Carb Heat
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A carburettor cannot sense density of the air - it senses airflow velocity only. If the intake air is less dense, the mixture will tend towards rich, because the same fuel flow (mass) will be drawn out from the jet for less mass of air.
That's why we need to "lean" an engine with increased altitude - we are not really making the engine run "leaner", rather we are keeping the air / fuel ratio where it should be, with a lower outside air density prevailing. The power produced will decrease but the still air fuel consumption will be better. (There will be a more beneficial IAS / TAS ratio which helps in that respect, also pumping losses are decreased due to lower exhaust back pressure).
Ice in a carburettor system can begin to block the filter, the intake pipe, the venturi or the throttle plate aperture. This is likely to cause the mixture to become much more rich as the jet experiences a stronger depression across it (think of the starting choke on a car - those who can remember manual chokes on cars will know, it's a bit like putting your hand over the end of the carb, causing the engine cylinder "suction" to pull much harder on the jet for a constant throttle setting), greatly increasing the amount of fuel drawn out.
I don't think going "mixture fully rich" will alleviate that symptom, it's going to make it worse - possibly to the extent that combustion doesn't take place and fuel goes unburnt out of the exhaust on a windmilling engine. This sounds like what probably occurred on A&C's flight!
This link is useful:
http://www.gasco.org.uk/upload/docs/...er%20final.doc
That's why we need to "lean" an engine with increased altitude - we are not really making the engine run "leaner", rather we are keeping the air / fuel ratio where it should be, with a lower outside air density prevailing. The power produced will decrease but the still air fuel consumption will be better. (There will be a more beneficial IAS / TAS ratio which helps in that respect, also pumping losses are decreased due to lower exhaust back pressure).
Ice in a carburettor system can begin to block the filter, the intake pipe, the venturi or the throttle plate aperture. This is likely to cause the mixture to become much more rich as the jet experiences a stronger depression across it (think of the starting choke on a car - those who can remember manual chokes on cars will know, it's a bit like putting your hand over the end of the carb, causing the engine cylinder "suction" to pull much harder on the jet for a constant throttle setting), greatly increasing the amount of fuel drawn out.
I don't think going "mixture fully rich" will alleviate that symptom, it's going to make it worse - possibly to the extent that combustion doesn't take place and fuel goes unburnt out of the exhaust on a windmilling engine. This sounds like what probably occurred on A&C's flight!
This link is useful:
http://www.gasco.org.uk/upload/docs/...er%20final.doc
Last edited by ShyTorque; 29th Oct 2005 at 22:18.
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A carburettor cannot sense density of the air - it senses airflow velocity only
The reality is that a carb measures something part-way between mass flow and plain velocity. Same for fuel injection incidentally.
The carb does however measure the FUEL by mass flow - it's easy to do with a liquid.
This disparity between the way the air is measured and the way the fuel is measured gives rise to the need to lean for increasing altitude.
There are altitude compensated carbs which implement a bodge to approximate true mass flow air measurement.
Direct air mass flow measurement is standard on cars but planes are some decades behind...
I don't really see a useful connection between carb icing and the mixture setting though. A lean mixture is likely to increase the EGT but the carb doesn't see any of that obviously. Higher EGT would mean a hotter exhaust pipe and if the carb is getting some exhaust-heated air then than a higher EGT would help but the effect is bound to be marginal. The change in EGT from fully rich to peak-EGT is only about 30%.
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IO,
You seem to like to split hairs. I don't think you would be flying your piston engined aircraft in the extremes of the Martian atmosphere or at 100,00 feet; not a good example. The point is that changes in air density caused by increased altitude effects makes little measurable difference to the fuel draw in the working environment of a piston engine carburettor. If it did then aircraft wouldn't need a mixture control. Aircraft carburettor designs became hugely complicated about fifty years ago, for just that reason.
"I don't really see a useful connection between carb icing and the mixture setting though."
This is basic stuff. A carburettor depends on the smooth passage of air through the venturi. Anything that alters or disrupts the flow has a marked effect on absolute mixture strength or on the formation of a uniform fuel/air emulsion in the manifold.
