Abstract:
A compression ignition dual fuel engine utilizes individual fuel injectors to inject both gaseous and liquid fuels into each engine cylinder. Each of the fuel injectors includes two electrical actuators that control pressure in a respective liquid control chamber and gaseous control chamber to control the opening movement of check valves to facilitate liquid and gaseous fuel injection events, respectfully. The control fluid for liquid injection events is liquid, whereas the control fluid for the gaseous injection event is gaseous. The used liquid fuel drained from each fuel injector is returned for recirculation and subsequent injection, whereas the used gaseous fuel that drains from the fuel injector is supplied to the intake manifold for burning as circumstances permit.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to compression ignition dual fuel engines that utilize liquid and gaseous fuels, and more particularly to a fuel injector structure that reduces sensitivity to pressure differences between the two fuels. 
     BACKGROUND 
     One class of engines utilize a small pilot injection quantity of liquid diesel fuel that is compression ignited to in turn ignite a larger charge of gaseous fuel. Because of spatial constraints in and around engine cylinders, there has been an effort to supply both of the fuels to each engine cylinder via individual fuel injectors with the ability to inject both gaseous and liquid fuels. U.S. Pat. No. 6,073,862 to teaches such a fuel injector for use in a compression ignition engine. The &#39;862 reference also teaches that injection events are controlled with two separate electrical actuators that control fluid pressure in control chambers that allow for the direct control of the opening and closing of separate check valves to facility liquid and gaseous injection events, respectfully. The control chambers associated with both the gaseous fuel and the liquid fuel injection events are filled with liquid fuel. This reference teaches the inclusion of annulus of pressurized liquid fuel surrounding the check valve for the gaseous injection in order to inhibit leakage of gaseous fuel into the liquid side of the fuel system. Because liquid fuel is utilized to control both gaseous and liquid injection events, pressure fluctuations in the liquid fuel may undermine effective control of gaseous fuel injection events. In addition, leakage of liquid fuel into the gaseous side of the dual fuel system may present other problems. 
     The present disclosure is directed toward one or more of the problems set forth above. 
     SUMMARY 
     In one aspect, a dual fuel injector includes an injector body that defines a first fuel inlet, a second fuel inlet, a first nozzle outlet set, a second nozzle outlet set, a first drain outlet and a second drain outlet, and has disposed therein a first control chamber and a second control chamber. A first direct operating check valve has a first check valve member positioned in the injector body with a first closing surface exposed to fluid pressure in the first control chamber. The first check valve member is movable between a closed position in contact with a first seat to fluidly block the first fuel inlet to the first nozzle outlet set, and an open position out of contact with the first seat to fluidly connect the first fuel inlet to the first nozzle outlet set. A second direct operating check valve has a second check valve member positioned in the injector body with a second closing surface exposed to fluid pressure in the second control chamber. The second check valve member is movable between a closed position in contact with a second seat to fluidly block the second fuel inlet to the second nozzle outlet set, and an open position out of contact with the second seat to fluidly connect the second fuel inlet to the second nozzle outlet set. A first control valve member is movable between a closed position at which the first control chamber is fluidly blocked to the first drain outlet, and an open position at which the first control chamber is fluidly connected to the first drain outlet. A second control valve member is movable between a closed position at which the second control chamber is fluidly blocked to the second drain outlet, and an open position at which the second control chamber is fluidly connected to the second drain outlet. 
