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The turbine in turbine engines can face temperatures from the combustion chamber up to 1700ºC, but I can't find any information on the operational temperature of free piston turbine engines.

I'm interested on the subject because if free piston engines don't use the same absurd amounts of heat turbine engines normally use, then more conventional materials could be used for the turbine instead of super alloys.

Camille Goudeseune
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Fulano
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2 Answers2

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Any sort of heat cycle engine (Brayton - gas turbine, Otto - gasoline engine, Diesel, Carnot, etc) will have peak efficiency and power output with a higher peak temperature.

Consequently, all of these kinds of engines push towards higher temperature materials.

Note that modern piston engines require hardened valve seats (particularly exhaust valve seats) in order to handle higher temperatures -- as compared to early piston engines that could get away with soft valve seats (typically cast iron).

Higher temperatures is one of the most fundamental ways to make an engine better.

Rob McDonald
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    This is the key - engines are designed to run at the highest temperature they can handle because it improves efficiency. Turbines are only hotter than piston engines because it's possible to build turbine blades to be more heat-resistant than pistons. Any sort of device that reduces the input temperature of the turbine will also reduce efficiency. – Robin Bennett Jan 26 '24 at 12:45
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    @RobinBennett Counter example on [Water Injection](https://en.wikipedia.org/wiki/Water_injection_(engine)) In reciprocating engines, the air pressure at sea level has too much oxygen, which means you have to run the engine lean to prevent overheating. This reduces efficiency as the combustion is detonating early. Injecting water cools the engine, making early detonation less likely to happen, which increases efficiency – Ferrybig Jan 26 '24 at 14:20
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    This does not answer the question. I don't think "higher" is what was desired. – Pilothead Jan 27 '24 at 20:59
  • The question has been edited since my answer was posted -- it does seem a bit of a moving target. My point is that adding a lot of heat is the point of an engine -- that is how you get high specific power (a necessity for flight) and high temperatures are also a good thing - they're how you get high efficiency. I.e. There is nothing about free turbines that would push them to use lower temperature materials. If there were, then optimizing that design would drive the temperatures back up. – Rob McDonald Jan 28 '24 at 04:48
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For a 20 kW engine with a 20 L tank and a bore of about 10 cm, the tank's temperature should be about 300 K (isothermal) to 400 K (adiabatic, more practically), says p. 11 of an analysis by Sandia Labs, "Optimizing the Scavenging System for a Two-Stroke Cycle, Free Piston Engine for High Efficiency and Low Emissions: A Computational Approach," by Goldsborough and Van Blarigan, SAE Trans. 112(3): J. Engines, pp. 1-20.
400 K is 125 ºC, comfortably cool.

Newcastle U. investigated (unpaywalled) cylinder wall temperatures from 370 K to 435 K (p. 383, top) and a modified "low heat rejection design" with wall temperatures up to "50 per cent" higher (p. 384), thus at most 650 K. (Mikalsen & Roskilly, "The fuel efficiency and exhaust gas emissions of a low heat rejection free-piston diesel engine," Proc. IMechE (A): J. Power and Energy 223:379-384.)

650 K is 375 ºC, still cool enough for non-exotic piston materials, despite the in-cylinder gas temperature ranging from 400 K to 1800 K through a piston stroke (p. 383, Fig. 4).

But Fig. 8 shows that the steady-state exhaust gas temperature is 1100 K to 1140 K (870 ºC), so the turbine itself must be made of sterner stuff.

Camille Goudeseune
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