Abstract:
An emission control valve is operated by an electric actuator that has an electric coil, stator structure, and a positioning mechanism, including an armature that is selectively positionable along an axis, for selectively positioning a valve element. The stator structure is separated from the armature by an air gap that includes a non-ferromagnetic guide sleeve that is in surface-to-surface contact with the armature for guiding armature motion along the axis. The guide sleeve and the stator structure are in surface-to-surface contact for mutually concentricity with the axis. Along a region of mutual overlapping a minimum air gap is provided between the guide sleeve and the stator structure by radial spacing between the stator structure and the guide sleeve. Various embodiments are disclosed.

Description:
FIELD OF THE INVENTION 
     This invention relates to electric-actuated emission control valves of automotive vehicles, especially to a valve that comprises a non-magnetic sleeve that guides motion of a magnetic armature that controls the extent to which the valve selectively restricts a flow passage. 
     BACKGROUND OF THE INVENTION 
     Controlled engine exhaust gas recirculation (EGR) is a known technique for reducing oxides of nitrogen in products of combustion that are exhausted from an internal combustion engine to atmosphere. A known 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 induction fuel-air flow entering the engine for combustion so as to limit the combustion temperature and hence reduce the formation of oxides of nitrogen. 
     Electric-actuated EGR valves (EEGR valves) are capable of controlling recirculation of exhaust gas with the precision needed to comply with relevant emission regulations. However, increasingly stringent regulations create need for further improvements in EEGR valves. An EEGR valve that possesses more accurate and quicker response can be advantageous in achieving improved control of tailpipe emissions, improved driveability, and/or improved fuel economy for a vehicle having an internal combustion engine that is equipped with an EGR system. 
     A known electric actuator for a valve, such as an emission control valve, is a solenoid actuator having an armature that is selectively positioned along an axis according to the extent to which an electric coil of the actuator is energized by electric current. Various patents disclose emission control valves having linear solenoid actuators for improved accuracy in positioning the armature. Where the armature travel is guided by some sort of guide, frictional forces can affect positioning accuracy. In certain actuators, the armature is guided by a non-ferromagnetic sleeve that spaces the armature from surrounding stator structure of the solenoid. The armature is in surface-to-surface contact with the guide sleeve that provides a close sliding fit of the armature within the guide sleeve. Various patents show arrangements for guiding an armature within a solenoid to reduce sliding friction, but they may involve the inclusion of additional parts such as bearing rings, spheres, etc. 
     SUMMARY OF THE INVENTION 
     The present invention relates to improvements for reducing the friction that is encountered by an armature of an EEGR valve when an electric control signal applied to the valve commands armature movement for changing the extent to which the valve restricts exhaust gas recirculation. The invention arises through the discovery that radial components of the magnetic field that act on the armature create radial force components that affect the friction that the armature encounters as it moves axially within a nonmagnetic sleeve that guides the axial armature motion. The extent to which the centerline of the armature departs from concentricity with the centerline of the electromagnet coil that creates the magnetic field also affects the friction. The invention provides a solution that reduces the influence of radial components of the magnetic field on the armature, and consequently diminishes the frictional forces that the armature encounters as it travels within the sleeve. It is believed that these reductions in friction can provide meaningful improvements in valve response and accuracy without the inclusion of additional parts such as bearing rings, and without significantly altering the functional relationship of axial force versus coil current. 
     While establishing the best concentricity of the armature to the coil and associated stator structure is also important in reducing armature friction, the invention is able to reduce armature friction in conditions of less than perfect concentricity. The invention accomplishes this by providing a minimum air gap between the stator structure and the armature, the minimum air gap being provided by spacing a hub of a stator pole piece from a non-ferromagnetic guide sleeve along a region of mutual axial overlap. Various specific embodiments are disclosed. 
