This application claims priority to Japanese patent application serial number 2009-292502, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present teaching relates to engine valves made of heat-resistant alloy and having a valve head portion that is hot-forged in continuity with one end of a shaft portion.
2. Description of the Related Art
A process of manufacturing this kind of poppet-type engine valve is hereinafter described. As shown in
When the heat resistant alloy is hot-forged, the base material is oxidized to form an oxidized layer on a surface layer portion of the valve intermediate body. The oxidized layer has a favorable heat insulation property. Therefore, in the case of an engine valve that is exposed to a high temperature environment and receives a high thermal load, it is preferable that the oxidized layer is left without being removed. Due to the heat insulation property of the oxidized layer formed in the surface layer portion, the amount of heat transmitted into the engine valve is reduced and increase of temperature of the engine valve is suppressed. Especially, the heat insulation effect in the head portion is significant because the head portion is easily heated to have a high temperature as it is directly opposed to a combustion chamber of the engine.
However, in the case of Japanese Laid-Open Patent Publication Nos. 3-86457 and 4-8402, the burr produced around the outer peripheral edge of the head portion is removed by a machining operation. Therefore, even if an oxidized layer 101b is formed at the surface layer of the base material 101a by the heat-forging process, it may be removed at an outer peripheral surface 103d of the head portion 103 by the machining operation as shown in
In order to avoid the pre-ignition, an oxidized layer may be formed again on the outer periphery surface (machined surface) after the burr has been removed from the head portion of the engine valve as disclosed in Japanese Laid-Open Patent Publication Nos. 3-86457 and 4-8402. However, the re-forming process is troublesome and leads to increase of the cost.
Therefore, there is a need in the art for an engine valve that is difficult to be heated due to a high heat insulation property of an oxidized layer formed on an outer peripheral surface of a head portion.
An engine valve according to the present teaching has a hot-forged head portion having an oxidized surface layer at a radially outer peripheral surface. The oxidized surface layer is formed during hot-forging of the head portion.
a) and 2(b) are views showing manufacturing steps of the engine valve;
a) and 3(b) are views showing manufacturing steps of a known engine valve; and
Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved engine valves. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful embodiments of the present teachings.
In one example, an engine valve includes a shaft portion and a head portion each made of heat resistant alloy. The head portion is disposed at one end of the shaft portion in continuity therewith and hot-forged without forming any burr at a radially outer peripheral surface (marginal surface) of the head portion. The outer peripheral surface of the head portion is not machined so that the outer peripheral surface of the head portion has an oxidized surface layer formed by oxidization of the heat-resistant alloy. The valve face of the head portion contacting the valve seat may be ground for removing the oxidized surface layer if necessary or desired. The radially outer peripheral surface of the head portion is a surface extending between the flat head surface for facing to) a combustion chamber and the valve face for contacting the valve seat.
If no burr is formed when the head portion is forged, the machining operation such as a cutting or grinding operation to remove the burr is not required. Therefore, the radially outer peripheral surface of the head portion is remained as it is forged. More specifically, the oxidized layer formed by oxidization of the surface layer of the base material during the hot-forging process is not necessary to be removed by the machining operation. In this way, the oxidized layer may be maintained on the outer periphery surface of the head portion. Because the oxidized layer having high heat insulation property is remained also at the outer peripheral surface of the head portion, increase of temperature of the head portion, which is easily heated to a high temperature, can be prevented. Accordingly, it is possible to lower the temperature of the engine valve (especially the head portion) during operation of the engine compared to the known techniques as disclosed in Japanese Laid-Open Patent Publication Nos. 3-86457 and 4-8402. In this way, a critical region for occurrence of pre-ignition is improved. If the critical region for occurrence of pre-ignition is improved, combustion of the fuel mixture within the engine is stabilized. Accordingly, an engine output and fuel consumption can also be improved by increasing the compression ratio of the fuel mixture. Further, because the oxidized layer formed during the hot-forging process is simply maintained, it is not necessary to form the oxidized layer again after the machining operation or it is not necessary to form a heat insulation layer separately. Accordingly, the engine valve can be manufactured efficiently and at a lower cost.
