Use of the carburator heat

[worldpilot]

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

[EGBKFLYER]

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;)

[robin]

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.

[IO540]

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...

[EGBKFLYER]

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

That's exactly what the leaflet says. I don't think they are saying partial heat is a 'bad thing', just giving sensible advice. The only physics I see is that partial heat will melt ice more slowly.

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.

[Mike Cross]

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

[Andy_RR]

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

[Mike Cross]

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).

[mad_bear]

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.

[drauk]

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:
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

[Andy_RR]

Mmmmm.....
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.


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.

[Mike Cross]

I bow to your superior knowledge Andy:D

[EGBKFLYER]

So in summary, if we do what it says in the Safety Sense leaflet, we won't go far wrong - or did I miss something? :uhoh:

[DRJAD]

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.

[tangovictor]

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.

I never did understand this subect that well, Thanks for your plain & simple explanation, when I was being taught to fly a R22 heli, carb heat was used on EVERY decent, the fixed wing im learning in now, has no carb heat function
so with low power decents, a burst of throttle now & then is taught

[IO540]

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.

[Mike Cross]

you make the mixture lean (using the carb heat).
By applying carb heat you make the mixture richer, not leaner. This is because the warm air is less dense so, though you are getting the same VOLUME of air into the cylinders you are getting less MASS.

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.

[pistongone]

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?

[bookworm]

Which is why some of us are saying that worldpilot's friend has got it wrong and using carb heat will not cause detonation.

Lycoming says (http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/operation/inductionIcing.html)

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.

[Lister Noble]

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?

I agree and was taught carb heat on for downwind checks then off.
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;) ;)
Lister

[Whirlybird]

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?

Read the POH for your particular aircraft. On some aircraft it's OK to push it in on the approach; if you did it on a C150 you'd very likely get carb icing before you landed! You need to know how your particular engine works.

[Andy_RR]

...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.

No, this doesn't prove any such thing. The reason fuel injected engines don't suffer carb icing is that the fuel does not evaporate on a relatively cold part of the engine (like a throttle blade, for example), but directly at the intake valve / port (which is quite hot - hotter than most carb-heat can manage!)

If you had a fuel injection system that delivered the fuel upstream of the throttle valve you would still get 'carb icing' even though you didn't have a carburettor or a metering venturi

A

[Darth_Bovine]

so with low power decents, a burst of throttle now & then is taught

I think you're probably flying a fuel injected engine and hence no requirement for Carb heat. The burst of throttle is more likely to avoid "shock cooling" of the engine at extended low RPM settings (such as glide approaches). This is certainly what I was taught anyway... Ask your instructor if this is the reason this is taught at your school. Also find out about the engine, always useful to know what sort of donkey you have up front! :}

Cheers,
DB.

[FullyFlapped]

EGBK,

I'm no mechanic, but I had hundreds of hours touring a 172 all over Europe, and I always frequently used full carb heat at cruise power. To me, it's a no-brainer : what else can you do if you start to lose RPM or pick up a bit of roughness in clear VMC, at cruise power and altitude (assuming you've already leaned correctly, of course) ?

FWIW, the engine on that 172 went happily to TBO ...

My current plane doesn't have carb heat, but I still occasionally find myself reaching for the knob during checks ! :ugh:

As for carb heat off to land, I understand why we're taught this, but .... It seems simple enough to me that if you need full power for a go-around, you just take in the CH selector before you hit the throttle - in a 172 they're next to each other and it's very easy to do both with one hand (even simultaneously). However, I'm not an instructor, do what it says in the POH, caveat emptor etc etc !

FF :ok:

[pistongone]

ANDYRR,
carb icing is not ONLY caused by fuel molecules. I think you might find that WATER in the atmosphere contributes a significant amount of the ice you might find in a carb:sad: therefore all your reasoning about fuel hitting throttle valves etc are flawed. Consider the suggested WEATHER related conditions that will prevail carb icing! Water makes ice and lots of it, the fuel flow at low throttle settings, where carb icing is MORE likely, is quite low and i think you will find that its the WATER that is the main contributor to the ice you find in the carb!