     In another aspect, a compression ignition dual fuel engine includes an intake manifold fluidly connected to a plurality of engine cylinders. A plurality of fuel injectors are each positioned for direct injection into one of the engine cylinders. Each of the fuel injectors includes an injector body that defines a liquid fuel inlet, a gaseous fuel inlet, a liquid nozzle outlet set, a gaseous nozzle outlet set, a liquid drain outlet and a gaseous drain outlet. Disposed within each fuel injector is a liquid control chamber and a gaseous control chamber. Each of the fuel injectors also includes a liquid direct operated check valve with a liquid check valve member. A gaseous direct operating check valve with a gaseous check valve member, a liquid control valve member and a gaseous control valve member. A gaseous fuel common rail is fluidly connected to the plurality of fuel injectors. A liquid fuel common rail is also connected to the fuel injectors. A gaseous fuel supply and pressure control system is fluidly connected to the gaseous fuel common rail. A liquid fuel supply and pressure control system is fluidly connected to the liquid fuel common rail. Each of the gaseous drain outlets is fluidly connected to the intake manifold. Each of the liquid drain outlets is fluidly connected to the liquid fuel supply and pressure control system. An electronic controller is in control communication with each of the plurality of fuel injectors, the liquid fuel supply and pressure control system, and the gaseous fuel supply and pressure control system. 
     In still another aspect, a method of operating the dual fuel compression ignition engine includes injecting liquid fuel through the liquid nozzle outlet set by fluidly connecting the liquid control chamber to the liquid drain outlet past the liquid control valve member. Gaseous fuel is injected through the gaseous nozzle outlet set by fluidly connecting the gaseous control chamber to the gaseous drain outlet past the gaseous control valve member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a dual fuel engine according to the present disclosure; 
         FIG. 2  is a perspective view of a portion of the engine and dual fuel common rail system for the engine of  FIG. 1 ; 
         FIG. 3  is a sectioned perspective view of a portion of the engine housing shown in  FIG. 2  to reveal structure for one fuel injector and engine cylinder; 
         FIG. 4  is a sectioned side view through a co-axial quill assembly according to another aspect of the present disclosure; 
         FIG. 5  is a sectioned front diagrammatic view of a fuel injector according to the present disclosure; and 
         FIG. 6  is an enlarged sectioned front diagrammatic view of the control features of the fuel injector of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIGS. 1-3 , a dual fuel engine  10  includes a dual fuel common rail system  20  mounted to an engine housing  11  that defines a plurality of engine cylinders  12 . The dual fuel common rail system  20  includes exactly one fuel injector  25  positioned for direct injection into each of the plurality of engine cylinders  12 . A gaseous fuel common rail  21  and a liquid fuel common rail  22  are fluidly connected to each fuel injector  25 . The dual fuel common rail system  20  also includes gas supply and pressure control devices  16  fluidly connected to the fuel gaseous common rail  21 , as well as liquid supply and pressure control devices  17  fluidly connected to the liquid for common rail  22 . Each of the fuel injectors  25 , the gas pressure supply and control devices  16  and the liquid supply and pressure control devices  17  are controlled by, and in communication with, an electronic engine controller  15  in a known manner. The gas supply and pressure control devices  16  may include a pressurized cryogenic liquid natural gas tank with an outlet fluidly connected to a variable delivery cryogenic pump. Devices  16  may also include a heat exchanger, an accumulator, a gas filter and a fuel conditioning module that controls the supply and pressure of gaseous fuel to gaseous fuel common rail  21 . The liquid supply and pressure control devices  17  may include a diesel fuel tank, fuel filters and an electronically controlled high pressure fuel pump that supply liquid fuel to, and control pressure in, liquid fuel common rail  22 . 
     Each of the fuel injectors  25  includes a gaseous drain outlet  143  that is fluidly connected to the intake manifold  13  by way of a gas passage  23 . A valve  24 , which could be a check valve or an electronically controlled valve, is positioned between gas passage  23  and intake manifold  13  to prevent the reverse flow of gas from intake manifold  13  into gas passage  23 . Each of the fuel injectors  25  also includes a liquid drain outlet  133  fluidly connected to the tank of the liquid supply and pressure control system  17  by way of a liquid return line  28 . For sake of clarity, only the end fuel injectors  25  in the upper bank have the liquid drain outlet  133  numerically identified, and only the end fuel injectors  25  in the lower bank have the gaseous drain outlet  143  numerically identified. Likewise, the liquid return line  28  is only shown with regard to the upper bank of fuel injectors  25 , but is not shown for the lower bank of fuel injectors. Also for the sake of clarity, the gas passage  23  is only shown with regard to the lower bank of fuel injectors, but is not shown with regard to the upper bank of fuel injectors in  FIG. 1 . 
