EGT and Carb Ice

[Tip]

I left my home field this morning at daylight. We had our first frost of the season and there was valley fog, 39 degrees and a 2 degree dew point spread. Forty five minutes into a two hour cross country (over West Virginia) I noticed that the right EGT display was flashing yellow at 1560 degrees. The left was a slightly lower. I was at 3200' and I throttled back from 5250 to 5000 RPM. At that point, the engine began too change pitch up and down. I couldn't see it on the tach, but there definitely was an issue. I pulled the carb heat on and the engine smoothed out and the EGTs dropped down to 1400. I ran with the carb heat on for probably 20 minutes. At 550 hours, this is the first time that I have experienced this. Carb ice with the Bings leans fuel mixture.

[Doug G.]

Leans? Wouldn't it make it richer as the throat narrows with ice?

[Anticept]

It depends on where the ice forms. Ice changes the airflow dynamics.

 

In a traditional float carb (i don't know enough about bing carbs to comment), ice tends to form on the throttle plate (located after the venturi) first due to fuel vaporization causing a rapid temperature drop. Located near the throttle plate is the idle fuel jet, which can be blocked by the ice. Then, the ice reduces the airflow through the venturi as well, so it won't pick up as much fuel from the main discharge nozzle. This has a leaning effect.

[Doug G.]

Interesting to ponder. Lots of possibilities on a Bing I would think. The coldest spot would probably be after the fuel leaves the needle jet which would inhibit fuel flow and atomization. I don't know if ice could block the operation of the piston.

I believe the throttle plate is ahead of the jet on a Bing. I would need to double check that, but if that is the case then the plate would not be involved in any icing. And, with constant velocity, I don't think the venturi effect exists. I will bow to those more familiar with the carbs than I.

[Tip]

I have been told and have read that carb ice is not a problem with the Rotax. Well it is a very uncomfortable feeling when it does!

[Roger Lee]

Carb ice in a variable throat carb (Bing 64) is less prevalent than a standard carb, but it can and does happen. The climate in the UK is prime for that. Engines run better with cooler outside air. Rotax engine's with the air filter inside the cowl directly attached to the carb receiving hot under the cowl air is like having carb heat on all the time, but that makes the carb run richer. Fully exposed Rotax engine's with the air filter attached don't get that benefit. There are two types of carb heat products on the market for the Rotax. One is electric, but it pulls power and adds to the overall power pull on the engine. The next one is a small block manifold that attaches inline at the rubber carb flange that has hot engine coolant running through it all the time. So you always have carb heat on,  but unlike the hot air type it does not affect the carb fuel/air mixture. Another factor is an exposed engine to a tightly cowled engine. The tightly cowled engine like the CT keeps the carb's quite hot so it is less likely than a fully open and exposed engine to see carb ice.

 

There are no absolutes here and given the right circumstances carb ice could form, but we are at least on the bottom end of that issue.

[Tip]

Anticept, we were out your way to Greene Co. (I19). Flew just south of Rickenbacker.

[Ed Cesnalis]

 

Applying too much heat too quickly can be just as bad as having ice to begin with...if applying carb heat results in loss of power, or even in significant "roughening" of the engine, immediately open the throttle and pull the mixture control out far enough to smooth out the engine. As the ice melts, restore the mixture gradually to the original position.

 

 

 

Which one is the mixture?

[FlyingMonkey]

Which one is the mixture?

 

It's the red knob...oh wait...

[Doug G.]

When you apply carburetor heat to melt ice you may notice that the engine begins to run even rougher. This happens because the fuel mixture, already enriched because the ice is choking off some of the induction air flow, is suddenly made even richer by the addition of hot air.

 

Applying too much heat too quickly can be just as bad as having ice to begin with...if applying carb heat results in loss of power, or even in significant "roughening" of the engine, immediately open the throttle and pull the mixture control out far enough to smooth out the engine. As the ice melts, restore the mixture gradually to the original position.

 

Carb ice was a major motivator for me to buy the fuel injected 912. I really didn't wanna deal with it, even if infrequently.

What is your experience with carb ice with Bing carbs?

