Perhaps I can shed some much needed clarity on exhaust treatment of diesels. Stage 1) was using exhaust gas recirculation (EGR) to reduce oxides of nitrogen (NOx). The recirculated mostly nitrogen lowered the peak combustion temperature, dramatically decreasing NOX.
It required engine and lube oil manufacturers to change things --a LOT for lube oil formulation scientists. Soot loading of the lube oil more than doubled, and the lube oil had to have MUCH better soot dispersion capability, and oil filters had to have much more soot holding capacity to prevent 'bore polishing' and resultant high lube oil consumption-- so much for Stage 1.
Remember, all along the engine folks were beating on refiners to make better diesel with much lower sulfur. They also improved fuel injector technology, piston crown shape and cooling, and other 'metal modifications' to improve fuel economy , durability at high soot loadings etc. There WAS a steep learning curve.
Stage 2 was to further control exhaust soot---particularly the very fine particles (less than 2.5 microns in diameter). Particulate filters were in addition to much improved very high pressure 'common rail' electronically activiated fuel injectors with better optimized spray patterns--which improved fuel economy and reduced particulates This is DPF.
The fundamental issue is how to filter well (to minimize small particles) without plugging up. A host of chemical and mechanical issues were/are at odds. The most common is now ceramic filters that a periodically 'burned clean' by injecting more fuel and air in the hot exhaust to burn off the soot. Now remember, the 'turbo' should extract a lot of energy (heat) from the exhaust to power the compressor side to improve engine power and efficiency. Then you have to add a lot of heat to burn off the particulate filter ever so often.
There are different methods used by different engines to determine when to do a burn-off.. Then the EPA mandated ultra-low sulfur diesel in all road trucks (and now off-road and railway, and ships too!). That caused lubricity related problems with certain injector pumps and fuel design scientists had to make and use 'lubricity additives. The ULSD is, by far, the lowest sulfur in any fuel gasoline, diesel, kerosene, lpg. Making it required refiners to spend $ Billions in research and new high pressure hydrotreaters, new catalysts, more hydrogen manufacture---all of which substantially increased the cost of making ULSD as compared to the old stuff.
At first, there was not enough ULSD to go around and prices soared (law of supply and demand) . While ULSD prices have lowered SOME, they are still way above the real COST of making it. Other things being kept equal, the true 'cost' to make ULSD should not be more than about $0.10 per gallon than regular unleaded gasoline. ULSD also substantially lowered particulate formation, so soot filters are not as problematic as they were.
Now comes Stage 3 where we are today. NOX levels were still too high according to the EPA. There are two (2) ways to lower NOx emissions---1 is to reduce the combustion temperatures--with the brief peak period making the lions share of the NOx. EGR does that , but also robs power from a given engine displacement (recirculated exhaust has much less oxygen than fresh air so it acts as a diluent--you are basically compressing Nitrogen and circulating it, taking horsepower to pump it around.
The other easy to reduce NOx is to convert it back to Nitrogen and water vapor, which is what Diesel ExhaustFluid (DEF) facilitates.
This requires a catalyst (reactor), injection , control, and monitoring systems, etc., but does not rob horsepower, and actually allows MORE power and better fuel efficiency---efficiency goes up and combustion temperature goes up, which makes more NOx , but you easily handle that with the DEF and catalyst. VIOLA for a measly $10,000 or more, you get better fuel economy, much lower NOx emissions, and much lower tiny particulates (and much lower sulfur oxides SOx emissions) all of which have dramatically improved air quality in about 2% of the US territory and 5% of the population.
Modern, up to date diesel engines running on ULSD ARE much cleaner running, give improved fuel economy, and-----only cost a 150 $Billion to invent and a whole lot of additional fuel costs (money transferred from 'the people' to refinery stockholders.), the majority of whom are the 1% folks. The additional capital investments this required have long been recovered and paid for.
