What is Clean Diesel?


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About Clean Diesel

Public Transportation

Because of its safety, reliability and efficiency, diesel is the predominant power source for public transit, school and intercity bus services nationwide. Among public transit agencies, diesel and diesel-hybrid buses account for about 75 percent of the national fleet.


According to the American Public Transportation Association (APTA), more and more Americans are taking public transportation. In fact, ridership is the highest it has been since 1956 and has been growing since 1995. In 2014, Americans took 10.8 billion trips on public transportation to get to work, to school and to neighboring cities.

America’s transit bus fleet is also one of the newest and cleanest heavy-duty fleets around. The new clean diesel bus technology of today is the result of an interconnected system of clean fuels, advanced engine design and exhaust or aftertreatment technologies working together to reduce emissions to near-zero levels. The latest federal standards virtually eliminate emissions from new diesel buses, reducing particulate matter (PM) and nitrogen oxides (NOx) by 98 percent from 1988 levels resulting in significant clean air benefits by reducing emissions of particulate matter and ozone forming compounds to near zero.

In 2013, 77 percent of transit buses were powered by diesel engines and fuel, or diesel hybrid engines. Of that, almost half were powered by a diesel engine that met or exceeded the first clean diesel standard for model year 2007 and another 27 percent of buses were powered by an engine that met or exceeded the stricter emission standard for model year 2010.

Many cities have begun using the latest in diesel-hybrid technology to allow their transportation systems to be not only more efficient but also more environmentally friendly. Also, a growing number of transit districts are incorporating the use of renewable biodiesel fuels into their diesel bus fleets, further improving their environmental and climate sensibilities.

New technologies are also being used to upgrade (or "retrofit") older diesel engines. Retrofitting devices are reducing key pollutants from existing bus fleets by up to 90 percent. Take a minute to learn more through Why Retrofit.

Clean Diesel's Advantage

Clean diesel buses offer significant operational advantages over many alternative fuels, and assure reliable, durable and cost-efficient bus transportation.

A community will get more clean air for the dollar with a clean diesel bus fleet compared to CNG. Clean diesel buses are 20 to 25 percent less expensive than CNG buses, and do not depend on the separate fueling infrastructure required for CNG. Buying new diesel buses and retrofitting older buses allows transit agencies to convert a greater portion of fleets to clean diesel in order to meet state emissions requirements.

An analysis by the Clean Air Task Force illustrated the major emissions gains clean diesel buses have achieved. The analysis shows the air quality benefits of replacing older buses with newer clean diesel technology and a comparison of clean diesel and CNG buses.

 2012 Clean Diesel Bus & 2012 CNG Bus Emissions Comparison
(Vs. Model Year 2000 Diesel Bus)
Vs. 2000 DieselNOxParticulate MatterHydrocarbon
2012 Clean Diesel -94% -98% -89%
2012 CNG -80% -99% -100%

Source: Clean Air Task Force – “Clean Diesel versus CNG Buses: Cost, Air Quality & Climate Impacts”

Recent 2014 and 2015 clean diesel bus orders include:

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