inner valve element when in a pre-injection open position to prevent flow through all of the injector orifices, resulting in an unnecessarily complex and expensive assembly. Also, this assembly is designed for use with two different sources of fuel requiring additional delivery passages in the injector. In addition, like Hapeman, this design requires the timing and rate of opening of at least one of the needle valves to be controlled by fuel injection pressure thereby limiting injection control. 
     U.S. Pat. No. 5,199,398 to Nylund discloses a fuel injection valve arrangement for injecting two different types of fuels into an engine which includes inner and outer poppet type needle valves. During each injection event, the inner needle valve opens a first set of orifices to provide a preinjection and the outer needle valve opens a second set of orifices to provide a subsequent main injection. The outer poppet valve is a cylindrical sleeve positioned around a stationary valve housing containing the inner poppet valve. 
     U.S. Pat. No. 5,899,389 to Pataki et al. discloses a fuel injector assembly including two biased valve elements controlling respective orifices for sequential operation during an injection event. A single control volume may be provided at the outer ends of the elements for receiving biasing fluid to create biasing forces on the elements for opposing the fuel pressure opening forces. However, the control volume functions in the same manner as biasing springs to place continuous biasing forces on the valve elements. As a result, the needle valve elements only lift when the supply fuel pressure in the needle cavity is increased in preparation of a fuel injection event to create pressure forces greater than the closing forces imparted by the control volume pressure. 
     U.S. Pat. Nos. 4,202,500 to Keiczek and 4,215,821 to Eblen both disclose injectors having two sets of injector orifices controlled by respective needle valves. 
     Although some systems discussed hereinabove create different stages of injection, further improvement is desirable. Therefore, there is need for a servo-controlled fuel injector for providing enhanced control over injection timing and flow rate, especially in variable supply pressure systems. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention, therefore, to overcome the disadvantages of the prior art and to provide a fuel injector which is capable of effectively and predictably controlling the rate of fuel injection. 
     It is another object of the present invention to provide a servo-controlled injector capable of effectively providing a dual injection so as to minimize emissions. 
     It is another object of the present invention to provide a fuel injection system capable of providing a variable supply pressure and selectively providing either a low fuel injection rate followed by a high fuel injection rate or only a single low fuel injection rate. 
     It is yet another object of the present invention to provide a fuel injection system capable of selectively producing a wide variety of injection flow rates or rate shapes depending on engine operating conditions while remaining compatible with existing fuel systems. 
     It is a further object of the present invention to provide an injector for use in a variety of variable pressure fuel systems, including common rail system and accumulator pump systems, which effectively controls the rate of injection at each cylinder location. 
     Still another object of the present invention is to provide an injector which is capable of selectively creating different injection rate shapes to optimize emissions and fuel economy. 
     Yet another object of the present invention is to provide an injector which is compatible with existing pilot activated fuel injection mechanisms and methodologies. 
     Another object of the present invention is to provide an injector which permits injection duration to be extended and fueling accuracy improved at part load conditions. 
     These and other objects of the present invention are achieved by providing a fuel injection system for injecting fuel into the combustion chamber of an engine comprising a variable pressure fuel supply for supplying fuel at various pressure levels and a fuel injector including an injector body containing an injector cavity and a plurality of injector orifices communicating with one end of the injector cavity to discharge fuel into the combustion chamber. The fuel injector also includes a plurality of injector orifices including a first set of orifices and a second set of orifices. The injector body also includes a fuel transfer circuit for supplying fuel to the injector orifices and a first needle valve element positioned in the injector cavity for controlling fuel flow through the first set of orifices. A first valve seat is formed on the injector body and the first needle valve element is movable from a closed position against the first valve seat blocking flow through the first set of injector orifices to an open position permitting flow through the first set of injector orifices. The fuel injector further includes a second needle valve element positioned in the injector cavity for controlling fuel flow through the second set of injector orifices. The fuel injector also includes a second valve seat formed on the injector body wherein the second valve element is movable from a closed position against the second valve seat blocking flow through the second set of injector orifices to an open position permitting flow through the second set of injector orifices. The fuel injector further includes a first control volume positioned adjacent an upper end of the first needle valve element for receiving fuel and a second control volume positioned adjacent an upper end of the second needle valve element for receiving fuel. The fuel injector also includes a drain circuit for draining fuel from the first and the second control volumes to a low pressure drain. The injector also includes an injection control valve positioned along the drain circuit for controlling the flow of fuel from the first and the second control volumes through the drain circuit to permit movement of the first and the second needle valve elements between the open and the closed positions. The first needle valve element may be telescopingly received within a cavity formed in the second needle valve element to form a sliding fit with an inner surface of the second needle valve element. A throttle passage may be provided in the second needle valve element to restrict fuel flow upstream of the first set of injector orifices during a fuel injection event. A first biasing spring may be provided for biasing the first needle valve element toward the closed position and a second biasing spring for biasing the second needle valve element toward the closed position. The first biasing spring may be positioned within the cavity of the second needle valve element. The first and the second biasing springs may be positioned in overlapping relationship. The injector may also include a separator positioned between the first and the second control volumes and biased into a position by the first biasing spring. The injector may also include a first control volume charge circuit including a charge groove formed in an inner surface of the separator. 
