Carburettor heat & rough running engine

[bjornhall]

When browsing through the February AAIB bulletins, I came across this one: http://www.aaib.dft.gov.uk/publications/bulletins/february_2008/spezio_dal_1__g_nobi.cfm

Apparently, the pilot left carburettor heat on during a circuit which, eventually, caused the engine to run roughly and lose power, accompanied by the release of dark smoke.

What is the mechanism that causes this to happen? In particular, why would the engine start to run roughly only half way through the circuit, and not on initial application of full power with carburettor heat on?:confused:

[BackPacker]

I guess two things can happen. Plug fouling and detonation.

Plug fouling would cause the engine to run rough, particularly if two plugs in one cylinder would be affected. But I don't think that would lead to black smoke. Probably more light grey.

Detonation will physically tear the engine apart. Several things that may go wrong first (crankshaft, conrods, valve rods, you name it), virtually all will initially lead to massive quantities of oil getting into the cylinders. This will lead to black smoke, and a lot of it.

Carb heat should never be applied above something like 70% power.

BTW quick rule of the thumb:
White smoke - water (coolant) getting into the cylinders. Not likely in a lycosaurus but I once drove behind a porsche who clearly needed new head gaskets, as everybody started to put their fog lights on behind this guy.
Grey smoke - incomplete combustion for some reason
Black smoke - oil being burned for some reason. May be a gasket problem or something more serious.

[modelman]

Usually white smoke has occurred with all the blown race engines (aircooled) that I have had.Black smoke associated with rich mixture.Maybe aero engines are different.Poss carb heat=warm, less dense air=rich mixture.

MM

[Shunter]

Plugs don't foul up immediately, it can take a few minutes before it starts getting lumpy. Putting carb heat on feeds the engine with hotter air which, by definition, is less dense. Therefore the fuel/air ratio changes, richening the mixture. Over-rich = mucky combusion = fouled plugs.

[SNS3Guppy]

Plugs can foul immediately with no warning, depending on a number of factors. Don't make the mistake of thinking they can't.

[bjornhall]

I see, thanks, makes good sense now!

*note to self - don't do that*

[SNS3Guppy]

In particular, why would the engine start to run roughly only half way through the circuit, and not on initial application of full power with carburettor heat on?


If you have carburetor ice, you may have little or no response during initial application of carb heat. Roughness develops as the ice melts.

If the mixture is rich already and a low compression engine is doing little work to scavenge an unburned mixture, and the plugs are low tension or weak or the gap is wide or there's little heat in the cylinders, or there's an oil fouling issue with leaky valve guides (and so on and so on), fouling can be an issue with one or more fouled plugs. Just one or two cylinders fouled can really shake an engine.

If the carb air temperature was outside the icing range when carb heat was applied, the carb air temperature can be moved into the icing range...and carb heat can cause ice to form. Lacking a carb air temperature gauge, you're taking a shot in the dark by applying carb heat. Generally the manufacturer tries to err somewhat on the side of caution by making it too warm rather than not warm enough or in the icing range...but on a cool, humid morning, you can cause ice under some circumstances by applying carb heat.

If ice has been forming and the throttle is opened, the engine may just quit. It may be running at a lower power setting, but you can see a variety of potential things occuring if an occlusion exists adjacent to or on the throttle plate, or in the venturi. Opening the throttle is throwing a lot if air into that engine which may not be matched by the fuel flow; you may just put the engine out, send a slug if ice down the carb, or have other problems occur.

Dark smoke is indicative of an excessively rich mixture, or a mechanical issue such as a hole in a piston or a failed valve guide.

[BEagle]

The symptoms are very similar to those associated with an unlocked primer....

Perhaps that was the initial cause and the pilot tried to cure the rough running by selecting carb air to Hot?