| 摘要 |
A low-friction, extended-life exhaust valve has a coating on the valve stem that is protected from hot exhaust gases. The valve stem slides axially in a valve guide pressed into the cylinder head, and the coating on the valve stem is receded away from the end of the valve guide exposed to the hot exhaust gases passing through the exhaust port. Protecting the coating from the hot exhaust gases allows the use of coatings that cannot normally withstand the harsh environment to which the exhaust valve is exposed. The preferred coating is a molybdenum oxide coating, which not only retains lubricating oil because of its porous nature, but also has solid lubricity which is useful in case of hydrostatic breakdown of the lubricant between engine cycles. It is preferred that the non-coated portion of the valve stem, which is located towards the end of the valve guide where the hot exhaust gases are exposed, be recessed so that the bore of the valve guide contacts only the coated portion of the valve stem, thereby further reducing wear. Other embodiments of the invention involve various coatings and configurations for the valve guide as well as the valve stem.
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