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How carb heat affects manifold pressure

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How carb heat affects manifold pressure

Old 3rd Nov 2020, 13:00
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How carb heat affects manifold pressure

Hello,
I'm a little bit confused of how applying carburetor heat affects manifold pressure in constant speed propeller aircraft (this particular matter concers C182 with O 470 engine).
Assume cruising with 2200 rpm, manifold stays within green range, lets say 19 inches.

Normally it should acts like this: aplying carb heat shouldnt change MP value (keeping constant pressure) either rpms. I wonder why sometimes after closing the heat, MP is rising above previous value even if no icing conditions occur (carb temp gauge shows 10 degrees or more, no moisture etc)?
Moreover why sometimes when applying carb heat, MP rises immediately, even 2, 3 inches? It should be correct for fixed pitch propeller but why is this happening while in governor range?
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Old 3rd Nov 2020, 16:01
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In general, when you apply carb heat you exchange the cold air from one source with the less dense warm air from another source, thus resulting in a reduction in power ( manifold pressure), This is most obvious with a fixed pitch propellor: i.e. a loss of RPM owing to the reduction of MP. The constant speed propellor will, of course, with a reduction of power then fine off maintaining the RPM. Carb heat enrichens the fuel/air ratio and so there is the possibility that for a brief period only on returning to cold air there is a temporary boost to the mixture (as with all instruments there can be a marked lag though), but

I suspect that you are running the engine over lean and, if so, adding carb heat will mean that you are returning the mixture to a more correct fuel/air ratio so the increase. To achieve best lean the process is often described as: first leaning off until the RPM drops and then to go back towards rich to find peak rpm.
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Old 3rd Nov 2020, 16:42
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Finding peak rpm can be complicated with a constant speed prop. Better check the POH for the correct leaning procedure

Edit: I just checked some sources and found this quote from John Deakin
But I’ve flown a couple of 182s that are simply hopeless, and I’m tempted just to leave the mixture full rich and forget it. Pity, the Skylane is a marvelous airplane, but that TCM O-470 engine has the worst mixture distribution of any powerplant on the planet.
I don't have any experience with that engine myself, but to elaborate on the previous post, you might see different effects based on where your mixture is, what the outside air is doing, whether you may have had a touch of icing perhaps... And if John's quote is correct you may see different effects based on the mixture distribution at that time. If three out of the six see a better mixture thanks to your carb heat application, that may lead to a higher MP but it could be the other way around as well.
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Old 3rd Nov 2020, 22:08
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I have personally never seen a 0 470 engine that did not lose MP when carb heat was applied. If MP did rise after application of carb heat then there was carb ice present. Of note carb heat can clear carb ice in only a few seconds and this engine can be quite susceptible to carb icing
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Old 4th Nov 2020, 10:20
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Originally Posted by hellkitty View Post
Hello,
I'm a little bit confused of how applying carburetor heat affects manifold pressure in constant speed propeller aircraft (this particular matter concers C182 with O 470 engine).
Assume cruising with 2200 rpm, manifold stays within green range, lets say 19 inches.

Normally it should acts like this: aplying carb heat shouldnt change MP value (keeping constant pressure) either rpms.
By switching to carb heat you are now bypassing the air filter so the assumption that MP shouldn't change is not generally valid. I have never flown a 182 so I am not saying what is normal response of MP in that particular installation.
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Old 4th Nov 2020, 12:29
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In general, when you apply carb heat you exchange the cold air from one source with the less dense warm air from another source, thus resulting in a reduction in power ( manifold pressure), This is most obvious with a fixed pitch propellor: i.e. a loss of RPM owing to the reduction of MP. The constant speed propellor will, of course, with a reduction of power then fine off maintaining the RPM.
This is the main theme applicable to the question. Bypassing the air filter, or the possible melting of carb ice are secondary to the basic question. With a constant speed prop (note the "constant speed" reference), the governor will govern the RPM to be constant when in the governing range of engine speed. So if you do anything which reduces power on the whole, the MP must reduce, as the RPM won't. The hot air produced by carb heat application reduces the intake air density, so the power must reduce a little, and the MP will reduce.

If you do the carb heat check at a lower RPM, when the prop is already on the fine pitch stops, then the RPM and MP will reduce a little together ('cause the prop can't move to be more fine to maintain the RPM). But a 470 has a lot of inertia, so watch carefully, and give it a moment, as the effect of carb heat is much less at low RPM, so you won't see it as much.

Remember that to optimize the effect (heat) of carb heat, lean the engine more once it is applied (because the application of carb heat enrichened the engine). Just make sure to enrichen the mixture again when the carb heat is selected cold again.
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Old 4th Nov 2020, 15:29
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Originally Posted by Pilot DAR View Post
This is the main theme applicable to the question
The question was actually "I wonder why sometimes after closing the heat, MP is rising above previous value even if no icing conditions occur". It is somewhat ambiguous because "closing" the carb heat implies switching it off. Nonetheless, the question seems to be concerned with the observed effect on MP as opposed to the well known effect of carb heat on power - which I assume the OP does actually understand.