Preferably, the outer peripheral surface of the head portion has a curved configuration. Because a high heat insulation property is ensured, it may be possible that the outer peripheral surface is pointed. However, if the surface is curved, occurrence of the pre-ignition of the air-fuel mixture may be more effectively prevented compared to the pointed surface As a heat-resistant alloy, a heat-resistant steel may be used.
An example of an engine valve of the present teaching is explained below with reference to the drawings.
As shown in
The engine valve 1 is made of heat-resistant alloy. For example, heat-resistant alloy may be heat-resistant steel, such as SUH3, SUH11 or SUH35 (in JIS classification). Alternatively, titanium alloy may be used as the heat-resistant alloy. The head portion 3 includes a flat head surface 3a, a valve face 3b and a neck portion 3c. The flat head surface 3a may face to the combustion chamber. The valve face 3b can contact the valve seat in order to close the intake port or the exhaust port of the combustion chamber. The neck portion 3c is formed in series from the face surface 3b to a shaft portion 2. The diameter of the neck portion 3c is gradually reduced towards the shaft portion 2. The head surface 3a and the face surface 3b are connected to each other via a radially outer peripheral surface 3d of the head portion 3. The outer peripheral surface 3d of the head portion 3 is configured as a curved surface as it is not machined. The engine valve 1 has an oxidized surface layer 1b formed throughout on its outer surface. The oxidized surface layer 1b is formed due to oxidization of a base material 1a of the engine valve 1. In particular, the outer peripheral surface 3d of the head portion 3 is covered with the oxidized layer 1b.
A manufacturing process of the engine valve 1 will be explained below with reference to
The hot-forging process of the head portion primary body 5 may be made at a temperature of 800° C. or more. The heat may only be applied to the head portion primary body 5 to be plastically deformed. Due to the hot-forging of the head portion primary body 5, the oxidized layer 1b is formed on the surface of the engine valve 1 as a result of oxidization of the base material 1a (e.g., heat resistant steel). In the present teaching, the burr is not formed at the radially outer peripheral surface 3d of the head portion 3 after the hot-forging process. Accordingly, it is not necessary to remove the burr from the outer peripheral surface 3d of the head portion 3 by a machining operation, such as grinding or cutting. In this way, the engine valve 1 having the oxidized layer 1b formed by the hot-forging process also at the outer peripheral surface 3d of the head portion 3 may be obtained. Because the oxidized layer 1b having a high heat insulation property is formed at the outer peripheral surface 3d of the head portion 3, any heat that may be applied to the outer peripheral surface 3d of the head portion 3 during the operation of the engine can be effectively insulated. Therefore, increase in temperature of the head portion 3 is also well prevented compared to the known engine valve. Accordingly, it is possible to improve a critical region for occurrence of pre-ignition.
The valve face 3b may be ground if necessary or desired. In addition to the oxidized layer 1b, another kind of heat insulation layer (a heat insulation film) may be formed on the surface of the engine valve 1. A nitriding treatment such as a soft-nitriding treatment in a salt bath and a gas soft-nitriding treatment may also be applied to the surface of the engine valve 1. As a material of an additional heat insulation layer, ceramic series oxide, such as alumina, cordierite, zirconia, zircon, oxidized titanium and magnesia; ceramic series carbide, such as silicon carbide; and ceramic series nitride, such as silicon nitride may be used. In addition to these materials, ceramic series materials having high heat resistance and heat insulation property, such as aluminum silicate, oxidized chrome, WC/Co alloy, WC/Ni/W/Cr3C2 alloy, and Cr3C2/Ni—Cr alloy are also appropriate to be used for the heat insulation layer. The heat insulation layer could be used as a single layer or a plurality of heat insulation layers made of different materials maybe layered. The heat insulation layer may be formed by using a suitable technique, such as gas flaring, arc spraying, plasma spraying, blast spraying, sputtering and ion plating techniques. Furthermore, it may be desirable to anneal the heat insulation layer at a low temperature, for instance, approximately at 400° C. for removing residual stress generated during the forging process.
Number | Date | Country | Kind |
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2009-292502 | Dec 2009 | JP | national |