Aircraft Carburetors
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Below lay out on the operation of the Carburetor

Induction systems

The induction system brings air from outside to inside in the engine, since the fuel mixes with the air and delivers the fuel / air mixture to the cylinder where combustion takes place. Outside air enters the induction system by means of an intake port in the front of the hood. This port usually contains an air filter that the entry of dust and other foreign matter inhibits. Because the filter may sometimes be clogged, an alternative source of air need to be available. Usually, the alternate air coming from inside the engine compartment, where it bypasses a clogged air filter. Some alternate air source function automatically, while for others operated manually.

Two types of suction systems are often used in small aircraft engines:

1. carburetor system, the fuel and air into the carburetor mixes before this mixture enters the intake manifold, and

2. The fuel injection system, so that the fuel and air mixing just before they in each cylinder.


Carburetors are classified as float type or pressure type. Pressure carburettors are not usually found on small aircraft. The basic difference between a pressure and a float-type carburetor is the pressure carburetor supplies the fuel pressurized by a fuel pump.

In the operation of the float-type carburetor system, the outside air first flows through an air filter, usually near the air inlet in the front portion of the bonnet. This filtered air flows into the carburetor and through a venturi, a narrow throat of the carburetor. When the air passes through the venturi, a low pressure area is created, in which the fuel flows is forcing through a central fuel jet located at the throat. The fuel then flows in the air stream, where it is mixed with the flowing air. The type Float Aircraft Carburetor

Float-type Carb

The fuel / air mixture is then drawn through the intake manifold into the combustion chambers, where it is ignited. The "float-type carburettor" gets its name from a float which rests on the fuel in the float chamber. A needle attached to the float opens and closes an opening at the bottom of the carburetor bowl. These meters the proper amount of fuel into the carburettor, depending on the position of the float, which is controlled by the fuel level in the float chamber. When the level of fuel is forcing to increase the float needle valve closes the fuel opening and closing the fuel to the carburetor. The needle valve opens again required when the engine extra fuel. The flow of the fuel / air mixture to the combustion chamber is controlled by the throttle valve, which is controlled by the throttle lever in the cockpit.


Carburetors are normally calibrated at sea level pressure, the correct fuel-air mixture ratio is determined by the mixture control to FULL RICH position. As altitude increases, the density of air decreases in the carburetor, while the density of the fuel remains the same. This creates a richer mixture, which can result in the engine roughness, and a considerable loss of power. The roughness is normally due to the plug fouling of excessive carbon build-up on the spark plugs. Carbon build-up occurs because the too rich mixture lowers the temperature in the cylinder, inhibiting complete combustion of the fuel. This condition may occur during pretakeoff run at high elevation airports and during climbs or cruise flight at high altitude. To maintain the correct fuel / air mixture, you must blend leaner with the mixture control. Leaned the mixture reduces fuel flow, which compensates for the reduced air density at altitude. During a descent from high altitude, the opposite is true. The mixture is enriched, or it may be too poor. A too lean mixture causes detonation, which can result in rough engine operation, overheating and loss of power. The best way to maintain the correct mixture to monitor the motor temperature and enrich the mixture if necessary. Proper control mix, and a reduced fuel consumption for fuel injection engines may be achieved by the use of an exhaust gas temperature gauge. As the process of adjusting the mixture by plane to another, it is important to refer to the flight manual (AFM) or Pilot Operating Handbook (POH) determine the specific procedures for a given airplane.


A disadvantage of the float-type carburetor icing is the trend. Carburetor ice occurs as a result of the effect of fuel evaporation and the decrease in the air pressure in the venturi, which causes a sharp decrease in the temperature in the carburetor. When water vapor in the air condenses when the carburetor temperature at or below the freezing point of ice on the internal surfaces of the carburetor, such as the throttle.

Carb Ice

[The Figure shows that the formation of carburetor ice can reduce or block them from the fuel / air flow to the engine.]

The reduced air pressure, as well as the evaporation of the fuel, to the temperature reduction in the carburetor. Ice normally forms in the vicinity of the throttle valve and the venturi throat. This limits the flow of the fuel / air mixture and reduces power. If enough ice builds up, the engine can no longer work. Carburetor ice most likely to occur at temperatures below 70 ° F (21 ° C) and the relative humidity is above 80 percent. Because of the sudden cooling which takes place in the carburetor of ice formation even at temperatures of up to 100 ° F (38 ° C) and humidity as low as 50 percent prevented. This drop in temperature can be up to 60 to 70 ° F. Therefore, when an outdoor air temperature of 100 ° F, a temperature drop of 70 ° F results in an air temperature in the carburettor of 30 ° F.

