Abstract:
A hydraulic control device which may be used in a number of applications, including with a fuel injector, includes a control valve having a first and a second independently shiftable valve member, the control member being configurable to define a plurality of actuating fluid flow paths for controlling hydraulic flow therethrough. A fuel injector includes the aforementioned control valve. A method of hydraulic control includes a number of steps, including independently controlling the shifting of two valves in a control valve assembly to selectively control the flow of actuating fluid to an actuator.

Description:
RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Application No. 60/134,763, filed May 18, 1999, and incorporated herein in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This concept is directed to a double-acting, two-stage flow control valve (DATS Valve) for use as a hydraulic control device. The present invention has use generally as a hydraulic control device and may be used, for example, in a camless engine. Additionally, the present application is directed specifically at the use of the hydraulic control device in combination with an intensified, low-pressure, common rail fuel injector used in a hydraulically-actuated, electronically-controlled unit injection (HEUI) system for an internal combustion engine, particularly a diesel engine, and the method of operating the control valve to selectively achieve pilot injection, rate shaping injection, far split injection, and single shot injection modes of operation of the fuel injector. 
     THE PRIOR ART 
     The prior art injectors used here for reference are the hydraulically-actuated, electronically-controlled unit injectors described in the following references, which are incorporated herein by reference: SAE paper No. 930270, “HEUI—A New Direction for Diesel Fuel Systems,” and SAE paper No. 1999-01-0196, “Application of Digital Valve Technology to Diesel Fuel Injection” and U.S. Pat. Nos. 5,271,371, 5,479,901, 5,597,118, and 5,720,261, and 5,720,318. 
     A prior art HEUI injector  200  is depicted in prior art FIG.  1 . HEUI  200  consists of four main components: (1) control valve  202 ; (2) intensifier  204 ; (3) nozzle  206 ; and (4) injector housing  208 . 
     The purpose of the control valve  202  is to initiate and end the injection process. Control valve  202  is comprised of a poppet valve  210 , having an attached armature  213 , and an electric control solenoid  212 . High pressure actuating oil from a high pressure rail  215  is supplied to the lower seat  214  of the poppet valve  210  through oil passageway  216 . To begin injection, the electric control solenoid  212  is energized moving the poppet valve  210  upward from the lower seat  214  to the upper seat  218 . This action admits high pressure oil to the spring cavity  220  and through the passage  222  to the piston chamber  223  of the intensifier  204 . Injection continues until the solenoid of the electric control  212  is de-energized and the poppet  210  moves from the upper seat  218  to lower seat  214 . Oil and fuel pressure then decrease as spent oil is ejected from the injector  200  through the open upper seat oil discharge  224  to the valve cover area of the internal combustion engine. The valve cover area is at ambient pressure. 
     The middle segment of the injector  200  includes the intensifier  204 . The intensifier  204  includes the hydraulic intensifier piston  236 , the plunger  228 , fuel chamber  230 , and the plunger return spring  232 . 
     Intensification of the fuel pressure to desired injection pressure levels is accomplished by the ratio of areas between the upper surface  234  of the intensifier piston  236 , acted on by the high pressure actuating oil and the lower surface  238  of the plunger  228 , acting on the fuel in chamber  230 . The intensification ratio can be tailored to achieve desired injection characteristics. Fuel is admitted to chamber  230  through passageway  240  past check valve  242 . Injection begins as the high pressure actuating oil is supplied to the upper surface  234  of the intensifier piston  236 . 
     As the intensifier piston  236  and plunger move downward responsive to the force exerted by the actuation oil, the pressure of the fuel in the chamber  230  below the plunger  228  rises dramatically. High pressure fuel flows in passageway  244  past check valve  246  to act upward on needle valve surface  248 . The upward force on surface  248  opens needle valve  250  and fuel is discharged from orifice  252  into the combustion chamber of the engine. The intensifier piston  236  continues to move downward until the solenoid of the electric control  212  is de-energized causing the poppet valve  210  to return to the lower seat  214 , thereby blocking actuating oil flow. The plunger return spring  232  returns the piston  236  and plunger  228  to their initial upward seated positions. As the plunger  228  returns upward, the plunger  228  draws replenishing fuel into the plunger chamber  230  across ball check valve  242 . 
     The nozzle  206  is typical of other diesel fuel system nozzles. The valve-closed-orifice style is shown, although a mini-sac version of the tip is also available. Fuel is supplied to the nozzle orifice  252  through internal passages. As fuel pressure increases, the nozzle needle  250  lifts from the lower seat  254  to its open position, thereby allowing fuel injection to occur. As fuel pressure decreases at the end of injection, the spring  256  returns the needle  250  to its closed position against the lower seat  254 . 
