Carburator Heat

[caioatpl]

Hey huys!

I'm struggling with this question.

During the restart following engine failure during cruise of a C152 we have to set CARBURATOR HEAT ON because there is a probability of carburator icing, but if the engine is down, where that heat comes from ?

 

[DaleB]

Hey huys!

I'm struggling with this question.

During the restart following engine failure during cruise of a C152 we have to set CARBURATOR HEAT ON because there is a probability of carburator icing, but if the engine is down, where that heat comes from ?

Residual heat from the exhaust system, if you're lucky and quick enough on the carb heat.

 

[jesse]

Ideally you'll catch it before it shuts down.

FWIW -- I've managed to restart an O-200 that had shut down (but was still windmilling) from carb ice via pumping of the primer. It delivered enough fuel to make the engine fire enough to get things cleared out.

 

[caioatpl]

is there any other source of heat ?

 

[tonycondon]

turning carb heat on bypasses the air filter which might have gotten blocked by something or been partially ingested into the intake, causing the failure

 

[DaleB]

is there any other source of heat ?

Nope. Engine heat is pretty much it. Exhaust is really hot, cylinder heads are moderately hot, oil is pretty warm. Are there planes with electric carb heat? I don't know. I've never seen it, but there are a lot of things I don't know about.

 

[ja_user]

is there any other source of heat ?

Your body heat will rise when it happens, but you can't redirect that to the carb to melt the ice..

 

[caioatpl]

hahaha true..

 

[caioatpl]

yeah.. you're right, thanks

 

[ja_user]

Oh.. And if you do have to leave the carb heat on (especially at altitude), remember to lean for best power.. Because you just richened the mixture when you applied the heat..

Not needed a lot of the time, but if you are going to cruise somewhere with it on for a while....

 

[caioatpl]

you're right, thanks

 

[denverpilot]

Hey huys!

I'm struggling with this question.

During the restart following engine failure during cruise of a C152 we have to set CARBURATOR HEAT ON because there is a probability of carburator icing, but if the engine is down, where that heat comes from ?

Does it say it's for icing in the book?

 

[Capt. Geoffrey Thorpe]

The pipes stay warm for a while. Just try grabbing one 5 minutes after you shut down...

I've brought back an engine that quit due to (I assume) carb ice. Just took a couple seconds (C-85).

 

[kgruber]

is there any other source of heat ?

Yes.

After the engine quits there is residual "heat" (bad term, sorry you thermodynamicists) everywhere in the engine compartment. If you are lucky, the ice will melt because the entire engine is warm.

"heat" is not a noun. There is no such thing as "heat." If you open an oven door and look in there is no "heat" in there. There IS high temperature air.

 

[Dan Thomas]

Yes.

After the engine quits there is residual "heat" (bad term, sorry you thermodynamicists) everywhere in the engine compartment. If you are lucky, the ice will melt because the entire engine is warm.

"heat" is not a noun. There is no such thing as "heat." If you open an oven door and look in there is no "heat" in there. There IS high temperature air.

If the carb iced up, it's not going to deice itself due to warm air in the engine compartment. The temperature drop in the carb is due to two factors: the pressure drop in the venturi (good for about 30°F, IIRC) and the evaporative cooling caused by vaporizing fuel (another 40°F or so). The total is around 70°F, and can cause ice IF the humidity is high enough, and the carb setup is prone to icing, on a 100°F day.

The only heat that reaches the carb that will remove already-formed ice is the carb heat, usually from a muff on the exhaust pipe or muffler. If you ever have a chance to examine those parts when they're off, you'll see that they are very thin and light and won't hold heat very long, especially with air flowing around them on the way to the carb. They can stay hot after landing, but there's no airflow cooling them at that point and it's not a good indication of residual heat available for deicing. Once an engine quits due to carb ice, you have very little time to retrieve it. Get the throttle open all the way, since part of the trouble is air starvation. Some carbs will ice up their fuel nozzles, too, and cut off the fuel flow, but that's less likely.

Sometimes the throttle is already all the way open when it quits, since the unsuspecting pilot kept opening it a bit more as the RPM was failing. I have seen that happen on the ramp after a runup; the engine tries to die when the pilot closes the throttle, and so he just opens it some to keep it going, not realizing that it's trying to tell him that it's going to die if he doesn't wake up. A long taxi after runup can also create enough ice that there isn't enough power available to get out of ground effect.

The sad thing is that there are far too many engine failures due to ice and the lack of pilots' understanding of it and how to handle it. Accident investigation reports are full of phrases like "carburetor icing was suspected, since conditions were conducive to induction icing and no mechanical, fuel or electrical faults were found with the engine or aircraft." The ice melts due to engine compartment heat (or environmental warmth) after the accident, and the proof is gone. And many times, if the accident didn't wreck the engine, it will start and run just fine afterward.

