Patent Publication Number: US-7219655-B2

Title: Fuel injection system including two common rails for injecting fuel at two independently controlled pressures

Description:
TECHNICAL FIELD 
   The present invention relates generally to fuel injection systems, and more specifically to a fuel injection system and a method of injecting fuel using two common rails at different controlled pressures. 
   BACKGROUND 
   Engineers are constantly seeking ways to reduce undesirable engine emissions. One strategy is to seek ways to improve performance of fuel injection systems. Over the years, engineers have come to learn that engine emissions can be a significant function of injection timing, the number of injections, injection quantities and rate shapes. However, it has been observed that an injection strategy at one engine operating condition may decrease emissions at that particular operating condition, but actually produce an excessive amount of undesirable emissions at a different operating condition. Thus, a fuel injection system with a variety of capabilities to produce a variety of injection strategies can better perform and reduce emissions at all engine operating conditions than a fuel injection system limited in its control over injection timing, number, quantity and rate shapes. Further, increases in the ability to vary injection rates, injection numbers, injection quantities and rate shapes can lead to more research on, and discovery of, improved injection strategies at different operating conditions. 
   One apparent attempt to provide a fuel injection system that can quickly vary the pressure of injections is disclosed in “Heavy Duty Diesel Engines—The Potential of Injection Rate Shaping for Optimizing Emissions and Fuel Consumption”, presented by Messers, Bemd Mahr, Manfred Durnholz, WilhelmPolach, and Hermann Grieshaber, Robert Bosch GmbH, Stuttgart, Germany, at the 21 st  International Engine Symposium, May 4–5. 2000, Vienna, Austria. This reference teaches a common rail system and a directly controlled fuel injector that purportedly has the ability to inject medium pressure fuel directly from the rail, or utilize the fuel common rail to pressure intensify fuel within the injectors for injection at relatively high pressures. The magnitude of high pressure of pressure intensified injection will be, in part, a function of the pressure of the fuel acting on a pressure intensifier within the fuel injector. 
   While this fuel injection system theoretically may have the ability to produce multiple injections, each at different pressures and close in time, the fuel injection system does have drawbacks. For instance, the fuel used to actuate the pressure intensified injection and the fuel being injected directly from the common rail have the same source, i.e., the common rail. Thus, they are both at common rail pressure. In situations in which there is insufficient time to alter the pressure within the common rail between injections, the high pressure of the pressure intensified injection is dependent on the medium pressure injection of the common rail injection, or vice versa. For instance, the pressure of a main injection that is pressure intensified is limited by the pressure of a pilot injection directly injected from the common rail. Thus, although the pressure of the high pressure injection is greater than the pressure of the medium pressure injection, the Bosch fuel injection system lacks the capability to vary the pressure of the high pressure injection without also varying the pressure of the medium pressure injection. 
   The present invention is directed to increasing the capabilities of fuel injection systems. 
   SUMMARY OF THE INVENTION 
   In one aspect of the present invention, a fuel injection system includes at least a first common rail and a second common rail. At least one fuel injector is fluidly connectable with the first common rail and the second common rail. The first common rail is at a first pressure, and the second common rail is at a second pressure that is independent of the first pressure. 
   In another aspect of the present invention, a fuel injector includes a fuel injector body that defines at least a low pressure outlet, a medium pressure inlet, a high pressure inlet, and a nozzle outlet. A direct needle control valve includes a closing hydraulic surface. 