     The present invention is also directed to the above described fuel injection system wherein the first or inner needle valve element is adapted to open at a predetermined low pressure level while the second needle valve element remains in the closed position. In addition, the second or outer needle valve element is adapted to move into an open position at a higher fuel supply pressure greater than the predetermined low pressure level. The present invention is also directed to a fuel injection system and fuel injector wherein the outer needle valve element is adapted to open at a low pressure level while the inner needle valve element remains in the closed position until a higher predetermined supply pressure level is supplied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged cross sectional view of the closed nozzle injector of the present invention; 
     FIG. 2 is an expanded view of the area A of FIG. 1; 
     FIG. 3 is an expanded view of the area B of FIG. 1; 
     FIG. 4 is a graph showing actuator control voltage, control valve position, inner and outer needle position, inner control volume pressure, outer control volume pressure, inner needle injection rate and outer needle injection rate versus time during an injection event in the single needle operation mode with the injector of the present invention; 
     FIG. 5 is a graph showing actuator control voltage, control valve position, inner and outer needle position, inner control volume pressure, outer control volume pressure, inner needle injection rate, outer needle injection rate and total injection rate versus time during an injection event in the dual needle operation mode with the injector of the present invention; 
     FIG. 6 is a graph showing instantaneous volumetric injection rate versus time for a given injection event at various supply pressures using the injector of the present invention; 
     FIG. 7 is a graph showing injected quantity versus injection duration at various supply pressures for the injector of the present invention; 
     FIG. 8 is an enlarged cross sectional view of a second embodiment of the closed nozzle injector of the present invention; 
     FIG. 9 is an expanded view of the area A of FIG. 8; 
     FIG. 10 is an expanded view of the area B of FIG. 8; 
     FIG. 11 is a graph showing actuator control voltage, control valve position, inner and outer needle position, inner control volume pressure, outer control volume pressure, inner needle injection rate and outer needle injection rate versus time during an injection event in the single needle operation mode with the injector of FIG. 8; 
     FIG. 12 is a graph showing actuator control voltage, control valve position, inner and outer needle position, inner control volume pressure, outer control volume pressure, inner needle injection rate, outer needle injection rate and total injection rate versus time during an injection event in the dual needle operation mode with the injector of FIG. 8; 
     FIG. 13 is a graph showing instantaneous volumetric injection rate versus time for a given injection event at various supply pressures using the injector of FIG. 8; 
     FIG. 14 is a graph showing injected quantity versus injection duration at various supply pressures for the injector of FIG. 8; 
     FIG. 15 is a graph showing the ability of the injector of the present invention to form a small detached pilot quantity followed by a main injection event by illustrating control valve, inner needle and outer needle positions, and inner needle, outer needle and total injection rates versus time; and 
     FIG. 16 is a graph showing normalized mass average injection pressure versus injection quantity using the system and injector of the second embodiment of the present invention as compared to a conventional single needle valve assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a variable pressure fuel injection system, indicated generally at  10 , including a variable pressure fuel supply  12  and a closed needle injector  14 , which is capable of effectively producing multiple fuel injection mass flow rates, extending injection duration and improving fueling accuracy at part load conditions, providing a low quantity detached pilot injection at all operating conditions and providing an attached pilot injection at moderate to high pressure operating conditions. Closed needle injector  14  of the first embodiment of the present invention generally includes an injector body  16  formed from a nozzle housing  18 , spring housing  20 , a spacer  22 , actuator housing  24  and retainer  26  for holding the various components in compressive abutting relationship. For example, retainer  26  may contain internal threads for engaging corresponding external threads on an upper barrel (not shown) to permit the entire injector body  16  to be held together by simple relative rotation of retainer  26  relative to the upper barrel. Injector body  16  includes an injector cavity, indicated generally at  28 . Injector body  16  further includes a fuel transfer circuit  30  comprised, in part, of delivery passages  32  formed in actuator housing  24 , transfer passages  34  formed in spacer  22  and delivery passages  36  formed in spring housing  20  for delivering fuel from a high pressure source to injector cavity  28 . Injector body  16  also includes a plurality of injector orifices  38  fluidically connecting injector cavity  28 , including a mini-sac  40 , with a combustion chamber of an engine (not shown). Injector  14  is positioned in a receiving bore (not shown) formed in, for example, the cylinder head of an internal combustion engine. Although variable pressure fuel injection system  10  is illustrated as including only one injector, system  10  may be used on an engine having any number of fuel injectors, for example, an injector for each engine cylinder such as a four, six, eight, ten or twelve cylinder engine. 
