1. A method of controlling engine NOx production, comprising: producing a first exhaust stream containing NOx from a first cylinder group; producing ammonia in an ammonia production source to react with the NOx in the first exhaust stream in a catalyst; determining a desired amount of NOx production from the first cylinder group based on the amount of ammonia produced by the ammonia production source; and determining at least one engine operating parameter to produce the desired amount of NOx using a feed-forward neural network, wherein the neural network determines the at least one engine operating parameter based on values of multiple engine operational characteristics at a current time and an earlier time.