     FIGS. 2 a ,  2   b ,  2   c , and  2   d  illustrate a prior art Digital Hydraulic Operating System (DHOS) injector and digital control valve operation. The intensifier and nozzle portions of the DHOS injector are similar to those of the HEUI injector and have been identified with the same reference numerals. However, in the DHOS injector, the poppet control valve  202  of the HEUI injector has been replaced by a spool type digital control valve  300  which is controlled by two solenoid coils  302 ,  304 , the valve spool  306  which is made of magnetic material, being the armature. Thus, as illustrated in FIG. 2 c , when the coil  302  is energized to begin an injection event or engine cycle during which an injection occurs, the valve spool  306  is pulled toward the coil  302  thereby open a fluid connection between the hydraulic fluid (high pressure lube oil) supply passage  310  and the working fluid passages  312  to the intensifier chamber  223  within the injector while isolating the vent passages  314 . When the coil  302  is de-energized, the valve spool will remain in the open position shown in FIG. 2 c  due to residual magnetism in the valve spool  306 . 
     To end the injection, the coil  304  is energized to pull the valve spool  306  rightward toward the coil  304  thereby establishing a fluid connection between the vent passages  314  and the working fluid passages  312  to the intensifier chamber  223  within the injector while isolating the hydraulic fluid supply passage  310 . 
     With either the HEUI or the DHOS injector, the size of the control valve normally is targeted for a single injection operation for achieving maximum injection pressure. And it is also sized for good performance at low temperature operation when hydraulic fluid is relatively viscous. Once the size of the control valve is selected, the fuel delivery quantity may be determined based on the actuation pressure and valve open duration (pulse width duration). The maximum fuel delivery for these type injectors could reach 200 mm 3 /stroke for full engine load condition. The minimum fuel delivery for engine at idle could be as small as 4 mm 3 /stroke. Especially for the DHOS injector, the digital valve is also responsible for pilot injection operation. The pilot injection quantity can be as small as 1 mm 3 /injection at maximum actuation pressure, approximately 3000 psi. 
     When a large size control valve is used for a small quantity of fuel delivery, significant performance variability is introduced during shot-to-shot and injector-to-injector operation. It is believed that this performance variability can be reduced if a smaller valve is used for small quantity operation and a large valve for full capacity operation. 
     SUMMARY OF THE INVENTION 
     The present invention is a valve for use generally as a hydraulic control device, such as, for example, in a camless internal combustion engine. One of the specific purposes of this invention is a control valve for a unit fuel injector, which can provide small flow when it is needed and can be switched to provide a larger flow rate when desired. Fundamentally, the control valve of the present invention has the ability to provide two-stage flow (high rate of flow and low rate of flow) with flexible controllability. 
     Many advanced diesel injector features, such as pilot injection, rate shaping, and efficient single shot injection, have been made available in various forms in prior injectors. All these features need to be available on a single injector for a diesel engine to achieve the goal of meeting ever more stringent emission regulations. With this invention, the user can flexibly choose between pilot injection, rate shaping injection, and single shot injection. The quantity of the fuel delivery and schedule of all events are flexibility selected and controlled. 
     This invention covers three different concepts. The first is a double-acting two stage (DATS) valve configuration as illustrated in the FIG.  3 . The second concept is the combination of a DATS valve with a low pressure, intensified, hydraulically-actuated, electrically-controlled, common rail diesel fuel injector as shown in FIG.  6 . The third concept is the operating strategies for the DATS injector to produce various modes of fuel injection as shown in FIG. 7 depending on various engine operating conditions. Although this valve concept can be used in many different applications, the direct application of this particular DATS valve is in diesel engine injection systems. 