Bypassing the air filter, or the possible melting of carb ice are secondary to the basic question.
Well, not if it answers the question. The elimination of the restriction that is the air filter is bound to increase the MP, it cannot possibly reduce it. On the other hand you are actually changing from one induction configuration to a completely different one so the net effect will be specific to the installation. For instance, the reduction of ram air pressure when switching to alternate air or the presuure drop through the alternate air passage versus the normal induction system are both factors. I am not saying I know what the net effect is for a 182 but it remains a fact that bypassing the air filter will tend to increase MP, and therefore it is a potential answer for the OP to consider.

With a constant speed prop (note the "constant speed" reference), the governor will govern the RPM to be constant when in the governing range of engine speed. So if you do anything which reduces power on the whole, the MP must reduce, as the RPM won't.
I certainly don't agree with that. Power is proportional to torque times RPM. With the CS prop, after application of carb heat per this thread, the BMEP would be reduced due to the lower density charge, whilst the RPM is maintained by reducing the blade pitch - the result being lower torque at same RPM. The reduction of blade pitch at constant RPM is a reduction of power. So, the reduced power resulting from appplication of carb heat has already been accounted for by a reduction in torque and it does not follow that "the MP must reduce, as the RPM won't'"
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Old 4th Nov 2020, 17:08
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been accounted for by a reduction in torque and it does not follow that "the MP must reduce,
In the case of a CS prop piston engine governing, the torque and MP indication are directly related. Yes, bigger piston engines do measure torque directly, but for the O-470, MP is your only indication of torque if the prop is governing RPM. Think about a manual turbo engine, again, the RPM will govern, so if you add too much power (open the throttle too much), you're over torquing the engine, and over boosting it. The over boost is indicated to you as MP exceeding the permitted limit [for the altitude]. A wastegate failure can get you there too.

Yes, if you had carb ice, and the application of cart heat melted it out, you'd have a higher MP after carb heat cold, compared to that when you selected it hot. The engine is again able to breath, and make more [full] power.

The carb heat induction air path delibertely does not go through the air filter (in case your filter ices over - 'happened to me once). Correct, there is no ram effect when running carb heat hot, but then at Cessna 182 speeds, there's not much ram effect carb heat cold, through the air filter either. If your air filter is truly clogged, then yes, carb heat hot would seem to result in an increase of MP for the same reasoning.

Remember that the "manifold pressure" is rather the induction vacuum below atmospheric pressure. So although it is reading a larger number, it's actually a lower amount of vacuum. It's a bit counter intuitive.
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Old 6th Nov 2020, 09:38
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Originally Posted by Pilot DAR View Post
In the case of a CS prop piston engine governing, the torque and MP indication are directly related.
They are not directly related. MP is proportional to torque if RPM and air density are held constant. But we are talking about applying carb heat which changes the inlet air density. Therefore the reduced torque resulting from this reduced air density does not show up on the MP gauge.

for the O-470, MP is your only indication of torque if the prop is governing RPM.
The MP is neither a direct indication of torque nor is it the only indirect indication of torque. It is proportional to torque if the RPM and air density are held constant. With a CS prop, a change in torque will show up on the ASI (note, I am definitely not implying that all changes in airspeed result from a change in torque). Eg you apply carb heat > BMEP reduces > CS unit fines off the blades > airspeeed reduces.

Measuring torque is not what I am talking about. The point I am making is it does not follow that application of carb heat results in a reduction of MP due to the reduction of power, which is what you wrote. The reduction in torque resulting from the reduction of inlet charge density does not show up on the MP gauge. But it will show up on the ASI. And I am not saying that cycling carb heat on and off does not change the MP. I am saying it does not change MP for the reason you think.

The carb heat induction air path delibertely does not go through the air filter (in case your filter ices over - 'happened to me once). Correct, there is no ram effect when running carb heat hot, but then at Cessna 182 speeds, there's not much ram effect carb heat cold, through the air filter either. If your air filter is truly clogged, then yes, carb heat hot would seem to result in an increase of MP for the same reasoning.
Right, so back to my original point: the pressure drop between upstream infinty and the bell mouth of the carb is inevitably going to be different for the normal versus alternate air. This will impact the MP. And because, for the reasons I spelled out above, the reduction in air density is not indicated on the MP gauge, the primary reason for any observed change in MP when cycling between normal and alternate air is........the change in air pressure delivered to the carb resulting from the difference in those two induction paths.
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Old 6th Nov 2020, 18:17
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the primary reason for any observed change in MP when cycling between normal and alternate air is........the change in air pressure delivered to the carb resulting from the difference in those two induction paths.
I see it differently:

The "pressure" being measured by the MP gauge is below atmospheric. So, yes, a change in induction air route could have a minor effect, or a major effect, if the air filter is blocked (which I have had with sudden freezing rain). Assuming that the air filter is not blocked, there is very little pressure drop across the filter (or it's really poorly designed). Similarly, there is no (well, shouldn't be) obstruction in the alternate air path, particularly as there is no filter at all. So, again, no pressure drop. However the heated carb heat hot air is much less dense. The MP gauge is not measuring that density, but it is measuring the engine performance running on the air at the actual density. Less dense air, less performance.

