Reverse carb heat !

[FullyFlapped]

On a couple of occasions recently, when applying carb heat shortly after startup (during power checks), engine RPM has increased. This is a standard C172, except for a modifed "tuned" exhaust system. If I wait a minute or so, then do the checks again, normal behaviour is resumed, and the drop in RPM is again normal.

Anyone have any ideas why the application of "warm" air (not that it would be very warm just after startup) - with mixture full rich - would cause a power increase ?

Cheers,

FF :ok:

[Sir George Cayley]

Have you in fact got icing almost immediately on startup.? So first application of Carb Heat clears it instantly and the revs climb?

Or is there a choke effect when selecting the other air source? Might this richen (?) the mixture relatively causing the increase assisted by the fact it hasn't warmed up?

Or maybe someone who knows about engines should answer!

Sir George Cayley

[Sunfish]

How "shortly" is "shortly"? If it was a second or two, then maybe you have a case. If it was a minute or more, then your exhaust was already warmed up and you had carburettor icing. In a C150 I've had ice while trundling from the flight line to the runup bay - about one minute's taxiing.

[DubTrub]

FF

Is it a Cont 0-300 powered 172?

[UV]

Actually not that unusual!

Our Grob 109B (powered by a Grob 2500cc 95 hp Grob 2500 engine) gives a 100 rpm INCREASE when carb heat is applied at most power settings, and in all temperatures and weathers, whilst on the ground.

Close inspection of the POH reveals that the Before Take Off Checks require selection of full power (to check rpm) and THEN to apply Carb Heat. Low and behold there is a 100 rpm LOSS.

Selection of Carb Heat in the air gives the normal response.

UV

[fireflybob]

Bear in mind also that humidity is generally highest near or after dawn so if you are starting at these times it is quite common to get some carb icing at this stage.

[FullyFlapped]

DubTrub, nope, it's a Lycoming IO-320.

Carb ice did occur, but the increase in RPM is instantaneous when I pull the knob out : unless ice can melt incredibly quickly I suspect something else must be going on.

However, you're all quite right, I always run the engine for a while before I do the run ups, so I guess it's a possibility.

Thanks all for the answers.

What do you think, Capn H ?

FF :ok:

[david viewing]

I suggest you have a good look at the little pinch bolt in the end of the carb heat quadrant. Mine stripped allowing the operating wire to slip through, giving all manner of strange behaviour and defying several engineer inspections (until it broke over the Irish sea!). I think it would be possible for the carb to stick in 'hot' after landing and not be noticed until the next day when exercising the control and it flops over to 'cold'.

[smarthawke]

FF

You say its an IO-320? If its an 'I' (injected) it won't be carb icing and you won't have carb heat to select (injected engines have 'alternate air'.

Only the latest 172s are injected (P's and earlier aren't) and they use an IO-360 engine.

As others have said some engines do seem prone to icing straight after start up and idle speed over damp grass will accentuate it. The Lycoming/Cessna carb heat system isn't great as all it does is drag air over the #4 cylinder exhaust down pipe. Immediately after start this won't be very warm air. If during power checks as you say, it sounds like you have picked up ice, the initial selection clears it and then you are back to normal.

If the mixture is a bit to lean for the engine's wants then selected hot air will increase the rpm as it enrichens the mixture. I had a 152 with an induction leak once - loose stub pipe in the sump, at 1700rpm the revs increased when hot air was selected, at low rpm the 'idle' was not smooth.

If you haven't got an engineer nearby and want to check the carb heat operation, you can normally see the airbox with the cowlings on through the exhaust stub hole (although with the modded exhaust you may not be able to see this). Get someone to operate the carb heat (ENGINE OFF!) whilst you watch the arm on the side and check it moves backwards and forwards with a clunk at each end as the selector flap moves. Also ensure the outer cable is held tight and not moving in the securing P-clamp. T

This is only a visual check and if there is any thoughts of a problem then get a qualified engineer to check the system.

