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The float-type carburetor has several distinct disadvantages. In the first place, imagine the effect that abrupt maneuvers have on the float action. In the second place, the fact that its fuel must be discharged at low pressure leads to incomplete vaporization and difficulty in discharging fuel into some types of supercharged systems. The chief disadvantage of the float carburetor, however, is its icing tendency. Since the float carburetor must discharge fuel at a point of low
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Float-type carb
uretorFUELFuel level is maintainedby a float-type device.FLOAT CHAMBERFuel is received intothe carburetor throughthe fuel inlet.FUEL INLETThe mixture needle controls fuel to the discharge nozzle. Mixture needle position can be adjusted using the mixture control.MIXTURE NEEDLEAir enters the carburetorthrough the air inlet.AIR INLETThe blend of fuel and air is routed to the combustion chambers to be burned. FUEL/AIR MIXTUREThe flow of the fuel/air mixture is controlled by the throttle valve. Thethrottle valve is adjusted from the flight deck by the throttle.THROTTLE VALVEFuel is forced through the discharge nozzle into the venturi by greater atmosphericpressure in the float chamber.DISCHARGE NOZZLEThe shape of the venturi creates an area of low pressure.VENTURIThe air bleed allows air to be mixed with fuel being drawn out of the dis-charge nozzle to decrease fuel density and promote fuel vaporization.AIR BLEED
Figure 6-10. Float-type carburetor.
pressure, the discharge nozzle must be located at the venturi throat, and the throttle valve must be on the engine side of the discharge nozzle. This means the drop in temperature due to fuel vaporization takes place within the venturi. As a result, ice readily forms in the venturi and on the throttle valve.
A pressure-type carburetor discharges fuel into the airstream at a pressure well above atmospheric. This results in better vaporization and permits the discharge of fuel into the airstream on the engine side of the throttle valve. With the discharge nozzle located at this point, the drop in temperature due to fuel vaporization takes place after the air has passed the throttle valve and at a point where engine heat tends to offset it. Thus, the danger of fuel vaporization icing is practically eliminated. The effects of rapid maneuvers and rough air on the pressure-type carburetors are negligible since their fuel chambers remain filled under all operating conditionsMixture Control
Carburetors are normally calibrated at sea-level pressure, where the correct fuel-to-air mixture ratio is established with the mixture control set in the FULL RICH position. However, as altitude increases, the density of air entering the carburetor decreases, while the density of the fuel remains the same. This creates a progressively richer mixture, which can result in engine roughness and an appreciable loss of power. The roughness normally is due to spark plug fouling from excessive carbon buildup on the plugs. Carbon buildup occurs because the rich mixture lowers the temperature inside the cylinder, inhibiting complete combustion of the fuel. This condition may occur during the pretakeoff runup at high-elevation airports and during climbs or cruise flight at high altitudes. To maintain the correct fuel/air mixture, the mixture must be leaned using the mixture control. Leaning the mixture decreases fuel flow, which compensates for the decreased air density at high altitude.
During a descent from high altitude, the mixture must be enriched, or it may become too lean. An overly lean mixture causes detonation, which may result in rough engine operation, overheating, and a loss of power. The best way to maintain the proper mixture is to monitor the engine temperature and enrich the mixture as needed. Proper mixture control and better fuel economy for fuel-injected engines can be achieved by use of an exhaust gas temperature (EGT) gauge. Since the process of adjusting the mixture can vary from one aircraft to another, it is important to refer to the airplane flight manual (AFM) or the pilot’s operating handbook (POH) to determine the specific procedures for a given aircraft.Carburetor Icing
As mentioned earlier, one disadvantage of the float-type carburetor is its icing tendency. Carburetor ice occurs due
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Incoming air
VenturiIceTo engineFuel/air mixtureIceIce
Figure 6-11. The formation of carburetor ice may reduce or block fuel/air flow to the engine.
Relativ
e humidityOutside air temperature100%50%80%60%70%90%20°F/-7°C32°F/0°C70°F/21°C100°F/38°CHigh carburetoricing potentialCarburetor icing possible
Figure 6-12. Although carburetor ice is most likely to form when the temperature and humidity are in ranges indicated by this chart, carburetor ice is possible under conditions not depicted.by a decrease in manifold pressure, but no reduction in rpm. Propeller pitch is automatically adjusted to compensate for loss of power. Thus, a constant rpm is maintained. Although carburetor ice can occur during any phase of flight, it is particularly dangerous when using reduced power during a descent. Under certain conditions, carburetor ice could build unnoticed until power is added. To combat the effects of carburetor ice, engines with float-type carburetors employ a carburetor heat system.
　
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