Internal-combustion engines with ringless carbon pistons
Efficiencies would be higher and weights lower than those of conventional engines.
Internal-combustion engines would be constructed with cylinders and ringless pistons made of lightweight carbon/carbon composite materials, according to a proposal. This proposal is a logical extension of previous research that showed that engines that contain carbon/carbon pistons with conventional metal piston rings running in conventional metal cylinders perform better than do engines with conventional aluminum-alloy pistons. The observed performance improvement (measured as increased piston life during high-performance operation) can be attributed mainly to the low thermal expansion of the carbon-carbon composite. Carboncarbon pistons can continue to operate under thermal loads that cause aluminum pistons to seize or sustain scuffing damage due to excessive thermal growth and thermal distortion.
In addition to having an extremely low coefficient of thermal expansion, carboncarbon is about 30 percent lighter than aluminum which provides the benefit of reduced reciprocating mass (lower reciprocating mass can potentially reduce vibration forces and increase r/min. capability). Carbon-carbon composite also has the advantage over aluminum that it fully retains room-temperature strength and stiffness at high temperatures. Furthermore, the strength, thermal expansion, and thermal conductivity of carbon-carbon composites can be tailored by orientation of the carbon fibers and selection of fiber type, matrix type, and processing methods.
The rings are needed on aluminum pistons to seal the clearance which must exist between the piston and cylinder wall to accommodate differential thermal expansions of the piston and cylinder material (conventionally, a cast iron sleeve in an aluminum block). Although cold-clearance can be reduced somewhat by substituting a carbon-carbon piston, rings will still be needed to obtain effective sealing. An advantage is potentially achievable in a four-stroke engine because a tighter piston fit reduces the so-called “crevice volume” or the gap between the piston and the cylinder wall above the top ring. Fuel mixture which enters this gap is not combusted and is exhausted as unburned hydrocarbon. If the metal block were to be fitted with a carbon-carbon sleeve, the cold clearance could be further reduced, but minimum clearance might be difficult to achieve because the sleeve shape could be offected by thermally-induced distortions in the surrounding metal block (there are also issues as to how the sleeve might be contained in the block). If, on the other hand, the metal cylinder block and sleeve were to be replaced with a cylinder block made entirely of carboncarbon, the thermal expansion differential between the piston and cylinder materials would virtually be eliminated, as would the potential for thermal distortion of either component. The clearance could then be reduced to the absolute minimum. Operation without rings, which would eliminate a source of power-robbing friction, can now be considered an intriguing possibility. Rings may ultimately be required in the fourstroke application to minimize combustion-gas blow-by and/or control oil consumption; however, the crevice volume, which is a major cause of hydrocarbon emissions, would be eliminated over the engine’s entire operating temperature range and ring performance could potentially be improved because of less piston rocking in the bore. Ringless operation would appear to be particularly attractive for high-r/min two-stroke engines where oil-wiper rings are not required and relatively more blow-by may be tolerable.
For simplicity, the figure illustrates a one-cylinder, air-cooled, two-stroke internal-combustion engine that might be built according to this concept (multicylinder and four-stroke engines are also possible). The cylinder barrel would be made of carbon-carbon composite sandwiched between an aircooled metal head and a metal crankcase. This assembly would be held together by long head bolts, which would pass through the head and through (or alongside) the carbon/carbon cylinder barrel into threaded holes in the crankcase. The carbon/carbon cylinder barrel could be sealed to the crankcase with an O-ring and to the head with a head gasket.
The cylinder block could be fabricated with one or more of many possible configurations of fibers in the carbon/carbon material. The simplest and most economical configuration would be a stack of plies in which all fibers are aligned perpendicular to the axis of the cylinder bore. The inherently low interlaminar strength of the carbon/carbon block would not be a major concern because the clamping force applied by the head bolts would negate cross-ply tensile stresses in the laminate. In principle, this configuration could likely be chosen to maintain the close-tolerance piston/cylinder clearance because it would exploit two features of carbon fibers that are very attractive in this application: high lengthwise thermal conductivity (for some fibers, greater than that of copper) and nearly zero lengthwise thermal expansion. This configuration would minimize thermal expansion of the cylinder bore while maximizing the outward conduction of heat through the cylinder barrel to the ambient air. In practice, some circumferentially oriented fibers would also be needed to provide reinforcement against hoop stresses, but the proportion of such fibers should be minimized.
Fabrication of the cylinder barrel could begin with stacking the plies in a mold that could include an inner mold die roughly the size of the cylinder bore. Alternatively, the cylinder bore could be machined somewhat undersize prior to carbonization. In either case, the initial formation of the bore would expose the inner edges of all the plies to impregnating materials, which would be applied during densification steps. Eventually, the cylinder bore would be machined to near the final diameter, then the inner surface of the cylinder would be treated in sealing and coating processes to reduce friction and protect against oxidation. The cylinder would then be honed to its final diameter.
Langley Research Center, Hampton, Virginia
This work was done by Philip O. Ransone of Langley Research Center. No further documentation is available.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Langley Research Center; (757) 864-3521. Refer to LAR-15094.
Copyright Associated Business Publications Aug 2002
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