Dan

 

[MAKG1]

"heat" is not a noun. There is no such thing as "heat." If you open an oven door and look in there is no "heat" in there. There IS high temperature air.

Sure there is. Heat is a form of energy. It can be a noun, verb, or adjective (that's "heat energy").

There are heat equations that show how heat flows from a hotter point to a lower point. We are working with heat engines; they work with temperature differences and take advantage of the heat flow that implies.

In your oven, there is a heat source (electrical, radiative, or from combustion).

Back to the point in question, wouldn't heat soak work to your advantage on a stopped engine? The hottest part of the engine is generally the exhaust ports, close to the valves. The coolest part on an air cooled engine will be the fins. Heat will conduct along the aluminum surfaces. More to the point, venturi and evaporation cooling don't work if air isn't flowing through the venturi.

While I was a student (soloing, right before my checkride), I had a carb ice scare. The ice formed while I was sequenced for departure, after run-up. It was a cool day, and fog had just burned off (so it was relatively humid). I crammed the throttle and made a decent takeoff. At about 400 AGL, the carb ice broke off and the engine missed. The aircraft stumbled, but kept flying. I flew it around the pattern once (as it was still flying normally), parked the aircraft, and went looking for a change of underwear....

 

[Dan Thomas]

Back to the point in question, wouldn't heat soak work to your advantage on a stopped engine? The hottest part of the engine is generally the exhaust ports, close to the valves. The coolest part on an air cooled engine will be the fins. Heat will conduct along the aluminum surfaces. More to the point, venturi and evaporation cooling don't work if air isn't flowing through the venturi.

Nope, heat soak won't work, at least not soon enough. If that heat soak was significant, your carb wouldn't ice up easily in the first place. Lycomings are a good example of that; their carbs are mounted on the oil pan and the hot oil helps keep the carb body a little warmer. Of course, the first flight of the day means colder oil and no help from that quarter. On most small Continentals, the carb is more isolated from the case and heat-soak is almost nonexistent and they'll ice up much more easily. In both engines, the induction tubes have rubber hose connectors that do not conduct heat worth two cents, so heat from the heads is useless to the carb. Besides, those tubes get really, really cold from the evaporation still taking place as the air/fuel mix runs through them.

If no air is flowing through the venturi, then yes, there's no pressure-drop or evaporative cooling, but then neither is the carb heat going to help.

Dan

 

[Capt. Geoffrey Thorpe]

Back to the point in question, wouldn't heat soak work to your advantage on a stopped engine? The hottest part of the engine is generally the exhaust ports, close to the valves. The coolest part on an air cooled engine will be the fins. Heat will conduct along the aluminum surfaces. More to the point, venturi and evaporation cooling don't work if air isn't flowing through the venturi.
.

If air isn't flowing through the venturi, then either the prop is stopped or you have 100% plugged the carb with ice. In that case carb heat won't do any good. But if one of these happens you have gone way beyond regular old carb ice.

Carb heat is available for a while - you have heat energy (enthalpy) stored in the cylinders / heads / exhaust pipes which will warm up the air to the carb a bit if you don't waste time and get the carb heat knob out when the engine first quits. (BTDT) Of course, every instructor will tell you to fiddle around getting into a stabilized glide first to give the engine time to cool off...

One of those times I regretted not having a camera handy - one of my POS cars was running poorly, I suspected carb ice. I made it home, whipped open the hood / air cleaner - and there it was. A nice chunk of ice sitting in the booster venturi. But by the time I ran for a camera it was gone. In this case, the heat did come from the engine itself but the car was shut off - something you don't want to happen in yea olde airplane while still in the air.

 

[rottydaddy]

turning carb heat on bypasses the air filter which might have gotten blocked by something or been partially ingested into the intake, causing the failure

Not as likely as carb ice, but if carb ice actually stops combustion, you've waited too long for the carb heat to do much good anyway, if you ask me. At that point, it's only worth trying to rule out blockage of the inlet with the filter on it. It might help with the ice, but that would mean a lot of ice, and less and less heat every second to use to melt it. Assuming the ice somehow melts, now you have a windmilling engine with water in the cylinders, which may shortly become an inert engine with water (or water vapor) in the cylinders. Might be a problem for a restart.

Works the other way if you just have a reduction in RPM... probably ice, so you apply carb heat... which will take care of the blocked-filter thing also, if that happens to be the real problem.


Bottom line is you can't afford to wait even a second when it comes to carb ice... I got it once, and I was lucky I remembered to hit the carb heat before even thinking about moving the throttle. This was in a 172, in lowish-power cruise at about 2000 MSL on a nice dry winter day, and the rpm dropped rather quickly, initially. Pretty scary, especially when the hot air reduced power even more, followed by a rough moment when the melted ice went into the engine.

