DEF stands for diesel exhaust fluid. It is a mix of urea and water that when sprayed on diesel exhaust changes the nitrogen oxide to nitrogen and water. Because the spraying happens after the diesel goes through the engine, it is part of an after-treatment system. The US Environmental Protective Agency requires most owners and drivers of diesel engines to use DEF without fail starting January 1, 2015. As a result, all diesel-powered vehicles and machine are equipped with DEF equipment. Below are the basics:
Diesel exhaust is treated using selective catalytic reduction (SCR). It only treats nitrous oxide, and not the other stuff; hence, the term “selective.” The diesel exhaust goes through a series of chambers that first filters the exhaust for large particulates before passing it through to the chamber where the DEF goes into action. It breaks down nitrous oxide into nitrogen, water, and a bit of carbon dioxide and other gases, which then exits the system. The exhaust is much cleaner than it would have been if it had not gone through the SCR system.
The SCR system needs DEF to work, just as the engine needs fuel. All vehicles and machines with an SCR system have a separate DEF tank, typically marked by a blue cap. It is not as big as the fuel tank because you only need about 2.5 gallons for every 800 miles. Back in the day, long-haul truckers had to have large DEF tanks to make sure they had enough to last them the whole trip, because DEF was not always available. Today, however, smaller DEF tanks are possible because DEF is readily available in most gas stations.
The rules require that DEF tanks have a gauge to monitor the level of DEF in the tank. Without DEF, the motor will not start, so it is as important as the fuel gauge. In general, the gauge looks much like any gauge, and you want to avoid the needle going down to “0” or “Empty.”
Basic DEF equipment is necessary to follow the new EPA regulations for diesel engines. But, you also have to make sure they are properly maintained. It is important to keep your DEF in pristine condition and to have your SCR system regularly checked to avoid trouble.