As a hobby I have worked on and modified carburettors and intakes for ground-based vehicles for many years; I used to be amazed how a seemingly minor modification to the intake, or even the filter type or position, can make a large difference in mixture strength - now I'm not at all surprised to hear how a small build-up of ice can stop an engine altogether.
........................
"ST, I see your reasons but the basic issue with partial carb heat is: The air will be warmed up but its dew point is still going to be what it was before".
A point you have perhaps missed is that there might well be liquid water (droplets) present in the ambient / intake air, especially so under a cu-nim cloud. Once the intake air becomes warmed it will contain more water than ambient. This is where the use of partial carb heat may put the air in the band for icing to occur (a small increase in the amount of water will absorb much more heat in the carb) and so the design of an effective carb heat system must allow a surprisingly large increase in temperature, to overcome the powerful cooling effects in the intake / carb combination.
As we know, the trick is to keep the intake temperature above zero degrees C so that ice cannot precipitate. The designer of an effective carb heat system will have to over-compensate for safety reasons, especially if a carb temperature gauging system is not fitted. If you look at a typical carburettor equipped engine hot air intake for a car, you might be surprised how closely the hot air shroud fits round the exhaust manifold. It looks like an over-compensation but it's like that for good reason.
The information is all out there; here is something else for you to read. This article mentions partial carburettor heat being possibly ineffective and also gives advice to lean mixture (rather than richen it) if icing is encountered:
www.casa.gov.au/fsa/2001/jul/28-31.pdf
My input now complete, I'll go back to tidying up my garden as I only fly turbines these days.
You seem to like to split hairs. I don't think you would be flying your piston engined aircraft in the extremes of the Martian atmosphere or at 100,00 feet; not a good example. The point is that changes in air density caused by increased altitude effects makes little measurable difference to the fuel draw in the working environment of a piston engine carburettor. If it did then aircraft wouldn't need a mixture control. Aircraft carburettor designs became hugely complicated about fifty years ago, for just that reason.
"I don't really see a useful connection between carb icing and the mixture setting though."
This is basic stuff. A carburettor depends on the smooth passage of air through the venturi. Anything that alters or disrupts the flow has a marked effect on absolute mixture strength or on the formation of a uniform fuel/air emulsion in the manifold.
As a hobby I have worked on and modified carburettors and intakes for ground-based vehicles for many years; I used to be amazed how a seemingly minor modification to the intake, or even the filter type or position, can make a large difference in mixture strength - now I'm not at all surprised to hear how a small build-up of ice can stop an engine altogether.
........................
"ST, I see your reasons but the basic issue with partial carb heat is: The air will be warmed up but its dew point is still going to be what it was before".
A point you have perhaps missed is that there might well be liquid water (droplets) present in the ambient / intake air, especially so under a cu-nim cloud. Once the intake air becomes warmed it will contain more water than ambient. This is where the use of partial carb heat may put the air in the band for icing to occur (a small increase in the amount of water will absorb much more heat in the carb) and so the design of an effective carb heat system must allow a surprisingly large increase in temperature, to overcome the powerful cooling effects in the intake / carb combination.
As we know, the trick is to keep the intake temperature above zero degrees C so that ice cannot precipitate. The designer of an effective carb heat system will have to over-compensate for safety reasons, especially if a carb temperature gauging system is not fitted. If you look at a typical carburettor equipped engine hot air intake for a car, you might be surprised how closely the hot air shroud fits round the exhaust manifold. It looks like an over-compensation but it's like that for good reason.
The information is all out there; here is something else for you to read. This article mentions partial carburettor heat being possibly ineffective and also gives advice to lean mixture (rather than richen it) if icing is encountered:
www.casa.gov.au/fsa/2001/jul/28-31.pdf
My input now complete, I'll go back to tidying up my garden as I only fly turbines these days.
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Unfortunately the PDF leaflet, written in the standard UK CAA-style patronising manner, contains the same old assertions
I am still looking for a proper physical proof of this, rather than a repeat of the same statement that's dished out everywhere.
As for my 100,000ft example, I was merely using the standard way of proving or disproving something easily, by considering obvious extreme cases
Only if the droplets are able to vapourise in the time it takes for them to pass from the part of the induction system where the air gets heated, to the part where ice would form which is the venturi. Can this happen? I don't know.
