Frank Stelzer did this first in 1961. Only he did it with opposed piston/cylinder units. Over the years, Frank built many prototypes, using his own funds. Being German in Germany, his plant was set up there.

Over the years, different magazines wrote up articles on his engine. Google or Youtube: "Frank Stelzer engine." Consider Wikipedia at https://en.wikipedia.org/wiki/Stelzer_engine.

Properly built, the Stelzer engine could be coaxed into doing something the Toyota engine probably could not. A Stelzer type engine could be built to harness the awesome forces of detonation, forces that could easily deliver 100 mpg on a standard mid-size car, forces that would normally destroy a standard piston engine because they are so powerful.

Is it any wonder that Frank Stelzer and his engine were ignored by the car companies (controlled by big oil)? And now that Toyota is making a half-hearted attempt, what is really behind their plans? Or is it all simply advertising?