Abstract:
Selective laser and induction hardening are applied to areas of a water jacketed engine cylinder liner to provide improved resistance to scuffing resulting from the rubbing contact of the walls and piston rings of an associated piston. An upper bore portion and port relief areas of the liner are fully induction hardened to improve wear resistance. The port area is then fully laser hardened to improve wear resistance. If desired, the laser hardening process may continue beyond the intake port area to the port relief areas to ensure full hardening of the port relief areas and continuous hardening between the intake ports and the port relief areas to improve wear resistance.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to internal combustion engines and, more particularly, to selective hardening of engine cylinder bores to improve resistance to scuffing and wear.  
       BACKGROUND OF THE INVENTION  
       [0002]     Water jacketed cylinder liners having annularly spaced air inlet ports intermediate their ends are known in the internal combustion engine art. Such liners are commonly made from an alloy cast iron having a medium hardness, as cast, in the range of from about 200 to 260 Brinell. It is also known in such liners to provide a diametrically relieved area of the bore at the ports and extending axially on either side thereof. This port relief area is smoothly curved and blended into the upper and lower liner bores and helps reduce scuffing originating in the port areas and resulting from the rubbing contact in service of the walls of an associated piston and its rings with the cylinder liner bore and port area.  
         [0003]     Various other methods have been employed to reduce cylinder liner scuffing. One such method for improving a cylinder liner to reduce scuffing involves plating the cylinder liner with a hard material such as chromium. However, this method is not generally desired because of cost.  
         [0004]     Another method of improving a cylinder liner to reduce scuffing involves laser hardening scuff prone surfaces of the bore. However, since lasers are commonly focused into a small diameter beam, the laser must make multiple passes or closed helical passes over the surface of the bore to adequately heat and fully harden the scuff prone surfaces of the bore. As a result, laser hardening is a costly and time consuming method.  
         [0005]     Another method of improving a cylinder liner to reduce scuffing involves induction hardening scuff prone surfaces of the bore. Induction hardening uses an electromagnetic coil, which rapidly heats adjacent surfaces of the bore to a hardening temperature. However, when a coil is used around the intake ports and the relieved areas of the cylinder bore, the varying dimensions and geometry of the intake ports and the relieved areas can create difficulties in the hardening process, in that the portions of the bore nearest the coil heat at a faster rate than the portions of the bore farthest from the coil. As a result, the surfaces farthest from the coil may not reach hardening temperature, causing inconsistencies in the quality of the heated surfaces.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a method utilizing a combination of laser and induction hardening to more efficiently provide a cylinder bore surface resistant to bore scuffing in a water jacketed cylinder liner for a two stroke-cycle diesel engine having annularly disposed air inlet ports with blended port relief areas adjacent the ports.  
         [0007]     In an exemplary embodiment, a cylinder liner is cast of an iron alloy. The cylinder liner includes a generally cylindrical cast iron body defining a generally cylindrical interior wall with a plurality of radially extending ports through the cylindrical interior wall and spaced annularly therearound to form a port area intermediate opposite ends of the interior wall.  
         [0008]     The cylinder liner is then machined to create upper and lower bore portions respectively above and below an annular band at and extending slightly above and below the port area. The annular band is formed having a slightly greater diameter than that of the upper and lower bore portions to blend the greater diameter of the of the port area into the slightly smaller diameter bore portions, thereby forming upper and lower blended port relief areas between the bore portions and the intake port area.  
         [0009]     After the cylinder liner is machined, the cylinder liner undergoes a hardening process according to the method of the present invention. The method includes an induction hardening step and a laser hardening step.  
         [0010]     The induction hardening step involves heating the surface of the upper cylinder bore and the upper and lower blended port relief areas to a hardening temperature using electromagnetic induction. The heated surfaces are then cooled to ambient temperature to create a scuff resistant hardened surface in the upper bore area and the blended port relief areas without significant distortion of the cast iron cylinder liner body and its previously machined surfaces.  
         [0011]     The laser hardening step is performed primarily to harden the intake port area. The laser hardening process locally heats the intake port area to a hardening temperature by traversing a laser beam across the inner surface of the intake port area in a closed helical pattern. After the laser adequately heats the intake port area to a desired hardening temperature, the intake port area is allowed to cool to ambient temperature to create a scuff resistant hardened surface on the intake port area without significant distortion of said cast iron cylinder liner body and its previously machined surfaces.  
         [0012]     Alternatively, the laser hardening step may be expanded to include traversing the laser beam from the intake port area to the upper and/or lower port relief areas to ensure continuous hardening of the annular band.  
         [0013]     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a cross-sectional view of a cast water jacketed ported cylinder liner having the cylinder bore selectively case hardened by a combination of induction and laser hardening in accordance with the present invention; and  
         [0015]      FIG. 2  is a cross-sectional view similar to  FIG. 1  but showing an alternative embodiment of the cylinder liner in which the selective hardening processes are overlapped between the induction hardened portion and the laser hardened portion of the cylinder bore. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     Referring now to  FIG. 1  of the drawings in detail, numeral  10  generally indicates a removable cylinder liner of a type used in certain well-known two-cycle diesel engines that are used in numerous applications, including the propulsion of railway locomotives, except for the cylinder bore hardening process, which will be subsequently described. The liner  10  includes a generally cylindrical cast iron body  12  defining internally an elongated generally cylindrical bore  14  having upper and lower bore portions  16 ,  18 , respectively, separated intermediate the ends by an annular row of generally rectangularly spaced air inlet ports  20 .  
