Piston engine and carburettor heater?

[jmmoric]

I had the most odd experience the other day, while running the checklist before flying.

When activating the carburettor heater, the engine RPM did not drop, on the contrary it rose ever so slightly, almost unnoticable on the RPM meter, but was able to hear the increase in RPM.

I've never tried that before, do anyone have any clue to why that happens?

[jonkster]

What were weather conditions? Is it possible you picked up some icing on taxi?

[jmmoric]

What were weather conditions? Is it possible you picked up some icing on taxi?

No, it was only a short taxi, and I checked it a few times to make sure it was not ice, kept it on for a short while in case there was.

And the weather was severe CAVOK...

[Procrastinus]

Carb heat mechanical connections not working?

[jmmoric]

Carb heat mechanical connections not working?

I was thinking the same, the feeling, force needed, when turning it on, was like it usually is when working. And yes, it´s definately mechanical, it´s a 1978 Piper 28, so nothing fancy here :)

Well, I've put a notice on it, and will have a mechanic look at it.

[18greens]

I had the most odd experience the other day, while running the checklist before flying.


When activating the carburettor heater, the engine RPM did not drop, on the contrary it rose ever so slightly, almost unnoticable on the RPM meter, but was able to hear the increase in RPM.


I've never tried that before, do anyone have any clue to why that happens?


Are you sure it wasn't already hot and you moved it to cold (easily done in a brain fart moment). Assuming the carb heat is wired the correct way if there is ice the action of checking carb heat should cause a drop , some rough running as the water runs through the engine, then when set to cold a rise against the original RPM. I've noticed a lot of people don't check the before and after rpm to see if there was ice. Also when testing it you should test it for 10 seconds to allow the ice to melt away. A lot of people just flick it on and off. Since carb ice is so rare ( i only notice it once or twice a year) the check gets shortened. Now does seem to be the season for it.


Does any of that help explain what you saw?

[uncle8]

If your mixture was set, for some reason or other, at a leaner than normal setting, applying carb heat will enrichen it so that it may return it to a more normal setting and a higher RPM. If that is the case, you should be able to replicate it.

[jmmoric]

Does any of that help explain what you saw?


It's a good suggestion, but I'm not that guy that just flicks it back and forth, especially not if it does not do as expected....
If your mixture was set, for some reason or other, at a leaner than normal setting, applying carb heat will enrichen it so that it may return it to a more normal setting and a higher RPM.


That could be the case actually, I do lean it a bit before taxi, but not very much.

For that to happen, wouldn't I have leaned it way too hard in the first place? The only reason I see to lean it hard during the run up, is if the engine runs rough checking the magnetos, to try and "burn off" residue from the spark plugs.

[uncle8]

I think that the mixture should be at full rich at run up carb heat check. Alternatively the mixture adjustment on the carb ( set by the engineer) may be set incorrectly.

[jmmoric]

I think that the mixture should be at full rich at run up carb heat check. Alternatively the mixture adjustment on the carb ( set by the engineer) may be set incorrectly.

You are correct.

The leaning before taxi was something I was taught during training, for whatever reason... it never appeared on the checklists, and therefore the only mentioning is the "full rich" when starting, and "set mixture" before take off.

I'll have to review my procedures, and just follow the checklists more strictly.

[Airbus38]

Often in the winter when starting from cold a slightly richer mixture than normal will be ideal. Used to fly an aeroplane with a choke which on initial start would give a very rich mixture ideal for starting and the first couple of minutes of running. Once started gradually losing the choke kept the rpm at peak. After a short warming period it would be far too rich and start spluttering black smoke.

Anyway, I found through experimentation that the carb heat would often have a similar effect on startup; the freezing cold (and therefore lovely and ‘thick’) air going in to the freezing cold block liked to have a much richer mixture than could be set with full rich in the mixture control; therefore warming the air going in to the carbs using a little carburettor heat had the effect of lowering the air density and therefore creating a mixture much closer to optimum for running. All a bit counter-intuitive really when seen in action but totally normal.

