Across all types and sizes, emissions from new diesel engines now achieve near-zero levels. As a result, diesel emissions are a declining portion of the emissions inventory, and the increasing use of advanced diesel engines contributes to clean air progress.

Meeting national clean air goals for fine particles and ozone depends on reducing emissions from the many sectors that contribute. Thanks to the introduction of cleaner fuels and near-zero emissions diesel technology over the last two decades, emissions from diesel engines are small and declining contributors to the emissions inventory. 

Fine Particles (PM 2.5)

Fine particle emissions come from many sources, including naturally occurring ones such as forest fires, road and agricultural dust, as well as man-made ones. These can include commercial cookstoves, fuels like natural gas and coal used in power generation and residential heating and mobile sources - cars, trucks, buses, airplanes, locomotives, marine vessels and off-road equipment. Fine particle emissions also come from brake dust and tire wear.

According to the latest emissions inventory published by the U.S. Environmental Protection Agency (U.S. EPA) in 2017, mobile sources account for just under 5% of the fine particle emissions inventory. Wildfires are the largest source of emissions making up 43% of the inventory.

Thanks to the introduction of cleaner fuels, advanced engine technology and particulate filters, fine particle emissions from diesel engines have declined by over 230,000 tons between 2008 and 2017. 
Within the variety of diesel applications, emissions form heavy-duty diesel trucks and buses have fallen the most. Impressive reductions between 2008 and 2017 have also been achieved from other applications like equipment and marine vessels as more older technologies have been replaced with more modern and cleaner diesel technologies. 

According to our most recent analysis of 2021 U.S. Vehicles in Operation TIPNet Data (Class 3-8 vehicles, Model Year 2010 and newer) provided by S&P Global Mobility, 53% of all diesel commercial vehicles are now of the newest generation that incorporates technology to achieve near-zero emissions for nitrogen oxides (NOx) and particulate matter (PM). 

As more of the fleet transitions to advanced diesel technology, we can expect further reductions in particulate matter. Similar control technologies are integrated into off-road machines and equipment along with engines that power marine vessels and locomotives. As more of these older diesel engines are replaced with new cleaner models, we can expect further reductions in fine particle emissions from diesel sources.

(U.S. EPA National Emissions Inventory 2017, latest available)

Oxides of Nitrogen (NOX)

Nitrogen oxides (“NOx”) is a category of nitrogen-based compounds that contributes to ozone “smog” formation. Sources of NOx emissions are many and varied. The EPA identifies 56 sources of NOx ranging from soil compost and industrial sources to mobile sources including cars, trucks, locomotives, airplanes, and equipment. 
According to the EPA’s most recent national emissions inventory, NOx emissions have fallen by 38% percent nationwide over the last decade. As a single category, coal fired power plants have contributed the most to falling emissions as many of these facilities have been retired and replaced with natural gas units or transitioned to renewable sources of power generation. Yet the reductions from the diesel sector have eliminated twice as many NOx emissions as from coal power plants.
(U.S. EPA National Emissions Inventory 2017, latest available)

Advanced Technology Diesel Engines: Achieving Near-Zero Emissions

Diesel technology has undergone a significant transformation from 2000 – 2014, with the introduction of ultra-low sulfur diesel fuel (2007/2010) and new emissions control technologies.  As a result, since 2011 for commercial vehicles like trucks and buses and since 2014 for off road engines and equipment have reduced emissions by over 98%.


For commercial vehicles, comprising everything from large pickup trucks to tractor-trailer size Class 8 trucks along with school and transit buses, relative to previous generations of technology, the latest near-zero diesel emissions options reduce fine particle and NOx emissions by 98%. This has resulted in substantial clean air benefits. 

According to research commissioned by the Diesel Technology Forum, heavy-duty diesel commercial vehicles powered by 2007-2020 MY engines has resulted in a reduction of 27 million tonnes of NOx, 1.6 million tonnes of PM and 202 million tonnes of CO2.

In its  Advanced Collaborative Emissions Study, the Health Effects Institute found that heavy-duty diesel truck engines manufactured to meet the model year 2010 emissions standard were delivering NOx emissions 60% below the maximum allowable standard and particulate matter emissions 90% below the standard.

The Next Chapter for Commercial Trucks Even Nearer-to-Zero

Through the EPA’s Cleaner Trucks Initiative, future diesel engines will be even nearer to zero. The proposed rule is expected to be finalized in 2022 and will establish future emissions standard levels for nitrogen oxides and other emissions along with new durability and testing requirements for manufacturers.  


Diesel engines and fuel power more than 95% of commercial vehicles and off-road equipment from construction, agricultural, warehouse and mining equipment, locomotive and marine engines and mobile and stationary generators. 

Technology developed to achieve near zero emissions in commercial vehicles have also been deployed in of off-road equipment. In 2014, most off-road equipment had to meet the latest emissions standards known as the Tier 4 standards. As of 2015, most large engines typically found in locomotives, marine applications and industrial engines must meet these standards.

As a result, emissions have been reduced anywhere from 86% - 96% for both NOx and PM thanks to the same cleaner diesel fuel, along with advanced engine and emissions control technology. 

Recent research confirms that replacing older engines in much larger applications, including locomotives and marine vessels, can yield substantial fine particle and NOx emission reductions. A single engine replacement for a marine vessel can reduce 15 tons of NOx emissions and is equivalent to taking more than 30,000 cars off the road for a year.


How Advanced Diesel Technologies are Delivering Clean Air to the Nation's Ports

Today 53% of commercial trucks on the road across the country achieve near zero emissions for fine particles thanks to emissions particulate filter technology.  

In 2005, trucks in the Ports of Los Angeles and Long Beach were the second leading source of all PM 2.5 emissions after ocean going vessels. In 2019, port trucks were the second smallest source of PM 2.5 emissions after cargo-handling equipment, according to emissions inventory data released. Over this period, fine particle emissions from port trucks fell more than 90% as a result of requiring that only 2007 and newer model year vehicles be used to service the port. In 2019, trucks serving the Port of Long Beach accounted for just 7 tons of fine particle (PM 2.5) emissions, down from 186 tons in 2005, thanks largely to the introduction of new technology diesel trucks even as cargo volumes in the port have expanded by 14%.

The benefits were created by the 90% of the trucks powered by diesel engines with the remainder powered mostly by natural gas. The benefits these trucks generate for near-port communities offers an interesting case study confirming the benefits of near-zero emissions diesel trucks.

Recent research confirms that replacing older engines in larger applications, such as locomotives and marine vessels, can yield substantial NOx reductions. For example, a single engine replacement in a marine vessel can reduce 15 tons of NOx emissions a year and is equivalent to taking more than 30,000 cars off the road for a year.