Half a croissant, on a plate, with a sign in front of it saying '50c'
h a l f b a k e r y
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Maximally efficient heat engine

Easier to build than a Carnot engine
  [vote for,

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.

caspian, Aug 19 2011

RoC hybrid RoC_20hybrid
My idea started out as a comment on the one linked here. [caspian, Aug 20 2011]

Vacuum engine Vacuum_20Engine
I since found this, which seems to exactly match part of my idea. [caspian, Aug 20 2011]




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