In the majority of scenarios for building sustainable mobility (decreasing emissions, increasing fuel efficiency, weaning from oil), there is a substantial short- to medium-term emphasis on advanced conventional technologies.
One of the promising engine technologies that has emerged over the past few years is called HCCI -- Homogeneous Charge Compression Ignition. To the right is a diagram comparing HCCI with spark ignition (your basic gasoline engine) and compression ignition (diesel). (Click to enlarge. Source.)
HCCI relies upon a very lean (high proportion of air to fuel) and well-mixed (Homogeneous) air-fuel mixture (Charge) that is compressed (Compression) until it autoignites (Ignition). The resulting spontaneous burn produces a flameless energy release in a large zone almost simultaneously -- very different than the spark/gasoline burn or the compression/diesel burn.
HCCI is a very efficient engine without the nasty NOx or PM emissions of a diesel. It can operate using a variety of fuels -- gasoline, diesel, natural gas, biofuels, hydrogen.
However, there are barriers to overcome -- such as reducing the amount of unburned hydrocarbons and figuring out the complex contorl mechanism. The latter is first a matter of understanding the dynamics of the chemistry and then figuring out how to best control it in different situations, with different fuels, etc. There is a great deal of research focus on HCCI, however, because the potential benefit in many different use cases is high. The Society of Automtoive Engineers (SAE) is hosting a special two-day HCCI symposium this August.
FreedomCAR & Vehicle Technologies
HCCI Lab at UC Berkeley