Patent Publication Number: US-6904897-B1

Title: Bearing porosity control in an exhaust gas recirculation valve

Description:
FIELD OF THE INVENTION 
   This invention relates generally to emission control valves that are used in emission control systems associated with internal combustion engines in automotive vehicles. The invention particularly relates to an exhaust gas recirculation (EGR) valve. 
   BACKGROUND OF THE INVENTION 
   Controlled engine exhaust gas recirculation is a known technique for reducing oxides of nitrogen in products of combustion that are exhausted from an internal combustion engine to atmosphere. A typical EGR system comprises an EGR valve that is controlled in accordance with engine operating conditions to regulate the amount of engine exhaust gas that is recirculated to the fuel-air flow entering the engine for combustion so as to limit the combustion temperature and hence reduce the formation of oxides of nitrogen. 
   Because they are typically engine-mounted, EGR valves are subject to harsh operating environments that include wide temperature extremes and vibrations. Tailpipe emission requirements impose stringent demands on the control of such valves. An electric actuator, such as a solenoid that includes a sensor for signaling position feedback to indicate the extent to which the valve is open, can provide the necessary degree of control when properly controlled by the engine control system. An EGR valve that is operated by an electric actuator is often referred to as an EEGR valve. 
   Individual parts of an EGR valve must not only be strong, tightly toleranced, thermally insensitive, and essentially immune to combustion products present in engine exhaust gases, but they must also be cost-effective. 
   A cost-effective construction for the bearing in an EEGR valve comprises fabricating the bearing using powdered metal technology. A powdered metal bearing fabricated from stainless steel powder is well suited for use in the harsh environment of hot engine exhaust gases. 
   Increasingly stringent demands on valve performance, life, and reliability have led to the discovery that even small amounts of exhaust gas leakage through an EEGR valve bearing can be detrimental to valve performance over time. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a solution for improving the resistance of a powdered metal EEGR bearing to exhaust gas leakage. The improvement allows the continued use of powdered metal technology for the fabrication of such bearings without the necessity of making major constructional modifications to either the bearing or the EEGR valve. 
   A powdered metal part possesses some inherent degree of porosity. Because of that porosity gases can infiltrate and migrate through the part. A result of such infiltration and migration that has been observed in an EEGR valve bearing is the accumulation of material on the bearing surface that guides the stem, or shaft, of the valve pintle. That is not to say that the accumulation of material is due solely to migration through the porous bearing material, but rather it means that at least some accumulation is believed attributable to bearing porosity. 
   U.S. Pat. No. 5,041,168 discloses a process for infiltrating material into the guide surface of a powdered metal engine valve guide for the purpose of improving certain characteristics of the stem/guide interface. The process involves rolling sheet material into a cylinder and inserting it into the valve guide bore. That patent does not appear to be concerned with controlling the bearing porosity in a manner that would resist leakage of combustion gases through surfaces other than the guide bore surface. A general aspect of the invention relates to an EGR valve comprising valve body structure providing an exhaust gas passage that is selectively restricted by a valve operated by an actuator via an operative coupling that comprises a linearly positionable shaft guided by a guide bore of a powdered metal bearing that is fit to the valve body structure in closure of an opening to the exhaust gas passage thereby exposing a surface portion of the bearing that is exterior to the guide bore to exhaust gas flow through the passage. 
   At least that surface portion of the bearing is sealed to essentially stop infiltration of exhaust gas through that surface portion into the powdered metal. 
   Another general aspect relates to a method of making such a bearing. 
   Still another general aspect relates to an engine having an exhaust gas recirculation system that comprises a valve having such a bearing. 
   The accompanying drawings, which are incorporated herein and constitute part of this specification, include one or more presently preferred embodiments of the invention, and together with a general description given above and a detailed description given below, serve to disclose principles of the invention in accordance with a best mode contemplated for carrying out the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front elevation view of an exemplary EEGR valve embodying principles of the invention. 
       FIG. 2  is an enlarged cross section view of the valve. 
       FIG. 3  is an enlarged cross section view of the valve bearing. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2  illustrate an exemplary EEGR valve  20  embodying principles of the present invention. Valve  20  comprises a base  22  and an elbow  24  assembled together to form a flow path  26  through the valve between an inlet port  28  provided in a flange at a side of base  22  and an outlet port  30  provided in a flange at one end of elbow  24 . 
   Base  22  is a metal part that has a main longitudinal axis  32 . Base  22  may be considered to have a generally cylindrical shape about axis  32  comprising a generally cylindrical wall bounding an interior space that is open at opposite axial end faces of the base. Base  22  is constructed so that its interior space is also open to inlet port  28 . 
