This is obviously a super-simplified version of how SCR works, but it's not unlike the way your gasoline-powered car's catalytic converter works, aside from the extra step of injecting urea into the exhaust stream.
Nitrogen is the primary component of the air we breathe and is harmless to the environment. The hot exhaust gas and DEF then enter the catalytic converter where the urea from the DEF and the exhaust gas react with a variety of metallic compounds to convert nitrogen dioxide and monoxide into nitrogen and water. Yes, urea is found in urine - quit giggling, please - but this is a refined form of the compound and is mostly used in the agricultural industry as a component of fertilizer.
DEF is made from deionized water and a very pure form of urea. That takes care of the "rolling coal" aspect of old diesel engines that made them relatively unpopular in the US in the 1960s, '70s and '80s.įrom the particulate filter, the exhaust gas travels past a nozzle which sprays diesel exhaust fluid into the stream of gases. So, in an SCR-equipped vehicle, the exhaust gas from the engine is routed first through a particulate filter to catch all the soot and ash generated from burning what is a relatively impure fuel. Filling up on DEF is as simple as popping the cap and pouring in a jug, but you'll do it a lot less often than you fill up your fuel tank.