Aircraft Systems

Two types of induction systems are commonly used in small fuel to flow through a main fuel jet located at the throat. The aircraft engines: fuel then flows into the airstream where it is mixed with the flowing air. [Figure 7-10] 1. The carburetor system mixes the fuel and air in the carburetor before this mixture enters the intake The fuel-air mixture is then drawn through the intake manifold. manifold and into the combustion chambers where it is 2. The fuel injection system mixes the fuel and air ignited. The float-type carburetor acquires its name from a immediately before entry into each cylinder or injects float that rests on fuel within the float chamber. A needle fuel directly into each cylinder. attached to the float opens and closes an opening at the bottom of the carburetor bowl. This meters the amount of Carburetor Systems fuel entering into the carburetor, depending upon the position Aircraft carburetors are separated into two categories: float- of the float, which is controlled by the level of fuel in the type carburetors and pressure-type carburetors. Float-type float chamber. When the level of the fuel forces the float carburetors, complete with idling, accelerating, mixture to rise, the needle valve closes the fuel opening and shuts control, idle cutoff, and power enrichment systems, are the off the fuel flow to the carburetor. The needle valve opens most common of the two carburetor types. Pressure-type again when the engine requires additional fuel. The flow of carburetors are usually not found on small aircraft. The basic the fuel-air mixture to the combustion chambers is regulated difference between a float-type and a pressure-type carburetor by the throttle valve, which is controlled by the throttle in is the delivery of fuel. The pressure-type carburetor delivers the flight deck. fuel under pressure by a fuel pump. The float-type carburetor has several distinct disadvantages. In the operation of the float-type carburetor system, the First, they do not function well during abrupt maneuvers. outside air first flows through an air filter, usually located Secondly, the discharge of fuel at low pressure leads to at an air intake in the front part of the engine cowling. This incomplete vaporization and difficulty in discharging filtered air flows into the carburetor and through a venturi, a fuel into some types of supercharged systems. The chief narrow throat in the carburetor. When the air flows through disadvantage of the float-type carburetor, however, is its the venturi, a low-pressure area is created that forces the icing tendency. Since the float-type carburetor must discharge Fuel-air mixture Float chamber Fuel inlet The blend of fuel and Fuel level is maintained Fuel is received into air is routed to the by a float-type device. the carburetor through combustion chambers the fuel inlet. to be burned. Throttle valve The flow of the fuel-air mixture is controlled by the throttle valve. The throttle valve is adjusted from the flight deck by the throttle. Fuel Venturi The shape of the venturi creates an area of low pressure. Mixture needle The mixture needle Discharge nozzle controls fuel to the Fuel is forced through discharge nozzle. the discharge nozzle Mixture needle position into the venturi by can be adjusted using greater atmospheric the mixture control. pressure in the float Air bleed chamber. Air inlet The air bleed allows air to be mixed Air enters the carburetor with fuel being drawn out of the discharge nozzle to decrease fuel through the air inlet. density and promote fuel vaporization. Figure 7-10. Float-type carburetor. 7-8