   In yet another aspect of the present invention, there is a method of injecting fuel. Fuel is injected at a first pressure, at least in part, by fluidly connecting a fuel injector to a first common rail. Fuel is injected at a second pressure, at least in part, by fluidly connecting the fuel injector to a second common rail. The second pressure is independent of the first pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of a fuel injection system according to the present invention; 
       FIG. 2  is a sectioned side diagrammatic view of a fuel injector according to the present invention; 
       FIG. 3  is the fuel injector of  FIG. 2  as viewed along a different section line; and 
       FIGS. 4   a – 4   e  are exemplary graphs illustrating fuel injection rate, plunger movement, flow control valve movement, needle control valve movement and sac pressure versus time, respectively, for an example injection sequence according to the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a six cylinder diesel engine  10  includes a double common rail fuel injection system  12 . The system  12  includes an individual fuel injector  14  for each engine cylinder, an oil circuit  38  including an oil common rail  16  fluidly connected to an oil sump  20 , and a fuel circuit  39  including a fuel tank  18  fluidly connected to a fuel common rail  17 . Those skilled in the art will appreciate that in other applications there may be two or more separate oil common rails and/or fuel common rails, such as a separate fuel and/or oil rail for each side of a “V” engine. Although the two common rails  16  and  17  are illustrated as including two different fluids, it should be appreciated that both common rails could include the same fluid, i.e., fuel. Further, it should be appreciated that if both common rails included fuel, both common rails could be fluidly connectable to one another as long as each common rail could deliver fuel to the fuel injectors  14  at a pressure independent of the other common rail. For instance, the common rails could be fluidly connectable to one another and to the fuel injectors with a pressure regulator positioned between the two common rails. In addition, if both rails contained fuel, the present invention contemplates fuel from both common rails being injected directly into the engine cylinder with or without an option of pressure intensification. 
   An electronic control module  22  controls the operation of fuel injection system  12 . The electronic control module  22  preferably utilizes advanced strategies to improve accuracy and consistency among the fuel injectors  14  as well as pressure control in oil common rail  16  and fuel common rail  17 . For instance, the electronic control module  22  might employ electronic trimming strategies individualized to each fuel injector  14 , to perform more consistently. Consistent performance is desirable in the presence of the inevitable performance variability responses due to such causes as realistic machining tolerance associated with the various components that make up the fuel injectors  14 . In another strategy, the electronic control module  22  might employ a model based rail pressure control system that breaks up the rail pressure control issue into one of open loop flow control coupled with closed loop error and pressure control. 
   When fuel injection system  12  is in operation, oil in an oil fluid circuit  38  is drawn from oil sump  20  by a low pressure oil circulation pump  24 , and the outlet flow is split between an engine lubrication passage  27  and a low pressure fuel injection supply line  28  after passing through an oil filter  25  and a cooler  26 . The oil in engine lubrication passage  27  travels through the engine  10  and lubricates its various components in a conventional manner. The oil in low pressure supply line  28  is raised to a medium pressure level by a high pressure pump  29 . This “medium pressure” is a relatively high pressure compared to oil drain pressure, but still lower than peak injection pressures. Pump  29  is preferably an electronically controlled variable delivery pump, such as a sleeve metered fixed displacement variable delivery pump. Thus, the pump  29  can vary the pressure of the oil being delivered from the pump  29 . High pressure pump  29  is connected to common rail  16  via a high pressure supply line  30 . Each of the individual fuel injectors  14  have an actuation fluid inlet  60  connected to common rail  16  via a separate branch passage  31 . After being used within individual fuel injectors  14  to pressurize fuel, the oil leaves fuel injectors  14  via an actuation fluid drain  62  and returns to oil sump  20  for recirculation via a return line  32 . 
   Fuel is drawn from a fuel tank  18  by a fuel transfer pump  36  and delivered to a high pressure fuel pump  19  via a fuel supply line  34  that includes a fuel filter  37 . The fuel transfer pump  36  and fuel filter  37  are preferably contained in a common housing. Fuel transfer pump  36  is preferably a constant flow electric pump, whereas the high pressure fuel pump  19  is preferably an electronically controlled variable delivery pump, such as a sleeve metered fixed displacement variable delivery pump. The high pressure fuel pump  19  pressurizes the fuel to a “medium pressure”, and delivers the pressurized fuel to a fuel common rail  17 . Although the fuel high pressure pump  19  can vary the pressure of the fuel being delivered from the pump  19 , the “medium pressure” of the fuel is less than peak injection pressure. It should be appreciated that the medium pressure of the fuel can be greater or less than the medium pressure of the oil from the oil common rail  16  depending on the desired injection strategy. Each of the fuel injectors  14  have a fuel inlet connected to the fuel common rail  17  via a separate fuel branch passage  15 . Any fuel not used by the fuel injectors  14  is recirculated to fuel tank  18  via fuel return line  35 , which is connected to low pressure fuel outlet  65 . 
