Use of the carburator heat
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Use of the carburator heat
I was flying with a friend of mine in a Cessna 172 and we came into argument about the use of the carburator heat. I used the carburator heat several time during cruise flight and he said that was dangerous because of detonation.
I usually use the carburator heat periodically during cruising just to make sure that I don't experience any carburator icing condition. In fact, there was a time when I flew a cessna 172 for almost an hour with the carburator heat engaged because of light rain encountered during the flight period.
I would like to hear you opinions on this issue.
WP
I usually use the carburator heat periodically during cruising just to make sure that I don't experience any carburator icing condition. In fact, there was a time when I flew a cessna 172 for almost an hour with the carburator heat engaged because of light rain encountered during the flight period.
I would like to hear you opinions on this issue.
WP
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This leaflet on piston engine icing is published by the UK CAA - it's very good and should answer your questions...
http://www.caa.co.uk/docs/33/SRG_GAD_WEBSSL14.PDF
The only other thinsg I would add is, as always, read the Pilot's Operating Handbook for the aircraft - it should give you a guide on how to use carb heat for that type - recommended techniques do vary.
Oh and for what it's worth - I would rather fly with you than your buddy
http://www.caa.co.uk/docs/33/SRG_GAD_WEBSSL14.PDF
The only other thinsg I would add is, as always, read the Pilot's Operating Handbook for the aircraft - it should give you a guide on how to use carb heat for that type - recommended techniques do vary.
Oh and for what it's worth - I would rather fly with you than your buddy
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Don't fly with your friend again.....
With his attitude towards use of carb heat, he should take a trip in a VW- or 0-200 powered aircraft on an icing day and see how long he lasts, although he probably isn't a pilot.
I use carb heat often, always when at engine revs below cruise power, and often when taxying if the engine sounds like it needs it.
In my aircraft, it isn't just a case of pulling the carb heat control for a few seconds. I increase the revs to warm the engine before I use carb heat.
With his attitude towards use of carb heat, he should take a trip in a VW- or 0-200 powered aircraft on an icing day and see how long he lasts, although he probably isn't a pilot.
I use carb heat often, always when at engine revs below cruise power, and often when taxying if the engine sounds like it needs it.
In my aircraft, it isn't just a case of pulling the carb heat control for a few seconds. I increase the revs to warm the engine before I use carb heat.
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I think the general idea is that carb heat produces a slight leaning of the mixture (because the mass of air sucked in is greater if the air is warm) which could push the engine into the detonation region.
However, I am sure that no engine with such a narrow detonation margin would pass the FAA certification.
Detonation is unlikely anyway during cruise power settings, which one would assume would be below 75% of rated max HP. Lycoming generally authorise leaning all the way to peak EGT at below 75%. If somebody doesn't like using carb heat during cruise, one assumes they have the red lever wirelocked in the fully forward position too
That CAA leaflet also repeats the old tale about partial carb heat being a bad thing. I would like to see the physics behind that explained...
However, I am sure that no engine with such a narrow detonation margin would pass the FAA certification.
Detonation is unlikely anyway during cruise power settings, which one would assume would be below 75% of rated max HP. Lycoming generally authorise leaning all the way to peak EGT at below 75%. If somebody doesn't like using carb heat during cruise, one assumes they have the red lever wirelocked in the fully forward position too
That CAA leaflet also repeats the old tale about partial carb heat being a bad thing. I would like to see the physics behind that explained...
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Always use full heat whenever carb heat is applied; partial hot air should only be used if an intake temperature gauge is fitted and only then if specifically recommended in the approved Flight Manual or Pilot's Operating Handbook
Hot air is also less dense than cold air, which means mass flow decreases with carb heat applied. Mixture therefore becomes slightly richer, not leaner, which still causes an rpm drop due to sub-optimal combustion conditions.
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I too would sooner fly with you than your friend.
I suspect he's getting confused between mixture and carb heat.
Pulling carb heat reduces the mass of air entering the engine (warm air is less dense than cold air) and therefore richens the mixture.
Pulling Mixture leans the mixture. Leaning it too much causes the cylinder head temps to rise and can give rise to detonation.
Carb heat, by richening the mixture actually cools the engine. Some of the fuel remains unburnt and cools the cylinder walls. The downside is that it also washes the oil film off them, increasing wear, and it increases fuel consumption.
Edited to add
IO540 has it wrong and needs to go back to Boyles Law
The reason for full carb heat and not partial is generally given thus:-
A gas cools as it expands (adiabatic cooling, one of the ways that clouds form). As the air passes through the carburettor venturi it expands and its pressure reduces. This reduced pressure is what draws fuel in to the air stream. A side effect of this expansion is that the air cools. If it is humid air the water vapour will crystallize and build up inside the carb. The carb heat system is therefore so designed that at any temperature likely to be encountered in normal flight the temperature of the air will be high enough to prevent ice forming when FULL carb heat is applied.
You'll be aware of airframe icing. This can occur when there are no ice crystals in the atmosphere but the air is moist. Crystals need something to form on, and if the air is clean there's little to start the crystals forming. An airframe gives them something to form on and ice starts to build.
Now imagine the same thing with partial heat. The supercooled air passes through the heater and is warmed so that it is above freezing. It then goes through the venturi where it is cooled again and its temp drops back below freezing. There is plenty in the carb body for ice to form on so it does so. The use of full carb heat would raise the entry temperature sufficiently to prevent the cooling effect of the venturi taking the temp below freezing.
Hope I've explained this well enough.
Mike
I suspect he's getting confused between mixture and carb heat.
Pulling carb heat reduces the mass of air entering the engine (warm air is less dense than cold air) and therefore richens the mixture.
Pulling Mixture leans the mixture. Leaning it too much causes the cylinder head temps to rise and can give rise to detonation.
Carb heat, by richening the mixture actually cools the engine. Some of the fuel remains unburnt and cools the cylinder walls. The downside is that it also washes the oil film off them, increasing wear, and it increases fuel consumption.
Edited to add
IO540 has it wrong and needs to go back to Boyles Law
The reason for full carb heat and not partial is generally given thus:-
A gas cools as it expands (adiabatic cooling, one of the ways that clouds form). As the air passes through the carburettor venturi it expands and its pressure reduces. This reduced pressure is what draws fuel in to the air stream. A side effect of this expansion is that the air cools. If it is humid air the water vapour will crystallize and build up inside the carb. The carb heat system is therefore so designed that at any temperature likely to be encountered in normal flight the temperature of the air will be high enough to prevent ice forming when FULL carb heat is applied.
You'll be aware of airframe icing. This can occur when there are no ice crystals in the atmosphere but the air is moist. Crystals need something to form on, and if the air is clean there's little to start the crystals forming. An airframe gives them something to form on and ice starts to build.
Now imagine the same thing with partial heat. The supercooled air passes through the heater and is warmed so that it is above freezing. It then goes through the venturi where it is cooled again and its temp drops back below freezing. There is plenty in the carb body for ice to form on so it does so. The use of full carb heat would raise the entry temperature sufficiently to prevent the cooling effect of the venturi taking the temp below freezing.
Hope I've explained this well enough.
Mike
Last edited by Mike Cross; 27th Sep 2006 at 09:54.
Carb ice?
Carb ice formation has little to do with the venturi. It mostly happens at low throttle openings where the airflow through the venturi is low such that there is also a low metering depression. If it were due to the metering venturi depression, then ice would form faster at full throttle.
Mostly carb ice would form due to the latent heat of evaporation of the fuel.
The other influencing factor is the pressure drop across the throttle valve, which is greatest at low power levels. This will cause the air/mixture to cool rapidly. This is a different pressure drop than that caused by the venturi!
Partial carb heat? Well, it doesn't matter how many times you heat and cool the air, the moisture content stays the same. I can't see how partial carb heat can be a problem, but what's the reason for using it? If carb ice is a risk, then manage it actively - don't just set and forget.
A
Mostly carb ice would form due to the latent heat of evaporation of the fuel.
The other influencing factor is the pressure drop across the throttle valve, which is greatest at low power levels. This will cause the air/mixture to cool rapidly. This is a different pressure drop than that caused by the venturi!
Partial carb heat? Well, it doesn't matter how many times you heat and cool the air, the moisture content stays the same. I can't see how partial carb heat can be a problem, but what's the reason for using it? If carb ice is a risk, then manage it actively - don't just set and forget.
A
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Mmmmm.....
At partial throttle openings the throttle butterfly makes a much bigger obstruction to the airflow and ice crystals are more likely to build up on it rather than pass straight through to the cylinders.
What has greatest pressure differential across it? A closed throttle or an open throttle? The greater the pressure differential the greater the cooling effect.
Agreed we should be talking about the combined effects of venturi & throttle.
For sure latent heat of vaporisation contributes towards it but more throttle=more fuel and in theory more icing whereas the risk of icing is greater at low throttle openings.
Heating of the carb body will also affect propensity to icing. Many Lycomings have the inlet manifold running through the sump meaning that the manifold and the carb get warmed by the oil. By contrast the small Continentals have a separate oil tank and the carb and inlet manifold hang out in the breeze, keeping them colder and forming a nice little ice factory (trust me, I know).
At partial throttle openings the throttle butterfly makes a much bigger obstruction to the airflow and ice crystals are more likely to build up on it rather than pass straight through to the cylinders.
What has greatest pressure differential across it? A closed throttle or an open throttle? The greater the pressure differential the greater the cooling effect.
Agreed we should be talking about the combined effects of venturi & throttle.
For sure latent heat of vaporisation contributes towards it but more throttle=more fuel and in theory more icing whereas the risk of icing is greater at low throttle openings.
Heating of the carb body will also affect propensity to icing. Many Lycomings have the inlet manifold running through the sump meaning that the manifold and the carb get warmed by the oil. By contrast the small Continentals have a separate oil tank and the carb and inlet manifold hang out in the breeze, keeping them colder and forming a nice little ice factory (trust me, I know).
Last edited by Mike Cross; 27th Sep 2006 at 10:44.
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I'm not much of a pilot, but I know a bit about how engines work, and I've got a Cessna 172 POH on my desk
Looking at the carb icing probability chart, it says that in an environment where relative humidity is over about 90% (i.e., dewpoint within a couple of degrees C of temperature) icing is a significant risk at any power, with temperatures between 0 C and 15 C. So icing is certainly possible at cruise power. There is a modest (up to about 30%) probability of light icing at cruise power, with temperatures between -10 C and 30 C (i.e., almost all temperatures routinely encountered in the UK). My understanding of the POH is that you wouldn't necessarily want to run at cruise speed with carb heat applied, although you should be alert to the symptoms of icing.
At descent power, the same temperature range gives a probaility > 40% of `serious' icing.
FWIW, my instructor insists on carb heat for all descents, and recommends that I switch it on periodically in the cruise to check whether icing is, in fact, present.
As for taxiiing, my understanding is that the 172 uses unfiltered air for the heated air intake, so there is a risk of sucking dust, etc., off the ground into the carburettor.
Looking at the carb icing probability chart, it says that in an environment where relative humidity is over about 90% (i.e., dewpoint within a couple of degrees C of temperature) icing is a significant risk at any power, with temperatures between 0 C and 15 C. So icing is certainly possible at cruise power. There is a modest (up to about 30%) probability of light icing at cruise power, with temperatures between -10 C and 30 C (i.e., almost all temperatures routinely encountered in the UK). My understanding of the POH is that you wouldn't necessarily want to run at cruise speed with carb heat applied, although you should be alert to the symptoms of icing.
At descent power, the same temperature range gives a probaility > 40% of `serious' icing.
FWIW, my instructor insists on carb heat for all descents, and recommends that I switch it on periodically in the cruise to check whether icing is, in fact, present.
As for taxiiing, my understanding is that the 172 uses unfiltered air for the heated air intake, so there is a risk of sucking dust, etc., off the ground into the carburettor.
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I suspect the advice to use 'full' carb heat is about the best one sentence generic advice one can give, but there are a lot of variations. If you have a carb ice temperature gauge you may find that running with partial carb heat is very useful.
The Cessna Pilots Assocation (www.cessna.org) has a lot of information on this. For example, as well as minimizing the risk of carb ice many 182 pilots will use partial carb heat to induce smoother running by evening out the difference in temperature between cylinders. But you need the instrumentation to measure these things.
Another interesting comment by the guy who runs the Advanced Pilot Seminars in the US was this:
The Cessna Pilots Assocation (www.cessna.org) has a lot of information on this. For example, as well as minimizing the risk of carb ice many 182 pilots will use partial carb heat to induce smoother running by evening out the difference in temperature between cylinders. But you need the instrumentation to measure these things.
Another interesting comment by the guy who runs the Advanced Pilot Seminars in the US was this:
I have written here before that the most common times for carb ice, in order of the most common to the least common are:
1) Startup
2) Takeoff
3) Cruise
4) Power reduction for descent
5) Power reduction to idle for landing
1) Startup
2) Takeoff
3) Cruise
4) Power reduction for descent
5) Power reduction to idle for landing
The key, Mike, is that at low throttle openings, the air-fuel mixture passes across a large pressure drop created by the throttle valve/blade. The sudden low pressure causes the fuel to boil (saturated vapour pressure > gas pressure at that point). The boiling/vapourising fuel absorbs heat from the air (and carb body) which lowers the surrounding temperature and causes the moisture content of the air to drop out (and freeze if the temperature is low enough).
At high throttle openings, you don't have the throttle valve causing this pressure drop so the fuel doesn't evaporate over the throttle blade.
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I'd be inclined to think of the safety sense leaflet as the basic, generic word on the subject - and couple it with any actual, experiential knowledge of the engine in question.
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The venturi has nothing to do with it. It is purely a fuel metering device to create a differential pressure that varies with airflow to push the required amount of fuel into the airstream. At low airflow, the differential pressure will also be low.
The key, Mike, is that at low throttle openings, the air-fuel mixture passes across a large pressure drop created by the throttle valve/blade. The sudden low pressure causes the fuel to boil (saturated vapour pressure > gas pressure at that point). The boiling/vapourising fuel absorbs heat from the air (and carb body) which lowers the surrounding temperature and causes the moisture content of the air to drop out (and freeze if the temperature is low enough).
At high throttle openings, you don't have the throttle valve causing this pressure drop so the fuel doesn't evaporate over the throttle blade.
The key, Mike, is that at low throttle openings, the air-fuel mixture passes across a large pressure drop created by the throttle valve/blade. The sudden low pressure causes the fuel to boil (saturated vapour pressure > gas pressure at that point). The boiling/vapourising fuel absorbs heat from the air (and carb body) which lowers the surrounding temperature and causes the moisture content of the air to drop out (and freeze if the temperature is low enough).
At high throttle openings, you don't have the throttle valve causing this pressure drop so the fuel doesn't evaporate over the throttle blade.
so with low power decents, a burst of throttle now & then is taught
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I may have got the enrich/lean effect of carb heat the wrong way round but I still can't see where detonation risk comes into it.
A typical fuel system would be set up to deliver about 150F ROP with all levers forward; this is the full-power-climb setup for a Lyco engine which keeps it reasonably cool while avoiding the detonation region which is centred on about 50 F ROP.
So, let's say you are climbing, full-bore, at 150F ROP and you make the mixture lean (using the carb heat). If that goes far enough then, at that high power setting, it could push the engine into detonation, by taking it into the 50F ROP region. But if carb heat has an enrichening effect, it will take the engine to say 200F or 250F ROP which will do nothing but waste fuel, reduce power a bit, and bung up the spark plugs.
A typical fuel system would be set up to deliver about 150F ROP with all levers forward; this is the full-power-climb setup for a Lyco engine which keeps it reasonably cool while avoiding the detonation region which is centred on about 50 F ROP.
So, let's say you are climbing, full-bore, at 150F ROP and you make the mixture lean (using the carb heat). If that goes far enough then, at that high power setting, it could push the engine into detonation, by taking it into the 50F ROP region. But if carb heat has an enrichening effect, it will take the engine to say 200F or 250F ROP which will do nothing but waste fuel, reduce power a bit, and bung up the spark plugs.
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you make the mixture lean (using the carb heat).
Which is why some of us are saying that worldpilot's friend has got it wrong and using carb heat will not cause detonation.
As you rightly point out, it is leaning the mixture that can cause detonation.
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Well my understanding of carburettors, is the venturi creates a low pressure area above the fuel nozzle that comes up from the float bowl. This sucks the fuel out of the float bowl and into the air stream going into the inlet manifold and then into the cylinder when the valve opens! The reason as i understand it, for getting carb ice is the air being expanded into the back of the venturi, small opening to large exit point, cools down, this expansion of the air is what causes the cooling effect which happens to be the principle of refrigeration. Just have a look at the top of a big Propane tank being used in anger and you will see ice on it even in relatively warm environments! Conversley, if you compress air it heats up, witness the old bicycle pump test, hold it near the end of the pump chamber and it gets very hot when inflating a bicycle tyre. You should avoid using carb heat on the ground as the air is un-filtered. Tango Victor the reason you dont have carb heat in some aircraft is if they are fuel injected, which also proves it is the venturi causing the ice as an injection system has inlet manifolds and no venturi's. Also i think "bumpffitch" recommends the use of carb heat in your down wind checks and i was taught to apply the C/H first and work through the rest finishing with the carb heat to cold. One last carb heat grey area< do you leave it on to the ground on approach or push it in at 300' so you have full power available for a goaround?
Do not use carburetor heat for takeoff or climb with a Lycoming engine as it is not necessary, and it may bring on detonation and possible engine damage.
I guess the issue is that the temperature of the mixture has an effect on the combustion process, but it seems unlikely to me.
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i think "bumpffitch" recommends the use of carb heat in your down wind checks and I was taught to apply the C/H first and work through the rest finishing with the carb heat to cold. One last carb heat grey area< do you leave it on to the ground on approach or push it in at 300' so you have full power available for a goaround?
On again when reducing power for lowering speed and operating flaps,off again on final.
I must admit I have sometimes forgotten to turn it off on final and never noticed any difference in climb out on a go around.
Surely it is safer to leave it on when engine is on idle,than risk icing at a critical moment for the sake of a small loss in power?
I am a low hours pilot but do know a bit about high performance engines,not that these are used in GA craft
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