brian-1927(at)lloyd.com
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 Posted: Thu Nov 29, 2007 4:37 pm Post subject: Thermodynamic efficiency (was: Re: Throttle setting at highe |
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This is one of those things that is well understood and it doesn't
really matter who says it, Vladimir notwithstanding.
HP output is going to be proportional to MAP and to RPM. If all other
things remain the same and you make a 10% reduction in manifold
pressure, you will get a 10% reduction in HP as you have reduced
available fuel/air by 10%.
Likewise, if you reduce RPM by 10% you are reducing the available
fuel and air by 10% as well, as the volume of fuel/air being pumped
into the cylinders by the cylinders is reduced by that amount too.
But several things happen that make this not quite linear. When you
close the throttle part way, the engine has to do more work to draw
in the fuel/air charge against the restriction caused by the throttle
valve. It also increases turbulence in the induction system. Both of
these reduce efficiency. Also, the cylinder pressure is lower while
the backpressure due to the ambient pressure remains the same. This
means that cylinder scavenging (getting rid of exhaust) is less
efficient. So, if you are seeking greatest efficiency, you need to
leave the throttle wide open.
Of course, the above assumes that we are operating the fuel/air
mixture at the most efficient as well. To extract maximum energy from
the fuel we must get complete combustion. That will only happen with
a mixture that is slightly on the lean side of peak EGT. (You need an
excess of O2 to make sure all the fuel molecules find O2 to combust
with.)
Now as for RPM, efficiency goes up when RPM goes down. Higher RPMs
produce higher frictional losses. Also, the higher flow rates in both
the induction and exhaust systems associated with higher RPMs reduce
efficiency as well. (The engine again experiences more pumping losses.)
So, you want to get the most "bang" for your gasoline "buck" the
solution is very simple:
1. Leave the throttle wide open. Climb to reduce MAP. This also gets
you a gain in TAS relative to dynamic pressure on the airframe (IAS).
2. Use the lowest RPM possible that will produce the desired speed.
3. Lean to a skosh lean of peak EGT. (Sorry, you can't do this in an
unmodified M14P.)
It appears to me that greatest efficiency would occur where you climb
to maximum altitude (engine producing only enough power to maintain
level flight at best L/D). That is certainly the most efficient
operating point for the airframe. I suspect that the engine will lose
enough efficiency by that point that one would get slightly better
efficiency at a little bit lower altitude. Still, going high is your
best bet for burning the least fuel possible on a long trip.
Brian Lloyd 3191 Western Drive
brianl AT lloyd DOT com Cameron Park, CA 95682
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
PGP key ID: 12095C52A32A1B6C
PGP key fingerprint: 3B1D BA11 4913 3254 B6E0 CC09 1209 5C52 A32A 1B6C
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