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A lean burn engine reduces nitrogen oxides by

  • A.Operating rich of stoichiometric
  • B.Recirculating exhaust gas only
  • C.Raising the compression ratio
  • D.Running very lean

Answer: D

Excess air dilutes and cools the charge, and the emission falls once the mixture is lean enough. The difficulty is that a three-way catalyst cannot reduce nitrogen oxides in the resulting oxygen-rich exhaust, so a lean NOx trap or reduction catalyst is needed.

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