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National Clean Diesel Campaign (NCDC)

Working Together for Cleaner Air

Diesel Retrofit Devices

Diesel retrofit devices for after-treatment pollution control can be installed on new or existing vehicles and equipment to reduce particulate matter (PM), nitrogen oxides (NOx), hydrocarbons (HC), or carbon monoxide (CO) as well as other air pollutants. The information below provides estimated emission reductions which may be used in the selection of appropriate technologies for air quality programs. It may not be used to support formal emission reduction claims for State Implementation Plans (SIPs), compliance programs, or consent decree projects.

Note: Actual emissions reductions and costs will depend on specific manufacturers, technologies and applications. EPA and the California Air Resources Board (CARB) verify the emissions performance of retrofit devices through specific testing protocols and statistical analysis. To learn more, visit EPA’s Verified Technologies List, CARB's Verified List Exit EPA Disclaimer, EPA’s Emerging Technologies List and NCDC’s Publications Page.

  Typical Emission Reductions (percent) Typical Costs ($)
Technology PM NOx HC CO  
Diesel Oxidation Catalyst (DOC) 20-40   40-70 40-60 material: $600-$4,000
installation: 1-3 hours
Diesel Particulate Filter (DPF)
Active or Passive
85-95   85-95 50-90 material: $8,000–$50,000
installation: 6-8 hours
Partial Diesel Particulate Filter (pDPF)
Partial or Flow-through
up to 60   40-75 10-60 material: $4,000–$6,000
installation: 6-8 hours
Selective Catalytic Reduction (SCR) *   up to 75     $10,000–$20,000 Urea $.80/gal
Closed Crankcase Ventilation (CCV) * varies        
Exhaust Gas Recirculation (EGR) *   25-40      
Lean NOx Catalyst (LNC) *   5-40     $6,500–$10,000
  • *May be combined with DOC or DPF systems to reduce PM, HC and CO emissions.

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Diesel Oxidation Catalyst (DOC)

Diesel oxidation catalysts (DOCs) are exhaust after-treatment devices that reduce emission from diesel engines. Typically packaged with the engine muffler, DOCs are widely used as a retrofit technology because they require little or no maintenance. Engine manufacturers have used DOCs in a variety of applications for many years.

DOCs consist of a flow-through honeycomb structure that is coated with a precious metal catalyst and surrounded by a stainless steel housing. As hot diesel exhaust flows through the honeycomb (or substrate), the precious metal coating causes a catalytic reaction that breaks down the pollutants. These devices may be formulated to operate with fuel sulfur levels of 500 ppm or less, but they are most effective when fuel sulfur is 15 ppm or less. DOCs can be coupled with closed crankcase ventilation, selective catalytic reduction or lean NOx catalyst technologies for additional emission reductions.

Fact sheets:

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Diesel Particulate Filter (DPF)

Diesel particulate filters (DPFs) are exhaust after-treatment devices that significantly reduce emissions from diesel-fueled vehicles and equipment. DPFs typically use a porous ceramic or cordierite substrate or metallic filter to physically trap particulate matter (PM) and remove it from the exhaust stream. DPFs can be installed on existing vehicles and must be used in conjunction with ultra-low sulfur diesel (ULSD) which has a sulfur content of less than 15 ppm. DPFs may require special mounting or brackets as they are typically heavier than a conventional muffler or DOC. In addition, an electronic back pressure monitoring and driver notification system must be used with a DPF.

DPFs use either passive or active regeneration systems to oxidize the PM accumulated in the DPF. Passive filters require operating temperatures high enough to initiate combustion of collected soot. Active regeneration uses other heat sources, such as fuel burning or electric heaters, to raise a DPF temperature sufficiently to combust accumulated PM. In addition, filters require periodic maintenance to clean out non-combustible materials, such as ash.

DPFs work best on engines built after 1995. Knowing the age and type of each engine in the fleet as well as the drive cycles of the vehicles is an important part of any retrofit project. Exhaust gas temperature data logging must be performed to determine if the exhaust temperature profile meets DPF-specific requirements. DPFs can be coupled with closed crankcase ventilation, selective catalytic reduction or lean NOx catalyst technologies for additional emission reductions.

Fact sheets:

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Partial Diesel Particulate Filter (pDPF)

Partial filters, also known as partial diesel particulate filters (pDPFs) or flow-through filters, provide moderate (30 to 50 percent) reduction of PM from diesel exhaust. Partial filters use catalyzed metal wire mesh structures or metal foil-based substrates with sintered metal sheets to reduce diesel PM. Technologies verified to date employ structures to briefly retain particles for oxidation, structures to promote turbulence and impaction, and catalysts to oxidize diesel particles.

Partial filters trap PM with lower efficiencies than DPFs. However, because they trap particles, partial filters are always subject to minimum temperature requirements necessary for periodic regeneration (i.e., combustion of collected PM). Partial filters should incorporate electronic back pressure monitoring equipment to signal vehicle and equipment operators when the devices need to be cleaned.

Selective Catalytic Reduction (SCR)

Selective Catalytic Reduction (SCR) Systems inject a reductant, also known as diesel exhaust fluid (DEF), into the exhaust stream where it reacts with a catalyst to convert NOx emissions to N2 (nitrogen gas) and oxygen. The catalytic reaction requires certain temperature criteria for NOx reduction to occur. As with DPFs, knowing the age and type of each engine in the fleet as well as the drive cycles of the vehicles is important. Data logging must be performed to determine if the exhaust gas temperatures meet the specific SCR system requirements.

SCR systems require periodic refilling of the DEF, and the system should ensure that the DEF never freezes. SCR systems are commonly used in conjunction with a DOC and/or DPF to reduce PM emissions. Because of new NOx standards, most 2010 and newer on-highway diesel engines come equipped with an SCR system. A DEF refueling infrastructure is in place, facilitating the use of SCRs.

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Closed Crankcase Ventilation (CCV)

In many older diesel engines, crankcase emissions, also known as "blow-by", are released directly from the engine into the atmosphere through a vent or the "road draft tube." Closed Crankcase Ventilation (CCV) systems capture the oil in blow-by gas, return it to the crankcase, then redirect these gaseous emissions back to the intake system for combustion instead of emitting them into the air.

CCV systems incorporate filter elements that must be periodically replaced. CCV system maintenance requirements must be reviewed for each manufacturer’s product and potentially for each configuration. Emissions will be further reduced if the CCV is paired with a DOC or DPF.

CCV systems can help keep engine compartments and components clean, and reduce oil usage. As EPA’s 2007 Highway Heavy Duty Diesel rule requires that engine manufacturers control crankcase emissions as a part of overall emissions control strategy, most highway engines manufactured since 2007 come equipped with CCV systems.

Exhaust Gas Recirculation (EGR)

Exhaust Gas Recirculation (EGR) redirects a portion of engine exhaust back into the engine to cool and reduce peak combustion temperatures and pressures, thereby reducing the production of NOx. EGR is commonly used by engine manufacturers as a method to comply with new engine emission control standards. As retrofit EGR systems may require major engine integration, the use of EGR as a retrofit technology is limited.

Lean NOx Catalyst (LNC)

Lean NOx Catalysts (LNC) use diesel fuel injected into the exhaust stream to create a catalytic reaction and reduce pollution. Verified LNCs are paired with either a DPF or a DOC. An LNC can also be paired with an active DPF to reduce NOx emissions and enable filter regeneration over a range of duty cycles. However, an LNC still requires specific exhaust temperatures for appropriate NOx emission control performance. LNCs can increase fuel usage by 5-7 percent.

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