Abstract:
A method of fabricating a piezo actuator is disclosed. The method includes forcing a blank against a mold to form a cylindrical casing having an open end and a closed end and rotating the cylindrical casing. The method also includes urging a die having a plurality of equally spaced protrusions into a surface of the cylindrical casing to form bellows in the cylindrical casing. The method further includes positioning a piezo element within the cylindrical casing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 11/473,135, filed Jun. 23, 2006, now U.S. Pat. No. 7,429,815, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure is directed to a fuel injector and, more particularly, to a fuel injector having an encased piezo electric actuator. 
     BACKGROUND 
     Common rail fuel systems typically employ multiple closed-nozzle fuel injectors to inject high pressure fuel into the combustion chambers of an engine. Each of these fuel injectors include a nozzle assembly having a cylindrical bore with a nozzle supply passageway and a nozzle outlet. A needle check valve is reciprocatingly disposed within the cylindrical bore and biased toward a closed position where the nozzle outlet is blocked. To inject fuel, the needle check valve is selectively moved to open the nozzle outlet, thereby allowing high pressure fuel to flow from the nozzle supply passageway into the combustion chamber. To move the needle check valve, a control chamber in fluid communication with a base of the needle check valve is selectively drained of pressurized fuel to create a force imbalance that biases the needle check valve toward the open position. 
     A piezo actuator is often used to drain the pressurized fuel from the base of the needle check valve. Specifically, the piezo actuator, upon being energized, expands to move a valve element from a first position at which pressurized fuel is directed to the base of the needle check valve, to a second position at which the pressurized fuel at the base of the needle check valve is directed to a drain. Although this configuration is effective for initiating the injection of fuel, it is critical that the piezo actuator remain isolated and protected from the fuel and other contaminates. In particular, fuel, if allowed to contact the piezo actuator, can short circuit the actuator or otherwise degrade the performance of the actuator. 
     One method utilized by injector manufacturers to isolate the actuator from fuel and other contaminates is described in U.S. Pat. No. 6,874,475 (the &#39;475 patent) issued to Katsura et al. on Apr. 5, 2005. The &#39;475 patent describes a fuel injector for an internal combustion engine. The fuel injector includes a piezo electric valve actuator enclosed within a housing. The housing is made of stainless steel cylindrical bellows consisting of large-diameter portions and small-diameter portions arrayed alternately. The bellows allow expansion and contraction of the piezo electric valve actuator through deformation. The housing is hermetically closed by an upper plate and a lower plate to minimize the ingress of fuel. The upper and lower plates are used to transfer force imparted by the piezo electric valve actuator. One of the lower and upper plates may be formed integral with the housing to improve air tightness. 
     Although the fuel injector of the &#39;475 patent may sufficiently inject fuel while minimizing piezo/fuel contamination, it may be problematic and costly. For example, because the one of the lower and upper plates and the housing are integral, the material of both the plate and the housing must be the same. This material, when optimized for the deformation described above, may not be optimal for force transmission. Similarly, this material, when optimized for the force transmission described above, may deform poorly. Components fabricated from material that is not optimally suited for intended operations may be prone to premature failure. In addition, because a first material used, for example, to fabricate the housing, may have a higher cost than a second material best suited for the plate, the integral component functioning as both the housing and the plate, which is made of only the first material may unnecessarily increase the cost of the fuel injector. Further, the process of fabricating this integral housing/plate component may be expensive. 
     The fuel injector of the present disclosure solves one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     One aspect of the present disclosure is directed to an actuator. The actuator includes a piezo element, a casing, and at least one end plate. The casing is fabricated through a deep draw process, has bellows, and is configured to house the piezo element. The at least one end plate is hermetically connected to an end portion of the casing. 
     Another aspect of the present disclosure is directed to a fuel injector. The fuel injector includes a nozzle member, a needle check valve, and an actuator. The nozzle member is configured to receive pressurized fuel and has at least one injection orifice. The needle check valve is disposed with the nozzle member and is movable between a flow blocking position at which fuel flows through the at least one orifice, and a second position at which fuel flow through the at least one orifice is blocked. The actuator is operatively connected to move the needle check valve between the first and second positions, and includes a piezo element, a casing, and at least one end plate. The casing is fabricated through a deep draw process, has bellows fabricated through a thread-rolling process, and is configured to house the piezo element. The at least one end plate is hermetically connected to an end portion of the casing and is operatively connected to the needle check valve. 
