He told me that the new generation diesel engines equal the emissions of the CNG powered buses
Thanks to the latest diesel engines going to pressurized common-rail direct fuel injection and the use of advanced emission control systems to remove harmful NOx and diesel particulates, the latest diesel-powered buses emit a tiny fraction of the air pollution of older buses. And companies like Cummins, Catepillar, Daimler-Benz, etc. are working on even more advanced technologies that could make diesel engines eventually meet the even more stringent EPA Tier 2 Bin 3 and CARB AT-PZEV standards for exhaust emissions by 2011, an amazing engineering feat.
He must have lousy mechanics ... two of the fleet managers (both small commercial operations) that I know rave about how NG is less wear and tear on their engines and require less maintenance.