PSA Peugeot Citroen, Bosch working on compressed-air, battery-free hybrid
I'm glad you mention something about 'heat expanding air'. I've been thinking about a series hybrid drivetrain comprised of a small turbine powered by hydrogen or whatever liquid/gaseous fuel and Di Pietro air engines. The turbine turns continuously on a fixed RPM and pumps air into canisters. The canisters act as batteries and when needed the air is channeled to the air motors. The exhaust heat from the turbine would heat the expanding air inside the air motors. You could even cool the turbine if you channel the air pipes around the turbine. Thermal efficiency should be pretty good compared to traditional ICE. Very simple system, uses no fancy materials (except in turbine maybe?) and should be lightweight and durable. So I'm happy to see PSA doing something like this.