Yet another regenerative braking strategy. The most proven method is electrical. Hrench suggested a flywheel. There are also the options of compressed air or even a spring. This idea uses compressed air, but differently than the conventional way. Instead of just using the pressure of the air to push a piston, you're suggesting using it as a supercharger. This might be possible, but it seems like it would require some pretty tricky engine modifications.

Normally, when you let up on the accelerator pedal, the engine is throttled. This doesn't cut off the fuel perse, it cuts down the air flow into the engine. Basically, the cylinder pulls a partial vacuum. The fuel/air mixture that enters the cylinder is still the normal ratio, there's just less of both.

What you would need to do to make your idea work would be to cut off the fuel, open the thottle, and provide a way for the air compressed in the cylinder to flow into a tank. One way would be to add an additional valve to the cylinder head that opens during the compression stroke. This valve could feed the air directly to the tank. The problem is the existing valves take up pretty much the entire face of the cylinder head, so there isn't much room for another valve. Another way would be to use the existing exhaust valve and design a system to shut the outlet of the exhaust manifold and vent the air compressed in the manifold into the tank. This might be workable, I don't know. There might be other ways too.

Superchargers still require fuel. In fact, in most cases superchargers need to run a bit rich to work well. Thus, it might not save fuel. However, engines do use a significant amount of energy to draw air into the cylinders, so using the compressed air to assist the air intake might actually help you get normal power with less fuel. And of course the supercharger effect could boost horsepower to above normal, though it will waste some fuel.

What I'm suggesting is really for fuel-injected engines. When the brake pedal is depressed (as opposed to when the foot is lifted off the throttle pedal) the fuel injector for the candidates cylinder(s) would be closed. Maybe there could be a bypass (like the idle air bypass on many cars) for the candidate cylinders that reroutes intake air around the closed throttle valve so the those cylinder(s) are not drawing against a vacuum or partial vacuum on the intake stroke. I am not suggesting modified cylinder heads to handle the exhaust- I think it should be doable with the existing exhaust valve. Just have some way of redirecting the "exhaust" (which equals compressed air) downstream of the exhaust valve, to the compressed air accumulator. After thinking about this a bit more on my drive home yesterday, I suppose another different option would be to drive a small air compressor off the flywheel ring gear during braking. If that compressor drive could be made to only engage the ring gear during braking there would be no drag/downside during normal operation. If that is impractical, maybe the drag could be minimized by using some sort of clutch like that on the A/C compressor to engage/disengage the drive. Imagine a starter motor in reverse with the much larger flywheel driving the much smaller gear during braking. That thing would spin very fast.

Surprisingly, this system seems to be half-designed when you look at diesel truck 'Jake' brakes. They operate a separate exhaust valve at the top of the compression stroke to exhaust the compressed air so there is no air-spring effect and the energy is removed, wasted. Diesel trucks have no throttle plate, so when the accelerator is released, there is no vacuum to cause engine brakeing.

If you could figure a way to capture that wasted compression--right now it goes out the exhaust system as a huge machine-gun sound.

Other companies offer exhaust compression brakes where the whole exhaust system is just throttled, but it's hard on engines and doesn't provide nearly as much braking. But again, they waste the energy.

http://en.wikipedia.org/wiki/Jake_brake