Abstract:
A cylinder liner for an opposed-piston engine, and corresponding methods of extending engine durability and thermal management therewith, has opposite ends and a bore with a longitudinal axis for supporting reciprocating movement of a pair of opposed pistons. An intermediate portion of the liner extends between the opposite ends and includes an annular liner portion within which the pistons reach respective TC locations. A liner ring is seated in a portion of the bore in the annular liner portion, between the TC locations, for scraping carbon from top lands of the pistons and/or increasing the thermal resistance of the annular liner portion.

Description:
RELATED APPLICATIONS/PRIORITY 
       [0001]    This disclosure includes material related to the disclosure of commonly-owned U.S. application Ser. No. 13/385,127, filed Feb. 2, 2012, and titled “Opposed-Piston Cylinder Bore Constructions With Solid Lubrication In The Top Ring Reversal Zones”, which is now U.S. Pat. No. 8,851,029 B2. 
     
    
     FIELD 
       [0002]    The field includes opposed-piston engines. More particularly, the field relates to a cylinder liner constructed to support sliding movement of a pair of opposed pistons. 
       BACKGROUND 
       [0003]    Construction of an opposed-piston engine cylinder is well understood. The cylinder is constituted of a liner (sometimes called a “sleeve”) retained in a cylinder tunnel formed in a cylinder block. The liner of an opposed-piston engine has an annular intake portion including a cylinder intake port near a first liner end that is longitudinally separated from an annular exhaust portion including a cylinder exhaust port near a second liner end. An intermediate portion of the liner between the intake and exhaust portions includes one or more fuel injection ports. Two opposed, counter-moving pistons are disposed in the bore of a liner with their end surfaces facing each other. At the beginning of a power stroke, the opposed pistons reach respective top center (TC) locations in the intermediate portion of the liner where they are in closest mutual proximity to one another in the cylinder. During a power stroke, the pistons move away from each other until they approach respective bottom center (BC) locations in the end portions of the liner at which they are furthest apart from each other. In a compression stroke, the pistons reverse direction and move from BC toward TC. 
         [0004]    A circumferential clearance space between pistons and cylinder liners is provided to allow for thermal expansion. After long hours of operation carbon builds up in this clearance space, on the top land of a piston. Carbon built up on the top land of a piston moving in this space can result in increased friction and ring wear; at worst it can cause ring jacking. In conventional four-stroke, single-piston engines, carbon removal from the top land is typically performed by scraper ring hardware mounted between the top of the cylinder liner and the cylinder head. In an opposed-piston engine, the possible sites for removing carbon are limited. An opposed-piston engine does not include a cylinder head where carbon scraper devices can be located. Liner construction further reduces the possibilities. It is preferable that carbon removal not occur near the BC locations of the pistons, where the ports are located. Carbon debris near the intake port can contaminate charge air entering the bore, thereby degrading combustion. Carbon debris in the vicinity of the exhaust port can be swept into the gas stream exiting the cylinder after combustion, thereby increasing exhaust emissions. It is therefore desirable to remove carbon from the piston top lands within the liner at locations distant from the intake and exhaust ports. 
         [0005]    Another factor that degrades engine performance throughout the operating cycle of an opposed-piston engine is related to loss of heat through the cylinder liner. Combustion occurs as fuel is injected into air compressed between the piston end surfaces when the pistons are in close mutual proximity. Loss of the heat of combustion through the liner reduces the amount of energy available to drive the pistons apart in the power stroke. By limiting this heat loss, fuel efficiency would be improved, heat rejection to coolant would be reduced, which can allow use of smaller cooling systems, and higher exhaust temperatures can be realized, which leads to lower pumping losses. It is therefore desirable to retain as much of the heat of combustion as possible within the cylinder. 
         [0006]    An opposed-piston engine cylinder liner constructed according to the present disclosure satisfies the objective of carbon removal, thereby increasing the durability of the engine relative to opposed-pistons of the prior art. An opposed-piston liner construction according to the present disclosure satisfies the objective of heat containment, thereby allowing opposed-piston engines to operate higher heat retention than opposed-piston engines of the prior art. In some aspects, an opposed-piston liner construction according to the present disclosure satisfies both of these objectives simultaneously. 
