To be strictly accurate, there are a lot of other influencing factors at work when you go lean. Your thermal efficiency will improve (quite dramatically) as you lean due to the factors stated in my previous post. However, on an engine with fixed ignition timing, the ignition will be further and further away from best torque/optimum - this means that you degrade BSFC as you enlean, which I think is probably the major effect on aero engines. If you could control ignition with air-fuel ratio (FADEC anyone?) you could gain a useful improvement here.
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Is LOP worth it? - My test results
- Thread starter Webb
- Start date
To be strictly accurate, there are a lot of other influencing factors at work when you go lean. Your thermal efficiency will improve (quite dramatically) as you lean due to the factors stated in my previous post. However, on an engine with fixed ignition timing, the ignition will be further and further away from best torque/optimum - this means that you degrade BSFC as you enlean, which I think is probably the major effect on aero engines. If you could control ignition with air-fuel ratio (FADEC anyone?) you could gain a useful improvement here.
Kevin Horton
Well Known Member
I agree that they are many factors at play which will cause bSFC to vary as we vary the mixture LOP. I don't have any data on most of them though, so it I don't know how to quantify the potential impact.
I do have some info on friction power for Lycoming engines. It can be found on one of the charts near the end of this Calculaging Lycoming Power from Fuel Flow document (3.2 MB pdf). For a 360 cubic inch displacement engine, at 2500 rpm, they show about 26 hp lost to friction. They claim that this is constant, no matter how much brake horsepower are being produced.
Running the calculation method in this document, if I am at a condition where a IO-360-A series engine produces 150 hp (75% power) at 2500 rpm with the mixture set for best power, and then lean to go LOP, the best bSFC is at about 135 hp, with fuel flow of 9.19 gph, and bSFC of 0.409. The engine would actually be producing an indicated power (the power produced in the cylinders, before losses due to internal friction) of about 135 + 26 = 161 hp, with an iSFC of about 0.343.
If we climb to higher altitude, or pull back the MP to find a condition where this same engine makes 110 hp with mixture set to best power, then pull the mixture back to LOP, we see best bSFC of about 0.441 at about 100 hp at 7.34 gph. The ihp would be 126 hp, and their claimed iSFC is 0.351.
Lycoming's claimed iSFC doesn't vary too much (0.343 vs 0.351), but the friction power becomes much more significant at lower power, which causes the bSFC to degrade from 0.409 to 0.441.
We're going to confuse ourselves with this discussion! 
Going back to Mehrdad's original post (#41) we are talking about ROP -v- LOP for the same power. Anything else is a bit apples and oranges.
Of course, when you lean from a constant throttle position (also MAP) then you'll see a change in BSFC as you effectively reduce the load/power you're running at. That effect is not unexpected, however your IAS/TAS will drop too, obviously. You are right of course, that the BSFC must increase at some point as you reduce load, since the limiting case is closed throttle, where power goes negative and BSFC ceases to have any meaning since it's fallen off the top of an asymptotic infinity and exploded into mathematical mystery. Because of this we're back demanding only apples or only oranges to create anything meaningful
BTW, interesting article you posted. I'm surprised at the FMEP that is quoted at 26hp (2500rpm). This is equivalent to 1.6Bar / 22.8PSI which is way higher than what I know is normally assumed for automotive engines. Tis also much higher than most diesel engines, which are traditionally high. I would expect it to be much closer to 1.0Bar / 14.5PSI which is more like 16-17hp at 2500rpm. If the Lyc really is up at that friction level, then there is huge scope for friction reduction...
A
Going back to Mehrdad's original post (#41) we are talking about ROP -v- LOP for the same power. Anything else is a bit apples and oranges.
Of course, when you lean from a constant throttle position (also MAP) then you'll see a change in BSFC as you effectively reduce the load/power you're running at. That effect is not unexpected, however your IAS/TAS will drop too, obviously. You are right of course, that the BSFC must increase at some point as you reduce load, since the limiting case is closed throttle, where power goes negative and BSFC ceases to have any meaning since it's fallen off the top of an asymptotic infinity and exploded into mathematical mystery. Because of this we're back demanding only apples or only oranges to create anything meaningful
BTW, interesting article you posted. I'm surprised at the FMEP that is quoted at 26hp (2500rpm). This is equivalent to 1.6Bar / 22.8PSI which is way higher than what I know is normally assumed for automotive engines. Tis also much higher than most diesel engines, which are traditionally high. I would expect it to be much closer to 1.0Bar / 14.5PSI which is more like 16-17hp at 2500rpm. If the Lyc really is up at that friction level, then there is huge scope for friction reduction...
A
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LOP descent with fixed pitch props?
Hi Group,
I've read all the Deakin articles and have been enjoying the benefits of operating LOP. My question is, what are those with fixed pitch props doing in descent? Deakin's "Descent" article only deals with a CS prop.
I have been pulling throttle and continue to enrich to maintain 50 degrees LOP on the way down. Once work load starts to increase around the pattern I just go to full rich.
Are there any better suggestions?
Thanks
Jon D.
Hi Group,
I've read all the Deakin articles and have been enjoying the benefits of operating LOP. My question is, what are those with fixed pitch props doing in descent? Deakin's "Descent" article only deals with a CS prop.
I have been pulling throttle and continue to enrich to maintain 50 degrees LOP on the way down. Once work load starts to increase around the pattern I just go to full rich.
Are there any better suggestions?
Thanks
Jon D.
