I think the best technological implementation of a turbo to improve fuel economy would create one hell of a complicated engine.
First, take a small atkinson cycle engine - 13:1 CR with an effective 8:1 CR. Use a small, efficient turbo ducted through an intercooler to raise the effective CR to ~10:1 (i.e. miller cycle). Use variable valve timing to increase the CR to 13:1 w/ turbo (which should already be operating in its most efficient range) and switch the injectors over to feed from a smaller tank holding 91 - 100 (avgas?) octane fuel during acceleration. Route the spent gasses through the primary turbo and feed the residual gas into a turbocompound that sends power back into the flywheel.
Ofcourse, the impact on backpressure will have to be balanced with gains, but it's an orgy for the mind
. The coolest implementation I have seen so far is the Napier Nomad engine, even though it's a two stroke. Essentially, a diesel engine is mated to a turbine that drives both a compressor and a CVT attached to the crankshaft. During light cruise, the turbine powers the efficient axial compressor and feeds residual energy back into the flywheel. During takeoff when extra power is needed, fuel is dumped into the turbine making a temporary (and inefficient) turboshaft engine.
Link:
http://en.wikipedia.org/wiki/Napier_Nomad
- LostCause