A new spark ignition could make hybrids (and others) burn cleaner than ever before. Honda is currently working on implementing the technology developed at the EPA, Stanford and Berkeley.
According to the site at Berkeley: "Unlike a traditional S.I. or Diesel engine, Homogeneous Charge Compression Ignition (HCCI) combustion takes place spontaneously and homogeneously without flame propagation. This eliminates heterogeneous air/fuel mixture regions. In addition, HCCI is a lean combustion process. These conditions translate to a lower local flame temperature which lower the amount of Nitric Oxide (NOx) produced in the process. NOx is a gas that is believed to be responsible for the creation of ozone (O3)."
Source on Honda implementation:Autopia
In October, the Wall Street Journal reported (as quoted by the Green Car Congress) that Honda was working on an HCCI engine, which the company confirmed to me but would not offer further details.