From: highflyer <highflyer@alt.net>
Newsgroups: rec.aviation.homebuilt
Subject: Re: Why carb ice in airplanes but not cars?
Date: Tue, 29 Sep 1998 07:58:22 -0500

John Haggerty wrote:

> Actually, car engines are susceptible to icing. It has to do with the drop
> in temperature caused by the faster flow of air near the air input to the
> carburetor, combined with amount of moisture in the air. Cars also have heat
> supplied to the air intake to help eliminate the icing. This could be a
> temperature sensitive air input from the exhaust manifold, a hot water line
> from the radiator, etc. Some of the old clunkers I used to drive didn't have
> all the correct parts sometimes, and it gets exciting when there's enough
> ice buildup to prevent your throttle releasing when you try to decelerate.
> Aircraft usually fly in lower temperatures than are found on the surface.


Actually, whether or not aircraft usually fly in lower temperatures
than are found on the surface, cars frequently run in lower temperatures
than light aircraft usually do.  Jets do get high enough to get into
the canadian arctic temperature range.  I NEVER do with my airplane.

Also carb ice is NOT a problem if the air is cold enough.  It is
usually only a problem from a few degrees below freezing up to about
the mid 60's fahrenheit.  Say from -5 to 15 Celsius.

There is a strong temperature drop in the carburetor throat.  Everyone
answering on this string seems to think the temperature drop is due to
the acceleration and resulting drop in pressure going through the
venturi.  While there may be a drop of a degree or two in temperature
due to that reason, what really causes the temperature to fall if the
fuel coming in through the carburetor.  It requires a LOT of heat to
vaporize all of that fuel.  That heat primarily comes from the air
passing through the carburetor.  The heat of vaporization will typically
drop the temperature of the incoming air from ten to fifteen degrees
celsius.  This temperature drop, caused when the gasoline evaporates,
condenses out the water vapor in the incoming air.  If the temperature
has been dropped below zero celsius, this condensation will immediately
freeze.  It will then SNOW inside  your carburetor.  Just like a snow
drift will form behind a snow fence, this snow will build up on the
back of the throttle plate.  That is why carb ice is most likely when
the throttle is NOT fully open.  When it is open, the snow just goes
on into the cylinder and cools the mixture a little, allowing you to
charge the cylinders with a denser mixture.

If you should ever have the carb heat cable break, like I did once
when flying in a snowstorm, and start to get carb ice, like I did
when flying in that snowstorm, and can't melt it without a carb heat
capability, do like I did.  If you open the throttle all the way and
fly as slowly as you can ( barely above stall speed ) so that you are
on the back side of the power curve, you do two things.  You increase
the temperature inside the cowling because of the high power settings
and low airflow, and you remove the throttle plate obstruction from
the incoming air so the ice tends to pass on into the, now hotter,
engine where it harmlessly melts.    When you get the ice melted,
transition back to normal cruise settings.  It worked for me.