sounds like the original hybrid drive system. I have a couple of tier 4 diesel units here and Nox reduction is the main reason for the DEF system in them. Works pretty good according to the environmental Nazis. Only problem we have is when some really smart idiot puts diesel in the DEF tank. Yep it is next to impossible yet it has happened. But as far as Exhaust is concerned, if he does not remember his posts and claims should tell you something about him.