[Cabotage Kid]

unless ice can melt incredibly quickly I suspect something else must be going on.
The ground response to pulling the heat lever is often instant. A tenner says it was ice.

[Send Clowns]

You hadn't left the mixture lean from taxiing had you? If not, then ice. Was there dew on the ground?

[VP959]

So far no one seems to have mentioned alternate air.

Quite a few installations bypass the air filter when carb heat is selected, so allowing the engine to breathe air directly, rather than through the slight restriction of the filter. If the outside air temperature is such that carb heat isn't going to change the inlet air temperature by much, then it's at least possible that the freer air flow resulting from bypassing the filter is allowing the small rpm increase observed.

It's worth remembering that selecting carb heat often bypasses the air filter, as it might just be useful should you experience a filter blockage in flight (flying through a sudden duststorm, perhaps?). The snag is that dust laden, unfiltered air, will fairly quickly result in excessive cylinder bore wear, but it might be a trick worth remembering if you ever get caught in something like a volcanic ash cloud (not likely, I know...............).

[Sunfish]

Dang! VP just beat me too it! I'm pretty sure that carb heat in a Cessna bypasses the air filter - which is why you need to be cautious about using it on the ground.

Selecting carb heat could be giving you a less restricted airflow - hence the rev jump.

[DubTrub]

Not so, Sunfish, hot air from the carb heater has less weight of air per unit volume, being warmer, and therefore there is less oxygen in this warm flow, so less fuel is burnt and therefore the rpm reduces. The mixture is richened by application of carb heat too, because more fuel remains unburnt in the oxygen-deprived flow.

But you and VP are correct in that carb heat does bypass the filter. I do not know of one installation where this is not the case.

[Cabotage Kid]

If I wait a minute or so, then do the checks again, normal behaviour is resumed, and the drop in RPM is again normal.
From the above quote in the original question is it boring old mundane ice. Nothing quite as exotic as a blocked airfilter.

:O

[FullyFlapped]

Smarthawke, you're quite right - finger trouble - there's no "I" in my engine, it's just an O-320.

To everyone else, thanks for all the good advice. I will monitor the situation and have the various items suggested checked by an engineer shortly.

Cheers guys,

FF :ok:

['I' in the sky]

Typical LIGHT carb ice, happens here in our PA28s on early morning start ups even in Spain - and we don't park on grass.

Carb Ice because it's early in the morning, temp and dewpoint still close together even though it might be quite warm, and a low power setting. The engine has some natural resistance to carb ice if the exhaust manifold is located very close to the carburretor. In the PA28 they as good as touch. However it takes a while for the heat to conduct through which is why it typically happens within two or three minutes after starting.

LIGHT carb ice because that's the symptom of light carb ice when you apply carb heat. It clears quickly without much fuss, allowing more mixture through, in this case enough to counteract the effect of adding the hot, less dense air, which isn't actually all that hot and less dense yet, and hence increase rpm.

Larger quantities of ice though on application of carb heat, dumping water into the fuel air mixture will cause the more commonly quoted 'further drop, rough running then increase in rpm'

[Sunfish]

I wonder if it might be gremlins?

[KiloDelta]

Hi,

it might be posible that your carb heat is not in a proper condition. i think it does not completly close the airflow from the filter and so you have two airstreams in carb direction (this increases cyl filling)
That also means that you not have full carb-deicing capabilities in a case were you really need it.
so please - contact an engineer before your next takeoff.

mhl
Kilodelta:cool:

[Mike Cross]

DubTrub is not quite right.

The original post said this happened before the engine had warmed up.

Situation therefore is that the engine is running full rich and pulling the carb heat will not have much effect on the intake air temp. It will however lean the mixture slightly by removing the choking effect of the intake air filter. Try leaning the mixture slightly in the same circumstances and I bet you get an increase in RPM.