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Diesel Emission Retrofits


The EPA has signed new stringent standards for model year 2007 and later heavy-duty diesel engines. The new regulations consist of two components, which include new emission standards and new diesel fuel regulations. The new standards will be phased into the sales of 2007 and later engines. Fleets with pre-2007 diesel vehicles can voluntarily reduce emissions by installing emission retrofit devices.

Diesel emission retrofit devices are designed to minimize particulate matter (PM), hydrocarbons, carbon dioxide (CO2), and carbon monoxide (CO) from diesel engine exhaust. The pollutant reduction is achieved by capturing and/or destroying these particles. These devices provide immediate and significant pollution reductions with no new infrastructure requirements. They are also a cost effective alternative to bringing existing fleets up to date with the new standard.

School Bus

Approximately one third of diesel emissions are a result of on-road diesel vehicles. Three highly effective diesel retrofit devices capable of reducing emissions are diesel oxidation catalysts (DOCs), diesel particulate filters (DPFs), and closed crank ventilation systems(CCVs).

Retrofit Quick Facts:

  • Diesel exhaust is a major source of PM2.5 emissions in urban areas. Approximately one third of diesel engine emissions are a result of on-road vehicles.

  • More than 100 million Americans live in areas where PM2.5 levels exceed the EPA's guideline, which poses a significant health risk to the public.

  • A diesel oxidation catalyst (DOC) is a porous honeycomb-like device that is covered with materials that catalyze a chemical process in order to break down pollutants in the exhaust stream into less harmful pollutants. DOCs can be used with regular diesel fuel. However, effectiveness can be increased if used with ultra low sulfur diesel fuels.

  • A diesel particulate matter filter (DPF) is a ceramic device that collects particulate matter in the exhaust stream. The ceramic structure heats up due to the high temperature of the exhaust stream, which then allows the particles within the exhaust stream to oxidize into less harmful pollutants. DPFs must be used with ultra low sulfur diesel fuel.

  • A closed crank ventilation system (CCV) is designed to capture and return the crankcase gases that have leaked through the engine piston rings. These crankcase emissions can be substantial, thus a CCV is used to control gases flow and return the emissions to the engine for combustion. This effectively prevents the crankcase emissions from entering the atmosphere.

  • DOCs are best suited for retrofitting older vehicles, whereas DPFs are highly efficient at reducing PM2.5 emissions where ultra low sulfur diesel fuel is available.

  • Most DOC and DPF retrofits come with a 100,000 - 150,000 mile warranty and can last between 7 and 15 years.


Est. Cost


Emissions Reductions




PM 20-50%
HC 60-90%
CO 60-90%



Yearly cleaning to remove built up ash

PM 80-90%
HC 60-90%
CO 60-90%


$1,200-$ 2,000

Periodic filter replacment

PM 10-15%
HC 30-40%
CO 50-90%

Each of these diesel retrofit technologies developed by the manufacturer must go through an independent verification of their effectiveness. This is done by either the Environmental Protection Agency (EPA) or the California Air Resources Board (CARB). The purpose of this verification is to determine the reduction of regulated emissions such as PM, Nitrous Oxide (NOx), and CO through testing. Additionally, testing is performed to verify how long these technologies can retain reduced emissions. This essentially determines how effective each emission control technology is in reducing emissions for an extended period of time.

Funding is available to purchase and install EPA verified or CARB certified DPFs, DOCs, and CCVs on school buses and trash trucks within the state of Oklahoma.

More information on Diesel Emission Retrofits

EPA-National Clean Diesel Campaign exit

EPA-Verified Retrofit Technologies exit

CARB-Verified Retrofit Technologies exit

Page last updated: March 19, 2013


Diesel Emission Retrofits


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