     As best shown in  FIGS. 1 and 2 , the blocks  31  of the co-axial quill assemblies  30  may be daisy-chained together with gaseous fuel line segments  18  and liquid fuel line segments  19  to define the gaseous fuel common rail  21  and the liquid fuel common rail  22 , respectively. The last co-axial quill assembly  30  in the daisy-chain may have a set of plugs in place of the fittings shown in  FIG. 2 . 
     Referring in addition to  FIG. 4 , the dual fuel common rail system  20  includes a co-axial quill assembly  30  with an inner quill  32  and an outer quill  33  in sealing contact with a common conical seat  27  of each fuel injector  25 . In the illustrated embodiment, a pressure damping chamber  48  consists of an upstream segment  49  of the gaseous fuel conduit  47  that has a flow area at least several times larger than the downstream segment  50  of the gaseous fuel conduit  47 . The gaseous fuel conduit  47  is fluidly connected to the gaseous fuel inlet  102  of each fuel injector  25 . The pressure damping chamber  48  may be defined in each co-axial quill assembly  30  in order to damp pressure waves moving from gaseous fuel common rail  21  toward the respective fuel injector  25 , especially during an injection event. The pressure damping chamber  48  has a volume greater than a gaseous fuel volume  26  (nozzle chamber, sac and gas passageways) within the respective fuel injector  25 . Those skilled in the art will appreciate that the available space constraints on fuel injector  25  limit the size of the gaseous fuel volume  26  within each fuel injector  25 . The gas volume  26  in each fuel injector may likely be many times less than a rated gaseous injection volume from injector  25 . 
     One strategy for sizing the pressure damping chamber  48  may start with the continuity equation, and then derive an equation for the pressure response of a particular fluid (e.g. natural gas) in a specific volume (the pressure damping chamber  48 ) to a flow rate arriving (from the rail  21 ) to a flow rate leaving the volume (injection rate). The idea is to reduce the pressure change reaction to the volume flow of the fluid to a satisfactory level. The pressure damping chamber  48  should provide sufficient absorption of arriving pressure waves to damp out reflective transients. Thus, one might consider a maximum rated volume of gaseous fuel delivery for fuel injector  25  in the engine  10 , and the gas injection pressure, and size a volume of the pressure damping chamber  48  that will provide sufficient absorption of the pressure waves. 
     Referring again to  FIGS. 2-4 , each co-axial quill assembly  30  may include a load adjusting clamp  34  with a pivot surface  75  in contact with a block  31  at a load adjustment location  56  that is intersected by the axis  29  of the inner quill  32 . The load adjusting clamp  34  may define a fastener slot  77  and a fastener bore  76  that receive a first fastener  81  and a second fastener  80 , respectively. The load adjustment clamp  34  pivots on load adjustment location  56  responsive to adjustments to the first and second fasteners  81 ,  80 . Fastener  80  may include a spherical washer and bolt, while fastener  81  may be a shoulder bolt that is utilized to set an attitude of load adjustment clamp  34 . For instance, the proper assembly may require connection of co-axial quill assembly  30  to engine housing  11  with first fastener  81 . Bolt  80  can then be tightened to a pre-determined torque that assures proper seating seal contact between outer quill  33  and inner quill  32 , independently but simultaneously, on common conical seat  27  of fuel injector  25 . During this process, load adjustment clamp  34  will pivot through some limited small angle. The fasteners  80  and  81  are received in fastener bore  54  and fastener slot  55 , respectively of blocks  31 . 
