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?


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

It’s sprayed into the exhaust gas to break down some climate killer components (Nitrooxides) of it.

Too high temperatures in turn break it down (it’s Urea, the stuff thats also in your pee). Above 130°C it turns into other chemicals, so it must be used once the gas became cooler than that

DEF is injected into your exhaust, not your intake manifold. If you added it to the diesel fuel it would be along for the ride as you go through your fuel pump, injectors, cylinder, combustion, and finally out through the exhaust. In a diesel truck the DEF reservoir feeds into the exhaust train upstream of your catalyst. It breaks down and makes ammonia and that combines with the nasty compounds in your diesel exhaust to produce nitrogen and water.

Combustion of diesel fuel produces nitrogen oxides and those are frowned upon by the EPA. DEF contains urea (yeah…you read that right) and that’s got ammonia in it. After injection into your exhaust the DEF becomes ammonia and isocyanic acid. From there the acid breaks down into carbon dioxide and ammonia. Those all flow through your catalyst and reduce the nitrogen oxides into nitrogen and water.

I copied this from another website so don’t think I just whipped this out off the top of my head, but the whole process is shown below. The goal is get rid of the 4NO.

2(NH2)2CO + 4NO + O2 → 4N2 + 4H2O + 2CO2 and 2(NH2) 2CO + 3NO2 -> 7/2N2 + 4H2O + 2CO2

You’ve got good answers already, but here’s another one, specifically for why it can’t be added to the fuel. The active ingredient, urea, is a solid. When fuel goes into the cylinder it needs to evaporate to burn quickly to push the piston down. If you had a solid component it would crystallize as the fuel evaporates. Very quickly that’s going to build up residue on the inside of the cylinder and the injectors. It would be similar to putting sugar the fuel tank!

How does it constantly spray DEF into your exhaust without you having to refill the DEF with every tank of gas?

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