Carb Heat

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cristi.neagu
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Carb Heat

Post by cristi.neagu »

I wanted to do some pattern works in "sunny" Pacific Northwest (yes, i'm using live weather), and i ran across an interesting problem. Engine power kept dropping until i was pushing the throttle all the way in but the engine was idling. Turns out the carb froze. Mind you, this is at 9C. So i was wondering: what's the proper way to use carb heat? Do you keep it in all the time?

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Oracle427
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Re: Carb Heat

Post by Oracle427 »

It is used as needed, so of you you need to keep it in, then yes, keep it in while you observe signs of power loss.

The outside temp generally must be higher than freezing for carb icing. The air temp drops when the pressure drops as the air passes through the restriction in the carburetor and fuel evaporates into the air further dropping the temperature as it absorbs heat from the air as it evaporates.

If the air entering the carb is already very cold, it won't have much moisture content to begin with so icing us usually less of a factor. The temp can drop by 15C+ as the air passes through the carburetor. When the throttle body is not fully open, there is a greater restriction and therefore a bigger temp drop. The plate also presents a nice large and cold surface for ice to stick to.
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AKar
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Re: Carb Heat

Post by AKar »

In A2A Comanche, it is a safe bet to pull the carburetor heat on whenever it is raining. I don't remember if I've ever encountered carburetor icing in it otherwise, certainly it doesn't happen too often.

As already mentioned, chances of carburetor icing often goes up with increasing temperature - this has to do with moisture content of the air that can increase dramatically with rising temperatures. Cold air only contains small amounts of moisture, so the icing effects of it are minimal in comparison.

Moist, rainy weather close to zero degrees centigrade makes a special case in that sense the parts in the induction system that are normally not too prone to icing may freeze over due to pressure drops in the induction/fuel system, and associated temperature drops.

The largest contributor to the carburetor icing has to do with the issue of fuel evaporating within the area of the throat and the throttle plate assemblies, which make vulnerable surfaces for ice to form. Heat of vaporization of the aviation gasoline is something like 350 kJ/kg. If we assume a fuel flow of 60 lbs/h, or about 0.45 kg/min, the energy absorbed by the fuel evaporation is roughly 158 kJ/min, or 2.6 kJ/s, or perhaps more illustratively (even if being somewhat funny use of units), 2.6 kilowatts, or 3.5 horsepower.

This is a lot of heat getting absorbed, equivalent to what a powerful kitchen water boiler takes from the mains. This is enough to cause an overall temperature drop of 20 °C ... 25 °C of the mixture, depending on the mixture itself, temperatures of its ingredients and many other things. Not coincidentally, typical carburetor icing charts show a reduction in icing hazard after we get much warmer than that.

In pressure carburetor, the issue of carburetor icing is significantly reduced by simply putting the point of fuel injection downstream of the carburetor itself.

-Esa

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cristi.neagu
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Re: Carb Heat

Post by cristi.neagu »

Thanks for the replies, guys. It all makes sense now.

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Re: Carb Heat

Post by n421nj »

Very informative but short answer is use as needed
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