Question Regarding Carb Heat

[RICK MARTIN]

At the risk of embarrassing myself, I'll ask this question:

Why do no other engines that I can think of require a carb heat system other than aircraft engines? And why not all aircraft engines? Is it only 4 stroke? Does it depend on attainable altitude?

[quadrirotor]

A very interesting question, because the answer is not always well understood! Not easy to be clear neither!
At the risk of embarrassing myself, I will answer this question...
The ice forms itself (most of the time) when the temperature is bellow 0° Celsius (let us say -2°C...-3°C), and the humidity of the air is close to the due point (let us say lot of water in the air)...
The capacity of the air to sustain water is less when depressed, is less when it is cold…The ice forms quickly itself when in contact with something which is bellow 0°C.
Evaporation of gas creates a lot of cold; depressing the air creates lots of cold and lower the capacity of the air to sustain water.

Let us say the outside air is 5°C (OAT), very humid, the air enters the carburetor, collides with the butterfly, behind the butterfly it depress and it is sucked, the temperature is lowered and the capacity to sustain water is lessened, moreover the gas evaporates, so the temperature get lower than -5°C…Formation of the ice where the ice can stick and built up!

Most of the engines have a carburetion heater (used when needed or automatic). Aircrafts, cars, boats, etc…
When the engine is fuel injected, the evaporation is so close to the intake valve that the gas does not lower the temperature of the intake, but in some severe conditions, ice could form behind the butterfly (the same for diesel, injected in the cylinder), some carb are more prone to icing (venturi)…

For engines where the oil is mixed with the gas, the ice has difficulties to stick to the metal, and is ingested, but sometimes the ice finds somewhere to stick…If the oil is not mixed, the ice can form itself and sticks inside, or outside…To be convinced, when a two strokes engine is used in an hovercraft, on a Canadian lake, at a temperature near 10°C, the ice can be seen all over the carburetors (inside and outside) and stalls if not heated!…
There is a chart giving the conditions prone to get ice in the carb, I’ll post it, if find it…

[mceagle]

Pre fuel Injection days all modern cars had carby heat. You probably didn't notice it because in most cases it was automatic. That is that air-duct hose running from the aircleaner snout to the exhaust. It was common for older cars than that to suffer carby ice. Carby ice is not considered to be a problem on Fuel Injected engines.

[KenSandyEggo]

Carb ice can also form at relatively warm temperatures if the dew-point is close to it. I've experienced it here in San Diego while taxiing when I had the carb on my Soob.

[John_Read]

Yes Ken.

Taxying is prime carb-ice time.

My aircraft has Rotax 532 with Bing Carbs, and has the habit of stopping on me after a long taxi unless I keep the revs up and wait a few moments before engaging the pre-rotator at the hold, I guess to allow for some heat to reach the carbs. There is no "carb heat" on this engine, though I would like some. I am very careful about how long I let the engine stay in "flight idle" when up in the air and doing descents.

I plan to fit the electric carb heat kit during the winter "refit".

John

[birdy]

Good idea John,don't let anyone tell you that 2 bangers won't ice up, even at cruise setting.

[Doug Riley]

I've never had a Rotax (2- or 4-stroke) ice up, but the VW on my first gyro would. The idle would get slow and ragged on approach on cool, damp day.

When I parked and looked down the Holly carb throat, I could see icy slush in the process of melting off the butterfly.

I had a FIAT station wagon once that used to ice up on rainy days. It would eventually quit cold and I'd have to wait for the ice to melt before it would start again. Its carb heat system was pretty crude and clearly inadequate.

[RICK MARTIN]

Thanks to all you great guys out there I now consider myself a little wiser regarding the carb heat issue.
Thanks again and again,

[KDOG]

Is the carb-heat systems availabe for gyroplane engines automatic or manual?

[John_Read]

The system I have seen available for the Rotax with Bing carbs is electric. Small heaters are fixed to the carbs and can be left on all the time, as they do not warm the air, but the metal. Therefore there is no performance penalty.

[barnstorm2]

Is there an electric CH device that will work on a Holley?