Carb heat on Warrior (LYCOMING O-320-D3G)?

[Contacttower]

I just can't seem to get a straight answer on this one....

I've always been taught apply carb heat on any carb engine as part of pre-take off, FREDA and downwind checks. This is how I've always operated and continue to operate.

I have the CAA AIC on carb icing and from time to time consult to remind myself of the most dangerous conditions for ice formation.

But I have heard different stories (this has been covered before on here) about the use of the carb heat on the carbed PA28s....

Considering the above imagine my shock at being told by my IR instructor in Florida upon seeing me reach for the carb heat during a FREDA check "nah it's too hot for carb heat" mentally I just said to myself WTF?!! (Temp about 30 dew point about 25). He also seemed to think that oil temperature had a bearing on whether ice would form or not....again something that really surprised me.

Reading the POH however revealed this:

The mixture control should be kept in full "RICH" position to ensure maximum acceleration if it should be necessary to open the throttle again. Carburetor heat should not be applied unless there is an indication of carburetor icing, since the use of carburettor heat causes a reduction in power which may be critical in the case of a go-around. Full throttle operation with carburetor heat can cause detonation.

Still not convinced I asked the examiner during my checkride "would you use carb heat now" when we where descending with about 1900RPM set and in close proximity to yet not actually in clouds. "No I would not" came to reply; he had asked the factory and in there opinion during normal descents the use of carb heat was not recommended.

This left me with the impression that Piper are pretty confident that the O-320 engine doesn't really suffer carb icing in normal conditions. How do they know this though? I've never seen a graph that describes ice forming conditions for the PA28....does one exist and what do other people think of this?

[DavidHoul52]

I think that... oops sorry !:oh:

[gasax]

There is a somewhaht different perception of carb icing in the US to the UK. I suspect some of that is the difference in temperatures, or rather the difference between ambient and dew point.

But there is also a big difference between a Lycoming and the Continental ice making machines. i've mainly flown behind the ice making machines - Continental and Gypsy. So your technique is largely the one I have used. But I have also had the same sort of comment in the US about the need for cab heat. Or rather the lack of need of carb heat. Given a good gap between ambient and dew point and a Lycoming then probably carb heat is largely not needed.

[Maoraigh1]

I've got an FAA diagram of temp, dew point, and carb icing probability. In the U.K. usually only in the coldest air am I outside the carb icing range. There is also high temp at which, regardless of humidity, carb icing cannot occur. The instructors is likely to be right.

[Contacttower]

I've got an FAA diagram of temp, dew point, and carb icing probability.

How different is it from the CAA one?

[BackPacker]

Carburetor heat should not be applied unless there is an indication of carburetor icing, ...

Carb ice is a real fickle thing. Hard to detect without actually applying carb heat and watching the results, unless you are really paying attention, and that's only possible if you have lots of experience on type and spare mental capacity. Not your average student.

The POH is right: it only makes sense to apply carb heat if there is an indication of carb ice, and it may actually be dangerous to apply carb heat unnecessarily - detonation comes to mind.

But in the absence of carb ice detectors and students who can make perfect judgement calls about whether the situation is conducive to carb ice forming, flying schools have developed some general guidelines which are not damaging to the aircraft and will prevent carb ice from becoming a problem if the conditions happen to be right.

So you and I have been taught to apply carb heat briefly during the runup checks (to melt the ice that may have formed during taxi, and to test the mechanism), to apply carb heat during FREDA in the cruise (to detect formation of carb ice during cruise settings), to apply carb heat on downwind (probably actually as part of the RPM reduction to 2000 or below), and to turn off carb heat just before or after landing so that a go-around is flown without carb heat.

That's all fine. Those practices won't damage the engine and in case the conditions are right for carb ice, they are really effective - provided that your FREDA checks are not too far apart. The fact that a carb heat check as part of FREDA at 30 degree ambient doesn't really makes sense, doesn't really matter.

There are other things that you may have learned to do by rote which do not make sense right now. Some schools insist that the pre-landing checks include a "gear down" check, even in aircraft that have fixed gear. It's nonsense right now, but may save your life (or at least prevent an expensive repair) eventually, some day, in a situation where your mental capacity just fell that tiny bit short.

[Contacttower]

There are other things that you may have learned to do by rote which do not make sense right now. Some schools insist that the pre-landing checks include a "gear down" check, even in aircraft that have fixed gear. It's nonsense right now, but may save your life (or at least prevent an expensive repair) eventually, some day, in a situation where your mental capacity just fell that tiny bit short.Indeed my thoughts exactly BP.

I'd just like to know more about the actual testing done on that particular engine to determine when and when it won't suffer carb icing. I know because of the location of the venturi the O-320 doesn't suffer like some Continentals do at 'normal' UK temps/dew points but surely at some temperatures (for example when one might think it is too cold for icing) this may actually make carb icing more likely.....just when you aren't expecting it.

What would be nice is an engine specific version of the CAA AIC on the issue.

[gasax]

It is a very cheap mode to add an optical icing sensor to a Marvel Schriber carb. One of my friends has done it to his last two aircraft.

Flying beind a B2B 0-320 he very rarely sees any sign of icing. But it does happen - especially when dew point and ambient are close.

My flying behind a C90 and O-200 and Gypsy - I've have significant carb icing many times. With the Gypsy continuous warm air is othen the only way to keep it at bay - but there is no way to fit one of those sensors.............

[Maoraigh1]

http://www.caa.co.uk/docs/33/ga_srg_07webSSL14.pdf

It's the same. I just got the FAA one years ago, before the CAA one. Link above to CAA site explains the lot.

[RMarvin86]

I have been taugh that the application of carb heat cannot be of any danger with the throttle open at 75% or less so I would say, WHEN in DOUBT of icing formation, just apply.

[SkyHawk-N]

Lycoming states that carburetor ice will occur in the 20 to 90 degrees F. temperature range but it also depends on the humidity. I've been suffering quite bad carb ice with my 0320 on the Northwest coast of the USA over the last week or so. It's been approximately 80+% humidity, 70 degrees F, dewpoint 64 degree F.

[Mark1234]

Surely the point in applying carb heat during the runup is to ensure that the carb heat selection is functioning, *should* you need it in flight...

[SkyHawk-N]

Surely the point in applying carb heat during the runup is to ensure that the carb heat selection is functioning, *should* you need it in flight...

Yes, but carb icing can also occur while taxying. Best to clear it before the take-off roll. ;)

[SNS3Guppy]

Mark 1234 and Skyhawk N are both correct; it's to check that it's working, it's also to ensure the carb ice is removed prior to beginning the takeoff roll.

Don't assume that the procedures for one airplane apply to all airplanes, or more specifically, simply because this or that is the way you've done it before, you should do it that way all the time.

Apply carb heat where you need it, when you need it.

Simply because you're not normally recommended as needing it during a go-around, for example doesn't mean you won't need it. I've had engines quit as I or a student executed a go-around, with power restored by the application of carburetor heat.

Use it when you need it.

[Pilot DAR]

Carb ice can certainly occur when not expected, taxiing included. The comments suggesting a reference to the POH are obviously the best to follow. Do remember that not only does the use of carb heat cause a slight loss of power, but to be the most effective, it is vital to lean/relean the engine after application, the obtain the greatest effect. If you actually need the carb heat to deice the carb, the engine may already be running a little more rich than is intended, due to restricted airflow. When you select carb heat, you introduce hooter air, which is less dense (similar to a higher altitude), so less power. Less power equals less exhaust gass temperature, which is the source of heat for the carb heat in the first place. So, to get the heat back, lean the engine. That will increase the EGT again, and you'll get much hotter air, and thus more effective carb heat.

Also, my certification testing of several carb heat systems, including my Cessna 150, indicates that if you really have to get rid of ice in the carb in flight, the very best result will be attained with a power setting much less than full power. I attribute this to the lower mass airflow through the exhaust heat exchanger and carb, which allows the air to heat up more, and carry more of that heat to the offending carb ice. This reduced power setting also allows many engines to be operated at, or beyond peak lean without damage (consult you engine's manual please). Therefore, without an exhaust gas temperature gauge, you can lean for heating effect simply by bringing the mixture toward lean until the engine stumbles, then richening it a little (with carb heat already hot).

