Abstract:
A fuel injector comprises a body having a longitudinal axis, a piezoelectric actuator that has first and second ends, a needle coupled to the first end of the piezoelectric actuator, and a hydraulic compensator coupled the second end of the piezoelectric actuator. The piezoelectric actuator includes a plurality of piezoelectric elements along the axis between the first and second ends. The needle is movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection. And the hydraulic compensator axially positions the piezoelectric actuator with respect to the body in response to temperature variation. Also, a method of compensating for thermal expansion or contraction of the fuel injector comprises providing fuel from a fuel supply to the fuel injector; and axially adjusting the piezoelectric actuator with respect to the body in response to temperature variation. The axially adjusting includes moving hydraulic oil through an orifice connecting first and second hydraulic oil reservoirs.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention generally relates to piezoelectric strain actuators. In particular, the present invention relates to a hydraulic compensator for a piezoelectric actuator, and more particularly to an apparatus and method for hydraulically compensating a piezoelectrically actuated high-pressure fuel injector for internal combustion engines.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is believed that a known piezoelectric actuator is includes a ceramic structure whose axial length can change through the application of an operating voltage. It is believed that in typical applications, the axial length can change by, for example, approximately 0.12%. In a stacked configuration, it is believed that the change in the axial length is magnified as a function of the number of actuators in the piezoelectric actuator stack. Because of the nature of the piezoelectric actuator, it is believed that a voltage application results in an instantaneous expansion of the actuator and an instantaneous movement of any structure connected to the actuator. In the field of automotive technology, especially, in internal combustion engines, it is believed that there is a need for the precise opening and closing of an injector valve element for optimizing the spray and combustion of fuel. Therefore, in internal combustion engines, it is believed that piezoelectric actuators are now employed for the precise opening and closing of the injector valve element.  
           [0003]    During operation, it is believed that the components of an internal combustion engine experience significant thermal fluctuations that result in the thermal expansion or contraction of the engine components. For example, it is believed that a fuel injector assembly includes a valve body that may expand during operation due to the heat generated by the engine. Moreover, it is believed that a valve element operating within the valve body may contract due to contact with relatively cold fuel. If a piezoelectric actuator stack is used for the opening and closing of an injector valve element, it is believed that the thermal fluctuations can result in valve element movements that can be characterized as an insufficient opening stroke, or an insufficient sealing stroke. It is believed that this is because of the low thermal expansion characteristics of the piezoelectric actuator as compared to the thermal expansion characteristics of other engine components. For example, it is believed that a piezoelectric actuator stack is capable of 30 microns of movement and that a valve element is capable of contracting 10 microns due to temperature fluctuations, in which case the piezoelectric actuator stack loses 30% of its overall movement. Therefore, it is believed that any contractions or expansions of a valve element can have a significant effect on fuel injector operation.  
           [0004]    It is believed that conventional methods and apparatuses that compensate for thermal changes affecting piezoelectric actuator stack operation have drawbacks in that they either only approximate the change in length, they only provide one length change compensation for the piezoelectric actuator stack, or that they only accurately approximate the change in length of the piezoelectric actuator stack for a narrow range of temperature changes.  
           [0005]    It is believed that there is a need to provide thermal compensation that overcomes the drawbacks of conventional methods.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a fuel injector. The fuel injector comprises a body having a longitudinal axis, a piezoelectric actuator that has first and second ends, a needle coupled to the first end of the piezoelectric actuator, and a hydraulic compensator coupled the second end of the piezoelectric actuator. The piezoelectric actuator includes a plurality of piezoelectric elements along the axis between the first and second ends. The needle is movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection. And the hydraulic compensator axially positions the piezoelectric actuator with respect to the body in response to temperature variation.  
           [0007]    The present invention also provides a method of compensating for thermal expansion or contraction of a fuel injector. The fuel injector includes a body that has a longitudinal axis, a piezoelectric actuator that has first and second ends, a needle coupled to the first end of the piezoelectric actuator, and a hydraulic compensator coupled the second end of the piezoelectric actuator. The piezoelectric actuator includes a plurality of piezoelectric elements along the axis between the first and second ends. The needle is movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection. The method comprises providing fuel from a fuel supply to the fuel injector; and axially adjusting the piezoelectric actuator with respect to the body in response to temperature variation. The axially adjusting includes moving hydraulic oil through an orifice connecting the first and second reservoirs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain features of the invention.  