   Fuel injection system  12  is controlled in its operation via an electronic control module  22  via control communication lines  40 ,  21  and  41 . Control communication line  40  communicates with oil high pressure pump  29  and controls its delivery, and hence the pressure in oil common rail  16 . Similarly, control communication line  21  communicates with fuel high pressure pump  19  and controls its delivery, and hence the pressure in fuel common rail  17 . Control communication line  41  includes four wires, one pair for each electrical actuator within each fuel injector  14 . Those skilled in the art will appreciate that by modifying control signals, a single pair of wires could be used to control two electrical actuators. In addition, there may be more wires, such as for carrying feed back signals to the electronic control module. These respective actuators within fuel injectors  14  control flow of actuation fluid to the injectors from rail  16 , and the opening and closing of the fuel injector spray nozzle. Electronic control module  22  determines its control signals based upon various sensor inputs known in the art. These include an oil pressure sensor  42  attached to rail  16  that communicates an oil pressure signal via sensor communication line  45 , and a fuel pressure sensor  51  attached to rail  17  that communicates a fuel pressure signal via a communication line  52 . In addition, an oil temperature sensor  43  and a fuel temperature sensor  53 , which are also attached to rail  16  and  17 , respectively, communicate an oil temperature signal and a fuel temperature signal to electronic control module  22  via sensor communication lines  44  and  54 , respectively. The electronic control module  22  receives a variety of other sensor signals via a sensor communication line(s)  46 . These sensors could include but are not limited to, a throttle sensor  47 , a timing sensor  48 , a boost pressure sensor  49  and a speed sensor  50 . 
   Referring in addition to  FIGS. 2 and 3 , each fuel injector  14  includes an injector body  61  that can be thought of as including a fuel pressurization portion  66  and an injection portion  68 . Fuel injector  14  can also be thought of as being divided between fuel pressurization assembly  67  and a direct control nozzle assembly  69 . In the fuel injector  14  illustrated, fuel pressurization assembly  67  is located in fuel pressurization portion  66 , whereas direct control nozzle assembly  67  is located in injection portion  68 . Although the fuel injector  14  shows the fuel pressurization assembly  67  and the direct control nozzle assembly  69  joined into a unit injector  14 , those skilled in the art will appreciate that those respective assemblies could be located in separate bodies connected to one another with appropriate lines. The fuel pressurization assembly  67  includes a pressure intensifier  70  and a flow control valve  74 , which is operably coupled to a first electrical actuator  72 . Direct control nozzle assembly  79  includes a needle control valve  76  that is operably coupled to a second electrical actuator  78 , which is located in and attached to injection portion  68 . In addition, a direct control needle valve  79  is controlled in its opening and closing by needle control valve  76 , and hence second electrical actuator  78 . Pressurized oil enters injector body  61  through its top surface at high pressure actuation fluid inlet  60 , and used low pressure oil is recirculated back to the sump  24  via a low pressure actuation fluid drain  62 . Fuel is circulated to and from the fuel injectors  14  via a medium pressure fuel inlet  64  and a low pressure outlet fuel outlet  65 . 
   Pressure intensifier  70  includes a stepped top intensifier piston  82  and preferably a free floating plunger  84 . Intensifier piston  82  is biased to its retracted position, as shown, by a return spring  83 . The stepped top of intensifier piston  82  allows the initial movement rate, and hence possibly the initial injection rate, to be lower than that possible when the stepped top clears a counter bore. Return spring  83  is positioned in a piston return cavity  86 , which is vented directly to the area underneath the engine&#39;s valve cover via an unobstructed vent passage  87 . Free floating plunger  84  is biased into contact with the underside of intensifier piston  82  via low pressure fuel acting on one end in fuel pressurization chamber  90 . Plunger  84  preferably has a convex end in contact with the underside of intensifier piston  82  to lessen the effects of a possible misalignment. In addition, plunger  84  is preferably symmetrical about three orthogonal axes such that fuel injector  14  can be more easily assembled by inserting either end of plunger  84  into the plunger bore located within injector body  61 . When intensifier piston  70  is undergoing its downward pumping stroke, fuel within fuel pressurization chamber  90  is raised to high pressure injection levels. Any fuel that migrates up the side of plunger  84  is preferably channeled back for recirculation via a plunger vent annulus  88  and a vent passage  92 . Pressure intensifier  70  is driven downward when flow control valve  72  connects actuation fluid passages  80 / 81  to high pressure actuation fluid inlet  60 . Between pressure intensified injection events, flow control valve  72  connects actuation fluid passages  80 / 81  to low pressure drain  62  allowing the intensifier  70  to retract toward its retracted position, as shown, via the action of return spring  83  and fuel pressure acting on the underside of plunger  84 . Thus, when pressure intensifier  70  is retracting, fresh fuel is pushed into fuel pressurization chamber  90  past check valve  93  via fuel inlet  64 . 
