> The Wave Disk Generator uses 60 percent of its fuel for propulsion; standard car engines use just 15 percent. As a result, the generator is 3.5 times more fuel efficient than typical combustion engines.

I'm skeptical.

Thermodynamic limits apply. While piston engines aren't near the limits, I'm pretty sure that they're around 50% of the limit and would be very surprised if this engine is over 80% of carnot.

So, the only way this engine can be 2x as efficient is if it runs at a significantly higher temperature (which moves the limit).

And, the only way that it can run without a transmission is if it has a very wide operating range. That's hard to do with combustion. And, unless the engine is both reversible and has torque at zero RPM, they'll need both a clutch and a reverse gear.

If they use it to run a generator, they need to account for the electric motor, said generator, and maybe batteries. See diesel-electric locomotives. The prius combines the electric motor and the generator, but needs a transmission.