The carburetor forms an integral part of the engine’s induction system, and its primary role is to mix air and fuel so that this mixture can be directed to each cylinder part where it is ignited as part of the four-stroke engine cycle. Carburetors are mechanical apparatuses that take advantage of the Venturi effect to atomize liquid fuel and mix it with air in a correct ratio for optimum combustion. In technical terms, a carburetor uses the velocity increase and corresponding pressure drop at the Venturi throat to suck fuel into the airstream where it mixes with intake air.
Carburetor Design and Operation
Named after the float used in the fuel chamber to regulate the fuel level, the float carburetor is the most common fuel system type found on light aircraft. In this design, the carburetor is split into two separate regions: the fuel chamber and the Venturi.
Fuel enters the fuel chamber via the fuel system. Within the chamber, a float regulates the fuel level, and the buoyant part of the float remains on the surface of the liquid fuel. When the fuel level in the float chamber rises or falls, the float moves in response to the fuel level, thereby opening or closing the valve. Generally, the float is constructed to maintain the fuel in the chamber below the level of the fuel discharge nozzle so that no fuel leaks from the carburetor when the engine is not running.
As intake air is accelerated by the Venturi section, passageways between the float chamber and the Venturi section of the carburetor provide a passage for liquid fuel to be drawn from the chamber to the discharge nozzle. The discharge nozzle is normally positioned in the throat where the pressure is the lowest, and this establishes a pressure gradient between the float chamber and the discharge nozzle, resulting in fuel being sucked out of the chamber, through the metering jet, and into the Venturi stream at the discharge nozzle.
The metering jet is an orifice that determines the maximum fuel flow rate from the float chamber to the discharge nozzle.
The pressure differential between the float chamber and the Venturi throat is called the metering force, which increases as the throttle is opened due to the increased air flow through the Venturi. At lower throttle settings, the metering force reduces and sufficient fuel may not be supplied to the engine. As such, air bleed in the diffuser nozzle assists in vaporizing the fuel and providing even fuel discharge via the full range of throttle settings.
The volume of air-fuel mixture entering the intake manifold and the ratio of air-to-fuel in this mixture are controlled by the throttle and the mixture control levers. The throttle and mixture control levers are located in the cockpit and enable the pilot to control the power output (throttle) and the air-to-fuel (mixture). The throttle lever operates the butterfly valve positioned in the Venturi portion of the carburetor that either lets more air-fuel to enter the combustion chambers or limits it.
An idle passage in the carburetor allows the engine to idle and provides a path for fuel to flow from the float chamber to the low-pressure side of the throttle valve. This low-pressure sucks fuel through the idle bypass and into the engine.
Mixture levers within the cockpit are typically painted red and are placed to the right of the throttle lever. A process known as “enriching the mixture” consists of pushing the lever forward to allow more fuel to enter the discharge nozzle, whereas a process known as “leaning the mixture” consists of pulling the lever slightly backward to limit the amount of fuel entering the Venturi. Finally, if the lever is pulled all the way back, no fuel will be discharged into the Venturi. There are two types of mixture control systems, those of which are needle type control and back-suction control systems.
When the throttle is opened too rapidly from a lower power setting, a large volume of air rapidly enters the Venturi. The fuel dispensing system in the carburetor, on the other hand, reacts to the throttle change more slowly than the air via the intake, causing a drop in the fuel-air ratio for a short period. This will temporarily lean the mixture and result in the engine responding slowly or “stuttering” in response to the abrupt throttle change. One way of overcoming this is by incorporating a small piston pump in the carburetor to inject fuel into the Venturi and enrich the mixture until the metering system can catch up.
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