Overview The concept of "retrofit" has typically been too narrowly defined. The term is most often used as a label describing various exhaust aftertreatment devices. While exhaust aftertreatment technology represents a very promising category of solutions for in-use emissions reduction, it is only one of a number of options available for fleet owners. Each of the Five "Rs" of diesel retrofit has its own advantages and strengths that can be tapped to tailor an emissions reduction program to specific air quality improvement goals, budgets and equipment owners' needs:

1. Repair/Rebuild - Regular engine maintenance plays a critical role in ensuring proper engine performance and engine rebuilding can significantly lower emissions in some cases.
2. Refuel - The use of advanced diesel fuels, such as ultra-low sulfur diesel (ULSD), can lower emissions with or without the addition of other emissions control devices.
3. Retrofit - The installation of exhaust aftertreatment technologies - such as particulate filters, oxidation catalysts, exhaust gas recirculation (EGR), selective catalytic reduction (SCR) devices, and NOx catalysts - can reduce emissions of particulate matter (PM) and nitrogen oxides (NOx).
4. Repower - Replacing the older engine with a new clean diesel engine can be a cost-effective option for certain highly-valuable pieces of diesel equipment.
5. Replace - Replacing entire vehicles or equipment may be the best, most cost-effective option for some of the oldest engines. Click here for a list of retrofit products that the Environmental Protection Agency or the California Air Resources Board (CARB) has verified.

Full Article: What is Retrofit?