What is DEF Fluid for in diesel trucks?


What is DEF Fluid for in diesel trucks?

In: Engineering

DEF stands for Diesel Exhaust Fluid. It’s a chemical that is injected into the exhaust stream to reduce emissions in diesel trucks. Trucks that have it (and use it properly lol) don’t put out nearly as much black smoke compared to one’s that don’t have it.

Diesel Exhaust Fluid (DEF) is something that gets added to the exhaust system of large diesel trucks in order to reduce the concentration of harmful nitrogen oxide (NOX) compounds such as nitric oxide and nitrogen dioxide. These compounds contribute to smog and acid rain formation, as well as deplete tropospheric ozone.

DEF’s active ingredient is urea, the main component in mammalian urine. When it’s exposed to hot exhaust gas, it undergoes a series of chemical reactions that convert the harmful NOX compounds into harmless nitrogen gas.

Diesel engines operate at very high temperatures and pressures compared to gasoline engines. They get so hot that they can actually manage to knock apart nitrogen molecules (which is tough to do because N2 is a very stable molecule, and why you don’t need DEF in gas engines) and combine it with oxygen, creating NO, NO2, NO3, collectively just referred to as NOx. NOx is a major pollutant and a big contributor to acid rain (NOx turns into nitric acid when dissolved in water).

DEF is basically urea (same stuff as in pee); the chemistry is complicated, but basically it prevents the nitrogen from uniting with the oxygen around it and prevents NOx from forming.

Sidenote, the big VW diesel scandal happened because they claimed they’d figured out a way to make a diesel engine that didn’t need DEF but still kept NOx under the legal limit, but it turns out they were cheating on the test (and as it turns out, so is everyone who’s ever made a diesel engine apparently).