Abstract:
Improved oil control piston rings with reduced friction compared to prior art rings are disclosed for use in liquid lubricated internal combustion engines, gas pumps, and gas compressors. The ring assemblies are interchangeable with conventional oil control rings and offer similar oil control performance. Like conventional oil control rings, they include a spring action expander that loads circular steel scraper rails against the cylinder bore to form a sliding barrier between the oil-filled crankcase and the combustion chamber and pressure sealing piston rings. Unlike conventional rings, the improved ring assemblies utilize means of supporting thinner scraper rails that form the sliding barrier with less contact force and resulting friction.

Description:
[0001]    This application claims the priority of U.S. provisional application 61/744,553. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to oil control piston rings that form sliding oil barriers in pistons operating in cylindrical bores in liquid lubricated internal combustion engines and reciprocating pumps and compressors. 
       BACKGROUND OF THE INVENTION 
       [0003]    The oil control piston rings in internal combustion engines are essential to minimizing lubricating oil consumption, but at the same time are a major contributor to engine friction that increases fuel consumption. This is particularly true for light-duty vehicle engines that typically operate at part load where parasitic oil ring friction becomes a larger part of the total engine output. Oil control ring friction consumes an estimated 1 to 3% of engine fuel at full load and 2 to 6% at light load. Two embodiments of the inventive reduced friction oil control piston ring offer a means of reducing friction and resulting fuel consumption without increasing lubricating oil consumption. Both are novel assemblies that incorporate thin, low contact force oil scraper rails to reduce friction while retaining oil control performance. One is a five-piece assembly similar in many respects to conventional three-piece oil control rings which have a formed sheet metal spring expander and are used predominantly in light-duty reciprocating pistons. The second is also a five-piece assembly similar in many respects to two-piece oil control rings which have a wire helical spring expander that are used predominantly in heavy-duty applications. Fuel saving are expected to provide a quick payback of the cost of the additional components. 
         [0004]      FIGS. 1 and 2  show a typical prior art three-piece oil control piston ring assembly  100 . It is installed in a groove  101  in a piston  102  below the two compression ring grooves  103  and  104  containing compression rings  108  and  109 , and is made up of two split circular steel scraper rails  105  spring-loaded against the cylinder bore  106  by an expander  107  through multiple contact points  115 . The expander also separates the two scraper rails and positions the rails so that they are in sliding contact with the upper and lower piston groove surfaces  116  and  117 . The expander  107  functions as a circumferential compression spring with a free diameter larger than the installed diameter. When the ring assembly  100  is compressed to the bore diameter, the scraper rings  105  circumferentially compress the expander  107  to the installed diameter, resulting in an outward radial force transferred from the expander  107  to the scraper rails  105 , urging them into tight contact with the bore  106 . The scraper rails are flexible in the radial direction and the radial expander force is set high enough that the scraper rails will conform to minor cylinder bore distortions. 
         [0005]    As the piston  102  reciprocates (downward motion  110  shown) in the bore  106 , the scraper rails  105  remove most of the oil  111  on the cylinder bore and return it to the crankcase, leaving a thin film (not shown) to lubricate the upper portion of the piston and the compression rings  108  and  109 . Oil  112  collected between the two scraper rails  105  is returned to the crankcase through oil drain holes  113  connecting the oil ring piston groove  101  to the inside of the piston  102 . The contact area between a scraper rail  105  and the cylinder bore  106  acts as an oil-lubricated slider bearing, where the thickness of the oil film in the bearing zone is a major factor in determining the thickness of the oil film left on the cylinder bore surface. In this hydrodynamic mode the ring acts as a linear slider bearing of the type described in the Standard Handbook for Mechanical Engineers, 7th Edition, edited by Theodore Baumeister. Page 8-171 (1960) published by the McGraw-Hill Book Company, New York. This lubrication theory indicates that the slider bearing oil film thickness h o  is: 
         [0000]    
       
         
           
             
               h 
               o 
             
             ∝ 
             
               L 
                
               
                 
                   
                     μ 
                      
                     
                         
                     
                      
                     υ 
                      
                     
                         
                     
                      
                     C 
                   
                   W 
                 
               
             
           
         
       
     