In my 150, an RPM of about 2100 and peak lean with carb heat hot gives the best result (I have a carb air temperature gauge). Incresing power fromthis setting will result in coller carb air temperatures, thus, one would presume, less effective carb deicing.

Also be cautious that the use of carb heat as a preventative measure, can create a different problem. If the moisture in the air is just below freezing (yes, you can have water drops in air which is jut below freezing), and the carb air temperature is also less than freezing, there is a chance that the water will go right through the carb without interfering with the way the engine ran. If you apply carb heat, you can warm the moisture, which then may refreeze in the carb. I have been able to produce this effect at times. In this case, carb heat cold would have been safer.

Also remember that the use of carb heat on the ground removes the air filter from the induction air circuit (on purpose), thus if it is dusty, that's wht's going through your engine. It is agreed though that dusty and moist do not usually occur at the same time in air.

For what it's worth, I (while using a carb air temperature gauge) never use carb heat, inless it's use is indicated by the conditions. I check it's working, and that's it. Only in moist conditions, when the carb air is right around freezing do I apply carb heat, and then only enough to bring the carb air up out of the freezing range.

The application of carb heat will not cause harmful detonation in the engine, if it is otherwise being run properly (correct octane gasoline and not over square for normaly aspirated engines)

Pilot DAR

[Mark1234]

Ok, I should have worded that more precisely :) What I meant to communicate was: Even if icing is unlikely, one should still test the carb heat during the runup to ensure it is functioning, in case you should for some reason need it later!

I confess I do mindlessly apply carb heat once I throttle back on base then knock it off turning final; I was taught not-above 2000rpm unless dictated by icing. I guess that's attempting to mitigate people feeding the engine excessively hot air at flat chat.

[Dan Winterland]

Carb heat also depends on the model of aircraft. In the PA28 series, the carb inlet is in a position in the cowling where warm air is drawn in so an element of carb heat already exists. The POH reflects this is it's advice already mentioned here.

Also, WRTo cooling in the carb, the venturi effect will cool about 15 degrees and evaporation about another 15. There is a school of thought that carb icing is not possible over 30 degrees OAT. (Not likely in the UK!).

Having said that, caution is the best policy.

[Final 3 Greens]

I've about 200 hours on PA28s with O320s and O360s, so not a huge amount compared to some of the other posters.

Having "gone through the motions" for over a 100 hours of carb ice free flying on PA28s, I was doing a few night circuits with an instructor on a lovely moonlit early November night.

Having started up and taxied to the hold, apparently all normal, I appplied the carb heat during the power checks.

There was coughing and misfiring as I'd never heard before :eek:

The instructor turned to me and said word to the effect that it was a surprise we'd picked up carb ice on a nice night like this, but it just goes to show how it can happen and why the checks exist.

ILAFT.

[gasax]

Reading the POH is usually the key - with a Gypsy it is recommended that you use hot air on the ground in dusty conditions! (the induction route has a sharp 90 deg bend which much reduces the dust ingested!).

Taxiing on wet grass is a classic location for severe carb icing, my C90 would require hot air if idling for any time.

[bArt2]

Carbu Heat should be used when the OAT is between +20 and -5ºC with visible moisture present. Visible moisture would be in the clouds, rain or bad visibilty. If you have 20 Km visibility there would not be a lot of moisture in the air.

If you use it to prevent ice forming AND you have a Carburetor temperature indicator you should apply just enough heat to get the temperature 5 degrees C above the yellow arc.

If you use it to get rid of icing that has accumulated allready you should use full heat, then when icing has disipated adjust heat to 5º above the yellow arc.

If you have no carbu temp indicator I would use full heat.

You should not use carbu heat on the ground for a prolonged time unless absolutely necessary. Because as you may or may not know, when using carbu heat the air going to the carburator does no longer pass through the air filter and therefore you can suck dust, sand or other particles in the engine.

If you use carbu heat (certainly full and with high powersetting) it may be necessary to lean because otherwise the mixture becomes too rich.

However it all depends on the engine type. In my experience with the SF260 (Lycoming O-540-E4 A5) it is very prone to get carbu icing. A symptom of carbu icing in that aircraft is that when you change power from about 75% power to full power the engine would start running rough and hesitating while changing the throttle.

Using full heat in this aircraft results in a huge loss of power and a very rough running engine so leaning is then absolutely necessary.

On the other hand, the C152 is in my opinion less likely to build up carbu ice and if you use full carbu heat you do not notice a lot of diference in how the engine runs.

If have no experience in the PA28

Hope this helps.

Bart

[BFPO]

I am taught to do the carb heat checks regardless of temperature if for no other reason than to put you in the habit. If it becomes a routine part of your drills, then you are less likely to forget to use it when you need to!

[DavidHoul52]

apply carb heat on downwind (probably actually as part of the RPM reduction to 2000 or below),


That was what I was taught to do. I see the pre-landing check list says "Check and set". Is there any point in checking it if you are going to set it to hot straight away? What would one do differently if the check showed there was a lot of icing?

[RatherBeFlying]

In the C-172 I simply put on full carb heat on downwind and do my best to remember to turn off carb heat for a go around -- actually the muscle memory has carb heat off with full throttle.

Flight below a ceiling is conducive to carb icing. And I definitely remember the engine stumbles I got after selecting carb heat turning downwind after a descent of a few minutes;)

[Final 3 Greens]

do my best to remember to turn off carb heat for a go around

Its easy to forget.

I normally return it to cold when taking land flap at about 300' - that then builds in a trap if I land with less than full flap, so have to remind myself at that stage.

I see the pre-landing check list says "Check and set". Is there any point in checking it if you are going to set it to hot straight away?

That line item reads a little ambiguously to me - I see why you read it one way, I could also read that as "check if HOT, if not set to HOT", or even later in the landing (very short final) "check if HOT, if HOT set to COLD."

In my opinion, on the 152, you need to achieve 2 things. One, don't have your engine quit through carb icing before you can make the runaway, so use carb heat during the descent and then (two) have full power available for any go around, since you'll need this, as she is adequately, rather than generously, powered.

The ability to make the runway is dependent on energy in the airframe, I've always liked to approach a little steeply so I'm less engine dependent, but that is a different discussion altogether.

[BeechNut]

If have no experience in the PA28

With all due respect, that pretty much invalidates your response as the query was specifically directed to the PA28, which IS very much different from a Cessna in this important aspect.

There are some key elements to keep in mind here:

1) the hot air circuit is unfiltered, so use of carb heat should be minimized to the extent necessary;
2) the O320 can detonate under full throttle with hot air;
3) the PA28 with the O320 is not exactly overpowered;
4) the design of the carb system on the O320 in the specific PA28 installation (does not apply to Cessna), makes carb ice very unlikely, though not impossible.

I have about 250 hours in my own PA28, operated summer and winter; in rain, snow, cool temperatures and hot temperatures, and freezing temperatures. Never once encountered carb icing, even once when inadvertently flying into unexpected icing conditions (alive to tell the tale: descended to a warmer altitude). I now operate a Beech C23 with the Lyc O360 and the POH states that carb heat should only be used on approach if required, not systematically, and with the Beech, again I have never encountered carb icing.

That said, that's not to say it's impossible, hence the reason for carb heat. I therefore do a brief check of the system during runup (as per checklist) and in the downwind leg to ensure it is functional, but would not fly the approach with carb heat unless called for by conditions.

My current bird has a carb temp gauge though, which is a real help and should be standard on C150s...

[Final 3 Greens]

PA28, which IS very much different from a Cessna in this important aspect.

I completely agree with your point Beechnut.

Having flown other aircraft with the Continental 0-100, I found that they needed constant checking for carb ice (i.e. regular additions of hot air in the cruise), whereas PA28s (O320 and 0360), as you say, are highly resistant to carb ice.

This doesn't make the 0-100 a bad engine, it just requires a different approach to engine management.

Thought I'd share that for any newbies/low hours on Pup series 1 or similar.