     Each block  31  of each co-axial quill assembly  30  may define a gaseous rail passage  45  that is oriented perpendicular to the axis  29  of inner quill  32  and fluidly connected to a gaseous fuel passage  46  that opens at one end into a quill chamber  52  outside of conical seat  53 . The gaseous rail passage  45  may extend completely through block  31  in order to facilitate the daisy chain connection structure shown in  FIGS. 1 and 2 . Each block  31  also includes a liquid rail passage  42 , which may extend all the way through, and that is oriented perpendicular to the axis  29  and fluidly connected to a liquid fuel passage  43  that opens on one end into quill chamber  52  through conical seat  53 . A segment of liquid fuel passage  43  may have an orifice segment  41 , as shown, to reduce a flow rate from the liquid rail  22  to help manage transients in the liquid quill  32 . The liquid fuel passage  43  is fluidly connected to the liquid fuel inlet  101  of each fuel injector  25 . Both liquid fuel inlet  101  and gaseous fuel inlet  102  open through common conical seat  27  of each fuel injector  25 . The minimum area required for the orifice  41  may be computed by dividing the total injection quantity by the injection duration, and sizing the orifice to allow that delivery with a minimum pressure drop. Thus, the sizing of that flow area may relate to the performance characteristics of fuel injector  25 . The inner quill  32  defines a liquid fuel conduit  44  extending between a first end  60  and a second end  62 . First end  60  includes an annular spherical surface  61  that rests in contact at a gage line  87  with, but remains unattached to, the conical seat  53 , and a gage line  85  on an annular spherical surface at second end  62  in contact with common conical seat  27  of fuel injector  25 . The outer quill  33  has a hollow interior  65  separating a first end  66  from a second end  67 . The first end  66  is received in the quill chamber  52 , and the outer quill  33  may be attached to block  31  with mating threads  51 . 
     Practical manufacturing limitations may forbid mass production of co-axial quill assemblies  30  in which either the inner quill  32  or the outer quill  33  are integrally formed with block  31 , or each other. Thus, an annular seal  71  serves to seal against leakage of gaseous fuel from between block  31  and outer quill  33  of co-axial quill assembly  30 . In this embodiment, annular seal  71  includes an o-ring  73  in a face seal configuration trapped between block  31  and outer quill  33 . In the illustrated construction, the inner quill  32  is out of contact with the outer quill  33  in each co-axial quill assembly  30 . A gaseous fuel conduit  47  is fluidly connected to gaseous fuel passage  46 , and also extends between outer surface  63  of inner quill  32  and the inner surface  69  of outer quill  33 . Spatial constraints in engine housing  11  may require that an upstream half  49  of the gaseous fuel conduit  47  have a pressure damping chamber  48  with a volume larger than a volume of a downstream half  50  of the gaseous fuel conduit  47 . Thus, a majority of the volume of the pressure damping chamber  48  may be located in an upstream half  49  of the gaseous fuel conduit  47  both within outer quill  33  and within quill chamber  52 . As stated earlier, the pressure damping chamber  48  should be of sufficient size and shape to damp pressure waves arriving from the gaseous fuel passage  46  in order to reduce variations in gaseous fuel injection rates and quantities. In this specific example, the available space in engine housing  11  may permit the relatively uniform wall thickness of the outer quill  33 , which is defined between an inner surface  69  and outer surface  68 , to include two step wise diameter reductions  70  along the axis  29  in a direction of second end  67 . Nevertheless, other engine housing geometries may vary substantially from that shown. The gaseous rail passage  45  of each block  31  may define a portion of the gaseous fuel common rail  21 . Likewise, the liquid rail passage  42  of each block  31  may define a segment of the liquid fuel common rail  22  as best shown in  FIGS. 1 and 2 . 
