2 ms·

Nevermind, got the answer here: http://spectrum.ieee.org/cars-that-think/transportation/advanced-cars/how-professors-caught-vw-cheating http://spectrum.ieee.o

• x0x0 11y ago
  from the article [Dan] Carder [director of West Virginia University’s Center for Alternative Fuels, Engines and Emissions (CAFEE)] did, however, shed light on diesel exhaust. The problem, he says, is one of thermal management. Catalytic converters break down NOX only when they’re hot, and keeping them hot may sometimes require that you burn more fuel than you need for the performance you want. You might want to run the engine on a lean fuel-air mixture, but you have to run it rich. “There’s also a trap for NOX that acts like a sponge,” Carder points out. “But if you saturate it with NOX, then you have to go to a rich-burning condition to use the catalyst to reduce the NOX” and thus wring out the sponge.