[Contacttower]

All good stuff guys....:ok:

I have occasionally run the engine at full power with the carb heat on for brief periods....classic example being after a stall recovery forgetting to take it off immediately. I know this carries the risk of detonation and should be avoided at all times.

However is one going to do the engine any damage by having the heat on for 15-30 seconds periodically during a flight in the 2200-2400RPM range?

I've always assumed that the answer is no....with the only danger being from foreign objects entering through the unfiltered entry (which I've always assumed is unlikely at altitude).

[robin]

I suppose the only comment I'd make is to avoid the ' I've never had that problem' attitude.

There is a real risk of getting casual with icing checks because you fly a 'resistant' machine. Over the past couple of years it does seem as though there have been a fair number of incidents leading to forced landings and worse which can be attributed to poor recognition and handling of icing conditions.

Having flown behind VWs and Continental 0-200s and having had the engine stop on occasion, I can testify to the way icing can creep up on you.....

Oh, and a friend told me that he was at full power in a Warrior on Saturday and experienced a marked power loss through icing.

[BeechNut]

Agree, doesn't mean it can't happen, hence the reason carb heat exists even on these fairly resistant engines.

My checklist calls for a carb heat check at runup, and I do one on downwind, and occasionally in cruise, but mainly to see if the system is working. I have a carb temp. gauge so I rely on that to see if there's a potential for icing. The test also verifies the functioning of the carb temp. gauge as application of heat should cause it to rise out of the danger zone.

The POH does say use it on approach if necessary. But only if necessary.

[DavidHoul52]

Perhaps I'm just stating the obvious but (according to this AAIB report) the standard 10 second setting to hot which we are taught is not necessarily enough in some conditions.


http://www.aaib.dft.gov.uk/cms_resources/Reims%20Cessna%20F152%20Aerobat,%20G-BFZT%2003-08.pdf

[bArt2]

With all due respect, that pretty much invalidates your response as the query was specifically directed to the PA28, which IS very much different from a Cessna in this important aspect.


I wrote about 13 lines.

Most of it was general information about using carbu heat and there was something about the Lycoming 540.

There was only 1 line about the C152, so I do not really understand why you interpret this post as being C152 specific.

completely agree with your point Beechnut.

Having flown other aircraft with the Continental 0-100,

By the way the C152 over here have a Lycoming 235, not a continental.

Bart

[Final 3 Greens]

bArt2

Please re-read the sub heading of my post, I wrote it for a reason ;)

[dont overfil]

We had an odd situation a few years ago with a C152 being flown in extremely cold winter weather, around -15 at 4000ft. When carb heat was applied the engine gradually began to lose power. The aircraft managed to limp back to the field and no fault was found. The theory given was due to the abnormally low temperature carb heat brought the temperature in the carb UP to the icing band.
I wouldn't have thought enough moisture would have existed in the air at that temperature.
Any thoughts?
DO.

[bArt2]

around -15 at 4000ft. When carb heat was applied the engine gradually began to lose power

That is why in my first reply in this thread I mentioned that you should use it if the temperature is between -5ºC and +20ºC. If you use it below -5ºC you would create the problem that you just brought up. Having rather low visibility due to moist air could be enough moisture.

I hope this doe not turn into a Cessna discussion just because I mentioned the C152 in one sentence.

@Final 3 greens

I am not shure what you mean with the sub-heading "not just the 150"

Greetings, Bart

[porch monkey]

Would someone mind explaining to me how the use of carb heat can cause detonation?

[barrow]

Would someone mind explaining to me how the use of carb heat can cause detonation?

1, increase in induction air temp.
2, decreasing power output.
3, increasing fuel/air ratio.
4, increase in cylinder temps/pressures for same output (no heat)
5, possible increase for detonation at high power outputs.

[bjornhall]

That is why in my first reply in this thread I mentioned that you should use it if the temperature is between -5ºC and +20ºC.

Do you mean only to use it in that band? According to the various carburettor icing graphs in circulation, such as those repeatedly linked to in this thread, it would appear carburettor icing is possible in the range from -15ºC to +35ºC...

[bArt2]

The rule that I am aware of is to use it when visible moisture is present AND the outside temperature is between -5 and +20ºC.

Now it seems that different rules circulate, such as -5º to +30ºC and -7ºC to +30º.

-15ºC seems a bit on the cold side I think.

So do not use carbu heat if you are above or below this range.
Using it above this range would not be a problem I guess as long as you make sure that you have no detonation and adjust mixture as required.
If you use it below the range however you could actually cause carburetor ice to be formed.

The reason that you should not use carbu heat below -5-ish temperature is that below this waterdroplets would normally be frozen and would just pass through the carburettor. If you would then use carbu heat, the droplets become liquid again and can then re-freeze in the carburetor.

As with most general rules, I guess that it is valid in most but not all cases b.

[SNS3Guppy]

Having flown other aircraft with the Continental 0-100, I found that they needed constant checking for carb ice (i.e. regular additions of hot air in the cruise), whereas PA28s (O320 and 0360), as you say, are highly resistant to carb ice.

This doesn't make the 0-100 a bad engine, it just requires a different approach to engine management.



Continental O-100, you say? Unique engine, don't you think? Did you have it especially installed? Is that possibly a 2 cylinder O-200? Sort of like a half-VW?

1, increase in induction air temp.
2, decreasing power output.
3, increasing fuel/air ratio.
4, increase in cylinder temps/pressures for same output (no heat)
5, possible increase for detonation at high power outputs.


Of the five reasons cited there for causing detonation, only one applies, two are incorrect, and one is a restatement that carb heat causes detonation.

Decreasing power output has nothing to do with causing detonation. Increasing the fuel-air ration, or in other words, enrichening the mixture by applying carburetor heat, does not cause detonation. Enrichening the mixture, unless you were already operating lean of peak, moves the engine farther from the possibility of detonation. By applying carburetor heat and increasing the induction air temperature, one draws less dense air into the carburetor; as you know, with less dense air, you have a richer mixture...and you have just enrichened the mixture by applying carburetor heat.

Detonation occurs at higher power settings, typically close to peak mixture, or stochiometric. If you've leaned your engine manually to a point rich of peak, which is where most typically operate, then by applying carb heat you're moving farther from Stoch, or in other words farther from peak, and decreasing the possibility of detonation.

As for increasing cylinder temperatures and pressures...again, no. If you enrichen the mixture, you decrease the combustion temperatures for a given power setting. Your cylinder pressures aren't increasing, either, and even if they did, you're not going to create a dangerous situation with respect to cylinder pressure in a normally aspirated engine, using carburetor heat. It sounds good, but it's not true. Additionally, think about when you're actually using the carburetor heat...most often at low power settings. If you apply carb heat at a low power setting, do you think you need to be concerned about cylinder head temperatures or cylinder pressures?

As for robbing performance, yes...you're increasing the effective density altitude of the induction air...it's hotter, less dense, and enrichened to boot...so yes, the engine is producing less power. At a low power setting this is really irrelevant. At a high power setting it's more meaningful...which is why you don't normally apply carburetor heat at high power settings at a critical time, such as on takeoff or a go-around.

Now...during takeoff or on a go-around, if you're operating lean of peak and you do apply carburetor heat, there does exist the possibility of creating a situation conducive to engine damage, and that does include potential detonation issues.

[barrow]

They are not reasons, they are bullet points of what happens when carb heat is on!
This first 4 are EXACTLY what happens with carb heat, which may lend itself to the 5th point of "possible detonation"
To say any are incorrect, shows a basic lack of understanding on the recip powerplant.

[Islander2]

They are not reasons, they are bullet points of what happens when carb heat is on!
This first 4 are EXACTLY what happens with carb heat, which may lend itself to the 5th point of "possible detonation"
To say any are incorrect, shows a basic lack of understanding on the recip powerplant.Barrow, please forgive my basic lack of understanding on the recip powerplant, then, but you've got me confused. It would be helpful if you could explain the mechanism that causes your bullet points 1 to 3 to give rise to bullet point 4 unless, as SNS3Guppy observes, you're applying carb heat when operating with one or more cylinders lean of peak.

