Can I just add one thing to that diatribe - please:

> The thermodynamic facts to take way here are:
> 1) An ideal compressor is Isothermal (no heat addition during compression)
> 2) An ideal Engine is Adiabatic (no heat loss during compression)

and 3) The theoretical efficiency of a heat engine is a function solely of
one thing -the difference between the starting and finishing temperatures.
Any analogies you can think of?

So aircraft cruise as high as they can (ATC permitting), as low intake temps
are vital for efficiciency, regardless of the low air density and pressure,
which is in fact an aerodynamic boon. Their intakes design and then
efficient compressors can (easily) make up the pressure, especially when
starting with very low temperatures.

Clive Smith