SACRAMENTO, Calif. -- The California Air Resources Board (CARB) pledged to rid the state of diesel-engine school buses and replace them with Compressed Natural Gas-powered buses. The board cites cleaner emissions on those buses, but are they really cleanier?

Navistar's Dan Herman said while that may be true when compared to the diesel engines of 20 years ago, it is certainly not the case today. Herman compares the emissions of a typical diesel engine vs. a typical CNG engine, breaking down the types of emissions and the amounts of each generated by both engines.

The levels of hydrocarbons (HC) emitted are equal, the CNG emits more carbon monoxide (Co), slightly less oxides of nitrogen (NOx), and slightly more particulate matter (PM).

The analysis, Herman said, shows there is no validity to the consistent claims CNG engines are cleaner than today's diesel engines.