The carburetor is the most commonly used device in the light-aircraft powerplant. It sits in the engine's induction system and forms the heart of the throttle system, where it measures incoming air and meters the correct amount of fuel. By accelerating the intake air through a miniature venturi, it tears liquid gasoline into a fine mist and blends it with air in the ratio needed for optimum combustion. Most trainers and utility aeroplanes therefore rely on a simple updraft, float-type, fixed-jet carburetor that draws fuel whenever the float drops below its preset level. Only a few installations substitute a pressure-type unit that feeds fuel under positive pump pressure.
What is a carburetor in an aircraft?
A carburetor is a mechanical apparatus for premixing vaporized fuel and air in proper proportions and supplying the mixture to an internal combustion engine. It feeds the mixture to each cylinder in the engine. Liquid fuel must be vaporized and mixed with the proper amount of air to burn properly in the cylinders, and it is the carburetor that performs this vital function. Flow of the fuel-air mixture to the combustion chambers is regulated by the throttle valve, which is controlled by the throttle in the flight deck. The carburetor forms part of the engine's induction system, drawing fuel from tanks through its inlet. Carburetor comes from the French word carburer meaning ‘to combine with carbon’.
In a carburetor system, the float-type chamber collects fuel so the reservoir holds a consistent volume of fuel, assuring steady supply. The venturi atomizes liquid fuel and mixes it with air. The mixture passes through the inlet valve(s) and finally into the combustion chamber.
How does a carburetor work on a plane?
A carburetor works on the Venturi effect. A venturi is a tube where the middle section is indented. This creates an area of decreased air pressure and increased air velocity. Fuel is then shot up through the middle of the carburetor venturi and mixes with the air. Because the air within the venturi is at a lower pressure than the air around it, it creates a vacuum that draws fuel out of the main discharge nozzle. It is fed into the engine’s cylinders during the intake stroke. The amount of fuel to be put into the mixture is controlled by the mixture lever in the cockpit. The amount of total fuel/air mixture fed to the engine is controlled by the throttle in the cockpit.
Carburetors are normally calibrated at sea-level pressure, where the correct fuel/air ratio is set with the mixture control at or near the "full rich" position. As altitude increases, the density of the air entering the carburetor decreases. If the amount of fuel entering the carburetor is not altered, the mixture will slowly become too rich. The mixture will become progressively richer as altitude increases, resulting in engine roughness and a reduction of power output. Rich mixtures slow the speed that the fuel/air mixture burns in the cylinder, leading to less power output. Running with an overly rich mixture for extended periods can lead to spark plug fouling, which creates a weaker spark in the combustion chamber, further reducing your power output.
To maintain a proper fuel/air ratio, it is important to lean the mixture. Leaning decreases fuel flow, compensating for the less dense high-altitude air. As the craft descends from high altitudes, one must enrich the mixture to prevent engine roughness. If the mixture is left leaned as the aircraft descends, eventually, the fuel/air will become too lean causing the engine to run rough and not produce sufficient power. It can also in some cases cause detonation, leading to potential engine damage.
What are the parts of an aircraft carburetor?
The parts of an aircraft carburetor are listed below.
- Venturi Tube: The venturi tube is a narrow point in the vertical carburetor pipe. It serves to speed up the flow of the fuel and oxygen mixture. As it passes through the constriction, the mixture is forced to move faster. This reduces the pressure in the area, creating a partial vacuum.
- Float Chamber: The float chamber measures and regulates the amount of fuel going to the engine’s cylinders.
- Jets: Jets are small brass fittings positioned between the float chamber and the venturi tube. They control the amount of fuel flowing into the carburetor housing as it is pulled through the venturi tube into the jets.
- Throttle Valve: The throttle valve (piston valve) regulates the air flowing into the carburetor.
- Choke: The choke is a butterfly valve whose role is to reduce the amount of air going into the carburetor so that the fuel-air ratio is richer in fuel. This allows a cold engine to be started more easily.
- Idle System: The idle system is responsible for monitoring the fuel supply when the engine is idling. This is necessary because when the engine is running at low RPM, the throttle is almost closed. This lowers the speed of the air flowing into the carburetor, which can interfere with the air-fuel mix and cause stalling.
- Linkages and Springs: Linkages are the mechanical arms of the system that open and close valves. Springs are used in conjunction with the linkages to enable certain automated movements and ensure that they maintain their proper positions.
