On the gas engine front, Volkswagen is also developing homogenous charge compression ignition (HCCI) engines. HCCI offers the advantage of diesel efficiency with virtually no soot or NOx emissions but works best in a very limited operating range. HCCI engines are optimal for constant speed operations but don't do so well in transients. VW is working on a multi-mode design that would switch back and forth between compression and spark ignition, but that requires very sophisticated engine controls and variable valve control. A pure HCCI engine would work best in an application were it can run at constant speed like a series hybrid where it only drives a generator. Both of these engine types are still at least 5-10 years from production.
[Source: Automotive News - Sub. req'd]