My first and hopefully last corral entry, and it's hypothetical
 

Wouldn't running variable 20something:1 (basically, maximum lean running) to 11:1 (rich max) AFM with a high compression ratio, electronic spark control, diesel-type exhaust scrubbing and multiport direct injection on the late side of the compression stroke (similar to diesel) have reasonable efficiency? I mean, it's running lean, so it is burning more of the fuel and it's probably producing way too much NOx... but would it burn less fuel per horsepower at part-throttle, where the throttle would be more open for the same power level?

I realise Honda did a less extreme version of this, and probably Chrysler too.

But this seems pretty #Unicorn to me.

It doesn't sound totally unicorn-ish at all.

Lean-burn works and is VERY fuel efficient, but EPA kills it because it doesn't play-well with catalytic converters and emits too much NOx.

you could still use a diesel-type exhaust system (2 way cat, and DEF system)

I'd rather use water injection instead of DEF.

If you combined a variable compression engine that uses a sliding piston in the head (to vary compression without affecting squish clearance) with a simulated atkinson-cycle (with VTEC engagement so you could still run normal cam timing in power mode) you could reap *huge* part throttle gains.

Remember the Prius engine is around 13:1 compression right now. I'm willing to bet that the only reason it isn't 15:1 (for better efficiency on a highway cruise) is because it would ping at full power (you could close the intake valve even later, but that would reduce max power further). With a variable compression engine and atkinson-cycle you could eliminate that shortcoming.

You would always be running the highest expansion ratio possible.

But that just mixed three patents from major companies that don't want you using their tech...

I'd like to demand data from God too, because he hasn't proven he exists yet.

Anyway... I had an even dumber idea involving the combination of a jet engine and a steam engine (using the jet engine to boil for the steam engine, and any waste exhaust, of which I hope there will be a lot, will be dumped into a recovery turbine attached to a 3 speed automatic transmission... the steamer would be run through a 5 speed manual)

I suspect the fuel economy would be horrific, and so would the emissions (NOx from turbo-steam-shaft boiler, water vapour and blowby from the steam engine crankcase, though the vapour should be fairly tame and condensable)

Sounds like you just need a diesel engine.
Why reinvent the wheel?

because i want gasoline, i want steam, i don't want a lungful of dieselbacco smoke (though I like diesel engines, I don't like the PM1 soot in Euro 6 dzl or PM25 in noncompliant dzl)

Water injection to mitigate pre-ignition associated to some extreme lean-burn would give you both the gasoline and the steam you want :D