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Old 04-06-2021, 01:47 PM
KayS KayS is offline
Join Date: Jul 2009
Location: lake constance
Posts: 361
Default higher specific fuel consumption with lower RPM

looking at the 360`s operators manual i have an hard time to understand pages 3-18 and 3-19.

so far my understanding was that specific fuel consumption (pounds per horsepower for one hour) decreases when you lower the RPM. lower RPM's result in less internal engine friction and less volume of air has to be pumped through the system.

but the manual shows clearly that the curves for the different RPM's have a wavy shape and come closer together when power output gets lower. at 40/45% power or so the lines even meet. if we just follow the curve for 2400 RPM the difference is very significant (0,43 for 75% and 0,58 for around 40%).

why do the lines meet at low power? is that caused by part throttle pumping losses at lower manifold pressure?

the graphs show that for full rich and fixed timing. would leaning and electronic ignition change the behaviour in general?

edit: i just discovered that my heading is misleading. it should read: "why is specific fuel consumption at different RPM's equal at low power settings?"


Last edited by KayS : 04-06-2021 at 01:54 PM.
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Old 04-06-2021, 02:12 PM
BobTurner BobTurner is offline
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,359

It’s because while the cylinders may produce a certain amount of power, what’s actually measured and used in the specific fuel calculation is ‘shaft horsepower’ - how much power is available to turn the prop. At very low power settings, the cylinders are still producing power, but it’s all going to engine friction, with nothing left over. Shaft power goes to zero, so spec fuel consump goes to infinity. You’re seeing the start of that trend.
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Old 04-06-2021, 06:32 PM
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rv6ejguy rv6ejguy is offline
Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,937

Best BSFC usually occurs at a composite of highest VE (torque peak rpm) and lowest frictional losses (somewhat lower than TP rpm usually) at WOT.

O-360 torque peak is around 1800 rpm, IO-360 at around 2600. 1800 is about the lowest possible rpm to be practical in flight.

Dave Anders gets some pretty incredible fuel flows flying at 2100 rpm WOT, LOP with more advanced timing and 10 or 12 to CR. I've run some crude numbers and get something in the .33 to .35 range which is right where most aero diesels are.

Dave continues to experiment on induction tube design and length to optimize cylinder filling and increase VE.

The Lycoming might be old and crude but it turns in some pretty impressive BSFC figures when slowed down, leaned down (with even mixture distribution) and with spark timing optimized.

Our flight experiments with the EFI RV-10 show about an 8% gain in fuel economy (at the same TAS) running 30-50 LOP with timing advanced to around 30 degrees.

Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 445.9 hrs. on the Hobbs,
RV10 95% built- Sold 2016

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