Reduction of tailpipe NO_x emissions always appears problematic when first tackled, but after effort and introduction of novel approaches, emissions targets have always been achieved. Successes relied on understanding nitrogen oxide chemistries both in cylinder NO_x formation and in aftertreatment NO_x destruction. Here these fundamentals are reviewed before detailing ways in which tailpipe NO_x levels have been controlled since the mid-1970s. The most recent NO_x challenges involve diesel engines, and technologies such as EGR and advanced common rail high pressure fuel injection have significantly reduced engine-out NO_x levels, and combined with aftertreatment developments, especially with ammonia SCR catalysts that enabled surprisingly low temperature operation needed for use on diesel cars, have enabled very low tailpipe NO_x levels. SCR catalysts can use ammonia generated during NO_x-trap regeneration, and they are being incorporated into diesel particulate filters as a contribution to reducing component count and cost. Recent work demonstrated high frequency rich pulsing of NO_x-traps provides good NO_x control at temperatures higher than bulk nitrates are stable, possible mechanism are considered including the speculative role ammonia might have here and in TWC control of NO_x. It is foreseen increasingly simple NO_x control systems will be devised in the future.
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