“soon a really radical design called homogeneous combustion compression-ignition ... 50 mpg”
I looked that up. The HCCI concept achieves its efficiency exactly as the diesel does: by a higher compression ratio.
So why should we use HCCI rather than just use diesels? HCCI engines, like diesels, need to be built stronger and heavier than the spark-ignition engines.
But I've read a few things on HCCI and one thing they're considering is using aerospace-quality ceramics for the hottest parts of an HCCI engine, namely the piston and piston liner. That way, you don't need an over-heavy engine block.
Anyway, I still remember in the early 1980's when sophisticated engine computers, four-valve per cylinder valvetrains, and port fuel injection were "pie in the sky" ideas that everyone thought were engine improvements with diminishing returns. Today, practically every NEW car has engines sophisticated engine computers, port fuel injection, four-valve per cylinder valvetrains and even variable valve timing (indeed, my 1998 Honda Civic HX coupe has all four of these technologies!). I expect the HCCI--which may cut fuel consumption 20% or more compared to today's engines--to be widely available by 2015-2016 time frame.
By the way, one big advantage of HCCI over a diesel engine is the fact cleaning up the exhaust emissions is much easier, since you don't have to deal with elevated NOx levels and diesel particulates, both of which can be quite expensive to remove in the exhaust stream.