Valve seat
Encyclopedia
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 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
) to be thinner and lighter.
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 concentric
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 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.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
gasoline
Spark-ignition
The term spark-ignition engine refers to internal combustion engines, usually petrol engines, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug...
or diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
is the surface against which an intake
Intake
An intake , or especially for aircraft inlet, is an air intake for an engine. Because the modern internal combustion engine is in essence a powerful air pump, like the exhaust system on an engine, the intake must be carefully engineered and tuned to provide the greatest efficiency and power...
or an exhaust
Exhaust system
An exhaust system is usually tubing used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes...
valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
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
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; a dripping tap is less significant than a leak from a six-inch pipe carrying high-pressure radioactive steam.ANSI specifies six different leakage...
will occur which will adversely affect the engine compression ratio
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
and therefore the engine efficiency
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...
, performance (horsepower
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
), exhaust emissions, and engine life.
Valve seats are often formed by first press-fitting an approximately cylindrical piece of a hardened
Hardening (metallurgy)
Hardening is a metallurgical and metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain...
metal alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
, such as Stellite
Stellite
Stellite alloy is a range of cobalt-chromium alloys designed for wear resistance. It may also contain tungsten or molybdenum and a small but important amount of carbon...
, into a cast depression in a cylinder head
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
above each eventual valve stem position, and then machining a conical
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
-section surface into the valve seat that will mate with a corresponding conical
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
-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
Valvetrain
Valvetrain is an all-encompassing term used to describe the mechanisms and parts which control the operation of the valves. A traditional reciprocating internal combustion engine uses valves to control air and fuel flow into and out of the cylinders, facilitating combustion.-Layout:Valvetrain: The...
) to be thinner and lighter.
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
Waviness
Waviness is the measure 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"...
, tilt of the machined surfaces relative to the valve guide hole axis, deviation of the valve seat surfaces from concentric
Concentric
Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another...
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 has traditionally been very difficult to achieve until the advent of digital holography
Digital holography
Digital holography is the technology of acquiring and processing holographic measurement data, typically via a CCD camera or a similar device. In particular, this includes the numerical reconstruction of object data from the recorded measurement data, in distinction to an optical reconstruction...
which has enabled high-definition metrology
High-definition metrology
High-definition metrology refers to measurement of dimensional or other attributes of a surface or an object in which measurements are made densely across the observable extent of that surface or object, so that the measured attribute of the surface or object can be portrayed with high-definition...
for measuring all of these listed deviations.