Function

One of the reasons of equipping a vehicle with a SCR (Selective Catalytic Reduction) catalyst might be to avoid a closed particulate filter. Additionally, there is no threat of clogging. Perhaps even more importantly, the manufacturer wants to limit the nitrogen oxides, which are formed especially during a relatively high combustion temperature in the engine. If it is possible to remove the nitrogen oxides well-aimedly afterwards, it might not be necessary to pay too much attention on this problem while designing the engine. A relatively high temperature of the combustion through a higher compression or boost pressure makes the engine more economical. However, the expenditure for the additional substance to be re-filled remains.

How it works

Fulfilling the truck Euro 5 exhaust emission codes is quite a challenge. Auxiliary tanks with 150 - 220 litres of volume (see picture) might be needed at gas stations until the introduction of a new infrastructure. The complete engine is being checked regarding its compression, firmness (engine mount, crank gear), air/fuel mixture and sealing.
What kind of miraculous potion is it that eliminates all our Diesel emission problems at once? It is clear, odourless, and not toxic. Its name is AdBlue, and it consists of one third urea and two thirds of water. It is added to the exhaust gases beyond the engine, just before the SCR-catalyst. It should be added in a ratio of 4 - 6 percent of the consumed Diesel fuel.In fact, ammonia is the nitrogen oxide 'killer', but it would be too dangerous to use it directly, thus AdBlue is used, which transforms into ammonia through 'catalytic hydrolysis'. Big particles are burned in the same process, too.
Besides this rather extensive solution, there is the option with a small auxiliary tank, filled by the garage for long intervals, automatically added to the fuel after every refill in very small amounts. It might not necessarily be a mixture with urea, it might have a Fe-basis. Two questions are unanswered so far: what happens if the auxiliary tank is empty, yet the driver keeps on going? Or if pure water instead of AdBlue is added?