     The present invention is a control valve assembly for use with a fuel injector, the fuel injector being controllable to define selected injection strategy of an injection event and includes a control valve having an inlet port and a drain port, the inlet port being in flow communication with a source of actuating fluid and the drain port being in flow communication with an actuating fluid drain having a first and a second independently shiftable valve member being configurable during an injection event to define a plurality of actuating fluid flow paths for controlling the injection event. The present invention is further a fuel injector that includes the aforementioned control valve. Additionally, the present invention is a method of controlling injection strategy of an injection event of a fuel injector which includes a number of steps, including the step of; 
     independently controlling the shifting of two valves in the control valve assembly to selectively control the flow of high pressure actuating fluid to the intensifier chamber to effect the desired injection strategy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional elevational view of a prior art HEUI injector; 
     FIG. 2 a  is a sectional elevational view of a prior art DHOS injector; 
     FIG. 2 b  is a sectional elevational view of the digital control valve portion of the prior art DHOS injector of FIG. 2 a;    
     FIG. 2 c  is a sectional elevational view of the spool valve digital control valve portion of the prior art DHOS injector in the open disposition; 
     FIG. 2 d  is a sectional elevational view of the spool valve of digital control valve portion of the prior art DHOS injector in the open disposition; 
     FIG. 3 is a sectional elevational view of the DATS valve; 
     FIG. 4 a  is a sectional elevational view of the DATS valve in the non-working (drain) mode of operation; 
     FIG. 4 b  is a sectional elevational view of the DATS valve in the pilot flow mode of operation; 
     FIG. 4 c  is a sectional elevational view of the DATS valve in the main flow mode of operation.; 
     FIG. 5 is a graphic representation of magnetic force as it relates to air gap; 
     FIG. 6 is a sectional elevational view of an exemplary injector incorporating the present invention; 
     FIG. 7 is a series of graphic representations of the energization states of the opening and closing coils as they relate to various modes of operation and rates of injection; 
     FIG. 8 is a schematic view of a sleeve design embodiment of the DATS valve at pilot flow mode; and 
     FIG. 9 is a right side view of sleeve wheel structure of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The double-acting two-stage (DATS) control valve assembly of the present invention is shown generally at  10  in the figures. The basic structure of the DATS control valve assembly  10  is a valve inside of another valve. As shown in FIG. 3, the main components in the control valve assembly  10  are a valve housing  12 , an outer spool valve  14 , and inner spool valve  16 , a push piston  18 , an inner spool valve spring  20 , a closing solenoid coil  22  and its end cap  24 , and an opening solenoid coil  26  and its end cap  28 . The valve housing  12 , end caps  24 ,  28  and push piston stop  32  are all stationary pieces. The opening coil  26  may also be considered to be the double acting coil  26 . 
     The outer spool valve  14  is shiftably disposed in a close fitting sealing relation with a cylinder bore  15  defined in the valve housing  12 . The inner spool valve  16  is shiftably disposed in a close fitting fluid sealing relation within an axial cylinder bore  17  of the outer spool valve  14  for axially slidable movement therein, the friction between the inner and outer spools being controlled to a minimum level. The opening coil  26  and closing coil  22  are disposed adjacent the ends of the housing  12  on both sides to control the position of the outer spool valve  14 . The push piston  18  includes an armature plate  19  disposed externally of the opening coil end cap  28  from the spool valves  14 ,  16  and a push pin  30  extending through the end cap  28  to contact one end of the inner spool  16 . The push pin  30  may be integrally formed with the armature plate  19 . The inner spool valve spring  20  is disposed in the bore  20  between the closing coil end cap  24  and the other end of the inner spool valve  16  to bias the inner spool valve  16  toward the opening coil  26  and the push piston  18  away from the opening coil  26  to a position abutting surface  31  of the push piston stop  32  disposed on the opening coil end of the housing  12  as shown in FIG.  3 . 
     Both end caps  24 ,  28 , valve housing  12 , outer spool valve  14  and push piston  18  are all made with the same type of magnetic steel. Such magnetic steel conducts magnetic flux when either coil  22  or  26  is energized. The inner spool valve  16  is made out of non-magnetic steel and therefore has relatively poor magnetic conductivity. Accordingly, energizing coil  26  or coil  22  produces a negligible amount of flux on the inner spool valve  16 . Motion of the inner valve spool  16  is caused only by the motion of the push piston  18  and by the bias of the spring  20 . Biased spring  20  keeps the inner spool valve  16  in very close contact with the push piston  18 . The push piston  18  and inner spool valve  16  move together under all operating conditions. Energizing the coil  22  attracts only the outer spool valve  14 . Energizing the coil  26  attracts the outer spool valve  14  from one side to initiate rightward motion and the push piston  18  from the other side to initiate leftward motion. This two-sided attraction feature resulting in concurrent oppositely directed motion is referred to as being double-acting with a single coil. Both coils  22 ,  26  are substantially identical. The magnetic force produced from either coil  22 ,  26  on the outer spool valve  14  is substantially the same under zero air gap conditions. 