     Yet another aspect of the present disclosure is directed to a method of fabricating a piezo actuator. The method includes forcing a blank against a mold to form a cylindrical casing having an open end and a closed end. The method also includes rotating the cylindrical casing and urging a die having a plurality of equally spaced protrusions into a surface of the cylindrical casing to form bellows in the cylindrical casing. The method also includes positioning a piezo element within the cylindrical casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic and diagrammatic illustration of an exemplary disclosed fuel system; 
         FIG. 2  is a cross-sectional diagrammatic illustration of an exemplary disclosed fuel injector for the fuel system of  FIG. 1 ; and 
         FIG. 3  is a cross-sectional illustration of an exemplary disclosed actuator for use with the fuel injector of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an engine  10  and an exemplary embodiment of a fuel system  12 . For the purposes of this disclosure, engine  10  is depicted and described as a four-stroke diesel engine. One skilled in the art will recognize, however, that engine  10  may embody any other type of internal combustion engine such as, for example, a gasoline or a gaseous fuel-powered engine. Engine  10  may include an engine block  14  that defines a plurality of cylinders  16 , a piston  18  slidably disposed within each cylinder  16 , and a cylinder head  20  associated with each cylinder  16 . 
     Cylinder  16 , piston  18 , and cylinder head  20  may form a combustion chamber  22 . In the illustrated embodiment, engine  10  includes six combustion chambers  22 . However, it is contemplated that engine  10  may include a greater or lesser number of combustion chambers  22  and that combustion chambers  22  may be disposed in an “in-line” configuration, a “V” configuration, or any other suitable configuration. 
     As also shown in  FIG. 1 , engine  10  may include a crankshaft  24  that is rotatably disposed within engine block  14 . A connecting rod  26  may connect each piston  18  to crankshaft  24  so that a sliding motion of piston  18  within each respective cylinder  16  results in a rotation of crankshaft  24 . Similarly, a rotation of crankshaft  24  may result in a sliding motion of piston  18 . 
     Fuel system  12  may include components that cooperate to deliver injections of pressurized fuel into each combustion chamber  22 . Specifically, fuel system  12  may include a tank  28  configured to hold a supply of fuel, and a fuel pumping arrangement  30  configured to pressurize the fuel and direct the pressurized fuel to a plurality of fuel injectors  32  by way of a common rail  34 . 
     Fuel pumping arrangement  30  may include one or more pumping devices that function to increase the pressure of the fuel and direct one or more pressurized streams of fuel to common rail  34 . In one example, fuel pumping arrangement  30  includes a low pressure source  36  and a high pressure source  38  disposed in series and fluidly connected by way of a fuel line  40 . Low pressure source  36  may be a transfer pump configured to provide low pressure feed to high pressure source  38 . High pressure source  38  may be configured to receive the low pressure feed and to increase the pressure of the fuel to the range of about 30-300 MPa. High pressure source  38  may be connected to common rail  34  by way of a fuel line  42 . A check valve  44  may be disposed within fuel line  42  to provide for one-directional flow of fuel from fuel pumping arrangement  30  to common rail  34 . 
     One or both of low pressure and high pressure sources  36 ,  38  may be operably connected to engine  10  and driven by crankshaft  24 . Low and/or high pressure sources  36 ,  38  may be connected with crankshaft  24  in any manner readily apparent to one skilled in the art where a rotation of crankshaft  24  will result in a corresponding rotation of a pump drive shaft. For example, a pump driveshaft  46  of high pressure source  38  is shown in  FIG. 1  as being connected to crankshaft  24  through a gear train  48 . It is contemplated, however, that one or both of low and high pressure sources  36 ,  38  may alternatively be driven electrically, hydraulically, pneumatically, or in any other appropriate manner. 
     Fuel injectors  32  may be disposed within cylinder heads  20  and connected to common rail  34  by way of a plurality of fuel lines  50 . Each fuel injector  32  may be operable to inject an amount of pressurized fuel into an associated combustion chamber  22  at predetermined timings, fuel pressures, and fuel flow rates. The timing of fuel injection into combustion chamber  22  may be synchronized with the motion of piston  18 . For example, fuel may be injected as piston  18  nears a top-dead-center position in a compression stroke to allow for compression-ignited-combustion of the injected fuel. Alternatively, fuel may be injected as piston  18  begins the compression stroke heading towards a top-dead-center position for homogenous charge compression ignition operation. Fuel may also be injected as piston  18  is moving from a top-dead-center position towards a bottom-dead-center position during an expansion stroke for a late post injection to create a reducing atmosphere for aftertreatment regeneration. 
