Patent Publication Number: US-6216585-B1

Title: Carbon-carbon engine components and method of fabrication

Description:
RELATED APPLICATION 
     This application is a Division of my U.S. Ser. No. 09/070,325, Filed: Apr. 30, 1998, entitled “CARBON-CARBON ENGINE COMPONENTS AND METHOD OF FABRICATION”, and it is being filed under the provisions of 35 U.S.C. 121, now U.S. Pat. No 6,029,346 on Feb. 29, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the fabrication of internal combustion engine parts from carbon-carbon composites. Carbon-carbon composites are made of carbon fiber reinforced in carbon matrix. 
     In the past, carbon-carbon parts have been used primarily in aeronautical and space applications because of their light-weight and high temperature properties. However, these characteristics are also extremely beneficial in industrial and automotive engines as evidenced by the fact that aluminum pistons(density 2.7 g/cm 3 ), which are also lighter than steel(8.0 g/cm 3 ), but heavier than carbon-carbon (1.7 g/cm 3 ), have achieved significant commercial success in these markets. However, aluminum has a number of disadvantages. The relative difference in thermal strength and coefficient of thermal expansion of the aluminum pistons with other mating engine components, require large clearances between the piston and the adjacent walls to eliminate interference and galling between the piston and the cylinder wall and the wrist pin. To improve engine efficiency, piston rings are used in these aluminum pistons to seal the gap between the piston and the cylinder wall. In fact, multiple rings with staggering gaps are required to prevent high pressure leakage and possible piston erosion from local high flow rates at the rings, piston and cylinder wall inner face. Because of the poor, high temperature strength of aluminum, it has been found necessary to lower the piston rings from the crown to prevent the rings from sticking in the ring grooves, and this has resulted in unburned hydrocarbon build-up in the space around the piston above the ring yielding reduced engine efficiency, noting that aluminum melts at 660 degrees C. (and the maximum application temperature is 300 degrees C.), which is well below the typical combustion engine temperature. Also, large amounts of lubricant are required to reduce the piston and cylinder wall temperature and wear rates in aluminum piston assemblies. 
     The carbon-carbon components are desirable in this environment because of their resistance to high temperature and thermal shocks, coupled with high temperature strength. In some cases, the carbon-carbon piston can eliminate the necessity of piston rings because of the negligible coefficient of thermal expansion of carbon-carbon(1-2 ppm), which is far less than aluminum(18-20 ppm). Even at high temperatures, the carbon-carbon parts uniquely maintain strength, allowing the piston to operate at both higher temperature and higher pressure than metal pistons. Thermal efficiency of the engine is also improved because of the high emittance and low thermal efficiency of carbon-carbon, resulting in less heat loss into the piston and the cooling system. 
     The carbon fibers in the carbon-carbon composite are known as precursors, and there are three different types; namely, rayon, polyacrylonitrile, and pitch. Rayon has been largely abandoned in recent years because of the resulting poor quality fibers so that today fibers are predominantly made from P.A.N.(polyacrylonitrile) or pitch. P.A.N. is preferred for high strength, whereas pitch derivatives are desirable for high modulus and high thermal conductivity. 
     In reality, however, the use of carbon-carbon composites in engine components in the industrial and automotive market has not been extensive primarily for two reasons. The first is cost. In the early 1990&#39;s, carbon fiber used to cost about $40/lb., and now costs $8-$9/lb., and the near term projections are for under $5/lb. This cost reduction and an increased demand for fibers, which is projected, should drive the fiber cost down further making the carbon-carbon composites a very strong engineering material to replace steel and aluminum in many applications. 
     The second reason why carbon-carbon composites have not achieved great commercial success is the inability of fabricators to optimize and reduce the cost of the fabrication process. This is due in part to the difficulties in processing techniques to convert the binder to complete carbon which can hold the fibers, so the fibers therein reinforce the binder in such a way to have suitable engineering properties. Traditional processing consists of mixing the fiber with resin and preform into the desired shape. These preforms are kept in a high temperature furnace and heat treated for several hours ranging from 800 to 2000 degrees C. After firing, the composites are placed in a CVD furnace and densified. CVD refers to chemical vapor deposition. Due to the nature of CVD, it is extremely difficult to fabricate thick specimens with uniform density. As such, even for thin samples the CVD process takes from a few days to several weeks to finish. The CVD is sometimes replaced by chemical vapor infiltration (CVI), which causes carbon to close on the outside walls of the preform and inhibit penetration to the inside walls. Thus, in addition to time costs, the resulting crusting problem and its removal made these processes highly labor intensive and not conducive to high volume production. 