     During operation, the push piston  18  may be either attracted against the external side  27  of the end cap  28  by the opening coil  26  or biased by the spring  20  and inner spool  16  against the push piston stop  32 . The push piston  18  has two positions. The first position is abutting the push piston stop  32  and the second position is magnetically latched on the external side of open coil end cap  28 . The larger diameter armature  19  provides sufficient magnetic force when opening coil  26  is activated to be attracted towards the open coil end cap  28  outer surface  27  by overcoming the biasing force of the spring  20  from other end of the inner spool valve  16 . The push piston air gap  40  is reduced to zero as push piston  18  is magnetically latched to the surface  27  of the end cap  28 . 
     The inward side of the closing coil  22  attracts the outer spool valve  14  when the closing coil  22  is energized. Since inner spool valve has relatively poor magnetic conductivity and is relatively far away from the end cap  24 , the magnetic force from the closing coil  22  acting on the inner spool valve  16  is negligible. When the coil  26  is activated, the outer spool valve  14  is attracted to the inward side of the end cap  28 . The push piston  18  is also attracted toward the outer side of the end cap  28 . The function of the coil  26  together with end cap  28  is to create an opposite direction of motion between the outer spool valve  14  and the inner spool valve  16 . The relative position between the outer spool valve  14  and the inner spool valve  16  changes as both the push piston  18  and the outer spool valve  16  move towards the end cap  28 . As the relative position between spools  14 ,  16  and valve housing  12  changes, the flow ports in the housing will open and close accordingly, as is described in detail below to effect the desired operating modes of hydraulic fluid flow. 
     FIGS.  4 ( a ),  4 ( b ) and  4 ( c ) illustrate exemplary movements of the inner and outer spool valves  16 ,  14  within the housing  12 . The flow area of a drain annulus H, an annulus to the bore  17  of the outer spool  14 , is determined by the relative positions of the inner spool valve  16  and the outer spool valve  14 . If the outer spool valve  14  is latched at the closing coil end cap  24 , as shown in FIG. 3, the drain annulus H may be closed by activating open coil  26  to move the inner spool valve  16  with the push piston  18  toward the left. When the push piston  18  latches against the opening coil end cap  28 , as shown in FIG.  4 ( b ), the inner spool valve  16  is at its full leftward travel position and the drain annulus H is completely closed. In this position, a pilot passage D between the inner spool valve annulus E and the housing passage A is completely open and actuating fluid flows from pressure inlet  36  through pilot passage D to intensifier chamber  223 , as indicated below. Motion of the inner spool valve  16  or of the outer spool valve  14  does not close the supply passage F defined in the outer spool  14 . Supply passage F aligns with a supply passage G which is in fluid communication with the intensifier chamber  223  of the injector  8  (see FIG. 6 for a depiction of chamber  223 ). 
     The valve housing  12  provides the communication between the high pressure hydraulic actuating fluid source (inlet  36 ), the drain (drain port J), and the intensifier chamber  223  of the injector  8 . Inlet port A is directly connected to high-pressure source  36 . Drain port J is linked to the drain or reservoir of the engine at nearly ambient pressure by preferably spilling from the injector  8  under the engine valve cover. Supply port C allows in-flow of high pressure actuating fluid from inlet port A to the intensifier chamber  223  of the injector  8 . 
     A second supply port G has a dual responsibility. It provides a fluid path for the high pressure flow from inlet port A through pilot passage D, annulus E, supply passage F to the intensifier chamber  223  of the injector  8 . Supply port G also provides the fluid vent path for the venting of the actuating fluid from intensifier chamber  223  to flow through supply passage F, annulus E, drain annulus H, and drain annulus I. Drain annulus I is fluidly connected thereto to drain port J by passage L. Flow in all of the flow ports A, C, G, and J on the valve housing  12  is directly controlled by the position of the outer spool valve  14  relative to the housing  12 . When the outer spool valve  14  shifts from abutting one end cap  24  or  28  to the other end cap  28  or  24  (as the case may be) either the supply annulus B or the drain annulus I on the outer spool valve  14  will be open to the ports, while the other annulus B or I is closed by the valve housing  12 . 
     Pilot passage D is always open to the high pressure inlet port A. However, whether the pilot passage D opens to the intensifier chamber  223  is determined by the position of the inner spool valve  16  relative to the outer spool valve  14 . When the push piston  18  is latched against the open coil end cap  28 , as shown in FIGS.  4 ( b ) and  4 ( c ), the pilot passage D is open to intensifier chamber  223  so that high pressure actuating fluid can flow from inlet port A through pilot passage D to inner spool annulus E to supply port G to the intensifier chamber  223  of the injector  8 . It is desired to make the flow area through pilot passage D very small, preferably about 10% of the flow area of the larger outer spool valve supply annulus B. With very restricted actuator fluid flow through the pilot passage D to the intensifier chamber  223  of the injector  8 , the actuation process of the intensifier  204  is controlled at a desirable relatively stable and slow rate. The outer spool valve  14 , along with the two end caps  24 ,  28  and coils  22 ,  26 , performs the basic digital valve concept as illustrated in prior art FIG.  2 . The outer spool valve  14  is attracted from one coil side to the other coil side depending on which coil  22 ,  26  is actuated. 