   Referring in addition to  FIG. 4 , flow control valve  72  includes the first electrical actuator  74 , which in the illustrated embodiment as a solenoid, but could equally be any other suitable electrical actuator known in the art including, but not limited to, piezos, voice coils, etc. Flow control valve  72  includes a valve body that includes separate passages connected to actuation fluid inlet  60 , actuation fluid drain  62  and actuation fluid passages  80 / 81 , respectively. Flow control valve  72  includes a spool valve member  113  that is biased to a first position that fluidly connects an actuation fluid passage  80 / 81  to actuation fluid drain  62 . When electrical actuator  74  is energized, the valve member  113  moves to a second position. At this energized position, spool valve member  113  closes the fluid connection between actuation fluid passage  80 / 81  and drain  62 , and opens high pressure inlet  60  to actuation fluid passages  80 / 81 . Control communication line  41  of  FIG. 1 , electronic control module  22 , and electric terminals that are attached to valve body  113  are electrically connected to the electrical actuator  74  in a conventional manner. 
   When pressure intensifier  70  is driven downward, high pressure fuel in fuel pressurization chamber  90  can flow via nozzle supply passage  107  to the nozzle chamber  105 , and out of nozzle outlets  104  if direct control needle valve  79  is in an open position. Further, even when pressure intensifier  70  is in the retracted position, “medium” pressure fuel that flows from the fuel common rail  17  to the fuel pressurization chamber  90  can flow via nozzle supply passage  107  to the nozzle chamber  105 , and out of the nozzle outlets  104  if direct control needle valve  79  is in the open position. When direct control needle valve  79  is in its closed position as shown, nozzle chamber  105  is blocked from fluid communication with nozzle outlets  104 . 
   Direct control needle valve  79  includes a needle valve member made up of a needle  112  separated from a piston  109  by a lift spacer  106 . Thus, the needle valve member in this embodiment is made up of several components for ease of manufactureability and assembly, but could also be manufactured from a single solid piece. The needle valve member includes an opening hydraulic surface  103  exposed to fluid pressure in nozzle chamber  105  and a closing hydraulic surface  101  exposed to fluid pressure in a needle control chamber  100 . The thickness of lift spacer  106  preferably determines the maximum opening travel distance of direct control needle valve  79 . The direct control needle valve  79  is biased toward its downward closed position, as shown, by a biasing spring  102  that is compressed between lift spacer  106  and a VOP (valve opening pressure) spacer  108 . Thus, the valve opening pressure of the direct control valve  79  can be trimmed at time of manufacture by choosing an appropriate thickness for VOP spacer  108 . 
   Needle control chamber  100  is fluidly connected to either low pressure fuel outlet  65  or to nozzle supply passage  107  depending upon the positioning of needle control valve assembly  76 . When needle control chamber  100  is fluidly connected to nozzle supply passage  107 , regardless of whether the nozzle supply passage  107  includes high pressure fuel or medium pressure fuel, direct control needle valve  79  will remain in, or move toward, its closed position, as shown, under the action of fluid pressure forces on closing hydraulic surface  101  and the spring force from biasing spring  102 . When needle control chamber  100  is fluidly connected to fuel outlet  65 , while nozzle supply passage  107  and hence nozzle chamber  105  are above a valve opening pressure, the fluid forces acting on opening hydraulic surface  103  are sufficient to lift the direct control needle valve  79  upward towards its open position against the action of biasing spring  102  to open nozzle outlets  104 . It should be appreciated that the nozzle control chamber  100  could be connectable to any passage being at a lower pressure than the “medium” pressure of the fuel common rail  17 . 
