Abstract:
A fuel injector in an engine includes a spring cavity, a piston, a plunger, a spring, a fuel cavity, and a stop plate. The piston is hydraulically controlled to force the plunger down to compress fuel in the fuel cavity. However, under certain conditions, the plunger can contact the stop plate and/or the spring can become overcompressed. Both of these conditions can cause damage to the fuel injector. The present invention locates a pressure equalization channel in such a way as to dampen the motion of the piston to prevent this damage to the fuel injector.

Description:
TECHNICAL FIELD  
         [0001]    This invention pertains to a pressure control for a piston-type device and, more particularly, to a pressure control which prevents the fuel plunger in a hydraulically-actuated fuel injector from contacting the stop plate of the fuel injector.  
         BACKGROUND  
         [0002]    A fuel injector is commonly used to pressurize and atomize fuel in an internal combustion engine. In a common hydraulically-actuated fuel injector, a piston and plunger system in a spring cavity transfers hydraulic fluid pressure to the fuel. The piston moves reciprocally up and down within the spring cavity, and the motion of the piston causes the plunger to move, as well. First, fuel is introduced into a fuel cavity beneath the plunger, and hydraulic pressure on the piston forces the plunger down into the fuel cavity to compress the fuel. Since the fuel cavity and plunger are of a smaller cross-section than the spring cavity and piston, the force from the piston through the plunger and to the fuel cavity is magnified accordingly in a known manner for greater efficiency of compression.  
           [0003]    Next, one of two things can happen. The plunger can contact, or “bottom out” on, a stop plate at the bottom of the fuel cavity and the plunger is consequently stopped and ready for the next stage of the compression cycle. Often the plunger/stop plate collision can damage one or both components, so this is generally only a secondary method of stopping the plunger. Alternately and usually preferably, the fuel or another fluid present in the fuel or spring cavity becomes pressurized until the fluid&#39;s resistance to further compression resists and/or stops the motion of at least one of the plunger and the piston. The latter condition is referred to in the art as a “hydraulic lock”, in which a fluid cannot be compressed any more by the outside pressure placed upon it, and is of primary interest in the below description.  
           [0004]    Regardless of the plunger stop mechanism, the compressed fuel is injected into the combustion chamber in a known manner at any suitable point in the plunger motion cycle, thereby vacating the fuel cavity. Finally, a piston spring in the spring cavity forces the piston back up to prepare the fuel injector for the next compression cycle. Hundreds or even thousands of these high-speed and high-stress reciprocal fuel compression cycles occur every minute, which makes efficient and robust operation of the various components of the fuel injector a priority.  
           [0005]    Often hydraulic fluid under pressure seeps past the piston and into the spring cavity below the piston during operation of the fuel injector. Since the hydraulic fluid could build up in the spring cavity and hydraulically lock the piston as described above before the fuel is fully pressurized for injection, it is common for a vent hole to be provided at the bottom of the spring cavity to carry any extant hydraulic fluid to a vent line, this evacuation being normally propelled by the downstroke of the piston. This vent hole may also function as an air intake to prevent a vacuum being formed in the spring cavity on the piston upstroke and slowing the motion of the piston.  
           [0006]    The stop plate mentioned previously is commonly located at an end of the fuel cavity opposite the plunger. The stop plate acts partially to form the fuel cavity and partially to halt motion of the plunger in a situation when the fuel or hydraulic fluid in the fuel cavity or the spring cavity is insufficient to hydraulically lock the plunger and piston in the preferred manner. Situations that can cause a low fuel situation and subsequent “bottoming out” of the plunger (allowing the plunger to contact the stop plate) include fuel transfer pump failure, air in the fuel supply line, fuel pressure regulator valve failure, the engine&#39;s being simply allowed to run out of fuel through neglect or malfunction, and the like. Additionally, while bottoming out is generally not a preferred plunger function, design features and choices with respect to other components may allow the plunger to occasionally bottom out in an otherwise normally functioning fuel injector.  
           [0007]    There are two main malfunctions that can result when a plunger bottoms out. The high impact velocity of the plunger on the stop plate can cause material failure and stress damage to one or both components, particularly if repeated contact occurs. Also, and more seriously, the piston spring can overtravel or become overcompressed, either of these causing a permanent reduction in the height of the piston spring or even breakage of that spring. Since the piston spring is the only force outside the hydraulic lock acting to resist downward motion of the plunger, a shortened piston spring will probably allow the plunger to bottom out repeatedly until the fuel injector totally fails because of component breakage. It is estimated that this total injector failure occurs within about twenty seconds of the piston spring failure, leaving little to no time for the problem to be detected and the engine shut down to prevent such failure. When the fuel injector fails, the engine effectively loses power in that cylinder and numerous well-known problems typically result.  