However, if the temperature of the carb internal surfaces was below 0C and there were liquid droplets in the air, one would expect to get extremely rapid carb icing - just like flying through freezing rain causes a very rapid clear ice buildup.
Under certain conditions, partial carburettor heat may be worse than none at all.
As for my 100,000ft example, I was merely using the standard way of proving or disproving something easily, by considering obvious extreme cases
A point you have perhaps missed is that there might well be liquid water (droplets) present in the ambient / intake air, especially so under a cu-nim cloud. Once the intake air becomes warmed it will contain more water than ambient
However, if the temperature of the carb internal surfaces was below 0C and there were liquid droplets in the air, one would expect to get extremely rapid carb icing - just like flying through freezing rain causes a very rapid clear ice buildup.
Last edited by IO540; 30th Oct 2005 at 10:22.
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The reality is that a carb measures something part-way between mass flow and plain velocity. Same for fuel injection incidentally.
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"Only if the droplets are able to vapourise in the time it takes for them to pass from the part of the induction system where the air gets heated, to the part where ice would form which is the venturi."
IO,
As long as water is present, liquid OR vapour (and don't forget that water can exist as supercooled droplets), it can form ice if the carb body temperature is low enough. Partial carb heat may not overcome the "refrigeration cooling" effect in the carb caused by the evaporation of fuel issuing from the jet, which is the main cause of the temperature drop therein, not the Bernoulli effect.
.................
"Can this happen? I don't know."
I'm certain it can. I've watched it happen in an SU carburettor with the air filter removed and the engine sucking in warm engine bay air while I tried to tune it. The engine misfired like hell after a couple of minutes at idle, so tuning it was impossible. I shone a torch in the venturi and noticed ice in there. The ambient conditions were cold and very wet, it was raining but I was inside the garage with the door open. I eventually gave up tuning for the night. It was fine next day once the ambient air was less humid and the engine cover was closed. Increased engine bay heat, coupled with lower humidity, was sufficient to prevent the icing problem.
IO,
As long as water is present, liquid OR vapour (and don't forget that water can exist as supercooled droplets), it can form ice if the carb body temperature is low enough. Partial carb heat may not overcome the "refrigeration cooling" effect in the carb caused by the evaporation of fuel issuing from the jet, which is the main cause of the temperature drop therein, not the Bernoulli effect.
.................
"Can this happen? I don't know."
I'm certain it can. I've watched it happen in an SU carburettor with the air filter removed and the engine sucking in warm engine bay air while I tried to tune it. The engine misfired like hell after a couple of minutes at idle, so tuning it was impossible. I shone a torch in the venturi and noticed ice in there. The ambient conditions were cold and very wet, it was raining but I was inside the garage with the door open. I eventually gave up tuning for the night. It was fine next day once the ambient air was less humid and the engine cover was closed. Increased engine bay heat, coupled with lower humidity, was sufficient to prevent the icing problem.
A point you have perhaps missed is that there might well be liquid water (droplets) present in the ambient / intake air, especially so under a cu-nim cloud. Once the intake air becomes warmed it will contain more water than ambient.
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Where does this "more water" come from then? If the water is there, as liquid or vapour, it will form ice in the carb due to the cooling. And it will do so whether you apply partial carb heat or no carb heat at all. How can partial carb heat be worse than no carb heat (in the absence of ice particles)?
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Unless the air is cooled sufficiently to cause ice precipitate to pass straight through the engine. It appears that with very cold air, the ice particles go straight by the throttle plate without sticking.
Aah, High Wing Drifter beat me to it.
Aah, High Wing Drifter beat me to it.
There isn't any more water. Merely that you have say +5deg ambient, in the carb it is now -25deg. At -25deg, ice will be much less likely to stick. My understanding is that ice sticks because when it impacts, the impact pressure causes the ice particle to quickly melt and then it re-freezes.
Ice doesn't stick. Supercooled liquid water sticks.
Unless the air is cooled sufficiently to cause ice precipitate to pass straight through the engine. It appears that with very cold air, the ice particles go straight by the throttle plate without sticking.