 

[rottydaddy]

Ideally you'll catch it before it shuts down.

FWIW -- I've managed to restart an O-200 that had shut down (but was still windmilling) from carb ice via pumping of the primer. It delivered enough fuel to make the engine fire enough to get things cleared out.

I missed this anecdote... I'm surprised that worked, and I wonder how, exactly, it would do the trick. What did you try before that?

 

[Dan Thomas]

Carb heat is available for a while - you have heat energy (enthalpy) stored in the cylinders / heads / exhaust pipes which will warm up the air to the carb a bit if you don't waste time and get the carb heat knob out when the engine first quits. (BTDT) Of course, every instructor will tell you to fiddle around getting into a stabilized glide first to give the engine time to cool off...

The carb heat relies on those hot exhaust pipes, which have 1400°F gases running through them. When the fire goes out after serious carb ice, the temp rapidly drops to less than CHT, so we'd see exhaust temps of maybe 250°F, not enough to do much good removing ice. Those exhaust pipes are far too light to hold much heat at all. Wall thickness of the typical pipe or muffler is only about .025" or .030", far thinner than automotive stuff.


Dan

 

[MAKG1]

One of those times I regretted not having a camera handy - one of my POS cars was running poorly, I suspected carb ice. I made it home, whipped open the hood / air cleaner - and there it was. A nice chunk of ice sitting in the booster venturi. But by the time I ran for a camera it was gone. In this case, the heat did come from the engine itself but the car was shut off - something you don't want to happen in yea olde airplane while still in the air.

That's why one should keep the heat riser repaired. It seems almost everyone with a carb neglects that.

 

[MDeitch1976]

"heat" is not a noun. There is no such thing as "heat." If you open an oven door and look in there is no "heat" in there. There IS high temperature air.

Actually, there is no cold. Everything above absolute zero is heat. Cold is just an adjective. Thus latent heat, and sensible heat. Therefore heat is a noun.



Sent from my iPad using Tapatalk HD

 

[dell30rb]

A long taxi after runup can also create enough ice that there isn't enough power available to get out of ground effect.

I've picked up carb ice during taxi many times on very humid summer mornings. Never a lot, but definitely noticeable amounts.

If there is any significant delay between run up and takeoff (lsa's flying bomber patterns etc..) I turn the carb heat on before taking the runway, then back off again before I go full throttle for takeoff. I'm surprised I have never seen this listed anywhere on a checklist

 

[dell30rb]

The carb heat relies on those hot exhaust pipes, which have 1400°F gases running through them. When the fire goes out after serious carb ice, the temp rapidly drops to less than CHT, so we'd see exhaust temps of maybe 250°F, not enough to do much good removing ice. Those exhaust pipes are far too light to hold much heat at all. Wall thickness of the typical pipe or muffler is only about .025" or .030", far thinner than automotive stuff.


Dan

I agree with dan, with a bunch of air flowing through the cowling and the very thin wall material exhaust (its an airplane, therefore it must be as light as possible) I would not expect heat to last long at all. Many vehicles have cast iron exhaust manifolds which are huge chunks of metal. The exhaust manifolds in the old ford 460 my uncle and I overhauled probably weighed 50lbs together

 

[kgruber]

In a Cessna 150 I will take the runway with carb heat on, go to full throttle with it still on and then push it off at the beginning of the T/O roll.

 

[Capt. Geoffrey Thorpe]

The carb heat relies on those hot exhaust pipes, which have 1400°F gases running through them. When the fire goes out after serious carb ice, the temp rapidly drops to less than CHT, so we'd see exhaust temps of maybe 250°F, not enough to do much good removing ice. Those exhaust pipes are far too light to hold much heat at all. Wall thickness of the typical pipe or muffler is only about .025" or .030", far thinner than automotive stuff.

Scud running across Michigan’s upper peninsula at a few hundred feet - not watching the engine as carefully as one should - then there was that sudden silence. One hand went to the carb heat, another to the fuel selector (there were two of us in the Cessna 120). A very short time later, the noise returned. Experimenting with the fuel selector did nothing so it seems fair to believe that it was the carb heat that did the job.
You may have theories to support your belief that it won't work, but my experience leads me to believe otherwise.

 

[kgruber]

Sure there is. Heat is a form of energy. It can be a noun, verb, or adjective (that's "heat energy").

Then you would have failed Thermodynamics 101 at the UW. "Heat" is not a person, place or thing. It is not a noun.

There is not even a unit for "heat." (Hint, it is not BTU)

 

[MAKG1]

Then you would have failed Thermodynamics 101 at the UW. "Heat" is not a person, place or thing. It is not a noun.

