[DUMBGRUNT]

—” Their thermal efficiency goes UP with the colder environmental temperature. “

Yes, a nice Delta-T and all that...

On an ICE most of the energy is lost to the atmosphere.

But... The hot-vapor engine did all this running unheard of high temperatures at an extremely lean air/fuel ratio, in seeming violation of accepted internal-combustion-engine physics.

https://www.motortrend.com/how-to/hrdp-1009-what-ever-happened-to-smokeys-hot-vapor-engine/#:~:text=Baker%20fully%20explained%20how%20Smokey,12%20to%2013%20seconds)%2C%20and

Smokey Y. was a true gearhead guru.
Some of his exploits will be remembered for many generations.

[ProtectOurFreedom]

Good old "Smokey" Yunik. He wrote great monthly articles for Popular Science magazine for years! I vaguely recall his "hot vapor" engine.

It makes me laugh a big because, as a power engineer, I worked with many regenerative air heaters. These are HUGE machines in power plants that capture heat from the exhaust flue gas to preheat the combustion air going into the boiler.