"Guy King" wrote in message
...
The message ews.net
from "Doctor Drivel" contains these words:

In co-axial flues the incoming air is heated by the exhaust air. Two pipe
flue systems are more efficient as the incoming cold stays cold until
burnt.

On large trucks there were experiments to us heat pumps to cool the
incoming
air. The larger the engine, and more fuel used, the more cost effective
these things are.

I'm fairly sure that the coolth of the incoming charge is only of
consequence on an expansion engine, where the idea is to get the
smallest most dense charge into the engine, then expand it to the
maximum amount practicable to extract the most energy possible.

Yep.

Boilers are different

Not really, although external combustion is far more efficient than internal
explosions.

- provided the fuel is fully burnt at the
stochiometric point or better (leaner),

A pre-mix fan can only deliver a ratio of air/gas because of the fan design.
Put more oxygen insthe air and more efficient combustion. Cold air is denser
containing more oxygen.

then all the thermal energy will
have been liberated and now the task
it to extract as much as
practicable.

All the sensible heat has been liberated, not to recover the latent heat.

This means that the closer the
exhaust temperature is to
ambient (above, of course) the
more energy has been retained for useful
purposes.

It means more latetent heat has been extracted

If you look at the design of boiler
systems for power stations you'll
see that they take great trouble to
preheat the imcoming air for
combustion to the maximum practicable.
This is a much closer analogy to
a domestic boiler than an internal
combustion engine.

It depends on the engines being used. Turbines are different again.