     One general aspect of the present invention relates to an emission control valve for controlling flow of gases with respect to combustion chamber space of an internal combustion engine. The valve comprises a housing having a passage that has an inlet port for receiving gases, an outlet port for delivering gases to the combustion chamber space, and a valve element that is selectively positioned by an electric actuator to selectively restrict the passage. The actuator comprises a solenoid having an electric coil, stator structure, and a positioning mechanism, including an armature that is selectively positionable along an axis, for selectively positioning the valve element. The stator structure and the armature cooperatively form a magnetic circuit in which the coil, when energized by electric current, creates magnetic flux for selectively positioning the armature along the axis. The stator structure is separated from the armature by an air gap that includes a non-ferromagnetic guide sleeve that is in surface-to-surface contact with the armature for guiding armature motion along the axis. The guide sleeve and the stator structure are mutually overlapping along a region of the axis and are fit to substantial mutual concentricity with the axis, and at that region, the air gap includes a minimum air gap provided by radial spacing between the stator structure and the guide sleeve. 
     Another general aspect of the present invention relates to an automotive vehicle emission control system that includes a valve, as described above, for controlling flow of gases with respect to combustion chamber space of an internal combustion engine that powers the vehicle. 
     Still another general aspect of the present invention relates to a method of reducing friction between an armature and a non-ferromagnetic guide sleeve of an electric actuator of an automotive vehicle emission control valve wherein the guide sleeve has surface-to-surface contact with the armature for guiding armature motion along an axis while separating the armature from stator structure of the actuator by an air gap. The method comprises disposing the guide sleeve and the stator structure in mutually overlapping axial relation along a region of the axis, fitting the guide sleeve and the stator structure to substantial mutual concentricity with the axis, and at the mutually overlapping region, providing a minimum air gap by radially spacing the stator structure from the guide sleeve. 
     The foregoing, and other features, along with various advantages and benefits of the invention, will be seen in the ensuing description and claims which are accompanied by drawings. The drawings, which are incorporated herein and constitute part of this specification, disclose a preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross section view through an exemplary automotive emission control valve, an EEGR valve in particular, embodying principles of the present invention. 
     FIGS. 2 and 3 are respective graph plots useful in appreciating how the present invention can provide improved control of an EEGR valve. 
     FIG. 4 is an enlarged view in oval  4  of FIG.  1 . 
     FIG. 5 is a view similar to FIG. 1, but with certain elements omitted, showing another embodiment. 
     FIG. 6 is a view similar to FIG. 5 showing still another embodiment. 
     FIG. 7 is a view similar to FIG. 5 showing still another embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an exemplary EEGR valve  10  that comprises a housing assembly  12  provided by a shell  14  having an open upper end that is closed by a cap  16 . Shell  14  further comprises a flat bottom wall  18  that is disposed atop a flat upper surface  20  of a base  22 . Fasteners  23  pass through holes in bottom wall  18  and an intervening spacer  25  to secure the shell on the base. Base  22  comprises a flat bottom surface  24  that is adapted to be disposed on a flat mounting surface  26  of a component of an internal combustion, such as a manifold  28 , typically with an intervening insulator gasket  30 . Apertured feet  32  protrude from the side of base  22  to provide for fastening of valve  10  to manifold  28  by threaded fasteners  34 . 
     Valve  10  comprises a flow passage  36  extending through base  22  between an inlet port  38  and an outlet port  40 . With valve  10  mounted on the engine, inlet port  38  is placed in communication with engine exhaust gas expelled from the engine cylinders and outlet port  40  is placed in communication with the intake flow into the cylinders. 
     A valve seat element  42  is disposed in passage  36  proximate inlet port  38  with the outer perimeter of the seat element sealed to the passage wall. Valve seat  42  has an annular shape comprising a through-hole. A one-piece valve member  44  comprises a valve head  46  and a valve stem  48  extending co-axially from head  46  along a central longitudinal axis AX of the valve. Head  46  is shaped for cooperation with seat element  42  to close the through-hole in the seat element when valve  10  is in closed position shown in FIG.  1 . 