     Variable pressure fuel supply  12  may be any fuel supply capable of supplying fuel at different pressure levels, for example, capable of varying the pressure between a low pressure level and a high pressure level including supplying fuel at any pressure between the low and high pressure levels, such as at a moderate supply pressure level. For example, variable pressure fuel supply  12  may be provided by the fuel system disclosed in U.S. Pat. No. 5,676,114 entitled  Needle Controlled Fuel System with Cyclic Pressure Generation , the entire contents of which is hereby incorporated by reference. Variable pressure fuel supply  12  may alternatively be in the form of any high pressure common rail or alternatively, a dedicated pump assembly, such as in a pump-line-nozzle system or a unit injector system incorporating, for example, a mechanically actuated plunger into the injector body so long as the system is capable of the variable pressure supply described above. 
     Closed needle fuel injector  14  also includes a first or inner needle valve element  42  and a second or outer needle valve element  44  both positioned for reciprocal movement within injector cavity  28 . Specifically, outer needle valve element  44  has a generally cylindrical shape forming an inner cavity  46  for receiving inner needle valve element  42 . Injector orifices  38  include an outer set of orifices  48  and an inner set of orifices  50 . An outer valve seat  52  is formed at the lower end of nozzle housing  18  for abutment by the lower end of outer needle valve element  44  when in a closed position so as to prevent fuel flow from injector cavity  28  through outer set of injector orifices  48 . An inner valve seat  54  is formed on the inner surface of the lower end of nozzle housing  18  for abutment by the lower end of inner needle valve element  42  when in a closed position to prevent fuel flow from injector cavity  28  through the inner set of injector orifices  50  via mini-sac  40 . A lower guiding surface  56  formed on inner needle valve element  42  is sized to form a close sliding fit with the inner surface of outer needle valve element  44  to provide a guiding function while permitting unhindered reciprocal movement of the needle valve elements. Likewise, an upper guiding surface  58  is formed on inner needle valve element  42  and sized to form a close sliding fit with the inner surface of a floating needle separator  60  positioned within the upper end of outer needle valve element  44  so as to create a very restrictive fluid passage. Likewise, the outer surface of floating needle separator  60  is sized to form a close sliding fit with the inner surface of outer needle valve element  44  while also creating a very restrictive fluid passage. 
     Closed needle injector assembly  14  also includes a first or inner needle biasing spring  62 , i.e. coil spring, positioned within inner cavity  46  of outer needle valve element  44  for biasing inner needle valve element  42  into the closed position as shown in FIG.  1 . The lower end of inner biasing spring  62  engages an inner needle shim or seat  64  positioned in abutment against a land formed on inner needle valve element  42 . The upper end of inner needle biasing spring  62  is seated against the lower end of floating needle separator  60 . Closed needle injector assembly  14  also includes a second or outer needle biasing spring  66 , i.e. coil spring, positioned in injector cavity  28  around the outer surface of outer needle valve element  44 . Thus, outer needle biasing spring  66  surrounds inner needle biasing spring  62  and is positioned in overlapping relationship with inner needle biasing spring  62  along the longitudinal axis of the injector. The inner end of outer needle biasing spring  66  engages a shim or seat  68  positioned in abutment against an annular land formed on outer needle valve element  44 . The upper end of outer needle biasing spring  66  engages spring housing  20 . 