       SUMMARY 
       [0007]    A cylinder liner for an opposed-piston engine constructed in accordance with the present disclosure increases durability of an opposed-piston engine by reducing or eliminating carbon build-up on the top lands of opposed pistons contained in the liner. The cylinder liner has a cylindrical wall with an interior surface defining a bore centered on a longitudinal axis of the liner. The bore has a first diameter. Intake and exhaust ports are formed in the cylindrical wall near respective opposite ends of the liner. An intermediate portion of the liner extends between the ends and includes an annular liner portion within which the pistons reach their TC locations. The annular liner portion is defined between first and second top ring reversal planes that orthogonally intersect the longitudinal axis. The first top ring reversal plane is at a first axial position where the topmost ring of a first piston is located when the piston is at its TC location. The second top ring reversal plane is at a second axial position where the topmost ring of a second piston is located when the piston is at its TC location. A liner ring is seated in a portion of the bore contained in the annular liner portion. The liner ring has an interior annular surface with a second diameter that is slightly fess than the first diameter. Thus, the liner ring slightly reduces the clearance space between the liner bore and top lands of the pistons. Since the liner ring includes the TC locations of the cylinder bore, the top land of each piston will only traverse the liner ring when the piston approaches and leaves TC. Therefore, the liner ring reduces the clearance where carbon collects so as to remove excess carbon as the top lands pass over the ring. 
         [0008]    The highest concentration of heat in the cylinder occurs in the annular portion of the liner between the TC locations of the pistons, where combustion takes place. Nearly half of the total heat flux into the liner occurs in this annular portion. Accordingly, construction of the liner ring in such a manner as to yield a high thermal resistance will reduce heat flux through the annular liner portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a cylinder in accordance with the present disclosure with a section removed to show a pair of opposed pistons disposed in a bore therein between bottom and top center positions. 
           [0010]      FIG. 2  is a perspective view of the cylinder of  FIG. 1  with a section removed to show a liner ring seated in the bore of the cylinder of  FIG. 1 . 
           [0011]      FIG. 3  is an enlarged side sectional view of an annular liner portion of the cylinder liner of  FIGS. 1 and 2  showing the liner ring in greater detail. 
           [0012]      FIG. 4  is the view of  FIG. 3  rotated axially by 90°. 
           [0013]      FIG. 5  is an enlarged side sectional view of a first alternate cylinder liner construction in accordance with the present disclosure. 
           [0014]      FIG. 6  is an enlarged side sectional view of a second alternate cylinder liner construction in accordance with the present disclosure. 
           [0015]      FIG. 7  is a schematic drawing of an opposed-piston engine  100  with one or more cylinder liners according to this specification. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    With reference to the drawings,  FIGS. 1, 2, and 3  show a cylinder liner  10  constructed in accordance with the present disclosure with a section removed to show a pair of opposed pistons  12 ,  14  therein between bottom and top center positions. Although not shown, the cylinder liner with the pistons therein would be retained in a cylinder tunnel of an opposed-piston engine, for example in the manner described and illustrated in commonly-owned U.S. application Ser. No. 14/450,572, filed Aug. 4, 2014 for “Opposed-Piston Engine Structure With A Split Cylinder Block.” The cylinder liner  10  has a cylindrical wall  20  with an interior surface defining a bore  22  centered on an imaginary longitudinal axis of the liner (represented by the line  24 ). The bore  22  has a first diameter D 1 . Longitudinally-spaced intake and exhaust ports  28  and  30  are formed or machined near respective ends  32  and  33  of the cylindrical wall  20 . Each of the intake and exhaust ports  28  and  30  includes one or more circumferential arrays of openings or perforations. In some other descriptions, each opening is referred to as a “port”; however, the construction of one or more circumferential arrays of such “ports” is no different than the port constructions shown in  FIGS. 1 and 2 . 