     Valve  10  further comprises a bearing member  50  which is basically a circular cylindrical member except for a circular flange  52  at its lower end. An upper rim flange of a multi-shouldered deflector member  53  is axially captured between flange  52  and a shoulder  54  of base  22 . Deflector member  53  is a metal part shaped to shield bearing member  50  and a portion of stem  48  below the bearing member. Deflector member  53  terminates a distance from valve head  46  so as not to restrict exhaust gas flow through passage  36 , but at least to some extent deflect the gas away from stem  48  and bearing member  50 . 
     Bearing member  50  further comprises a central circular through-hole, or through-bore,  56  with which stem  48  has a close sliding fit. Bearing member  50  may comprise a material that possesses some degree of lubricity providing for low-friction guidance of valve member  44  along axis AX. 
     Valve  10  further comprises an electromagnetic actuator  60 , namely a solenoid, disposed within shell  14  coaxial with axis AX. Actuator  60  comprises an electromagnetic coil  62  and a polymeric bobbin  64 . Bobbin  64  comprises a central tubular core  66  and flanges  68 ,  70  at opposite ends of core  66 . Coil  62  comprises a length of magnet wire wound around core  66  between flanges  68 ,  70 . Respective terminations of the magnet wire are joined to respective electric terminals mounted side-by-side on flange  68 , only one terminal  72  appearing in the view of FIG.  1 . 
     Actuator  60  comprises stator structure associated with coil  62  to form a portion of a magnetic circuit path. The stator structure comprises an upper pole piece  74 , disposed at one end of the actuator coaxial with axis AX, and a lower pole piece  76  disposed at the opposite end of the actuator coaxial with axis AX. Shell  14  comprises a side wall  78 , a portion of which extends between pole pieces  74 ,  76  to complete the stator structure exterior of the coil and bobbin. 
     An annular air circulation space  80  is provided within shell  14  axially below actuator  60 . This air space is open to the exterior by several air circulation apertures, or through-openings,  82  extending through shell  14 . Side wall  78  has a slight taper that narrows in the direction toward bottom wall  18 . In the portion of the shell side wall that bounds space  80 , several circumferentially spaced tabs  84  are lanced inwardly from the side wall material to provide rest surfaces  86  on which lower pole piece  76  rests. Proximate its open upper end, the shell side wall contains similar tabs  88  that provide rest surfaces  90  on which upper pole piece  74  rests. Cap  16  comprises an outer margin that is held secure against a rim  92  at the otherwise open end of the shell side wall by a clinch ring  94 . A circular seal  96  is disposed between the cap and shell to make a sealed joint between them. 
     The interior face of cap  16  comprises several formations  98  that engage upper pole piece  74  to hold the latter against rest surfaces  90  thereby axially locating the upper pole piece to the shell. Cap  16  comprises a first pair of electric terminals, only one terminal  100  appearing in FIG. 1, that mate respectively with the terminals on bobbin flange  68 . The cap terminals protrude externally from the cap material where they are bounded by a surround  102  of the cap material to form a connector adapted for mating connection with a wiring harness connector (not shown) for connecting the actuator to an electric control circuit. 
     Cap  16  also comprises a tower  104  providing an internal space for a position sensor that comprises plural electric terminals, only one terminal  106  appearing in the Figure, that protrude into the surround for connecting the sensor with a circuit via the mating wiring harness connector. 
     The construction of valve  10  is such that leakage between passage  36  and air circulation space  80  is prevented. Bearing member through-hole  56  is open to passage  36 , but valve stem  48  has a sufficiently close sliding fit therein to substantially occlude the through-hole and prevent leakage between passage  36  and air circulation space  80  while providing low-friction guidance of the stem along axis AX. 
     Within space  80 , a deflector  108  circumferentially bounds the portion of stem  48  that passes through the space. The construction of deflector  108  comprises a circular cylindrical thin-walled member whose opposite axial ends are fit to the lower pole piece and the bearing member thus forming a barrier that prevents foreign material, muddy water for example, from intruding into space  80  and fouling the stem. 