     Referring to FIG. 2, closed needle injector assembly  14  also includes a first or inner control volume  70  formed within floating needle separator  60  adjacent the upper end of inner needle valve element  42  and a second or outer control volume  72  positioned outside separator  60  adjacent the upper end of outer needle valve element  44 . A control volume charge circuit  74  is provided for directing fuel from fuel transfer circuit  30  into inner control volume  70  and outer control volume  72 . Specifically, control volume charge circuit  74  includes a first charge passage  76  comprised of a slot formed in the inner surface of floating needle separator  60  for delivering supply fuel from inner cavity  46  to inner control volume  70 . It should be noted that supply fuel is delivered from injector cavity  28  to inner cavity  46  via a cross passage  78  formed in outer needle valve element  44 . Control volume charge circuit  74  also includes a second charge passage  80  formed in lower spacer  22  for connecting fuel transfer circuit  30  to outer control volume  72 . Second charge passage  80  includes an outer inlet control orifice  82 . Floating needle separator  60  is maintained in sealing abutment against the lower surface of spacer  22  by inner biasing spring  62 . 
     Closed needle injector assembly  14  also includes a drain circuit, indicated generally at  84 , for draining fuel from inner control volume  70  and outer control volume  72  to a lower pressure drain. Specifically, drain circuit  84  includes a first drain passage  86  formed in spacer  22  for draining fuel from inner control volume  70 . First drain passage  86  also functions as an inner outlet control orifice. Drain circuit  84  also includes a second or outer control volume drain passage  88  for draining fuel from outer control volume  72  and functioning as an outer outlet control orifice. In the exemplary embodiment shown in FIG. 2, outer control volume drain passage  88  extends through spacer  22  to connect with the upper end of first drain passage  86 . The lower end of outer control volume drain passage  88  communicates with an annular groove  90  formed in the upper end of floating needle separator  60  which, in turn, communicates with outer control volume  72 . 
     Closed needle injector assembly  14  of the present embodiment also includes an injection control valve, indicated generally at  92 , positioned along drain circuit  84  downstream of the intersection of drain passages  86  and  88  for controlling the flow of fuel through drain circuit  84  so as to permit the controlled movement of inner needle valve element  42  and outer needle valve element  44  as described hereinbelow. Injection control valve  92  includes a control valve member  94  biased into a closed position against a valve seat formed on spacer  22 . Injection control valve  92  also includes an actuator assembly  96  capable of selectively moving control valve member  94  between open and closed positions. For example, actuator assembly  96  may be a fast proportional actuator, such as an electromagnetic, magnetostrictive or piezoelectric type actuator. Actuator assembly  98  may be a solenoid actuator such as disclosed in U.S. Pat. No. 6,056,264 or U.S. Pat. No. 6,155,503, the entire contents of both of which are incorporated herein by reference. 
     Inner needle valve element  42  is of the conventional mini-sac type design whereas outer needle valve element  44  is of the valve covered orifice (VCO) design. Also, the number and size of holes specified for the inner set of spray orifices  50  and the outer set of spray orifices  48  are selected to provide reduced fuel injection rates when the inner needle valve element  42  is operated alone, and conventional fuel injection rates when both inner needle valve element  42  and outer needle valve element  44  are operated as described hereinbelow. 
     As shown in FIG. 3, a feed passage  53  is formed in outer needle valve element  44  to fluidically connect an outer supply cavity  55  to an inner supply cavity  57  formed between inner needle valve element  42  and outer needle valve element  44 . Feed passage  53  extends transversely through outer needle valve element  44  and preferably perpendicular to a longitudinal axis of the injector. Feed passage  53  is preferably sized larger than the total cross sectional flow area of the inner set of injector orifices  50  to avoid feed passage  53  functioning to restrict fuel flow while inner needle valve element  42  is in the open position. Alternatively, feed passage  53  may be sized, relative to the total flow area of injector orifices  50 , to produce a flow induced pressure drop upstream of inner needle valve element  42  in inner supply cavity  57 . This pressure drop and thus the corresponding lower fuel pressure in inner supply cavity  57 , reduces the pressure acting on the lower face of inner needle valve element  42  to improve its closing responsiveness. Although the present invention relies on the different biasing spring preloads, needle area ratios and control orifice flow coefficients in combination with variable pressure fuel supply  12  to permit selective control of the needle valve elements  42 ,  44 , feed passage  53  may also be sized to produce a sufficient pressure drop in inner supply cavity  57  adequate to provide further control, such as ensuring outer needle valve element  44  does not lift before inner needle valve element  42  has reached its uppermost open position. 
