Internal-combustion engines – Particular piston and enclosing cylinder construction – Piston
Reexamination Certificate
2001-02-16
2003-08-12
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Particular piston and enclosing cylinder construction
Piston
Reexamination Certificate
active
06604501
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a piston of finest grain carbon and a method for producing a piston blank and a polyaromatic mesophase powder for producing a piston.
Special aluminum alloys have been used up to now as material for pistons in combustion engines. Disadvantageously, they have relatively large specific mass, require high production accuracy, and nevertheless exhibit high frictional loss.
Carbon pistons have been proposed to obtain pistons having a small specific mass for facilitating mass compensation and reducing the frictional loss. Moreover, exhaust gas contaminants should be reduced.
The development of carbon materials for such combustion engines demanded not only improved mechanical properties but also special thermo-physical properties, in particular high heat conductivity. The reason for the latter requirement is that combustion processes in the cylinder can result in so-called “knocking” in consequence of overheating. Pistons of aluminum alloys have heat conductivities of approximately 140 to 160 W/mK. The development of carbon materials to replace pistons made from aluminum alloys necessarily requires a heat conductivity of at least 60 W/mK. The minimum requirements with respect to bending strength are values of more than 120 MPa in connection with a Weibull parameter of more than 20.
As is known from the technology of producing carbon or graphite materials, the demands for a higher bending strength contradict those for high thermal conductivity. The latter is achieved only through high temperature treatment at temperatures in excess of 2500° C. With such high temperatures, re-crystallization of the graphite matrix considerably impairs the mechanical properties, such as the bending strength.
Carbon or graphite materials have been produced by mixing, compacting and subsequently carbonizing grained carbon materials (such as coke, carbon black or graphite) with a binding agent, usually a thermoplastic resin. To obtain graphitic material, high temperature treatment up to a temperature range of more than 2500° C. follows. This has the above-mentioned disadvantages. DE 30 34 359 C2 proposes the production of carbon materials by pulverizing coke, forming with the addition of binder resin, baking of the shaped body in a first baking stage at 450-700° C., re-impregnating the baked material with resin following an obligatory previous cooling, and subsequently baking the impregnated material in a second baking stage at at least 1000° C. for carbonization, wherein a graphitizing step may follow at a temperature of up to 3000° C.
DE 196 28 965 C2 discloses a method for the manufacture of a hollow tubular body made from carbon having high density, high strength and high heat conductivity. A green body is pressed, carbonized and subsequently graphitized. A self-blocking fine carbon powder, without binder, (preferably a carbon mesophase) having a powder density in accordance with DIN 51 913 of about 1 g/cm
3
and with an average grain size between 5 and 20 hydrometer is thereby pre-compressed. The pre-compressed powder is pressed under a pressure of between 50 and 150 MPa about a rigid plunger to generate the hollow green body. The pressure is subsequently reduced steadily at a rate between 0.19 and 6 MPa/Min. For carbonization, the green body is then initially heated in an environment at a rate of 25 K/Min. to a temperature of 200° C. and subsequently up to a holding temperature between 500 and 700° C. at an extremely slow heating speed between 0.05 to 0.5 K/Min. The holding temperature is maintained for a certain holding time. The temperature is then increased to a carbonizing temperature between 800 and 1200° C. at a rate of 0.05 to 1 K/Min. and likewise held at this temperature. The carbon body is subsequently heated to a graphitizing temperature between 2000 and 3000° C. in an inert atmosphere. Although the procedure may be useful for its intended purpose of producing head-shaped hollow bodies, it is not applicable to pistons of motor vehicles. In particular the publication provides only little information concerning strength values resulting from the procedure. No bending values are indicated. The results may be adequate for a container, but certainly not for a motor vehicle piston.
The known method is extremely demanding due to the mixing requirement and, in particular, due to the impregnation with resin, since this requires intermediate cooling after the first baking stage. The extremely long impregnation and heating up times are also inefficient, the latter taking up to several days. The carbon material obtained is not intended for pistons and is not suitable therefor, since the bending strength is far below that which is required.
DE 44 37 558 A1 also describes production of graphite by mixing coke with a resin binder. The disclosed process is also extremely demanding. The coke powder used has an average particle size of 1 &mgr;m (which is actually irrelevant from a technical point of view). Mixing with resin must be effected through kneading under increased pressure and the mixture must be cooled down and re-powdered to an average particle size of 4 &mgr;m. Compacting of this powder is not possible at an earlier stage. Due to the high final treatment temperature of 2800° C., this material most certainly has a high heat conductivity on the order of 60 W/mK, although this is not stated.
Furthermore, the use of carbon fiber reinforced carbon (CFC) has been proposed (e.g. WO97/32814 A1). Such materials are extremely expensive due to the carbon fibers and also due to the high production costs per se, as the matrix is usually formed by inside pore separation from the gaseous phase. These materials are thus not suited for an economical production of pistons which can compete with aluminum pistons. The behavior during use is also not known.
The production of carbon materials on the basis of polyaromatic mesophase has also been suggested.
Wolf, R. et al, “Development of Binderless Carbon-Mesophase for Production of High Strength Graphites” (Mater.; Funct.Des.; Proc. Eur. Conf. Adv. Processes Appl., 5
th
(1997), volume 2, 2/341-2/344. Editor(s): Sarton, L. A.; Zeedijk, H. B., Publisher: Netherlands Society for Materials Science, Zwijndrecht, Netherlands) describe carbon materials having a bending strength between 75 and 125 MPa and thermal conductivities of 45 to 60 W/mK as well as 15 W/mK. The materials, having a thermal conductivity of 45 and 60 W/mK, are proposed for use as pistons in combustion engines. They have, however, low bending elongation of e.g. 0.625%, (estimated from the bending strength and the elasticity modulus using Hooke's Law).
Mörgentaler, K. D. “Die Entwicklung einer Technologie für die konturnahe Herstellung von Kolben für Verbrennungsmotoren aus hochfesten Feinstkornkohlenstoffen” (The Development of a Technology for Close-Tolerance Production of Pistons for Combustion Engines of Highly Rigid Finest Grain Carbon) (Werst. Verkehrstech., Editor(s): U. Koch, Publisher: DGM Informationsgesellschaft, Oberursel Symp. 2, Werkstoffwoche '96 (1997) Meeting Date 1996, 67-72) discloses a raw material named CARBOSINT which allegedly has properties similar to those of a polyaromatic mesophase powder. This raw material leads, however, to extremely hard and brittle carbon material. The extensive hardness requires extremely demanding processing such that these carbons are poorly suited for the mass production of pistons which are in any event, unacceptably brittle. CARBOSINT is intended to have a portion of toluene-insoluble components (TI) of 97% and a portion of quinoline-insoluble components (QI) of 57%. Thus, the difference between the toluene-insoluble components and the quinoline-insoluble components is 40%. After sintering, small bodies show a strength between 181 and 197 MPa in the 3 point bending strength test. Isostatically pressed large bodies having a size up to 90×90×110 mm show a strength in the 3 point bending strength test of 148 to 152 MPa after graphitization, wherein processing times of 3 months are r
Goetz Ulrich
Hegermann Rainer
McMahon Marguerite
Sintec Keramik GmbH & Co. KG
Vincent Paul
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