Check out page 7 of this pdf:

http://www.eere.energy.gov/vehiclesa...deer_duffy.pdf

There are still some problems with HCCI (homogenenous charge compression ignition), but the people developing them think they can be solved fairly easily with the next generation of intake / exhaust sensors.

The major people working on diesel HCCI are the Japanese manufacturers (inc Toyota). Should they succeed in cracking the other problems of HCCI, we could have diesel engines with lower NOx outputs than current spark-ignition gasoline engines.

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm