[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.