There seems to be a simple vocabulary issue. There is of course a "maximum
power" for an engine in the sense that running it at even higher power
would melt the metal parts, or some such awful consequence. This is not the
sense of the "maximum-power" condition explored by Curzon and Ahlborn. To
put it very concretely, they asked what should be the temperature
differences between heat source and engine and between engine and heat sink
in order to maximize the power output for given temperatures of the heat
source and sink. The cross-sectional area of the thermal contact between
source and engine (and between engine and sink) determines the actual power
input and output in watts, which can be tiny, or just below the level that
would destroy the engine, or beyond the level that would destroy the
engine. Am I still missing something?
There are of course a host of practical engineering design issues. The
value of the insight of Curzon and Ahlborn is to sharpen the theoretical
limit on what can be achieved, as the Carnot limit does not give a very
good limit in the practical sense, since an engine that runs at zero power
isn't of much practical use.
I remember being told as a student that "practical engines don't come close
to achieving the Carnot efficiency" followed by some vague statement about
friction as an explanation. I was glad to see the role of thermal
resistance highlighted by Curzon and Ahlborn. Or to put it another way, the
inefficiency (below the Carnot limit) is due to entropy production in the
Universe. Running the Carnot engine doesn't increase the entropy of the
Universe, but running a nonzero-power engine does, due to thermal
resistance.