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Diesel’s Shrinking Share of Emissions Inventory

Diesel continues to serve as an efficient fuel source with near-zero emissions helping to provide clean air benefits to communities here at home and around the globe. Thanks to decades of collaboration between regulators and industry, these clean air benefits can be delivered today, as diesel is one of the fastest falling sources of emissions.


Diesel is a Rapidly Declining Share of National Emissions Inventory 

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 near-zero emissions diesel technology over the last two decades, emissions from diesel engines now are one of the fastest declining sectors, making the introduction of new technology diesel engines a key factor in improving air quality in every community.

Fine Particles (PM 2.5)

Fine particle emissions come from many sources, including forest fires, road and agricultural dust, commercial cookstoves, fuels in power generation, residential heating and mobile sources - cars, trucks, buses, airplanes, locomotives, marine vessels and off-road equipment. 

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 five percent of the fine particle emissions inventory. Wildfires are the largest source of emissions making up 43 percent of the inventory.

Diesel-powered vehicles and equipment are among the fastest falling sources of PM 2.5. 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.

Fine Particle Reduction

(2017, latest U.S. EPA data available)

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. 

More than half of the diesel commercial truck and bus fleet come with technology to achieve near-zero emissions of fine particles as required for new trucks manufactured beginning in 2007. 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.

Diesel Sources of PM 2.5

(2017, latest U.S. EPA data available)

Oxides of Nitrogen (NOx)

Nitrogen Oxides (or NOx) is a ground level ozone forming compound that contributes to smog. There are no fewer than 56 sources of NOx tracked by U.S. EPA, ranging from rotting plants and soil compost to the biggest sources of emissions - mobile sources including cars, trucks, locomotives, airplanes, and equipment. Combustion of fuel in the industrial sector ranks as number two. The top five leading contributors to NOx emissions make up about 95 percent of all emissions.

Top Sources of NOx

(2017, latest U.S. EPA data available)

New Technology Diesel Engines: Building on a Strong Tradition of Low Emissions for Heavy-Duty Commercial Vehicles & Off-Road Equipment

New technology diesel engines deliver impressive emission reductions and the latest technologies developed to meet the latest emissions requirements are just one chapter in the clean diesel story. Whether it is heavy-duty commercial vehicles or off-road equipment, new technology diesel engines are the latest point in a downward trend for tailpipe emissions.

Commercial Trucks & Buses

Clean Diesel Progress - Heavy Duty On-Highway (Trucks, Transits & School Buses)

For commercial vehicles, comprising everything from large pickup trucks to even larger Class 8 tractors, along with school and transit buses, diesel technology is ready and able today to meet the near-zero fine particle (PM 2.5) emissions standards were established for model year 2007 and the near-zero NOx standard set for model year 2010. Relative to previous generations of technology, the latest near-zero diesel emissions options reduce fine particle and NOx emissions by 98 percent.

Meeting and Beating the Standard

Recent research estimates that heavy-duty diesel engines actually beat these emissions requirements handily. The Advanced Collaborative Emissions Study found that heavy-duty diesel engines manufactured to meet the model year 2010 emissions standard demonstrate that these engines deliver NOx emissions 60 percent below the standard and particulate matter emissions 90 percent below the standard.

The population of these clean diesel vehicles in service today are delivering substantial clean air benefits. According to research commissioned by the Diesel Technology Forum, heavy-duty diesel commercial vehicles powered by an engine that meets or beats the model year 2010 standard has resulted in a reduction of 1.5 million tons of NOx between 2010 and 2014 while vehicles powered by engines that meet or beat the 2007 standard have reduce emissions of particulate matter by 40,000 tons since 2007.

While these clean diesel vehicles are generating substantial clean air benefits, roughly one-in-five heavy-duty vehicles on the road today is powered by an engine that meets or beats the strict model year 2010 standard. Clearly, even more substantial savings can be realized if more of these clean vehicles enter into service. In fact, air quality regulators in the Los Angeles area estimate that NOx emissions attributable to diesel-powered commercial vehicles in the South Coast basin could fall by 70 percent if all commercial vehicles in the region were powered by one of these engines.

How Clean is Clean Diesel? About as Clean as Battery Electric Solutions

When it comes to reducing fine particle emissions, near-zero emissions diesel technology has shown to be nearly as effective as zero-emissions solutions. According to U.S. EPA data, when replacing an older school bus with a variety of technologies, near-zero emissions diesel technology removes about as much fine particle emissions as a battery-electric option and more effective than a propane model. In fact, recent research suggests that far more fine particle emissions are generated from brake and tire wear than from the tailpipe of a truck or bus.

