The valve seat in an

internal combustion An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...

gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...

diesel engine The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-call ...

is the surface against which an

intake An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...

or an

exhaust Exhaust, exhaustive, or exhaustion may refer to: Law *Exhaustion of intellectual property rights, limits to intellectual property rights in patent and copyright law **Exhaustion doctrine, in patent law ** Exhaustion doctrine under U.S. law, in p ...

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...

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 Valve leakage refers to flow through a valve which is set in the 'off' state. The importance of valve leakage depends on what the valve is controlling. For example, a dripping tap is less significant than a leak from a six-inch pipe carrying high ...

will occur which will adversely affect the engine

compression ratio The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values. A fundamental specification for such engines, it is measured two ways: the stati ...

and therefore the

engine efficiency Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines- #Internal combustion (gasoline, diesel ...

, performance (

engine power Engine power is the power that an engine can put out. It can be expressed in power units, most commonly kilowatt, pferdestärke (metric horsepower), or horsepower. In terms of internal combustion engines, the engine power usually describes the ' ...

and engine torque), exhaust

emissions Emission may refer to: Chemical products * Emission of air pollutants, notably: **Flue gas, gas exiting to the atmosphere via a flue ** Exhaust gas, flue gas generated by fuel combustion ** Emission of greenhouse gases, which absorb and emit rad ...

, and engine life. Valve seats are often formed by first

press-fitting An interference fit, also known as a pressed fit or friction fit is a form of fastening between two ''tight'' fitting mating parts that produces a joint which is held together by friction after the parts are pushed together. Depending on the am ...

an approximately cylindrical piece of a hardened metal

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

, such as

Stellite Stellite is a range of cobalt-chromium alloys designed for wear resistance. The alloys may also contain tungsten or molybdenum and a small, but important, amount of carbon. History Stellite is a trademarked name of Kennametal Inc. Prior to tha ...

, into a cast depression in a

cylinder head In an internal combustion engine, the cylinder head (often abbreviated to simply "head") sits above the cylinders and forms the roof of the combustion chamber. In sidevalve engines, the head is a simple sheet of metal; whereas in more modern ov ...

above each eventual valve stem position, and then machining a conical-section surface into the valve seat that will mate with a corresponding conical section of the corresponding valve. Generally two conical-section surfaces, one with a wider cone angle and one with a narrower cone-angle, are machined above and below the actual mating surface, to form the mating surface to the proper width (called "narrowing" the seat), and to enable it to be properly located with respect to the (wider) mating surface of the valve, so as to provide good sealing and heat transfer, when the valve is closed, and to provide good gas-flow characteristics through the valve, when it is opened. Inexpensive engines may have valve seats that are simply cut into the material of the cylinder head or engine block (depending on the design of the engine). Some newer engines have seats that are sprayed on rather than being pressed into the head, allowing them to be thinner, creating more efficient transfer of heat through the valve seats, and enabling the valve stems to function at a lower temperature, thus allowing the valve stems (and other parts of the

valvetrain A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) ...

) to be thinner and lighter.Corollaland "Toyota Corolla Engines, 1993-2007"
/ref> 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 that they deviate unacceptably from perfect roundness or

waviness Waviness is the measurement of the more widely spaced component of surface texture. It is a broader view of roughness because it is more strictly defined as "the irregularities whose spacing is greater than the roughness sampling length". It can ...

, tilt of the machined surfaces relative to the valve guide hole axis, deviation of the valve seat surfaces from

concentric In geometry, two or more objects are said to be concentric, coaxal, or coaxial when they share the same center or axis. Circles, regular polygons and regular polyhedra, and spheres may be concentric to one another (sharing the same center point ...

ity 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 was historically very difficult to achieve until the advent of

digital holography Digital holography refers to the acquisition and processing of holograms with a digital sensor array, typically a CCD camera or a similar device. Image rendering, or reconstruction of object ''data'' is performed numerically from digitized interfero ...

, which has enabled high-definition metrology for measuring all of these listed deviations.

References

{{Reflist Engine valves