     FIG. 5 illustrates the theory that the magnetic force is function of the air gap for a given current level. As depicted in FIG. 3, the shifting of the spool valves  14 ,  16  variously opens and closes push piston air gap  40 , open solenoid air gap  41  and close solenoid air gap  42 . The theory of FIG. 5 applies to each of the air gaps  40 - 42 . The magnetic force level is significantly less if the spool valve is at the remote position (air gap is large). The maximum force level will be reached when spool valve is latched to the end cap of a coil which is energized. 
     It is highly desirable that the closing coil  22  generate equal or greater maximum magnetic force (force at zero gap) than the force generated by the opening coil  26 . By doing this, the following features are achieved: 
     (1) If the opening coil  26  is de-energized and the closing coil  22  is energized, the outer spool valve  14  will be latched at the closing coil side end cap  24 . Since the opening coil  26  is de-energized, the inner spool valve  16  along with push piston  18  will be pushed to the push piston stop  32  (away from the opening coil  26 ) by the pre-loaded force of the spring  20 . The spools  14 ,  16  will thus be in the positions shown in FIG.  4 ( a ). 
     (2) If the closing coil  22  is energized and the outer spool valve  14  is latched on the closing coil side end cap  24 , simultaneously energizing the opening coil  26  cannot cause the outer spool valve  14  to move because due to magnetic force and gap theory. The magnetic force produced on the closing coil side  22  is greater than on the opening coil  26  side because there is no air gap between the spool  14  and end cap  24  while there is a maximum air gap on the opening coil side between spool  14  and end cap  28 . See FIGS. 3,  4   a , and  4   b . However, energizing the opening coil  26  will move the push piston  18  to engage the external side  27  of the opening coil end cap  28 , resulting in the spool valves  14 ,  16  assuming the positions shown in FIG.  4 ( b ). 
     (3) If the outer spool valve  14  is on the closing coil side (see FIG. 4 b ), and the closing coil  22  is not energized, energizing the opening coil  26  will move both the outer spool valve  14  and the push piston  18  toward opening coil. This causes both the spool valves  14 ,  16  to move in relatively opposite directions to achieve the relative positions shown in FIG. 4 c . The outer spool valve  14  shifts rightward and the inner spool valve  16  shifts leftward responsive to energizing the open coil  26 . 
     FIG. 6 shows the DATS control valve  10  mounted to in a HEUI injector  8 , including an intensifier chamber  223 , an intensifier piston  236  operatively connected to intensifier plunger  228  so that, upon high pressure actuating fluid being supplied to the intensifier chamber by the DATS control valve  10 , the intensifier piston forces the plunger  228  into the fuel chamber  230 , there by causing the fuel to enter the injection nozzle  206 , lift the needle valve  250  and eject fuel from the nozzle  206 . Operation of the intensifier and nozzle portions of the injector  8  is similar to those portions of the prior art injectors described above. 
     DATS Injector Operation 
     FIGS.  4 ( a ),  4 ( b ), and  4 ( c ) illustrate the operation of the DATS valve  10  of the present invention for obtaining flexible control of different stages of fuel injection flow rates and volumes. 
     FIG.  4 ( a ) shows both spool valve  14 ,  16  positioned in the drain configuration or non-working mode of the injector. In this drain mode position, the intensifier chamber  223  of the injector  8  is vented to the ambient pressure through drain passageways G, F, E, H, I, J, and K. During the drain process, the closing coil  22  is energized, and the opening coil  26  is de-energized. Consequently, the outer spool valve  14  is magnetically latched in the most leftward disposition to the closing coil end cap  24  while the inner spool valve  16  and the push piston  18  are being pushed by the spring  20  against the push piston stop  32  (the most rightward disposition). The pilot passage D is sealed by the land  43  of the inner spool valve  16 . The drain annuluses H and I are wide open. The main flow port A is also fully sealed by land  44  of the outer spool valve  14 . The closing coil  22  is de-energized when the spool valve  14  is in the drain position. The outer spool valve  14  will remain latched to the closing coil end cap until the next injection event due to residual magnetic force. 
