In light-aircraft powerplants the carburetor remains the standard device that atomizes and meters the fuel-air charge. Outside air enters below the propeller spinner, passes through an air filter, and is ducted to the carburetor throat where a low-pressure zone draws fuel through the main jet; the throttle valve downstream then fixes the volume of the mixture delivered to the cylinders. To offset the carburetor's principal weakness - ice formation in the induction tract - airframes carry a carburetor-heat system that warms the incoming airflow and lowers the freezing risk. Compared with fuel-injection, the carburetor is simpler and lighter, yet it yields less-precise mixture control and is more prone to icing. Although injection systems resist evaporative icing and offer finer metering, they can suffer vapor locks on hot ground operations.
What are the components of an aircraft carburetor induction system?
The carburetor induction system has three main parts: air scoop, carburetor, and intake manifold. Outside air enters the system through a ram air intake located at the lower front nose cowling so that the air is forced into the induction system, giving a ram effect caused by the aircraft moving through the air. The air scoop collects inlet air and normally contains an air filter that inhibits the entry of dust and other foreign objects. Air is passed through the air filter into air ducts leading to the carburetor; these air ducts consist of a fixed duct riveted to the nose cowling and a flexible duct between the fixed duct and the carburetor air valve housing. The carburetor meters fuel in proportion to air and mixes air with the correct amount of fuel. In a float-type carburetor, outside air first flows through the air filter and then through the venturi, a narrow throat that creates a low-pressure area and forces fuel to flow through the main fuel jet. Fuel is discharged on the engine side of the throttle valve, which regulates the flow of the fuel-air mixture to the combustion chambers. The intake manifold delivers the fuel-air mixture to the cylinders, completing the induction process.
What are the advantages and disadvantages of an aircraft carburetor induction system?
The advantages of an aircraft carburetor induction system include that the system requires only basic pilot actions like checking for an RPM drop during run-up, and carburetor heat serves as an alternate air source if the intake filter clogs. Correct fuel-to-air mixture ratio is established with the mixture control set in the FULL RICH position, and enriching mixture as needed maintains proper mixture. Carburetor systems offer automatic mixture control. Rapid maneuvers have negligible effects on pressure-type carburetors, and large fuel lines rarely clog. Carburetor systems require less maintenance.
The disadvantages of an aircraft carburetor induction system include that carburetor icing will cause gradual loss in RPM in fixed-pitch propellers, shifts in manifold pressure for constant-speed propellers, and loss of engine power. Carburetor ice forms with temperatures as high as 100°F (37.8°C) and humidity as low as 50%; carburetor icing can occur in all phases of flight. Ice forms on the induction intake or air filter, and enough ice buildup causes the engine to cease to operate. Fuel distribution is not as accurate as fuel injection, and the carburetor system mixture is less precise for each cylinder. Fuel must be discharged at low pressure leading to incomplete vaporization, and low pressure leads to difficulty discharging fuel into some supercharge systems. As altitude increases, the density of air entering the carburetor decreases, while fuel density remains the same, so mixture must be leaned using the mixture control. Richer mixture results in engine roughness, loss of power, and fouled spark plugs. Carbureted engines are less efficient during flight, and fuel injection systems generally allow greater power than a same-displacement carbureted engine. Fuel injection systems are expensive to maintain, replace, or overhaul, and more complicated than carburetors.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.