Hey! That seems like a really good idea.
There are the so called "air cars" that try to run a piston motor entirely on pre-compressed air that seem dubious to me, but using engine braking to store compressed air to be used as a "turbo boost" for acceleration seems like a much more practical idea. Getting an "in-town" 20%+ efficiency boost and saving the brake pads by way of computer control engine braking seems like a win-win without having to include batteries, inverter and eMotor to get the job done. I hope they get that out of the lab and into production, although pure EV is still the way to go...