     In CVD, hydrocarbon gas is sent through the preform to crack it with high heat. This breaks the carbon down from hydrogen. 
     Another deficiency in the prior art of carbon-carbon high temperature components is the failure to optimize the performance of the component by controlling and varying the performance characteristics at specific locations on each part. For example, a rotating shaft under load will run hotter in the area of the bearings than at a point midway between the bearings. Prior art methodology for constructing such a shaft would result in homogenous physical properties throughout the shaft, and it is this approach that has in the past contributed to the high cost and less than optimum performance for carbon-carbon composites. 
     The Taylor, U.S. Pat. No. 4,683,809, assigned to NASA, shows a light-weight carbon-carbon piston with no piston rings. The piston is constructed in one piece and the fibers are laid up randomly throughout the piston. The methodology of fiber lay-up tends to disburse the fibers randomly resulting in internal cracks, unreliability, and low strength. The resulting piston component is heavy with poor fracture toughness. Taylor also suggests in this patent a carbon-carbon cylinder wall  60 , but is silent as to how the cylinder wall or sleeve is formed or how its performance optimized. 
     Another Taylor patent, also assigned to NASA, is U.S. Pat. No. 4,736,676, which discloses a composite piston structure including a carbon-carbon or ceramic piston cap  11  with a metallic piston body  13 . This piston is quite complicated and too difficult to manufacture in commercial production. 
     A later Taylor, et al., U.S. Pat. No. 4,909,133, also assigned to NASA, discloses a carbon-carbon piston that has a tubular closed ended knitted preformed sock of carbon fibers  11  imbedded within the matrix of the piston structure on the piston crown side wall in the inside surface. 
     Finally, the Fluga, U.S. Pat. No. 5,154,109, discloses a method of manufacturing a piston and piston rod in which a layer of carbon fibers is triaxially braided on a mandrel with a cylindrical body. A second layer of carbon fibers is triaxially braided over the first layer. The fiber layers are spaced from one another and impregnated with a thermo set resin. The preform is unidirectional in the sense that it does not have a uniform axial diameter. These are extremely difficult to manufacture and difficult to densify using CVD. Furthermore, the design is not flexible because the whole structure is made of one type of material, and thermal expansion is difficult to predict, and in some cases, may expand obliquely. 
     It is the primary object of the present invention to ameliorate the problems noted above and provide an improved method and structure for fabricating carbon-carbon light-weight components intended for high temperature environments. 
     SUMMARY OF THE PRESENT INVENTION 
     In accordance with the present invention, specific internal combustion components including piston, wrist pin, and sleeve are constructed of matching carbon-carbon composites. The piston is a two-piece assembly divided in crown and skirt, each fabricated to optimize the most desirable properties into different cylinder areas in which they operate. The crown is fabricated by placing carbon fiber and carbon matrix into a compression mold and pyrolizing(heating) the resulting preform at a high temperature in the range of 1500 to 2000 degrees C. to achieve high temperature strength and high thermal conductivity that continue after machining to the finished crown part. The skirt, a separate piece, on the other hand is fabricated differently to seek higher lubricity along the crown wall with lower thermal conductivity and lower high temperature strength. The skirt preform is fabricated by wrapping fiber or fabric around a mandrel followed by successive stages of heating and pyrolizing at temperatures substantially less than the cracking temperature of the crown preform resulting in substantial time and labor savings. The skirt preform is machined and then assembled to the crown to complete the piston. 
     The piston skirt, the wrist pin, and the cylinder sleeve are also fabricated using a matching fabrication technique to minimize tolerances between these parts. The three parts are separately preformed on mandrels utilizing the same wrapping angles to achieve matching properties. Then, the resulting preforms of each are preheated and pyrolized at the same temperatures to almost equate the elevated temperature strength and most importantly, coefficients of thermal expansion. This technique permits the engine designer to reduce the clearances between these parts, minimize blow by, reduce lubrication requirements, and increase horsepower. 