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With reference to Bookworm's last statement, surely it doesn't matter that the clear air being flown through is unsaturated. It still contains water, however little.
So if it is -25 deg and applying carb heat raises the temperature to -5 deg, then the carb itself will be at about the right temp for water in the air (however little) to freeze on contact with parts in the carb and slowly accumulate. Again if this was the case, then it would be hard to melt the ice (without descending) , as the carb heat is already on full.
Now am I thinking right, or does the air have to be saturated, ie water droplets be present for the ice to actually buiild up?
This thread certainly makes you think!
So if it is -25 deg and applying carb heat raises the temperature to -5 deg, then the carb itself will be at about the right temp for water in the air (however little) to freeze on contact with parts in the carb and slowly accumulate. Again if this was the case, then it would be hard to melt the ice (without descending) , as the carb heat is already on full.
Now am I thinking right, or does the air have to be saturated, ie water droplets be present for the ice to actually buiild up?
This thread certainly makes you think!
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If the statement
"Under certain conditions, partial carburettor heat may be worse than none at all."
is correct, which for reasons given above I don't think to be the case, then surely
"Under certain conditions, full carburettor heat may be worse than none at all."
is equally true (or more likely false).
"Under certain conditions, partial carburettor heat may be worse than none at all."
is correct, which for reasons given above I don't think to be the case, then surely
"Under certain conditions, full carburettor heat may be worse than none at all."
is equally true (or more likely false).
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Maybe I haven't expressed myself properly. From your post I am given to understand that you were suffering from Carb Ice so you immediately applied full carb heat. That is not farting about! Trying to prevent carb ice by trying other methods first is!
Now am I thinking right, or does the air have to be saturated, ie water droplets be present for the ice to actually buiild up?
However, the cases we're comparing are:
1) Take air at a particular temperature and humidity and cool it by 30 degC in the carb. (carb heat off)
2) Take air at the same temperature and humidity, heat it by (say) 20 degC and then cool it by 30 degC in the carb. (partial carb heat)
The amount of water available in the carb as supercooled droplets to cause icing is the same in each case (or in fact, rather more in case 1), unless there are ice crystals in the starting sample of air.
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Water droplets have to be present for ice to build up. Because of the cooling in the carb, unsaturated ambient air can become saturated and cause icing.
However, if the air humdity is high enough, then it is possible for the pressure drop in the venturi to be sufficient to condense the moisture in the air and so form droplets?
However, if the air humdity is high enough, then it is possible for the pressure drop in the venturi to be sufficient to condense the moisture in the air and so form droplets?
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Yes absolutely. But heating the air first (with partial carb heat) can only reduce the amount of water that condenses as (supercooled) droplets.
perfect for a special serving of Carb Sorbet
I think the bottom end of the carb icing envelope is determined by the amount of water vapour in the air.
1) If you start at an OAT of 25 degC (saturated or near) and cool to -5 degC in the carb, there's 20 g/m^3 of water that condenses out into supercooled droplets ready to form ice.
2) If you start at an OAT of 5 degC (saturated or near) and cool to -25 degC in the carb, there's something like 5 g/m^3 of water that condenses out. That's much less of a problem from an icing point of view, which is why the -25 degC in the carb is less troublesome than -5 degC in the carb.
3) If you start at an OAT of 5 degC (saturated or near), heat to 25 degC and then cool to -5 degC in the carb, there's only about 4 g/m^3 of water that condenses out, even less than in the previous case.
You might argue that a greater proportion of the water in case 2 would spontaneously form ice than in case 3, but I think the processes that go with glaciation are much slower than the time in which the air is cooled in the venturi and then slams into surfaces that can collect ice. You'd expect all the condensed water to be capable of causing ice.
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Bookworm,
I don't have the knowledge to argue about how much water condenses out for a given temp drop. Suffice to say that I'm pretty sure that it doesn't matter what order you heat and cool, the amount of water in the air doesn't change (unless it rains which is another somewhat mysterious process). For a given pressure and RH at 5 deg you will get RH' at -5deg and RH'' at -25deg regardless of what route you followed to finally end up at the temprature.
