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Anyhow, when the butterfly is
closed, the engine still needs to be
fed with fuel, so there is an idle jet
that bypasses it to keep the engine
idling. It also helps the venturi, since
the airflow at idle is quite small (the
jet is actually a hole next to the
butterfly, and it's sometimes called
the slow running jet). Also, when you
need power in a hurry, there is a
small lag due to inertia between the
time you open the throttle and the
time the engine starts to speed up,
because the air supply responds
more quickly than the fuel, which
gives you a weak cut (a momentarily
weak mixture), so a small squirt of
fuel is delivered separately to
compensate for this, from an
accelerator pump. When starting an
engine from cold, therefore, resist
the temptation to pump the throttle,
because all you will do is flood it
with large drops of fuel. A better
tactic, if you need the throttle open,
is to do so very slowly, so the pump
doesn't kick in.
Because aeroplanes go up, and
because air gets less plentiful at
height, there is a danger of the
46 JAR Private Pilot Studies
fuel/air mixture getting out of
balance as you climb. A mixture that
has too much fuel against air is rich,
while one the other way round is
weak. The mixture control is provided
to adjust for this – for example, you
would have it set fully rich for
takeoff and landing. At height, the
engine will not work at all if the ratio
of fuel to air is not correct.
Leaning makes the engine run hotter
and give you more power for less
fuel; a 112 hp aircraft cruising at
4000 feet and 85 knots will burn 5
gallons an hour when rich, but only
4.5 when leaned, giving a range of
116 miles as opposed to 100—a
saving, or an increase, of 16%.
Although you could lean off slightly
in the climb for better economy,
never take off with reduced power
or too lean a mixture. It may save
fuel, but petrol has a high latent heat
content, and the excess inside a
cylinder from a rich mixture has a
cooling effect when it evaporates.
The "normal" mixture is about 15:1
of air to fuel by weight, but this is
not critical over quite a wide range
(some say 14:1 is correct).
The mixture control has a secondary
function, which is to cut fuel from
the engine on the ground when you
want to stop it (you don't just switch
the magnetos off). The Idle Cut Off
(ICO) in the carburettor is joined to
the mixture lever with a Bowden
cable. When the lever is operated at
the end of a flight, the engine is
starved of fuel, and stops.
Carburettor Icing
This is actually one aspect of
induction system icing. The other
two are fuel icing, arising from
water suspended in fuel, and
impact ice, which builds up on the
airframe around the various
intakes that serve the engine.
Even on a warm day, if it's
humid, carburettor icing is a
danger, especially with small
throttle openings where there's
less area for the ice to block off
in the first place (as when
descending, etc.). Also, the
temperature drop (between the
OAT and that in the venturi)
can be anywhere between 20-
30°C, so icing (in an R22,
anyway) can happen even when
the OAT is as high as 21°C
(70°F), or more. Tests have
produced icing at descent power
at temperatures above 30°C,
with a relative humidity below
30%, in clear air. Because it is
more volatile, and likely to
contain more water, you can
expect more fuel and carb icing
with MOGAS than AVGAS.
It usually arises from the action
of the venturi in the throat, just
before the butterfly valve, which
regulates the amount of fuel
into the engine. You will
remember the venturi's purpose
is to accelerate airflow by
restricting the size of the
passageway, which has the
effect of reducing the pressure
and pulling the fuel in.
Unfortunately, this process also
reduces the temperature, as
does the fuel vapourisation,
hence the problem (the lower
temperature means greater
relative humidity, and closeness
to the dewpoint, and the
vapourisation takes its latent
heat from the surroundings,
making the situation worse). In
Engines & Systems 47
fact, the vapourisation (and
cooling) can carry on most of
the way to the cylinders, causing
the problem to persist,
especially with the butterfly
semi-closed, which produces
　
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