The message
from The Natural Philosopher contains these words:

We know, that at idle, producing no actual acceleration, the powertrain
is necessarily 0% efficient.

WE suspect that, at full power, its less efficient than part power. So
the curve of efficiency is definitely sort of parabolic. Where IS the
most efficient part?


Intelligent guesswork suggests its not close to idle at all. There
frictional loses in the engine will be nearly all the losses.

It is more than 40 years since I did any thermodynamics and we didn't do
much on IC engines anyway but through the mists of time ISTR that an IC
engine is most efficient at max bmep (brake mean effective pressure
IIRC) and max bmep equates very closely to max torque.

One of the reasons I remember this at all is because it seems to
conflict with the age old advice that the way to save fuel is to get
into as high a gear as possible as early as possible.

--
Roger Chapman