[Andy_RR]

Pistongone, I'm so glad you reminded me it takes water to make ice - I'd nearlly forgotten that! Of course! That's why I wrote:

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).

The water vapour in the air doesn't just freeze of it's own accord. It takes a mechanism to extract its latent heat to cause it to condense/freeze. If it isn't the vapourising fuel that's doing this, can you please explain what is?

[pistongone]

OK Andy, i will try to explain it as i understand it.
first of all i must admit to missing your reference to water vapour, sorry for that. But the carb is constructed with the float bowl at the bottom, with a nozzle going upwards into the venturi. Also in the float bowl is a float mechanism which is basically like a ball cock in your toilet to regulate the supply of fuel to the carb. Then in the venturi you have a butterfly valve, commonly refered to as the throttle valve.I have just thought of a way for you to visualise what is going on. If your petrol cap came off in flight the fuel would be sucked up out of the tank yes? Well this is because the wing creates a low pressure area on its top surface and the pressure differential causes the fuel to be sucked out. Well the venturi works in much the same way, by creating a low pressure area on top of the fuel nozzle, and its this low pressure that causes the fuel to rise from the float bowl and enter the airstream, in sodoing it mixes with the air which is called atomising. Now as i explained previously it is this low pressure and EXPANSION of the air which causes the lowering of temperature. I must admit it was a long time ago i was at engineering college but i think you will find this is something like how it happens. I worked on a motorcycle race team as mechanic, WSB and GP's and i must admit the carburretors on those machines were different in many ways to an aero engine carb, due to different performance parameters, but surely the principle is the same? Or can you explain what forces the fuel to RISE out of the float bowl, which is against the law of physics, and into the cylinders?
But whatever machine we are talking about here, if you compress gas it will heat up and expanding gas will cool down, that will always be the case, and you are expanding it into the venturi, which is a word with aerodynamic meaning, and i am quite sure its this expansion of the air which is the problem.
One little experiment you could try to see this principle in action is this. Get a can of coke and a straw, with the straw in the can blow accross the top of the straw and watch the liquid rise up the straw, this is beacause of the low pressure area you are creating at the top of the straw. One last interesting point about carbs, the manoever we love to see Spitfires do, the wing over or peel off, was performed simply to keep positive G, as negative G meant the fuel in the float bowl would rise up to the top of the chamber and starve the engine, which wasnt very convienient when chasing Gerry in an ME109 which had an injection system. If you are around Sywell on Sunday i would love to discuss this with you further, but i think i have taken up more than enough space on this thread now:ok: I will be in the C150 G-BDFZ, weather permitting.

[High Wing Drifter]

As you rightly point out, it is leaning the mixture that can cause detonation.Over-leaning causes pre-ignition not detonation. Detonation is the delayed spontaneous burning of unburnt fuel after ignition and my general understanding is that this is most likely when over-rich at high power settings (like climbing with carb-heat) and/or overboosting.

Andy,
The water vapour in the air doesn't just freeze of it's own accord. It takes a mechanism to extract its latent heat to cause it to condense/freeze. If it isn't the vapourising fuel that's doing this, can you please explain what is?
My understanding is that the drop in pressure in the venturi is the primary cause of the temprature drop in the carb. When air temp drops it may fall below the DP and so contained water condenses out. I believe the net effect is in the region of up to about a 35C loss. That means a humid OAT of 20-25 places you at the most risk of carb icing.

[IO540]

It's time that bookworm dropped in here and sorted out the physics :)

The burst of throttle is more likely to avoid "shock cooling" of the engine at extended low RPM settings (such as glide approaches). This is certainly what I was taught anyway...

I was taught that too but it is almost certainly wrong. Shock cooling (or, more precisely, damage caused by shock cooling) is an issue only if the engine it hot enough to start with.