     Referring to FIGS. 1-4, during operation, with pressurized supply fuel from variable pressure fuel supply  12  present in fuel transfer circuit  30 , outer supply cavity  55  and inner supply cavity  57 , and with injection control valve  92  in its normally closed position blocking flow through drain circuit  84 , all volumes within injector cavity  28  are pressurized to the supply fuel pressure level. Hydraulic fuel pressure forces are developed on the active surfaces of both needle valve elements  42 ,  44 , i.e. surfaces not excluded by a respective seat area and specifically those valve element areas exposed to the pressurized fuel in inner control volume  70  and outer control volume  72 . The hydraulic forces combined with the preloads of inner biasing spring  62  and outer biasing spring  66  to ensure that sufficient needle seating forces are generated to prevent leakage regardless of supply operating pressure. Different modes of operation will now be described based on the level of supply pressure and/or the number of needle valves operating during a given injection event. Importantly, fuel injection system  10  of the present invention permits several different modes of operation to provide a wide variety of rate shape choices to better match the injection rate shape to a particular set of operating conditions thereby permitting reductions in emissions, particulates and combustion noise while also improving brake specific fuel consumption (BSFC). The first mode of operation to be discussed with reference to FIGS. 1-4 is a low pressure, single needle fuel injection mode. Again, with a source of pressurized fuel from variable pressure fuel supply  12  available in fuel transfer circuit  30  and injection control valve  92  in its normally closed position, all volumes within the injector cavity  28  are pressurized to the supply level. This operating state is indicated at A in FIG.  4 . From this state, a fuel injection sequence is initiated by energizing actuator assembly  96  (B) to open injection control valve  92  (C). The fuel pressure in both inner control volume  70  and outer control volume  72  drops (D) as fuel flows through the respective unobstructed inner and outer drain passages  86  and  88  respectively, to a low pressure drain. Repressurization of inner control volume  70  and outer control volume  72  is prevented by first charge passage  76  and second charge passage  80  which function as restriction control orifices. The net hydraulic force that held inner needle valve element  42  against its inner valve seat  54  quickly changes direction to oppose the preload of inner needle biasing spring  62  thereby lifting inner needle valve element  42  (E) and allowing fuel to pass from outer supply cavity  55  through feed passage  53 , inner supply cavity  57 , mini-sac  40  and inner injection orifices  50  (F) into the combustion chamber. Outer needle valve element  44  is prevented from responding in similar fashion, i.e. opening, due to a higher minimum set opening pressure. That is, the fuel supply pressure must be at a higher pressure to affect the opening of outer needle valve element  44  due to the higher preload of outer needle biasing spring  66 . 
     Inner needle valve element  42  hovers (G) in a state of force equilibrium near its upper stop, i.e. the lower surface of spacer  22 . Force equilibrium is established and maintained by inner needle valve element  42  as it restricts flow to first drain passage  86 . When the equilibrium is disturbed so as to cause inner needle valve element  42  to move toward its upper stop, the flow restriction across the top of inner needle valve element  42  increases, correspondingly increasing the fuel pressure in inner control volume  70  and increasing the resulting hydraulic force imbalance tending to close inner needle valve element  42 . Conversely, as the equilibrium is disturbed so as to cause inner needle valve element  42  to move away from its upper stop, the flow restriction into first drain passage  86  decreases, correspondingly decreasing the fuel pressure in inner control volume  70  and decreasing the resulting hydraulic force imbalance tending to close inner needle valve element  42 . Inner needle valve element hovering minimizes control flow rate and the associated energy loss required to maintain the injection. The termination of fuel injection is initiated by de-energizing actuator assembly  96  (H). Afterward, injection control valve  92  and thus control valve member  94  closes (I), and fuel flowing through first and second charge passages  76 ,  80  repressurizes inner and outer control volumes  70 ,  72  respectively (J). As a result, inner needle valve element  42  closes (K) and the fuel injection event is terminated (L). The previously described hovering action maintains the inner control volume  70  in a pressurized state during the injection process to improve closing responsiveness. Also, as noted hereinabove, feed passage  53  may be sized to function as a feed orifice restricting flow to inner supply cavity  57  to improve low fuel quantity metering performance. Outer needle valve element  44  remains closed during the single needle injection mode to provide a mechanical guide for inner needle valve element  42 . Fuel flowing in the vicinity of outer valve seat  52  provides a cooling effect to reduce the tendency for coking and plugging during prolonged periods of single needle operation. Coking and plugging of the outer injection orifices  48  may be avoided altogether during extended single needle operation by flexibly and intermittently operating outer needle valve element  44 . 
