And what of the Stirling?
“What about the Sterling Engine?”
The Sterling Engine - by definition - is an external combustion engine. The advantage - and about the only one - is that any fuel can be used, whereas for INTERNAL combustion, the range of fuels is more limited.
Since there is the heat conduction issue - from external combustor to the working fluid - the power per unit weight/volume is low. The internal combustion engine does not have this heat transfer issue and therefore more powerful than the Sterling Engine. In the past 30-40 years, there has been some experimental sterling-engine cars built but their performance were lacking.
There are some persistent Sterling Engine groupies around and I am amused.
The new disc gas engine combines the compression stage typical of a conventional gas turbine with the combustion/expansion stage of a rocket engine.
I do not see the new disc engine being that much efficient than the conventional gas turbine.
There are 2 goals in engine design: efficiency and power. The greater the power (work per unit time), the less energy efficient; and vice versa.