Internal-combustion engines – Valve – Reciprocating valve
Reexamination Certificate
2000-09-29
2002-04-02
Kamen, Noah P. (Department: 3747)
Internal-combustion engines
Valve
Reciprocating valve
C264S624000, C264S625000, C264S626000, C264S628000
Reexamination Certificate
active
06363902
ABSTRACT:
BACKGROUND
1. Technical Field
This invention relates to intake and exhaust valves for an internal combustion (IC) engine, and more particularly, to such valves formed of a fiber reinforced ceramic matrix composite (FRCMC) material and methods for making them.
2. Background Art
In a typical four-stroke gasoline-powered internal combustion (IC) engine, an intake valve is designed to open during the intake stroke of the engine to allow a fuel mixture to enter the engine cylinder. Once the intake stroke is complete, the intake valve closes to seal the engine cylinders so that the fuel mixture can be compressed during a compression stroke of the engine. Upon completion of the compression stroke, the fuel mixture is ignited causing an expansion of gases which push the piston down in the cylinder. At this point an exhaust valves opens to allow exhaust gases to be expelled from the cylinder as the piston moves up in the cylinder (i.e. the exhaust stroke). Other types of IC engines work similarly, some having only one valve per engine which acts as both the intake and exhaust valve, while others have multiple intake and exhaust valves per cylinder. The basic structure of a valve, as shown in
FIG. 1
, includes a stem
12
and a head
14
. The head
14
has a bottom surface
16
which is directed toward the interior of the cylinder of an IC engine when the valve is installed therein. In addition, the head
14
of the valve has tapered side surface
18
which interfaces with the valve seats of the engine in order to form the aforementioned seal when the valve is closed.
The valves in any type of IC engine operate in a violent environment characterized by extreme operating temperatures and temperature variations, excessive gas pressures, corrosive fuel components, and intense hammering caused by the opening and closing of the valves. As such valves need to be very tough and durable. Most valves are made of metal, often involving complex multi-part constructions and exotic alloys. For example the head of the valve could be made of a high temperature resistant alloy to withstand the temperatures found inside the cylinder, whereas the stem could be made of a alloy which is stiffer and provides good bearing qualities. These characteristics are desirable as the stem resides within a valve guide which assists in aligning the valve and sealing the lower portion of the valve and engine cylinder from oil in the upper part of the engine. Thus, the stem must withstand the reciprocal motion between it and the valve guide. Valves also often have coatings or hollow portions filled with heat dissipating salts for the same reasons.
Although metal valves can be made more durable by the use of various alloys, coatings, etc., they still have limits as to the temperatures which they can withstand. However, the performance and fuel economy of an IC engine can be improved by increasing the temperature of the combustion chamber (i.e. the cylinder) beyond the limits of metal valves. This improvement in performance and fuel economy results because the higher chamber temperatures cause a more complete burning of the fuel. Therefore, more energy is released and less fuel is required to drive the engine. However, because of the temperature limitations of metal valves, operating at these higher temperatures has not been possible because, among other things, the valves would fail (which is often termed “burning the valves”). Typically, the valves fail because the increased temperatures cause the valves to expand to a degree that they no longer form a seal with the associated valve seat. This mismatch occurs because the head of an IC engine, which includes the valve seats, is cooled, typically by water or cooling fluid circulating through channels in the head. As the head is cooled, the valve seats do not tend to expand significantly while the engine is operating. However, the intake and exhaust valves of the engine are not cooled like the valve seats. As a result, the valves expand as the engine heats up. The tapered surface of the head of a valve which interfaces with the valves seats is often specially ground by an expensive and complex process referred to as a triple grind. The special grinding ensures the valve head will seal with the valve seat even though it expands as the engine heats up. However, if the temperatures become too great, even special grinding cannot accommodate the expansion of the valve head. If the valve head expands to the point that it not longer seats into the valve seat, the valve and the valve seat can become physically damaged by the mismatch, and hot exhaust gases can leak through any gaps formed between the two structures causing localized burning of the metal forming the valves and valve seats. The resulting damage can increase pollutants emitted by the engine, reduce engine performance, or even cause the engine to fail completely.
One attempt to resolve the problems associated with conventional metal valves has been to make them from a monolithic ceramic material. For example, valves formed of silicon nitride are commercially available. Monolithic ceramic valves have the advantage of being extremely resistant to damage by heat in that ceramic material has a low thermal conductivity and will not readily absorb heat. In addition, ceramic materials are thermally stable in that they exhibit a low coefficient of thermal expansion and so do not expand significantly as the temperature increases. Thus, even at higher engine operating temperatures, ceramic valves will not expand to an extent which would jeopardize their sealing with the valve seats. As a result there is no damage or burning of these structures. Additionally, monolithic ceramic valves made of materials such as silicon nitride tend to be hard, while at the same time having external surfaces which exhibit a low coefficient of friction (i.e. slipperiness). The hardness of the ceramic material is advantageous as it makes the bottom face of the head portion of the valve resistant to the violent environment of the cylinder of the engine. The low coefficient of friction or slipperiness of the material is advantageous for two reasons. First, the slipperiness of the surface facilitates the sliding of the tapered surface of the valve head in and out of the valve seat without causing any abrasive grinding between the interfacing surfaces or a valve sticking condition. In addition, the stem of the valve which resides within the aforementioned valve guide would advantageously have a slippery surface. For example, the slipperiness of a valve made from a silicon nitride ceramic material results in the valve stem sliding easily within the valve guide. As such the stem and the valve guide will not be damaged which could otherwise cause a misalignment of the valve and/or oil to drip down the valve stem from the upper part of the engine.
Ceramic valves also have another advantage in that they weigh less than metal valves. The weight and size of the entire valve train is effected by the weight of the valves. Lighter valves allow the use of a smaller, lighter valve springs, which in turn means the camshaft does not have to be as stiff. Thus, a smaller, lighter camshaft can be employed. In addition, the rocker arms and push rods (if employed) can be smaller and lighter as they do not have as much weight to push around. Thus, the entire valve train can be made lighter by employing lighter valves. Reducing the weight of the valve train makes for a lighter, more efficient engine.
However, monolithic ceramic valves present unique problems of their own. Monolithic ceramic structures tend to be porous and brittle, and extremely difficult to form without structural flaws. These structural flaws make the material subject to cracking. Thus, the monolithic ceramic valve is susceptible to catastrophic failure when impacted, or otherwise subjected to even moderate forces. They are also strain intolerant and cannot be deflected more than 0.1 percent without being fractured. These are very undesirable characteristics for a moving part such as a intake/exhaust val
Atmur Steven Donald
Strasser Thomas Edward
Anderson Terry J.
Hoch, Jr. Karl J.
Kamen Noah P.
Northrop Grumman Corporation
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