I'm stepping into dangerous territory of guesswork from 1st principles here!!!: Your pointing out of supercooled droplets triggered a memory and got me looking up me old notes.
Something to bear in mind, is that when water condenses it releases latent heat into the atmosphere. The same is true when supercooled water droplets (SCWD) impact and then move to the stable ice state. This process of releasing latent heat (80 cal for deposition?) is the problem for ice formation, because if there is enough released heat to partially melt the now iced droplet, it will re-freeze and this it is the re-freezing that forms the dangerous clear ice. The portion of the SCWD that was instantly frozen is fluffy and aerated (rime ice) and probably gets blown down the tube almost instantly. At -25deg, significantly less of the SCWD is melted and re-frozen preventing serious ice formation. At -5deg the opposite is true and the majority of the SCWD ends up as clear ice.
Obviously, that last paragraph assumes that wing icing and carb icing are two sides of the same coin.
With that in mind, your point about how much time this takes is taken. But I guess if ice forms on the wing of a jet liner at 450kts TAS then 150kts inside the carb presents no real barrier. A quick and rough calculation of carb body at 150kts and airliner wing at 450kts (TAS of course) suggests that they both form ice in about 0.01 seconds!
I don't have the knowledge to argue about how much water condenses out for a given temp drop. Suffice to say that I'm pretty sure that it doesn't matter what order you heat and cool, the amount of water in the air doesn't change (unless it rains which is another somewhat mysterious process). For a given pressure and RH at 5 deg you will get RH' at -5deg and RH'' at -25deg regardless of what route you followed to finally end up at the temprature.
I'm stepping into dangerous territory of guesswork from 1st principles here!!!: Your pointing out of supercooled droplets triggered a memory and got me looking up me old notes.
Something to bear in mind, is that when water condenses it releases latent heat into the atmosphere. The same is true when supercooled water droplets (SCWD) impact and then move to the stable ice state. This process of releasing latent heat (80 cal for deposition?) is the problem for ice formation, because if there is enough released heat to partially melt the now iced droplet, it will re-freeze and this it is the re-freezing that forms the dangerous clear ice. The portion of the SCWD that was instantly frozen is fluffy and aerated (rime ice) and probably gets blown down the tube almost instantly. At -25deg, significantly less of the SCWD is melted and re-frozen preventing serious ice formation. At -5deg the opposite is true and the majority of the SCWD ends up as clear ice.
Obviously, that last paragraph assumes that wing icing and carb icing are two sides of the same coin.
With that in mind, your point about how much time this takes is taken. But I guess if ice forms on the wing of a jet liner at 450kts TAS then 150kts inside the carb presents no real barrier. A quick and rough calculation of carb body at 150kts and airliner wing at 450kts (TAS of course) suggests that they both form ice in about 0.01 seconds!
With that in mind, your point about how much time this takes is taken. But I guess if ice forms on the wing of a jet liner at 450kts TAS then 150kts inside the carb presents no real barrier. A quick and rough calculation of carb body at 150kts and airliner wing at 450kts (TAS of course) suggests that they both form ice in about 0.01 seconds!
I was thinking that one might claim that the condensed water might have already frozen at -25 degC, but not at -5 degC, since we normally observe glaciated (pure ice) cloud at -25 degC, but a high proportion of SCWD at -5 degC, which is what makes clouds much more dangerous to the airframe at -5 degC than -25 degC. That's because the SCWD soon (seconds or minutes) finds an ice nucleus and sticks to it. It has already done that by the time your airliner wing hits it.
But the time over which the cooling occurs in a carb venturi is of the order of 0.1 milliseconds (length of a carb venturi divided by the speed of sound) and I don't think this is long enough for the newly condensed SCWD to find an ice nucleus. So in the carb, I don't think one can take much consolation from the -25 degC.
The latent heat issue is a different one, and a good point. I agree you would expect to deposit clear ice in a carb at -5 degC and rime ice in a carb at -25 degC. While clear ice is more dangerous for the wing because of runback, I don't know how much difference it would make in a carb, nor whether rime ice "probably gets blown down the tube almost instantly" -- you may have a case. My vague recollection of carb ice photos from experiments is that it looks milky like rime.