Urea and water gives carbon dioxide gas and ammonia
CO(NH2)2 + H2O => CO2 + 2NH3
Diesel catalyst assisted conversion NOx (nitrous oxides) and the ammonia from above give nitrogen gas and water vapor.
4NO + 4NH3 + O2 => 4N2 + 6H2O
4NO2 + 4NH3 + O2 => 3N2 + 6H2O
The diesel sulfur fuel and ammonia, if at wrong temperature of exhaust gases can form ammonium sulfates, as sulfur can condense out if temperatures are low.
(I see this sulfur form in steam boiler flues if flue temperature of flue gas gets too low and water formed in combustion of flue gas cakes this sulfur deposits, when burning a sulfur based fuel like diesel or coal, this can also form some sulfuric acid to etch at metal surfaces).
If exhaust gases were too hot ammonia would just evaporate when liquid is injected into hot exhaust gases, so reaction to convert the nitrous oxide gases to nitrogen may not occur.
(I am unsure how they control the exhaust gas temperature, beside maybe by adjusting fuel to air ratio of the engine to get complete combustion of fuel with maybe some excess air for pollution and to cool cylinders some),
I am wondering if this is not in part driven by the price of the catalytic precious PGM metals and manufacturing saving money, shifting the cost onto the consumer having to maintain and make repairs later after the truck drives off of the sales lot, this system to me sounds like a high maintenance system, and potential to cost more in maintenance and repairs of the truck.
Of course I hardly ever like the new things, to me the old things always worked better. I do not take change easily.