     As illustrated in  FIG. 2 , each fuel injector  32  may embody a closed nozzle unit fuel injector. Specifically, each fuel injector  32  may include an injector body  52  housing a guide  54 , a nozzle member  56 , a needle valve element  58 , and an actuator  59 . 
     Injector body  52  may be a cylindrical member configured for assembly within cylinder head  20 . Injector body  52  may have a central bore  60  for receiving guide  54  and nozzle member  56 , and an opening  62  through which a tip end  64  of nozzle member  56  may protrude. A sealing member such as, for example, an o-ring (not shown) may be disposed between guide  54  and nozzle member  56  to restrict fuel leakage from fuel injector  32 . 
     Guide  54  may also be a cylindrical member having a central bore  68  configured to receive needle valve element  58 , and a control chamber  71 . Central bore  68  may act as a pressure chamber, holding pressurized fuel that is continuously supplied from a fuel supply passageway  70 . During injection, the pressurized fuel from fuel line  50  may be allowed to flow through fuel supply passageway  70  and central bore  68  to nozzle member  56 . 
     Control chamber  71  may be selectively drained of or supplied with pressurized fuel to control motion of needle valve element  58 . Specifically, a control passageway  73  may fluidly connect a port  75  of control chamber  71  with actuator  59 . Control chamber  71  may also be continuously supplied with pressurized fluid via a supply passageway  77  that is communication with fuel supply passageway  70 . A diameter of supply passageway  77  may be less than a diameter of control passageway  73  to allow for a pressure drop within control chamber  71  when control passageway  73  is drained of pressurized fuel. 
     Nozzle member  56  may likewise embody a cylindrical member having a central bore  72  that is configured to receive needle valve element  58 . Nozzle member  56  may include one or more orifices  80  to allow the pressurized fuel from central bore  68  into combustion chambers  22  of engine  10 . 
     Needle valve element  58  may be an elongated cylindrical member that is slidingly disposed within housing guide  54  and nozzle member  56 . Needle valve element  58  may be axially movable between a first position at which a tip end  82  of needle valve element  58  blocks a flow of fuel through orifices  80 , and a second position at which orifices  80  are open to allow a flow of fuel into combustion chamber  22 . 
     Needle valve element  58  may be normally biased toward the first position. In particular, as seen in  FIG. 2 , each fuel injector  32  may include a spring  90  disposed between a stop  92  of guide  54  and a seating surface  94  of needle valve element  58  to axially bias tip end  82  toward the orifice-blocking position. A first spacer  96  may be disposed between spring  90  and stop  92 , and a second spacer  98  may be disposed between spring  90  and seating surface  94  to reduce wear of the components within fuel injector  32 . 
     Needle valve element  58  may have multiple driving hydraulic surfaces. In particular, needle valve element  58  may include a hydraulic surface  100  tending to drive needle valve element  58  toward the first or orifice-blocking position when acted upon by pressurized fuel, and a hydraulic surface  104  that tends to oppose the bias of spring  90  and drive needle valve element  58  in the opposite direction toward the second or orifice-opening position. 
     Actuator  59  may be disposed opposite tip end  82  of needle valve element  58  to indirectly control the motion of needle valve element  58 . In particular, actuator  59  may include a three position proportional valve element  106  disposed within control passageway  73  between control chamber  71  and tank  28 . Proportional valve element  106  may be actuated to move between a first position at which fuel is allowed to flow from control chamber  71  to tank  28 , a second position at which pressurized fuel from fuel line  50  flows through control passageway  73  into control chamber  71 , and a third position at which fuel flow through control passageway  73  is blocked. The position of proportional valve element  106  between the first, second, and third positions may determine a flow rate of the fuel through control passageway  73 , as well as the flow direction. Proportional valve element  106  may be movable between the first, second, and third positions in response to an electric current applied to a piezo device  108  associated with proportional valve element  106 . It is contemplated that proportional valve element  106  may alternatively embody a two-position valve element that is movable between only a control chamber draining position and a control chamber filling position, if desired. It is further contemplated that piezo device  108  may directly move needle valve element  58 , without the use of proportional valve element  106 , if desired. 
