I’ve never owned a diesel engine vehicle. What is the purpose of diesel exhaust fluid?
In: Engineering
Diesel engines operate differently from gasoline engines and are potentially strong emitters of pretty nasty chemicals. So we’ve developed a few different technologies that operate together to reduce the amount of nastier emissions.
Specifically we solve one problem inside the engine that causes a new issue where nitrogen-containing chemicals are produced in the exhaust, which is still bad. So Diesel Exhaust Fluid is sprayed into the exhaust gases and the heat from the exhaust breaks DEF down into smaller chemicals which will now react with any nitrogen-containing chemicals they find – ripping the nitrogen atoms off and forming water out of what’s left.
If everything works as it should, the nitrogen atoms will bind together in pairs forming molecular nitrogen (N2) which is naturally like 80% of the air anyway and the rest turns into water and so the DEF has fully “digested” the nasty Nitrogen-chemicals into harmless byproducts.
It serves a very similar function to the catalytic converter in a gas powered vehicle: changing Bad Gasses in the exhaust to slightly less bed gasses.
It’s to remove nitrogen oxide pollution from the exhaust.
The heat produced in diesel engines cause a small amount of nitrogen in the air to react with oxygen to produce a small amount of nitrogen oxides. These are hazardous to human health and also help produce ozone which at low altitudes is not only poisonous but helps produce yet more pollutants and speeds up the aging of plastics and rubber causing them to crack and weaken. This is bad news for everyone.
The diesel exhaust fluid (often sold as AdBlue) is a mix of urea and water designed to remove those nitrogen oxides by chemically reacting with them before they can leave the exhaust.
The diesel exhaust fluid is sprayed into the hot exhaust as it leaves the engine. In the heat, urea breaks down to produce a molecule of ammonia and a molecule of something called isocyanic acid. The isocyanic acid reacts with water to produce another molecule of ammonia and a molecule of carbon dioxide.
There are a number of reactions between ammonia and nitrogen oxides, some of which need oxygen to work. The end result is harmless nitrogen and water which come out of the exhaust pipe.
The chemical reaction is made more efficient by a process called selective catalytic reduction; where a chemical (known as the catalyst) helps the reaction along. The catalyst is found in a catalytic converter on the exhaust which contains a porous ceramic material through which the exhaust passes. The catalysts themselves are often vanadium or tungsten; but might include silicon-based chemicals called zeolites.
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