In summary, the way I visualised it was: Very cold dense air in to a cold engine = mixture a little lean of peak. Slightly warming the incoming air before sending through the carburettor => richens mixture slightly taking it closer to peak. Once the engine warms up to operating temperature, mixture will be rich of peak and therefore getting rid of carb heat will make the engine respond in the way you would expect.

Interesting to play around with in any case - hope that helps!

[aterpster]

No, it was only a short taxi, and I checked it a few times to make sure it was not ice, kept it on for a short while in case there was.

And the weather was severe CAVOK...

You can get carburetor ice on a CAVOK day, especially when descending at low power settings. The only time you won't is when the atmosphere is extremely dry.

[jonkster]

I fly 2 aircraft that regularly will quickly build up carb ice on certain CAVOK Australian mornings while taxiing. Some Continental engines are well known for this. As aterpster mentions, certain power settings temps and appropriate relative humidity and you are potentially in the zone, even when temps are warm.

NB Not saying icing is the issue for the OP's particular issue but FWIW I regularly see students and long time pilots on engine runups going through the ritual of pulling the carb heat for a second, saying RPM drops, then immediately pushing it forward, without actually checking to see if they may have built up ice. The check should not just be to check the revs drop. It can be an eye opener for them when I get them to hold the carb heat on and they start to see the engine RPM increase and when they return to cold see the RPM now higher than their initial setting.


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[spornrad]

You are correct.

The leaning before taxi was something I was taught during training, for whatever reason... it never appeared on the checklists, and therefore the only mentioning is the "full rich" when starting, and "set mixture" before take off.

I'll have to review my procedures, and just follow the checklists more strictly.

Leaning for taxi is a very good advice to avoid fouling the spark plugs at low engine speeds. Learned it and do it routinely. However, the old hands highly recommend to lean very aggressively for ground ops, not just a bit. Why? Because the day will come and you will forget to enrich before takeoff, and that may put the "slightly leaned" engine straight into detonation. If leaned aggressively, it will just cough and quit when trying to set take-off power.

[Chu Chu]

Denser air calls for a richer carburetor setting regardless of how long the engine's been running. How much fuel is metered through the carburetor jet depends on the volume of air flowing through the venturi. The colder air is, the more oxygen molecules (and nitrogen molecules) there are in a given volume. More oxygen mixing with the same amount of fuel means a leaner mixture. So the carburetor needs to be set richer to get the same fuel:air ratio. And since it's the temperature in the venturi that matters, warming up the engine doesn't change things (unless you use carb heat, of course).

A cold engine has a different effect. Fuel vaporizes better when it's hot. When the engine's cold, some fuel pools in the intake, and some settles on the cylinder walls. Since less of the fuel that comes out of the jet actually burns, more is needed to get the same effective mixture. That's why carburetors have chokes.

Of course, cold weather means that the engine is colder when it starts (and perhaps really cold weather means that some fuel doesn't vaporize even after the engine warms up).

[jmmoric]

Thanks all for the answers, really appreciate it.

Only experienced carburettor icing once, and that was a place I usually not fly. But very aware of it still. For some reason I still get anxious when the engine starts sounding just a little different, and start scanning and checking, including trying the carburettor heater... mostly it's just because I fly into another mass of air over water or something like that, but it gets my attention :)

[Maoraigh1]

Continental O200 engines are bad for carb Ice at startup, before there's heat in the exhaust. At Inverness I've often had the engine stop shortly after first start. With cowling off, frost can be seen forming on the outside of the carb.
You could easily have ice on a short taxi.
Carb Ice is common on the O200 in the cruise in the Scottish climate.
I fly two different aircraft, and the installation makes a difference.
I suspect Konsin on the surface makes carb Ice possible at temperatures too low for it to show on the diagrams.

[Gauges and Dials]

My guess would be that the normal intake path is slightly obstructed (e.g. by a filthy air filter, or a foreign object somewhere in the plumbing) and that switching to carb heat draws air via a less obstructed pathway.