     As illustrated in  FIG. 3 , piezo device  108  may include a hermetically sealed assembly of multiple component. In particular, piezo device  108  may include a piezo element  110  pre-tensioned by one or more springs  112 , an outer casing  114 , a first end cap  116 , a second end cap  118 , and electrical leads  120 . Piezo element  110 , together with springs  112 , first end cap  116 , second end cap  118 , and electrical leads  120 , may be supplied together as a sub-assembly for insertion into and sealing within casing  114 . It is contemplated that springs  112  may be omitted and the function of pre-tensioning alternatively performed by casing  114 , first end cap  116 , and second end cap  118 , if desired. 
     Piezo element  110  may include one or more columns of piezo electric crystals. Piezo electric crystals are structures with random domain orientations. These random orientations are asymmetric arrangements of positive and negative ions that exhibit permanent dipole behavior. When an electric field is applied to the crystals, such as, for example, by the application of a current, the piezo electric crystals expand along the axis of the electric field as the domains line up. Conversely, as the electric field is removed from the crystals, the piezo electric crystals retract along the same axis. The piezo electric crystals may be stacked and compressed a predetermined amount by springs  112 . 
     Casing  114  may house piezo element  110  and provide protection against environmental hazards (e.g., fuel contamination, physical damage, etc.). Casing  114  may include a generally cylindrical wall portion  122  and an end portion  124 . Wall portion  122  may include a plurality of alternating large and small diameters that together form bellows  126 . In one example, bellows  126  may extend along casing  114  about the same length as piezo element  110 , when assembled, to accommodate the expansion and retraction described above. End portion  124  may be integral to wall portion  122 , formed of the same material, and bent inward from wall portion  122  toward a central axis (not shown). In one example, end portion  124  may be bent inward through an angle greater than 90 degrees for engagement with first end cap  116 . 
     Wall and end portions  122 ,  124  may be formed through a deep draw process. Specifically, a metallic blank (not shown) such as, for example, an aluminum blank, may be forced against a mold (e.g., into a female mold or over a male mold) to form a substantially cylindrical-shaped object having an open end and a closed end. In the particular example depicted in  FIG. 3 , the aluminum blank was forced into a female mold such that end portion  124  was bent through the appropriate angle described above. Once the cylindrical-shaped object is formed, a hole  126  having a diameter less than an inner diameter of wall portion  122  may be made through the closed end of the cylindrical-shaped object, such that only an annular lip structure remains. Hole  126  may be made through a shearing process, reaming process, boring process, or any another known hole-making process. It is contemplated that wall and end portions  122 ,  124  may alternatively be formed from a metal blank other than aluminum such as, for example, from stainless steel, if desired. 
     Bellows  126  may be formed within wall portion  122  through a thread-rolling process. In particular, the cylindrical-shaped object described above may be mounted within a machine to rotate or otherwise be spun about its central axis. During this rotation, one or more dies having a plurality of equally spaced, ridge-shaped protrusions may be urged into an outer and/or inner surface of the cylindrical casing, thereby deforming the surface to create bellows within casing  114 . 
     First end cap  116  may be operatively connected to piezo element  110 . First end cap  116  may be connected to piezo element  110  to transfer the force associated with the expansion and contraction of piezo element  110  to proportional valve element  106  (referring to  FIG. 2 ). To withstand the forces generated by the expansion of piezo element  110  and the chemical environment within piezo device  108 , first end cap  116  may be fabricated from, for example, stainless steel. 
     To minimize the likelihood of fuel leaking into and contaminating piezo device  108 , first end cap  116  may be hermetically sealed to casing  114 . Specifically, first end cap  116  may include an inner face  128 , an outer face  130 , and a cylindrical surface  132  connecting inner and outer faces  128  and  130 . Outer face  130  and/or cylindrical surface  132  may be welded, chemically joined, or otherwise sealed to wall and/or end portions  114 ,  116 , respectively. It is contemplated that multiple sealing locations between casing  114  and first end cap  116  may provide improved leakage protection for piezo element  110 , as compared to a single sealing location. 
     Similar to first end cap  116 , second end cap  118  may likewise be connected to piezo element  110  and hermetically sealed to casing  114 . Second end cap  118  may be connected to an end of piezo element  110  opposing first end cap  116  to transfer the force associated with the expansion and contraction of piezo element  110  in reverse direction to a support of fuel injector  32  (referring to  FIG. 2 ). To withstand the forces generated by the expansion of piezo element  110  and the chemical environment within piezo device  108 , second end cap  118  may also be fabricated from stainless steel. An outer cylindrical surface of second end cap  118  may be welded, chemically joined, or otherwise sealed to wall portion  114 . 
