I do enjoy d4a. Here's another recent video that has me stoked: https://youtu.be/bwGQlf0y6zE

It's about a non-boxer opposed flat engine, which is an incredibly compact design. That makes it very flexible to package for range extension for hybrids.

I'm interested because I want to one day build an electric rail (light sports car), and I want to minimise battery weight. 80 horse of generation could get you 400 peak at the wheels, which would be quite a ride if you can keep under 700Kg...

Interestingly the site only shows 125cc 23hp versions. The video shows a 500cc version with a whole lot more power. I guess the 500cc version will be available to the public later.