Homogeneous Charge Compression Ignition, or HCCI, holds out a great deal of hope for green automotive enthusiasts. Mercedes-Benz has shown concept vehicles using the technology, which they refer to as DiesOtto, in their F700 concept vehicle pictured above. Like diesels, HCCI engines eschew spark-plugs in favor of compression ignition. Unlike diesels, HCCI burns gasoline, which could make them a popular choice in countries like the U.S. where gasoline is more widely available than diesel (and, currently in most areas, cheaper). HCCI engines could potentially allow for the emissions levels of gasoline with the efficiency of diesel and could be cheaper to produce than a comparably-powered diesel engine.
Over the hump to Number 5.