I think it is important for us to realize what BSFC is, and what it isn't.
BSFC is simply a measure of how good your engine is at turning gas into power
(vs. noise or heat). When you want to compare the relative merits of two
engine designs in terms of efficiency, BSFC curves are the usual method.
Most engine manufacturers generate these curves using dynanometer runs and
they strive for standard day sea level conditions (or apply corrections to
the data). This is important for creating curves that can be used "apples to
apples." However, and this is a big however, standard day sea level BSFC
curves have relatively little to do with telling you what the efficiency of
your airplane will be with that engine installed.
For the most part, engine BSFC comes down to two variables: (1) friction;
and (2) volumetric efficiency. The ideal engine has high volumetric
efficiency to generate decent power, and low internal friction so that the
power generated can perform useful work through the shaft.
Friction is pretty straight forward. In general, the faster you spin the
more your friction losses are and the worse your BSFC becomes. The effect of
this is readily seen in the curves. One example is the IOL-300 curves that
Paul sent out which showed an amazing .345 BSFC. However, what was equally
amazing was the RPM . . . 1600!! I also published some Lycoming 360 series
numbers early on in this thread (which just refuses to die). Pretty
respectable numbers of around .4. But these numbers were for 2000 RPM.
Increase that RPM to 2700 while retaining best economy mixture and 55% power
and we're now looking at a .46!! So, why are large bore aircraft engines
able to generate such good BSFCs? Simple . . . they generate their
horsepower at low RPMs which minimizes friction losses. No rocket science
here. (To take this to the logical extreme, consider the BSFCs attained by
locomotive or very large oceangoing marine diesels)
The thing about friction is that, assuming the same RPM, it doesn't care
about the density of the air that the engine is breathing. (Actually, it
does a little . . . an engine generating a lot of power by breathing dense
air, i.e., low altitude or turbocharged/superchared, will have a little more
friction because the bearings are under heavier load.) So, let's apply that
to a normally aspirated airplane engine. At sea level, the engine makes a
ton of power and a relatively small percentage of the power goes to
overcoming friction. However, at 10,000 feet, the engine's ability to
generate power has declined substantially, but the friction it has to
overcome has barely changed. This, of course, impacts the BSFC attainable at
altitude quite a bit.
An example of these effects is the post about a 500 mile time trial in an
O-320 powered Long-EZ. If it is like an O-360, I would expect it to have a
dyno-BSFC of around .4. The Long-EZs flight profile consisted of a
taxi/takeoff/climb (rich), a sustained cruise at 10,000 feet and 2600RPM
(lean), followed by a descent/landing/taxi (rich). Paul came up with a
"mission" BSFC of around .48, which sounds about right to me. However, it
should be clear to everyone that, even though the Long-EZ flew a profile that
substantially diminished the engine's BSFC, it also flew a very efficient
profile for the airplane. Hence the rub, engine BSFC and aircraft efficiency
are very different animals.
My point is to use BSFC curves for what they were intended, very low-level
comparison of relative efficiencies obtainable by different engines. Keeping
things apples to apples is important. Comparing a "mission" BSFC of an
engine installed in an airplane flying a profile to a dynanometer BSFC of a
different engine is of little value. However, a controlled experiment of
similar airplanes equipped with different engines (similar HP) flying the
same flight profile would be extremely interesting, and the most useful
comparison of all.
Rick
Excellent description of how the efficiency of an engine is affected by
the various factors Rick. I think I will plagiarize it sometime in
the future :-)> Can I post this to the ACRE NL web site?
Here is a little table that I generated with a simple program.
Its not perfect. The BSFC slope if any is a guess. Ideally there
would be a lookup table for BSFC as a function of engine HP
leaned to optimum and allowed by the engine manufactures.
The program does a trial and error iteration to compare drag with
available thrust out of the prop until it is less than some error.
It is still a work in progress.
As you can see slowing down just a few MPH can gain you far more
in MPG than several points in the BSFC number.
Paul Lamar
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org
Flat Plat = 1.6
HP FPS MPH thrust drag BSFC pounds gal MPG
65.0 245.5 167.4 116.5 116.6 0.415 27.0 4.5 37.2
70.0 251.7 171.6 122.4 122.5 0.420 29.4 4.9 35.0
75.0 257.5 175.5 128.2 128.2 0.425 31.9 5.3 33.0
80.0 263.1 179.4 133.8 133.9 0.430 34.4 5.7 31.3
85.0 268.5 183.1 139.3 139.4 0.435 37.0 6.2 29.7
90.0 273.5 186.5 144.8 144.7 0.440 39.6 6.6 28.3
95.0 278.5 189.9 150.1 150.0 0.445 42.3 7.0 27.0
100.0 283.4 193.2 155.3 155.3 0.450 45.0 7.5 25.8
105.0 288.1 196.4 160.4 160.5 0.455 47.8 8.0 24.7
110.0 292.5 199.4 165.5 165.4 0.460 50.6 8.4 23.6
115.0 297.0 202.5 170.4 170.6 0.465 53.5 8.9 22.7
120.0 301.1 205.3 175.4 175.3 0.470 56.4 9.4 21.8
125.0 305.2 208.1 180.2 180.2 0.475 59.4 9.9 21.0
130.0 309.2 210.8 185.0 184.9 0.480 62.4 10.4 20.3
135.0 313.2 213.5 189.7 189.7 0.485 65.5 10.9 19.6
140.0 317.1 216.2 194.3 194.4 0.490 68.6 11.4 18.9
145.0 320.8 218.7 198.9 199.0 0.495 71.8 12.0 18.3
150.0 324.4 221.1 203.5 203.5 0.500 75.0 12.5 17.7
155.0 327.9 223.6 208.0 207.9 0.505 78.3 13.0 17.1
160.0 331.5 226.0 212.4 212.5 0.510 81.6 13.6 16.6