« PreviousContinue »
engine performance. Issues such as oxidation life, oil, and fuel infiltration effects and cyclic endurance require continued study, but the prognosis for CC pistons is very promising. An illustration of a CC piston together with an aluminum counterpart is shown in figure 7. Carbon-carbon pistons for experimental motorcycle engines were also developed for Harley-Davidson vehicles with the CC fabrication carried out at HITCO.
In another engine application, CC is being evaluated for use on rotary valves in two- and four-stroke-cycle small engines. The CC rotary valve drives off the crankshaft and, as a replacement for the conventional poppet valve, it offers weight savings, quieter engine operation, and a reduced torque load. As in the CC piston, the achievable low thermal expansion and good strength retention at high temperatures allow the close tolerances needed for the rotary valve function. Development work in this area is being conducted at the Nematec Company in Westland, Michigan.
A very promising area for CC engine components is in diesel engines. Potential benefits that could be derived from CC components have been identified in both adiabatic and low-heat rejection designs. The adiabatic diesel engine is based upon the adiabatic or no-heat-loss process. Essentially, this process involves an insulated combustion chamber from which the high-temperature exhaust gases are passed, first through an insulated duct system and then through turbine wheels, to extract additional energy before finally being expelled. The turbines power the compressor and the crankshaft. A basic diesel engine has approximately 36 percent thermal efficiency and the adiabatic diesel has approximately 48 percent thermal efficiency; this thermal measurement is equivalent to a turbine inlet temperature of over 4000°F (2205°C) for a gas turbine engine.
The turbocharger and pistons are CC components. The maximum temperature anticipated, with advanced materials, is approximately 2000°F (1093°C) compared with current maximum diesel temperatures of approximately 600°F (316°C). High-temperature resistant materials such as CC would permit reduced airflow and reduced engine volume. Thermal shockresistant material is required and, because reduced heat transfer is a key consideration, low thermal conductivity CC or shield materials also may be required.
Other diesel-engine designers have considered CC to be a viable material for use on advanced engines but have emphasized a low-heatrejection diesel design rather than an adiabatic diesel design. A low-heatrejection diesel engine retains heat within the engine and is a passive means for improving efficiency. The temperatures of interest are on the order of 1600°F (871°C). The low-heat-rejection engine would offer a thermal efficiency of 45 percent; this efficiency is achieved with less complexity than would be required for the adiabatic diesel engine to achieve an anticipated 48 percent thermal efficiency.
Components that would benefit from the use of CC would be valves, piston crowns, and, possibly, cylinder liners. Another interesting application would be on the foredeck of the diesel engine, which is currently cast iron. The CC composition and constructions would have to be carefully designed because of the necessity for a low thermal expansion material. The chief benefits of CC are the weight savings and the efficiency accrual that are the result of high-temperature operations.
The majority of the uses of CC to date involve relatively low-volume production (i.e., Space Shuttle and rocket motor components) in which performance requirements outweigh production efficiencies. One wellknown use of CC which involves high production, however, is in aircraft wheel brake sets. The B.F. Goodrich Company and HITCO are two of the major producers of these disk-like components. The CC brakes consist of either laminated fabric constructions or chopped carbon fabric moldings. The CC brake rotors weigh 20 percent less than comparable steel parts and, because of the higher temperature capabilities of CC (2.5 times the heat capacity of steel), durability is increased, which permits up to 3000 jet aircraft landings compared with approximately 1500 landings for metal rotors (ref. 14). On the Boeing 767 aircraft, using CC yields a weight savings of 871 lb over steel brake systems (ref. 15). The market for CC brakes has also expanded and includes automotive brakes for highperformance cars such as racers as well as CC clutch assemblies. A CC clutch assembly and brake components, which HITCO fabricated, are illustrated in figure 8. The potential for significant growth in these areas is further reinforced by the interest shown by domestic and foreign passenger car manufacturers in utilizing CC in their advanced automotive braking systems.
The aerospace field continues to be one of the primary areas for use of CC. In addition to the Space Shuttle that was discussed previously, CC has been successfully used in solid-propellant rocket motor nozzles and exit cones, and as ablative nosetips and heat shields for reentry vehicles. Solid rocket motor gas temperatures on the order of 5400°F (3000°C), coupled with near-isotropic thermomechanical loads, have made 3-D CC an attractive material for nozzle throat and exit areas. While 2-D CC exit
A relatively new application of CC that is currently being explored is its use as a structural material for advanced spacecraft. Items for CC applications might include thin-wall tubes, angles, and panels for use as booms, trusses, equipment support mounts, and thermal management components. This new interest in CC use is based on the demonstrated fabrication of thin-ply CC shapes and the use of the newer high-modulus graphite (E = 100 + msi) fibers as reinforcement materials. Additionally, the high-temperature capabilities of CC coupled with the dimensional stability achievable through the use of high-modulus, negative-thermal expansion fibers make it one of the most resistant materials to high thermal pulse environments. For many space applications, CC does not outgas; therefore, CC does not present a contamination problem for sensitive optical surfaces (ref. 16).
A related application was demonstrated, based on achievable dimensional stability and high electrical conductivity. Through a cooperative effort, the Aerospace Corporation, Ford Aerospace Corporation,* and HITCO designed and fabricated a parabolic radio frequency (RF) antenna reflector for satellite communications. This component (fig. 10) was shown at the 33rd International Symposium of the Society for the Advancement of Material and Process Engineering.
Key properties of CC, such as good strength retention at temperatures approaching 5000°F (2760°C), low density, dimensional stability, and complex shape fabricability, make CC an appropriate material for a variety of evolving applications. The use of RF heating in the Tokomak Fusion Reactor requires RF limiters to act as plasma shields to protect the RF launchers. Earlier limiters were made of Poco or ATJ graphite tiles fastened to a water-cooled Inconel structure. Substitution of CC limiters
Now called Space Systems/Loral.