     Referring more specifically to  FIG. 4 , reliable sealing contact between the co-axial quill assembly  30  and fuel injector  25  against leakage of both gaseous and liquid fuels may be accomplished by tightening only a single fastener  80  to a predetermined torque load. This may be accomplished by locating the gage line  85  at the second end  62  of the inner quill  32  to extend a predetermined target distance Δ beyond the gage line  86  at the second end  67  of the outer quill  33 . The gage line  85 ,  86  is the sealing contact line. A predetermined load may be placed on block  31  by load adjusting clamp  34  acting along axis  29  so that the outer and inner quills  33 ,  32  seat and sealingly engage on common conical seat  27  at their respective gage lines  85 ,  86 . Tightly controlling the predetermined target distance Δ may be accomplished in a number of ways. In the illustrated embodiment, target distance Δ is held to a tolerance d that is a stack up of tolerance e, β and α. Dimension distance E +/−tolerance e corresponds to the distance between the gage line of conical seat  53  and the shoulder face against which o-ring  73  seals on block  31 . Dimension distance B +/−tolerance β corresponds to the distance from the shoulder surface of outer quill  33  to the gage line  86  at second end  67  of outer quill  33 . Dimension distance A +/−tolerance α corresponds to the distance between the gage lines  87 ,  85  at opposite ends of inner quill  32 . Provided that the distances A, B and E can be held within reasonable tolerances, the tolerance stack up d on target distance Δ can be made acceptable such that proper sealing at conical seat  27  of fuel injector  25  is reliably made. Tolerance stack up d equals e plus β plus α. During preassembly, the predetermined target distance Δ may be set within an acceptable tolerance d by selecting a block  31  with an appropriate dimension distance E +/−e, an outer quill  33  with an appropriate dimension distance B +/−β, and a inner quill  32  with an appropriate dimension distance A +/−α. Provided that the tolerance stack up of e+β+α yields an acceptable tolerance d, simple nearly fool proof installation may be assured by simply tightening a single fastener  80  to an appropriate torque load to apply an appropriate load along centerline  29 . 
     Those skilled in the art will appreciate that the inner and outer quills  32 ,  33  may have different spring rates and may require different load levels to ensure proper sealing at common conical seat  27 . Therefore, some differential length, which may be positive, negative or zero, depending upon the specific design, quill materials and geometries may need to be added to the above described dimensions in order to ensure proper sealing contact at fuel injector  25 . 
     In order to trap metallic debris often liberated into the fuel flows during the first time operation of engine  10  after being built, co-axial quill assembly  30  may include a gaseous fuel edge filter  36  and a liquid fuel edge filter  37 . In the illustrated embodiment, liquid fuel edge filter  37  may be positioned in the liquid fuel conduit  44  defined by inner quill  32 . The gaseous fuel edge filter  36  is shown positioned within outer quill  33  between the two step wise diameter reductions  70 . In the illustrated embodiment, gaseous fuel edge filer  36  may have a combined dual purpose by including a retainer  38  that can be thought of as in contact with the inner surface  69  of outer quill  33  and of the outer surface  63  of inner quill  32 . In this embodiment, retainer  38  may include an o-ring  91  that encourages gaseous fuel traveling along gaseous fuel conduit  47  to move through filter passages  93  between edge filter  36  and outer quill  33  to trap metallic debris upstream from fuel injector  25 . The outer surface of retainer  38  includes a plurality of filter passages  93  that are distributed around, and oriented perpendicular to the axis  29 . In this embodiment, retainer  38  may comprise a suitable metallic piece, such as steel, that is machined to the shape as shown and also includes an o-ring  91  that grips the outer surface  63  of inner quill  32 . Retainer  38  may be connected to the outer quill  33  with a metal to metal interference fit  95 . 
     Because inner quill  32  is unattached to either outer quill  33  or block  31 , co-axial quill assembly  30  may include the retainer  38  that is in contact with the outer surface  63  to maintain the inner quill  32  with the block  31  and outer quill  33  during pre-installation handling. In other words, retainer  38  may inhibit inner quill  32  from falling out of outer quill  33  during pre-installation handling. The edge filter  36 /retainer  38  of the disclosure allows the co-axial quill assemblies  30  to be preassembled with a precisely predetermined target distance Δ so that installation is made easy and simple without the need for custom adjustments at each co-axial quill assembly  30 . In the illustrated embodiment, consistent leak free installation may only require torqueing fastener  80  to a predetermined load, without any other considerations. 
