Valve used to control the air pressure in the intake manifold of an internal combustion engine
In
internal combustion engines
with
carburetors
, a
choke
valve
or
choke
modifies the air pressure in the
intake manifold
, thereby altering the
air?fuel ratio
entering the engine. Choke valves are generally used in
naturally aspirated
engines to supply a richer fuel mixture when starting the engine. Most choke valves in engines are
butterfly valves
mounted upstream of the carburetor jet to produce a higher partial vacuum, which increases the fuel draw.
[1]
In heavy industrial or fluid engineering contexts, including oil and gas production, a
choke valve
or
choke
is a particular design of
valve
with a solid cylinder placed inside another slotted or perforated cylinder.
Carburetor
[
edit
]
A choke valve is sometimes installed in the carburetor of internal combustion engines. Its purpose is to restrict the flow of
air
, thereby enriching the
fuel-air mixture
while starting the engine. Depending on engine design and application, the valve can be activated manually by the operator of the engine (via a
lever
or pull handle) or automatically by a
temperature
-sensitive mechanism called an automatic choke.
Choke valves are important for naturally-aspirated
gasoline
engines because small droplets of gasoline do not evaporate well within a cold engine. By restricting the flow of air into the throat of the carburetor, the choke valve reduces the pressure inside the throat, which causes a proportionally greater amount of fuel to be pushed from the main
jet
into the combustion chamber during cold-running operation. Once the engine is warm (from
combustion
), opening the choke valve restores the carburetor to normal operation, supplying fuel and air in the correct
stoichiometric
ratio for clean, efficient combustion.
The term "choke" is applied to the carburetor's enrichment device even when it works by a totally different method. Commonly,
SU carburettors
have "chokes" that work by lowering the fuel jet to a narrower part of the needle. Some others work by introducing an additional fuel route to the constant depression chamber.
Chokes were nearly universal in
automobiles
until
fuel injection
began to supplant carburetors. Choke valves are still common in other internal-combustion engines, including most small portable engines,
motorcycles
, small propeller-driven
airplanes
, riding
lawn mowers
, and normally-aspirated marine engines.
Industrial
[
edit
]
In the
extraction of petroleum
(and other heavy-duty fluid handling contexts), a choke
valve
(or "choke") is an adjustable
flow limiter
that is designed to operate at a large
pressure drop
, at a large
flow rate
, for a long time. A choke is often a part of the "
Christmas tree
" at the
wellhead
.
The most familiar choke design is a solid cylinder (called a "plug" or "stem") that closely fits inside another cylinder that has multiple small holes through it (the "cage"). Gradually withdrawing the plug uncovers more and more holes, progressively reducing the resistance to flow.
[2]
[3]
If the holes are regularly placed, then the relationship between the position of the valve and the
flow coefficient
(C
v
) (the flow rate per unit pressure) is roughly linear. Another design places a closely fitted cylindrical "sleeve" around the outside of the cage rather than a plug inside the cage.
[4]
A choke may also include a conical valve and valve seat, to ensure complete shutoff.
Fluids flowing into the cage (through all uncovered holes) enter from all sides, producing fluid jets. The jets collide at the center of the cage cylinder, dissipating most of their energy through fluid impinging on fluid, producing less friction and cavitation erosion of the metal valve body. For highly erosive or corrosive fluids, chokes can be made of
tungsten carbide
or
inconel
.
References
[
edit
]
External links
[
edit
]