Carburetor Heat
Many light aircraft are equipped with a control
which is mysterious to the generation of pilots who grew up after the CD was
invented – the carburetor heat control.
These pilots probably never saw or operated a
carburetor until they arrived at the airport for flight training. Their automobiles and even motorcycles are
electronically fuel injected.
For these pilots, a little background is in
order. A carburetor is a mechanical
device designed to evaporate liquid gasoline into a combustible mixture which
is sucked into the engine and burned to produce power. This is different than fuel injection, where
the fuel is pumped into the intake port just before it is sucked into the
cylinder to be burned.
Some aircraft – especially newer ones – are equipped
with a simple mechanical fuel injection, which is really nice, but much more
expensive and complicated than a carburetor.
Fuel injected engines will not have a “Carb Heat”
control knob. They might have an “Alternate
Air” control – or they might not. Some
aircraft simply use a spring to automatically open the alternate air when the
air intake of a fuel injected engine becomes blocked (e.g. during severe icing
conditions).
But if your aircraft has a “Carb Heat” control, it’s
got a carburetor, and there are a few really important things you need to learn
about it, to avoid losing engine power.
In theory, a carburetor is pretty simple. It uses something called a venturi, which creates
suction and pulls liquid gasoline from a float bowl inside the carburetor, just
like you suck on a straw to drink a soda.
The fuel is evaporated and mixes with the air rushing through the
carburetor into a combustible mixture for the engine to burn.
In practice, a carburetor is a little trickier than
that – it has all sorts of gimmicks and tricks, like an accelerator pump, to
deal with transient conditions.
One very practical fact about a carburetor is that
it is a little refrigerator. If you put
your hand on a carburetor of a running engine – or right after it stops – you will
notice that the carburetor is cold. The
reason for that is because of the venturi decreasing the pressure of the air,
and the energy taken to evaporate the fuel.
This is actually pretty easy to understand. Have you ever pushed a tire valve in, and let
the air out of a tire? The valve will
get cold, because the pressurized air in the tire that was at room temperature
is now spread out into a larger volume, but with the same amount of
energy. That results in cooling. That’s exactly what a venturi does, with its
pressure drop. It’s called adiabatic
expansion, if you are interested in such arcane details.
Also, it takes energy to evaporate the liquid fuel,
just like you have to heat up the element on your kitchen stove to heat up and
boil water in a pot or kettle. That’s an
endothermic reaction, in physical chemistry.
Depending upon the conditions, you can actually see
condensation on the outside of the carburetor, if it’s cooled below the
dewpoint of the air mass. Heck, you can
even see ice on the outside of a carburetor.
And you can even get ice forming inside the carburetor, which is really
bad.
Remember that an internal combustion engine is
really just an air pump. And it’s got to
be able to get air into and out of the engine.
Any kind of plugging of the intake or exhaust of an engine is really bad
news.
That’s why it is a legal requirement for any
certified aircraft with a carburetor, to be equipped with a carburetor heat
system, to melt the ice which can plug it up.
You would be insane not to have a carb heat system on a homebuilt. Incredibly, many ultralight aircraft do not
have carb heat. They scare me, for many
reasons.
The carb heat system is actually very simple. It consists of a piece of bent metal around
the very hot exhaust tubes. Air flows
around the hot exhaust tubes and is heated up.
Just before the carburetor, there is a flapper box which allows you to
select either normal cold filtered air, or to select the heated air.
When you pull the carb heat knob out, a cable moves
the flapper and hot air goes into the carburetor to melt any ice that may have
formed.
The problem is that it can be hard to tell how much
ice is forming, and how fast. Some
aircraft are equipped with a carb heat temperature gauge which will allow you
to intelligently select partial carb heat, but they are rare in my experience.
Without a carb heat temp gauge, it is best to use
the carb heat knob as a binary control – either the carb heat is fully off
(knob in, cold air) or the carb heat is fully on (knob out, hot air). Generally I like to use full carb heat
whenever the RPM is below the green arc.
The POH of course is golden – do what it says.
Different engines and different installations of the
same engine will be more or less susceptible to carburetor icing. The Continental engines (e.g. O-200 or O-470)
with the carburetor hung out in space below the engine will make good power,
but tend to ice up pretty badly. The
Lycoming engines (e.g. O-235, O-320, O-360) bolt the
carburetor right to the oil sump, and run the intake tubes through it, so as
soon as the oil is up to temperature, they are pretty resistant to carburetor
icing, but it can still happen. You have
to watch Lycomings on the first takeoff of the day, when the oil is still cold.
What I do, when the conditions are ripe for
carburetor icing (cool, low dewpoint spread) is after taxiing out for takeoff –
when the carburetor may have filled up with ice – is to position for takeoff on
the runway and select the brakes and push the throttle in for 1500 RPM and
select carb heat and lean the mixture for max RPM. I count to ten, then all the knobs go into
the panel and I take off. This ensures
that I don’t have any carb ice, at least at the start of the takeoff roll.
Let’s look at what I did above. Something I did which might surprise you, but
is very important, is that I leaned the mixture after selecting carburetor
heat. No one does this, but it is very
important because when you selected the carb heat, you enrichened the mixture
with the hot, less dense air and the same amount of gasoline. A richer mixture will cool the engine, and
that’s not what we want right now. We
want the exhaust to be hot, not cold, to produce maximum carb heat.
So the lesson for you today is that every time you
select the carb heat, you must also lean the mixture for max RPM, in order to
produce hot air. This may be
counter-intuitive in a descent, when you are used to enrichening the
mixture. And in the circuit, you
probably are really uncomfortable with anything less than full rich
mixture. And that’s ok, but just be
aware that you are not producing as much hot air as you could be.
Most of the time that doesn’t matter, but if you
ever have a real carb icing encounter (which is indicated by a loss of RPM with
a fixed pitch prop, or a loss of manifold pressure with a constant speed prop)
please do remember to lean the mixture for max RPM with a fixed pitch prop, or
max airspeed with a constant speed prop.
--
acboyd@gmail.com Jan 2012