     Referring now in addition to  FIGS. 5 and 6 , each fuel injector  25  includes an injector body  100  that defines a liquid fuel inlet  101  ( FIG. 4 ). A gaseous fuel inlet  102  ( FIG. 4 ) a liquid nozzle outlet set  132  ( FIG. 5 ), a gaseous nozzle outlet set  142  ( FIG. 5 ), a liquid drain outlet  133  ( FIGS. 1 and 5 ) and a gaseous drain outlet  143  ( FIGS. 1 and 5 ). Disposed within each injector body  100  is a liquid control chamber  134  and a gaseous control chamber  144 . Each of the fuel injectors  25  includes a liquid direct operated check valve  135  with a liquid check valve number  136 , and a gaseous direct operating check valve  145  with a gaseous check valve number  146 . Although the liquid check valve number  136  and the gaseous check valve number  146  are shown as made up of two parts, those skilled in the art will appreciate that each of the check valve members  136 ,  146  could also be made of a unitary body without departing from the intended scope of the present disclosure. In addition, the direct operated checks  135  and  145  are shown in a side-by-side relationship, but coaxial direct operated checks would also fall within the intended scope of the present disclosure. In the illustrated embodiment, liquid check valve member  136  is positioned in parallel with, but spaced apart from gaseous check valve member  146 . Also in the illustrated embodiment, the first electrical actuator  126  and the second electrical actuator  127  are solenoids that utilize a shared stator and act along a common centerline  115 . Nevertheless, those skilled in the art will appreciate that the two electrical actuators could be piezos and/or be arranged differently, such as in a side-by-side relationship, without departing from the intended scope of the present disclosure. 
     In the illustrated embodiment, all of the first linkage  128  that operably couples the first electrical actuator  126  to the liquid control valve member  137  is exposed to fluid pressure in liquid drain outlet  133  via passageways that may not be visible in the sectioned views of  FIGS. 5 and 6 . A part  150  of second linkage  129  that operably couples the second electrical actuator  127  to the gaseous control valve member  147  may also be exposed to fluid pressure in liquid drain outlet  133 . However, a remaining part  151  of second linkage  129  may be exposed to fluid pressure in gaseous drain outlet  143 . In particular, second linkage  129  may include a piston  153  with a top end exposed to liquid drain pressure in liquid drain outlet  133  via passages not visible in  FIGS. 5 and 6 , whereas an opposite end of piston  153  may be exposed to fluid pressure in gaseous drain outlet  143  as best shown in  FIG. 6 . Piston  153  may move with a tight clearance in a bore  154  over a sufficient length to limit substantial leakage along the outer surface of piston  153  between liquid fuel from above and gaseous fuel from below. It might be desirable to maintain the liquid drain outlet  133  pressure slightly higher than the pressure in gaseous drain outlet  143  in order to inhibit migration of gaseous fuel into the liquid side of fuel system  20 . 
     In the illustrated embodiment, both the liquid control valve associated with liquid control valve member  137  and the gaseous control valve associated with gaseous control valve member  147  are two-way valves. Nevertheless, those skilled in the art will appreciate that three-way valves could be substituted in place of either valve without departing from the intended scope of the present disclosure. In the illustrated embodiment, the liquid fuel inlet  101  is always fluidly connected to the liquid control chamber  134  through a Z-orifice  160  and passageways not visible in the sectioned views of  FIGS. 5 and 6 . When liquid control valve member  137  is in its open position, liquid control chamber  134  becomes fluidly connected to liquid drain outlet  133  through an A-orifice  161 . The gaseous side works in a similar manner. In particular, the gaseous fuel inlet  102  is always fluidly connected to the gaseous control chamber  144  through a Z-orifice  165 . When the gaseous control valve member moves to its open position, the gaseous control chamber  144  becomes fluidly connected to gaseous drain outlet  143  through an A-orifice  166 . Although gaseous control valve member  147  moves into and out of contact with a flat seat  170 , those skilled in the art will appreciate that alternatively shaped control valve member that could be utilized that moves into and out of contact with a conical seat without departing from the intended scope of the present disclosure. 