         [0000]    where L is the bearing contact zone width, μ is the oil viscosity, v is the sliding velocity, C is the bearing zone circumference and W is the total radial force on the bearing zone. Oil film thickness therefore decreases with a narrower contact zone or a higher radial force. Higher radial force, however, increases the friction force given by: 
         [0000]      F∝√{square root over (WμνC)}
 
         [0006]    These equations show that a narrow ring with a low radial force is the lowest friction means of producing a given oil film thickness. There are limits, however, to how much the scraper rail bearing contact zone width and radial force in conventional three-piece oil control rings can be reduced. The scraper rails  105  must be thick enough to withstand the axial friction loads without excessive axial deflection in the unsupported bridge areas between expander  107  contact points, limiting how thin the rails can be. The outside diameter  114  of a standard thickness scraper rail can be beveled or profiled to form a narrower bearing contact zone to allow reduced radial force and friction, but this approach has drawbacks. The bearing zone width will tend to increase with ring wear, increasing the oil film thickness and oil consumption. Also, the scraper rail radial stiffness will not decrease in proportion to the reduced contact zone width and radial force. This limits the ability of the ring to conform to cylinder bore distortion, increasing oil film thickness and consumption in portions of the rail circumference having reduced contact force. 
         [0007]      FIGS. 3 and 4  show a typical prior art two-piece oil control piston ring assembly  300 . It functions in much the same way as the three-piece rings described above, but has higher flexibility and wear tolerance making it more suitable for heavy duty applications requiring long service life. It is installed in a groove  101  in a piston  102  below the two compression ring grooves  103  and  104  containing compression rings  108  and  109 , and is made up of a split circular scraper ring  301  spring-loaded against the cylinder bore  106  by a helical expander spring  302 . The scraper ring  301  is in sliding contact with the upper and lower piston groove surfaces  116  and  117 , and incorporates an integral upper scraper  303  and lower scraper  304  separated by a groove  305 . Radial oil return slots  306  connect the groove  305  to the inner surface  307  of the scraper ring. The expander spring  302  is a circumferential compression spring with a free diameter larger than the installed diameter. When the ring assembly  300  is compressed to the bore diameter, the scraper ring  301  circumferentially compresses the expander spring  302  to the installed diameter, resulting in an outward radial force transferred from the expander spring to the scraper ring  301 , urging the scrapers  303  and  304  into tight contact with the bore  106 . The scraper ring  301  is flexible in the radial direction and the expander spring force is set high enough that the scrapers will conform to minor cylinder bore distortions and maintain oil control. 
         [0008]    As with three-piece rings, there are limits to how much the width of the scrapers  303  and  304  contact zones and radial force in conventional two-piece oil control rings can be reduced. The scrapers  303  and  304  must be thick enough to withstand the axial friction loads without excessive bending stress and with a radial extent sufficient to allow for wear over the service life of the ring. The outside diameter of standard thickness scrapers rail can be beveled or profiled to form a narrower bearing contact zone to allow reduced radial force and friction, but this approach has drawbacks. The bearing zone width will tend to increase with ring wear, increasing the oil film thickness and oil consumption. 
       SUMMARY OF THE INVENTION 
       [0009]    The proposed reduced friction oil control piston rings combine a narrow contact zone that is insensitive to wear with the radial flexibility to conform to cylinder bore distortion with reduced radial force. Like conventional control rings, they are installed in a piston groove below the two compression ring grooves and include a spring action expander. The difference is that the two individual circular steel scraper rails in three piece rings or the integral scrapers of two-piece rings are replaced by thin, flat scraper rail rings. The expander circumferential spring tension is reduced to provide the required oil film thickness with the thin scraper rails, thereby reducing ring friction while remaining flexible enough to conform to cylinder bore distortions. Additional elements are added to the ring assemblies to support the thin scraper rings. The first embodiment is a five-piece assembly in which the thin scraper rails are spring-loaded against the cylinder bore by the expander and in sliding contact with the adjacent piston groove surface. Thicker support flat rails that are not spring-loaded against the cylinder bore by the expander, and lightly loaded against the cylinder bore by their own elastic tension, are positioned between the scraper rails and the expander. The expander contacts only the sides of the two support rails, and positions them in the axial direction so that they are in sliding contact with the thin scraper rails. Radial clearance is provided between the support rail