     FIG.  4 ( b ) shows the pilot mode configuration of the control valve  10 . This position is preferably commanded in the initial portion of an injection event. Very often, a small volume of actuator fluid flow into the intensifier is preferred during the initial portion of an injection event. This small flow stage is operated in following way. The close coil  22  is energized first and is kept on for a predetermined time during the pilot injection portion of the injection event. The open coil  26  is de-energized. The outer spool valve  14  is thus attracted to the closing coil side and is latched to the end cap  24  to assure the main inlet flow port is initially fully closed. At this point, the pilot passage D is also fully closed by land  43  of inner spool valve  16  as depicted in FIG. 4 a.    
     With the outer spool valve  14  secured on the closing coil side end cap  24 , the opening coil  26  is energized to attract the push piston  18 , thereby moving the inner spool valve leftward compressing spring  20  to open the pilot passage D. High pressure actuating fluid is admitted through the pilot passage D, E, F, G, to the intensifier chamber  223 . The flow rate at this condition is limited to a small and very stable and controllable level. The motion of the intensifier piston  236  will be relatively slow due to the slow flow rate of actuating fluid flow through the pilot passage D. At the end of the pilot injection portion of the injection event, the opening coil  26  is de-energized. The inner spool valve  16  then shifts rightward under the bias of the spring  20 , sealing off the pilot passage D and to provide a dwell period between the pilot injection portion and either the main injection portion of the injection event or a subsequent pilot injection portion of the injection event or to end the injection event, as desired. The rightward shifting of the inner spool valve  16  terminates pilot injection. 
     FIG.  4 ( c ) shows main flow configuration for the main injection portion of the injection event. Under this condition, a larger volume of high pressure actuating fluid is allowed to flow into the intensifier chamber  223  of the injector  8  through both main flow passages C and G. To achieve this, the closing coil  22  is de-energized and the opening coil  26  is energized. Both the outer spool valve  14  and the push piston  18  are latched against opening coil end cap  28 . The outer spool valve  14  is in its rightmost disposition. The inner spool valve  16  is in its leftmost disposition, compressing spring  20 . In this position, the main flow annulus B is open to actuating fluid supply inlet port A. The pilot passage D is still open, augmenting the main flow while the drain annulus H is closed. However, it should be noted that if the pilot passage size is very small, the pilot passage flow may be negligible compared to the main flow. 
     DATS Valve Application on Fuel Injection 
     The DATS valve  10  of the present invention has a broad range of application in the field of hydraulic control. The fundamental feature of this valve  10  is its ability to provide two-stage flow with flexible controllability. When a small flow rate is desired, the DATS valve  10  can be locked in a first position to provide, for example, a pilot mode of operation. When a large flow quantity is desired, the DATS valve  10  can be locked in a second position to provide, for example, a main flow mode of operation. The duration of each mode of operation is flexibly controlled through a pulse-width control modulation to the coils  22 ,  26 . 
     A direct application of the DATS valve  10  is in the diesel fuel injection area. As indicated through the analysis of the prior art injector, it is highly desirable to improve the prior art digital spool valve control for flexible injection operation. The small flow mode is used for pilot injection operation to achieve both controllability and stability. The larger flow mode can be used for main injection operation to achieve high injection pressure and improve injection efficiency. 
     The opening coil  26  and the closing coil  22  of the DATS control valve  10  are energized and de-energized under the control of a programmed engine control microprocessor (not shown) to provide various methods of operation of the DATS injector  8  and the engine. As shown in FIG. 7, the coils  22 ,  26  are energized at E and de-energized at 0. The following fuel injection strategies are possible with the DATS control valve  10 : 
     (1) Single Shot Injection 
     Prior to the start of an injection event, both the inner and outer spool valves  14 ,  16  are in the drain configuration shown in FIG.  4 ( a ). The open coil  26  is energized first attracting both the outer spool valve  14  and the push piston  18 , acting on the inner spool valve  16 , to move to the open coil end cap  28 . The main injection configuration shown in FIG.  4 ( c ) is then achieved. In this configuration, a large flow of high pressure actuating fluid flows into the intensifier chamber  223  of the injector  8 . With a high flow rate and high pressure at the intensifier chamber, the injection pressure at the nozzle  206  builds up quickly and fuel injection occurring under this condition is eruptive and very efficient. Most engine operation under high speed conditions utilize this injection strategy. At end of the injection event, the closing coil  22  is energized and the opening coil  26  is de-energized. The outer spool valve  14  returns to the closing coil end cap  24 . The inner spool valve  16  moves in the opposite direction due to the spring  20  and both the main flow port A and pilot passage D are closed while the drain annuluses H and I open up to vent the intensifier chamber  223  to end the injection event, thereby leaving the components in the drain configuration. Subsequently, the closing coil  22  is de-energized until the next injection event, residual magnetism holding the control valve  10  in the configuration of FIG. 4 c.    
