Valve seat

From Wikipedia, the free encyclopedia

This article does not cite any references or sources. (November 2006) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.

The valve seat in an internal combustion gasoline or diesel engine is the surface against which an intake or an exhaust valve rests during the portion of the engine operating cycle when that valve is closed. The valve seat is a critical component of an engine in that if it is improperly positioned, oriented, or formed during manufacture, valve leakage will occur which will adversely affect the engine compression ratio and therefore the engine efficiency, performance (horsepower), exhaust emissions, and engine life.

Valve seats are often formed by first press-fitting an approximately cylindrical piece of a hardened metal alloy, such as Stellite, into a cast depression in a cylinder head above each eventual valve stem position, and then machining several conical-section valve seat surfaces to form the valve seat into a shape that will match the mating surface of the corresponding valve. These several conical-section surfaces are each designed to mate with or to clear the valve that will be subsequently inserted into the valve guide hole that is below or above the valve seat in the cylinder head.

There are several ways in which a valve seat may be improperly positioned or machined. These include incomplete seating during the press fitting-step, distortion of the nominally circular valve seat surfaces such they deviate unacceptably from perfect roundness or waviness, tilt of the machined surfaces relative to the valve guide hole axis, deviation of the valve seat surfaces from concentricity with the valve guide holes, and deviation of the machined conical section of the valve seat from the cone angle that is required to match the valve surface. Automated quality control of inserted and machined valve seats has traditionally been very difficult to achieve until the advent of digital holography which has enabled high-definition metrology for measuring all of these listed deviations.