Nutation is what a coin does toward the end of its motion when you spin it, when it wobbles around the edge, and rotates at the same time.
The Nutating engine is similar to a Wankel rotary, only instead of having a triangular rotor that orbits and spins around the crankshaft, it has a disk that wobbles as it rotates (bottom pic for animation).
The military is funding the development of an aluminum prototype, which promises to double (or more)the power to weight ratio for small engines, where turbines lose much of their advantage.
Right now, they are looking at a power to weight ratio of 1.6:1 or so, which is a pretty remarkable number.
A NASA study (PDF) is also available.
looks like the inside of a Badger meter water meter
Not surprising.
Basically, any pump is like a motor, and fluid meters function like hydraulic motors.
Research the history of the engine – starting in the 1600s in England – and you'll find that the water meter
was derived from it. Same principle.
Fascinating design; but, still hopelessly inefficient, in most applications, due to thermal energy loss.