How Does an Aircraft Carburetor System Work?

Carbureted engines have long been used as one of the main power sources for light aircraft. Due to their simple and reliable nature, carbureted engines have been employed as a primary power source on airplanes and motor vehicles for over 100 years. Given their ubiquity and rich history in aviation, it is important for anybody interested in the field to understand how these brilliantly designed engines work. In this blog, we will highlight everything you need to know about the aircraft carburetor.

Fuel delivery is arguably the most crucial step in the aircraft engine cycle. With air pressure fluctuations and a narrow window for optimal efficiency, ensuring a proper fuel-to-air ratio is both difficult, and necessary. For light aircraft, two fuel-delivery systems predominate, the fuel-injection system and the carburettor. While they have taken over in popularity in the automotive realm, fuel-injection systems still trail behind the carbureted design in aviation.

The carburetor is a device that mixes fuel and air into a homogenous mixture before delivering it to the combustion chamber. To accomplish this, carburetors use the physics of the Venturi and Bernoulli's principles which define the relationship between pressure, area, and velocity. The Venturi principle states that in a closed system, Area1Velocity1=Area2Velocity2. Therefore, carburetors are designed with a choke point which can be represented as Area2. With this in mind, the resulting velocity is defined by a ratio of Area1/Area2, meaning the fuel speeds up as it travels through the carburetor. Meanwhile, Bernoulli's equation can be manipulated to show an inverse relationship between pressure and velocity, displaying a decreased pressure at the choke point.

While there are several carbureted engine designs, the most common by far is the float carburetor which uses a float placed in the chamber to help regulate the amount of fuel entering the system. With this engine design, fuel first enters the fuel chamber through an inlet from the fuel system where it is regulated by a float device. This float sits on top of the fuel at any given level and is connected to a needle valve, that of which only allows a set amount in the chamber at any given time; thus, it keeps the fuel level constant throughout flight. Fuel then passes through the metering jet which regulates the flow rate as it passes through the rest of the system. This variable can be controlled directly from the cockpit when pilots need to lean or enrich the mixture.

Ambient air enters the carburetor through the air inlet, where it immediately passes through a choke point; thus, it increases the air’s velocity while decreasing pressure. The fuel discharge nozzle is positioned where air pressure is lowest, creating a gradient between the two, and causing fuel to get sucked into the Venturi chamber. As these two elements mix, the fuel vaporizes and creates an ideally combustible mixture. Since air pressure decreases with altitude, the fuel-to-air ratio entering the chamber must be changed throughout the flight. Most carbureted aircraft have a mixture needle in the cockpit just right of the throttle. Typically, a richer mixture is preferred during takeoff and landing, while a leaned engine performs better at cruising altitude.

Whether you are looking to upgrade or replace your carbureted engine, ASAP Purchasing has all the components you need to keep you operational. As a leading distributor for the civil and defense aviation markets, we offer customers high-quality products with rapid lead times and unmatched cost savings. You can also shop with confidence at ASAP Purchasing, thanks to our robust quality assurance program and strict NO CHINA Sourcing pledge. Submit an RFQ through our website today to experience a truly streamlined purchasing process.

January 12, 2023
December 7, 2022

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