Diesel Exhaust Fluid: What is it?

If you haven’t heard of Diesel Exhaust Fluid (DEF), chances are you will soon. DEF is required in new diesel engines and chances are your vehicle or fleet will soon be required have to have the new product.

What is Diesel Exhaust Fluid?

Diesel exhaust fluid is one of the key elements of the Selective Catalytic Reduction (SCR) process used by most medium and heavy‐duty engine builders to meet EPA 2010 regulations. DEF is a nontoxic solution of 67.5% purified water and 32.5% ultra pure automotive grade urea. DEF is not a fuel or fuel additive. Instead, when injected into the exhaust stream and passed over a catalyst, DEF helps convert NOx into nitrogen gas and water vapor – two harmless and natural components of the air we breathe.

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.

What is urea?

Urea is a compound of nitrogen that turns to ammonia when heated. It is used in a variety of industries, including as a fertilizer in agriculture. Urea is naturally occurring, but is also commonly made from natural gas.

Why not use a 100% urea solution?

DEF provides the highest degree of practical NOx reduction in real life automotive conditions. Using DEF at a 32.5% solution in water is cost‐effective. Also, it allows the engine to perform at the highest levels of effi‐ ciency and at the widest range of climatic temperatures – anything above 12 degrees Fahrenheit.

How does DEF work within an SCR system?

The goal of an SCR system is to reduce levels of NOx (oxides of nitrogen emitted from engines) that are harm‐ ful to our health and the environment. SCR is an after treatment technology that treats exhaust gas down‐ stream of the engine. Small quantities of DEF (automotive grade urea solution) are injected into the exhaust stream and when mixed with the NOx changes into harmless nitrogen and water.