Choke valve
Encyclopedia
A Choke valve is a type of valve
designed to create choked flow
in a fluid
. Over a wide range of valve settings the flow through the valve can be understood by ignoring the viscosity
of the fluid passing through the valve; the rate of flow is determined only by the ambient pressure on the upstream side of the valve.
In automotive contexts, a choke valve
modifies the air pressure in the intake manifold of an internal combustion engine
, thereby altering the ratio of fuel and air quantity entering the engine. Choke valves are generally used in naturally aspirated engines with carburetor
s, to supply a richer fuel mixture when starting the engine. Most choke valves in engines are butterfly valve
s mounted in the manifold above the carburetor jet, to produce a higher partial vacuum and thereby draw more fuel into the intake stream.
In heavy industrial or fluid engineering contexts, a choke valve is a particular design of valve
that raises and lowers a solid cylinder (called a "plug" or "stem") which is placed around or inside another cylinder that has holes or slots. The design of a choke valve means fluids flowing through the cage are coming from all sides and that the streams of flow (through the holes or slots) collide with each other at the center of the cage cylinder, thereby dissipating the energy of the fluid through "flow impingement". The main advantage of choke valves is that they can be designed to be totally linear in their flow rate.
of internal combustion engine
s. 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 autochoke.
Choke valves are important for normally 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.
Note that the term "choke" is applied to the carburetor's enrichment device even when it works by a totally different method. Commonly SU carburetors 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 applications, including most small portable engines, motorcycle
s, small propeller-driven airplane
s, riding lawn mowers, and normally aspirated marine engines.
or inconel
.
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...
designed to create choked flow
Choked flow
Choked flow is a compressible flow effect. The parameter that becomes "choked" or "limited" is the velocity or the mass flow rate.Choked flow is a fluid dynamic condition associated with the Venturi effect...
in a fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
. Over a wide range of valve settings the flow through the valve can be understood by ignoring the viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of the fluid passing through the valve; the rate of flow is determined only by the ambient pressure on the upstream side of the valve.
In automotive contexts, a choke 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...
modifies the air pressure in the intake manifold of an internal combustion engine
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...
, thereby altering the ratio of fuel and air quantity entering the engine. Choke valves are generally used in naturally aspirated engines with carburetor
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
s, to supply a richer fuel mixture when starting the engine. Most choke valves in engines are butterfly valve
Butterfly valve
A butterfly valve is a valve which can be used for isolating or regulating flow. The closing mechanism takes the form of a disk. Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves are generally favored because they are lower in cost to other valve...
s mounted in the manifold above the carburetor jet, to produce a higher partial vacuum and thereby draw more fuel into the intake stream.
In heavy industrial or fluid engineering contexts, a choke valve is a particular design of 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...
that raises and lowers a solid cylinder (called a "plug" or "stem") which is placed around or inside another cylinder that has holes or slots. The design of a choke valve means fluids flowing through the cage are coming from all sides and that the streams of flow (through the holes or slots) collide with each other at the center of the cage cylinder, thereby dissipating the energy of the fluid through "flow impingement". The main advantage of choke valves is that they can be designed to be totally linear in their flow rate.
Automotive
A choke valve is sometimes installed in the carburetorCarburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
of internal combustion engine
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...
s. Its purpose is to restrict the flow of air, thereby enriching the fuel-air mixture
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
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
Lever
In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...
or pull handle) or automatically by a temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
-sensitive mechanism called an autochoke.
Choke valves are important for normally aspirated gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
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
Jet (fluid)
A jet is an efflux of fluid that is projected into a surrounding medium, usually from some kind of a nozzle, aperture or orifice. Jets can travel long distances without dissipating...
into the combustion chamber during cold-running operation. Once the engine is warm (from combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
), opening the choke valve restores the carburetor to normal operation, supplying fuel and air in the correct stoichiometric ratio for clean, efficient combustion.
Note that the term "choke" is applied to the carburetor's enrichment device even when it works by a totally different method. Commonly SU carburetors 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
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
began to supplant carburetors. Choke valves are still common in other internal-combustion applications, including most small portable engines, motorcycle
Motorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...
s, small propeller-driven airplane
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
s, riding lawn mowers, and normally aspirated marine engines.
Industrial
Heavy duty industrial choke valves control the flow to a certain Flow Coefficient (Cv) determined by how far the valve is opened. They are regularly used in the oil industry and for highly erosive and corrosive purposes, they are often made of tungsten carbideTungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
or inconel
Inconel
Inconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco"...
.
External links
- Short video of a choke valve operating on a chainsaw carburetor from HowStuffWorksHowStuffWorksHowStuffWorks is a commercial edutainment website that was founded by Marshall Brain with the goal of giving its target audience an insight into the way in which many things work. The site uses various media in its effort to explain complex concepts, terminology and mechanisms, including...