Clean Diesel versus CNG Buses: Cost, Air Quality, & Climate Impacts

The analysis finds that while CNG buses offer long-term fuel cost savings and lower long-term GHG emissions (GWP100), they have higher upfront costs and higher short-term climate impacts (GWP20) due to fugitive methane emissions. In capital-constrained environments, new diesel buses may provide greater fleet-wide reductions in criteria pollutants because more units can be purchased and older buses retired per dollar spent.

• CNG transit buses have higher initial costs, averaging $70,000 more per bus than diesel equivalents, plus fueling infrastructure costs of $25,800 or more per bus. However, they offer annual fuel savings of over $11,000 per bus, leading to a pay-back period of five to eight years and lifetime net savings of $50,000 - $80,000 per bus over a 12-15 year lifespan.
• Both new diesel and CNG buses significantly reduce NOx, PM, and HC compared to model year 2000 diesel buses. On a per-bus basis, new CNG buses provide slightly better reductions in PM and HC, while new diesel buses provide better reductions in NOx.
• In capital-constrained scenarios, purchasing new diesel buses can be more effective for air quality because the lower cost allows more older buses to be retired. For every $10 million in funding, an agency can buy 26 diesel buses versus 21 CNG buses, resulting in fleet-wide reductions of NOx, PM, and HC that are 47%, 23%, and 11% higher, respectively, when choosing diesel.
• The climate impact of CNG buses depends on the global warming potential (GWP) time horizon used. Using GWP100, new CNG buses have 5% lower wells-to-wheels GHG emissions (3,655 g CO2-e/mile) than new diesel buses (3,840 g CO2-e/mile). However, using GWP20, CNG buses have higher emissions (4,643 g CO2-e/mile) than new diesel buses (3,981 g CO2-e/mile).
• Fugitive methane emissions from natural gas production are a critical driver of the climate impact of CNG buses. If fugitive methane leakage from production were assumed to be zero, CNG bus GHG emissions would be 16% lower than new diesel buses under GWP100.

APA

Lowell, D. (2012). Clean Diesel versus CNG Buses: Cost, Air Quality, & Climate Impacts. Clean Air Task Force. https://cdn.catf.us/wp-content/uploads/2019/02/21093821/CATF_Pub_Diesel_VS_CNG.pdf

Chicago

Lowell, Dana. Clean Diesel versus CNG Buses: Cost, Air Quality, & Climate Impacts. Clean Air Task Force, 2012. https://cdn.catf.us/wp-content/uploads/2019/02/21093821/CATF_Pub_Diesel_VS_CNG.pdf.

Wikipedia

{{cite report |last1=Lowell |first1=Dana |title=Clean Diesel versus CNG Buses: Cost, Air Quality, & Climate Impacts |publisher=Clean Air Task Force |date=22 February 2012 |url=https://cdn.catf.us/wp-content/uploads/2019/02/21093821/CATF_Pub_Diesel_VS_CNG.pdf |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{lowell2012clean, author = {Lowell, Dana}, title = {{Clean Diesel versus CNG Buses: Cost, Air Quality, \& Climate Impacts}}, institution = {Clean Air Task Force}, year = {2012}, month = feb, url = {https://cdn.catf.us/wp-content/uploads/2019/02/21093821/CATF_Pub_Diesel_VS_CNG.pdf}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }