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.

Abstract: Coking in and around the injector nozzles of indirect injection compression ignition engines is reduced by means of distillate fuel with which has been blended suitable concentration of:(a) organic nitrate ignition accelerator, and(b) the condensation product of a phenol, preferably a high molecular weight alkylphenol, and aldehyde and an amine having a H-N< group.Also described are additive mixtures of (a) and (b) for use in distillate fuels in amounts sufficient to reduce the coking tendencies of such fuels when used in the operation of indirect injection compression ignition engines.