DEF is a non-hazardous, nonflammable mixture made from urea and deionized water. DEF is sprayed into the exhaust stream of diesel powered vehicles via the Selective Catalytic Reduction (SCR) system which in turn reduces the levels of harmful nitrogen oxides emitted by the combustion of diesel fuel and converts it into eco-friendly nitrogen and water. These SCR systems can be found on diesel powered vehicles manufactured in the year 2010 or later. DEF is not a fuel additive and it never comes in contact with the diesel fuel. It is carried on board SCR-equipped vehicles in a separate designated tank which typically has a blue filler cap.
How DEF is used in the SCR system
The Selective Catalytic Reduction (SCR) system uses a catalyst system to break down harmful nitrogen oxides NOx. The chemical reactions required to achieve this requires a constant source of ammonia gas. The SCR system introduces the ammonia into the exhaust stream using a urea solution called Diesel Exhaust Fluid (DEF). The DEF is sprayed into the exhaust by an advanced injection system and then converted into ammonia gas on a catalyst which then breaks down the NOx into nitrogen and water.
Diesel Exhaust Fluid (DEF) levels need to be refilled periodically. The consumption is measured as a ratio to diesel fuel use. Typically a medium to heavy-duty diesel vehicle will have a 2-3% DEF to diesel fuel ratio. A rule of measure recommended by many manufactures is to refill the DEF tank at the oil change service intervals, however, consumption will vary based on vehicle type, usage and driving cycles.
If DEF levels in your vehicle get low, there will be a series of warnings that will be displayed on the dashboard (similar to when fuel levels are low). Depending on the make and model of vehicle this could be a gauge or warning light. As the DEF levels get increasing low, the warning lights will become more prominent and if the levels are allowed to get too low, the engines power will be reduced and the vehicle will go into “limp mode” until DEF is added to the designated tank. Once the tank is replenished, normal engine power will be resumed.
Diesel Exhaust Fluid (DEF) significantly cuts down the nitrogen oxides (NOx) produced from the combustion of diesel fuel. This enables our diesel engines to be kinder to the environment and our health.