     According to the present invention and based on a differential analysis of piston environment, the property requirements for the skirt are high self-lubricious properties and improved wear resistance while for the crown high thermal strength and high thermal conductivity. To achieve this, the crown is fabricated in a compression mold separately from the skirt. This permits the piston designer to select the fiber orientation in the preform for the crown and the subsequent heat treating, pyrolizing and CVD steps to best accommodate the temperature and pressure conditions above the crown. Usually, this dictates a pyrolizing temperature of about 2000 degrees C. 
     The skirt, on the other hand, is manufactured by wrapping fibers on a mandrel with the fiber layers running along different directions. Building the skirt on a mandrel enables a hybrid of fiber additions, such as stabilized P.A.N., low temperature pitch or a high temperature pitch fiber in such a way to tailor the properties for specific strength, thermal conductivity, and thermal expansion, etc. This usually dictates a significantly lower heat treating temperature in the range of 600 to 800 degrees C. After machining the crown and skirt, they are threaded to one another and the threads welded permanently in a heat treating furnace at 400 degrees C. 
     This method of fabrication has several advantages. One, it enables tailorable piston properties at different parts of the piston to achieve superior performance characteristics which is otherwise not able to achieve from prior art. Other significant advantages are production volume is significantly increased due to the smaller preforms making heat treatment and C.V.D. more efficient and less time-consuming. The lower temperature skirt curing also incrementally increases production efficiency. Separation of crown and skirt also enables the manufacturer to mix and match skirts and crowns to increase assembled part variations. A still further advantage is that this method of fabrication lends itself more to automation than prior techniques. 
     A binder is also used for the threads between the crown and the skirt, which may be pitch, phenolic, or any high temperature resin. 
     Other objects and advantages of the present invention will be more clearly apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sub-assembly of engine components according to the present invention including piston, wrist pin and cylinder sleeve; 
     FIG. 2 is a sub-assembly cross-section of the piston crown illustrated in FIG. 1; 
     FIG. 3 is a right side view of the piston crown illustrated in FIG. 2; 
     FIG. 4 is a left side view of the piston skirt illustrated in FIG. 5; 
     FIG. 5 is a sub-assembly longitudinal section of the piston skirt illustrated in FIGS. 1 and 4; 
     FIG. 6 is a partly fragmented sub-assembly of the piston skirt illustrated in FIGS. 1,  4  and  5 , rotated 90 degrees in two orthogonal planes from the FIG. 5 view; 
     FIG. 7 is a sub-assembly of the piston wrist pin; 
     FIG. 8 is a perspective view of the cylinder sleeve illustrated in FIG. 1; 
     FIG. 9 is an exploded view of the layers in the crown preform; 
     FIG. 10 is an exploded view of a compression mold forming the crown preform; 
     FIG. 11 is an end view of a mandrel upon which the carbon fibers are layered to form the piston skirt and wrist pin; 
     FIG. 12 is a perspective view of the mandrel wrapping illustrated in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings and particularly FIG. 1, a partial piston and cylinder assembly  10  is illustrated including a carbon-carbon cylinder sleeve  11 , and a piston assembly  12  consisting of a carbon-carbon crown  14  threaded into a carbon-carbon skirt  16  carrying a carbon-carbon wrist pin  18 . The crown  14  has a domed upper surface  19 , a groove  20  that define with top surface  21  of the skirt  16 , a groove for a piston ring  23 . A second piston ring  24  is received in annular groove  25  near the top of the skirt  16 . The rings  23  and  24  and the grooves  20  and  25  are positioned close to the top of the piston to reduce blow by and unburned hydrocarbons around the periphery of the piston, made possible by the high thermal conductivity and high heat strength of the crown  14 . 
     The crown has a depending annular flange  27  that is threaded at  28  received in a threaded annular counter bore  30  in the top of the skirt  16 . Skirt  16  has an enlarged through bore  32  to reduce skirt weight. Skirt  16  also has a cross recess  34 , as seen in  66 , in its lower surface that also contributes to weight reduction and port relief in some applications. 
     Skirt  16  has a cross bore  35  there-through for receiving the wrist pin  18 , and it has spaced annular grooves  37  and  38  therein for receiving snap rings  40  for holding the wrist pin in position. Snap rings  40  fit into spaced annular recesses  42  in the wrist pin  18 , as seen in FIG.  7 . 