           [0008]    Additionally, there are many other applications in the field for a piston assembly such as that described above. Any hydraulic piston assembly working to compress a fluid in much the same manner, perhaps in an injection molding or glue-applying situation, would be subject to these or similar difficulties. Since the overall structure of these piston assemblies is analogous to the fuel injector described, it is intuitively obvious that many different applications can be effected by piston assembly failure as described. Therefore, a solution to the piston assembly failure is widely sought.  
           [0009]    The present invention is directed to overcoming one or more of the problems as set forth above.  
         SUMMARY OF THE INVENTION  
         [0010]    In an embodiment of the present invention, a hydraulic piston assembly is disclosed. The hydraulic piston assembly includes a piston body, a cavity disposed within the piston body, a piston disposed within the cavity and moveable between a first position and a second position, and a vent hole in the piston body which selectively connects the cavity to a low pressure.  
           [0011]    In an embodiment of the present invention, a hydraulic piston assembly is disclosed. The hydraulic piston assembly includes a piston body, a cavity disposed within the piston body, a piston disposed within the cavity and separating the cavity into a first subcavity and a second subcavity, and a piston hole in the piston.  
           [0012]    In an embodiment of the present invention, a hydraulically-actuated fuel injector is disclosed. The hydraulically-actuated fuel injector includes a piston body defining a piston axis, a spring cavity located inside the piston body and having a first cavity end and a second cavity end spaced apart from the first cavity end along the piston axis, a piston located substantially inside the spring cavity and moveable along the piston axis between a first position and a second position, and a pressure equalization channel.  
           [0013]    In an embodiment of the present invention, a method of controlling the motion of a piston in a fuel injector, wherein the fuel injector includes a spring cavity having a first cavity end and a second cavity end, is disclosed. The method includes the steps of locating the piston within the spring cavity, moving a plunger with the piston, and providing a piston spring within the spring cavity and adapted to provide positive pressure to the piston in a first direction. The method also includes the steps of providing pressurized hydraulic fluid at a first portion of the spring cavity located near the first cavity end, exerting positive pressure on the piston in a second direction, and allowing the pressurized hydraulic fluid to enter a second portion of the spring cavity located between the piston and the second cavity end. The method also includes the steps of substantially equalizing the pressures between the first and second portions of the spring cavity and slowing the piston.  
           [0014]    In an embodiment of the present invention, a hydraulic damping device for a piston mechanism is disclosed. The hydraulic damping device includes an elongate piston body, a piston, a hydraulic source, and a pressure equalizing system. The elongate piston body has a first end and a second end. The piston is adapted to move reciprocally between the first and second ends, thereby defining a variable volume first chamber adjacent the first end and a variable volume second chamber adjacent the second end. The hydraulic source is adapted to supply hydraulic fluid to the piston body. The pressure equalizing system is adapted to substantially equalize pressures of the hydraulic fluid in the first and second chambers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a cutaway side view of a fuel injector incorporating a preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 2 a  is a partial cutaway side view of a fuel injector incorporating a preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 2 b  is a partial cutaway side view of a fuel injector incorporating a preferred embodiment of the present invention; and  
         [0018]    [0018]FIG. 3 is a partial cutaway side view of a fuel injector incorporating another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 1 depicts a fuel injector  100  having a piston body  102  and a spring cavity  104 . The piston body  102  defines a piston axis  106 . The spring cavity  104  includes a first cavity end, shown generally at  108 , a second cavity end spaced apart from the first cavity end  108  along the piston axis  106 , shown generally at  110 , and a cavity midsection located along the piston axis  106  between the first cavity end  108  and the second cavity end  110 , and shown generally at  112 . The operation of a fuel injector is substantially described above, but certain aspects of the operation will be further clarified as needed.  
         [0020]    A piston  114  is located, at least initially, near the first cavity end  108  and is adapted to travel in a reciprocating motion within the spring cavity  104 , driven by hydraulic fluid. The piston  114  will be located in the cavity midsection  112  during at least a portion of the reciprocating travel. A stop plate  116  is located near the second cavity end  110 . A plunger  118  is attached to, or in a contacting relationship with, the piston  114 . The plunger  118  may be the same diameter as the piston  114 , but is preferably of a reduced diameter as shown in the Figs., for pressure intensifying reasons well-known in the art.  
         [0021]    The plunger  118  and the stop plate  116 , along with portions of the spring cavity  104 , define a fuel cavity  120  near the second cavity end  110 . A barrel  122  at least partially surrounds the fuel cavity  120  and at least a portion of the plunger  118  to help define the fuel cavity  120  and to guide the plunger  118 . Preferably, the barrel  122  creates a reduced-diameter fuel cavity  120  as shown so that the pressure-intensifying effects of the varied diameters of the piston  114  and plunger  118  may be utilized. The barrel  122  is adapted to provide fuel to the fuel cavity  120  from a fuel source (not shown), in a known manner.  