     Referring to FIGS. 1-3 and  5 , a moderate to high fuel pressure, dual needle fuel injection mode will now be described. At higher operating pressures, for example greater than 100 MPa, both inner needle valve element  42  and outer needle valve element  44  are activated to maximize spray hole utilization and fuel injection rate, and to minimize opportunities for unused spray holes to coke and plug. The injection sequence is initiated by opening injection control valve  92 . Inner needle valve element  42  responds first due to the lower preload of inner biasing spring  62 , followed by the movement of outer needle valve element  44  after a brief pressure dependent time delay. Both inner and outer needle valve elements  40 ,  42  hover near the maximum extent of their respective strokes. The hovering actions maintain much of the inner control volume  70  and outer control volume  72  in pressurized states to improve closing responsiveness and minimize controlled flow rates. As shown in FIG. 5, injector cavity  28  is pressurized (A) and actuator assembly  96  energized (B) to cause injection control valve  92  and thus control valve member  94  to open (C) causing the fuel pressure in inner control volume  70  and outer control volume  72  to decrease (D) at different rates. Consequently, inner needle valve element  42  lifts (E) and inner needle injection begins (F). The rate of pressure decay in outer control volume  72  and drain circuit  84  is reduced during the opening motion of the inner needle valve element  42  as a result of the pumping action of the moving inner needle valve. Inner needle valve element  42  then hovers near its stop (G). The fuel pressure in outer control volume  72  continues to decrease (H) causing outer needle valve element  44  to lift (I) causing an outer needle injection to begin (J). Outer needle valve element  44  then hovers near its upper stop (K). To terminate injection, actuator assembly  96  is de-energized (L), causing injection control valve  92  and thus control valve member  94  to move into the closed position (M) causing inner needle valve element  42  to also move into a closed position (N). As a result, the inner needle injection ends (O) and then the outer needle valve element  44  closes (P) ending the outer needle injection event (Q). 
     Another mode of operation is to provide a single needle operating event at moderate to high pressure. Single needle operation can be achieved at moderate to high supply pressures provided that the commanded injection duration is short enough to prevent activation of outer needle valve element  44 . This operating mode may be desirable during transient engine conditions when it may not be possible, practical or efficient to vary the fuel supply pressure using variable pressure fuel supply  12 . 
     FIGS. 6 and 7 illustrate hydro-mechanical simulation results including plots of injected quantity versus supply pressure and actual injection duration. The supply pressure was varied from 35 to 200 MPa in six steps and the ratio of outer to inner spray hole areas was set to ½. Consequently, the ratio of single needle to dual needle injection rates was ⅓. The drain pressure was set to 0.1 MPa and the outer needle parameters were set to prevent operation when the supply pressure was less than approximately 110 MPa. In these cases, the rate shape is square with a sharp end of injection. Dual needle injection is evident in the rate shapes produced at the higher pressures of 134, 167 and 200 MPa. In these higher pressure cases, the rate shape includes a small leading boot of single needle injection and a trailing feature related to the early closing of inner needle valve element  42 . The plots clearly show favorable low fueling controllability. FIG. 7 shows overlap in injected quantity between low pressure with long duration and high pressure with short duration. 