     The liquid control valve number  137  is operably coupled to a first electrical actuator  126  to control pressure in liquid control chamber  134  to control the timing and duration of liquid injection events through liquid nozzle outlets  132 . Likewise, the gaseous control valve member  147  is operably coupled to second electrical actuator  127  to control pressure in gaseous control chamber  144  to control the timing and duration of gaseous injection events through gaseous nozzle outlet set  142 . 
     Each liquid check valve member  136  includes a liquid closing hydraulic surface  138  exposed to fluid pressure in liquid control chamber  134 . The liquid check valve member  136  is movable between a closed position as shown in contact with a liquid seat  139  to fluidly block the liquid fuel inlet  101  ( FIG. 4 ) to the liquid nozzle outlet set  132 , and an open position out of contact with the liquid seat  139  to fluidly connect the liquid fuel inlet  101  to the liquid nozzle outlet set  132  to facilitate a liquid fuel injection event. The gaseous check valve member  146  includes a gaseous closing pneumatic surface  148  exposed to fluid pressure in gaseous control chamber  144 . The gaseous check valve member  146  is movable between a closed position (as shown) in contact with a gaseous seat  149  to fluidly block the gaseous fuel inlet  102  ( FIG. 4 ) to the gaseous nozzle outlet set  142 , and an open position out of contact with the gaseous seat  142  to fluidly connect the gaseous fuel inlet  102  to the gaseous nozzle outlet set  142 . The liquid control member  137  is normally biased upwards into contact with a conical seat  171  corresponding to a closed position to fluidly block the liquid control chamber  134  to the liquid drain outlet  133 . When first electrical actuator  126  is energized, a linkage  128  pushes liquid control valve member  137  out of contact with conical seat  171  to an open position at which the liquid control chamber  134  is fluidly connected to the liquid drain outlet  133  to reduce pressure acting on liquid closing hydraulic surface  138  to facilitate a liquid injection event. Thus, first electrical actuator  126  can be thought of as being operably coupled to liquid control valve member  137 . Although liquid control valve member  137  is shown moving into and out of contact with a conical seat  171 , those skilled in the art will appreciate that an appropriate control valve member and a flat seat could be substituted without differing from the intended scope of the present disclosure. 
     The gaseous control valve member  147  is movable between a closed position in contact with a flat seat  170  at which the gaseous control chamber  144  is fluidly blocked to the gaseous drain outlet  143 . When second electrical actuator  127  is energized, a second linkage  129  is moved upward allowing gas pressure in gas control chamber  144  to push gas control valve member  147  at a flat seat  170  to an open position at which gaseous control chamber  144  is fluidly connected to gaseous drain outlet  143  to reduce pneumatic pressure acting on closing pneumatic surface  148  to facilitate a gaseous injection event. 
     Industrial Applicability 
     The present disclosure finds potential application in any fuel injector that is to be utilized to inject two fuels that differ in at least one of pressure, chemical identity and matter phase. The present disclosure finds specific applicability to dual fuel injectors associated with compression ignition engines to inject liquid diesel fuel at a first pressure, and gaseous fuel (e.g. natural gas) at a second pressure. Finally, the present disclosure finds potential application in dual fuel compression ignition engines that seek to utilize a small injection of liquid diesel fuel that is compression ignited to in turn ignite a larger charge of gaseous fuel. Finally, the present disclosure finds specific applicability to dual fuel common rail systems. 