- Gaskets: Gaskets are a type of seal that prevent air or fuel leakage between the carburetor and the engine.
- Fuel Inlet: The fuel inlet is the entrance to the carburetor to which the fuel inlet lines attach, thus feeding fuel to the carburetor. The fuel goes directly into the float chamber, and the fuel inlet regulates the flow rate.
- Fuel Filter: A fuel filter prevents debris from entering the carburetor by removing foreign objects like rust particles or dust from the fuel.
- Air Cleaner: The air cleaner filters out impurities like dust and regulates air flow into the carburetor. This helps prevent dirt from clogging the carburetor components.
What are the types of carburetors in aviation?
There are two types of carburetors in aviation. Primary types of carburetors include fixed-venturi and variable-venturi. Fixed-venturi carburetors are the simplest and most common type. Fixed-venturi carburetors are characterized by a fixed-size venturi. Variable-venturi carburetors, known as constant-depression carburetors, offer more precise control over the air-to-fuel mixture.
The second type of carburetors are float type. The majority of carburetors used in general aviation are the float-type. Two carburetor mixture control systems are needle type control and back-suction control. Needle type control and back-suction control are carburetor mixture control systems primarily seen in light aircraft.
How does a float-type carburetor work in aviation?
A float-type carburetor uses a floating bopper to maintain proper fuel level. Fuel is initially stored in a fuel bowl or float chamber, a holding vessel designed to provide a nearly constant level of fuel to the main discharge nozzle. The float rests on the fuel within the chamber; as the fuel level increases, the float rises, and the needle valve attached to it by a lever closes the fuel opening, shutting off fuel flow. When the fuel level drops, the float falls, the needle valve opens, and fuel is allowed to flow again, assuring the float chamber level is controlled by the position of the float.
Air travels through the venturi, creating a low-pressure zone that sucks fuel from the bowl through the fuel nozzle. The fuel is sent to the main discharge nozzle located in the lowest pressure portion of the venturi. The metering force required to raise fuel from the float chamber to the nozzle outlet is provided by this pressure differential. The pilot's mixture control regulates the amount of fuel allowed to travel from the float chamber through the main discharge nozzle, allowing precise control of the air-fuel mixture.
The float mechanism, hinged on the rear to pivot up and down meters the correct amount of fuel into the carburetor depending on the position of the float. However, if the float loses buoyancy and sinks, it causes the air/fuel mixture to become too rich. If the float or its needle valve becomes stuck, it causes the mixture to become lean or overly rich. Fuel adheres to the walls of the discharge nozzle and breaks off intermittently in large drops rather than forming a fine spray, affecting engine performance.
I remember learning about the functioning of a float-type carburetor. A float-type carburetor maintains a continuous amount of fuel. The float ascends with the gasoline depth and closes when the basin is full. When the motor consumes the mixture, the float comes down, allowing more gasoline to flow in. This device maintains the balance of the gasoline in the basin at all times.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is a pressure-type carburetor in aircraft?
A pressure-type carburetor delivers fuel under pressure by a fuel pump. Fuel is discharged into the airstream at a pressure well above atmospheric via a fuel pump. The discharge nozzle is located on the engine side of the throttle valve, so fuel is forced out into the venturi after the throttle butterfly. Because the fuel pump pressure is higher than atmospheric pressure, the danger of fuel vaporization icing is eliminated. Any temperature drop due to fuel vaporization takes place after the air has passed the throttle valve, and engine heat offsets that drop. Rapid maneuvers and rough air have negligible effects on pressure-type carburetors because the fuel chambers remain filled under all operating conditions. The Bendix-Stromberg pressure carburetor, found on large radial reciprocating engines, uses a four-chamber regulator to meter the proper amount of fuel to the downstream discharge nozzle.
How to overhaul an aircraft carburetor?
The easiest way is to have the carburetor overhauled by a factory-authorized service center, a process that returns the unit to factory wear limits. The bowl drain plug is first removed to allow fuel and sediment to drain out, the fuel strainer is lifted and cleaned, and the air box is inspected before and after every flight. Worn or damaged parts must be detected and replaced. Floats are upgraded to solid epoxy models, new gaskets and hardware are installed, and the internal passages are slushed with soluble corrosion-preventive oil before the carburetor is placed in storage. The entire process is performed using methods acceptable to the FAA, and a release certifies that the complete overhaul has been accomplished. This guards against water contamination and corrosion that will otherwise cause detonation and damage to freshly overhauled engines.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
