In the heavy-truck world, there were 2 schools of thought going into the new EPA tier system: Aftertreatment and EGR. Aftertreatment involves the urea/DEF/catalyst systems discussed above, and the primary advantage was that as an add-on system, it allowed the engine builders to meet emissions standards without making MAJOR changes to the actual engine designs (so they could stick with continued, incremental development). Navistar was the major (lone?) holdout and decided to go with advanced EGR instead, which involved a serious redesign of its basic engines, but negated dependance on (complicated and oft-unreliable, as detailed above) aftertreatment systems. The result? It was a disaster for Navistar... and they've since (as of early 2013) reversed course and started down the aftertreatment route.

W.R.T. the EPA... this is all from the same folks who brought us ethanol-blend gasoline, which IMHO constitutes a flat-out FRAUD on the American driver and taxpayer.