What is Clean Diesel?

Share This Page


Why Retrofit?

Modernizing and upgrading existing diesel engines and equipment can be cost effective and offer immediate, significant, emissions reductions to some diesel engines, vehicles, and equipment

The Opportunity

While the environmental performance of new diesel engines is constantly improving, many older generations of technology remain in use today. Diesel technology is known for its reliability and durability, with engines often lasting hundreds of thousands of miles or running for hundreds of thousands of hours. Even as investment in new vehicles and equipment increases, a sizable fleet of older generation equipment remains in operation. Some of the same advances used to improve new engines can be applied to this existing fleet.

In 2005, Congress passed the Diesel Emissions Reduction Act (DERA) Program (known as "DERA") under the Energy Policy Act of 2005.This act gave EPA new grant and loan authority for promoting diesel emission reductions and authorized appropriations to the Agency of up to $200 million per year.  In 2020, DERA was reauthorized for up to $100 million annually through 2024. The program will continue to award grants and rebates to achieve diesel emissions reduction.

Both the US EPA and California Air Resources Board have many resources and a listing of the requirements governing the retrofit options and modernizing and upgrading engines and equipment. This includes incentive funding, verified device databases, diesel emissions quantifiers to evaluate options, and other assistance.

The term “retrofit” is often used in a general sense to cover several strategies to reduce emissions from older engines, vehicles and equipment. The term is frequently used to describe the practice of applying a new exhaust emissions control device such as a diesel oxidation catalyst or particulate filter to an older generation existing engine, it can also encompass a broader range of options to reduce emissions, including re-powering, and rebuilding existing equipment.

Rebuild: Engines face normal wear and tear and need to be rebuilt to operate efficiently returning to the manufacturer's original specifications. During the course of a rebuild, equipment owners may choose to install new engine components that restore both the fuel economy and emissions profile to the original levels. US Environmental Protection Agency (EPA) rules outline emission requirements for rebuilt engines to ensure emissions reductions. At a minimum, existing engines that are rebuilt are returned to their original emissions performance designs. In some instances, depending on engine and application, an existing engine can be rebuilt to have better emissions performance than its original design. 

Repower: For some applications such as commercial heavy-duty trucks, industrial engines, marine and some larger off-road machines, the life and value of the equipment, vehicle or vessel exceeds that of the original engine. In this instance, replacing the original existing engine with a newer generation diesel engine that meets the original performance specifications is very cost effective, and can immediately boost performance, reduce fuel consumption and restore emissions performance to original levels. In some instances, the repowered engine may be upgraded in emissions performance to even lower emissions than the original design. Each application is different and requires consideration of a broad range of economic and operating and technical parameters. Local engine and equipment dealers should be consulted to help equipment operators make informed decisions about the best options.

Replace: In certain instances, based on the age of the equipment or vehicle, its overall condition, replacing the engine would not be cost-effective. Instead, the most cost-effective strategy to reduce emissions may be to replace the entire vehicle or equipment with a newer model.

Retrofit: The installation of various emission control technologies may also improve emissions from older existing diesel engines. Many diesel oxidation catalysts, particulate filters and some lean-NOx traps have been successfully applied to existing diesel engines that power school buses, commercial vehicles and some off-road engines and equipment. Generally, retrofitting of these devices can only be considered for engines and equipment made from 1988 through 2014 model years. Engines sold after this time in both on and off-road applications come already equipped with advanced emissions control systems.  

Immediate Significant Reductions

If suitable, retrofit devices can be installed on some 1988-2014 vehicles and equipment and can reduce in-use emissions of some compounds by as much as 85%, depending on the technology and the characteristics of the vehicle or equipment. The suitability of retrofitting requires careful consideration of the technical feasibility of each desired application, including whether or not EPA and CARB have verified and approved devices for a specific engine family or vehicle type. CARB maintains a comprehensive retrofit device verification database that must be consulted. Not every device can be safely applied to every engine or equipment type.

Flexible and Voluntary Programs

Voluntary approaches to diesel emission reductions are successful throughout the United States, thanks in large part to the Diesel Emissions Reduction Act noted previously. The types of equipment and vehicles that have benefitted from these programs are diverse and include heavy-duty trucks, forklifts, port material handling equipment, bucket loaders, tractors, wheel loaders, refuse trucks, as well as transit and school buses and marine vessels and locomotives.

>Diesel Retrofit Technologies

There are several categories of retrofit technologies. Not every type of technology will work in every engine or machine. Retrofit technologies can generally be considered for engines and equipment manufactured from ~1988 – 2012 model year.

Particulate filters capture up to 95% of particulate matter, or soot. According to the Manufacturers of Emission Controls Association, hundreds of thousands of  diesel particulate filters have been sold nationwide over the last 20 years, helping to improve the emissions of diesel vehicles and equipment.

Exhaust gas recirculation (EGR) recirculates a portion of engine exhaust back into the engine diluting the oxygen content of the fuel-air mixture. EGR technology significantly reduces both NOx and Particulate Matter.