[Final 3 Greens]

Sort of like a half-VW?

No, that would be a V\

[Pace]

Barrow

Dont mix it with SNS3Guppy you will loose := crossed swords with him a couple of times but quite like the guy ;)

Pace

[SNS3Guppy]

To say any are incorrect, shows a basic lack of understanding on the recip powerplant.


REALLY??

Why don't you go ahead and provide a basic understanding of the "recip powerplant," then. This ought to be good.

First, explain why applying carburetor heat will "increase in cylinder temps/pressures for same output (no heat)."

Next, why don't you explain how enrichening the mixture, or "increasing the fuel air ratio" will cause detonation, "higher cylinder temps," or "higher pressures." That should really be interesting.

Perhaps you can explain how all this happens in a normally aspirated engine with a fixed pitch prop, to keep it simple. I'm looking forward to a good education.

[bigfoot01]

So flying a C150/152 or PA28, is pulling the carb heat every 10 minutes or so while doing a FREDA check wrong (in the UK?)

[Contacttower]

4, increase in cylinder temps/pressures for same output (no heat)

One hears this a lot.....but is it true?

When one applies carb heat the power goes down and the mixture is slightly richer; so do the cylinders really get hotter or do they actually get colder?

So flying a C150/152 or PA28, is pulling the carb heat every 10 minutes or so while doing a FREDA check wrong (in the UK?)I don't think so as such and certainly I will continue to operate like that. However I would like to know more about engine specific carb icing characteristics and perhaps debunk some myths along the way....

[SNS3Guppy]

One hears this a lot.....but is it true?

When one applies carb heat the power goes down and the mixture is slightly richer; so do the cylinders really get hotter or do they actually get colder?



Again...what happens when the mixture is enriched? Unless you are already operating lean of peak...where most pilots do NOT operate, when you increase the richness of the mixture, you're going to cause a decrease in the engine temperature.

Not an increase.

The only time that making the mixture richer will cause the engine to get warmer is if one is operating lean of peak. In that case, increasing the mixture towards rich moves the mixture closer to peak EGT and peak cylinder temperatures...and you're going to see an increase in CHT and EGT.

If you're operating with a lean mixture, which is lean of peak, and you lean the mixture, you'll see a temperature decrease. If you're operating rich of peak and lean the mixture, you'll see a temperature increase.

If you're rich of peak and apply carburetor heat, you're making the mixture richer...and cooling the engine.

I know it seems to make sense that increasing the temperature of the air entering the engine might make it run hotter, but that's not the case, and it's a minor thing with respect to the internal operating temperatures. Remember that there's already a fire inside making that engine warm, and that fire is what's determining the CHT and EGT...not the induction air temp. Changing the temperature of the fire inside the engine, the combustion temperature, by changing the mixture ratio, has a far greater effect, because after all, that's what's making the engine hot in the first place. Cool the mixture by moving it farther to the rich side of peak (enrichening a mixture that's already operating rich of peak), and you cool the engine...which is what occurs when you apply carburetor heat.

[barrow]

REALLY??

Why don't you go ahead and provide a basic understanding of the "recip powerplant," then. This ought to be good.

First, explain why applying carburetor heat will "increase in cylinder temps/pressures for same output (no heat)."

Next, why don't you explain how enrichening the mixture, or "increasing the fuel air ratio" will cause detonation, "higher cylinder temps," or "higher pressures." That should really be interesting.

Perhaps you can explain how all this happens in a normally aspirated engine with a fixed pitch prop, to keep it simple. I'm looking forward to a good education.

Can you answer a few simple questions for me?

What happens to EGT when only one plug fires?
What happens to CHT when only one plug fires?
Does Engine speed have an effect on timing?
Does the mixture have an effect on timing?
Does the angle of the valve placement have an effect on CHT + EGT?

[SNS3Guppy]

We get it. You're an automotive genius.

You can't adjust the valve "angle of placement" from the cockpit. Neither can you turn on or off one plug, nor does the firing of one or two plugs have anything to do with adjusting the mixture. Engine timing and speed are equally irrelevant to the use of carburetor heat, or mixture setting in the cockpit. Timing of magnetos is fixed in relation to the engine on reciprocating aircraft engines, and the use of the carburetor heat and the mixture controls is likewise unrelated to timing.

You made some very incorrect assertions...so instead of attempting to introduce even more and tap dance around, why don't you back up about what you said in the first place, instead of defending it.

Show us how application of carburetor heat increases "cylinder temperatures and pressures." Can you do this? You were just very adamant that this is a true statement and that you were not incorrect. Show us how.

[barrow]

SNS3Guppy, could you please answer the questions, one by one?

[BackPacker]

Barrow, I don't know what you want to achieve here, but if you want to discuss optimum valve angles vs. CHT & EGT, you might be better off in the tech forum.

[jxk]

Can you answer a few simple questions for me?

What happens to EGT when only one plug fires?
That would depend on whether the EGT is fitted to all exhaust pipes; it's usually only one at the rear of block. The probe is mounted a few inches from the exhaust outlet. And don't forget that there are two plugs per cylinder so there would only be a minuscule amount of change to performance and temperature of any particular cylinder.
What happens to CHT when cylinder only one plug fires?
The CHT is a thermocouple usually attached to the spark-plug copper washer and I doubt whether losing one plug would make any difference; again it's usually only fitted to one cylinder. I believe cylinders would be cracking on a regular basis if there was a real difference between different parts of the cylinder head temperature. And the whole purpose of the fins on the cylinders is for cooling and even dissipation of the temperature.
Does Engine speed have an effect on timing?
No, not if using magnetos, although with electronic ignition it would be possible. However, there are not many aircraft using this as mags are pretty reliable and don't require a battery. Most aircraft engines only rev at a max of 2700 rpm so there is little need for messing about with the timing. If you are considering diesel engines like the Thielert then they rev higher and have a reduction gear to keep propellor speed from going supersonic and they do have electronic ignition with separate emergency battery.
Does the mixture have an effect on timing?
None at all.
Does the angle of the valve placement have an effect on CHT + EGT?
Not sure which valve you mean. If you mean the butterfly valve in the carb/fuel injection system then 'yes' because you are changing the amount of fuel going into the engine.

I'm only considering the general case here but of course there are many exceptions.

[dbee]

As an FI, I would expect to see carb heat selected to full ON before closing the throttle. Never takeoff with carb heat on (except on the RAF's Chipmunks, when it was wired ON). IT is better to land with it ON than to end up without response on a go-around (you select COLD as you advance the throttle). :ok::ok:dbee

[BackPacker]

If you are considering diesel engines like the Thielert then they rev higher and have a reduction gear to keep propellor speed from going supersonic and they do have electronic ignition with separate emergency battery.
I'm only considering the general case here but of course there are many exceptions.

Diesels don't have electronic ignition but electronically-controlled injection. Is that one of the exceptions you mean?:ok:

[SNS3Guppy]

SNS3Guppy, could you please answer the questions, one by one?


No, as you've made ridiculous assumptions and defended your errors, and refuse to explain yourself...and can only ask more and more irrelevant questions...I think your credibility in the matter may safely be dismissed. Thus ends any further conversation with you on the subject.

[barrow]

SNS3Guppy, take your ball and go home.

[Islander2]

Can you answer a few simple questions for me?

What happens to EGT when only one plug fires?
What happens to CHT when only one plug fires?
Does Engine speed have an effect on timing?
Does the mixture have an effect on timing?
Does the angle of the valve placement have an effect on CHT + EGT?Okay barrow, notwithstanding that by disagreeing with you I have apparently demonstrated a "basic lack of understanding on the recip powerplant", I'll play!

The answers are:
a) Increases
b) Slightly decreases
c) If you mean the crank angle at which the plug sparks, not materially across the range of speeds we operate our engines
d) If you mean the crank angle at which the plug sparks, no
e) Yes

Having got that out of the way, any chance you can answer the one question we've been asking you? If you're operating a cylinder ROP (which is quite likely if you haven't yet selected carb heat on, since carb heat and a reduced throttle setting are both generally required to stand much chance of getting a typical carburetted Lycoming or Continental engine to run LOP;)), what is it that leads you to believe the cylinder temperatures and pressures will increase when carb heat is applied, given that power output will reduce significantly (maybe by 15%)?