     (2) Pilot Injection 
     Pilot injection is achieved by the following operation strategy. The closing coil  22  is energized first to assure that the outer spool valve  14  shifts leftward and stays latched on the closing coil side end cap  24 . See FIG. 4 b . When the outer spool valve  14  is latched on the closing coil side end cap  24 , energizing the opening coil  26  can only make the inner spool valve  16  move leftward to open pilot passage D so that a small quantity of high pressure actuating fluid flows from the high pressure input port A into the intensifier chamber  223 . With a small actuating fluid flow rate, fuel injection starts slowly and very steady. The opening coil  26  is de-energized when the desired quantity of pilot fuel injection is achieved which is proportional to the pulse width duration applied to the opening coil  26 . Such de-energization frees the spring  20  to shift the inner spool valve  16  rightward, sealing off the pilot passage D. See FIG. 4 a . Pilot injection ends when the drain port J opens as the inner spool valve  16  returns to the drain configuration. 
     The injector  8  is in the dwell period between injection events. Both the opening and closing coils  22 ,  26  may be de-energized. At the end of the dwell period, the opening coil  26  is energized again while the closing coil  22  stays de-energized at the initiation of the succeeding injection event. The outer spool valve  14  and the push piston  18  are thereby caused to shift toward the opening coil end cap  28  resulting in the main injection configuration. The outer spool valve  14  is in its rightmost disposition and the inner spool valve  16  is in its leftmost disposition. As above, the main flow of high pressure actuating fluid flows from the high pressure input port A in to the intensifier chamber  223  through both the main flow path (passage A to B to C) and the pilot flow path (passage A to D to E to F) to provide main injection. At end of main injection, the closing coil  22  is energized and the opening coil  26  is de-energized. The intensifier chamber  223  is vented through the drain annuluses H and I and all components go back to the drain configuration. Pilot injection strategy is regarded as the most important injection strategy to provide low noise and low emissions from the engine. 
     Boot or Rate-shaping Injection 
     Boot or rate-shaping injection is similar to pilot injection described above but without an obvious dwell period between the pilot injection and the main injection. Boot injection is characterized by a small injection flow rate occurring before the main injection starts (the rate of injection curve over time appearing similar to the outline of a boot). It is highly desired to have flexibly control both the initial low rate of fuel injection and the subsequent high rate of fuel injection. With the injector  8  having the DATS control valve, the small quantity of the initial portion of injection is achieved by the flow through pilot passage D and thence to passages E and F to chamber  223 . Similar to pilot operation discussed above, the closing coil  22  is energized first to latch the outer spool valve  14  on the closing coil side end cap  24 . The opening coil  26  is then energized resulting in L to deliver the pilot flow quantity. Injection starts but at a very small injection flow rate. When the desired initial low rate of injection duration is achieved, the closing coil  22  is then de-energized to release the outer spool valve  14 . Since the opening coil  26  is still energized, the outer spool valve  14  soon shifts to latch on the opening coil side end cap  28 . The main injection flow starts as a function of the shifting of the outer spool valve  14  while the pilot flow still continues. The end of the injection event is achieved by de-energizing the open coil  26  and energizing the close coil  22 . The control valve  10  reverts to the disposition of FIG. 4 a.    
     (4) Far Split Injections 
     This injection strategy is very often used at engine idle and cold engine operations. Far split injection is two single injections of low (but greater than pilot quantity) occurring in close sequence within the same injection event. The operation of the DATS control valve  10  for this strategy is to operate the Single Shot Injection strategy described above and, at the end of the injection described above and within the same injection event or engine cycle, de-energizing the closing coil  22  and energizing the opening coil  26  to achieve a second single shot injection. The far split injection is ended by de-energizing the opening coil  26  and energizing the closing coil  22  to end the injection event with the control valve  10  in the drain configuration after which the closing coil may be de-energized to await the next injection event. 
     DATS Valve  10  with Sleeve Design 
     FIG. 8 illustrates a schematic of the DATS valve  10  with a sleeve design, a further embodiment of the present invention. A sleeve  50  is placed between the outer spool valve  14  and the inner spool valve  16 . The sleeve  50  is a simple cylindrical shape having an axial bore defined in the center. The sleeve  50  is preferably made out of non-magnetic material and is stationary in all modes of operation. There are several flow passages defined in the sleeve body  52  to provide flow communication between the inner spool valve  16  and the outer spool valve  14 . The DATS valve including the sleeve  50  provides at least three advantages. 
