There is an ongoing effort to develop internal combustion engine parts, such as valves, pistons and connecting rods, that are light-weight and that enhance engine performance at high temperatures. One such effort is in the reduction of the weight of engine parts that are constrained in reciprocating movements within an internal combustion engine.
Inlet and exhaust valves for internal combustion engines are currently made from steel and other metals. Valves made from metals are heavy and exhibit poor structural properties at high temperatures. Due to their weight, metallic valves acquire enormous inertial loads as engine speed increases. The enormous inertial loads adversely effects the amount of power the engine can produce, by limiting the engine's capability to reach higher revolutions per minute (RPM). In addition, high levels of friction are created by the traditional metallic valves. The heavy weight of metallic valves, high levels of friction associated with metal valves and the temperature limitations of metallic valves result in an overall decrease in the efficiency of internal combustion engines at high temperatures.
In response to the problems inherent to the use of metallic valves, those skilled in the art have sought to replace the traditional metallic valves with valves made from light-weight materials, that can operate reliably and increase internal combustion engine performance at extremely high temperatures.
One such replacement for metallic valves are valves made from titanium or titanium aluminide. Although valves made from these materials are light-weight and can operate reliably at high temperatures, the costs of such materials has limited their use in the industry.
Another non-metallic material proposed as a replacement for metallic valves are ceramic materials. Although valves made from ceramic materials are lighter than metallic valves and can operate at high temperatures, they have 30 to 50% the density of the traditional metallic valves, have proven to be brittle and are subject to catastrophic failure at such extreme temperatures.
To improve the brittle nature of ceramic valves, Berneburg et al., U.S. Pat. No. 4,928,645, disclosed a ceramic valve with a reinforcing woven carbon fiber sleeve. The ceramic valve disclosed by Berneburg et al. includes (a) an elongated valve stem comprising fibrous ceramic sleeving which is packed with an axially aligned unidirectional cluster of ceramic reinforcing fibers; and (b) a ceramic head containing fibers which is molded onto the valve stem. The use of discontinuous ceramic fibers from the valve stem to head results in an inherently weak valve structure. The discontinuous fibers offer low stiffness and their distribution can vary from valve to valve in production, with low resin pockets yielding voids during carbonization of the phenolic precursor.
Another possible replacement for metallic valves is disclosed by NASA, PCT/US97/03965, which teaches a carbon fiber reinforced carbon composite valve for internal combustion engine comprising a valve stem and head. The valve includes braided carbon fiber material over axially aligned unidirectional carbon fibers, forming a valve stem. The braided and unidirectional carbon fiber are "broomed" out at one end of the valve stem to form the valve head. The valve structure is subsequently densified with a matrix of discontinuous carbon fibers. This type of valve construction results in poor structural strength for the simple reason that as the fibers transition from the valve stem, nominally 6 mm in diameter, to the valve head, nominally 40 mm in diameter, the fibers have to transition from a smaller volume to a larger one. As a result, the fiber volume in the valve head is decreased by a factor of 44. The biaxial strength is decreased dramatically and is essentially that of the matrix in these regions. Such a dramatic decrease in the fiber volume results in a negligible reinforcement of the valve seat.
Therefore, it is desirable to develop a light-weight composite valve structure for internal combustion engines that is capable of reliable operation at extremely high temperatures, and that allows the engine to obtain higher revolutions per minute (RPM), thus improving the overall efficiency of the engine. It is further desirable to develop a light-weight composite valve structure for internal combustion engines that are more reliable at high temperatures than the carbon fiber reinforced ceramic valves disclosed in the prior art.