NOx reductant generation in a compression-ignition engine by hydrocarbon injection during the expansion stroke

The invention relates to a method for producing NOx reductants by injecting hydrocarbon into a diesel engine's combustion chamber during the expansion cycle. The secondary injected hydrocarbon is formed, under appropriate conditions, into oxygenates and olefins that are suitable for reducing NOx species in the presence of conventional exhaust catalysts. The appropriate injection timing is obtained by referring to a calibration obtained by direct engine measurements.