     FIGS. 8-10 illustrate a second embodiment of the present fuel injection system and fuel injector which is very similar to the system and injector of the first embodiment with the primary exceptions of a different charge circuit for supplying fuel to the control volumes and the preload setting of the biasing springs resulting in a different sequential order of operation of the needle valve elements. It should be noted that features of the present embodiment which are the same as the previous embodiment of FIG. 1 will be identified with like reference numerals. The variable pressure fuel injection system  100  of the present embodiment likewise includes variable pressure fuel supply  12  and a closed nozzle fuel injector  102  having many of the same features and components as closed nozzle injector  14  of the first embodiment. However, in the present embodiment, control volume charge circuit  74  includes a first charge passage  104  extending from annular groove  90  to inner control volume  70 . Thus, inner control volume  70  is supplied with pressurized supply fuel via outer control volume  72  and first and second charge passages  104  and  80 , respectively. It should also be noted that outer control volume drain passage  88  and inner control volume drain passage  86  extend through spacer  22  without intersecting but opening at the upper end of spacer  22  in close proximity so that control valve member  94  can effectively seal a valve seat surrounding the openings when control valve member  94  is in a closed position against the valve seat formed on spacer  22 . 
     More importantly, the biasing spring preloads are set such that a lower fuel pressure affects the opening of outer needle valve element  44  while the preload of inner needle valve element  42  is set to require a higher minimum opening pressure. It should be noted that although the biasing spring preloads primarily determine the fuel pressure at which the valve elements  42 ,  44  open and close, the needle area ratios and control orifice flow coefficients also affect the opening pressure threshold and response characteristics for each valve element. 
     Referring to FIG. 10, it should also be noted that the present embodiment does not include a feed passage extending through the lower end of outer needle valve element  44  to supply fuel to inner supply cavity  57 , since outer needle valve element  44  is the first valve to open and, once opened, inner supply cavity  57  will be exposed to supply fuel prior to the opening of inner needle valve element  42 . It should be noted also that the diameter of inner needle valve element  42  at lower guiding surface  56  may be sufficiently less than the corresponding inner diameter of outer needle valve element  44  to permit fuel flow from inner cavity  46  into inner supply cavity  57 . 
     In the present embodiment, during the low pressure, single needle fuel injection mode, the sequence of operation is similar to that of the previous embodiment except that the outer needle valve element  44  opens and closes without the opening and closing of the inner needle valve element  42 . Specifically, initially with the operating state at A in FIG. 11, the actuator assembly  96  is energized to open injection control valve  92  causing the fuel pressure in inner and outer control volumes  70  and  72 , respectively, to decrease as fuel flows through inner drain passage  86  and outer drain passage  88 . Repressurization of the control volumes is prevented by a restrictive outer inlet control orifice  82  positioned in second charge passage  80 . The net hydraulic force that previously held outer needle valve element  44  against its valve seat quickly changes direction to oppose the preload of outer biasing spring  66  thereby permitting outer needle valve element  44  to lift (E) permitting fuel to pass from outer supply cavity  55  to the combustion chamber via the outer set of injection orifices  48  (F). Inner needle valve element  42  is maintained in a closed position and prevented from opening due to a higher minimum set opening pressure determined by the preload of inner biasing spring  62 , and the relative size of the control orifices permitting flow into the control volume and out of the control volumes. Outer needle valve element  44  hovers (G) in a state of equilibrium near its upper stop. This hovering effect is similar to that described hereinabove with respect to the first embodiment. The termination of fuel injection is initiated by de-energizing actuator assembly  96  (H) causing the closing of injection control valve  92  (I). Fuel flowing through second charge passage  80  and then through first charge passage  104  repressurizes outer and inner control volumes  72 ,  70  (J). Outer needle valve element  44  then closes (K) and a fuel injection event is terminated (L). Inner needle valve element  42  remains closed during single needle injections to provide a mechanical guide for outer needle valve element  44  thereby reducing variations in spray plume geometry and atomization. As with the previous embodiment, fuel flowing in the vicinity of inner valve seat  54  provides a cooling effect to reduce the tendency for coking and plugging during prolonged periods of single needle operation. Coking and plugging of the inner needle injection orifices  50  may be avoided altogether during extended single needle operation by flexibly and intermittently operating inner needle valve element  42  as described hereinbelow. 