     Upper pole piece  74  is a ferromagnetic part that comprises a central, cylindrical-walled, axially-extending hub  110  and a circular radial flange  112  at one end of hub  110 . Hub  110  is disposed co-axially within the upper end of a circular through-hole in bobbin core  66  concentric with axis AX, and flange  112  is disposed against bobbin flange  68 , thereby axially and radially relating bobbin  64  and upper pole piece  74 . Flange  112  has a clearance slot for the bobbin terminals. 
     Lower pole piece  76  comprises a ferromagnetic part having a central cylindrical hub  116  and a circular flange  118  at the lower end of hub  116 . An annular wave spring  120  is disposed around hub  116  and between flange  118  and bobbin flange  70  for the purpose of maintaining bobbin flange  68  against flange  112  while allowing for possible effects of differential thermal expansion. In this way, a controlled dimensional relationship which is insensitive to external influences, such as temperature changes, is maintained between the two pole pieces and the bobbin-mounted coil. 
     Hub  116  extends from flange  118  into the bobbin core through-hole, but stops short of hub  110  of upper pole piece  74 . Hub  116  comprises a circular through-hole that is concentric with axis AX and that has a shoulder  122  facing the end of the through-hole that is toward upper pole piece  74 . The radially outer surface of the hub wall has a frustoconical taper  124  that extends from flange  118  to the end of the hub that is disposed within the bobbin core through-hole. This imparts a narrowing taper to the hub wall in the direction of upper pole piece  74 . Above shoulder  122 , the through-hole in hub  116  has a diameter that is substantially equal to the nominal diameter of a circular through-hole in hub  110 , with both being concentric with axis AX. 
     A non-ferromagnetic part  126  axially spans hubs  110  and  116  to provide both an armature guide  128  for a magnetic armature  130  of actuator  60  and a spring seat  132  for one end of a helical coil spring  134  that acts on valve element  44  to bias valve head  46  toward seating closed on seat element  42 . Spring seat  132  has a central clearance hole for valve stem  48 . A separate spring seat element  136  is secured to stem  42  beyond spring seat  132  to provide a seat for the other end of spring  134 . Part  126  may comprise aluminum or aluminum alloy that can be drawn to the illustrated shape. Part  126  comprises a circular cylindrical sleeve forming a side wall that is fit to the through-holes in the respective hubs  110 ,  116  so as to make armature guide  128  concentric with axis AX. Where seat  132  joins guide  128 , part  126  has an undulating flange for seating part  126  on shoulder  122  of lower pole piece  76 . 
     Armature  130  cooperates with the stator structure to form the magnetic circuit of actuator  60 . Armature  130  comprises a circular cylindrical outer wall  138  of suitable radial thickness for the magnetic flux that it conducts. Midway between its opposite ends armature  130  has a transverse wall  140  that serves to provide a point for operative connection of stem  48  to the armature such that motion of the armature along axis AX is transmitted through stem  48  to position valve head  44  relative to seat element  42 , thereby setting the extent to which valve element  44  allows flow through passage  36 . Wall  140  also provides a means for transmitting armature motion to the position sensor housed within tower  104 . The outside diameter of wall  138  is dimensioned for a close fit within armature guide  128  so that the latter can provide precise axial guidance of armature travel. 
     FIG. 1 shows the closed position of valve  10  wherein spring  134  is pre-loaded, forcing valve head  46  to seat on seat element  42 , closing passage  36  to flow between ports  38  and  40 . The effect of spring  134  also biases the end of stem  48  against transverse wall  140  of armature  130  to form a single load operative connection between the armature and the stem. The nature of such a connection provides for slight relative movement between the two such that force transmitted from one to the other is essentially exclusively axial. 
     As electric current begins to increasingly flow through coil  62 , the magnetic circuit exerts increasing force urging armature  130  in the downward direction as viewed in FIG.  1 . Once the force is large enough to overcome the bias of the pre-load force of spring  134 , armature  130  begins to move downward, similarly moving valve element  44  and opening valve  10  to allow flow through passage  36  between the two ports. The extent to which the valve is allowed to open is controlled by the electric current in coil  62 , and by tracking the extent of valve motion, the position sensor can provide a feedback signal representing valve position, and hence the extent of valve opening. The actual control strategy for the valve is determined as part of the overall engine control strategy embodied by an associated electronic engine control. One or more through-holes  142  that extend through wall  140  provide for the equalization of air pressure at opposite axial ends of the armature. 