         [0009]    [0009]FIG. 1 is a cross-sectional view of a fuel injector assembly having a piezoelectric actuator stack and a hydraulic compensator unit.  
         [0010]    [0010]FIG. 2 is an enlarged view of an embodiment a hydraulic compensator assembly.  
         [0011]    [0011]FIG. 3 is an enlarged view of an alternative embodiment of a hydraulic compensator assembly.  
         [0012]    [0012]FIG. 4 is an enlarged view of a tube spring for a piezoelectric stack. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    [0013]FIG. 1 is a cross-sectional view of a fuel injector assembly  100  having a piezoelectric actuator stack  22  and a hydraulic compensator assembly  16 .  
         [0014]    The fuel injector assembly  100  includes inlet cap  14 , injector housing  11 , and valve body  8 . The inlet cap  14  includes a fuel filter  23 , fuel passageways  27  and  30 , and a fuel inlet  26  connected to a fuel source (not shown).  
         [0015]    Injector housing  11  encloses the piezoelectric actuator stack  22  and the hydraulic compensator assembly  16 . Valve body  8  is fixedly connected to injector housing  11  and encloses a valve needle  6 .  
         [0016]    The piezoelectric actuator stack  22  includes a plurality of piezoelectric actuators that can be operated through contact pins (not shown) that are electrically connected to a voltage source. When a voltage is applied between the contact pins (not shown), the piezoelectric actuator stack  22  expands in a lengthwise direction. A typical expansion of the piezoelectric actuator stack  22  may be on the order of approximately 30 microns, for example. The lengthwise expansion can be utilized for operating the injection valve needle  6  for the fuel injector assembly  100 .  
         [0017]    [0017]FIG. 4 is an enlarged view of a tube spring  17  for pre-compressing the piezoelectric actuator stack  22 . Tube spring  17  prevents the piezoelectric actuator stack  22  from being placed in tension and thus cracking. Tube spring  17  has holes  31  uniformly distributed over its entire surface. The holes  31  are of a “dumb-bell” shape and run through the tube spring  17  at right angles relative to the axis of the spring. The holes  31  provide assurance that the tube spring  17  has sufficient elasticity for allowing for elongation of the piezoelectric actuator stack  22  and that the tube spring  17  has a negligible interference on the elongation characteristics of the piezoelectric actuator stack  22 . The elasticity of the tube spring  17  can be adjusted by the number and size of the holes  31  to permit a desired elongation of the biased piezoelectric actuator stack  22 . Tube spring  17  is made preferably from spring steel, which has excellent high strength characteristics. Alternatively, other materials, such as materials with a low elasticity modulus (e.g., copper-beryllium alloys), can be used as well for tube spring  17 .  
         [0018]    Piezoelectric actuator stack  22  is guided along housing  11  by means of guides  25 . The piezoelectric actuator stack  22  has a first end in operative contact with valve needle  6  by means of bottom  3 , and a second end that is operatively connected to hydraulic compensator assembly  16  by means of a top  15 .  
         [0019]    Fuel injector assembly  100  further includes an inner spring  18 , an outer spring  19 , a spring washer  1 , a keeper  2 , a bushing  4 , a lower bellows  5 , a valve needle seat  7 , a bellows weld ring  9 , and an O-ring  20 . O-ring  20  may be preferably an “Apple” type O-ring. Nested inner and outer springs  18  and  19 , respectively, allow for a relatively high spring factor and small overall spring diameter as compared to a single spring with the same overall spring factor.  
