The news source for this tidbit is not called “Yahoo” for nothing.
You want REALLY clean-burning Diesel engines? Fuel them with Compressed Natural Gas.
In fact, CNG should be much more widely available than it now is, for both spark-ignition and compression-ignition vehicle engines.
That aside, urea-injection exhaust systems do a very good job of cleaning up NOx emission, they are widely used on light-duty pickup trucks and on Mercedes Diesels sold in this country.
I still believe that a lot may be done using either a fully recirculating steam power unit with a condensation unit, based on the Abner Doble designs of the 1920’s.
Without researching further, I'd guess these are the non-urea engines that have been the subject of speculation because most other diesels these days require urea injection.