If you have been flying full-bore for a bit and then close the throttle, the engine will cool rapidly from a high CH temperature, and that is where the danger lies (of cracks). A momentary power increase will do little to change this.

If you have planned this in advance and have reduced the power to something well below cruise, say 45%, and got the engine to stabilise there, then you can do more or less what you like in terms of power reduction, without damage. If not, you would damage the engine every time you landed (due to the engine power reduction or near-shutdown on short final).

Over-leaning causes pre-ignition not detonation. Detonation is the delayed spontaneous burning of unburnt fuel after ignition and my general understanding is that this is most likely when over-rich at high power settings (like climbing with carb-heat) and/or overboosting.

Detonation is the burning of the charge at an excessive speed. It can be caused by excessive leaning combined with a high power setting, particularly if combined with a high CHT. Detonation is not normally evident to the pilot - other than through a high CHT.

Pre-ignition is ignition taking place before it is supposed to, e.g. from an incandescent piece of material (say, a piece of lead stuck to the spark plug, or a damaged cylinder head) in the combustion chamber igniting the charge by itself. Not likely to happen otherwise unless you use the wrong fuel, and not related to leaning. Pre-ignition can damage an engine within minutes.

[Whirlybird]

Am I the only person who is now seriously confused? :confused: And I really, really thought I understood about carb icing. :{

I just looked at Andy RRs website, and it certainly sounds to a non-engineer like me that he ought to know what he's talking about. OTOH, almost everyone else's explanations agree with what I was taught, what my books say........and what I now teach my students! Help!!!!!!!!

I might just start telling my students that this is the way I understand it, though not everyone agrees, but it doesn't matter - they should use carb heat as advised in the POH for the aircraft which they fly.

And to go back to the beginning of this thread, if I remember it, you certainly DO need to use it frequently in the cruise in every small aircraft that I can think of!

[pistongone]

The simple solution to all this is fit the GA fleet out with PT6A's, it would also cure the loading probs on a PA28, no need for tabs:} :}

[Say again s l o w l y]

Gosh, this is a bit complicated.

In most light a/c carb ice can happen at any time (for example I had it yesterday in a 152 in the climb with full power.)

Whatever the mechanism that causes the temperature reduction in the venturi the simple fact is that it happens.

Partial carb heat is only useful if you have a good carb temperature guage. Heli's such as the R22 can use partial carb heat at any time because they have a temperature guage, but more importantly, they have derated engines. Meaning that the carb heat doesn't affect the power out-put in the same way it would with a fixed wing machine.

As has been put here already, read the POH for your machine, but if needs be use carb heat whenever you suspect that you have ice. Don't muck about it, use full heat and clear it as quickly as possible.

One last thing, none of that "put the carb heat to cold at 300' on the approach" rubbish. What people should do is to always think about where the carb air is whenever they make a power setting change. If you want to go around, then carb air to cold and then full power. A go around is rarely an instantaneous emergency, you can invariably see it coming a mile away, so you should have plenty of time to think about your actions.

[Andy_RR]

Pistongone,

Thankyou for your explanation. There are some things I disagree with which are fundamental to the problem.

The throttle valve is not in the venturi. It is downstream of the venturi and through it passes the atomised air-fuel mixture. - do you agree?

I agree that the venturi is used to draw fuel from the float chamber into the airstream. No arguments there!

The reason that a venturi is used is that the pressure differential it creates - that which is used to do the drawing of the fuel - is in some way proportional to the speed of the air flow through it. The more airflow through the venturi, the bigger the pressure differential, just like the more airspeed you have, the more lift you get.

So, if the most significant temperature drop is caused by the 'expansion' of the air through the venturi, why is carb icing more prevalent at low throttle openings (=> low airflow => low pressure differential) than at high throttle openings?

I suggest to you, the reason is because the 'expansion' through the venturi is not the most significant cause of temperature reduction in this process and that fuel vapourization is! Fuel vapourization that is caused by it boiling at the low pressures (e.g. low manifold pressure), which occur at or downstream of the throttle valve.

A

[pistongone]

AndyRR, i think i will have to take advice on this as we are entering the area of chemical reactions which i dont have great knowledge of. What i do know just to try to find an answer to the throttle valve question, which i think may be a bit of a red herring, is this. On Kawasaki motorcycles they had a problem with a certain model suffering carb icing and they passed hot water from the cooling system through it to solve the problem. The reason i am telling you this is that a motorcycle carb doesnt have a butterfly valve it has a cylindrical valve with a tappered needle attached to it which goes into the fuel nozzle to automatically lean and richen as required. So this Needle valve assembly is directly above the nozzle and therefore in the middle of the venturi. I dont think from your posts that we are talking about the same thing when we say Venturi? As the venturi is the shape of the air chamber within which the throttle valve and fuel nozzle are situated, it begins at the air inlet and ends at the inlet manifold. As for carb ice being more prevelant at low engine speeds, could that be a product of less engine heat to stop the icing? All of which is very interesting stuff which i now want to get to the bottom of:ok: as i am not totaly sure of the fuel vaporisation element in all of this? Just checked a book i have and it shows the ice building up in the back of the venturi as it drops away from the throttle valve, and states this is where it is most likely to happen as its the area with the greates pressure drop! Seems to be pointing towards my theory i think?

[Hampshire Hog]

Many many months ago, when I did my first solo, the PA28 in front of me had an engine failure just above the runway as they ballooned and opened the throttle to go around. Cause - carb ice. Not nice, even though it didn't result in any damage to anyone.

I leave the heat on until landed now. Check it is off once power set for the go-around - although I don't seem to have so many of those now:ok:

[Andy_RR]

Some useful links:

http://www.atsb.gov.au/aviation/editorials/e00016.aspx

and

http://www.ellison-fluid-systems.com/faqs/carburetor_heat.htm

BTW, Pistongone, I'd love to meet up with you at Sywell, but I'm currently in India, unfortunately with limited opportunities to test carb ice theories! :{

[robin]

Many many months ago, when I did my first solo, the PA28 in front of me had an engine failure just above the runway as they ballooned and opened the throttle to go around. Cause - carb ice. Not nice, even though it didn't result in any damage to anyone.
I leave the heat on until landed now. Check it is off once power set for the go-around - although I don't seem to have so many of those now:ok:

On some days and with certain engine/aircraft combinations the engine will stop when in the flare, even with carb heat applied.

We have had enough experience of this now to be prepared for it. So apologies to anyone who has had to go around because our Jodel is stopped on the runway......

[pistongone]

Well have a good trip AndyRR,
Maybe we can see if we can make it a PPRune technical Bash one day! Deliberately sabotage an engine and see if people can guess why it wont go!:confused: :confused: :confused: What say you?
Are you in the north of India? I only ask as i think being near mount Everest is one of those things which you just have to do in your life.
Cheers for now.

[High Wing Drifter]

IO540,
It can be caused by excessive leaning combined with a high power setting
Thanks for the correction. I just checked the Lycoming site. It is interesting that being over rich and over boosting was given as a reason during my ATPL! Maybe I wasn't paying attention or a slip on the instructors part :rolleyes:

[Mike Cross]

Am I the only person who is now seriously confused? And I really, really thought I understood about carb icing.
Part of the rich tapestry of aviation whirly!
An interesting discussion that has ranged about all over the place. For the puspose of instruction you need a simple explanation that the stude can understand. The fact is that the air is cooled as it passes through the carb. This is caused by a number of factors.

It's undoubtedly the case that the reduced air pressure in the venturi causes the temp to drop.
It's also undoubtedly the case that the vaporisation of fuel causes the temperature to drop
I would add that the pressure differential across a closed throttle butterfly also causes the temperature to drop, cooling the butterfly and the surrounding area.


How much detail you need to go into when explaining this to students is a matter for debate.

There are a couple of ways to illustrate these effects.

Pour a little fuel on to your hand (not too often as it's not very nice stuff) and it will feel cool. This is caused by the fuel evaporating, heat from your hand is being used to make the fuel vaporise.
Get hold of an aerosol and give it a good prolonged squirt (again not too often on account of the greenhouse effect). The can will get noticeably colder. A more eco-friendly thing would be to demonstrate the opposite by pumping up your bike tyres, the pump will get hot, illustrating that compressing a gas causes its temp to rise while expanding it causes it to drop.


As to which of these has the most effect? I suspect you could argue that 'til the cows come home.

[bookworm]

It's undoubtedly the case that the reduced air pressure in the venturi causes the temp to drop.

True, but it's reversible: the temp rises again when the air leaves the venturi. Thus unless the ice is actually forming in the venturi, that can be discounted.

My understanding is that most "carb ice" is in fact throttle icing, and occurs at the throttle butterfly, not in the body of the venturi. The reason why carburetor but not injected engines are affected is that the fuel vaporisation cooling occurs before the throttle butterfly in carb engines, after it in injection engines.

[Mike Cross]

the temp rises again when the air leaves the venturi
Errr..... by what mechanism?

The mixture is only going to move if there is a pressure differential, ergo the pressure has to be reducing all the way along the path (unless we're talking total improbability drives). If the air is flowing from an area of high pressure to one of lower pressure it's expanding, and if it's expanding, it's cooling.

[Andy_RR]

Mike, it's old Bernoulli again!

You've got an adiabatic (nearly) situation there, so when the air flows through the vena contracta, to maintain the same mass flow, it must speed up. Now old 'B' says that the sum of the energies must remain constant. So you're actually trading the fluid enthalpy (pressure and temperature in this case) for kinetic energy (velocity)

When you slow it back down again at the exit of the venturi, it being a reversible process trades the kinetic energy back again for pressure and temperature.

A

[pumper_bob]

RTFQ!!!:ugh: :ugh:
The original question was "to heat or not to heat?"
Heat the air if ice is there.
Heat the air if you think ice is there
Heat the air if you want to make sure ice isnt there.
So to sum up use carb heat every 15 mins in your freda and dont be shy, give it a good 10-15 seconds and check for a rise in RPM.
All the ballony over venturis and fuel vapourisation:=
Air expands causing a drop in temperature, so your right that this would promote ice.
Fuel vapourisation causes lower temps on the surface of cold metal objects in the area, so more potential ice!
I think both are valid points on a percentage of 75/25% water winning 3-1:D :D :D Ice does happen, we know why, protect against it, move on!!!

[shortstripper]

RTFQ!!! :ugh: :ugh:
The original question was "to heat or not to heat?"


No, it was asking us to settle an aurgument over the use of carb heat at cruise rpm and whether it causes detonation. The answer is that it doesn't and carb heat can quite safely be used at high rpms. Some just like to pontificate over the science of why, that's all. :E

SS

[bookworm]

There a NACA paper (http://naca.larc.nasa.gov/reports/1950/naca-report-982/naca-report-982.pdf) on induction icing who prefer science to pumper_bob's assertions of what he thinks is valid.

[Andy_RR]

As an interesting twist to this, I read on the Vans Airforce website a few guys using carb heat during cruise to increase the manifold pressure and reducing the pumping work for improved fuel consumption.

They would select carb heat, set mixture to LOP and reset cruise engine speed. I guess as long as cylinder head temperatures didn't go through the roof, you could be reasonably sure that it wasn't knocking it's bollox off.

The improved mixture preparation at higher air temperatures also improved the ability to run LOP.

They were claiming improvements of up to 1 USgal/h, although this seems a little optimistic, but there's certainly something in it.

Not according to the POH, but in an experimental, why not experiment! :)

A

[jamestkirk]

Pumper Bob has answered everyones question.

Why is everyone getting so anal about the mechanics then reading a tech manual and posting it on pprune.

Read pumper bob's explanation and leave it at that.

Also, if you are in a long descent, remember to warm the engine if you are descending from outside carb heat not required range to ensure some hot air going in. That mistake has been the downfall of many a practise forced landing exercise.

IT'S BETTER TO USE IT A LITTLE TOO MUCH THAN TOO LITTLE.

[IO540]

That URL appears duff, bookworm.

[pumper_bob]

There a NACA paper (http://naca.larc.nasa.gov/reports/1950/naca-report-982/naca-report-982.pdf) on induction icing who prefer science to pumper_bob's assertions of what he thinks is valid.
Well Bookworm, I hope the scientific people are better at getting their experiments on carb ice working better than their link:} :} :} Also i would have expected a bookworm to be reasonably accurate in their GRAMMAR:= I would have thought "There's" would have been the correct word at the beggining of your post. If your going to throw stones, move out of the glass house my boy! (what is the term for a young worm?):mad: :mad: Also, what assertions of my post exactly are you refering to as not being valid? Just curious bookworm.

[pumper_bob]

Quote "I used the carburator heat several time during cruise flight and he said that was dangerous because of detonation."
ShortStripper,
The author said he used carb heat and his passenger said he shouldn't use carb heat by way of inference! His statement of the risk of detonation infered that he didnt think he should be using it. So "to heat or not to heat" that is the question:ugh: :ugh: :ugh: And all i was doing was playing on some words a Gentleman known as "The Bard" once wrote:p :p

[bookworm]

Get back under yer bridge... ;)

[bookworm]

That URL appears duff, bookworm.

I think the URL is fine, but the site's down at the moment.

[pistongone]

Maybe the scientists didnt know how to use the carb heat after all and their computer crashed:} :} :} Whilst we are waiting, Bookworm can you tell us what these "scientists" actually had to say about the subject?

[gasax]

Just to further muddy the water (apart from Pumper Bob - who was spot on!).

Fly a Gipsy powered machine and for a lot of the time you'll have to have hot air applied. to be strickly fair it is really 'warm air' but it is so necessary in that engine that the RAF wired their Chipmunks to give warm air continuously.

I suspect its problems come from the engine capacity which for its power is significantly bigger than even a Lycon. Either way the engine handbook talks about continuous hot air.

I do wonder however why such myths as hot air causes detonation can ever start - does no one read the engine manual? I recall when I learnt to fly the POH was regarded as something that mere students should not be allowed to read but is that still really the case? Do pilots not have any level of curiousity about the machinery that hauls them aloft (just enough to read the manual)?
Are instructors so ignorant that they perpetuate this rubbish?

[Crash one]

The way I read this is. The air flows from the top of the carb down to the narrower neck of the venturi increasing in pressure as it goes, at the narrow point it rapidly increases in velocity & pressure drops, sucking fuel into the stream, & cooling down adiabatically, as it gets below the venturi it must begin to increase in pressure & decrease in velocity (slightly) as the throttle barrel increases in dia, then, passing through the throttle butterfly there is a much more marked difference in area / volume so the venturi (cooling) effect is much greater at this point.
The more the throttle butterfly is closed the greater this difference is, so the greater chance of icing at low throttle settings, around & below the butterfly.
If a throttle barrel / venturi has more marked differences in dia ie:-a relatively smaller waist, then icing could ocur at open (cruise) settings.
Is there an optimum venturi shape / profile that is universal or do carburettor designers choose their own? POH aside I'm just going by my understanding of how a throttle works.
I am being taught to close the heat at 300ft (c.r.a.p) checks on final, (C152)
I am also under the impression that detonation is caused by hot spots & pre-ignition by overheating / too lean a mixture.
By the way I am not overly confused by this, if the engine sounds a bit rough I'll use the heat regardless of throttle setting, if that doesn't work I'll get out & clean the plugs!!

Trevor, dumb stude!

