Bluetec is an emission-control technology from Mercedes-Benz that enables a diesel to be as clean as a gas engine.
According to Mercedes' website, "Breakthrough BlueTEC technology controls emissions by driving exhaust gases through filters to the catalytic converter, where they mix with a new water-based additive called AdBlue. This process converts polluting nitrogen oxides into harmless nitrogen and water. The result is a significant reduction in soot and pollutants, making the BlueTEC engine smoother, more efficient, quieter - yet also more robust - than its predecessors. So regardless of which of the three luxurious models (above): the ML350 BlueTEC, the GL350 BlueTEC, and the R350 BlueTEC."