[Ian]

........

 

Applying too much heat too quickly can be just as bad as having ice to begin with...if applying carb heat results in loss of power, or even in significant "roughening" of the engine, immediately open the throttle and pull the mixture control out far enough to smooth out the engine. As the ice melts, restore the mixture gradually to the original position.

 

........

 

Utter Bullshit!

[Anticept]

Actually, Ian, he is right on that count. Not in Rotax engines, mind you, but in some turbo engines, apply carb heat can significantly increase the risk of detonation at higher power settings.

 

Carb heat should be applied in accordance to the POH. It will say what you need to do, as well as precautions. Leaning out a little bit is a technique used to help smooth out the overly rich engine, but it too should be used with caution in higher power engines.

[Ed Cesnalis]

I was not aware of the risk of detonation in some high power engines.  My experience of additional roughness ( then normal power ) after applying carb heat has always been attributed to the ice losing its grip and being ingested.

 

My question about the mixture was just a snarky way of point out that the advice was odd for the CT forum because none of us have mixture control.

[Anticept]

most air cooled engines use the additional fuel to cool the engine, especially valves. Rich mixtures are also harder to detonate.

 

In turbo engines, you are already pushing some serious heat. Lean mixtures burn slower, raising the egt temps very high. Combined with the carb heat and high upper deck pressure, you are ingesting air that may be hotter than what the intercooler can take. Now you want to pass hot air into the cylinders in a leaned condition? Detonation chances are much higher now.

[Anticept]

Interesting to ponder. Lots of possibilities on a Bing I would think. The coldest spot would probably be after the fuel leaves the needle jet which would inhibit fuel flow and atomization. I don't know if ice could block the operation of the piston.

I believe the throttle plate is ahead of the jet on a Bing. I would need to double check that, but if that is the case then the plate would not be involved in any icing. And, with constant velocity, I don't think the venturi effect exists. I will bow to those more familiar with the carbs than I.

I forgot to comment on this.

 

At lower power settings, a throttle plate acts like a venturi, lowering the air temperature and extracting moisture. Fuel vaporazation in regular carbs accelerates this effect, though.

 

If the throttle plate comes before the jet, then it will act like a choke, where the engine vacuum would draw more fuel out. However, since I do not know much about the bing carbs (yet, going to training in the next couple weeks), i don't know if there is another mechanism used to retard the fuel delivery as vacuum forces increases.

[Doug G.]

If I have this right the throttle plate is ahead of the jet. The poison, which lifts the tapered needle in the jet, is lifted by the air drawn across the bottom of the piston which creates a relative vacuum inside the piston thereby drawing the needle higher and allowing more fuel to flow.

There is a narrowing of the throat, not as pronounced as non CV carbs, but the piston would also provide some century effect.

It would be interesting to see exactly how and where the ice would form.

[Tom Baker]

If I have this right the throttle plate is ahead of the jet. The poison, which lifts the tapered needle in the jet, is lifted by the air drawn across the bottom of the piston which creates a relative vacuum inside the piston thereby drawing the needle higher and allowing more fuel to flow.

There is a narrowing of the throat, not as pronounced as non CV carbs, but the piston would also provide some century effect.

It would be interesting to see exactly how and where the ice would form.

Doug, I think you have it backwards. The throttle plate is on the engine side of the carburetor. The piston and needle are on the intake side of the carburator

[Doug G.]

I stand corrected, thanks.

[FastEddieB]

I, too, doubted it for a second and had to Google a diagram.

 

Yes, the butterfly is on the engine side.

[Anticept]

That makes much more sense. Butterfly valves before carbs are for chokes (again, bing does things differently, so i do not know if this applies) so that the engine vacuum draws more fuel for starting. Cold engines tend to have fuel collect on the walls, so the extra fuel is needed to saturate the air.

[Doug G.]

Just curious...what is the difference if the butterfly is before, or after the jet on a CV carb? Either would decrease air flow and reduce the flow that lifts the needle. Is this the reason for using a starting circuit rather than a choke?

[FastEddieB]

Good Bing overview here:

 

http://contrails.free.fr/temp/type64e.pdf