How does DEF work and what’s the point of it? Why can’t it just be added to the diesel when it’s processed?
In: Chemistry
DEF is a mixture of urea and deionized water used to catalytically destroy a pollutant known generically as NOx, or nitrogen oxides, present in diesel exhaust.
When DEF is injected into the exhaust stream, it reacts with those NOx to break them apart into harmless Nitrogen and water molecules before it leaves the tailpipe. The entire system is known as SCR- selective catalytic reduction.
The primary reason for not mixing it with diesel fuel is that it can’t affect anything on that end of the combustion cycle. If you injected DEF into the combustion chamber and burned it with diesel fuel, you’d thermally destroy the urea and it would be useless for its intended purpose on the other side.
Another reason is that modern common-rail diesel engines rely on highly pressurized fuel systems (tens of thousands of psi) and extremely fine-tolerance pumps to run them. Urea is a very nasty compound to have floating around in such sensitive systems- it’s highly corrosive and forms abrasive crystals wherever it touches- so it can (and does) rapidly destroy fuel systems when inadvertently mixed into the fuel.
Source: Own and work on diesel engines with SCR
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