[SNS3Guppy]

since carb heat and a reduced throttle setting are both generally required to stand much chance of getting a typical carburetted Lycoming or Continental engine to run LOP



Hmmm...no; carburetor heat doesn't contribute to moving a rich mixture toward a lean of peak setting, and reducing throttle doesn't either.

Any recip engine can easily be run lean of peak...simply by adjusting the mixture to a lean of peak condition. An engine operated lean of peak runs cooler than an engine run at peak...just as an engine run rich of peak runs cooler.

Lean of peak operation is generally recommended by light airplane engine manufacturers, not because it's harmful to the engine if done properly but because pilots don't tend to do it properly...and there's more potential for pilot error. It's actually a much more efficient way to run an engine, when properly done, under the proper circumstances.


---for the benefit of Barrow, edited to say the above should read "Rich of peak operation is..." A typo.

[barrow]

SNS3Guppy
Hmmm...no; carburetor heat doesn't contribute to moving a rich mixture toward a lean of peak setting, and reducing throttle doesn't either.

Any recip engine can easily be run lean of peak...simply by adjusting the mixture to a lean of peak condition. An engine operated lean of peak runs cooler than an engine run at peak...just as an engine run rich of peak runs cooler.

Lean of peak operation is generally recommended by light airplane engine manufacturers, not because it's harmful to the engine if done properly but because pilots don't tend to do it properly...and there's more potential for pilot error. It's actually a much more efficient way to run an engine, when properly done, under the proper circumstances.Name one POH for a carb single or multi that says this!

[SNS3Guppy]

That should have said "rich of peak operation is..."

A typo.

But then lean of peak operation has been taught since long before you began flying. It's seldom taught today...probably something you wouldn't know about or understand.

You're still unable to provide an answer, then. Like I said...no credibility for you.

[Contacttower]

barrow would you concede that one some types at least the EGT goes down on application of carb heat?

[IO540]

In the Lyco/Conti context, I don't see how carb heat could lead to detonation, unless the following are also true

1) the engine is running at high power - over 80%

2) the CHT is very high - around 500F which is right at the top end

3) the mixture was leaned, enough to lead to 2) above

4) the airflow was poor, also leading to 2) above

and then the carb heat leans the mixture just enough to bring it on.

But not otherwise. Most engines are set up so that during climb (all 3 fully forward) the operating point is about 150F ROP and it would take a lot more than a bit of carb heat to bring on detonation in that condition.

Incidentally, LOP is negligibly more efficient than peak EGT. I have done some very careful tests and on the IO-540 I find that, relative to peak EGT, 75F ROP is about 10% worse (on MPG), while 25F LOP (the theoretical most efficient point for any petrol engine) is so close to the peak EGT efficiency that any gain is below the measurement resolution of about 1%. I plan to do additional tests using a different method but I am not expecting to see a better gain than 1% by using LOP.

The crucial thing in this kind of measurement is to keep the IAS and RPM constant. This ensures identical conditions of airframe drag, thrust, and prop efficiency. If you don't do that, it is dead easy to find LOP being a lot more efficient than peak-EGT because the power output is lower, the IAS is lower, and of course the plane is more efficient when flying slower - the most efficient IAS for any airframe is equal to Vbg.

[barrow]

That should have said "rich of peak operation is..."

A typo.

But then lean of peak operation has been taught since long before you began flying. It's seldom taught today...probably something you wouldn't know about or understand.

You're still unable to provide an answer, then. Like I said...no credibility for you.Well, Lindbergh flew lean of peak all the time, so it was taught before I was born!
I'm well aware of how to operate LOP in fuel injected birds with gami's, and know it's
impractical with carburetors unless you use heat.
Both LYC and TCM say to follow the POH for how to operate their engines, every POH for every carbed engine I've flown says:

Full power with heat MAY cause detonation.

A pilot is landing, the usual ISA SL conditions in his warrior.
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder,
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.

[Islander2]

Oh dear, it's difficult to know where to start here! Now let's see, SNS3Guppy said:

Hmmm...no; carburetor heat doesn't contribute to moving a rich mixture toward a lean of peak setting, and reducing throttle doesn't either.Absolutely correct ... and absolutely irrelevant! I am left with the strong impression that you've never, ever tried to operate a carburetted Lycoming or Continental LOP! They won't, because the fuel/air distribution on these engines is so appallingly variable that, in practice, each cylinder is a different engine. By the time you've leaned sufficiently far to get the second or third 'engine' LOP, the first 'engine' has experienced a lean cut - result is such an interesting amount of vibration that you most definitely aren't going to fly with it.

Those people that have doggedly pursued LOP operation of carburetted engines have established two interesting facts that can make it possible in some engine installations: a) heating the air through use of carb heat improves fuel atomisation, helping to even out the mixture distribution; and b) 'cracking' the throttle a little closed from the wide-open position creates air turbulence around the butterfly valve that also improves the mixing of the fuel and air.

There you go, SNS3Guppy, seems you don't know it all, after all. Get out there and talk to a few pilots that actually fly LOP in carburetted engines!

Any recip engine can easily be run lean of peak...simply by adjusting the mixture to a lean of peak condition.SNS3Guppy, that's total bo**ocks, it makes you come across as a troll whose only knowledge has come from textbooks! I presume that's not the case, so please go out and fly a carburetted Lycoming or Continental that's fitted with a multi-cylinder engine analyser and try to get all cylinders LOP without using the above techniques. Chances are you won't.

Lean of peak operation is generally recommended by light airplane engine manufacturersAnd that, SNS3Guppy makes you look like a troll that's not even very good with the textbooks! It's simply so far from the truth to be laughable. With the exception of a quite small number of specific engines, the manufacturers (Lycoming in particular, because their fuel/air distribution tends to be worse) expressly recommend against LOP operation. Try taking a look at Lycoming's 'enlightening' document SSP700, for example.

And barrow, this is terrific stuff:

A pilot is landing, the usual ISA SL conditions in his warrior.
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise ....... Eloquently put and absolutely true - but unfortunately irrelevant because every bit of it is just as true when advancing the throttle without carb heat applied! The end difference, however, is that with carb heat applied you are generating around 15% less power - with all that that means for lower temperatures and pressures. So your conclusion:
....... which leads to detonation, because of carb heat at full power.simply doesn't follow!

[SNS3Guppy]

I'm well aware of how to operate LOP in fuel injected birds with gami's, and know it's
impractical with carburetors unless you use heat.


Another winning, nonsensical statement. How do you suppose Lindberg and thousands upon thousands of others did it...all without the benefit of magical GAMI injectors?

How does the use of carburetor heat make operating lean of peak practical? Have you thought that through, at all? You understand that when operating lean of peak, operation of carb heat causes the engine to run hotter and closer to peak, right?

You state that lean of peak operations are practical without carburetor heat. This is absolutely nonsensical.

a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!


The "extra gas" isn't "cooling the head." The combustion temperature is lower with a richer mixture, and a lower combustion temperature doesn't heat the cylinder as much. That's all.

Some carburetors use enrichment valves or economizer valves, but not all.

You understand that in a normally aspirated engine, the cylinder acts as a vacum pump, do you not? When the throttle is closed, it works very much like putting your hand over the end of the hose on a vacum cleaner; manifold pressure drops because the engine is producing suction, just like the pressure drops in the vacum hose. Opening the throttle is nothing more than taking away the obstruction in the line; it's allowing ambient air pressure to return once again...hardly a dramatic rise, hardly a mind bending change in the manifold or cylinder pressure, and hardly a danger...carb heat or no.

The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder,
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.


You googled that, did you?

Not unless you're already operating very close to peak at high power settings and low altitudes in normally aspirated power plants...on a cold day.