There is not even a unit for "heat." (Hint, it is not BTU)

Umm, I beg to differ. The BTU is an inconvenient, but perfectly legitimate unit for any form of energy, including heat. Most of the people I work with prefer to use kilocalories or Joules, though.

I think you heard the argument that temperature is not energy, but misunderstood. Temperature is an expression of heat, but it is not heat.

I passed three years of thermodynamics, up to and including shock waves and turbulence, with no undue trouble.

It sounds like your instructor has some odd definitions of heat, or more likely you didn't understand it.

 

[kgruber]

Umm, I beg to differ. The BTU is an inconvenient, but perfectly legitimate unit for any form of energy, including heat. Most of the people I work with prefer to use kilocalories or Joules, though.

I think you heard the argument that temperature is not energy, but misunderstood. Temperature is an expression of heat, but it is not heat.

I passed three years of thermodynamics, up to and including shock waves and turbulence, with no undue trouble.

It sounds like your instructor has some odd definitions of heat, or more likely you didn't understand it.

If heat is a "person, place or thing," please wrap some up and mail it to me. Your superficial "understanding" is duly noted.

 

[MAKG1]

If heat is a "person, place or thing," please wrap some up and mail it to me. Your superficial "understanding" is duly noted.

You think all nouns are tangible? Please don't tell me you learned grammar from Schoolhouse Rock.

Is "knowledge" a noun? Heat is a "thing" in the same sense. Good luck trying to mail a "centimeter" or a "concept."

Much of physics education is dealing with abstract concepts. They are just as real as tangibles; they have measurable effects, but they are not objects you can hold.

Grammar is not understanding. Do you know what heat is? At least the standard statistical mechanics definition?

 

[Capt. Geoffrey Thorpe]

Carb Heat is the name of a knob you pull to get the noise to resume.

 

[DaleB]

Carb Heat is the name of a knob you pull to get the noise to resume.

The noise from the big spinny thing up front, not the noise made by the terrified pax.

 

[dell30rb]

Scud running across Michigan’s upper peninsula at a few hundred feet - not watching the engine as carefully as one should - then there was that sudden silence. One hand went to the carb heat, another to the fuel selector (there were two of us in the Cessna 120). A very short time later, the noise returned. Experimenting with the fuel selector did nothing so it seems fair to believe that it was the carb heat that did the job.
You may have theories to support your belief that it won't work, but my experience leads me to believe otherwise.

I picked it up once while skimming the tops of a low layer in a 152 at about 65% power cruise flight. All the classic signs of carb ice - motor started losing power, pulled heat and briefly ran rough and then smoothed out.

 

[MAKG1]

The noise from the big spinny thing up front, not the noise made by the terrified pax.

Yep, that second one is the "crew isolate" switch.

 

[jesse]

I missed this anecdote... I'm surprised that worked, and I wonder how, exactly, it would do the trick. What did you try before that?

I was idle with carb heat on and the engine carb iced up and quit. Honestly at this point I can't remember if it was windmilling or not but I remember trying to pump the throttle and then I remember pumping the primer which seemed to deliver enough fuel to make the engine barely run. From there I turned the carb heat off and kept with the primer until things were back to normal.


It was rather cold and seemed to be carb heat induced carb ice.

 

[dell30rb]

I was idle with carb heat on and the engine carb iced up and quit. Honestly at this point I can't remember if it was windmilling or not but I remember trying to pump the throttle and then I remember pumping the primer which seemed to deliver enough fuel to make the engine barely run. From there I turned the carb heat off and kept with the primer until things were back to normal.


It was rather cold and seemed to be carb heat induced carb ice.

If you were idle throttle for awhile that would make sense.. probably the exhaust header temps fell quite a bit after being at idle for awhile - which would be like partially applying carb heat, which as we know can cause or even worsen ice formation

 

[MBDiagMan]

Yes.

After the engine quits there is residual "heat" (bad term, sorry you thermodynamicists) everywhere in the engine compartment. If you are lucky, the ice will melt because the entire engine is warm.

"heat" is not a noun. There is no such thing as "heat." If you open an oven door and look in there is no "heat" in there. There IS high temperature air.

I thought that there IS such thing as heat, which is energy. There is no such thing as cold. Cold is a lack of heat. This is my laymans physics anyway.

 

[ChrisK]

I thought that there IS such thing as heat, which is energy. There is no such thing as cold. Cold is a lack of heat. This is my laymans physics anyway.

http://www.rogermwilcox.com/darksucker.html

 

[Capt. Geoffrey Thorpe]

I thought that there IS such thing as heat, which is energy. There is no such thing as cold. Cold is a lack of heat. This is my laymans physics anyway.

http://en.wikipedia.org/wiki/Heat

Seems to be a reasonable enough description.