         [0017]     The liner body  12  includes a generally cylindrical inner wall  22 , defining the cylinder bore  14  and a plurality of flanges  24 ,  26 ,  28 ,  30  which extend outwardly from the inner wall and upwardly or downwardly to receive upper and lower closure sleeves  32 ,  34 . The sleeves are brazed to the flanges and cooperate therewith to define upper and lower annular cooling jackets  36 ,  38  surrounding the inner walls  22  adjacent the upper and lower bore portions  16 ,  18  of the liner  10 . The cooling jackets  36 ,  38  provide for coolant flow through the lower and upper jackets and between the ports for cooling the liner  10  during engine operation.  
         [0018]     While the described embodiment of cylinder liner  10  includes brazed on cooling jacket sleeves, it should be understood that the method of the present invention is also applicable to other forms of coolant jacketed, ported cylinder liners having cast iron bores, including fully cast liners.  
         [0019]     The cylinder liner  10  is machined to create upper and lower bore portions  16 ,  18  respectively above and below an annular band  40  at and extending slightly above and below an intake port area  42  defining the inlet ports  20 . The annular band  40  is formed having a slightly greater diameter than that of the upper and lower bore portions  16 ,  18  to provide diametral relief at the port area  42 . The diameter of annular band  40  varies to blend the greater diameter of the of the port area  42  into the slightly smaller diameters of the upper and lower bore portions  16 ,  18 , thereby forming upper and lower blended port relief areas  44 ,  46  between the bore portions and the intake port area  42 .  
         [0020]     The present invention improves the liner  10  by providing a method, which increases the hardness of the upper bore  16  and the annular band  40  after the liner has been fully machined to provide a scuff resistant surface thereon.  
         [0021]     The cylinder liner  10  undergoes a case hardening method, which involves a combination of induction hardening and laser hardening to create a scuff resistant surface in the upper bore  16  and the annular band  40 . One step of the hardening process utilizes an electromagnetic induction coil or other known induction devices to heat the upper bore  16  and the blended port relief areas  44 ,  46  of the liner  10 . The coil traverses the bore  14  of the liner  10  for a period of time to adequately heat by inductance the upper bore  16  and the blended port relief areas  44 ,  46 . Once these areas are locally heated to a hardening temperature, the coil is removed from the bore  14 . The heated surfaces are then cooled to ambient temperature to create a scuff resistant hardened surface on the upper bore area  16  and blended port relief areas  44 ,  46 .  
         [0022]     In an additional step, the intake port area  42  is laser hardened to create a scuff resistant case hardened inner surface in the intake port area. In particular, the intake port area  42  of the liner  10  is fully case hardened through localized heating and ambient cooling of the surface. The heating is accomplished by a traversed laser beam, which is moved along the liner surface in a combination of orbital and axial motion to form a helical pattern  48  covering the intake port area  42 . As the laser beam heats the surface to a hardening temperature, the laser is advanced in a helical pattern  48  to allow the heated surface to cool to ambient temperature. Thus, a scuff resistant hardened surface is provided in the intake port area  42  without significant distortion of the cast iron cylinder liner body and its previously machined surfaces. To provide complete hardening of the intake port area, the hardened bands formed by traversing the laser beam over the surface are edge-connected by providing a closed helix without spaces between the hardened bands. With this pattern, the full surface of the intake port area  42  may be hardened in a single pass of the laser beam across the surface.  
         [0023]     Preferably, the laser hardening step is continued beyond the intake port area  42  into the blended port relief areas  44 ,  46 . Advantageously, the closed helical pattern  48  created by the laser beam is extended through one blended port relief area, such as area  44 , through the intake port area  42  and the other blended port relief area, such as area  46 , as shown in  FIG. 2 . This overlap insures a continuously hardened surface between the induction hardened surfaces of the blended port relief areas  44 ,  46  and the laser hardened surface of the intake port area  42 .  
         [0024]     The percent of overlap needed between the intake port area and the blended port relief areas  44 ,  46  depends on the effectiveness of the induction coil in heating the larger diameter blended port relief areas. When the upper and lower bores  16 ,  18  have a diameter substantially similar to the diameter of the blended port relief areas  44 ,  46 , the induction process will be more likely to adequately heat the blended port relief areas and thereby form an adequately hardened surface on the relief areas. In such a case, minimal or no laser hardening may be required to provide a continuously hardened surface between the intake port area  42  and the port relief areas  44 ,  46 , as shown in  FIG. 1 . However, if the diameter of the blended port relief areas  44 ,  46  are substantially greater than the diameter of the upper and lower bores  14 ,  16 , the induction process may not adequately heat the larger diameter portions of the port relief areas to provide a fully a sufficiently hardened surface in the relief areas. In this case, the laser hardening step may be extended farther into the blended port relief areas  44 ,  46  to ensure a fully hardened surface of the port relief areas and a continuously hardened surface between the intake port area  42  and the port relief areas, as shown in  FIG. 2 .  
         [0025]     The resulting liner, after the hardening process, has improved wear characteristics over the prior art in that the surfaces of the upper bore  16  and the annular band  40  are fully case hardened and, thus, more resistant to scuffing.  
         [0026]     While initial practice of the method has involved performing the induction hardening step first and the laser hardening step second, it should be understood that the order of these steps could be reversed without departing from the concepts involved in the invention.  
         [0027]     If desired, after the hardening process, the hardened surfaces of the liner  10  may be honed to roughen the bore  14  surfaces to provide for proper break-in of the rubbing components during initial engine operation.  
         [0028]     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.