Patent Publication Number: US-2010116240-A1

Title: Multi-piece thin walled powder metal cylinder liners

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This claims the benefit of U.S. Provisional Patent Application No. 60/910,100 filed Apr. 4, 2007, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to sintered powder metal manufacturing and in particular to powder metal cylinder liners for an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     The use of sintered powder metal (PM) parts has accelerated in the recent past for components difficult to manufacture by other methods as PM components can offer a cost effective alternative to other metal formed components. Some advantages of powder metallurgy include lower costs, improved quality, increased productivity and greater design flexibility. These advantages are achieved in part because PM parts can be manufactured to net-shape or near-net shape which yields little material waste, and which in turn eliminates or minimizes machining. Other advantages of the PM manufacturing process and parts produced there from, particularly over other metal forming processes, include greater material flexibility including graded structures or composite metal, lighter weight of the parts, greater mechanical flexibility, reducing energy consumption and material waste in the manufacturing process, high dimensional accuracy of the part, good surface finish of the part, controlled porosity for self-lubrication or infiltration, increased strength and corrosion resistance of the component, and low emissions, among others. 
     Internal combustion engine manufacturers have sought more efficient, cost effective and viable ways to reduce cost and weight in engines without sacrificing performance and/or safety. One of the largest and most important components of the engine is the cylinder block. In the past, cylinder blocks had been formed from cast iron, which provided strength, durability and long service life. However, as can be appreciated, cast iron is quite heavy. Further, cast iron has a relatively poor thermal conductivity. Consequently, alternatives to cast iron cylinder blocks are sought. 
     One such alternative is to form the blocks from aluminum alloy. Aluminum alloy is very lightweight and has good thermal conductivity, each of which are desirable features in the engine industry. However, aluminum alloy is relatively soft and easily scratched and thus does not provide the strength, durability and long service life required for use in a cylinder block, particularly with respect to the requirements of the cylinder bores in the block. Further, aluminum alloy has a relatively high coefficient of thermal expansion compared to iron, which can increase blowby between a cylinder and piston during combustion at high operating temperatures, thereby increasing emissions. 
     As an alternative, engine manufacturers have used more wear resistant cylinder liners within the cylinder bores of an aluminum block. Cylinder liners are typically in-cast into aluminum engine blocks to provide improved wear resistance compared to the aluminum bore that is present without the liner. A cast iron, machined cylinder liner is typically used for engines that require a cylinder liner. However, these cast iron cylinder liners have a less than desirable mechanical bond with the aluminum engine block which leads to less than desirable heat transfer properties. Further, features are required on the outside of the cast iron cylinder liner to “lock” in place in the aluminum block, and these features can create an uneven heat transfer from the cast iron cylinder liner to the aluminum block, or undesirable voids or local hot spots can be created between the liner and the aluminum. Additionally, the alloys used in cast iron cylinder liners are not optimum relative to strength and stiffness, resulting in bore distortion during combustion, more blow-by and higher emissions. 
     The inherent porosity of a powder metal iron alloy part, when in-cast into an aluminum casting, allows the molten aluminum to infiltrate the matrix of the PM part to improve the bond between the surrounding aluminum alloy and the PM part. Allowing penetration of the molten aluminum alloy into the cylinder liner porosity also takes advantage of the desirable machinability of the impregnated PM matrix. 
     Although PM technology has the potential of overcoming some of the problems with cast iron cylinder liners, production of PM cylinder liners by conventional compaction to net shape or near net shape has not been commercially feasible. One reason is that the high length to wall thickness ratio results in excessive difficulties filling the compaction die with metal powder. In addition, compacting from the ends of a part with a high aspect ratio results in an unacceptable density gradient along the length of the cylinder liner, and inadequate green strength of the compact. These problems can be somewhat overcome using cold isostatic compaction plus subsequent secondary manufacturing operations, but can be too costly in comparison with cast cylinder liners. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cylinder liner construction that can be used to make cylinder liners having a high length to wall thickness ratio, out of powder metal. The liner is made of multiple powder metal cylinder liner pieces, placed end to end coaxially, to form the cylinder liner. 
     In one aspect, the invention provides a cylinder liner that has a powder metal composition formed into a cylinder, where the cylinder includes a wall thickness and a length, and a ratio of the length to the thickness is greater than 12. Each piece, on the other hand, would typically have a ratio of less than 20. 
     In another aspect, the invention provides an internal combustion engine that has an engine block with at least one combustion cylinder liner of the invention. 
     An advantage of the present invention is being able to make a low density powder metal cylinder liner (e.g., nominally 6.3 g/cc) to improve the bond between the surrounding aluminum alloy and the cylinder liner by allowing penetration of the molten aluminum alloy into the cylinder liner PM matrix porosity. 
     Another advantage of the present invention is that the resulting improvement in bonding reduces or eliminates the need for outside diameter features, and improves uniformity of heat transfer from the combustion chamber to the surrounding aluminum. 
     Another advantage of the present invention is providing a powder metal component that has acceptable density, and preferably relatively uniform density, along the length of the wall from end to end. 
     Another advantage is being able to make the sintered powder metal liner pieces to near their final machined thickness, to reduce subsequent machining operations and material waste. 
     The present invention provides the advantages discussed above relative to sintered powder metal cylinder liners. 
