A throttle is located on the instrument panel in the cockpit. When the
throttle is closed, it is pulled rearward toward the pilot until it is stopped
by mechanical means. At this setting, the engine continues to run, but at “idle”
speed (a few hundred RPM). As the throttle is moved forward, the throttle valve
in the carburettor opens allowing more air into the carburettor, 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.
The the air is routed from the intake through ducts into the
carburettor. The
carburettor on most engines are of the updraft type; i.e. the
carburettor is mounted on the bottom of the engine, and the fuel/air mixture is
sucked upward to the engine.
When the carburettor heat control in the cockpit is pulled on, heated air
enters the carburettor. 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 carburettor icing or the normal intake
duct become blocked by ice at the induction port and air filter.
The carburettor 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 carburettor 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 carburettor. 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 carburettor. The fuel vaporizes into fine particles in the intake air flow.
This atomised 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 carburettor 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.
