Pistons for internal combustion engines have been made of such materials as cast iron, steel and aluminum. Most pistons manufactured today are made of aluminum. However, aluminum has low strength and stiffness at elevated temperatures, and aluminum has a large coefficient of thermal expansion. A new piston concept, made of carboncarbon refractory-composite material, has been developed that overcomes a number of the shortcomings of aluminum pistons. Carbon-carbon
material, developed in the early 1960's, is lighter weight than aluminum, has higher strength and stiffness than aluminum and maintains these properties at temperatures over 2500°F. In addition, a low coefficient of thermal expansion and a high thermal conductivity give carbon-carbon material excellent resistance to thermal shock. An effort, called the Advanced Carbon-Carbon Piston Program was started in 1986 to develop and test carboncarbon pistons for use in two-stroke-cycle and fourstroke-cycle engines. The carbon-carbon pistons developed under this program were designed to be replacements for existing aluminum pistons, use standard piston pin assemblies and use standard ring sets. The purpose of the engine tests was to show that pistons made of carbon-carbon material could be successfully operated in a two-stroke-cycle engine and a four-stroke-cycle engine.
Carbon-carbon pistons can potentially enable engines to be more reliable, to be more efficient (lower hydrocarbon emissions, greater fuel efficiency), and to have greater power output. By utilizing the unique characteristics of carbon-carbon material -- very low expansion rate, low weight, high strength and stiffness at elevated temperatures, and high thermal conductivity -- a carbon-carbon piston can (1) have greater resistance to structural damage caused by overheating, lean air-fuel mixture conditions and detonation, (2) be designed to be lighter-weight than an aluminum piston thus, reducing the reciprocating mass of an engine, and (3) be operated in a higher combustion temperature environment without failure.