My most extreme example of this was a Cessna 185 I test flew with a normally aspirated, carburetted engine, running on Mogas. I climbed it to 20,800 feet, just to see how high it would go on Mogas - that high. While up there, full open throttle (carb heat cold) was around 13" MP, at 75MPH, with the stall warning horn screaming - less dense air.

The MP is neither a direct indication of torque nor is it the only indirect indication of torque.
I agree that MP is not an absolute means of measuring torque, but it's as close a GA plane needs. Note that the Cessna performance charts present RPM, MP, and %BHP in relation. In a given row, the RPM is defined, while the MP and %BHP vary in relation.
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Old 11th Nov 2020, 15:31
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Originally Posted by Pilot DAR View Post
I see it differently:

The "pressure" being measured by the MP gauge is below atmospheric. So, yes, a change in induction air route could have a minor effect, or a major effect, if the air filter is blocked (which I have had with sudden freezing rain). Assuming that the air filter is not blocked, there is very little pressure drop across the filter (or it's really poorly designed). Similarly, there is no (well, shouldn't be) obstruction in the alternate air path, particularly as there is no filter at all. So, again, no pressure drop. However the heated carb heat hot air is much less dense. The MP gauge is not measuring that density, but it is measuring the engine performance running on the air at the actual density. Less dense air, less performance.
Also:
With a constant speed prop (note the "constant speed" reference), the governor will govern the RPM to be constant when in the governing range of engine speed. So if you do anything which reduces power on the whole, the MP must reduce, as the RPM won't.
The MP is a function of the RPM, throttle position, and delivery pressure to the carb. In the scenario given, the RPM and throttle are held constant but delivery pressure to the carb will vary if the PD of the alternate versus the normal induction system is different. If the carb heat is supplied at the same pressure as normal air, the MP will not change. If the MP does change it is due to the different pressure drop of the alternate vs normal induction system. It is not because “the MP must reduce, as the RPM won't”. A reduction in density will cause a reduction in mass flow rate and power but unless the volumetric flow rate is changed, the MP remains the same. The volumetric flow rate will change if RPM, or throttle position, or pressure upstream of the carb change. RPM and throttle are assumed constant in the scenario of the OP, so that only leaves the upstream pressure to explain the change in MP..

So getting back to the OP:

Moreover why sometimes when applying carb heat, MP rises immediately, even 2, 3 inches? It should be correct for fixed pitch propeller but why is this happening while in governor range?
In theory, this excessive increase of MP suggests that selecting alternate air is bypassing a significant restriction in the normal induction system and returning the pressure delivered to the carb to near normal levels. However, it seems unlikely that being 2 - 3 inches down on MP would go unnoticed to begin with. In my experience, something like this happens every now and then and doesn't add up until finally some quite obscure fault is found and solves the mystery.
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Old 12th Nov 2020, 01:47
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The MP is a function of the RPM, throttle position, and delivery pressure to the carb
And, density of the air going into the carb. Heating the air, will reduce its density.

The "pressure" of air going into the carb of a normally aspirated engine will be atmospheric plus a very minor ram pressure which will be a factor of airspeed. The maximum ram pressure which one could expect in the speed range of a Cessna 182 would be just under 1 inch of mercury, low airspeed to fast airspeed. Detectable on the MP gauge, yes, though probably less effect than the changed density of the carb heat hot heated air if selected.

If the induction air filter becomes suddenly blocked in flight - freezing rain on the filter (and windshield!), which I have encountered, MP will suddenly drop, just as closing the throttle the engine can't get enough air to run at high power. Selection of carb heat is also alternate air. This will bypass that blockage, and return you to maximum power, less the effect of the less dense hot air. If you have selected carb heat hot, then cold again, and had a net increase of MP, you had carb ice, and melted it away. The RPM remained the same the whole time, as the propeller governed as intended. On a fuel injected engine, there is no carb heat control. There may be an alternate air control, which if used, should have no effect on MP, as the alternate air would be the same density, just a different source. Some fuel injected engine installations have no alternate air control, the alternate air door in the induction system is automatic (held closed by a spring and/or magnet).

If you change the RPM by moving the propeller control, the MP will increase with RPM decrease, as with the increased blade angle of lower RPM, the engine has to produce more power to turn the more coarse prop.

As I took off once in my 150 on a damp day, I suspected carb ice, so I selected carb heat hot. The engine quit a moment later, and I conducted an EFATO forced landing, which was a success. When I stopped rolling, the engine was running at a stumbling idle, but would not respond to throttle movement at all. I towed the plane back to the hangar and took the cowls off to investigate. There was gasoline soaked fiberglass insulation wadded up in the throat of the carb. Mice had made a nest in a hot air hose, from insulation pulled from under the glareshield. They did it fast, as I'd flow he plane a few days earlier with no problem. I cleaned out the fiberglass from the carb, and the engine ran fine. I made a screen in the hot air hose elbow to prevent a future occurrence, and have not had a problem since. The wadded insulation acted like a close throttle, and changed the engine power to less than idle. Obviously, this was downstream of the the alternate air, so that would not help the situation. Now I do not select carb heat until I'm well up in other planes I fly, just in case!
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