Particularly in the case of engines equipped with an economizer valve (enrichment feature), the use of carburetor heat moves the engine operating environment FARTHER from a detonation regime, and does NOT increase CHT or EGT. Further, there is no dramatic pressure or temperature rise, particularly in a normally aspirated powerplant, owing to a fixed timing, or to an enrichened mixture.

Detonation occurs as uncontrolled, explosive burning of the fuel air mixture in the combustion chamber. If the engine is properly timed (even in a fixed magneto installation), detonation may occur only in a narrow mixture range close to stochiometric, or peak...and doesn't occur during carb heat application in normal operation.

Now from your comments, it might occur when you're operating the engine...but that sounds more the result of someone who has no earthly idea how to operate their engine, than the result of error on the part of the average pilot.

If the engine is being operated at any significant density altitude, it's difficult, if not nearly impossible, to cause detonation damage in a normally aspirated light engine powerplant...which is part of the reason that manufacturers recommend not leaning until several thousand feet...it's nothing more than a way of idiot proofing the engine. From your description, something they haven't yet accomplished, apparently.

[Contacttower]

Lean of peak operation is generally recommended by light airplane engine manufacturers

And that, SNS3Guppy makes you look like a troll that's not even very good with the textbooks! It's simply so far from the truth to be laughable. With the exception of a quite small number of specific engines, the manufacturers (Lycoming in particular, because their fuel/air distribution tends to be worse) expressly recommend against LOP operation. Try taking a look at Lycoming's 'enlightening' document SSP700, for example.

To be fair to Guppy if you look back to an earlier post of his he makes it clear that he didn't mean to write that and in fact meant the opposite....I had actually read it as he thought he'd written it anyway.

[SNS3Guppy]

SNS3Guppy, that's total bo**ocks, it makes you come across as a troll whose only knowledge has come from textbooks! I presume that's not the case, so please go out and fly a carburetted Lycoming or Continental that's fitted with a multi-cylinder engine analyser and try to get all cylinders LOP without using the above techniques. Chances are you won't.



Reading comprehension was never your long suit, clearly enough.

From my perspective as a pilot, mechanic, and instructor with ample experience operating, maintaining, servicing, building, rebuilding, and caring for powerplants...you might need to get out a little more and fly.

I've operated recip engines lean of peak from small displacement engines to large radials...each with carburetors, sometimes fully outfitted with individual cylinder instrumentation, sometimes not. And yes, it really can be done. Just not by you, it would seem. By thousands of others...yes. But not by you. And that's okay...because nobody is forcing you to operate that way, and you don't need to do so.

That's really not relevant to the discussion, however, nor to the turn taken in the thread by Mr. or Mrs barrow....however misguided his or her insight may be.

[Islander2]

To be fair to Guppy if you look back to an earlier post of his he makes it clear that he didn't mean to write that and in fact meant the oppositeOh please, you're being a touch gullible, aren't you?

This is what SNS3 wrote:

Lean of peak operation is generally recommended by light airplane engine manufacturers, not because it's harmful to the engine if done properly but because pilots don't tend to do it properly...and there's more potential for pilot error. It's actually a much more efficient way to run an engine, when properly done, under the proper circumstances.

---for the benefit of Barrow, edited to say the above should read "Rich of peak operation is..." A typo.If you insert the word 'not' before 'generally' in the first line, the paragraph is correct and makes perfect sense.

If instead you substitute 'ROP operation' for 'LOP operation' in the first line, as SNS3 would now have us believe he meant, can you tell me how, in respect of ROP operation, that pilots don't tend to do it properly and why there's more potential for pilot error (than with LOP) and on what basis you believe it's actually a much more efficient way (than LOP) to run an engine, when properly done, under the proper circumstances?

Ho, ho.

[Contacttower]

Oh please, you're being a touch gullible, aren't you?

Perhaps.....:oh:

But we seem to have concluded that it is actually very difficult indeed to damage the engine I originally started the thread about (the ever ubiquitous O-320) by using the carb heat.

[Islander2]

I've operated recip engines lean of peak from small displacement engines to large radials...each with carburetors, sometimes fully outfitted with individual cylinder instrumentation, sometimes not. And yes, it really can be done. Just not by you, it would seem. By thousands of others...yes. But not by you. And that's okay...because nobody is forcing you to operate that way, and you don't need to do so.

You misunderstand me, SNS3Guppy. I have been quite happily operating an IO-550 lean of peak for many years, but that of course is fuel injected.

My knowledge on operating the carburetted Lycoming and Continental flat fours and sixes LOP stems largely from the feedback from a very large number of graduates of the Advanced Pilot Seminars on Engine Management that are run by Walter Atkinson, George Braly and John Deakin. If you've no objection, I'll take that very large sample size any day over your sample of one!

[Islander2]

But we seem to have concluded that it is actually very difficult indeed to damage the engine I originally started the thread about (the ever ubiquitous O-320) by using the carb heat.
Well, that bit I do agree with ;)

[Mark1234]

A pilot is landing, the usual ISA SL conditions in his warrior.
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder,
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.

Hmm.. One small error there that shoots the whole thing down in flames: the most rapid burn occurs at the stoichometric(sp?) ratio - something like 14.7:1, near enough to peak. Don't believe me, look it up. Over lean, OR over rich mixtures burn slower; *that* is why you run super rich at wide open throttle, have WOT enrichment etc.

The 'extra gas' 'cools' the head not directly, but by altering the combustion characteristics.

carb heat will enrich the mixture further, due to reduced density of air, moving you further away from detonation margins. Don't be fooled into thinking the hot air makes things hotter - the increased heat energy of the incomming air is insignificant compared to the combustion temperatures.

Also remember that while the spark is 'timed' at (let's say) 25 BTDC, the period it takes from that point to get the combustion going is measured in time (i.e. hundredths of seconds), not degrees - if the prop is spinning rapidly, the timing is effectively later - the piston will have travelled further in the interval of time required for the spark to get going. That's why running CSU engines wide open throttle with low RPM *can* be a bad thing.

[Pilot DAR]

I support what Mark1234 has written, though I'm not knocking the other posts, Mark's is just the easiest to understand correctly.

I have purposefully dentonated a carburetted Continental 520 engine (we took the injection system off), and a 470 as well. For all of the factors I had to very carefully control to create detonation, carb heat really was not one of them. I'm sure it has an effect, but a minor one, compared to the other critical factors. Carb heat hot = richer mixture = greater detonation margin.

The factor which I used to most precisely cause and termnate detonation was RPM, realtive to manifold pressure (with temps carefully controlled as well) It was also necessary to run 80 octane gasoline in the 8.5:1 TCM 520 engine to get detonation. We did consistently create detonation, and measured it, and then ran our test, and proved that we had the required margins while operating in accordance with our instructions. This mod was STC approved based upon this, and other testing I did.

It is important to know, that when detonation begins, the engine will take on a life of it's own, and may continue to detonate after the adverse conditions which caused the detonation, are removed. The engine will get hotter, and continue to detonate, even after the extreme damands for power are removed. We did not cause damage to the engine doing this testing (teardown inspection) but the effects of detonation were visible on the pistons. None of the detonation we created and measured, could be detected in the cabin in any way other than the special equipment I had installed for the test. If you are detonating your engine, you will not know this until you have really damaged it - stay within the operating limits! The damage will be a hole in a piston, and all of that metal spread through your engine.

Also, the mixture distribution of carburetted engines can be affected by the RPM. Where there may be a few rich cylinders at full power, and leaning it will not affect thise relationship, a power change may effect the distribution. With an all cylinder EGT, it should be possible to find a power setting which provides more equal mixture distribution. On the big Continental engines, there is a "balance tube" which connects the left and right induction manifolds. There are three different sizes of balance tubes, and the correct size is an important factor in mixture distribution.

I do not operate engines lean of peak. We did not operated lean of peak when we flew pistons transatlantic. I would rather waste a little fuel, and have to flight plan more carefully, then to cause engine stress for a very minor benefit, and actualy have an engine problem (particularly there). Fuel is cheaper than a broken engine!

Pilot DAR

[Islander2]

That's an interesting post, Pilot DAR, but I would take you to task on two important points.