Selective Catalyst Reduction (SCR) is an advanced active emissions control technology system that injects a liquid-reductant agent, known as diesel exhaust fluid (DEF) through a special catalyst into the exhaust stream of a diesel engine. The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2). Those are natural components of the air we breathe, which is then expelled through the vehicle tailpipe.

Diesel Oxidation Catalysts (DOC) are similar to converters installed on light-duty vehicles. Exhaust travels through a honeycomb like structure that attracts fine particles. Diesel oxidation catalysts are capable of eliminating particulate matter by 20-50%. The US EPA estimates that tens of thousands of DOCs have been installed on older vehicles and equipment since 2008 making this technology one of the more popular and cost-effective retrofit options.

Visit the EPA website to learn more about retrofit technologies, including other programs ad installation costs. 

Key Programs

The federal government provides a significant source of funding for diesel retrofits through several programs including the Congestion Mitigation and Air Quality (CMAQ) Improvement Program, the Diesel Emissions Reduction Act¸ and the Environmental Quality Incentives Program (EQIP). Many states and localities provide retrofit assistance, too.

Federal Programs

Diesel Emission Reduction Act 

The Diesel Emission Reduction Act (DERA) was created in 2005 to help diesel equipment owners retrofit older equipment. The programs is managed by the US EPA. Congressional funding has varied over the years, but is generally in the tens of millions of dollars. The EPA also manages smaller set-aside programs to improve diesel emissions in construction equipment and port equipment found at work in areas known for poor air quality. The US EPA estimated that DERA retrofit activities improved emissions of more than 50,000 vehicles and equipment while reducing 203,000 tons of oxides of nitrogen and 12,500 tons of particulate matter between 2008 and 2012. 

CMAQ Improvement Program 

The other primary source of federal funding for retrofits comes through the Federal Highway Administration. Congress created the Congestion Mitigation and Air Quality (CMAQ) program in 1991 to help states with non-attainment areas improve air quality. The legislation lists a menu of possible policies that states and metropolitan planning agencies may use when applying for CMAQ funds through the Federal Highway Administration (FHWA). Diesel retrofits are included in that list. The most recent surface transportation reauthorization bill, enacted into law in July 2012, allows states with non-attainment areas for particulate matter to use a portion of their CMAQ funds to retrofit older construction equipment. The provision does not mandate that states do so. Please visit the FHWA website to learn more about the CMAQ program.

Environmental Quality Incentives Program (EQIP) 

Congress authorized the EQUIP in the 2002 Farm Bill. The current Farm Bill authorizes funding for the Natural Resources Conservation Service to manage a program to help agricultural equipment owners to retrofit older diesel-powered equipment found at work in rural areas with poor air quality. EQIP is subject to the annual appropriation process. Please visit the Natural Resources Conservation Service website to learn more about the EQIP program. 

State Programs

In addition to federal financial assistance programs for diesel retrofits, many states also provide financial assistance to help the owners of diesel equipment purchase retrofit technologies. Below is a listing of a few of those programs. If you’re an equipment owner seeking financial assistance for retrofit technologies, please contact your state department of environment or air quality. 

California - Carl Moyer Program 

The California Air Resources Board (CARB) administers funds to help the owners of heavy-duty diesel equipment purchase retrofit or newer engines that improve emissions. Funds are administered through California's various air districts. Legislation precludes applicants from seeking funds needed to bring non-compliant equipment into compliance with California's air quality regulations for particulate matter, oxides of nitrogen (NOx) and reactive organic gas emissions (ROG). The Carl Moyer Program was reauthorized by the California legislature in 2013 and received $2 billion in funding expiring in 2023. 

Thanks in part to the Carl Moyer Program, California is a large market for retrofit technologies. According to the Manufacturers of Emission Controls Association, more than 55,000 diesel particulate filters have been installed on diesel vehicles and equipment in California. To learn more about the program, please visit the Carl Moyer Program website

Texas - Texas Emissions Reduction Plan (TERP) 

In 2001, the Texas legislature created the TERP to help diesel equipment owners retrofit older equipment. The program is similar to the Carl Moyer Program except that TERP focuses exclusively on the potential for NOx reduction in choosing recipients. The program is funded from title fees on the sale of cars and trucks and various state truck tax revenue and is administered by the Texas Council of Environmental Quality (TCEQ). To learn more about the program, please visit the Texas Emissions Reduction Plan (TERP) website.

New Jersey - Stop the Soot Campaign 

On April 11, 2011, New Jersey Governor Christie signed Executive Order #60 creating a pilot project to test the feasibility of mandating the adoption of retrofit equipment deployed on off-road equipment used in a select few public works projects located near large urban centers. To learn more about the pilot project, please visit New Jersey’s website



Policy Overview

Sign up for diesel direct

weekly analysis & commentary from the diesel technology forum