Public Transit Buses: Diesel or CNG?

A Clean Air Task Force (CATF) study indicates that new CNG buses are not necessarily superior to new diesel buses regarding air quality or climate impact. While CNG buses may have marginally lower particulate matter and volatile organic compound emissions, they can have higher nitrogen oxide and greenhouse gas emissions. Furthermore, the higher purchase price of CNG buses may limit the number of older, high-emitting vehicles retired per dollar of capital funding, potentially resulting in lower fleet-wide emission reductions compared to a diesel fleet update. The climate impact of CNG is heavily dependent on methane leakage rates during production and the choice of Global Warming Potential (GWP) timeframes (GWP20 vs GWP100). Despite these factors, low fuel costs and federal funding structures incentivize transit agencies to choose CNG.

• A study commissioned by CATF found that new CNG buses are not inherently better for climate impact or air quality than new diesel buses; CNG buses may have higher nitrogen oxide and greenhouse gas emissions, despite marginally lower volatile organic compound and particulate matter emissions.
• Due to higher purchase costs and lower efficiency, CNG buses may result in fewer older vehicles being retired per unit of capital funding. For every $10 million spent, an agency could buy 26 diesel buses versus 21 CNG buses, meaning fleet-wide reductions in PM, NOx, and HC could be 23%, 47%, and 11% higher respectively if diesel is chosen.
• The climate impact of CNG buses is highly sensitive to the assumed methane leakage rate from production and transportation, as well as the Global Warming Potential (GWP) metric used. Using GWP20 (short-term) favors diesel, while GWP100 (long-term) suggests CNG may provide 18% greater annual fleet-wide GHG reductions given the $10 million funding scenario.

APA

Schneider, C. (2012). Public Transit Buses: Diesel or CNG? Clean Air Task Force. https://www.catf.us/2012/03/public-transit-buses-diesel-or-cng/

Chicago

Schneider, Conrad. Public Transit Buses: Diesel or CNG? Clean Air Task Force, 2012. https://www.catf.us/2012/03/public-transit-buses-diesel-or-cng/.

Wikipedia

{{cite press release |last1=Schneider |first1=Conrad |title=Public Transit Buses: Diesel or CNG? |publisher=Clean Air Task Force |date=12 March 2012 |url=https://www.catf.us/2012/03/public-transit-buses-diesel-or-cng/ |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@misc{schneider2012public, author = {Schneider, Conrad}, title = {{Public Transit Buses: Diesel or CNG?}}, publisher = {Clean Air Task Force}, year = {2012}, month = mar, url = {https://www.catf.us/2012/03/public-transit-buses-diesel-or-cng/}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }