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Old 01-19-2021, 10:18 PM
Bicyclops Bicyclops is offline
Join Date: Oct 2012
Location: LA, California
Posts: 361

Originally Posted by Freemasm View Post
I am aware, Sir. Sorry if I'm being dense but I'm going to be stubborn here. Bare with me.

For a given RPM, the IC engine is a constant volume machine. (Probably) No one will argue that. The volumetric flow through the venturi (thus ~ fluid speed) does not changep with throttle position alone. I’m aware that usually RPM will not stay that constant Air density on the other hand... Your position assumes that the static P effects are limited to the venturi. That is far from true. At some point of closure, the throttle losses become the predominant pressure loss; very high velocities occur around the butterfly valve. Static P drops proportionately to the square of this velocity.

So I'll ask again. Does anyone have data that supports the previous statement that fuel evaporative effects are the primary driver? I'll state again, the engine OEMs operation recommendations and the experiences posted here by others (at least anecdotally) backs that up as does axial compressors icing, i.e.closing inlet guide vanes increases this risk still without no latent heat effects.

Sorry for the back and forth. Respectfully submitted. I’m really trying to understand the entire proportional relationships.
As you point out, a closed throttle causes lowered pressure and causes the moisture in the air to freeze out. This is why throttle icing tends to occur at mostly closed throttle and on the back side of the throttle plate where the pressure is the lowest. Fuel vaporization icing occurs at the discharge nozzle and will be more likely to occur at higher throttle settings because the pressure is low enough in the venturi to draw the fuel from the float chamber and cause it to vaporize. lowering the temperature enough to freeze the moisture in the air. With the throttle closed, there isn't enough delta P to draw fuel through the main jet reliably and so an idle circuit with an outlet right by the edge of the closed throttle is necessary. The idle port could still ice at the same time the throttle is icing for a different reason.

So the question that I see you raising is which effect predominates. It depends on throttle position. I don't think it matters much to the pilot which effect is causing the power loss or making the throttle immovable. Yes, full carb heat is what you need for either cause.

Ed Holyoke

Last edited by Bicyclops : 01-19-2021 at 10:25 PM.
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