[AT] Carburetor Icing (Off Topic)

[Dudley Rupert]

Several of the recent posts on wind chill and oil change veered into
carburetor icing which I found to be an interesting subject in and of
itself.  For my own enlightenment I summarized/paraphrased some of those
posts, added input from my own rusty background and checked a few web sites.
You're welcome to read if you feel so inclined.
Dudley
Snohomish, Washington

The cause of ice is simple:  The presence of moisture/water and a low enough
temperature.  In the case of carburetor icing, both inside and outside, we
might ask the question "How does the temperature get low enough on a
relatively mild day to form ice on/in the carburetor?"

Several posts suggested the venturi effect as being the/a cause of this drop
in temperature, at least one post suggested the Intake Stroke/Vacuum Effect
as being the culprit and one suggested the vaporization of the gasoline.
Let's take a gander at these -

Venturi Effect - Simply put, when a constriction is placed in the path of a
fluid flowing through a tube two things happen:  1) the velocity of the
fluid increases through the constriction and 2) its' pressure decreases.
In the carburetor the constriction is the throttle plate and, it goes
without saying, the more it is closed (i.e., the closer to idle) the greater
the restriction.  Drop in temperature occurs anytime there is a drop in
pressure.  And this drop in temperature will, if the humidity is high
enough, result in condensation of the water vapor.

Intake Stroke/Vacuum Effect - As described above one by-product of the
Venturi Effect is a drop in pressure and this is also what occurs during the
intake stroke.  Thus, the results described above for the Venturi Effect
also apply for the effect of the intake stroke.

I was/am curious as to which causes the greater loss in pressure (and thus
greater drop in temperature):  the Venturi Effect or the Intake
Stroke/Vacuum Effect.  I pursued a dozen or so web sites with no luck.  I
did find, however, one site that indicated the temperature drop due to the
Venturi Effect was probably no more than a degree or two and a couple of
sites that simply said that the temperature drop due to the Venturi Effect
was insignificant.  What was not clear to me, however, was whether the sites
were speaking only for the result of the Venturi Effect or for the results
of both the Venturi Effect and the Intake Stroke/Vacuum Effect.  But the
bottom line is that several sites indicated that the vaporization of the
gasoline (discussed below) makes the biggest contribution to the overall
temperature drop across the carburetor.

Vaporization of Gasoline - Liquid gasoline is ingested into the carburetor
venturi where it is vaporized, that is, where its' state is changed from a
liquid to a gas.  This transformation requires a relatively huge amount of
energy, most of which comes from the air passing through the venturi.

By way of example let's look for a moment at water.  If we had one gram of
liquid water at 0 degrees C and wanted to raise its' temperature to 1 degree
C it would require 1 calorie of heat (assuming a standard day and all that
good stuff).  If we wanted to raise the temperature of this same one gram of
liquid water from 0 degrees C to say 100 degrees C if would take 100
calories of heat.  But, if we wanted to change the state of this one-gram of
liquid water at 100 degrees C to one gram of steam at 100 degrees C it would
take 540 calories.  My point in this mind-numbing excursion is simply to
note that it takes nearly five and a half times as much heat to change the
state of liquid water at the boiling point to steam at the boiling point as
it does to raise the temperature of liquid water from the freezing point to
the boiling point.  Now back to the vaporization of Gasoline.

I am too tired and/or lazy to look it up but I seem to recall that the
Specific Heat (i.e., the energy required to raise one gram one degree C) of
generic gasoline is approximately half that of water and its' Latent Heat
(i.e., the energy required to change the state) is approximately one sixth
that of water.  My point here is that while it does not take as much heat to
vaporize gasoline as it does water it still takes a bunch.

Conclusions:
1) The total temperature drop across a carburetor w/o any heat sources can
range from 15 to 25 degrees C and
2) The primary cause of this temperature drop is the vaporization of the
gasoline.
(The above as read in three web sites).

Now if I could of just stated those two conclusions and left all the
rambling out...