         [0022]    Should the fuel injector  100  not include a barrel  122 , the fuel cavity  120  may be formed as an extension of the spring cavity  104  or by any other suitable means. In this case, fuel may be provided to the fuel cavity  120  in any other suitable manner, and a component should be provided which separates the fuel cavity  120  from the spring cavity  104  to contain the fuel.  
         [0023]    Preferably, a piston spring  124  or other suitable resistor member is located substantially within the spring cavity  104  and positioned so as to provide positive pressure in a first direction  126  to the piston  114 . The piston spring  124  has a first spring end  128  which contacts the piston  114  and a second spring end  130  spaced apart from the first spring end  128  along the piston axis  106  which contacts the barrel  122  or, if the fuel injector  100  does not include a barrel  122 , contacts the second cavity end  110 . The piston spring  114  commonly surrounds the plunger  118  within the spring cavity  104 .  
         [0024]    Pressurized hydraulic fluid is provided to the spring cavity  104  near the first cavity end  108 . The hydraulic fluid builds up between the first cavity end  108  and the piston  114  to overcome the pressure provided by the piston spring  124  and propel the piston  114  along the piston axis  106  in a second direction  132 . The motion of the piston  114  causes the plunger  118  to move in the second direction  132  and subsequently reduce the volume of the fuel cavity  120 , therefore increasing the pressure of the fuel within the cavity. When the pressure in the first direction  126  becomes substantially equal to the pressure in the second direction  132 , no disparate hydraulic force is acting on the piston  114  in either the first or second directions  126 , 132 , and the piston  114  will naturally cease motion because of the lack of a “pushing” force. The pressurized hydraulic fluid from area of the first cavity end  108  is then released in a known manner as, or after, the now-pressurized fuel is transferred to the combustion chamber of the engine. Then, the pressure in the second direction  132  overcomes the pressure in the first direction to push the piston  114  back toward the first cavity end  108 . A new fuel injector cycle then begins.  
         [0025]    The pressure in the first direction  126  is substantially provided by a combination of the fuel&#39;s resistance to pressure (if there is fuel in the fuel cavity  120 ), the spring force provided by the piston spring  114 , and the resistance to pressure of any hydraulic fluid which happens to be extant in the spring cavity  104  below the piston  114 . By extant, what is meant is that the hydraulic fluid has either seeped past the piston  114  as described above or has been purposely routed past or through the piston; either way, the “extant” hydraulic fluid has come to be present in the spring cavity  104  between the piston  114  and the second cavity end  110 . The pressure in the second direction  132  is mainly from the pressurized hydraulic fluid which drives the piston  114 . In order to equalize these two pressures, a pressure equalization channel adapted to facilitate the transfer of hydraulic fluid in a desired manner is provided by the present invention.  
         [0026]    In a first preferred embodiment of the present invention shown in FIGS. 1, 2 a,  and  2   b,  the pressure equalization channel takes the form of a vent hole  134  provided in the cavity midsection  112 . The vent hole  134  is fluidically connected to a vent line  136  or other low pressure in a known manner. The precise location and dimensions of the vent hole  134  are important to the proper functioning of the present invention but are highly dependent upon the relative dimensions of the other components of the fuel injector  100  and thus do not form a necessary component of the present invention. It is intuitively obvious that experimentation will enable the proper placement of the vent hole  134  in the cavity midsection  112  in practice. It is advantageous, as described below, for the vent hole  134  to be located such that the piston  114  completely covers and blocks the vent hole  134  when the piston  114  is at or near a lower limit of travel in the second direction  132 .  
         [0027]    In a second preferred embodiment of the present invention shown in FIG. 3, the pressure equalization channel takes the form of a piston hole  342  in the body of the piston  114 . The piston  114  divides the spring cavity  104  into first and second portions or subcavities  238 , 240 , as shown best in FIGS. 2 a,    2   b,  and  3 . The first and second subcavities  238 , 240  are variable in volume as the piston  114  moves through its reciprocal cycle. The pressure in each subcavity comes from the sources described above—normally either pressurized hydraulic fluid driving the motion of the piston  114  or hydraulic fluid which has become extant in the spring cavity  104  below the piston  114 . The piston hole  342  directs pressurized hydraulic fluid through the piston  114  and into the second subcavity  240  in order to controllably set up a hydraulic lock situation which will stop the piston  114  when the pressures in the first and second subcavities  238 , 240  are substantially the same.  
