take advantage of different types of fuel induction systems as per an aircraft’s needs. Within small aircraft engines in particular, there are two main types of induction systems, those of which include the carburetor system and the fuel injection system. Today, we will be outlining how fuel induction systems bring in outside air, mix it with fuel, and deliver an optimal fuel-air mixture to the engine cylinders, as well as provide a brief overview of carburetor and fuel injection systems.
There are two primary types of carburetors, those of which are classified as either float type or pressure type carburetors. The float type is equipped with idling, accelerating, mixture control, idle cutoff, and power enrichment systems, which make it the most convenient option. Meanwhile, pressure type carburetors are not as common and are usually only found in larger aircraft. The main difference between both types of carburetors is the method they utilize to deliver fuel. For instance, pressure type carburetors deliver fuel by harnessing the pressure of a fuel pump.
Within a float type carburetor system, the outside air first flows through an air filter that is located at an air intake in the front part of the engine cowling
. This filtered air makes its way into the carburetor and through a venturi, a narrow throat in the carburetor. As the air flows through the venturi, a low-pressure area is created which forces the fuel to flow through a main fuel jet located at the venturi. At this point, the fuel flows into the airstream where it is mixed with flowing air.
The fuel-air mixture is then sucked into the intake manifold and into the combustion chambers where it is ignited. Moreover, the float type carburetor gets its name from a float that rests on top of the fuel within the float chamber. A needle attached to the float opens and closes an opening at the bottom of the carburetor bowl. This measures the proper quantity of fuel in the carburetor. The flow of the fuel-air mixture to the combustion chambers, on the other hand, is regulated by the throttle valve.
A major advantage of float type carburetors is its icing tendency. Since the float type carburetor must discharge fuel at a point of low pressure, the discharge nozzle
must be positioned at the venturi, and the throttle valve must be on the engine side of the discharge nozzle. Keep in mind that any drop in temperature due to fuel vaporization takes place within the venturi. As such, ice quickly forms in the venturi and on the throttle valve. If water vapor in the air condenses when the carburetor temperature is near below freezing, ice will form on internal surfaces of the carburetor, including the throttle valve.
Fuel Injection System
In a fuel injection system, fuel is directly injected into the cylinders, or just ahead of the intake valve
. The air intake for the fuel injection system is similar to that of a carburetor system, wherein an alternate air source is located in the engine cowling, and this source is typically used when the external air source is obstructed. The alternate air source is generally operated automatically, with a backup manual system that can be utilized if the automatic feature malfunctions.
A fuel injection system consists of six basic components
: an engine-driven fuel pump, a fuel-air control unit, fuel manifold, discharge nozzles, an auxiliary fuel pump, and fuel pressure/flow indicators. The auxiliary pump provides fuel under pressure to the fuel-air control unit for engine starting and/or emergency use. This control unit essentially replaces the carburetor and meters fuel based on the mixture control setting, sending it to the fuel manifold valve at a rate controlled by the throttle.
After reaching the fuel manifold valve, fuel is distributed to the individual fuel discharge nozzles. The discharge nozzles, which are found in each cylinder head, inject the fuel-air mixture into each cylinder intake port. A fuel injection system is less prone to icing than the carburetor system, but impact icing on the air intake can still occur in both systems. Impact icing takes place as ice forms on the exterior of the aircraft and blocks the air intake for the injection system.
Disadvantages of this system include the difficulty of starting a hot engine, vapor lock during ground operations on hot days, and issues with restarting an engine that has to quit due to fuel starvation.
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