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? |
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. |
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 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
|
Originally Posted by hellkitty
(Post 10917877)
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. |
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. 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. |
Originally Posted by Pilot DAR
(Post 10918586)
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. |
been accounted for by a reduction in torque and it does not follow that "the MP must reduce, 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. |
Originally Posted by Pilot DAR
(Post 10918723)
In the case of a CS prop piston engine governing, the torque and MP indication are directly related.
for the O-470, MP is your only indication of torque if the prop is governing RPM. 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. |
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. 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. |
Originally Posted by Pilot DAR
(Post 10920446)
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. 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. 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? |
The MP is a function of the RPM, throttle position, and delivery pressure to the carb 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! |
Originally Posted by Pilot DAR
(Post 10924541)
And, density of the air going into the carb. Heating the air, will reduce its density.
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. 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. The power output goes down when you reduce RPM. (Note: I am not saying you cannot achieve a higher power setting at lower RPM by increasing the MP with the throttle). If you select lower revs with the prop control, the new torque resulting could be more, less, or the same as the initial torque. It depends where the initial and final RPMs are on the engine torque curve. Blade drag varies with the square of RPM, so it follows that lower revs require more pitch for a given torque, but it does not follow that the torque will be higher than before, nor does it follow that the engine has to produce more power. You said in your first post: if you do anything which reduces power on the whole, the MP must reduce, as the RPM won't |
Has anybody else noticed that when you switch off one mag the MP INCREASES...
|
I’ve read a lot of potentially confusing terminology on this thread. I’ve never liked the term “vacuum”, especially because the aircraft doesn’t have a vacuum gauge.
During my days of teaching piston engine theory to basic students, some of whom had no prior knowledge, yet were learning to fly from scratch on aircraft with a CS propellor, I found it useful to point out that the engine is actually a petrol powered air pump and the MAP gauge simply measures the state of affairs inside the inlet manifold. The engine is able to pump out air from the inlet manifold, this is indicated by a reduction in MAP. The atmosphere is naturally trying to refill the manifold, indicated by an increase in MAP. The throttle plate is a valve which restricts the rate at which the atmosphere can refill the manifold. The MAP shows the result of a balance between how hard the engine can suck (RPM related) and how quickly the prevailing atmosphere can find its way past the air intake assembly (density related) and the position of the throttle plate (pilot controlled). |
I'm completely with oggers. To restate from my own words, more succinctly, may add some clarity.
We must differentiate between power input vs. power output. Assuming constant RPM (we're within governing range), the inputs are: Manifold pressure (Which is itself affected by ambient pressure, throttle, and other obstructions in the flow like filters, different plumbing, etc. in the alternate air path) Density (temperature) Spark timing One vs. two sparks Mixture Probably others I'm missing off the cuff The only result: Torque Anything that changes power, obviously changes the result, torque. But we don't have a measure of that. A second best correlation (but not measure) is manifold pressure. Assuming that MP is the only thing that changed, we can use that as a power correlate. However, it is not an indication or measure, because if any if the other inputs changed, that also changes the result, which we have no way of accounting for. For example, we can pull the mixture way over-lean into an obvious power loss (that shows up on the airspeed indicator/VSI) but it won't show up on the manifold pressure. For a stronger example, when we shut down the engine upon parking, the manifold pressure goes to full whereas the power goes to zero! Therefore it's incorrect that "if you do anything which reduces power on the whole, the MP must reduce." The MP is a good-enough power measurement substitute, only if we know nothing else has changed. (Succinctly, MP affects power, but power doesn't necessarily affect MP.) For the question of what does carb heat do to MP, this condition (only one variable changed) is not met. With carb heat application, we know the density decreases (duh) but at the same time we change the plumbing which has its own effects, maybe more than one. A filter goes away, which is a gain. But maybe we go through a more circuitous hose route with more bends and an intake location that does not act as ram air, which is a loss. Which predominates? We can't know experimentally without isolating every variable. The only way to really find out, is to change to a carb heat system of (for example) electrical heating pads around the induction that can be switched on and off, so only the presence of heating changes, with no other changes of plumbing. Impractical, but it would be one way to isolate the variable. Maybe there are others? But as it stands, you can't turn the carb heat on, see the MP change by X, and conclude that that's the result of density. |
All times are GMT. The time now is 20:26. |
Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.