New diesel trucks (at least big, commercial ones) need to meet certain EPA emission standards.
One way some trucks are meeting the N-oxide specification is with a Selective Catalytic Reduction (SCR) system that uses a Diesel Exhaust Fluid (DEF). DEF is a 32.5% mixture of urea, (H2N)2,CO, and water.
The process is pretty simple. The engine exhaust goes through a filter to remove the carbon particulates (AKA soot). The exhaust is then sent into the SCR where it is mixed with the DEF, where the N-Oxides are converted to nitrogen, carbon dioxide, and water.
I assume that the reaction would be:
2(H2N)2CO + 4NO + O2 = 4N2 + 2CO2 + 4H2O
If this works, it would be a pretty elegant solution to nitrogen oxide emissions. I haven’t seen any data, though.
It seems like it’s an awful lot to ask of a pretty small reaction chamber.