What is the Principle of a Carburetor?

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Principle of Carburetor:

The carburetor works on Bernoulli’s principle: the faster air moves, the lower its static pressure, and also the higher the dynamic pressure is. The throttle (accelerator) linkage doesn’t directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being carried into the engine. The speed of this flow, and so its (static) pressure, determines the number of fuel drawn into the airstream.

When carburetors are employed in aircraft with piston engines, special designs and features are needed to forestall fuel starvation during inverted flight. Later engines used an early type of fuel injection system referred to as a pressure carburetor.

Most production carbureted engines, as opposition fuel-injected, have one carburetor and an identical manifold that divides and transports the air/fuel mixture to the intake valves, though some engines (like motorcycle engines) use multiple carburetors on split heads.


Updraft Carburetors:

Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the highest. This had the advantage of never flooding the engine, as any liquid fuel droplets would fall out of the carburetor rather than into the intake manifold; it also lent itself to the employment of an oil bath air filter, where a pool of oil below part below the carburetor is sucked up into the mesh and therefore the air is drawn through the oil-covered mesh; this was an efficient system in the time when paper air filters didn’t exist.

Outboard motor carburetors are typically sidedraft because they need to be stacked one on top of the opposite to feed the cylinders during a vertically oriented engine block.



The main disadvantage of basing a carburetor’s operation on Bernoulli’s Principle is that being a fluid dynamic device, the pressure reduction during a venturi tends to be proportional to the square of the intake airspeed. The fuel jets are much smaller and fuel flow is proscribed mainly by the fuel’s viscosity so the fuel flow tends to be proportional to the pressure difference. So jets sized for full power tend to starve the engine at lower speed and part throttle. Most ordinarily this has been corrected by using multiple jets.

An airflow resisting valve called a choke, kind of like the throttle was placed upstream of the most jet to scale back the manifold pressure and suck additional fuel out of the jets.

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