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old man emu

Engaging Carby Heat

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Here's an interesting video dealing with the use of carby heat.


The points to be taken from it are the need to know the path that the air travels before it gets to the carby, and what happens to the temperature of the air as it travels that path.


The video also highlights important differences between engine management in a PA-28 and a C-150/172, normally aspirated engine. It shows the necessity of a thorough knowledge of the content of the POH.



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 Might be a good idea to see the engine of the plane you fly with the Cowling off and have someone show you the ducting etc. .Cessna's use both Lycoming and Continentals The Lycoming usually has the inlet passages passing through the sump where it's Quite hot. IF the air coming in is from a hot source you are getting less power than if it's cold air.

 Also it's correct that with most engines, carb heat on will reduce power available to the extent the go round will be compromised. Always do throttle FULL ..check Carb heat  OFF     AUTOMATICALLY as if it's one action.. Every time.

 Yes some engines will get dust from application of the hot air function but IF you need hot air use it. If you apply it after a long glide there won't be much effect as the hot bits you relied on have cooled off. It's also FULL ON, NOT partial,,  preferably with power still at cruise  for a minute or more, and IF you are in really bad conditions do the descent with some power,  when you can and heat on.. IF you  Have to go around it's covered by the full throttle, heat off action. All take offs and go rounds should include an engine power check as part of it .You may find the heat on or the throttle is not fully open but FLY the plane as well. You have control and flaps etc to worry about there.  Nev

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Why take off with "Carby - Cold"?


It comes back to the effects of Density Altitude on engine performance. Increasing the temperature of the air, by using carby heat, reduces its density. Less dense air means less air passing through the carby venturi, lessening the relative pressure between the venturi tube and the float bowl. Less pressure in the venturi tube means less suck of fuel from the float bowl. Less fuel means a weaker power output in the cylinder. Also, less dense air means less O2 per intake of air into the cylinder of air. Less O2 per intake stroke of air means that the Chemically Correct air/fuel mixture will not be reached. This means less effective fuel burn in the cylinder; which means less pressure on the piston top; which means less force on the crankshaft; which means less torque; which means less work from the propeller.



I have been careful to use the term "fuel burn" to describe the combustion of the fuel/air mixture in the cylinder. Abandon the old description of the Otto Cycle as Suck; Squeeze; Bang; Blow. In an efficient engine the sequence is Suck; Squeeze; Burn; Blow. If you have "Bang", you have detonation and an engine on its way to falure.

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