     Electrical leads  120  may embody positive and negative conductors that extend through second end cap  116  to direct current into and out of piezo element  110 . Electrical leads  120  may be alternatingly connected to layers of crystals within piezo element  110  to create a circuit through each crystal. Electrical leads  120  may be connected to the crystals of piezo element  110  in any manner known in the art. 
     Industrial Applicability 
     Although illustrated and described above as being utilized in conjunction with a common rail type fuel injector, the disclosed piezo device may be applicable to any fluid system where it is advantageous to isolate the associated piezo element from the fluid and/or other contaminates of the system. By isolating the piezo element from the fluid while still allowing movement of the piezo element, the piezo device may function as intended and have prolonged component life. In addition, by producing the isolation solution (e.g., the case housing the piezo element) from low cost materials through low cost manufacturing methods, the fluid system incorporating the piezo device may be economical. The operation of fuel injector  32  will now be explained. 
     Needle valve element  58  may be moved by an imbalance of force generated by fluid pressure. For example, when needle valve element  58  is in the first or orifice-blocking position, pressurized fuel from fuel supply passageway  70  may flow into control chamber  71  to act on hydraulic surface  100 . Simultaneously, pressurized fuel from fuel supply passageway  70  may flow into central bore  68  in anticipation of injection. The force of spring  90  combined with the hydraulic force created at hydraulic surface  100  may be greater than an opposing force created at hydraulic surface  104 , thereby causing needle valve element  58  to remain in the first position and restrict fuel flow through orifices  80 . To open orifices  80  and inject the pressurized fuel from central bore  68  into combustion chamber  22 , proportional valve element  106  may be moved to selectively drain the pressurized fuel away from control chamber  71  and hydraulic surface  100 . This decrease in pressure acting on hydraulic surface  100  may allow the opposing force acting across hydraulic surface  104  to overcome the biasing force of spring  90 , thereby moving needle valve element  58  toward the orifice-opening position. To close orifices  80  and end the injection of pressurized fuel, proportional valve element  106  may be moved to stop fuel from draining away from control chamber  71  and to, instead, fill control chamber  71  with pressurized fuel. 
     Proportional valve element  106  may be directly moved by piezo device  108 . In particular, as current is applied to the crystals of piezo element  110  via electrical leads  120 , the crystals within piezo element  110  may expand, resulting in the axial extension of piezo device  108  and the corresponding connected movement of proportional valve element  106 . In contrast, as current is removed from the crystals within piezo element  110 , the crystals may contract, resulting in the axial retraction of piezo device  10  and the corresponding connected movement of proportional valve element  106 . It is contemplated that piezo device  108  may alternatively move proportional valve element  106  indirectly by way of pilot fluid, if desired. 
     As the crystals within piezo element  110  expand or contract, casing  114  may accommodate the corresponding change in length. In particular, in order to avoid damage to piezo device  108 , as the crystals within piezo element  110  expand, bellows  126  of casing  114  may deform to increase the length of casing  114 . Conversely, as the crystals within piezo element  110  contract, bellows  120  may return casing  114  to its original shape to decrease the length thereof. 
     The ability of bellows  126  to deform during the expansion and contraction of piezo element  110  may provide the bias utilized to pre-tension piezo element  110 , when springs  112  are omitted. In other words, after fabrication of casing  114 , after first end cap  116  has been joined to casing  114 , and after piezo element  110  has been inserted into casing  114 , second end cap  118  may be inserted into casing  114  a predetermined distance or inserted with a predetermined force such that bellows  126  are slightly deformed and the crystals of piezo element  110  are pre-tensioned when second end cap  118  is joined to casing  114 . Pre-tensioning may be implemented to accommodate manufacturing tolerances and ensure that a majority of the piezo movement is transmitted to proportional valve element  106 . 
     The processes and materials used to fabricate casing  114  may reduce the cost and improve the reliability of piezo device  108  and fuel injector  132 . Specifically, because deep drawing and thread rolling are both relatively low cost manufacturing methods, and because aluminum is a relatively low cost material, the cost of casing  114  may likewise be inexpensive. In addition, because the material used for casing  114  is dissimilar to the material used for first and second end caps  116 ,  118 , each component may be fabricated from the material best suited for its intended function, without unnecessarily driving up the cost piezo device  108 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the fuel injector of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the fuel injector disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and their equivalents.