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 84 percent of the national fleet.

According to the American Public Transportation Association (APTA), more and more Americans are taking public transportation. In 2019, Americans took 9.9 billion trips on public transportation to get to work, to school and to neighboring cities. More and more cities are adding new routes to help meet the demand to move people.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 2020, 84 percent of transit buses were powered by diesel engines and fuel, or diesel hybrid engines. Of that, about 44 percent diesel buses are of the latest clean diesel technology powered by an engine that meets strict emission standard for model year 2010 and later.

 2020 transit bus stats infographic

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.

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 alternatives including CNG. Clean diesel buses are 20 to 25 percent less expensive than CNG buses, and two or three times less expensive than all-electric models. Clean diesel buses do not depend on the separate fueling or recharging infrastructure required for alternatives. Replacing older buses with new clean diesel buses can help transit agencies provide the most emission reductions for the communities they serve while providing reliable and low cost transit needs.

Recent research demonstrates that the clean diesel option reduces far more emissions than available alternatives thanks to clean diesel’s cost advantage. As the table below shows, for a $10 million investment in new buses, a transit district can replace many more older and higher emitting buses with the clean diesel option and generate greater emission reduction than with expensive alternatives. 

How to Make the Most of a $10 Million Investment for Immediate NOx Reduction

    Price Per Application # of New Buses Placed into Service Anticipated NOx Reduction Per Year Per Project Cost to Remove Each lb of NOx ($/lb) Total NOx lbs. Reduction Per Year
Replacing a MY 2000 Bus with...            
Clean Diesel   $370,000 27 1,062 $348 28,703
CNG   $460,000 22 925 $497 20,109
Hydrogen Fuel Cell   $1,200,000 8 1,162 $1,033 9,683
Battery-Electric   $880,000 11 1,162 $757 13,205


Source: (1) "Clean Diesel Versus CNG Buses: Cost, Air Quality and Climate Impacts" Clean Air Task Force (2012). (2) "From Deceit to Transformation: How Connecticut Can Leverage Volkswagen Settlement Funds to Accelerate Progress to a Clean Transportation System, CONN PIRG, January 18, 2017. (3) "Consortium to Fund New Flyer Hydrogen Buses to ACTransit", Passenger Transport, February 24, 2017.



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