Abstract:
An exhaust gas recirculation valve assembly including a bushing and pintle shaft sub-assembly having a pintle shaft supported by a sleeve bushing, the contact area between the shaft and bushing being reduced such that coking deposits that form on the remaining contact area can be broken by the force of the valve actuator, thereby preventing sticking of the valve from coking accumulations. Either the shaft surface or the bushing surface is relieved in a pattern of incuse areas and residual bearing areas. A currently preferred pattern is a helix.

Description:
RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS  
       [0001]    The present application claims priority from U.S. Provisional Patent Application, Serial No. 60/415,584, filed Oct. 2, 2002. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to pintle valves; more particularly, to exhaust gas recirculation (EGR) pintle valves for internal combustion engines; and most particularly, to an arrangement of the pintle shaft and the pintle bushing to prevent valve failure from shaft coking by exhaust gas components.  
         BACKGROUND OF THE INVENTION  
         [0003]    Pintle valves are well known for use in controlling flow of fluids, and especially gases. Some applications can expose a valve&#39;s internal surfaces and moving parts to fouling materials which can give rise to deposits, resulting in impaired action or outright failure of the valve. In a particularly severe application, exhaust gas recirculation pintle valves on internal combustion engines are known to accumulate coking deposits on the pintle shaft in the region of the pintle support bushing. Having this area of the pintle unobstructed by coking deposits is a requirement for smooth, reliable action of the valve in controlling exhaust gas flow. In severe cases, coking is known to immobilize a pintle shaft in a bushing bore, resulting in complete failure of the valve.  
           [0004]    It is known in the art to provide means for causing the pintle shaft to be scraped clean inherently by the axial motion of the shaft as the valve is exercised. Such means typically are disposed upstream of the sensitive area, such as a shaft bushing or a valve actuator.  
           [0005]    U.S. Pat. No. 5,511,531, issued Apr. 30, 1996, discloses an annular scraper element 74 for the purpose of scraping off any residue that may accumulate on shaft 60 so that such residue is prevented from passing onto actuator 12. At its center, scraper element 74 has a circular hole with which shaft 60 has a close sliding fit. The radially outer margin of element 74 is captured such that it is constrained against any significant axial motion. A problem with such a scraper is that it represents essentially a line contact around the shaft, and any material that escapes past that line is then free to migrate or be deposited further along the shaft and/or within the actuator. Further, scraper element 74 has no radial resilience and is subject to wear with use, such that its scraping action becomes progressively less effective over time.  
           [0006]    U.S. Pat. No. 4,253,642, issued Mar. 3, 1981, discloses an arrangement of wire bristles impinging radially on a valve pintle shaft “to clean threads or the like of the valve stem and remove debris therefrom.” A problem with using wire bristles is that, while the shaft may be scrubbed thereby, the bristle pattern is entirely porous, allowing particulate-laden gases to permeate through and cause coking downstream of the bristles.  
           [0007]    U.S. Pat. No. 6,212,881 B1, issued Apr. 10, 2001, discloses a “stopper” 72 having a central hole 72 b  through which the valve pintle shaft 56 a  passes. The diameter of the hole 72 b  is slightly larger than the diameter of shaft 56 a . When the valve shaft is moved axially, the inner surface of the hole removes foreign matter such as deposits from the surface of the valve shaft. This is substantially the same line-contact scraper mechanism as is disclosed in U.S. Pat. No. 5,511,531 discussed above.  
           [0008]    U.S. Pat. No. 5,666,932, issued Sep. 16, 1997, discloses a programmed feature of an engine start-up cycle wherein the EGR valve is violently opened and then slammed closed, which purports to jar or shear any contaminant buildup so as to reduce such buildup. No contact means is disclosed for actively scraping or otherwise mechanically removing contaminant buildup.  
           [0009]    U.S. Pat. No. 4,408,627, issued Oct. 11, 1983, discloses a gate-type EGR valve operated by a linear actuator attached to a valve shaft. A pair of nylon scraper rings 28,29 are positioned around the valve shaft “to remove any carbon deposits” from the valve shaft and prevent them from entering the actuator. The rings are separated by a compressed coil spring which urges the rings against respective axial sealing faces in the valve body.  
