The City of New York Department of Transport owns 1,280 buses and moves 114 million passengers annually, but the buses are actually operated by seven private companies. The department started a Clean Fuel Bus Program in order to migrate to being the cleanest bus fleet in the world. They wanted to reduce emissions, improve service and reliability, and reduce cost. Their plan was open to all technologies, and looked at replacing or retrofitting the diesel bus fleet with new technologies. The three main paths are expanding the use of CNG buses, buying hybrid buses, and getting clean diesel buses.
In service they found that the CNG buses were less fuel efficient than the diesel, less reliable and needed more service. The hybrids performed smoothly and performance could be customized in software. They did find that the batteries needed to conditioned periodically and more work was needed on the control software. The clean diesels eliminated most of the emissions but required low sulfur diesel fuel. Comparisons of emissions between the CNG and clean diesels generally favored the diesels. The two had similar particulate emissions and the diesel was much better in CO, HC and Carbonyl emissions. The CNG only had an advantage in NOx emissions, but not by much.