     The processes for fabricating the sleeve  11 , the piston  12 , and the wrist pin  18 , will be described as follows in conjunction with FIGS. 9,  10 ,  11  and  12 . 
     The preform for the crown  14  is fabricated in a compression mold  50  having an upper pressure platen  51 . In one method to achieve high thermal conductivity and high temperature strength, the carbon fibers are layered with a pitch matrix in mold  50 , as indicated by layers  52 ,  53 ,  54 ,  55 , etc. The fibers in each layer are orthogonally related to the fibers in the adjacent layers. 
     Alternatively, the mold  50  can be filled with chopped fibers in a pitch matrix in any orientation desired. It should be understood that the fibers in the crown preform  56  are generally perpendicular to the axis of the final piston assembly  12 . The fiber orientation in the preform  56  can be tailored for a particular engine; for example, the crown fiber orientation for gasoline engines are positioned for high thermal conductivity and for lower conductivity in diesel applications. The crown preform  56  is heated in a compression molding machine associated with mold  50  to about 600 degrees C. The preform  56  is thereafter cracked by pyrolization in a furnace at 1500 to 2000 degrees C. 
     The crown is then machined from the preform  56  utilizing conventional machining techniques. 
     The piston parts, as well as the liner  11  and the wrist pin  18 , can be coated if desired based on requirements to increase their performance characteristics by a CVD of carbon, SiC, Si 3  N4, or by electrolytic deposition of metals and their compounds such as Ni or Cu or by sol gel and other commercially available coatings. 
     Preforms for the cylinder sleeve  11 , the skirt  16 , and the wrist pin  18 , are formed by wrapping on a mandrel, such as mandrel  60 , illustrated in FIGS. 11 and 12. 
     In one method illustrated in FIGS. 11 and 12, a fiber sheet  61  is directed to the mandrel at +45 degrees while fiber sheet  62  is directed at the mandrel at −45 degrees, and fiber sheet  63  is directed at the mandrel at 0 degrees. The fibers in each layer or sheet run longitudinally in the sheets as shown. 
     Building the sleeve preforms from a bobbin-like mandrel enables a hybrid of fiber additions, such as stabilized PAN, PAN, low temperature pitch or high temperature pitch fiber in such a way to tailor the properties for specific strength, thermal conductivity, thermal expansion, etc. Chopped fibers may also be used with the mandrel technique by applying the chopped fibers uniformly to sheeting. 
     This mandrel process can be done in a machine similar to a lathe so that the sleeves are fabricated uniformly and rapidly with consistent quality. The resulting preform  65  can be preheated if desired on the mandrel  60  to stretch the fibers in the preform, which increases high heat strength for the resulting part. 
     Thereafter, the preforms are placed in a furnace and heated at 600 to 800 degrees C. for the final cure removed from the mandrel  60 . 
     The cylinder sleeve  11  and the skirt  16  have their preforms  65  made in exactly the same way, and are cured for the same time at the same temperature to achieve the optimum matching of piston skirt to cylinder sleeve. This technique minimizes the required piston-cylinder clearance by minimizing differential thermal growth between the piston and the cylinder sleeve. 
     The piston is assembled by threading the crown  14  to the skirt  16  with a binder such as pitch, phenolic or any high temperature resin. These parts are subsequently heat treated in the range of 400 to 1000 degrees C. depending upon the nature of the binder and the requirements. Subsequent to this assembly, the piston is further machined with wrist pin bore  35  and grooves  20 ,  25 . A separate heat treatment of the skirt results in a cost reduction because is is no longer required to be made to the high temperature requirements of the crown. Because the piston is made in two parts, the smaller preforms are simpler and faster to heat treat and CVD, if necessary. Also as noted above, different crowns and different skirts can be interchanged to provide a variety of products having different characteristics for different applications; i.e., a custom assembly system. It is also easier to create combustion bowl and valve relief on the piston crowns because of this fabrication method. 
     Furthermore, the piston rings  23  and  24  can be moved upwardly compared to aluminum piston technology for reduced blow by improved ring pack, and reduced clearance volume. This carbon-carbon assembly enables the engine to run lean, and as a result of the reduced weight, considerable horsepower will be realized with improved thermal efficiency, improved fuel consumption, and reduced emissions.