DEF - Usage

The DEF is only to reduce NOX emissions. Pre DEF systems did this by delaying injection to reduce peak pressures, and pumping a lot of exhaust back into the engine with the EGR. The DEF and SCR allow lower EGR flows, and more advanced timing, which puts the peak pressure where it makes more power, not less NOX. The DEF system then treats the exhaust to break down most of the NOX, to be below EPS levels. Advancing timing also helps with soot production, as the fuel has longer to burn in high temp / high pressure of the engine, before being shot out into the relatively low pressure exhaust system. Remember, more power for the same fuel is the same as less fuel for the same power.