         [0020]    [0020]FIG. 2 is an enlarged view of a first embodiment of a hydraulic compensator assembly  16 . Hydraulic compensator assembly  16  includes a bellows  50 , a piston  51 , a bellows weld ring  52 , an orifice screw  53 , O-rings  54  and  55 , a compression spring  56 , hydraulic oil  57 , an orifice  58  and a supply reservoir  59 . O-ring  54  may be a “Parker” type O-ring, and O-ring  55  may be an “Apple” type O-ring. Bellows  50  may be used in the hydraulic compensator assembly  16  because of its superior wear-resistant properties as compared to an O-ring. Piston  51  can be operatively connected to top  15  of piezoelectric actuator stack  22  so that any axial translation of piston  51  is directly transmitted to piezoelectric actuator stack  22 . Hydraulic oil  57  may be Silicon oil, but can alternately be any type of fluid with similar fluid properties, e.g., substantially non-compressible.  
         [0021]    During operation of the first embodiment of the hydraulic compensator  16 , fuel is introduced at fuel inlet  26  from a fuel supply (not shown). Fuel at fuel inlet  26  passes through a fuel filter  23 , through a passageway  30 , through a passageway  27 , through a fuel tube  10 , through a passageway  28 , and out through a fuel outlet  29  when valve needle  6  is moved to an open configuration.  
         [0022]    In order for fuel to exit through fuel outlet  29 , voltage is supplied to piezoelectric actuator stack  22  causing it to expand. The expansion of piezoelectric actuator stack  22  causes bottom  3  to push against valve needle  6  and allow fuel to exit the fuel outlet  29 . After fuel is injected through fuel outlet  29 , the voltage supply to piezoelectric actuator stack  22  is terminated and valve needle  6  is returned under the bias of inner and outer springs  18  and  19 , respectively, to close fuel outlet  29 . Specifically, the piezoelectric actuator stack  22  contracts when the voltage supply is terminated, and the bias of the inner and outer springs  18 ,  19 , which hold the valve needle  6  in constant contact with bottom  3 , also biases the valve needle  6  to the closed configuration.  
         [0023]    During engine operation, as the temperature in the engine rises, inlet cap  14 , injector housing  11  and valve body  8  experience thermal expansion due to the rise in temperature. At the same time, fuel traveling through fuel tube  10  and out through fuel outlet  29  cool the internal components of fuel injector assembly  100  and cause thermal contraction of valve needle  6 . Referring to FIGS. 1 and 2, as valve needle  6  contracts, bottom  3  tends to separate from its contact point with valve needle  6 . Piezoelectric actuator stack  22 , which is operatively connected to the bottom surface of piston  51 , is pushed downward by means of piston  51  of hydraulic compensator  16 . The increase in temperature causes inlet cap  14 , injector housing  11  and valve body  8  to expand and cause further compression of compression spring  56 . The compression force on compression spring  56  is transferred to hydraulic oil  57  by means of upper bellows  50 . Thus, hydraulic oil  57  is pushed from supply reservoir  59 , down through orifice  58 , to a working reservoir that forms a “shim” of hydraulic oil against the bottom end of orifice screw  53  and against the top surface of piston  51 . Because of the virtual incompressibility of hydraulic oil and the relatively small diameter of orifice  58  (approximately 30 microns), the “shim” of hydraulic oil against the top surface of piston  51  acts as a substantially solid structure and thus maintains the axial orientation of piston  51  during subsequent energizing or deenergizing of piezoelectric actuator stack  22 .  
         [0024]    During subsequent fluctuations in temperature around the fuel injector assembly  100 , any further expansion or contraction of inlet cap  14 , injector housing  11  and valve body  8  causes the hydraulic oil  57  to travel from or into reservoir  59 , through orifice  58 . Thus bottom  3  is maintained in constant contact with the contact surface of valve needle  6 .  
         [0025]    [0025]FIG. 3 is an enlarged view of a second embodiment of a hydraulic compensator assembly  70  according to the present invention. Hydraulic compensator assembly  70  includes a piston  71 , a back-up piston  72 , a plug  73 , an orifice screw  74 , O-rings  75 - 78 , a compression spring  79 , hydraulic oil  80 , a supply reservoir  81 , and an orifice  82 . O-rings  75  and  77  may be preferably “Parker” type O-rings, and O-rings  76  and  79  may be preferably “Apple” type O-rings. Piston  71  can be operatively connected to top  15  of piezoelectric actuator stack  22  so that any axial translation of piston  71  is directly transmitted to piezoelectric actuator stack  22 . Hydraulic oil  80  may be Silicon oil, but can alternately be any type of fluid with similar fluid properties, e.g., substantially non-compressible.  
