Choke Valves

The choke valve completely or partially blocks the opening to the intake manifold to alter the amount of airflow and create a better fuel-air mixture. Choke valves are found either above the intake manifold or inside the carburetor.

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Detailed Description for Choke Valves

A choke valve is a type of valve designed to create a choked flow in a fluid line in an automobile. The viscosity of the fluid passing through the valve is irrelevant in understanding how the mechanism works. 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 carburetors to supply a richer fuel mixture when starting the engine. Most choke valves in engines are butterfly valves mounted in the manifold above the carburetor jet to produce a higher partial vacuum, which increases the fuel draw.

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.

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 or automatically by a temperature-sensitive mechanism called an autochoke.

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.

Choke valves were commonly used in automobile engines until the end of the 1980s. Choke valves help restrict the flow of air pulled into the engine through the intake manifold. The choke valve completely or partially blocks the opening to the intake manifold to alter the amount of airflow and create a better fuel-air mixture. Choke valves are found either above the intake manifold or inside the carburetor. Modern vehicles have switched out choke valves and carburetors for fuel-injection systems, though certain machines still use choke valves to function.

In an engine, fuel and air mix together to create the power that runs the machine. The engine sucks in more fuel or air depending on the pressure placed on the chamber. In a naturally aspirated engine, pressure from the outside air causes this reaction. New engines use superchargers or turbochargers to create pressure and force the proper amount of air and fuel into the cylinder for maximum power.

Engines without a supercharger need a way to pull in the right amount of air and fuel, especially when the engine is still cold and needs to warm up. The choke valve is used to restrict the air flow and alter the pressure. This creates better engine performance when the engine is still running cold. Once it warms up, the choke valve opens to let more air flow into the engine.

Control of the choke valve is performed either manually by the engine or automatically through the use of electronics. When controlled manually, the valve is connected by a handle or a lever to part of the engine which in turn activates the choke valve during use. The automatic version uses a sensor, known as an autochoke, to control the opening and closing of the choke valve.

Modern vehicles use a fuel-injection system with poppet valves to allow a specific air and fuel mixture into the cylinder. It controls the pressure and the amount of air and fuel in the mixture, thus eliminating the need for a choke valve. Other machines, such as certain lawn mowers and small airplanes, still use the choke valve in their engines.

 

 


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