           [0010]    U.S. Pat. No. 4,061,157, issued Dec. 6, 1977, discloses a reciprocating valve having an outwardly facing conical surface disposed about the valve stem, which surface terminates in a knife-like edge that functions to remove accumulations of foreign material from exposed portions of the valve stem as it oscillates past the edge.  
           [0011]    What is needed in the art is a means for limiting buildup of coking contaminants on a valve pintle shaft and/or shaft bushing to prevent immobilization of the shaft and failure of the valve.  
           [0012]    It is a principal object of the present invention to increase the reliability of an exhaust gas recirculation valve by preventing unacceptable buildup of deposits on shaft and bearing surfaces thereof.  
         SUMMARY OF THE INVENTION  
         [0013]    Briefly described, in an exhaust gas recirculation valve having a pintle shaft supported by a sleeve bushing, the contact area between the shaft and bushing is minimized such that coking deposits that form on the remaining contact area can be broken by the force of the valve actuator, thereby preventing sticking of the valve from coking accumulations. Either the shaft surface or the bushing surface is relieved in a pattern of incuse areas and residual areas. A currently preferred pattern is a helix.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 is an elevational cross-sectional view of a prior art pintle-type valve;  
         [0016]    [0016]FIG. 2 is a detailed view of Area “ 2 ” shown in FIG. 1, in partial cross-section, showing an improved pintle shaft in accordance with a first embodiment of the invention;  
         [0017]    [0017]FIG. 3 is a cross-sectional view of the embodiment shown in FIG. 2, showing the pintle shaft in retracted position in the shaft bushing;  
         [0018]    [0018]FIG. 4 is a cross-sectional view like that shown in FIG. 3, showing the pintle shaft in extended position in the bushing;  
         [0019]    [0019]FIG. 5 is a cross-sectional view showing a second embodiment in accordance with the invention; and  
         [0020]    [0020]FIG. 6 is a view like that shown in FIG. 2, showing a third embodiment in accordance with the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Referring to FIG. 1, a prior art pintle-type valve  10  includes a valve body  12  comprising a first chamber  14  and a second chamber  16  separated by a first annular valve seat  18 . A pintle shaft  24  having a surface  23  is slidably disposed in first axial bore  25  in bushing  26  which is mounted in a second axial bore  28  in valve body  12 . Valve head  30  is fixedly attached to shaft  24  for axial movement therewith and is matable with valve seat  18  to regulate flow across seat  18  between chambers  14  and  16  in response to actuation by solenoid actuator assembly  32 .  
         [0022]    A pintle valve such as valve  10  may be mounted on an internal combustion engine  34  for use as an exhaust gas recirculation valve in known fashion. In such use, chambers  14  and  16  are fully exposed to engine exhaust gases. The purpose of the invention is to prevent gases from chamber  16  from causing unacceptable levels of coking along shaft surface  23  within bushing bore  25 , which surface is subject to reciprocation into and out of bushing  26  during operation of the valve.  
         [0023]    While not essential to the invention, the following explanation for success of the invention is currently believed to be correct.  
         [0024]    Coking products are chemical compounds similar to polymeric compounds in that they may be characterized in terms of shear strength and modulus. Shear strength is a fundamental property of a compound and may be used to determine a force required to break loose an assembly, such as a shaft in a bushing, which has seized due to formation of coking products at the shaft/bushing interface. The force required is a function of the shear strength of the coking material and also the total surface area clogged by coking. Further, the axial force available from a solenoid actuator assembly, such as assembly  32 , may be readily determined. Hence, immobilization of a shaft in a bushing by coking may be prevented by limiting the area of shaft/bushing interface such that the shear resistance of the total coking products in that area can always be overcome by the solenoid actuator.  
         [0025]    Limiting the area cannot be done, however, simply by shortening the axial extent of the shaft in the bushing because the axial bearing extent is what guides the valve head into the seat and prevents wobble and runout of the shaft in the bushing. Therefore, what is needed is a means for maintaining or even extending the axial length of the interface between the shaft and bushing while reducing very substantially the total interface area for debilitating accumulation of coking products.  