     The foregoing and other advantages of the invention appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a cylinder of an internal combustion engine with a cylinder liner of the invention cast in place; 
         FIG. 2  is a cross-sectional view of a cylinder liner of the invention illustrated apart from the cylinder of the engine; 
         FIG. 3  is a detail cross-sectional view of the joint between the two ends of the two cylinder pieces that make up the cylinder liner of  FIGS. 1 and 2 ; and 
         FIG. 4  is a cross-sectional view of one of the cylinder liner pieces apart from the other piece that makes up the liner of  FIGS. 1-3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , an internal combustion engine  10  has a combustion cylinder  12  that is made by casting aluminum alloy  22  around the outside of a sintered powder metal cylinder liner  18 . The aluminum alloy  22  infiltrates pores in the liner  18  to hold it firmly. A piston  14  with rings  16  reciprocates in the cylinder as the engine operates. 
     The liner  18  is made of two cylinder liner pieces  24 , which are cylinders of the same general shape as the liner  18 , but shorter. The two pieces  18  are placed end to end coaxially, preferably so that their ends abut. Preferably, as illustrated in  FIG. 2 , the ends are stepped, with mating male  30  and female  32  ends. As illustrated in  FIG. 2 , the bottom (female) end of the upper piece  24  fits around and mates closely with the upper (male) end of the lower piece  24 . As illustrated in  FIG. 3 , at the outer edge of the joint, a small gap, e.g., 0.050 at the edge, is created to permit brazing material to infiltrate the gap to hold the two pieces. The brazing may be sinter brazing, i.e., brazing that is performed during the sintering process, or post-sinter brazing. Alternatively, it may not be necessary to affix or bond the pieces  24  to one another prior to casting them into the cylinder  12 , if for example they are placed on a core rod or other device to align and abut them end to end and then affixed to one another by the aluminum alloy  22  during the cylinder casting process, or they could be press fit to one another prior to casting into the cylinder  12 , and then cast into the cylinder. 
     Conventional powder metal compaction and sintering processes can be used to make each piece  24 . The die cavity would have the shape of one of the pieces  24 , filling would be from the top, and compaction may be from both ends. Further, in a conventional powder metal compaction operation, for a part with a high aspect ratio, there would typically be density variations in the wall of the part along the length, with higher densities at the ends than at the middle of the part. By making each piece  24  shorter than the whole liner  18 , density thoughout the part is made more uniform. 
     The powder metal composition of the pieces  24  can include approximately between 85% and 99% sponge iron powder, approximately between 0.1% and 2.0% graphite, and approximately between 0.1% and 2.0% a synthetic wax such as ethylene bis-stearamide wax (synonymous with N, N′ethylene bis-stearamide; N, N′distearoylethyelendiamine; EBS). More specifically, powder metal composition  34  can include approximately 98.1% sponge iron powder, approximately 0.9% graphite, and approximately 1.0% ethylene bis-stearamide wax. Sponge iron powder results from the direct reduction of high grade magnetite iron ore. This process results in spongy particles (as viewed in photomicrographs, for example) which have good compressibility, exceptionally good green strength and produces parts with good edge integrity. Ancor MH-100 is an example of such a sponge iron powder. 
     The synthetic wax powder is used as a lubricant and binder for the compaction of powdered metal parts, such as Acrawax® lubricant. The graphite is a high quality powder graphite for sintering and alloy control, such as Asbury 3203 graphite. Powder metal composition  34  can additionally include up to 0.5% phosphorus. 
     Powder metal cylinder liner  22  consequently has a relatively uniform density along the length of the cylinder liner  18 . The density can be approximately between 5.8 g/cm 3  and 6.8 g/cm 3 , and more specifically, the density is approximately 6.3 g/cm 3 . Prior to machining the inside diameter, the wall thickness  50  may be, for example, just slightly more than the post machining thickness, for example each piece  24  may have an ID of 2.608 inches and an OD of 2.818. The machining operation may only remove about 2-10% of the wall thickness, or no machining may be necessary prior to casting the liner  18  into the cylinder block. Length of the liner  18  may be 3.582 inches for the whole liner, with a length of approximately half of that for each piece  24 . More than two pieces could be used to produce a liner, but acceptable filling, compaction and density uniformity will be possible in many cases with just two pieces  24 . The cylinder liner  18  can have a ratio of length to wall thickness  50  greater than 12, and the same ratio for each piece  24  should be less than 20. Also, preferably the wall thickness of the powder metal compact of each piece  24  (prior to sintering or any machining) should have a wall thickness of less than 0.20 inches. 
     The green compact powder metal cylinder liner pieces  24 , either alone or put together, typically requires sintering at an elevated temperature to strengthen them, as is well known. It&#39;s possible however that the sintered part could be made so near net shape that the machining step prior to in-casting could be eliminated, with the only machining being done after the sintered PM liner  18  is cast into the cylinder  12 . 
       FIG. 1  illustrates an internal combustion engine  10  according to the present invention which includes a cylinder  12  with at least one combustion cylinder bore having therein piston  14 , and at least one cylinder liner  18 . Internal combustion engine  10  can include other elements such as a fuel system, crankshaft, lubrication system, cooling system and other elements as are known. As stated, the cylinder bore defined by cylinder liner  18 , the aluminum alloy that impregnates it and the surrounding aluminum of the cylinder may require additional machining after the liner is cast into the cylinder  12 . 
     The liner  18  should be long enough so that at bottom dead center of the piston  14 , all of the rings  16  of the piston are axially overlapping the liner  18 , as they should also be overlapping at top dead center of the piston  14 . 
     A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiments described.