System for reducing NOx transient emission

In a method for reducing NOx emissions produced by a vehicle powered by a diesel engine having a de-nox reducing catalyst with an acidic washcoat which does not substantially store NOx and through which the exhaust gases pass so that reduction of NOx occurs by reacting a fuel reductant with the NOx over the catalyst bed, and a separate hydrocarbon trap catalyst immediately upstream of and in direct fluid communication with the de-nox catalyst or the de-nox catalyst having a hydrocarbon trap catalyst portion upstream of the de-nox catalyst, the method comprising the steps of;

• a) during a portion of the time the engine is warming up and prior to the time the engine reaches operating temperature, absorbing hydrocarbons in said hydrocarbon trap catalyst and thereafter, as said engine continues to warm up, desorbing the hydrocarbons from said hydrocarbon trap catalyst to cause an increase in temperature of said de-nox catalyst and said de-nox catalyst, at a set temperature of about 200° C. achieved during warm-up, producing a transient pulse of NOx;
• b) measuring the temperature of one of the exhaust gas, hydrocarbon trap catalyst and de-nox catalyst to determine the temperature of the de-nox catalyst during engine warm-up;
• c) detecting a set rise in the measured temperature during warm-up of the engine prior to the time said de-nox catalyst produces said transient pulse of NOx;
• and,
• d) Injecting a pulse of diesel fuel of a quantity that is matched with and sufficient to reducingly react with said pulse of NOx transient emission in said de-nox catalyst, said fuel pulse injected as a function of said set rise in temperature to occur at a time prior to and not later than the time that said transient pulse of NOx Is produced so that both diesel fuel reductant and NOx transient pulses exist simultaneously in said de-nox catalyst.