Exhaust emission control system and method for internal combustion engines

x x 2 x x x x x x x xxAn NOstorage-and-reduction type catalyst is used which exhibits a saturated NOstorage amount of 5 g or more as NOwith respect to 1 liter of a catalyst volume at 500° C., and rich spiking is controlled so that an actual NOstorage amount of the NOstorage-and-reduction type catalyst becomes 50% or less of the saturated NOstorage amount.Since the saturated NOstorage amount is large, the NOstorage amount is large even when it is 50% or less, it is possible to prolong intervals of the rich spiking. Then, since NOstorage component stores NOpreferentially into the sites which are likely to store and release NO, a reduction efficiency is high. Therefore, while prolonging the intervals of the rich spiking and sustaining an effect of mileage improvement, it is possible to improve the reduction efficiency of NO.