TEXTRON LYCOMING OPERATOR'S MANUAL
SECTION 3
0-360 and ASSOCIATED
MODELS
c. Use of Carburetor
Heat Control - Under
certain moist atmospheric
conditions at temperatures
of 20
°
to 90
°
, it is possible
for ice to form
in the induction system, even in summer weather. This is due to the
high air velocity through the carburetor venturi and the absorption of
heat from this air by vaporization of the fuel. The temperature in the
mixture chamber
may drop as much as
70°F. below the temperature
of
the incoming air. If
this air contains a large amount
of moisture, the
cooling
process can cause precipitation
in the form of ice. Ice formation
generally begins in the vicinity of the butterfly and may build up to
such an extent that a drop in power output could result. A loss of
power is reflected by
a drop in manifold pressure
in installations
equipped
with constant
speed propellers
and a
drop in manifold
pressure and RPM
in installations equipped with
fixed pitch propellers.
If not
corrected, this condition
may cause complete engine stoppage.
To avoid this, all installations are equipped with a system for
preheating
the incoming air supply
to the carburetor. In
this way
sufficient heat
is added to replace the heat loss
of vaporization of fuel,
and the mixing chamber temperature cannot drop to the freezing point
of water. This air preheater is essentially a tube or jacket through which
the exhaust pipe from one or more cylinders is passed, and the air
flowing over these surfaces is raised to the required temperature before
entering the carburetor. Consistently high temperatures are to be
avoided because of a loss in power and a decided variation of mixture.
High charge temperatures also favor detonation and preignition, both of
which are to be avoided if normal service life is to be expected from the
engine. The following outline is the proper method of utilizing the
carburetor heat control.
(1) Ground Operation - Use
of the carburetor air heat
on the ground
must be held to an absolute minimum. On some installations the air
does not pass through the air filter, and dirt and foreign substances
can be taken into the engine with the resultant cylinder and piston
ring wear. Carburetor air heat should be used on the ground only to
make certain it is functioning properly.
(2) Take-Off- Take-offs and full throttle operation should be made
with carburetor heat in full cold position. The possibility of
expansion or throttle icing at wide throttle openings is very remote,
so remote in fact, that it can be disregarded.
3-10
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