Carb heat hot = richer mixture = greater detonation margin.That's too simplistic and takes the squabble with barrow way too far. Of the thermodynamic variables that come into play when carb heat is selected, mixture strength is but one. What actually happens is:

1. an increase in temperature of the incoming charge increases CHTs;
2. a change to the combustion event from enrichening the mixture (due to the reduced air density) reduces CHTs; and
3. a reduction in power (due both to the reduced mass flow and to the over-richness) reduces CHTs.

It's a complex interaction but allow me, too, to fall foul of massive oversimplification by hanging some numbers on these factors following selection of carb heat to full when the engine is developing, say, 85% rated BHP:

1. temperature of incoming charge air will increase by circa 100degF ... because of the complex interaction with other variables, I don't believe this will be fully reflected in increased CHTs, but a significant increase will occur;

2. mixture will enrichen by circa 15% which, assuming the cylinders were already operating rich of peak CHT, will reduce CHTs by circa 15degF;

3. power output will reduce by circa 13% (according to Lycoming), which will reduce CHT's by circa 15 degF

Simplistic? Of course, but nonetheless I would expect CHTs to rise slightly following the application of carb heat ... although certainly not by a sufficient amount to cause detonation problems on a 4-cylinder, naturally-aspirated Lycoming or Continental. It is certainly the case that I see higher CHTs on take-off in Southern Europe compared with the UK, where differences in charge temperature could be in the order of 50degF. I think the admonition not to use carb heat at high power due to detonation risk is a hangover from the days of the big radials, where the combination of charge-temperature rise from both the supercharger and carb heat very definitely could take you into detonation territory.

But hey, there's no need to speculate on this. Would someone out there with a carburetted Lycoming or Continental, in an aeroplane fitted with a multi-cylinder engine analyser, please go out, experiment and report back!

And the other thing I must take you to task over is:

I do not operate engines lean of peak. We did not operated lean of peak when we flew pistons transatlantic.It's not clear if you are talking there about specific transatlantic operations that you've been involved in, or are referring to the piston airliner heydays of DC-7s, Constellations, etc. If the latter, then you are simply wrong! The reason we know so much about lean of peak operation and, with certain safeguards, can undertake it quite comfortably (in the process, reducing fuel consumption and increasing cylinder life) is because literally millions of hours were accumulated operating Wright Cyclones, Pratt & Whitney Wasps, etc lean of peak. It's your choice not to operate LOP, but quite wrong to suggest that in, former times, it wasn't the norm.

[Pilot DAR]

Hi Islander 2,

Sorry for the confusion, I should have clarified that the piston aircraft to which I refer were all flat engined GA aircraft. I do agree that the round engines of old were regularly operated lean of peak. It is my opinion that they are outside the scope of this discussion, as they are different enough that some operating characteristics may not be comparible.

Having reviewed your numbers, I cannot present direct evidence to the contrary, though I hold the opinion that there are too many other variables at play to allow your rationale to be considered valid in isolation. Perhaps in a test cell they would be closer to reality. I can say with confidence that in practical application, the application of carb heat to an engine which is otherwise running within the recommended operating range, will not increase the chances of detonation in any meaningful way.

If I have time, I'll fly the Teal today, (O-360) and watch it's scanner. It does not lean with great precision, but I should be able to form an opinion.

Pilot DAR

[Ringway Flyer]

This is all very interesting, but isn't the reason for setting the carb heat to cold at the decision height to give full power in the event of a go-around? Nothing to do with damaging the engine..... (apart from possibly ingesting undesirable stuff when on the ground).
:rolleyes:

[SNS3Guppy]

I do not operate engines lean of peak. We did not operated lean of peak when we flew pistons transatlantic. I would rather waste a little fuel, and have to flight plan more carefully, then to cause engine stress for a very minor benefit, and actualy have an engine problem (particularly there). Fuel is cheaper than a broken engine!


One won't break the engine if one operates it properly.

Studies in the past determined that the ideal range for detonation occurs within about 11% of stoichiometric, or peak mixture...that's 11% rich, not lean. This is only at high power settings. It's nearly impossible to cause detonation uner normal operating cruise conditions...and both Continental and Lycoming have pernitted and even recommended peak operation at cruise power settings between 65% and 75% power for a long time...again, a big part of the reason that you'll find recommendations against leaning below three or four thousand feet...above those altitudes in a normally aspirated engine you aren't going to achieve adequate power to cause detonation. It's simply idiot-proofing the operating practices against ham-fisted pilots who don't know how to operate their engines.

I think the admonition not to use carb heat at high power due to detonation risk is a hangover from the days of the big radials, where the combination of charge-temperature rise from both the supercharger and carb heat very definitely could take you into detonation territory.


In large radial operation, we adjusted carburetor heat by the carb air temperature gauge; it's absolutely true that applying too much carb heat at high temperatures in a large radial can cause all kinds of woes, but it's not done that way. The carb is adjusted specific to a narrow temperature range individually for each engine, by watching the carb air temperature gauge. Then again, it's well to remember that part of the normal induction of most large radials is a supercharger that comprises the back part of the engine case...even if the engine isn't turbocharged. In that respect the radial is different, though that factor is inconsequential when considering operation at the cylinder and at the carburetor with respect to carb heat at lower power settings...settings which approximate what you're seeing in say, an 0-320 or 0-520. Simply put, when running a radial below barometric (below about 29" Hg Mp at sea level, for example), it's operated in the same manner as the engine in your Skyhawk or Warrior or Cherokee.

The carburetion on a large radial is different; it's a pressure carburetor rather than a float carburetor, but that doesn't make a difference in the way the carb heat is used, either...so yes, the carb heat techniques on a radial engine, being a piston engine, apply to a small flat horizontally opposed engine, too. Mixture techniques, too. A big difference is that the big pressure strom carburetors utilized autorich and autolean settings...which were routinely overridden for manual leaning in flight. I often set mixture in cruise at night by the color of the flame out the exhaust first, and then final adjustments by instrumentation.

While you can't do that on a light single or twin (see exhaust flame), you can safely operate at peak at reduced power, and no, carburetor heat isn't going to tear the engine apart. The ideal would be a carb air temp gauge, but for cost and simplicity most light airplanes are equipped with a very crude carb air box (which typically leaks on most airplanes after a while anyway). It's either full on or full off...and if you need carb heat during takeoff, during climb, during cruise, during descent, then use it.

You can think of it this way; while the greatest possibility of detonation occurs at higher power settings, and one may reasonably guess that the greatest harm might occur with at high power settings, think about why you're applying the carb heat in the first place. If you have carb ice, then you need carb heat, period. When is the greatest potential for icing? When the greatest temperature drop occurs...which occurs at high power settings when the greatest airflow takes place through the venturi. Does carb ice form at high power settings? You bet, and carb heat is needed to get rid of it.

Carb ice is common during ground operations, too, but forms a little differently, for the same reason that carb fuction at idle is different than with an open throttle. Idle fuel is obtained from a different fuel jet, with a different, independent fixed-mixture setting, by utilizing calibrated airflow leakage past the throttle plate in the carburetor. Rather than using high airflow to create a pressure drop through a venturi to draw fuel into the induction airstream, the low manifold pressure and a calibrated leak past the throttle plate combine to draw fuel in a different place than with the open throttle...and ice forms in a different place and in a different manner. Carburetor icing and heat useage therefore, is a different subject when considering throttle open and throttle closed operation.

It becomes a little more complex, however. I see far too many instructors that teach their students to reduce power to idle abeam the numbers and make a prolonged idle descent. This is hard on engines. Often the drill is to reduce to idle power by closing the throttle, apply carburetor heat, and leave it on through the descent. This is poor airmanship and poor practice, and really should be reserved only for training for engine failures. It's a time when backlash issues come into play in the engine (timing can change as much as 20 degrees or more over time with a worn woodruff key in a mag driveshaft, for example, strictly due to backlash), piston ring flutter occurs, and though few realize it, idle mixture changes substantially (resulting in a leaner idle mixture than on the ground at idle). Prolonged operation during a descent at idle is a bad idea and poor practice.