Annual Tailpipe Emissions

(Data generated using U.S. EPA’s Diesel Emission Quantifier)

A Bright Future for Diesel and Cleaner Air

While the latest near-zero diesel innovations are capable of reducing NOx emissions by 98 percent relative to previous generations of technology, even further NOx reductions are achievable in the future. The leaders in diesel innovations are supporting the work of the U.S. EPA in developing a new emissions standard to reduce NOx emissions to closer to zero levels.

Through the Cleaner Trucks Initiative, work is ongoing to refine emissions reduction technology to achieve even greater reductions to deliver cleaner air for us all. Through the Cleaner Trucks Initiative, these closer-to-zero diesel innovations will meet a more stringent durability requirement to operate as intended years after manufacture while new test procedures are in development to ensure that these new innovations will deliver clean air benefits wherever they operate.

Next Up: Off-Road Equipment

Clean Diesel Progress - Small/Medium Off-Road Equipment

Clean Diesel Progress - Large Off-Road Equipment

Clean Diesel Progress - Locomotives

Today, diesel engines and fuel power over 95 percent of commercial vehicles yet they also power a similar share of off-road equipment from construction, agricultural, warehouse and mining equipment, locomotive and marine engines and mobile and stationary generators. Technology developed to meet strict emissions standards for commercial vehicles are 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.

Relative to the first generation of emission standards for this large off-road equipment, emissions have been reduced anywhere from 86 percent to 96 percent for both NOx and PM thanks to the deployment of clean diesel technology. 

Much like technologies developed to meet the model year 2010 emission standard for commercial vehicles, off-road engine and equipment manufacturers have coupled near-zero emission technology with substantial fuel savings capabilities. Many of these fuel savings capabilities involve advanced engine designs but also more efficient hydraulic systems, telematics and even hybridization. This equipment will deliver clean air benefits to many communities as more of these Tier 4 engines, and the equipment powered by them, enter into service. 

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.

What Would Happen if All Trucks Came With Near-Zero Emissions Diesel Solutions?

While more than half of all commercial trucks on the road across the U.S. come with the latest near-zero fine particle emissions reduction technology to meet the model year 2007 emissions standard, all of the 16,000 trucks serving America’s largest seaport complex - the Ports of Los Angeles and Long Beach, CA – come with this technology. 90 percent of these trucks are diesel 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.

In 2005, trucks 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 over 90 percent.

Port of Long Beach Emissions

(2019 Port of Long Beach Emissions Inventory)

In 2019, trucks serving the Port of Long Beach accounted for just seven 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 percent. In 2005, trucks were the second leading source of all PM 2.5 emissions after ocean going vessels.

Diesel Sources are the Fastest Falling Sources of NOx Emissions

Over the last 10 years, NOx emissions have fallen by 38 percent nationwide. 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 much NOx emissions as from coal power plants.

Leading Sources of NOx Reduction

(2017, latest U.S. EPA data available)

This dramatic reduction in NOx emissions from the diesel sector is attributable to the introduction of near-zero emissions diesel technologies. Thanks to the transition to cleaner technology, today’s diesel engines emit a tiny fraction of emissions relative to older technology - just 1/60th the amount of emissions compared to a 1988 model. You need 60 new tractor trailer trucks to equal the emissions of just one truck made in 1988.

60 trucks infographic

Today, just over 40 percent of all diesel commercial vehicles come with technology to achieve near-zero NOx emissions required of new trucks manufactured beginning in 2010. As more of the truck fleet transitions to the latest near-zero emissions technologies we can expect further dramatic improvements in NOx reduction.

Much like trucks, the latest off-road diesel engines that power equipment, locomotives and marine vessels also come with near-zero emissions technologies. We can expect even greater dramatic improvements in reduction in emissions from diesel sources as more of the engines that power the fleet of machines, locomotive vessels transition to the latest near-zero diesel innovations.

Recent research confirms that replacing older engines in much larger applications, including locomotives and marine vessels, can yield substantial NOx 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.

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Policy Overview

Hot Issues

"I don’t have concerns
with diesel technology in general"

When asked during the Congressional hearings on the VW situation about its random vehicle audit of all light-duty diesel, Chris Gundler, U.S. EPA Director of the Office of Transportation & Air Quality, replied:

“I don’t have concerns with diesel technology in general. I don’t expect to find widespread problems but we are going to be taking a very close look.”

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