         [0028]    The exact configuration of the piston hole  342  is not important, so long as it fluidically connects the first and second subcavities  238 , 240 , though it is obvious that a piston hole  342  substantially parallel to the piston axis  106  will provide a direct path for the hydraulic fluid to travel quickly through the piston  114 . A piston hole  342  using a labyrinthine structure, an integral valve, or the like would be considered a pressure equalization channel, as well.  
         [0029]    The substances used in the operation of the fuel injector  100  have been described as “fuel” and “hydraulic fluid”, but the exact nature of the substances is inconsequential, except as their properties affect other operations of the engine or another larger device encompassing the present invention. The substances may be different from one another or may be the same substance. Oils, petroleum distillates, water, compressed air, other fluids, and the like may be used without affecting the operation of the present invention.  
         [0030]    Industrial Applicability  
         [0031]    [0031]FIGS. 2 a  and  2   b  depict different stages in the reciprocal compression cycle of the piston  114  within the fuel injector  100  in the first preferred embodiment of the present invention. In FIG. 2 a,  hydraulic fluid enters the spring cavity  104  at the first cavity end  108  and forces the piston  114  in the second direction  132 . The piston  114  then pushes on the first spring end  128 , causing the piston spring  124  to compress.  
         [0032]    As the pressurized hydraulic fluid enters the spring cavity  104 , often a portion of the hydraulic fluid seeps past the piston  114  and becomes extant in the cavity midsection  112 . This seepage is an inherent characteristic of a hydraulically-actuated fuel injector  100 . In the embodiment shown in FIG. 3, hydraulic fluid is also purposely directed into the second subcavity  240  to supplement the seepage and remains in the spring cavity  104 . However, in the embodiment shown in FIGS. 2 a  and  2   b,  at least a portion of the hydraulic fluid that becomes extant within the spring cavity  104  is forced out of the vent hole  134  and carried away by the vent line  136  in a known manner as the piston  114  travels in the second direction  132 .  
         [0033]    [0033]FIG. 2 b  depicts the piston  114  at or near a lower travel limit. The piston spring  124  in FIG. 2 b  is almost fully compressed and may overtravel or become overcompressed, probably causing permanent damage to the fuel injector  100 , if the piston  114  continues in the second direction  132 . However, the vent hole  134  is now blocked by the piston  114 , and hydraulic fluid is therefore trapped in the spring cavity  104 . The trapped hydraulic fluid becomes pressurized by the piston  114  action, and the pressure equalizes in the first and second subcavities  238 , 240 , thus providing a damping function to prevent the piston  114  from further travel. The damping function slows or stops the piston  114  because the trapped hydraulic fluid sets up a hydraulic lock in the second subcavity  240 . The pressure of the fluid in the second subcavity  240  acts oppositely on the piston  114  as does the pressurized fluid pushing the piston  114  in the second direction  132 . When these oppositely directed forces become substantially equal, that is, when the pressures in the first and second subcavities  238 , 240  are about the same, there is no disparity of pressure pushing the piston  114  in a certain direction. The piston  114  thus is forced to slow or stop, as any pressure on the piston  114  from the second direction  132  is opposed or canceled out by pressure from the first direction  126 .  
         [0034]    Preferably, the pressure equalizing channel will be located and the piston spring  124  sized to allow the plunger  118  to travel far enough to compress fuel in the fuel cavity  120  as desired, but not far enough that the plunger  118  contacts the stop plate  116 . The damping function provided by the plunger cavity pressure control of the present invention can prevent the piston spring  124  from overcompressing in a no-fuel situation for in the range of 5-10 minutes, rather than the approximately twenty seconds provided by the prior art. This extra time allows for remedial action to be taken before the fuel injector  100  suffers expensive and wasteful damage.  
         [0035]    While aspects of the present invention have been particularly shown and described with reference to the preferred embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated without departing from the spirit and scope of the present invention. For example, the dimensions of the vent hole  134  or piston hole  340  could differ, the fuel injector  100  could be of another known type, the piston assembly could be used in an application other than a fuel injector (such as injection molding, glue application, metering substances, or the like), or the various fluids involved could be supplied or vented in a different manner. However, a device or method incorporating such an embodiment should be understood to fall within the scope of the present invention as determined based upon the claims below and any equivalents thereof.  
         [0036]    The apparatus and method of certain embodiments of the present invention when compared with other methods and apparatus may have certain features worthy of incorporating into the design, manufacture, and operation of fuel injectors. In addition, the present invention may contain other properties that have not been discovered yet. It should be understood that while a preferred embodiment is described in connection with a fuel injector, the present invention is readily adaptable to provide similar functions for other mechanisms. Other aspects, objects, and advantages of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.