     In accordance with certain principles of the invention more fully seen in FIG. 4, a minimum air gap  150  is provided between the stator structure and armature  130 . The minimum air gap is defined between the radially inner surface of hub  110  of upper pole piece  74  and the radially outer surface of armature guide  128  along a portion of the length of the axial overlap of the two respective parts  74  and  126  by a groove  152  in the radially inner surface of the former part. The groove extends around the full circumference of hub  110  and is rectangular in cross section. The combination of the minimum air gap and of substantial axial concentricity of armature guide  128  to coil  62  and its associated stator structure established in any suitable manner, such as by surface-to-surface fitting of part  126  to at least one of the pole pieces, is believed to provide a magnetic circuit flux whose radial components have reduced influence on the armature, thereby reducing surface friction between the armature and the armature guide. By avoiding the inclusion of additional parts such as bearing rings or the like, the valve can be more compact and cost effective. 
     Experimental testing has shown that the upper pole piece  74  has substantial influence on valve operation. FIG. 2 comprises two graph plots relating flow through the valve to the degree of modulation of a pulse width modulated duty cycle signal that energizes the solenoid coil. One graph plot  200  shows the substantial hysteresis that is present in a prior valve when relatively higher radial components of magnetic force, and resulting friction, are present between the guide sleeve and the armature. Such higher force and friction are attributable to lack of concentricity and of minimum air gap between the stator pole piece and the armature. Graph plot  202  shows how hysteresis can be significantly reduced by the present invention. 
     FIG. 3 shows graph plots  204 ,  206 , correlated with graph plots  200 ,  202  respectively, of flow as a function of valve travel, as measured by the position sensor in cap  16 . This Figure discloses that the inclusion of minimum air gap reduces hysteresis without significantly altering the overall functional relationship between flow through the valve and the position of the armature. 
     FIG. 5 illustrates a second example where a first minimum air gap  150  is provided between the radially inner surface of hub  110  of upper pole piece  74  and the radially outer surface of armature guide  128  along a portion of the length of the axial overlap of the two respective parts  74  and  126 , and a second minimum air gap  154  is provided between the radially inner surface of hub  116  of lower pole piece  76  and the radially outer surface of armature guide  128  along a portion of the length of the axial overlap of the two respective parts  76  and  126 . The respective minimum air gaps are created by forming respective beads  156 ,  158  in the sleeve of part  126  that forms armature guide  128 . Each bead extends around the full circumference of the sleeve and bulges radially outward in a generally semi-circular cross section. The crest of each bead has surface-to-surface contact with the inner surface of the respective hub. 
     FIG. 6 illustrates a third example where a first minimum air gap  150  is provided between the radially inner surface of hub  110  of upper pole piece  74  and the radially outer surface of armature guide  128  along a portion of the length of the axial overlap of the two respective parts  74  and  126 . The minimum air gap is created by dimensioning the outside diameter of the sleeve of part  126  less than the inside diameter of hub  110  by the thickness of a circular cylindrical spacer  160  that is disposed between the two parts  74 ,  126 . The spacer may be any suitable non-ferromagnetic material, and it may be fit, or applied, to either part. Tape is one example of a suitable spacer material. 
     FIG. 7 illustrates a fourth example where a first minimum air gap  150  is provided between the radially inner surface of hub  110  of upper pole piece  74  and the radially outer surface of armature guide  128  along a portion of the length of the axial overlap of the two respective parts  74  and  126 . The minimum air gap is similar to the first example of FIG. 4 except for the fact that the groove is extended to the inner end of hub  110 . 
     While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles are applicable to other embodiments that fall within the scope of the following claims. For example, it is believed that principles of the invention may be incorporated in various forms of automotive emission control valves.