         [0026]    During operation of the second embodiment of the hydraulic compensator  70 , fuel is introduced to the fuel inlet  26  from a fuel supply (not shown). Fuel at fuel inlet  26  passes through fuel filter  23 , through passageway  30 , through passageway  27 , through fuel tube  10 , through passageway  28  and out through fuel outlet  29  when valve needle  6  is moved to the open configuration.  
         [0027]    In order for fuel to exit through fuel outlet  29 , voltage is supplied to piezoelectric actuator stack  22  causing it to expand. The expansion of piezoelectric actuator stack  22  causes attached bottom  3  to push against valve needle  6  and allow fuel to exit the fuel outlet  29 . Upon fuel release through fuel outlet  29 , the voltage supply to piezoelectric actuator stack  22  is terminated and valve needle  6  is returned to its original position to close fuel outlet  29  under the bias of inner and outer springs  18 ,  19 . Specifically, the piezoelectric actuator stack  22  contracts when the voltage supply is terminated, and the bias of the inner and outer springs  18 ,  19 , which hold the valve needle  6  in constant contact with bottom  3 , also biases the valve needle  6  to the closed configuration.  
         [0028]    During engine operation, as the temperature in the engine rises, inlet cap  14 , injector housing  11  and valve body  8  experience thermal expansion due to the rise in temperature. At the same time, fuel traveling through fuel tube  10  and out through fuel outlet  29  cool the internal components of fuel injector assembly  100  and cause thermal contraction of valve needle  6 . Referring to FIGS. 1 and 3, as valve needle  6  contracts, bottom  3  tends to separate from its contact point with valve needle  6 . Piezoelectric actuator stack  22 , which is operatively connected to the bottom surface of piston  71 , is pushed downward by means of piston  71  of hydraulic compensator  70 . The increase in temperature causes inlet cap  14 , injector housing  11  and valve body  8  to expand and cause further compression of compression spring  79 . The compression force on compression spring  79  is transferred to hydraulic oil  80  by means of back-up piston  72 . Thus, hydraulic oil  80  is pushed from reservoir  81  down through orifice  82  to a working reservoir that forms a “shim” of hydraulic oil against the top surface of piston  71 . Thus, as compared to the first embodiment, instead of using a bellows to push the hydraulic oil out of the reservoir, the alternate embodiment of FIG. 3 uses a “Parker” type O-ring  77  and a backup piston  72  to push hydraulic oil  80  through orifice  82 . Because of the virtual incompressibility of hydraulic oil and the relatively small diameter of orifice  82  (approximately 30 microns), the “shim” of hydraulic oil against the top surface of piston  71  acts as a substantially “solid” rest structure and thus maintains the axial orientation of piston  71  during subsequent energizing or de-energizing of piezoelectric actuator stack  22 .  
         [0029]    During subsequent fluctuations in temperature around the fuel injector assembly  100 , any further expansion or contraction of inlet cap  14 , injector housing  11  and valve body  8  causes the high viscosity hydraulic oil  80  to travel from or into reservoir  81 , through orifice  82 . Thus bottom  3  is maintained in constant contact with the contact surface of valve needle  6 .  
         [0030]    Referring also to FIG. 1, fuel injector assembly  100  further includes a crush ring  12  and an adjusting screw  13 . Crush ring  12  adjusts the axial positioning of hydraulic compensator assembly  16  (or  70 ) relative to the housing  11 . Adjusting screw  13  allows pre-adjustment of the axial location of hydraulic compensator assembly  16  (or  70 ) relative to piezoelectric actuator stack  17 , as well as pre-adjustment of the spring factor of compression spring  56  (or  79 ).  
         [0031]    While the present invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims, and equivalents thereof.