inside diameter and the expander. The thicker support rails serve to bridge the unsupported areas between expander contact points and prevent excessive deflection of the thin scraper rails caused by axial friction forces. The second embodiment is a five-piece assembly in which the thin scraper rails are spring-loaded against the cylinder bore by a helical coil spring expander acting through an intermediate bridge ring expander. The scraper rails are supported and held in sliding contact with the upper and lower piston groove surfaces by a floating spacer ring that separates the two thin scraper rings. The spacer ring is lightly loaded against the cylinder bore by its own elastic tension, but is not loaded by coil spring expander so it generates little friction. The spacer ring contacts only the sides of the two scraper rails, and supports them against axial friction forces. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0010]    The appended claims set forth those novel features that characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of preferred embodiments. The accompanying drawings, where like reference characters identify like elements throughout the various figures in which: 
           [0011]      FIG. 1  illustrates a three-piece prior art oil control piston ring in an internal combustion engine; 
           [0012]      FIG. 2  provides a perspective view of the three-piece prior art oil control piston ring components; 
           [0013]      FIG. 3  illustrates a two-piece prior art oil control piston ring in an internal combustion engine; 
           [0014]      FIG. 4  provides a perspective view of the two-piece prior art oil control piston ring components; 
           [0015]      FIG. 5  illustrates a first embodiment of the reduced friction oil control ring according to this invention in an internal combustion engine; 
           [0016]      FIG. 6  provides a perspective view of the first embodiment of the reduced friction oil control piston ring components; 
           [0017]      FIG. 7  provides a perspective view and details of the first embodiment of the reduced friction oil control piston ring assembly; 
           [0018]      FIG. 8  illustrates a second embodiment of the reduced friction oil control ring according to this invention in an internal combustion engine; and 
           [0019]      FIG. 9  provides a perspective view of the second embodiment of the reduced friction oil control piston ring components: 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Upon examination of the following detailed description the novel features of the present invention will become apparent to those of ordinary skill in the art or can be learned by practice of the present invention. It should be understood that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only. Various changes and modifications within the spirit and scope of the invention will become apparent to those of ordinary skill in the art upon examination of the following detailed description of the invention and claims that follow. 
         [0021]    The prior art and the invention are described with reference to internal combustion engines, but it is to be understood that the invention is applicable to liquid lubricated oil control piston rings in other applications including gas compressors. In the description “upper”, “top”, “above” and “head” refer to the direction towards the combustion chamber, and “lower” and “downward” refer to the direction towards the crankcase. 
         [0022]      FIG. 5 ,  FIG. 6  and  FIG. 7  show the first embodiment of the reduced friction oil control piston ring  507 . Like the conventional three-piece oil control ring, this five-piece ring is installed in a piston groove  101  below the two compression ring grooves  103  and  104  containing compression rings  108  and  109 , and includes a spring action expander  500 . The difference is that the two circular steel scraper rails  105  are each replaced by a pair of rails. The outer rails  501  in each pair are thin scraper rails spring-loaded against the cylinder bore  106  by the expander  500  through multiple contact points  115  and in sliding contact with the adjacent upper and lower surfaces  116  and  117  of piston groove  101 . The inner rail  502  in each pair is a thicker support rail that has a radial clearance  503  with the expander  500  so that rail  502  is not spring-loaded against the cylinder bore  106  by the expander, and lightly loaded against the cylinder bore by its own elastic tension. The expander  500  contacts only the sides of the two support rails  502  at multiple points, and positions them in the axial direction so that they are in sliding contact with the thin scraper rails. As shown in  FIG. 7 , the thicker support rails  502  serve to bridge the unsupported areas  701  of the thin scraper rails  501  between expander contact points  700  and prevent excessive axial deflection of the thin scraper rails caused by frictional forces between the scraper rails and the cylinder bore  106 . The expander  500  circumferential spring tension is reduced to provide the required oil film thickness with the thin scraper rails  501 , thereby reducing ring friction. The light radial loads on the support rails  502  minimize their contribution to friction. 