The first indication of the carburetor icing in an airplane with a fixed propeller is a decrease in the engine speed, which may be followed by the engine roughness. In a plane with a constant speed propeller, carburetor icing is usually indicated by a decrease in manifold pressure, but no reduction rpm Pich propeller is automatically adjusted to compensate for the loss of power. Thus, a constant r.p.m. is maintained. Although carburettor ice can occur during any phase of flight, it is particularly dangerous when using a reduced flow during a descent. Under certain circumstances, the carburetor can build ice unnoticed until you try to give it to power. To combat the effects of carburetor ice, float-type engines with carburetors use a carburetor heat system.


Carburettor heat is an anti-icing system that heats the air before it reaches the carburetor. Carburetor heat is intended to keep the fuel / air mixture above the freezing temperature in order to prevent the formation of ice in the carburetor. Carburettor heat can be used to melt ice already formed in the carburetor. The emphasis is on using carburettor heat as a preventive measure.

The carburetor heat should be checked during the engine run up. In the use of carburetor heat, following the recommendations of the manufacturer.

When conditions are conducive to carburetor icing during the flight, periodic checks should be done to detect his presence. If confirmed, the full carburetor heat is applied immediately, and it should be left in the ON position until you are sure all the ice has been removed. When ice is present, the application of a partial heat or leaving heat would worsen the situation in a sufficient time. In extreme cases of carburetor icing, even after the ice has been removed, full carburetor heat can be used to prevent further formation of ice. A carburetor temperature, if installed, is very useful in determining when to use carburetor heat.

When the throttle is closed during the flight, the engine quickly cools and fuel evaporation less complete than when the engine is warm. Also, in this state, the engine is more susceptible to carburetor icing. Therefore, if you suspect carburetor icing and anticipate closed throttle control, adjust the carburetor heat to the full ON position for closing the throttle, and leave it on during closed-throttle operation. The heat will assist in the vaporization of the fuel, and to help prevent the formation of the carburetor ice. Periodically, open the throttle smoothly for a few seconds to keep the engine warm, otherwise the carburetor heater can not generate enough heat to prevent icing.

The use of heating carburettor causes a power loss, sometimes up to 15 percent, because the heated air is less dense than the outside air that was in the engine. This enriches the mixture. When ice is present is used in an airplane of fixed proppellor carburetor and heat, there is a decrease in rpm, followed by a gradual increase in rpm, the ice melts. The engine must also float after the ice has been removed operation. If ice is not present, the r.p.m. decreases, then remains constant. When carburettor heat is used in a plane with a constant speed propeller and ice is present, a decrease in the inlet pressure will, it is noted, followed by a gradual increase. If carburetor icing is not present, the gradual increase manifold pressure will not be clear until the carburetor heat off. It is necessary that a driver recognizes carburettor ice as it forms during the flight. Moreover, power loss will, height and / or air velocity arise. These symptoms may be accompanied by vibrations of the engine roughness. Once a power loss is noticed, action must be taken immediately to remove the ice already formed in the carburetor and prevent further icing. This is achieved with the full carburetor heat, which will lead to a further reduction of the energy costs and possibly engine roughness as melted ice cream passes through the motor. These symptoms can last from 30 seconds to several minutes, depending on the severity of the icing. During this period, the pilot must resist the temptation to the carburetor to reduce heat consumption. Carburettor heat must be returned in the full-hot position to normal flow.

ince the use of carburetor heat tends to reduce the output of the engine and also to increase the working temperature, carburetor heat should not be used at full power necessary (such as during take-off) or during normal engine operation, except for the presence or carburettor ice to delete.


Some aircraft are equipped with a carburetor air temperature gauge, which is useful in detecting icing conditions. Typically, the face of the meter is calibrated in degrees Celsius (° C), with a yellow bow in which the carburettor temperatures at which ice can form. This yellow arc typically varies between -15 ° C and + 5 ° C (5 ° F and 41 ° F). When the air temperature and the humidity of the air are such that the carburettor icing is unlikely, the motor can be operated with the indicator in the yellow range with no adverse effects. Where the atmospheric conditions are conducive carburetor icing, the indicator must be kept outside the yellow arc by application of heat carburetor.

Some meters have a red radial carburetor air temperature, which is the maximum permissible carburettor inlet air temperature is recommended indicated by the manufacturer of the motor; Also, a green arc can be included to indicate the normal operating range.


Most aircraft are also equipped with an outside temperature (OAT) meter calibrated in both Celsius and Fahrenheit. It provides the outside or ambient temperature to calculate true airspeed, and is useful in detecting potential icing too. (Thanks to the FAA)




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