Engines with Carburetors

Most aircraft piston engines of less than about 200 hp use carburetors to combine air and fuel to create a combustible mixture that burns in the cylinders. The Cessna Skylane RG in Flight Simulator has an engine with a carburetor.

How a Carburetor Works

Outside air flows through an air filter, then into the carburetor. The air flows through a venturi, a narrow throat in the carburetor. The air accelerates in the venturi and its pressure drops according to Bernoulli's principle. The partial vacuum forces fuel to flow through a jet into the airstream where it mixes with the flowing air. The air/fuel mixture then flows into the intake manifold, which routes it to each cylinder.

The Right Ratio

The carburetor mixes air and fuel by weight. Piston engines generally produce maximum power when the air/fuel mixture is about 15:1.

Carburetors are calibrated at sea-level pressure to meter the correct amount of fuel with the mixture control in the full rich position. As altitude increases, air density decreases. To compensate for this difference, the pilot uses the mixture control to adjust the air/fuel mixture entering the combustion chamber.

To control the amount of fuel that mixes with the air, most carburetors use a float in a fuel chamber. A needle attached to the float opens and closes an opening in the fuel line, metering the correct amount of fuel into the carburetor. The position of the float, controlled by the fuel level in the float chamber, determines when the valve opens and closes.

Running Rich

A air/fuel mixture that is too rich—too much fuel for the current weight of air—causes excessive fuel consumption, rough engine operation, and loss of power. Running an engine too rich also cools the engine, causing below-normal temperatures in the combustion chambers, which leads to spark plug fouling among other problems.

Running Lean

Operating with the mixture too lean—too little fuel for the current weight of air—results in rough engine operation, detonation, overheating, and a loss of power.

Carburetor Ice

Vaporization of fuel and expansion of the air in the carburetor causes sudden cooling of the air/fuel mixture. The temperature may drop as much as 60 degrees F (15 degrees C) within a fraction of a second. This cooling causes water vapor in the air to condense, and if the temperature in the carburetor reaches 32 degrees F (0 C) the water freezes inside the carburetor passages. Even a slight accumulation of this deposit can restrict the flow of air into the carburetor, reducing power. Carburetor ice may also lead to complete engine failure, particularly when the throttle is partly or fully closed.

Icing Conditions

On dry days, or when the temperature is well below freezing, the moisture in the air generally doesn't cause carburetor ice. But if the temperature is between 20 degrees F (–7 degrees C) and 70 degrees F (21 degrees C), with visible moisture or high humidity, the pilot should be constantly on the alert for carburetor ice.

Indications of Carburetor Icing

For airplanes with fixed-pitch propellers, the first indication of carburetor icing is a drop in RPM on the tachometer. For airplanes with controllable pitch (constant-speed) propellers, the first indication is usually a drop in manifold pressure. In both cases, the engine may start to run rough. RPM remains constant in airplanes with constant-speed propellers.

Thawing Out

To prevent carburetor ice from forming and to eliminate ice that forms, carburetors are equipped with heaters. The carburetor heater preheats the air before it reaches the carburetor. This preheating melts ice or snow entering the intake, melts ice that forms in the carburetor passages (provided the accumulation is not too great), and keeps the air/fuel mixture above freezing to prevent formation of carburetor ice.

Using Carburetor Heat

When flying in conditions conducive to carburetor icing, monitor the engine instruments to watch for signs that ice is forming. If you suspect that carburetor ice is present, apply full carburetor heat immediately. Leave it on full until you're certain that all the ice has been removed. Applying partial heat or leaving heat on momentarily might aggravate the situation.

When you first apply carburetor heat expect a drop in RPM in airplanes equipped with fixed-pitch propellers; in airplanes equipped with controllable pitch propellers, expect a drop in manifold pressure. If no carburetor ice is present, RPM or manifold pressure will remain lower than normal until the carburetor heat is turned off. If carburetor ice is present, expect a rise in RPM or manifold pressure after the initial drop (often accompanied by intermittent engine roughness). When you turn carburetor heat off, the RPM or manifold pressure rises above the value before heat was applied. The engine should also run more smoothly after the ice has melted.

In extreme cases of carburetor icing, after the ice has been removed you may need to apply just enough carburetor heat to prevent further ice formation.

Carburetor Heat as a Precaution

Whenever the throttle is closed during flight, especially as you prepare to land, the engine cools rapidly and vaporization of the fuel is less complete than if the engine is warm. If you suspect carburetor icing conditions, apply full carburetor heat before closing the throttle and leave it on.

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