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Aircraft Carburetor

Aircraft Engine Carburetor

Before gasoline can burn in a piston engine it needs to be vaporized and mixed with oxygen in the right quantities. This process is done by either a carburetor or a fuel injection system. For this process to be almost perfect the system needs to take into account, power setting, mixture control and such.

The evaporation of fuel inside of the carburetor causes a drop in temperature possibly forming ice if the humidity and outside air temperature conditions are right. External heat must be added to prevent this from happening.

Piston powered aircraft engines can use either a carburetor or a modern fuel injection system, this page here will go into the details of the carburetor, mixture control, idling system, accelerator pump and much more.



Operating Principles

Based on Bernoulli's principle the carburetor mixes fuel and air in the right ratio. This range is in between 1:9 (rich, 1 part fuel to 9 parts air by weight) and 1:18 (lean). The chemically correct (aka stoichiometric) ratio for gasoline is 1:14.7 by weight and provides the exact balance of oxygen and fuel for the most complete combustion and highest exhaust temperatures.

Fuel air mixture

In a rich situation, the extra fuel cools the engine even more. In a lean mixture oxygen is 'left over'. In the real world, practical experience found that the maximum power output is reached with a ratio of 1:12 (a bit rich) and best economy (best BSFC) with a ratio of 1:16 (a bit lean).

Normally aircraft carburetors are setup to operate in rich mixture when idling and on high power settings, lean is used for cruising. The reason for rich idling is that exhaust gases will be inhaled during valve overlap. Keep in mind that the spark plugs can and most probably will foul up by this. Leaning the mixture during taxi really helps to burn off this deposited carbon, and keeps the engine running smooth.

At high power settings the mixture is enriched to prevent detonation and this keeps the combustion/ exhaust temperatures somewhat lower. During cruise the mixture is leaned to extend the aircraft range and lower fuel consumption.

Float carburetor

Aircraft Float Carburetor

This type of carburetor uses a chamber with a float. The chamber is filled with fuel and the float regulates the amount of fuel in the chamber. The fuel enters the venturi through a metered jet in the chamber. In the venturi air pressure has dropped by Bernoulli's principle. The fuel then vaporizes and the resulting mixture is fed to the cylinders.

There is a throttle downstream of the venturi in the carburetor and connected to the throttle lever in the cockpit. For this simple carburetor to work satisfactory we need some extra systems.

Atomization

The main jet outlet needs a bit of help so that the fuel can atomize and diffuse to the maximum extent possible. The metering jet in the chamber is therefore equipped with an air bleed so that air bubbles are introduced and this improves vaporization of the fuel.

Carburetor heat

Due to Bernoulli's principle (lower air pressure due to acceleration) and evaporation of fuel (taking heat from its surroundings) ice may form and this process causes a temperature drop in the venturi and downstream on the throttle valve, causing ice to form here if there is enough moisture. Applying carburetor heat will take care of that.

Acceleration, idling & power enrichment

When the throttle is nearly closed the air flow in the venturi decreases so much that the fuel flow through the main jet becomes unreliable. There remains an air gap where the throttle valve almost touches the wall of the carb throat, this is where an outlet for fuel is created. This idling jet is also equipped with an air bleed for good fuel vaporization.

During rapid movement of the throttle air flow accelerates in the carburetor throat but as the fuel has more mass than air it is slower to move. The resulting mixture is too lean for the engine. To compensate this an accelerator plunger pump is added to the carburetor and this pump introduces extra fuel in parallel with the normal main jet.

Power enrichment is used at high manifold and RPM settings to avoid detonation and overheating. This is done by increasing the fuel flow when the throttle is almost at its wide open range.

Mixture control

Manual mixture control is needed because at higher altitudes the volume of air is the same but its density is less. Therefore the amount of fuel must be reduced to prevent the mixture to become too rich. This usually done by back suction from the throat of the venturi or a needle which reduces the fuel flow to the main jet.

To relieve the pilot from this task, some carburetors (Rotax uses Bing) use an automatic system with an aneroid capsule reducing fuel flow by back suction or with a needle valve on the main jet.

written by EAI.





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