Carburetor heat
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Carburetor (or Carburettor) heat (usually abbreviated to 'carb heat') is a system used in piston-powered light aircraft to help prevent or clear carburetor icing. It is usually manually controlled by the pilot. It consists of a flap which diverts warm air from around the exhaust manifold into the engine's air intake. The warmer air will usually clear any icing present within the carburetor.
Icing occurs in certain conditions due to the venturi within the carburetor, which raises the velocity of the air in the carburetor, which lowers its pressure (see Bernoulli's principle) and hence temperature (Boyle's Law). If the outside air is already at a low temperature, the temperature in the carburetor can drop below the freezing point of water, and if the air is humid, ice can form inside the carburetor, narrowing the aperture of the throat, which can create an even stronger venturi effect, and so forth. If left unchecked, the carburetor will eventually malfunction which will cause an engine failure, an emergency situation. Temperature drops of 20 °C or more are often encountered within the carburetor, and so icing can occur even on relatively warm days. Also, the adiabatic lapse rate (temperature drop) is around 2 °C per thousand feet, so it is really the humidity of the air which is the more important indicator of potential icing conditions. Perhaps paradoxically, winter flying is often less prone to icing, since cold weather is rarely associated with high humidity, and the air temperature can drop so far below freezing that there can be little or no water vapour in the air to begin with.
Diverting warm air into the intake will usually clear any icing present, though in some conditions still may not be sufficient. The wise pilot will not attempt to fly into known icing conditions if his aircraft is not equipped to deal with it.
The diversion of warm air into the intake reduces the thermodynamic efficiency of the engine, which will be manifest as a slight reduction in power while carb heat is applied. The reduction in power indicates to the pilot that there is no icing present, a reassuring piece of information. If there is icing, applying carb heat may not show this initial reduction, and as the ice clears there may be an increase in power. In the case of icing, it must be kept in mind that the ingestion of small amounts of water into the engine following melting in the carburetor may cause an initial period of rough running for us much as one or two minutes before the power increase is noted. Again, the pilot will note this as evidence that icing conditions are present. However, more than one pilot, when confronted with a rough running engine has mistakenly turned the carburetor heat back off, thereby exacerbating the situation.
If carb icing results in an engine stoppage, one of the first things the pilot will do is apply carb heat in an attempt to clear the icing, though as the engine will not be running, it is possible that the exhaust will cool sufficiently quickly that clearing the icing will not be possible. In any case the pilot will be carrying out the emergency landing procedure, including a possible engine restart. The outcome depends on the conditions, prompt action and skill. Again, avoiding icing is far better than trying to clear it in an emergency.
Applying carb heat as a matter of routine is built into numerous in-flight and pre-landing checks (e.g. see BUMPH and GUMPS).
The intake air of an aircraft engine equipped with a turbo-supercharger is heated through compression, preventing ice formation and offering a natural means to supply carb heating without having to duct air from the exhaust manifold. A turbocharged aircraft is likely to have no 'carb heat' switch, instead the pilot will have to increase power into the turbocharger stage to bring de-icing into effect.
Engines equipped with fuel injection do not require carb heat or an analogous system as they are not prone to icing.
Some car intake manifolds had this, for the same reasons that aircraft do. Many Chevrolets from the early 50's had it, but so do a number of aftermarket performance manifolds from Clifford Performance.