         [0017]    As is typical, the piston  12  includes at least one annular ring groove  40  with a piston ring  42  retained therein. The piston  12  has a circular peripheral edge  43  where the piston crown  45  meets the end surface  46  of the piston. An annular uppermost top land  47  of the piston extends between an upper surface  48  of the ring groove  40  and the peripheral edge  43 . An imaginary annular top ring reversal plane (represented by the circular line  49 ) that extends around the bore  22  and generally orthogonally to the longitudinal axis  24  indicates an axial location (with respect to the axis  24 ) where the upper surface  48  of the top ring groove  40  instantaneously comes to rest when the piston  12  reverses direction and begins to move away from TC. Similarly, the piston  14  includes at least one annular ring groove  50  with a piston ring  52  retained therein. The piston  14  has a circular peripheral edge  53  where the piston crown  55  meets the end surface  56  of the piston. An annular uppermost top land  57  of the piston extends between an upper surface  58  of the ring groove  50  and the peripheral edge  53 . An imaginary annular top ring reversal plane (represented by the circular line  59 ) that extends around the bore  22  and generally orthogonally to the longitudinal axis  24  indicates an axial location (with respect to the axis  24 ) where the upper surface  58  of the top ring groove  50  instantaneously comes to rest when the piston  14  reverses direction and begins to move away from TC. 
         [0018]    An intermediate portion  60  of the liner extends between the ends  32  and  33  and includes an annular liner portion  62  of the cylinder wall  20  within which the pistons  12  and  14  reach their TC locations The annular liner portion  62  is defined between the first and second top ring reversal planes  49  and  59 . As per  FIGS. 2, 3, and 4 , at least one fuel injector port  63  is provided through the annular liner portion  62  in which a fuel injector nozzle (not shown) is seated when the engine is assembled. In the example shown in these figures two fuel injector ports  63  are provided at diametrically-opposed locations in the annular liner portion  62 . A liner ring  70  is seated in a portion of the bore contained in the annular liner portion  62 . The liner ring  70  has an interior annular surface  72  with a second diameter D 2  that is slightly less than the diameter D 1  of the bore  22 . Thus, the liner ring  70  slightly reduces the clearance between the liner bore  22  and top lands  49 ,  59  of the pistons  12 ,  14 . Since the liner ring  70  extends between the top ring reversal planes, the top land of each piston will only traverse the liner ring when the, piston approaches and leaves TC. Therefore, the liner ring reduces the clearance where carbon collects so as to remove excess carbon as the top lands  49 ,  59  pass over the liner ring  70 . As can be seen in  FIGS. 3 and 4 , the liner ring  70  also includes one or more ports  71  for passage of fuel into the bore. The ports  71  are aligned with the fuel injector ports  63  in the annular liner portion  62 . In a preferred construction for seating the liner ring  70  in the bore  22 , the liner  10  includes an annular groove  73  in the portion of the bore  22  contained in the annular liner portion  62 . The liner ring  70  is received and retained in the annular groove  73 . 
         [0019]    The annular liner portion  62  defines space inside the bore where combustion occurs. In order to enhance the thermal resistance of this portion of the liner  10 , the liner ring  70  can be made to reduce heat flux through the annular liner portion  62  by elevating its thermal resistance with respect to that of the liner itself. In this regard, the material of which the liner ring  70  is made may be selected for a higher thermal resistance than the material with which the liner is made. Alternatively, as shown in  FIGS. 2 and 3 , the liner ring  70  may be provided with one or more grooves  74  on its outer annular surface with which to form one or more annular air-filled chambers (“air resistors”)  75  with the bore  22 . Of course, both thermal management options may be used in constructing the liner ring  70 . As a result thermal management is enabled during combustion of a mixture of fuel and air between the end surfaces of a pair of pistons disposed in the cylinder liner when the pistons are near respective top center locations in the annular liner portion of the cylinder liner by impeding flow of heat through the cylinder liner with a higher resistance in the annular liner portion than in the rest of the cylinder liner. 
         [0020]    This cylinder liner construction can provide an added structural element where maximum compression and peak cylinder pressures occur and so may eliminate the need for an additional external liner sleeve to provide this support. Furthermore, scraping carbon off of the piston top lands will reduce the occurrences of ring jacking, and thereby improve the durability of an opposed-piston engine. Finally, the liner ring can reduce the heat flow through the cylinder liner, between the top ring reversal locations, where nearly half of the total heat lost into the liner occurs. 
         [0021]    The body of the cylinder Liner may be made from cast iron, or other suitable material. The liner ring  70  may be made from steel, titanium, or other suitable material such as Inconel, to ensure structural integrity of the cylinder liner in the area of maximum pressures during combustion. 
