Carb Air Temp oddness.

[Guest iholrf]

Hi all,I hacked together a new carb air temp guage for my panel last night, and discovered that it doesn't seem to make sense to me. I used (A:Recip carburetor temperature:1,celsius)The net result is the temp of the carb air seems to go to about 0degC after engine start up and stays there. This occurs regardless of location or altitude, though at 20000 it did drop a degree or two.Figuring I must have done something wrong, I looked at the DC3 and the De Havilland buth of which have carb air guages and noticed they were displaying the same behavior.Now, perhaps I am on crack, but this does not make sense to me. Unless the carb is sitting in an air conditioner, when the outside temp is 32degC (90F) how is the temp dropping to zero on engine start and staying there? Should it not be somewhat relative to outside temperature?A search here on the forum has not turned up any relevant results, so I thought I would ask directly.TIAShad

[Guest Slacktide]

>Hi all,>>I hacked together a new carb air temp guage for my panel last>night, and discovered that it doesn't seem to make sense to>me. I used >>(A:Recip carburetor temperature:1,celsius)>>The net result is the temp of the carb air seems to go to>about 0degC after engine start up and stays there. This occurs>regardless of location or altitude, though at 20000 it did>drop a degree or two.>>Figuring I must have done something wrong, I looked at the DC3>and the De Havilland buth of which have carb air guages and>noticed they were displaying the same behavior.>>Now, perhaps I am on crack, but this does not make sense to>me. Unless the carb is sitting in an air conditioner, when the>outside temp is 32degC (90F) how is the temp dropping to zero>on engine start and staying there? Should it not be somewhat>relative to outside temperature?That's exactly correct, the carb IS an air conditioner. The pressure inside the throat of the carburetor is lower than the ambient pressure. The first law of thermodynamics says that the temperature drops adiabatically with manifold pressure. This is why induction system icing is a problem at low power settings (i.e. when you reduce power for decent, MP drops, temperature decreases, and if the humidity is high enough, ice may form) Induction icing is actually a bigger problem in summer months than winter, specifically because of the greater amount of moiusture that can be held in warmer air.Dan

[Guest iholrf]

>That's exactly correct, the carb IS an air conditioner. The>pressure inside the throat of the carburetor is lower than the>ambient pressure. The first law of thermodynamics says that>the temperature drops adiabatically with manifold pressure. >This is why induction system icing is a problem at low power>settings (i.e. when you reduce power for decent, MP drops,>temperature decreases, and if the humidity is high enough, ice>may form) Induction icing is actually a bigger problem in>summer months than winter, specifically because of the greater>amount of moiusture that can be held in warmer air.>>DanThanks Dan,I suspected that might be the case, but thought I would ask. I was just surprised that there seemed to be almost no variation in its value from idle to max, from 0ft to 20,000, from -30 to + 30.As manifold pressure changes should there not be some variation in the air temp? As the external environment changes should the values also not change?CheersShad

[Guest Ron Freimuth]

>>That's exactly correct, the carb IS an air conditioner. >The>>pressure inside the throat of the carburetor is lower than>the>>ambient pressure. The first law of thermodynamics says that>>the temperature drops adiabatically with manifold pressure. >> .........>>Dan>>>Thanks Dan,>>>As manifold pressure changes should there not be some>variation in the air temp? As the external environment changes>should the values also not change?>>Cheers>Shad I think the temp drop is mainly to evaporation of the fuel. Higher throttle setting means both more fuel and air flow. The ratio stays at the fuel/air ratio (about 15:1). And, the temperture drop would stay about the same. Leaning can help carb icing, more air relative to the evaporating fuel. A rich mixture cools the engine more. Ron

[Guest iholrf]

> I think the temp drop is mainly to evaporation of the fuel. >Higher throttle setting means both more fuel and air flow.>The ratio stays at the fuel/air ratio (about 15:1). And, the>temperture drop would stay about the same.>> Leaning can help carb icing, more air relative to the>evaporating fuel. A rich mixture cools the engine more. >> Ron>That makes sense. So it appears it is *me* who is on crack. :)Thanks RonCheersShad