NOVELTY - The flow of exhaust gas from the engine into a nitrogen-oxide absorber (46) and bypass flow of the exhaust gas around the absorber are calculated based on speed, load of engine. A preset quantity of reductant such as hydrogen, methane is supplied into an exhaust path (22) upstream to the absorber. The reductant is oxidized so as to maintain target regeneration flow.    USE - For regenerating nitrogen oxide absorber used with internal combustion engine.    ADVANTAGE - Enables to maintain required regeneration flow for generating rich exhaust gas environment due to oxidation of the reductant. Enables to reduce consumption of hydrocarbon and regeneration time period.    DETAILED DESCRIPTION - The target regeneration flow of the exhaust gas through the nitrogen-oxide absorber is determined. The exhaust gas is passed through the absorber when the calculated target regeneration flow is more than the flow of the exhaust gas from the engine. The flow of exhaust gas from the engine into nitrogen-oxide absorber and bypass flow of the exhaust gas around the absorber are calculated based on speed, load of engine, temperature and pressure of intake manifold , intake mass flow, fuel flow into engine and flow of exhaust gas.    INDEPENDENT CLAIMS are also included for the following:    (1) operating method for internal combustion engine which involves directing exhaust gas generated during combustion of fuel through a lean nitrogen oxide absorber. The absorber is regenerated periodically. The starting and ending time of the regeneration cycle are set based on the concentration of reductant downstream to the absorber, below and above a preset threshold concentration. The regeneration flow is controlled by a bypass valve (14) in a bypass path (12) and an exhaust valve in the exhaust path; and    (2) after-treatment system which has regeneration catalyst (42) arranged in the exhaust path for absorbing reductant. A flow control valve is arranged in a reductant line for regulating flow of reductant into the exhaust path. A controller (70) regulates operation of each valve based on control information from a sensor (71).    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the nitrogen-oxide control system.    bypass path (12)    bypass valve (14)    exhaust path (22)    regeneration catalyst (42)    nitrogen oxide absorber (46)    controller (70)    sensor (71)