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It would require a positive displacement supercharger that used no reciprocating parts and had super lightweight components.
A fan won't do the job. It might work with a rotary vane pump. Variable displacement would work even better. On the same shaft add the steam engine driven by exhaust gas residual heat.
Both of those energy sources are from the same energy that is normally wasted in a conventional setup. BMW was working on a steam powered accessory drive system that used exhaust heat. They claimed a 15% mileage improvement.
67% (EPA figures) heat energy losses in a typical passenger car would provide a lot of power for the initial acceleration to reduce the dramatic amount of fuel lost in those circumstances.
Additional electrical loads could be compensated for by increasing the alternator charging load on any DFCO event, which would save friction brakes.
I doubt any forced induction will make much of a difference in mileage, unless you had a smaller displacement engine. Ford is going that route with their eco-boost engines, if I understand their developments correctly.
Take the average pickup truck, SUV sized vehicle and use a 2.5 liter diesel engine. Maybe even 2 liter. Get it to produce 200 HP and good torque when you really need it, but also give you great higher load BSFC cruise capability at highway speeds.
I think that is the way they will get those full sized vehicles up to the 30-35 MPG range.
Just thinking out loud.
regards
Mech
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