   In the illustrated embodiment, the needle control valve  76  is a two position three way valve that includes a needle control valve member  114  that is moveable between contact with a high pressure seat and a low pressure seat. Depending upon the position of valve member  114 , needle control chamber  100 , is fluidly connected to nozzle supply passage  107  via connection passage  110  or to fuel outlet  65  via low pressure passage  111 . A needle control passage (not shown) fluidly connects the needle control chamber  100  to either the connection passage  110  or the low pressure passage  111 . Needle control valve  76  includes the second electrical actuator  78  which in the illustrated embodiment is a solenoid subassembly, but could also be another type of electrical actuator, such as a piezo, a voice coil, etc. The solenoid subassembly includes a stator, a coil and a pair of electrical connectors. The female electrical connectors, which could instead be male, permits an electrical extension to mate with solenoid subassembly within injector body  71  while providing exposed terminals for insulated conductors  95  outside of fuel pressurization portion  66 . Valve member  114  is biased downward to close a low pressure seat and low pressure passage  111 . When the second electrical actuator  78  is energized, valve member  114  opens the low pressure seat (passage  111 ) and closes the high pressure seat (passage  110 ). 
   INDUSTRIAL APPLICABILITY 
   During each engine cycle, each fuel injector  14  has the ability to inject up to five or more discrete shots. While a majority of these injection events will take place at or near the transition from the compression to power strokes, injection events can take place at any timing during the engine cycle to produce any desirable effect. For instance, an additional small injection even elsewhere in the engine cycle might be useful in reducing undesirable emissions. Although the present invention finds a preferred application in four cycle engines, it could also be used in two cycle engines. For the purposes of illustrating the present invention, the injections discussed will be classified into two categories: (1) pressure intensified injections, generally producing a relatively high pressure injection; and (2) common rail injections, generally producing a relatively low pressure injection. During each engine cycle, a number of basic steps can be performed to create both types of injections or a hybrid of the types of injections, and each of those steps is performed at a timing and in a number to produce a variety of fuel injection sequences. 
   Referring to  FIGS. 4   a – 4   e , there are shown exemplary graphs illustrating fuel injection rate, plunger movement, flow control valve movement, needle control valve movement and sac pressure versus time, respectively, for an example injection sequence according to the present invention. The example injection sequence includes a pilot injection  115 , a main injection  116 , and a post injection  117 . The pilot injection  115  and the post injection  117  are illustrated as common rail injections, and the main injection  116  is illustrated as a hybrid of a pressure intensified injection and a common rail injection. However, it should be appreciated that the teachings of the present invention can be used to create various injection sequences that include any number of injections at various pressures. For instance, by controlling the timing of the signals to the flow control valve and the needle control valve, a main injection event can include various rate shapes, including but not limited to boot, ramp and square rate shapes. Further, an injection sequence can include more than one pilot and/or post injection, or include no pilot and/or post injection. An injection sequence that may be desirable to reduce emissions at one operating condition may not be desirable at another operating condition. Thus, the electronic control module  22 , in a conventional manner, will control the signals to the flow control valve  72 , the needle control valve  76 , the oil high pressure pump  29 , and the fuel high pressure pump  19  in order to create an injection sequence that is desirable for the particular operating condition. It is known in the art that the electronic control module  22  is programmed to determine the proper injection sequence for the particular operating condition based on the sensed parameters being communicated from the various pressure sensors  42 ,  43 ,  47 ,  48 ,  49 ,  50 , and  51 . 
   Referring also to  FIGS. 1–3 , prior to the beginning of the example injection sequence, fuel has been drawn from the fuel tank  18  by the fuel transfer pump  36  and delivered to the high pressure fuel pump  19 . The high pressure fuel pump  19  will raise the pressure of the fuel to be delivered to the fuel common rail  17 . Because the output of the high pressure fuel pump  19  is electronically controlled, the pressure within the fuel common rail  17  is controlled by the electronic control module  22 . As discussed earlier, the electronic control module  22 , using any method known in the art, will determine the desired pressure at which the pilot injection  115  should occur and adjust the signal to the high pressure fuel pump  19  accordingly. Prior to the injection sequence, oil will have been drawn from the oil sump  20  by the low pressure oil pump  24  and delivered to the high pressure oil pump  29 . The electronic control module  29  will determine the desired pressure of the common rail  16  based on the desired pressure of the main injection event  116  and adjust the signal to the oil high pressure pump  29  accordingly. It should be appreciated that the pressure within the fuel common rail  17  and/or the oil common rail  16  could be controlled by apparatuses other than variable delivery pumps,  19  and  29 , respectively, including, but not limited to, electronically controlled valves. Further, it should be appreciated that the pressure of the oil within the common rail  16  can be higher or lower than the pressure within the fuel common rail  17  depending on the desired injection sequence. The pressurized oil will be delivered to the fuel injectors  14  via the oil common rail  16  and the branch passages  31 , and the pressurized fuel will flow through the supply branches  15  and into the respective fuel injectors  14 . 