     The direct friction is avoided between the inner spool valve  16  and the outer spool valve  14  that would otherwise arise due to oppositely directed motion. By eliminating this friction, motion variability due to relative motion is minimized. 
     The design also provides manufacturing simplicity. As shown on FIG. 3, an internal groove drilling process is required to produce the groove R to drain the fluid to ambient. This internal drilling process can be relatively difficult when the diameter of the inner spool valve  16  is relatively small. With the DATS valve including sleeve  50 , all internal drillings are replaced by external grooves and bores, which are much easier to form during manufacturing. As shown on FIG. 8, bores R and outer groove K are used to replace the inner groove R on FIG.  3 . 
     The sleeve  50  has a simple cylindrical body  52  with a wheel type structure on the double-acting coil  26  side. The cylindrical body  52  has an axial bore  54 . The inner spool valve  16  is translatably disposed in the bore  54 . FIGS. 5,  8  and  9  show a schematic of the wheel type configuration of the body  52 . The wheel structure  55  includes a plurality of spokes  56 . Each spoke  56  has a tip  58  having an end margin  60  that abuts the surface  31  of the stop  32 . The wheel structure  55  and the end cap  28  are preferably bonded together through a proper welding technique. When the push piston  18  moves towards end cap  28 , the push piston  18  contacts the wheel spokes  56  and does not directly contact the end cap surface  62  as shown by a small air gap  64  on FIG. 8 in the right lower corner. There is a very small gap  64  remaining between push piston  18  and end cap  28 . Due to this slight air gap  64 , the maximum magnetic force is slightly reduced (on the order of approximately 5%). This reduction can be considered to be negligible. The wheel type structure  55  secures the overall assembly structure of the valve  10  and prevents any structural damage caused by a high speed impact of the push piston  18  on the end cap surface  64 . Such impact would occur absent the interventions of the spokes  56  to arrest the leftward travel of the push piston  18 . Since the sleeve wheel structure  55  is non-magnetic and the wheel structure  55  has only few wheel spokes  56 , the magnetic flux path remains nearly the same as the path of the embodiment of FIG.  3 . There is enough magnetic area for flux to directly travel through the air gap or to go around the wheel spokes  56  through the air gap to the push piston  18  to generate sufficient magnetic force on the push piston  18 . 
     During pilot flow operation, the outer spool valve  14  is secured at the end cap  24  by energizing the closing coil  22 . This latches the outer spool valve  14  The main flow port B is closed. The opening coil  26  is then turned on. The push piston  18  starts to move leftward towards the opening end cap  28  under influence of the magnetic force generated by the opening coil  26 . As the inner spool valve  16  moves to the left with the push piston  18 , the inner spool valve  16  opens the pilot flow hole D and closes venting hole R. A limited flow rate passes from the inlet  36  through A, D 1 , D 2  and the restricted area D. Flow is then through F 1 , F and G to the actuator (chamber  223 ). 
     The drain passages R, K J, and I are completely shut off when the push piston  18  is arrested on the spokes  56  of the wheel structure  55 . In this mode of operation, the flow from inlet  36  to the actuator (intensifier chamber  223 ) is controlled at a selected relatively small flow rate. The size of pilot bore D is used to achieve the desired small flow rate. 
     This pilot flow mode is ended by de-energizing the coil  26 . The spring  20  then pushes the inner spool valve  16  and the push piston  18  to the rightmost position, thereby closing bore D and opening vent bore R. Actuating fluid is then vented from G to F and F 1 , to R and then outward through I and J to the outlet. 
     During main flow operation, the closing coil  22  is de-energized and the opening coil  26  is activated. Both the outer spool valve  16  and the push piston  18  are simultaneously moved towards the end cap  28 , the outer spool valve  16  moving rightward and the push piston  18  moving leftward on both the inner and outer surfaces of the stationary sleeve  50 . This countermotion causes the main flow port B to open and a significant amount of flow occurs from inlet  36  through A, through groove B to actuation port C and then to the actuator (chamber  223 ). At the same time, pilot flow also flows through bore D 1 , sleeve groove D 2 , pilot bore D, annulus E, bore F 1  and F to port G to chamber  223 . The venting port R is blocked by the inner spool valve  16  completely. End of the main flow is achieved by energizing the coil  22  and at the same time de-energizing the coil  26