         [0022]    The liner illustrated in  FIGS. 1-3  may be assembled by attaching the liner ring  70  to the liner  10  either with a mechanical fastener or with an interference fit. For an interference fit, the following steps illustrate a preferred method of constructing a cylinder liner according to this disclosure:
       1. The liner is constructed with intake and exhaust ports and the bore  22  is initially honed.   2. The annular groove  73  is formed by machining or etching the bore portion of the annular liner portion  62 .   3. The bore  22  is honed after the annular groove  73  is formed.   4. The liner is heated to increase inside diameter D 1  and the liner ring  70  is heated to increase its formability.   5. The liner ring  70  is placed in the center of the cylinder liner over the annular groove  66 .   6. The liner ring  70  is swaged into the annular groove  73  by driving tapered mandrels through the center of the liner ring  70  so as to expand the liner ring  70  into the annular groove  66 .   7. The liner  10  and the ring  70  are cooled.   8. From either end of the liner  10 , punches with the approximate shape of the piston top land profile are driven to the liner ring  70 . This will accomplish three goals:
           a. It will complete the swaging process,   b. It will fully embed the liner ring  70  into the annular groove  66 .   c. It will properly size the inner diameter of the liner ring  70 .   
           9. Form one or more injector ports through the annular liner portion  62  and the liner ring  70 .       
 
         [0035]    Alternatively, if the liner ring  70  is formed of a ceramic material, it would be made so that the outer ends of the insert were slightly higher than the body of the insert so that a scraping interference will occur between the insert ends and the piston lands. 
         [0036]    A first alternate cylinder liner construction according to this disclosure is shown in  FIG. 5 . In this construction the liner bore diameter is enlarged slightly by machining from one end of the liner into the annular liner portion  62 . This allows the liner ring  70  to be installed directly from the one end of the cylinder without the need to fabricate it with a slightly smaller outer diameter than the bore and then be enlarged by a mandrel to fit into the groove in the annular liner portion. Once the liner ring  70  is secured in the interior of the liner annular liner portion  62 , an inner liner sleeve  90  having an interior diameter equal to that of the rest of the cylinder is then installed up to the liner ring  70  and is secured therein. The liner ring could be attached to the cylinder liner with mechanical fasteners or seated therein by means of an interference fit. An interference fit could be accomplished by either super cooling the sleeve, (using liquid Nitrogen as an example), to shrink its outside diameter before placing it in the enlarged bore portion and then letting it reach room temperature. Alternatively, the liner could be heated to increase its inside diameter before inserting the sleeve and then both the liner and the inserted sleeve would be cooled. 
         [0037]    A second alternate cylinder liner construction according to this disclosure is shown in  FIG. 6 . In this construction the liner bore diameter D 1  is enlarged slightly to D 3  by machining from one end of the liner part way into the annular liner portion  62 . The bore diameter increases to D 4  for the remainder of annular liner portion  62 . As can be seen in  FIG. 6 , D 1 &lt;D 3 &lt;D 4 . The liner ring  70   a  is formed with an outside diameter that steps from D 2  to D 3  and is installed in the annular liner portion  62  as shown in  FIG. 6 . This construction requires pistons with unequal diameters, and also requires that the liner ring  70   a  have a stepped interior diameter such that in a first portion, the interior diameter is equal to or slightly greater than the diameter of the top land of the first piston and, in a second portion, the interior diameter is equal to or slightly greater than the diameter of the top land of the second piston. One or more air resistors may be formed between the outer surface sections of the liner ring  70   a  and the respective opposing sections of the bore  22 . 
         [0038]      FIG. 7  illustrates an opposed-piston engine  100  with three cylinders  101 , in which each cylinder comprises a cylinder tunnel  103  in a cylinder block  105  and a cylinder liner  107  according to this specification seated in the cylinder tunnel. Of course, the number of cylinders is not meant to be limiting. In fact, the engine  100  may have fewer, or more, than three cylinders. 
         [0039]    The scope of patent protection afforded these and other cylinder liner embodiments that accomplish one or more of the objectives of durability and thermal resistance of an opposed-piston engine according to this disclosure are limited only by the scope of any ultimately-allowed patent claims.