[shortstripper]

Pumper bob,

I can't resist jumping in to defend Bookworm over your attack on his grammar. This forum isn't an English language one (or literature for that matter, "ref, your reference to the bard").We all make minor mistakes and your ... or should that be you're? no exception. := :p ;)

SS :ok:

[pistongone]

Well well, SS! You're telling Pumper its not a grammar forum, then go on to correct a minor mistake. So i suggest you look at your AURGUMENT in post 46:} :} :} Also i tend to agree with Bob, in so much as it is a question of when to use carb heat, with the resultant detonation or not, being his passengers reason for not using it! Glass houses as was said earlier!:E :E

[Crash one]

sodthegrammerwhycantwegetbacktothethreadanleaveoutthebickeri ng.

Trevor,

[shortstripper]

Has everyone lost their sense of humour? I was being ironic and having a friendly dig at Pumper Bob. Why do you think I added a :p and a ;) ?

Oh well! never mind :ugh:

SS

[Crash one]

sorry:uhoh:

[ShyTorque]

The way I read this is. The air flows from the top of the carb down to the narrower neck of the venturi increasing in pressure as it goes, at the narrow point it rapidly increases in velocity & pressure drops, sucking fuel into the stream, & cooling down adiabatically, as it gets below the venturi it must begin to increase in pressure & decrease in velocity (slightly) as the throttle barrel increases in dia, then, passing through the throttle butterfly there is a much more marked difference in area / volume so the venturi (cooling) effect is much greater at this point.
The more the throttle butterfly is closed the greater this difference is, so the greater chance of icing at low throttle settings, around & below the butterfly.
If a throttle barrel / venturi has more marked differences in dia ie:-a relatively smaller waist, then icing could ocur at open (cruise) settings.
Is there an optimum venturi shape / profile that is universal or do carburettor designers choose their own? POH aside I'm just going by my understanding of how a throttle works.
I am being taught to close the heat at 300ft (c.r.a.p) checks on final, (C152)
I am also under the impression that detonation is caused by hot spots & pre-ignition by overheating / too lean a mixture.
By the way I am not overly confused by this, if the engine sounds a bit rough I'll use the heat regardless of throttle setting, if that doesn't work I'll get out & clean the plugs!!

Trevor, dumb stude!

Crash 1, I don't think one part of your post is quite correct. How would the pressure increase as it enters the carburettor body towards the venturi? The only influence on it, in a normally aspirated engine, is atmospheric pressure.

If anything, the air leaving the intake plenum and entering the carb will accelerate, reducing its pressure and temperature. Entering the venturi will increase the velocity further, reducing the pressure and temperature even more.

BTW, the venturi size depends on the purpose for which the carburettor is designed. For example, I have two Weber 34 ICH downdraught carbs in my garage. One has a 29 mm venturi, the other is only 25 mm. The difference is the engine for which they were designed. The first is for a 2.25 litre Landrover and the second for a 1.3 litre Ford Sierra.

The reason for the existence of the venturi is to cause enough of a depression to draw sufficient fuel, in aerosol form, from the fuel jet into the airflow throughout the operating rpms of the engine. A smaller cylinder engine (less suck each cycle) will require a smaller venturi to keep the depression sufficient for a high enough gas speed for the engine to operate at low speed. One could use a bigger main jet instead of a smaller venturi, one might think - but that would give too rich a mixture at high rpm and poor air/fuel mixing at low rpm.

At the end of the day, carburettor design and application is (was, mainly) theory followed by quite a lot of experimentation.
:)

[rotorfan]

A couple of points I haven't seen mentioned yet:

1) Partial vs. full carb heat - partial carb heat can sometimes raise the temp just enough to start melting the ice, only to have it refreeze deeper into the induction tract. If you have ice, or think you do, use full heat. As a couple of other R22 drivers here have mentioned, if your aircraft is equipped with a carb temp guage, partial heat can be applied, as you're able to tell just what the temp is inside. However, below 18" of MP, full heat should be used on the Robbie.

