Basics

This page lists the various directly harmful (ie not: nitrogen, oxygen, argon, co2) emissions from diesel engines (and many other ice's)
This is mainly as a lead in for Catalytic Converters , Diesel Particulate Filters, and Diesel Exhaust Fluid systems
Isn't too much of a small scale issue, but with a highway full of cars etc, it builds up
IS FIXABLE, but there are many issues involving a Developer-User Disconnect Issues (may need better name) where it works on paper, but not i certain real life situations, or logistical issues of the supplies/maintenance needed, or hard to maintain systems etc

Emissions

Nitrogen Oxides
Often Abbreviated as NOx
Caused by nitrogen and un-combusted oxygen being heated in the combustion chamber and reacting to form nitrogen dioxide, tridoxide etc\
Main solution is Catalytic Converters, although others can be implemented depending on the predicted amounts produced (a sort of balancing act for the others)
- The type of catalytic converter that does this "three way converter" doesn't work with high exhaust oxygen, as it would react with the catalyst

Caused by incomplete combustion due to too little oxygen
Mainly Carbon Black , but due to the impure nature of the fuel, and the combustion environment, it can often contain heavy metals, and other pollutants
Balancing act here is lower oxygen in the ratio means less "pre-treatment system" NOx but more soot etc
Thus either a perfect to lean fuel-oxygen ratio, or a Diesel Particulate Filter is needed
For more in depth information on this, see Atmospheric Particulate Matter

Uses catalysts to convert Carbon Monoxide, or uncombusted hydrocarbons into water and carbon dioxide
- This is referred to as a "Two-way" converter, although 3 way is mandatory in most places, and thus should be easier to find?
- May be cheaper due to less catalysts?
More recent ones add the additional capability on NOx emission conversion via the following reactions being catalyzed

2 CO + 2 NO → 2 CO2 + N2 hydrocarbon + NO → CO2 + H2O + N2 2 H2 + 2 NO → 2 H2O + N2

This may have higher cost, and also must alternate between lean and rich to have the right amount of oxygen for each reaction
These also tend to not work well with diesel engines due to the exhaust oxygen percentage

Essentially a Gas Scrubber but scaled down to car etc sized
Some SERIOUS logistical issues (no nearby places to buy the fluid, no "emergency" mode due to software locking etc)
Some design issues (design allowing for easy accidental mixing, little storage, no way of knowing if the fluid is "not expired" ie all the ammonia has evaporated off)
If improperly managed can emit ammonia gas (this is an issue of management of the system however)
Educational Issues (people don't know what it does, think it's a gimmick, using expired fluid etc)
It is a workable solution, just needs better design (WITH user feedback), logistical fixes, and ease of use (that or use another system if this proves to be the more difficult option)