It also depends very much on how the solution is defined. In the instant case of locomotive Tier 4 final standards, it might be observed that EMD missed being able to meet the NOx standard without SCR/urea on the 710 family by about 0.3%. Considering the absolute emission contribution of NOx from locomotive hours operating in the ‘excess’ part of operation, only a small percentage of an already small number of grams, and the substantial cost in many respects for EMD to design something to achieve it, I have to wonder if adjusting the standard to match engineering reality is something the government should have ‘listened to’. It’s not that different from setting the national speed limit at ‘double nickels’ and then providing weasel incentives for speed-trap local enforcement when it would have been both easier and more sensible at 100km/h.
As noted, if there had not been a considerable amount of convergent evolution in other tech fields, meeting ‘lower pollution’ standards would probably still be impossible for practical, maintainable IC automobile engines that meet typical US-American expectations for driveability. I well remember two sets of research: stratified-charge enrichment for practical lean-burn, and the application of high static electrical charge to the fuel plume to facilitate carburetion and decrease wetting-out. Remarkable what modern hard coatings, digital injection control, and the like make easy.
There is a related point: when the government provides a market-based carrot-and-stick, as with the progressive regulation of atmocatalytic CFCs by increasing the effective ‘tax’ on the material dramatically. I’m still expecting massive raise in the government motor-fuel tax once the various