Been reading and speculating about these pneumatic/hydraulic hybrid motors for years. Here's the French inventor with a air-piston engine and his evolving vehicle prototypes. An Italian engineer's very light air-rotary piston engine used in many applications. And an enigmatic but very interesting hydraulic engine called Gigadron drive. Good luck on your next-gen project. It would be very interesting to see.
Automotive - Gig high density thermo-hydraulic power for green, safe VTOL aircraft, land transport, electricity, air-con & atmospheric drinking water.