enginehistory.org

[lhusser]

Hello,

I'm new to the group. I own an Cessna 195 with Jacobs R-755A2...so, by default, an Aircraft Engine History enthusiast.

I'm looking for references in order to help solve a debate within the 195 community. Is there a meaningful efficiency gain using carb heat in cruise? Many run these engines with CATs from 70-130F, then leaned. I've found a NACA TR or War Report that had test data for a 9 cyl. wright, where they measured EGTs (or O2?) as a proxy for charge distribution on a test stand at varying charge temperatures. They found a significant gain, from what I remember: 90F for best power (probably increasing MP to account for lower charge density) and something like 100F for best economy. However, I have not been able to find that report searching the NACA database.

Is anyone familiar with a reference (NACA or otherwise) that has hard data?

I've played around with varying CAT/RPM and MP, and have not seen a meaningful increase in miles per gallon with the use of carb heat.

[sithomas]

I haven't got any experience with the R-755, but from what I have gleened from the BeechTalk forum was that the efficiency increase is more due to the increased turbulence in the induction from having the carby heat cracked open, rather than a benefit from the hotter inlet charge.

FWIW they also say that the early E-185/E-225 can also gain some efficiency from having the throttle ever so slightly closed - again due to increased turbulence.

If you are looking for increased efficiency, this could be a good segway into the ROP/LOP debate


Simon

[dpennings]

in carburated cars with a singe carburator for several cylinder, heating a hot spot under the carburator was very common. Gasified Petrol wan't unmix on the way to the cylinders, whereas fuel droplets tend to hit the outer walls in every curve of the intake system und thus giving uneven distribution. The ideal case, would be no cooling but even distribution, unfourtunally this can't be achieved with a singe carburator (unless you don't use excessive supercharging wich heats up the intake air).

I believe it is true, that slightly heating will improve economy.