Carb heat - what temp rise should I see?

For the grimy UK weather, I just fitted a carb heat jacket to my RV-6 (this jacket: https://www.vansaircraft.com/cgi-bi...2-135&browse=heatvent&product=cabin-heat-muff)

I took the air supply for the carb heat from the baffle above cylinder 4, low down on the baffle to get some heat from the flow over the cylinder.

I also have a Dynon with carb heat sensor I just fitted.

At cruise power, I saw a 9 deg F rise in carb temperature when selecting carb heat. Now, that does not strike me as very effective.

I've had it suggested I should restrict the flow of air into the carb heat inlet, but this wont work - the rate of flow of air down there is more or less fixed by the engine and what it is drawing.

What sort of temp rise should I be seeing? Anyone faced this challenge?

JP

FAA certification standards require 90*F carb heat heat rise for normally aspirated engines when the ambient air is 30*F and clear of moisture. My experience is this delta is tough to to realize on many of the homebuilts i've been associated with. Far be it for me to advise on what is acceptable or not, but when I've been able to obtain 75-80F delta I'm a happy camper. The best I've seen was a O-320 D3G equipped RV-4 I helped a friend with - he was able to reliably achieve over 100*F temp rise with his carb heat system.

deek said:

FAA certification standards require 90*F carb heat heat rise for normally aspirated engines when the ambient air is 30*F and clear of moisture. My experience is this delta is tough to to realize on many of the homebuilts i've been associated with. Far be it for me to advise on what is acceptable or not, but when I've been able to obtain 75-80F delta I'm a happy camper. The best I've seen was a O-320 D3G equipped RV-4 I helped a friend with - he was able to reliably achieve over 100*F temp rise with his carb heat system.

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Wow thanks. Well, at a 9deg F rise, I'm a ways short of what I need.

What have you used for carb heat, deek? Not the Vans jacket here then?

carb heat

If you have a typical crossover system on a Lycoming with induction on bottom of sump, adequate heat can be obtained with a hose or solid duct CLOSE to the crossover directly above the airbox. In 40 years I have never had carb ice with a Lycoming when operating in VMC conditions. IMC in freezing temperatures is a different scenario. So it depends on whether you want to fly IFR as well as other weather considerations.
Best solution of all is fuel injection.

20F to 30F is about all you will get. I have always questioned how the FAA measures their 90F. I am not sure I believe certified planes actually achieve that in flight. Maybe sitting on the ground with a lower throttle setting...
I would not take the air from above the cylinder. The engine pulls in the air it needs, so you do not need any ram air. I would get your air from the lower side of the engine. You can also put some stainless steal scouring pads inside that heat muff to improve the heat transfer from the exhaust pipe to the air. Check how well your air box seals off the cold air supply when you select carb heat. It does not take much leak-by to cool off your warm air.

pa38112 said:

20F to 30F is about all you will get. I have always questioned how the FAA measures their 90F. I am not sure I believe certified planes actually achieve that in flight. Maybe sitting on the ground with a lower throttle setting...
I would not take the air from above the cylinder. The engine pulls in the air it needs, so you do not need any ram air. I would get your air from the lower side of the engine. You can also put some stainless steal scouring pads inside that heat muff to improve the heat transfer from the exhaust pipe to the air. Check how well your air box seals off the cold air supply when you select carb heat. It does not take much leak-by to cool off your warm air.

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Thank you - great advice. I'll give it a go. I'll make those changes and report back - but just wondering if that Vans muff is up to the job or I need something better.

I had the robbin's wing small muff on the cross-over pipe on my O-320. I was able to consistently get a 40 RPM drop during run-up. The key was to seal up the flange on the FAB, so that the vacuum is pulling air through the muff and not through the open area of the flange. The flange included in the Van's FAB kit is for a forced air system and most RV implementations use a pull through system.

While I never measured it at higher power settings, I am sure the temp rise was higher, as the EGT's would have been much higher than at the 1800 RPM run-up level.

Larry

Keep an eye on your temperature when you slow the plane down and configure it for landing.

I noticed that with a Lycoming powered RV (OK, my RV), the carb temperature rises when the air through the cowl slows down.

RV8JD said:

I have on a few occasions, in the wet, cool Pacific NW. Others should be careful with this statement.

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I've needed it in the past. You want to make sure it works.

How long should it take to come up the required amount of degrees? Should you see this temperature rise in the few seconds that it takes for the carb heat check during run up?

I have the Robbins Wings heat muff and a Sam James cowl. During run up, I do see a slight decrese in rpm and a very slow increase in carb temperature. I've never run it long enough to see just how high the temperature gets.

Don't know about the Dynon probe but some time ago there was a discussion about some carb temp probes measuring the temp of the carb body where the probe screws in rather than the temp of the air in the venturi at the tip of the probe. If it measures the temp of the carb body you will get a slow/smaller?? change with carb heat applied. I have found this to be the case with different probes I have tried.

Currently I have a JP Instruments thermocouple type K carb temp probe which responds quickly and significantly when heat is applied.

Fin
9A