To maintain good fuel efficiency, engineers design diesels to run lean, but the consequent increased availability of oxygen promotes the formation of NOx during fuel combustion.

• 为维持较佳的省油效果，工程师会将柴油引擎设计成在稀油状态下运转，但如此一来，氧含量提高，在燃料燃烧时会促使氮氧化物形成。