Likewise, operation with the carb heat simply left on, mindlessly, while doing prolonged idle descents is also bad idea. Use of carb heat of it's own accord isn't a bad idea, but for heaven's sake, know why it's being used and what it's doing. Simply applying the heat because the instructor said to apply it there is never wise...especially since the instructor is probably only doing what he does because he saw someone else do it...the heritage of inexperience. It's that same heritage that teaches us that peak or lean of peak operations are dangerous, and a host of other myths that pervade the hangar talk that goes on today. (Along with destructive practices like pulling the propeller through to "limber up the oil" as part of a preflight, etc).

This is all very interesting, but isn't the reason for setting the carb heat to cold at the decision height to give full power in the event of a go-around? Nothing to do with damaging the engine..... (apart from possibly ingesting undesirable stuff when on the ground).


Well, yes, and no. You're right that the chief concern is making sure you have adequate power to go around. I cringe when I see idle descents made to a go-around. Going from a nice, cool or cold engine to full power on the go-around is not good for the engine for a host of reasons. However, when you need to go around, you want that power available, and carb heat robs power. If you're operating an airplane close to sea level and you're operating very lean, and at high power settings, you can put yourself into a regime where it can contribute to detonation, but it's difficult to do...nearly impossible with most properly maintained light airplane engines with leaded fuels. Further, contrary to some of the incorrect information that's been posted in the thread, you have NOTHING to fear from "increased cylinder pressures or temperatures" resulting from carb heat...doesn't occur, and presents no hazard.

Use of carburetor heat is there for two reasons; to help remove the ice that may have formed and to prevent it's formation by operating the carburetor in the ideal temperature range. That range isn't very big, and what manufacturers do when giving a simple butterfly carb air box with no carb air temperature gauge, is take a shotgun approach. Rather than provide you with the means to properly set carb temp, they shotgun it with extra heat in a form of overkill...apply enough to melt everything.

During a power transition, however, you pass through the ideal ranges and the less than ideal ranges, as airflow through the carb is changing, and the temperature in the cowl area (from whence the heated carb air comes) is changing...and you not only pass through the ideal carb air temp range for avoiding ice...but quite possibly both sides of that range where the ideal temperature and conditions for FORMING ice occur...you can cause ice to form.

I've seen this occur during a go-around during the right humid conditions following a rain, while doing flight training with student's, before. Application of power to go around produced a smooth transition to the climb, followed shortly thereafter by an engine failure with a very rapid buildup of carb ice. Reapplication of heat as we set up for the forced landing on the remaining runway, resulted in a restoration of power. This isn't common in my experience, but I've certainly experienced it. Use carburetor heat when and where you need it, but don't use it indiscriminately.

Having said all this, if you have instructions for your aircraft specifying a carb heat-on descent for landing, then do that, but take the time to get to know your system and the why's of what you're doing. Tools such as mixture and carb heat are NOT absolutes; just like the ideal application of carb heat is partial heat (which can only be done in conjunction with a carburetor air temperature gauge), the same applies to mixture. It's there to be adjusted, to be used as needed, and to be used with care and judgement. It's there to give you options...not simply there to be thrown on or off with abandon, or by rote.

[Will88]

When is the greatest potential for icing? When the greatest temperature drop occurs...which occurs at high power settings when the greatest airflow takes place through the venturi.

The greatest potential for carb ice is at low power settings, not high.

The venturi effect is more pronounced when the throttle butterfly is restricting the flow of the air, as opposed to when the throttle is wide open.

[SNS3Guppy]

Not true; that's a common misconception. If you'll re-read, I addressed why.

Idle plate icing takes place in a different location, and for a different reason than icing through and past the venturi. It's affects the airflow differently, affects the fuel flow differently, and is not the same. Completely different icing scenarios.

A common misconception is that the high airflow through the carburetor venturi or throat will prevent icing at high power settings. This is when the greatest temperature drop and the greatest pressure drop occurs...the faster the airflow occurs through the venturi, the greater the pressure drop. Some will argue that it's the volume that makes the difference, but this isn't the case. Icing at high power settings is very possible.

Icing at idle is an entirely different mechanism, when the throttle plate is closed, and has been previously addressed.

[Pilot DAR]

I took the Teal for a spin today. It is equipped with an O-360 Lycoming. It has a bar graph engine monitor. When operating about 50 degrees rich of peak lean at 2500' feet, the application of carb heat caused an indicated EGT rise of one to three degrees, and did not change the relationship from hot to cool cylinder. It caused a CHT rise of 2 degrees once stabilized. There was about a 1 inch MP decrease, so it did result in a power reduction.

At peak lean, the results seemed to be about the same - nothing dramatic.

I tried to operate lean of peak, but the engine just would not run smoothly. It stumbled badly, so I stayed over the water to be safe. No matter how precisely I leaned past peak, it would stumble right away, and the EGT would drop right off. Richen up, and it ran like a top. The temperature range of leaning seemed to provide about 150 degree EGT rise from rich to peak, All of the bars moved about the same about, with the hottest and coolest always remaining the same.

All of this was done at 2000 feet, 16C, with no vizible moisture, and an engine with 33 hours since overhaul.

I therefore suggest that on this engine, carb heat has little to do with EGT or CHT, affects power as expected, and is unlikely to be a cause for detonation.

I hope that this informal test helps a little.

Pilot DAR

[eharding]

(Along with destructive practices like pulling the propeller through to "limber up the oil" as part of a preflight, etc).


Care to take this one off to a separate thread? - obviously, radials require this, but a broader debate might be of benefit.

[SNS3Guppy]

Actually no, radials don't require this...especially as they aren't even wet sump...but the point was it's a fallacy that's often bantied about in the hangar or pilot's lounge when talking about light airplane engines, and is representative of many myths regarding light airplane engine operation.

Radial engines are pulled through, incidentally, to check for hydraulic lock; some must be pulled through on the starter to protect the engine, as the starter will slip, whereas using the propeller and the leverage it provides may damage the engine. Other engines are pulled through by hand. You're right, however; a detailed discussion of that topic merits a different thread...but there's nothing to debate there, so it may be a very short thread.

[SkyHawk-N]

but there's nothing to debate there, so it may be a very short thread.

SNS3Guppy, I bet you $10 that it wouldn't! It will be just like a thread discussing carb heat. :E

[SNS3Guppy]

You may be right, but then I don't have the ten bucks...I fly for a living.

[cjboy]

You may be right, but then I don't have the ten bucks...I fly for a living

You must get plenty of time off judging by the length of your posts! :}

[Islander2]

It caused a CHT rise of 2 degrees once stabilizedPilot DAR, that's interesting data. It confirms a slight rise in CHT from use of carb heat, but even smaller than I had expected. No significant reduction in detonation margin ... at least in that engine.

I tried to operate lean of peak, but the engine just could not run smoothly ........ no matter how precisely I leaned past peak, it would stumble right away.Now come on, Pilot DAR, you just don't fly right! ;) Remember, from the gospel according to SNS3Guppy:

Any recip engine can easily be run leak of peak, simply by adjusting the mixture to a lean of peak condition.
If you can't this sorted, you'll get an outpouring from SNS3Guppy similar to the one I got:

I've operated recip engines lean of peak from small displacement engines to large radials...each with carburetors, sometimes fully outfitted with individual cylinder instrumentation, sometimes not. And yes, it really can be done. Just not by you, it would seem. By thousands of others...yes. But not by you. And that's okay...because nobody is forcing you to operate that way, and you don't need to do so.Or some other such 'knowledgeable' insight .....

:ugh:

[Contacttower]

Thanks Pilot DAR - very interesting as Islander2 says. Your description of leaning is also pretty much what I've experienced as well with both the O-320 and O-360.

[SNS3Guppy]

You must get plenty of time off judging by the length of your posts!


More time on than off, but there's time in the hotels between legs.

[Final 3 Greens]

More time on than off, but there's time in the hotels between legs.

I know that this guy is a giant, but he can fit hotels between his legs:ooh:

[beatnik]

Maybe they are Monopoly hotels. They'd be small enough ;)

[SNS3Guppy]

I think the thread has run it's course, then.