[etskinner]

I too was frustrated that they didn't say what the fuel was. It led me to believe that it's ethanol too. They just didn't want people to see the article as the nothing burger that it is.

I looked at several other sources covering this and they're all similarly quiet about what the fuel actually is. Looking at they supplier's (Coryton's) website, they boast about 'bespoke' synthetic fuels. So I'm guessing it's Mazda that's being shy about the details.

[hasmanean]

Yup. The "innovation" here is in the car, not the fuel.

There is another engine design, a linear piston engine, which is just a piston connected to a magnet and coil. The engine generates electricity (at a constant speed, constant load) and this is fed to an electric motor. So the car is a hybrid. It's more efficient than a regular engine (which turns a large crankshaft etc.) and the fuel probably can be more finicky since the combustion cycle can be tuned for one operating point.

https://en.wikipedia.org/wiki/Free-piston_engine