9. Reactivation of unconventional concepts

Some old technologies are once again in vogue as part of the new energy transition paradigm. These include, but are not limited to, opposed piston engines, free piston engines and, of course, the Wankel rotary piston engine [BM 2 593] . These original technologies are back in the limelight with innovations in design and hybridized operating modes, whether or not coupled with electric or pneumatic power, and they can use bio-based fuels as an alternative to hydrocarbons.