Carb. Icing

G

gnx99

New Member
Why is carb icing only an issue at lower power settings like 1500 RPM, and not at higher RPMs, like 2500?
 
Trying to remember this from my systems class. Someone tell me I'm full of it because this is probably wrong: throttle plate being closed and the air being so cold from the venturi gives a nice flat, cold place for humidified air to freeze on and completly block off the carb?

That's probably wrong though, 'cause then the idle jet would still work but I thought when you got some good carb ice you're whole engine stops no matter what.

But there is my bad guess.

Cheers


John Herreshoff
 
Actually the decrease in air pressure as air flows through the venturi, plus fuel vaporization, causes carburetor ice. Decrease in air pressure causes temp. to go down. Like if I'm shuttin down the garage at work, and I drain the air compressor, air is moving fast through the valves and increased velocity of a fluid means decreased pressure, and that causes ice to form on the pipes.

Carb. heat uses air heated by the exhaust to go into the carburetor, which prevents ice from forming, but results in dcreased rpm because it is less dense than the regular outside air.

But I have no idea why it is not an issue at higher rpms. So, basically, um, I guess I failed to answer your question...

Someone else then!
 
That is such an excellent question I had to Google it. Here's what I found. Seems to make sense, and goes along with John's idea about closed throttle plate = lower pressure = lower temp. Tell me what you think! I'm a bit of a gearhead, and looove physics, so this thread really piqued my curiosity! John- do you still have any of those systems class books laying around??
 
Heya Jim,


You know, I don't think I have the system's one here anymore but I'll look. I am pretty sure I have my aerodynamics book 'cause I think the binding ripped right before I was going to sell it back, so I couldn't get rid of it if I wanted to. The system's book was written by Jepp, though, so it's a pretty avalible book I'm sure. Not quite sure of the title, but if you're interested in it I can find out.

Cheers


John Herreshoff
 
I remember reading about this very question. I "THINK" the reason NO carb-heat is required at a high power setting is that the ice does not have a chance to form and gather in great enough quantity to cause problems. Any Carb-Ice that tries form during a high power setting is sucked through the induction system and never given a chance accumulate. Does that make sense? I think the environment inside the carb at high power settings is not good for carb ice.

Tony
 
Another reason is due to the butterfly value no longer being in a full open position when you are not in full throttle mode. Picture this value as being slightly closed and therefore creating an eddy of air on the back side of the value. This eddy allows the formation of ice. In full throttle position, the air pressure is enough to not allow the formation of ice. Remember, the ice is a precipitate of water on to cold metal of the carb. If you don't allow the air to slow down it will not precipitate.
 
This thread reminds me of an instructor I had in Altlanta that suspected carb ice, and pulled the carb heat on for about two seconds, and then turned it off again.
confused.gif
It was obvioulsy not enough time to have any effect, and the instructor obviously had no idea what indicated the onset of carb ice, or the proper method of countering it.
I've experienced it in a 172, and although a bit worrisome, is really a non-event provided you recognize it early and counter it effectively
 
Machado's private book explains it well (pictures and all
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): It's the valve.... when you're full open the air rushes past all the parts

When you're at low RPMs the valve is flatter, giving ice the chance to form on the backside.
 
Three Types of Carb Icing: Fuel evaporation ice=caused by the decrease in air temperature resulting from the evaporation of the fuel after it is introduced into the airstream.Throttle ice= forms on the rear side of the throttle when the throttle is partially closed. The rush of air across and around the throttle valve causes low pressure on the back side. this creates a pressure differential across the throttle which causes a cooling effect on the fuel and air mixture.Impact ice= formed as a result of snow, sleet, or water that remains on the surface when temperatures are below 30'F. Usually builds on the carb screen.
So, carbs are prone to ice up at any throttle setting, just that at partial power operations there is a greater chance due to the explanation above!
wink.gif
 
Yup you can get icing at any setting - it's just more likely at low power settings.

We have Carb Temp guages and SOP is to put the heat on (we also have high temp boxes that allow us - in combination with the temp guage - to vary the amount of heat we're using) if the temp gets into the yellow arc - which I believe starts at about 10C. The hot boxes can generate heat up to 200F.

One day on a flight from Laughlin to PHX we were hanging out at about 9,500 under some overcast and sure enough we were getting into the yellow and I'm pretty sure we were starting to get some ice. Pulled the heat on, the roughness went away and we kept on goin.
 
[ QUOTE ]
This thread reminds me of an instructor I had in Altlanta that suspected carb ice, and pulled the carb heat on for about two seconds, and then turned it off again.
confused.gif
It was obvioulsy not enough time to have any effect, and the instructor obviously had no idea what indicated the onset of carb ice, or the proper method of countering it.
I've experienced it in a 172, and although a bit worrisome, is really a non-event provided you recognize it early and counter it effectively

[/ QUOTE ]

Which instructor was that??
 
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