Reduction of no.sub.x emissions from gasoline engines

Abstract

NO.sub.x emissions are reduced in two stages during operation of an engine equipped with an exhaust gas catalyst system. In the first stage, a reduction in NO.sub.x emissions is effected in the exhaust leaving the combustion chamber. In the second stage a further reduction in NO.sub.x emissions is effected in the exhaust passing through the exhaust catalyst system. Both of the two-stage reductions are brought about in the same way, namely (a) by use of a fuel composition containing at least one fuel-soluble cyclopentadienyl manganese tricarbonyl compound in an amount equivalent to up to 1/32 gram of manganese per gallon of fuel; and (b) by enhancing the NO.sub.x control activity of at least the catalyst by prior operation of the engine on such fuel composition, either intermittently or continuously. Such prior operation constitutes a run-in or conditioning stage of sufficient total duration (continuous or interrupted) so that substantial reductions in NO.sub.x emissions are thereafter continuously achieved in subsequent operation of the engine. The efficacy of the invention was demonstrated by conducting a fleet test involving 48 cars, each operated for 75,000 miles during the test. Dramatic reductions in NO.sub. x were achieved. Moreover, no significant catalyst pluggage, or increase in exhaust gas back pressure, or adverse effect upon oxygen sensors were experienced in the fleet test.