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carburetor opens allowing more air into the carburetor, thus increasing the RPM. When the throttle is full
forward maximum RPM results. The throttle can be locked into a set position with a friction lock so that
in cruise flight the power setting will remain set. This relieves the pilot from constant attention to the
throttle.
Carburetor
The carburetor provides 2 principal functions.
· It mixes the fuel with the air in the proper proportion
· It regulates the amount of air (and thus fuel) that enters the engine.
The the air is routed from the intake through ducts into the carburetor. The carburetor on most engines
are of the updraft type; i.e. the carburetor is mounted on the bottom of the engine, and the fuel/air
mixture is sucked upwardto the engine.
When the carbuerator heat control in the cockpit is pulled on, heated air enters the carbuerator. The air
source comes from inside the cowling, and passes through a “heat” box to warm the intake air. The
heated air can be selected when atmospheric conditions are conducive to carburetor icing or the normal
intake duct become blocked by ice at the induction port and air filter.
The carburetor is equipped with a small chamber containing fuel and a float valve. The valve maintains a
constant amount of fuel in the chamber. This provides a constant and sufficient source of fuel to satisfy
the fuel demands of the engine.
The main air duct of the carburetor is a tubular structure which decreases in diameter near the middle of
the duct, then increases in diameter near the intake manifold end of the carburetor. This is called the
“venturi”. This decreased diameter creates a vacuum in accordance to the Bernoulli principle. The fuel
intake port is located in this section. A metered amount of fuel is sucked into the carburetor. The fuel
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vaporizes into fine particles in the intake air flow. This atomized fuel and air mixture is of proper
proportion to cause correct burning of the fuel/air mixture in the engine.
The fuel / air mixture is set by design to be correct for operation at sea level. As the engine is operated
over a range of altitudes and air densities, the pilot can adjust the mixture via manual means in the
cockpit. It is called the “mixture control”. The correct mixture adjustment procedure is covered in the
Pilot Operating Handbook (POH) for the given aircraft. Some aircraft are equipped with an Exhaust Gas
Temperature Gauge in the cockpit. A proper fuel/air mixture will produce a given exhaust gas
temperature. The pilot can adjust the fuel / air mixture to a fairly accurate measurement by observing that
the exhaust gas temperature is within the proper range.
The throttle regulates the amount of fuel/air that enters the engine, thereby controlling the power that the
engine develops. On aircraft with a “fixed pitch” propeller, the throttle directly controls the engine RPM.
On aircraft with a variable pitch propeller, a Manifold Pressure Gauge directly measures the engine
power being developed. A propeller pitch control controls the propeller blade angle. The power setting of
the engine requires adjustment of both the throttle and propeller pitch control.
The carbuerator has an accelerator pump which will provide a “burst” of additional fuel for quick
development of maximum horsepower, such as performing a go around from landing approach. An
economizer valve allows the engine to idle when the throttle closed.
Icing
The predominate forms of icing affecting engine operation are carbuerator ice, throttle ice and induction
ice.
Carbuerator Icing
Carburetor icing is a constant concern to the pilot when operating in high
humidity and visible moisture conditions. Whenever the outside
temperature is 20° to 70° F, ice creation in the throat of the carburetor is a
possibility. Due to the Bernoulli effect and the vaporization of the fuel in
the venturi, the temperature of the fuel / air mixture can be as much as 50
degrees lower than the outside air.
Induction Icing
INDUCTION SYSTEM
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The air induction port at the front of the cowling can become partially or totally clogged with ice when
air temperatures are 32° F or below while flying in visible mousture. This is known as “impact” ice, and
is most prevalent when the Outside Air Temperature (OAT) in around 25° F and super-cooled moisture
exists.
Throttle Ice
Throttle ice in the carburetor occurs most often when the throttle is partially closed. This can occur at
low cruise speeds or near idle situations such as approach to landing. Some manufacturers recommend
　
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