     Referring again to all of the  FIGS. 1-6 , a method of operating dual fuel engine  10  includes injecting liquid fuel through the liquid nozzle outlet set  132  by fluidly connecting the liquid control chamber  134  to the liquid drain outlet  133  past the liquid control valve member  137 . Gaseous fuel is injected through the gaseous nozzle outlet set  142  by fluidly connecting the gaseous control chamber  144  to the gaseous drain outlet  143  past the gaseous control valve member  147 . Each liquid fuel injection event is ended by blocking the liquid control chamber  134  from the liquid drain outlet  133  with the liquid control valve member  137 . Likewise, gaseous fuel injection events are ended by blocking the gaseous control chamber  144  to the gaseous drain outlet  143  with the gaseous control valve member  147 . During and between liquid fuel injection events, the liquid fuel common rail  22  is fluidly connected to the liquid control chamber  134  through Z-orifice  160 . Likewise, during and between gaseous fuel injection events, the gaseous fuel common rail  21  is fluidly connected to the gaseous control chamber  144  through Z-orifice  165 . During gaseous fuel injection events, the amount of gaseous fuel used with the control function is moved from gaseous drain outlet  143  toward, and eventually into, intake manifold  13  for burning as circumstances permit, rather than being vented to atmosphere. 
     Each liquid injection event is facilitated by hydraulically pushing on a hydraulic opening surface  116  of liquid check valve member  136  toward an open position with liquid pressure from the liquid fuel common rail  22 . Likewise, gaseous fuel injection events are facilitated by pneumatically pushing on an opening pneumatic  117  of the gaseous check valve member  146  toward its open position with gaseous pressure from gaseous fuel common rail  21 . Thus, between liquid injection events, both ends of the liquid check valve member  136  are exposed to fluid pressure in the liquid fuel common rail  22 . Likewise, between injection events both ends of the gaseous check valve member  146  are exposed to fluid pressure in gaseous fuel common rail  21 . This construction may eliminate a potential need for a hydraulic lock feature associated with the check valve member  146  to inhibit exchange or leakage of gaseous fuel into the liquid side of fuel system  20 . The gaseous check valve member  146  may also be fluidly isolated from both the liquid fuel inlet  101  and the liquid drain outlet  133 . Likewise, the liquid check valve member  136  may be fluidly isolated from both the gaseous fuel inlet  102  and the gaseous fuel drain outlet  143 . Separation of the two fuels is partially accomplished by exposing all of the first linkage  128  that operably couples the first electrical actuator  126  to the liquid control valve member  137 . Part  150  of the second linkage  129  that operably couples the second electrical actuator  127  to the gaseous control valve member  147  is likewise exposed to fluid pressure in liquid drain outlet  133 . However, the remaining part  151  of the second linkage  129  is exposed to fluid pressure in the gaseous drain outlet  143 . It is these features that help maintain the two fuels isolated and inhibit leakage between the same during normal operation of engine  10 . 
     The fuel injector  25  of the present disclosure utilize liquid as the control fluid at the check top on the liquid side, but utilize gas as the control fluid at the check top of the gas side. This feature helps eliminate the potential need for a hydraulic lock mechanism since gas is used on both sides of the gaseous check valve member  146 . Also, due to the fact that gas is used as the control fluid for gaseous injection events, the gas injection events are insensitive to fluctuations in liquid rail pressure. Hence, even a small pilot liquid injection event prior to a gas injection event may not cause much shot-to-shot variation in gaseous fuel delivery than might what occur if liquid fuel were used as a control fluid for both liquid and gaseous fuel injection events. During normal operation, one might expect the liquid common rail  22  to be maintained at a higher pressure than the gaseous fuel common rail  21 . For instance, these pressures might be 35 and 30 MPa, respectively. On the other hand, if the engine  10  is operating in a so called limp-home mode in which only liquid fuel is injected, the liquid rail pressure may be increased to about 100 MPa, while the gaseous side of the fuel system  20  may be reduced to gaseous drain pressure. Because of the fluid isolation features taught in the present disclosure, the separation of the two fuels made maintained when operating in a normal mode and when in operating in a limp-home mode. Thus, even during limp-home mode, the fluid isolation feature of the present disclosure may inhibit leakage of liquid diesel fuel into the gaseous supply and drain side of fuel system  20 . 
     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 disclosure in any way. Thus, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims.