BMW 335d< one of the top green vehicles...BluePerformance uses a solution called AdBlue, which is basically urea (yes, that urea) that is injected into the exhaust gas to turn nitrous oxides into nitrogen gas and water vapor. This works with a diesel particulate filter to reduce emissions. The AdBlue solution should only need to be replenished during regular service intervals, and BMW will include free refills in its 4-year/50,000-mile maintenance program.