1. Field of the Invention
The present invention relates to a variable valve device for an internal combustion engine, which controls a valve.
2. Description of the Related Art
A reciprocating engine mounted in an automobile is an example of an internal combustion engine. In some reciprocating engines, in order to achieve better mileage, the engine can be run in a cylinder-suspension mode in which some of the cylinders are suspended under driving conditions wherein a large output is not required.
In the running in the cylinder-suspension mode, frequent lifts of inlet and exhaust valves of the cylinder are stopped with variable valve devices to decrease pumping loss. That is, opening and closing of the valve are stopped.
Unlike the inlet valve, there are few demands for the exhaust valve that a valve lift amount and open-close timing are precisely controlled according to a running state of the engine. Therefore, in order to simplify the structure of a rocker shaft for stopping the exhaust valve lift, a rocker arm incorporated into the rocker shaft is divided into a cain follower rocker which follows a cam and a valve drive rocker which drives the valve. A displacement of the cam follower rocker is transmitted to or cut off from the valve drive rocker through a switching portion (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2005-90408).
When the cam follower rocker and the valve drive rocker are connected to each other with the switching portion, the displacement of the exhaust cam is transmitted from the cam follower rocker to the exhaust valve through the valve drive rocker. When the cam follower rocker and the valve drive rocker are separated from each other by the switching portion, the cam follower rocker strikes the air for the valve drive rocker. Therefore, the displacement of the exhaust cam is not transmitted to the valve drive rocker.
Usually the two exhaust valves are tend to be used to enhance exhaust performance of the burned gas. Therefore, frequently a front-end side of an arm is branched in the valve drive rocker of the exhaust valve, and front-end portions of the branched arm are arranged in an upper end portion of the valve. This enables the two exhaust valves to be simultaneously driven (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2005-90408).
In the divided-type rocker arm structure, frequently a body is formed by casting with an iron-based metal material such as steel because strength is increased.
Because the as-prepared iron-based metal material such as the steel has difficulty in the strength, usually machining is carried out to secure accuracy after quenching is carried out as heat treatment. Specifically, because the iron-based metal material for which the quenching is carried out has high hardness, the machining is carried out on the iron-based metal material not by cutting but by polishing.
In the case where both the cam follower rocker and the valve drive rocker are made of the steel, the cam follower rocker has a portion, to which a high impact load is imposed, including a region which receives the displacements of the cam and an abutment which abuts on the piston. Therefore, from the viewpoint of strength, the use of the steel for which the quenching is carried out is required.
On the other hand, the valve drive rocker has no portion to which the high impact load is imposed. Specifically, the load imposed on the piston is received by a wide area such as a portion in which the piston is accommodated. Therefore, such high strength is not required compared with the cam follower rocker, but instead the valve drive rocker has a portion for which high accuracy is required.
In the case where the cam follower rocker and the valve drive rocker are made of the steel, as with the cam follower rocker, the quenching and the polishing are carried out even on the valve drive rocker in which the product accuracy is emphasized, which results in the problem that machining man-hours are increased, leading to higher cost in the variable valve device.