Product Code Database
In internal combustion engines with , a choke valve or simply 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 mounted upstream of the carburetor jet to restrict air flow there and produce a higher partial vacuum downstream, which increases the fuel draw.
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.
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 stoichiometry 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, 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 until fuel injection began to supplant carburetors. Choke valves are still common in other internal-combustion engines, including most small portable engines, , small propeller-driven , riding , and normally-aspirated marine engines.
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. Mokveld Product animation: angle choke valve Mokveld Product animation: axial choke valve If the holes are regularly placed, then the relationship between the position of the valve and the flow coefficient (Cv) (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. External Sleeve Choke Valve 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.
|
|
