For a majority of modern aircraft, the thrust and power necessary for achieving flight are generated through the combustion of compressed fuel-and-air mixtures. In order for proper combustion, a system must be in palace for the mixing of jet fuel and compressed atmospheric air. Most general aviation aircraft utilize a carburetor for this, that of which is a mechanical device that atomizes and mixes liquid fuel with air to achieve an optimal ratio. In this blog, we will discuss the general functionality of the aircraft carburetor, as well as the common types that one may find on an aircraft.
While the fuel injection system and approach has made the carburetor a fairly obsolete choice for many modern aircraft, light piston aircraft engines continue to rely on conventional carburetion for their needs. Carburetors operate through the use of a venturi which is a simplistic device with an inlet, thin throat, and outlet that allows for velocity, pressure, and area to be manipulated through the conversion of mass and Bernoulli’s equation. As air is incompressible, it will retain a set mass and density when passing through a venturi, but the velocity will be greater at the throat of the device as compared to the inlet. Additionally, a pressure drop will occur at the throat as velocity increases, leading to a situation where fuel is sucked into the airstream as pressure drops so that it will mix together with air.
Across most light aircraft, the most common carburetor type is the float-type variation. These devices get their name as a result of the float that is placed in the fuel chamber for the regulation of levels. As fuel enters the chamber through fuel system lines, it will be regulated by the float which opens and closes a valve based on the fuel quantity. Fuel is always kept below the nozzle to prevent a leak, and passageways present between the float chamber and venturi provide a path for fuel to be sucked into the discharge nozzle during operations. To manage the fuel flow rate from the chambers, a metering jet is used. The metering jet is an orifice that adjusts its diameter to determine the amount of fuel that will enter the throat venturi for atomization and mixing. Once this is done, the mixture is then sent through the intake manifold to be ignited within the engine combustion chamber.
The other common type of carburetor is the pressure-type variation, and they are designed to discharge fuel into a stream of air while it is above atmospheric pressure levels. This method of mixing ensures optimal evaporation and results in a temperature drop as vaporization occurs after air has already passed the throttle valve. This is highly beneficial for the fact that vaporization icing is avoided, making operation more reliable and safe. Despite their various benefits, pressure-type carburetors have been mostly superseded by fuel injection systems and are rarely found on modern engines as a result.
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