   Because the injector  14  is between injection events, the pressure intensifier  70  including the plunger  84  will be either in the retracted position or in the process of retracting. If the plunger  84  is in the process of retracting, the fuel will be drawn into the fuel pressurization chamber  90  past the check valve via  93  the fuel inlet  64 . If the plunger  84  has already retracted, the fuel pressure produced by the high pressure fuel pump  19  is sufficient to push the fuel into the fuel pressurization chamber  90  past the check valve  93 . Because the needle control valve  76  is biased to open needle control chamber  100  to nozzle supply passage  107 , the medium pressure fuel within the nozzle supply passage  107  will be insufficient to open the direct control needle valve  79 . Nozzle outlet  104  will remain closed and the fuel will not be injected. Further, because the flow control valve  72  is biased to fluidly connect the actuation passages  80 / 81  with the actuation fluid drain  62 , the plunger  84  will not advance and further pressurize the fuel within the fuel pressurization chamber  90 . 
   In order to initiate the pilot injection  115 , the electronic control module  22  will energize the second electrical actuator  78  operably coupled to the needle control valve  76 . The needle control valve  76  will move from its biased position blocking the low pressure passage  111 . Although the present invention illustrates the needle control valve  76  being biased to block the low pressure passage  111 , it should be appreciated that the needle control valve  76  could be biased to block the high pressure passage  110 . The closing hydraulic surface  101  of the direct control needle valve  79  will be exposed to low pressure within the fuel drain fluidly connected to the fuel outlet  65 , and thus, the pressure of the fuel within the nozzle chamber  105  acting on the opening hydraulic surface  103  of the needle  112  will be sufficient to lift the direct control needle valve  79  against the spring  102  and the low pressure acting on the closing hydraulic surface  101  in order to open the nozzle outlet  104 . The fuel from the fuel common rail  17  can then be directly injected into the engine cylinder as the pilot injection  115 . Because pilot injections generally are of smaller quantity than the main injection, the lower pressure at which the pilot is being injected can exploited to improve the accuracy of the injection quantity. 
   In order to end the pilot injection  115 , the electronic control module  22  will cease the supply of electric current to the second electrical actuator  78 , causing the needle control valve  76  to move to its biased position exposing the closing hydraulic surface  101  of the direct control needle valve  79  to pressure within the nozzle supply passage  107  via passage  110 . The pressure acting on the opening hydraulic surface  103  of the direct control needle valve  79  will be insufficient to keep the direct control needle valve  79  open, and the needle  112  will close, blocking the nozzle outlet  104 . During the pilot injection  115 ,  FIG. 4   b  illustrates that the plunger  84  of the pressure intensifier  70  does not advance, and  FIG. 4   c  illustrates that the flow control valve  72  does not move from its biased position. Further,  FIG. 4   e  illustrates that, during the pilot injection  115 , the sac pressure, which is the pressure below the tip of the needle  112 , is at the common rail pressure. 
   When the electronic control module  22  determines that the main injection  116  event illustrated in  FIG. 4   a  is desirable, the electronic control module  22  will again energize the second electrical actuator  78  via communication line  41 . Again, as illustrated by  FIG. 4   d , the needle control valve  76  will move to fluidly connect the needle control chamber  100  to the fuel outlet  65 , and thus expose the closing hydraulic surface  101  to low pressure, causing the direct control needle valve  79  to open the nozzle outlet  104 . As illustrated in  FIG. 4   b , the plunger  84  still remains in the retracted, biased position, and thus, the pressure within the fuel pressurization chamber  90  is low enough that the fuel from the common rail  17  can flow past the check valve  93  into the fuel pressurization chamber  90  and the nozzle supply passage  107 . The fuel from the common rail  17  will be injected into the engine cylinder to begin the main injection event  116 . 
