What is Diesel Exhaust Fluid? DEF is the key operating fluid for the Selective Catalytic Reduction (SCR) process used by most medium and heavy-duty engine builders to meet EPA 2010 regulations. It is a non-hazardous urea solution that chemically transforms harmful Nitrogen Oxide emissions into harmless water vapor and nitrogen–two harmless and natural components of the air we breathe. According to engine manufacturers, SCR combined with DEF improves overall fuel economy by approximately 5% compared to competing technologies, and achieves NOx reductions in excess of 90%.   DEF is stored in a dedicated tank next to the fuel tank on the driver's side. The tanks range in size from 6 to 23 gallons depending on the truck's application. The DEF tank fill opening is designed to accommodate a DEF fill nozzle to ensure only DEF is put into the tank. A diesel fuel nozzle will not fit into the DEF tank opening. Not sure of the amount of DEF you need? If you are a fleet operator needing DEF for your SCR trucks or if you require advice on which package size or storage system is most suitable for your operation, we can help you with various options and give  a full breakdown of cost. Please call our Keller-Heartt Customer Service Center at (800) 423-7513 to help you with your best solution. Are You Ready? 2010 trucks will have SCR systems requiring Diesel Exhaust Fluid (DEF).