         [0023]    The scraper rails  501  are supported on one side by the upper and lower piston groove surfaces  116  and  117  of piston groove  101  and on the other side by a thicker support rail  502 , and therefore may be very thin and still withstand the axial friction loads and loads imposed by radial expander  500 . Axial clearances are set such that the scraper rails  501  are free to slide radially relative to the support rails  502  and the piston groove surfaces  116  and  117 . Similarly, the support rails are free to slide radially relative to the expander and the scraper rails. Oil control performance is maintained over the life of the ring assembly, since scraper rail  501  wear does not affect the width of the slider bearing zone. The radial stiffness of the thin scraper rail  501  decreases in proportion to the decreased bearing zone width and reduced radial force. This characteristic allows it to retain the ability of conventional three-piece ring assemblies to conform to cylinder bore distortions, but with reduced radial force and friction. 
         [0024]    The trailing support rail  502 A is pushed radially inward by the oil  505  collected by the adjacent trailing scraper rail  501 A, and forms a dynamic gap  506  with the cylinder bore  106  that allows the oil to flow to the piston groove drain holes  113 . This dynamic gap is shown for the down-stroke in  FIG. 5 , and is larger than the oil film thickness in the scraper rails  501  slider bearing zones because of the low outward radial force of the support rail  502 . The leading support rail  502 B is not pushed in, and slides on the thin oil film left by the leading scraper rail  501 B. 
         [0025]      FIG. 8  and  FIG. 9  show the second embodiment of the reduced friction oil control piston ring  800 . Like conventional two-piece oil control rings, this five-piece ring is installed in a piston groove  101  below the two compression ring grooves  103  and  104  containing compression rings  108  and  109 , and includes a helical spring expander  801 . The difference is that the pairs of circular steel scrapers  303  and  304  are each replaced by thin, flat scraper rails  802  and  803 . These thin scraper rails are spring-loaded against the cylinder bore  106  by the expander  801  through an intermediate bridge ring  804 . The expander  801  exerts an outward radial force on the inside diameter of the bridge ring  804 , which in turn exerts an outward radial force on the inside diameters of the scraper rails. The bridge ring is thin in the radial direction, making it radially flexible along its circumferential extent so that it has a small effect on the distribution of force transferred from the expander  801  to the scraper rings  802  and  803 . A spacer ring  805  separates the scraper rings  802  and  803 , and keeps them in in sliding contact with the upper and lower surfaces  116  and  117  of piston groove  101 . The spacer ring  805  has a radial clearance  806  with the bridge ring  804  so that the spacer ring is not spring-loaded against the cylinder bore  106  by the expander  801 , and lightly loaded against the cylinder bore by its own elastic tension. The bridge ring  804  and the spacer ring  805  incorporate openings to facilitate oil flow from the annular volume between the scraper rails  802  and  803  to the inner diameter of the piston groove  101  and back to the crankcase through the piston oil drain holes  113 . The bridge ring  804  includes edge notches  807  and the spacer ring  805  includes radial slots  808  to provide these functions, but it is obvious that holes or other geometric features could provide similar functions. The expander spring  801  circumferential spring tension is reduced to provide the required oil film thickness with the thin scraper rails  802  and  803 , thereby reducing ring friction. The light elastic radial self-loading of the spacer ring  805  minimizes its contribution to friction. 
         [0026]    The net effect is the same as in the first embodiment: The scraper rails  802  and  803  are supported on one side by the upper and lower surfaces  116  and  117  of piston groove  101 , and on the other side by the spacer ring  805 , allowing the scrapers to be very thin and still withstand the axial friction loads and imposed radial loads. Axial clearances are set such that the scraper rails  802  and  803  are free to slide radially relative to the spacer ring and the piston groove. Similarly, the spacer ring position is independent of the bridge ring  804 . Oil control performance is maintained over the life of the ring assembly, since wear of the scraper rails  802  and  803  does not affect the width of the slider bearing zone. The radial stiffness of the thin scraper rails  802  and  803  decreases in proportion to the decreased bearing zone width and reduced radial force. This characteristic allows it to retain the ability of conventional two-piece ring assemblies to conform to cylinder bore distortions, but with reduced radial force and friction. 
         [0027]    As with the support rails  502 , the spacer ring  805  is pushed radially inward by the oil  809  collected by the adjacent trailing scraper rail  802 , and forms a dynamic gap  810  with the cylinder bore  106  that allows the oil to flow to the piston groove drain holes  113 . This dynamic gap is shown for the down-stroke  110  in  FIG. 8 , and is larger than the oil film thickness in the scraper rails  802  and  803  slider bearing zones because of the low outward radial force of the spacer ring  805 .