   An end of elbow  24  that is opposite the end containing outlet port  30  is fastened in a sealed manner to the lower end face of base  22  so that the interior of elbow  24  is open to the interior space of base  22 . A cover  34  is fastened in a sealed manner to the upper end face of base  22  to close that end of the interior space of base  22  while providing a platform for the mounting of an electric actuator  36  on the exterior of the cover. 
   Actuator  36  comprises a solenoid  37  that, when the valve is installed on an engine in a motor vehicle, is electrically connected via an electric connector  38  to an electrical system of the motor vehicle to place the valve under the control of an engine controller in the vehicle. 
   A bearing  40  is centrally fit to cover  34  such that a guide bore  41  (see  FIG. 3 ) of the bearing is coaxial with axis  32 . Bearing  40  serves to axially guide a double-pintle  42  of valve  20  along axis  32  via a guiding fit of the bearing guide bore to an upper portion of a stem  44  of double-pintle  42  that extends completely through the bearing guide bore from an armature  43  of solenoid  37  into the interior space of base  22  where upper and lower pintles  46 ,  48  are disposed on stem  44 . 
   A double-seat element  50  is fit to base  22  within the latter&#39;s interior space. Element  50  has a generally cylindrical wall  52  that is coaxial with axis  32  and that is open at opposite axial ends. Element  50  comprises axially spaced apart upper and lower seats  54 ,  56  with which pintles  46 ,  48  respectively cooperate. 
   Wall  52  comprises two pairs of openings, or apertures: an upper pair  58 ,  60 , and a lower pair  62 ,  64 . The lower pair are arranged axially between seats  54 ,  56  to provide for the open interior of element  50  that is circumscribed by wall  52  between seats  54 ,  56  to communicate through the opening in base  22  to inlet port  28 . The upper pair  58 ,  60  are arranged axially beyond seat  54  relative to the lower pair  62 ,  64  to provide for the open interior of element  50  that is circumscribed by wall  52  beyond upper seat  54  to communicate with respective entrances to an internal passageway than runs within base  22  internally through a portion of the generally cylindrical wall of the base that is in the semicircumferential portion of that wall opposite inlet port  28 . Apertures  62 ,  64  are in registration with inlet port  28 . 
   With the solenoid not being energized, each of the two pintles  46 ,  48  seats on the respective seat  54 ,  56 , closing the respective through-hole. Armature  43  is biased by a spring  82  to urge the pintles against the seats with an appropriate amount of force. A flange, or rim,  84  at the lower end of bearing  40  fits to the open upper end of seat element  50 . 
   When valve  20  is operated open, the entering exhaust gas flow divides more or less equally as it passes through seat element  50 . The mounting of bearing  40  exposes its lower axial end portion to exhaust gas flow that has passed through upper seat  54 . 
   In accordance with principles of the invention, the exposed surface of at least that portion of the bearing is sealed to stop infiltration of diesel exhaust gas into the powdered metal of the part and ensuing migration of the gas through the part. Depending on the nature of the specific process that is performed to accomplish the sealing and/or on relevant specifications applicable to the part, sealing of the surface of guide bore  41  may or may not occur. 
   The sealing process will seal at least the axial end face and an adjoining portion of the axial exterior surface that extends from the end face. That sealed surface portion is represented by the zone marked  86  in FIG.  3 . The sealing may even be so extensive as to seal the entire surface that is external to guide bore  41 , including a further zone marked  88  in FIG.  3 . Where the bearing has counterbores at the opposite axial end as in the illustrated example, their surfaces may or may not be sealed. Likewise the small lead at the lower end of the guide bore may or may not be sealed. 
   Sealing may be accomplished by a mechanical process or by application of a suitable sealant. An example of mechanical sealing is a tumbling process or shot peening process where one or more bearings to be processed are tumbled in a container. The container contains media that repeatedly impact the exterior surfaces of the bearings to effectively close the pores present in those surfaces without adversely impairing the dimensional integrity of the part. The sealed surface would be represented by zones  86 ,  88 . 
   A suitable sealant must be able to withstand conditions to which an EGR valve is subjected when in use with an engine. Examples of suitable sealants and processes for applying them can be performed by a company like Allegheny Coatings, Ridgway, Pa., using its “1092 inorganic presealer” or “Sermaguard”. Introduction of sealant fills at least the surface pores, and may extend even deeper into the powdered metal. 
   While the foregoing has described a preferred embodiment of the present invention, it is to be appreciated that the inventive principles may be practiced in any form that falls within the scope of the following claims.