The concept of "retrofit" has typically been defined broadly. While the term is frequently used as a label describing various exhaust emissions control devices such as the DOCs and particulate filters previously outlined, it can also encompass a broader range of options to reduce emissions, including re-powering, rebuilding and in some instances replacing 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 improve both the fuel economy and emissions profile.  In fact, U.S. Environmental Protection Agency (EPA) rules outline emission requirements on rebuilt engines to insure emissions reductions.

Refuel:  Since 2008, the use of Ultra Low Sulfur Diesel fuel (ULSD) has been mandated in the U.S.  The use of ULSD greatly reduces sulfur dioxide, a key component in smog, and also allows for the operation of other emission control technologies.

Repower: Replacing an older engine with a new diesel engine that meets the most current EPA emissions criteria will greatly reduce emissions.

Replace:  In certain instances, the most cost effective strategy to reduce emissions may be to replace the vehicle or equipment.

Retrofit:  The installation of various emission control technologies may also improve emissions from older diesel engines.  The mandated use of ULSD has paved the way for many emission control technologies to reduce criteria pollutant emissions from older diesel engines.

Diesel Retrofit Technologies:

Particulate filters are capable of capturing up to 95 percent of particulate matter, or soot.

Exhaust gas recirculation 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), natural components of the air we breathe, which is then expelled through the vehicle tailpipe.

Read more about SCR.

Oxidation Catalysts 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 percent.