     During the moderate to high pressure dual needle fuel injection mode, both inner needle valve element  42  and outer needle valve element  44  are activated to maximize spray hole utilization and fuel injection rate, and to minimize opportunities for unused spray holes to coke and plug. The injection sequence is initiated by opening injection control valve  92 . The areas of the needle valve elements exposed to fuel pressure and the biasing spring preloads are selected so that outer needle valve element  44  lifts into the open position and hovers near its upper stop provided by spacer  22  before inner needle valve element  42  responds. Inner needle valve element  42  then lifts after a brief pressure dependent time delay. Specifically, the hovering outer needle creates an additional flow restrictive mechanism which acts in series with the first charge passage  104  to reduce the pressure in the inner control volume  70  to a level which forces the inner needle  42  to open. Inner needle valve element  42  stops against spacer  22  rather than hovering near spacer  22  as the outer needle valve element  44  does. The fuel injection sequence is terminated by closing injection control valve  92  to block the drain flow of control fuel through first and second drain passages  86 ,  88 . The combination of a pressure excluded area on the top of inner needle valve element  42  and the restricting effect of first charge passage  104  (which functions as a control orifice), delays the closing of inner needle valve element  42  until outer needle valve element  44  closes far enough to significantly restrict fuel flow to inner valve seat  54 . In this way, the low flow outer needle valve element  44  is the first to open and the first to close. 
     FIG. 12 highlights the sequence of operations for dual needle fuel injection. The injector exists in a pressurized state (A), followed by the energization of actuator assembly  96  (B) which causes injection control valve  92  to open (C) resulting in a decrease in the fuel pressure in inner control volume  70  and outer control volume  72  (D). Outer needle valve element  44  then lifts (E) initiating a pilot injection (F) followed by outer needle valve element  44  hovering near its stop (G). Meanwhile, fuel pressure in inner control volume  70  continues to drop (H) causing inner needle valve element  42  to lift (I) thereby initiating the main injection (J). At a predetermined time, actuator assembly  96  is de-energized (K) causing the injection control valve  92  to close (L) and outer needle valve element  44  to move into the closed position (M). The first charge passage or control orifice  104  restricts the flow of control fuel into inner control volume  70  thereby delaying the repressurization of inner control volume  70  (N). Subsequently, however, eventually inner control volume  70  becomes repressurized sufficiently to cause the closing of inner needle valve element  42  (P) thereby ending the inner needle injection (Q). 
     The moderate to high pressure single needle operation mode, again, can be achieved at moderate to high supply pressures provided that the commanded injection duration is short enough to prevent the opening of inner needle valve element  42 . Again, this operating mode may be desirable during transient engine conditions when it may not be possible, practical or efficient to rapidly change fuel supply pressure using variable pressure fuel supply  12 . 
     FIGS. 13 and 14 contain plots of injected quantity versus supply pressure and actual injection duration for various test cases similar to FIGS. 6 and 7 of the previous embodiment. FIG. 13 illustrates the well behaved, pressure controlled, dual injection rate capabilities of the nozzle assembly as well as clearly illustrating the various operating modes including a single needle injection with short to long injection durations and low supply pressures; single needle injection with short injection durations and moderate to high supply pressures; and dual needle injection with moderate to long injection durations and moderate to high supply pressures. Further, the plots suggest that a one firing cycle pressure response capability, as provided by U.S. Pat. No. 5,676,114, the entire contents of which is hereby incorporated by reference, would be beneficial to minimize injected quantity variations during transitions between single and dual needle operation. However, without such a pressure response capability, a conventional injected quantity estimation and control method could be used. 
     FIG. 15 demonstrates the capability of the first embodiment of the present invention to produce a small quantity, i.e. 8 mm 3 , high peak pressure, i.e. 174 MPa, a detached (200 μs separation) pilot quantity followed by a large primary injection quantity, i.e. 304 mm 3 , soft start, high peak pressure, i.e. 198 MPa, main injection when the supply pressure is 200 MPa. FIG. 16 contains a comparison of normalized mass average injection pressure versus injected quantity obtained with the first embodiment of the present invention (FIG. 1) and a conventional single valve nozzle. The plots demonstrate the capability of the present invention to minimize internal pressure drop and thereby maximize mass average injection pressure as injected quantities approach zero. Pairs of plots corresponding to the present invention demonstrate the minimum injected quantity and mass average pressure reducing effects of increasing the flow resistance of the outer needle lower feed orifice. 
     INDUSTRIAL APPLICABILITY 
     It is understood that the present invention is applicable to all internal combustion engines utilizing a fuel injection system and to all closed nozzle injectors and especially applicable to fuel injection systems supplied with high pressure fuel at a controlled variable pressure level. This invention is particularly applicable to diesel engines which require different fuel injection rates in order to minimize emissions. Such internal combustion engines including a fuel injector in accordance with the present invention can be widely used in all industrial fields and non-commercial applications, including trucks, passenger cars, industrial equipment, stationary power plant and others.