produce a desired amount of reformate at the reformer 16 the controller 30 may control for example the flow control valve 14 and or components of the additive supply device 18 to yield the desired amount of reformate the desired amount of reformate may correspond to a substantially stoichiometric amount of hydrogen required to react with the high nox exhaust at the ammonia producing catalyst 22 it is understood however that the components of the ammonia production system 10 may be controlled to maximize the ammonia formed by the ammonia producing catalyst 22 and that in order to maximize the production of ammonia more than a stoichiometric amount of reformate may be produced by the reformer 16 the flow control valve 14 may direct the slip stream 36 to the reformer 16 and an exhaust flow 38 containing a relatively higher concentration of hydrogen hydrocarbons and carbon monoxide and a relatively lower concentration of water and oxygen may pass from the reformer 16 to the high nox exhaust line 12 the exhaust flow 38 may combine with the exhaust flow 32 from the lean cylinder 8 upstream of the ammonia producing catalyst 22 to form a mixed flow 39 to optimize ammonia production in an exemplary embodiment the mixed flow 39 may have an overall composition such that its measured lambda is less than 1 in such an exemplary embodiment the amount of reductants from flow 38 plus flow 32 is greater than the amount of oxidants from flow 38 plus flow 32 the mixed flow 39 may then pass to the ammonia producing catalyst 22 where its components may react with the catalyst materials therein to form ammonia an exhaust flow 40 may then pass from the ammonia producing catalyst 22 to.