[AT] Carburetor Icing (Off Topic)

[Gene Dotson]

    Restriction and constriction are two seperate items.
    Restriction, as provided by the throttle plate reduces the pressure 
within the entire intake system and is what we read on a vacuum gauge or 
load monitor.
    Constriction is a localized pressure and velocity change whereby the 
flow is restricted and then returned to the previous pressure and velocity. 
Venturi pull is what draws the fuel into the airstream as it passes through 
the carburetor because this the lowest pressure and temperature in the 
system. Venturi pull is greatest at the highest air flow, therefor drawing 
more fuel for higher power operation.
    The absence of a venturi in the system is the primary reason fuel 
injected engines do not suffer intake icing.
    Older aircraft used a vacuum venturi as a means of powering flight 
instruments. This was mounted on the outside of the airplane in the 
airstream and would provide approximately 5 inches of vacuum by this alone. 
This was also subject to freezing at ambient temperatures above 32 degrees

                        Gene





----- Original Message ----- 
From: "Dudley Rupert" <drupert at premier1.net>
To: "Antique tractor email discussion group" <at at lists.antique-tractor.com>
Sent: Sunday, December 09, 2007 4:33 AM
Subject: [AT] Carburetor Icing (Off Topic)


?"
>
> 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.