Ambient temperature air comes into the engine, it is heated with
the engine's heat recovery counterflow heat exchanger to a pre-
compression temperature, then compressed, then heated with
the
main energy source, then expanded to the pre-compression
temperature, then cooled at the heat recovery
counterflow heat
exchanger to ambient temperature, then compressed to ambient
pressure, then cooled with a counterflow heat exchanger with
outside cooling to ambient temperature.
This is an ideal heat engine along the lines of the Carnot heat
engine, but with several differences:
Limiting the pressure required for high temperature operation, so
less strength is required. High temperature operation is required
for
high efficiency, and this makes it more practical.
Preferring counterflow heat exchangers over constant
temperature
heat sinks and sources. Each direction of the heat exchanger
would
be constant pressure. A counterflow heat exchange works at a
range of temperatures, one side starting at the hottest and
changing to the coldest, and the other side going the opposite
way,
so rather than being sure of the exact efficiency based on the
hot
and cold temperatures as with a Carnot cycle, there is a hot
range
and a cold range, and the efficiency can be bounded between
the
Carnot efficiency for the coolest hot and warmest cold, and the
Carnot efficiency for the hottest hot and the coldest cold.
And two parts I haven't specified:
The main heat source can be applied at constant pressure (easier
with a turbine system) or constant volume (if it's a piston
system).
At the end of the cycle, the air is atmospheric pressure and
temperature. It can be exchange for fresh air, which is needed
for
internal combustion, or reused, which can work in other
situations
and maintains internal air purity.
The heating by the main energy source increases the internal
entropy, the cooling by outside cooling decreases internal
entropy,
all other steps neither increase it or decrease it in principle.
In addition to pistons and turbines there's one other option for
some
of the compressor and expander functions: travel within a
centrifuge.
If the gas is traveling along a pipe that leads from the centre of
the
centrifuge, to the rim, and back to the centre, it will be at
higher
pressure as it passes the rim than it is when it's in the centre.
If it travels to the rim, is heated, and travels back to the centre,
it
will be at higher pressure at the centre after heating than the
gas
at the centre before heating. So it'll need one of the other
expanders, such as a turbine, if you want it back to the original
pressure.