2) It's important to get the heat on NOW if you suspect a power loss. As the power output decreases, so does the very heat that is going to solve your problem. This is also why, when doing a routine power reduction, have the carb heat on for 15 seconds or so to give the carb parts time to warm up before the power reduction. If you pull the power back and apply heat at the same time, again, you'll be cutting back on the amount of heat available, just when you're more likely to need it.

3) I've read some studies on carb icing where the temp drop inside the carb was as much as 70ºF/21ºC. This means that you could be sweating during a hot 90ºF/32ºC day, even though it's below freezing inside the carb. You can have carb icing a large percent of the time if the relative humidity is high enough.

4) An engine driving a fixed-pitch prop should show an RPM reduction when heat is applied (less dense mixture=less oxygen=less power=less revs). If ice did exist, then when it melts, the RPM should increase. A constant-speed prop can disguise the building ice, since as the power output drops, the prop just flattens to keep the revs up.

I've only had carb ice once, and that was enough. It was morning, about 65ºF/18ºC in the summer, so high humidity here. About 2300' MSL. The engine went from smooth to choking in about three seconds. :{ I had not sensed any RPM loss, or roughness, leading up to the choking. If I wasn't fully awake before, I was now terrified. :eek: As training had taught me, I had the carb heat out in under a second (kinda like spring-loaded to dump that R22 collective) even as I was looking back over my shoulder at the stadium parking lot I had just passed over, planning on landing between light poles. In another three or four seconds, she was running clean again. :D This was in a 172, perhaps one of the more susceptible planes.

Rotorfan

[Final 3 Greens]

Arrrrrrrrrrrrrrrrrrrrrrrrrrrrrgh!

After reading this thread I am only going to fly fuel injected planes.

My brain hurts.:{

[IO540]

After reading this thread I am only going to fly fuel injected planes.

Notwithstanding the amount of junk science written here, that's an excellent policy anyway.

One does wonder how many fatal accidents were caused by carb icing. Of course the stuff melts, leaving no evidence. The end result appears like a botched forced landing, and AAIB reports pretty routinely speculate regarding pilot incapacitation.

[pistongone]

If you look at footage of a Space shuttle launch, there is loads of ice falling off the thing:confused: :confused: I didnt know rockets had carbs? Confuscious Say:Just because ice is white doesn't mean anything white is ice:confused:

[Final 3 Greens]

The boiling point of hydrogen is -252.7 degrees celsius.

The space shuttle main engines are powered by liquid hydrogen, which is stored in a ******* big tank at less than its boiling point.

Confucious knows **** all about rocket engines.

[Whirlybird]

Arrrrrrrrrrrrrrrrrrrrrrrrrrrrrgh!

After reading this thread I am only going to fly fuel injected planes.

My brain hurts.

My brain hurts too! But for those of us who prefer flying to technical detail, let's go over the basics....

1) You don't need to understand about carb icing in order to use carb heat effectively.
2) You can get carb icing in just about any weather possible in the UK.
3) You need to apply full carb heat whenever you have low power settings. At other times, use it frequently; the detrimental effects of too much carb heat are far, far less than the catastrophic effects of too little.
4) If in doubt about any of the above, if your engine sounds funny, if you haven't done it for a bit, if you just have a strange feeling something might be wrong....apply carb heat.
5) For minor differences to the above, read the aircraft POH, and do what it says, not what an instructor or another pilot or anyone on PPRuNe says.
6) If still in doubt, another blast of carb heat won't do any harm. :ok:

[pistongone]

So just to re-cap then Whirley, are you recommending the use of carb heat or not:} :} :} and i am just seeing if we can get this one into the hundred plus replies league:ok:

[Final 3 Greens]

Whirly

I don't agree with your statement that "You don't need to understand about carb icing in order to use carb heat effectively."

I believe that "Every pilot who flies an aircraft powered by a carbureted engine should be thoroughly educated about carburetor ice."