         [0026]    Beneficially, it has been found that the shear strength of coking products is very much a function of thickness and diminishes dramatically in inverse proportion to thickness, especially beyond a very thin “skin layer” having a thickness of about 0.05 mm. Any buildup beyond the skin layer is crumbly and not very strong. Unfortunately, in prior art valves, the shaft clearance within the bushing bore is on the order of about 0.05 mm, such that prior art valves are easily choked by coking products.  
         [0027]    Referring to FIGS. 2 and 3, a first embodiment of a bushing and pintle shaft sub-assembly  40  in accordance with the invention is suitable for substitution into a prior art valve assembly such as assembly  10 . First embodiment  40  comprises a pintle bushing  26 ′. An improved pintle shaft  24 ′ is disposed conventionally for axial actuation within bore  25 ′ in bushing  26 ′. A bushing cap  42  is urged against bushing  26 ′ by a hookian element such as coil spring  44 . A central aperture  46  in cap  42  is close-fitting to surface  23 ′ of pintle shaft  24 ′ to minimize escape of gases along bearing bore  25 ′.  
         [0028]    Referring to FIG. 3, a first portion  48  of surface  23 ′ within bore  25 ′ and below aperture  46  is relieved to define residual bearing areas  50  having a first radius  52  equal to the original radius  54  of shaft  24 ′, and incuse areas  56  having a second radius  58  less than first radius  52 . A second portion  60  of surface  23 ′ within bore  25 ′ is unrelieved to provide for sealability of the shaft in the bushing, in cooperation with cap  42 . Preferably, the axial length  62  of portion  60  is less than or equal to the shaft diameter (twice radius  52 ).  
         [0029]    In a currently preferred embodiment, residual bearing area  50  defines a clockwise helical pattern or “thread” having a relatively long pitch. Further, the helical width  64  of the residual area  50  is preferably relatively narrow, to provide radial support for shaft  24 ′ in bore  25 ′ while simultaneously minimizing the surface of residual bearing area  50  available for coking build-up. Further, the helical nature of area  50 , being inclined to the path of axial travel of the pintle shaft, acts inherently to plow coking deposits from the surface of the bearing bore during actuation; and the helical pattern, being open at the inboard end of the bushing, permits scraped deposits to escape back into valve chamber  16  from whence they are swept by the flow of gases through the valve.  
         [0030]    Coking products should be expected to accumulate in incuse areas  56 ; however, because the depth of such areas exceeds the thickness of a skin layer, such crumbly and incompetent accumulations are inconsequential to causing the shaft to become immobilized in the bushing.  
         [0031]    Referring to FIG. 4, shaft  24 ′ continues to be fully supported by bushing bore  25 ′ when the shaft is fully extended at valve closure, as shown in FIG. 1.  
         [0032]    Referring to FIG. 5, in a second embodiment of a bushing and pintle shaft sub-assembly  40 ′ in accordance with the invention, incuse portions  56 ′ and residual bearing portions  50 ′ are formed in the bore  25 ′ of bushing  26 ″ rather than in the pintle shaft  24  which may be smooth and unfeatured as in prior art pintle shaft  24 .  
         [0033]    In FIGS.  3 - 5 , a cap spring  44  is assumed, but omitted for clarity.  
         [0034]    Referring to FIGS. 2 and 6, a result of the helical arrangement shown in FIG. 2 is that a torque may be imposed on pintle shaft  24 ′ during actuation. Such torque may be beneficial, causing the shaft to rotate and thereby progressively clean the entire smooth bearing surface during repeated actuations. In applications wherein no rotation is desired, in a third embodiment  40 ″ as shown in FIG. 6, a second opposite-handed helical residual bearing area  70  may also be provided such that there is no net torque on pintle shaft  24 ″ during actuation. Preferably, the helical crossings  72  are open to provide circulation of gases and drainage of deposits along the incuse areas  56 . An important consideration in providing a double-helix arrangement such as is shown in FIG. 6 is to continue to minimize the total residual area  50 , 70  made available to coking, and to keep such total area small enough that the shear resistance of the accumulated coking deposits cannot exceed the axial strength of the actuator.  
         [0035]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.