Hybrid diesel-electric engines and compressed natural gas engines both offer significant benefits over current diesel engines in lower emissions and improved fuel economy, according to a year-long study of bus engines.

The Hybrid-Electric Drive Heavy-Duty Vehicle Testing Project put "state of the art" technologies through extensive testing. It was managed by the Northeast Advanced Vehicle Consortium and conducted by an independent team from M.J. Bradley & Associates and West Virginia University.
Particulate emissions from diesel-electric hybrids using low-sulfur diesel fuel were 50% to 70% lower than conventional diesel, while NOx emissions were 30% to 40% lower. The carbon monoxide emissions were 70% lower than a conventional diesel bus.
Particulate emissions from the CNG buses showed an 80% to 90% reduction from conventional diesel, and NOx emissions were 50% to 60% lower.
Hybrid diesel engines had a couple of advantages over CNG, according to the study. They offered 30% to 65% better fuel economy than conventional diesel, and as much as 100% over a comparable CNG bus when operated on severe duty cycles. The hybrid vehicle also put out significantly lower greenhouse gas emissions than either a conventional diesel or CNG bus, according to the report.
Los Angeles and other cities have been converting their entire bus fleets to compressed natural gas. New York City, however, has resisted. New York City Transit runs a fleet of more than 4,000 buses, the largest in the country. Ninety of them are powered by natural gas, and six are being tested with diesel-electric hybrid engines. Although natural gas powered buses are cheaper to buy than hybrid buses, it costs a lot of money to convert a city bus depot to refuel and service natural gas vehicles.
Environmental advocates criticized the study, saying it did not analyze the toxic qualities of the particulate matter.