   In the illustrated example, the pressure of the main injection  116  increases over time. In order to increase the pressure, the electronic control module  22  will send electric current to the first electrical actuator  74  operably coupled to the flow control valve  72  while continuing to energize the second electrical actuator  78  operably coupled to the needle control valve  76 . As illustrated in  FIG. 4   c , the flow control valve  72  will move from its biased position in which the actuation passages  80 / 81  are fluidly connected to the fluid actuation drain  62  to its energized position in which actuation passages  80 / 81  are fluidly connected to the fluid actuation inlet  60 . The high pressure oil being delivered to the fuel injector  14  has been pressurize by the high pressure oil pump  29 . The high pressure oil flowing from the oil common rail  16  to the fuel injector  14  via the flow control valve  72  will act on the hydraulic surface  85  of the pressure intensifier  70 , causing the plunger  84  to advance as illustrated in  FIG. 4   b . As the plunger  84  advances, the pressure of the fuel within the fuel pressurization chamber  90  increases. Due to the increased pressure, check valve  93  closes, and fuel from the fuel common rail  17  cannot flow into the fuel pressurization chamber  90 . The fuel within the fuel pressurization chamber  90  will ultimately increase to a relatively high pressure that is a function of the oil pressure acting on the hydraulic surface  85 . Thus, the pressure of the main injection  116  results from the electronic control of the high pressure oil pump  29 . Although the “high pressure” may vary, the pressure intensified fuel is injected at a higher pressure than the “medium pressure” fuel that is not pressure intensified by the pressure intensifier  70 . 
   The pressure intensified fuel will flow into the nozzle supply passage  107  and act on the opening hydraulic surface  103  of the needle  112 . As illustrated in  FIG. 4   d , the needle control valve  76  will, thus, remain in the open position allowing the pressure intensified fuel to be injected into the engine cylinder. As illustrated in  FIG. 4   e , the sac pressure of the main injection  116  has increased from the common rail pressure to the intensified pressure due to the advancement of the plunger  84  during the main injection  116 . 
   In order to decrease the pressure of the main injection  116  to a third pressure, being less than the “medium” pressure of the fuel common rail  17 , the electronic control module  22  may cease sending electric current to the first electrical actuator  74  operably coupled to the flow control valve  72 . Thus, as illustrated in  FIG. 4   c , the flow control valve  72  will return to its biased position blocking the flow of high pressure oil from the actuation passages  80 / 81 . Because there is not high pressure acting on the hydraulic surface  85 , the plunger  84  will retract to its upward position under the return action of the spring  83  and fuel pressure acting on plunger  84 . As the plunger  84  retracts, pressure within the fuel pressurization chamber  90  drops, and fuel from the fuel common rail  17  will be drawn in through the fuel inlet and past the check valve  93 . Further, as the plunger  84  retracts, the fuel injector  14  will continue to inject fuel from the fuel pressurization chamber  90  and the nozzle supply passage  107  because the direct control needle valve  79  will remain in the open position. Thus, the fuel being injected as the plunger  84  retracts may be injected at a pressure less than the “medium pressure” of the fuel common rail  17  due to the retracting plunger  84 . Thus, by injecting fuel as the plunger  84  retracts, fuel can be injected into the engine cylinder at the third, relatively low pressure. 
   To end the main injection  116 , the electronic control module  22  will cease sending electric current to the second electric actuator  78 . As illustrated in  FIG. 4   d , the needle control valve  76  will return to its biased position in which the needle control chamber  100  is in fluid communication with the nozzle supply passage  107  and blocked from fluid communication with the fuel outlet  65 . Therefore, the pressure within the nozzle supply passage  107  acting on the opening hydraulic surface  103  of the direct control needle valve  79  is insufficient to overcome the pressure acting on the closing hydraulic surface  101  and the bias of the spring  102 . Thus, the direct control needle valve  79  will move to the closed position blocking the nozzle outlet  104 . 
   The illustrated injection sequence includes a post injection  117 . Although post injections can be of varying quantity, timing, and pressure, post injections often occur with smaller quantities of fuel and at lower pressures than do main injections if possible. In the illustrated example, prior to the post injection  117 , both the flow control valve  72  and the needle control valve  76  are in their biased positions. Thus, the plunger  84  is still retracting, and the nozzle outlets  104  are closed. Due to the, retracting plunger  84 , the pressure within the fuel pressurization chamber  90  is sufficiently low that fuel can flow past the check valve  93  into the fuel pressurization chamber  90  and the nozzle supply passage  107 . In order to initiate the post-injection  117 , the electronic control module  22  will energize the second electrical actuator  78 . The needle control valve  79  will open fluid communication between the needle control chamber  100  and the fuel drain  65 , causing the fuel within the nozzle supply passage  107  to lift the needle  112  and to inject into the engine cylinder. The example shows the post injection event occurring at a relatively low pressure due to the plunger retraction. The electrical actuator  74  coupled to the flow control valve  72  will not be energized. Thus, the plunger  84  will not advance, but will continue retracting as illustrated by  FIG. 4   b , and the post-injection  117  will occur at the “third pressure” below that in the fuel common rail  17  as illustrated by  FIG. 4   e . To end the post injection  117 , the electronic control module  22  will stop energizing the second electrical actuator  78  coupled to the needle control valve  79  in order to close the nozzle outlet  104 . 
   The present invention is advantageous because it provides greater variety of fuel injection strategies available to the fuel injection system  12 , which may lead to further reduction in undesirable emissions and better performance. Although the operation of the present invention was described for an injection sequence including one pilot injection  115 , one post injection  117 , and one main injection  116  being injected at varying pressure, the present invention can be used to create injection strategies with varying injection numbers, quantities, and pressures. In fact, the present invention can create a wider variety of injection strategies because the present invention includes two independent means, i.e., the oil high pressure pump  29  and the fuel high pressure pump  19 , for controlling the pressure of the injection. For instance, the electronic control module  22  can set the pressure of the pilot injection  115  at the ideal pressure for the particular operating condition by adjusting the signal to the fuel high pressure pump  19 . The electronic control module  22  can set the pressure of the main injection  116  at the pressure found to be ideal at the particular operating condition regardless of the pressure of the pilot injection  115  by adjusting the signal to the oil high pressure pump  29 . Thus, two injections very close in time, or even one injection, can include two independently selected and controlled pressures, creating a greater variety of possible injection strategies. 
   Moreover, by providing more variability in the control over fuel injections, engineers can create and test new injection strategies that could lead to even further advancements in performance and undesirable emission reductions. For instance, the present invention provides engineers with the ability to research hybrid pressure injections in which the pressure of the injection changes between the fuel common rail pressure, the intensified pressure and the third pressure being a function of the common rail pressure and the rate of retraction of the plunger. Engineers can adjust the fuel common rail pressure and/or the oil common rail pressure to create a multitude of rate shapes, leading to knowledge about which rate shapes perform better and reduce undesirable emissions at which operating conditions. Further, the present invention is advantageous because it provides more variability over the control of the fuel injection system without requiring significant alterations to the design of the system. Because many fuel injection systems include two separate fluid circuits, the present invention can be implemented by ensuring that needle control valve  76  is fluidly connectable to a pressure source that is lower than the pressure within the fuel common rail so that injections directly from the common rail can occur. 
   The present invention is also advantageous because it can increase performance of the fuel injection system  12 . Because the fuel injection system  12  can inject fuel with or without the use of the pressure intensifier  70 , the fuel injection system includes a broad range of pressures at which fuel can be injected. Thus, the fuel injection system  12  can more accurately inject fuel at the pressure required to maintain the desired engine speed and load. For instance, when the vehicle is idling, the pressure intensifier  70  can remain stationary during the entire injection sequence, resulting in an injection at the lower common rail pressure. Further, the operation of the fuel injection system  12  at the common rail pressures can lead to more accurate small injection quantities without demanding that valves exhibit quicker responses to control signals from the electronic control module  22 . Thus, at lower pressures, multiple injections can occur closer in time and the greater accuracy. 
   It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. For instance, if check valve  93  were replaced with a different valve, fuel could be displaced from an injector back to one of the rails to possibly pressurize the same. In other words the injectors could act as unit pumps for one of the common rails. Thus, those skilled in the art will appreciate that other aspects, objects, and advantages of the invention can be obtained from a study of the drawings, the disclosure and the appended claims