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Systems and methods for reducing NOx emissions using a branched exhaust system with a first and second turbine including an emission-control device containing a zeolite, are described. In one example approach, a method comprises: during a first duration when exhaust temperature is below a first temp

Systems and methods for reducing NOx emissions using a branched exhaust system with a first and second turbine including an emission-control device containing a zeolite, are described. In one example approach, a method comprises: during a first duration when exhaust temperature is below a first temperature threshold, directing exhaust gas through the second turbine and the emission-control device, and adjusting the second turbine to control intake boost; and during a second duration following the first, directing exhaust gas through the first turbine, and adjusting the first turbine to control intake boost. In this way, the first and second turbines may provide a greater degree of boost control in order to reduce boost fluctuations while enabling storing cold start NOx emissions for later reduction.

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1. A method for an exhaust system having a first turbine and a second turbine, comprising: during a first duration when exhaust temperature is below a first temperature threshold: directing exhaust gas through the second turbine and an emission-control device;adjusting the second turbine to control

1. A method for an exhaust system having a first turbine and a second turbine, comprising: during a first duration when exhaust temperature is below a first temperature threshold: directing exhaust gas through the second turbine and an emission-control device;adjusting the second turbine to control intake boost;during a second duration following the first where exhaust gas does not flow through the emission-control device: directing exhaust gas through the first turbine; andadjusting the first turbine to control intake boost; andpurging the emission-control device when exhaust temperature is above a second temperature threshold higher than the first temperature threshold. 2. The method of claim 1 wherein the emission-control device includes a zeolite. 3. The method of claim 1 wherein the emission-control device is a selective catalytic reducing device. 4. The method of claim 1 wherein the emission-control device is a diesel oxidation catalyst. 5. The method of claim 1 further comprising adjusting an expansion across the second turbine to control a rate of the purge and adjusting an expansion across the first turbine in response to the adjustment of the expansion across the second turbine. 6. The method of claim 1 wherein the purging occurs during low vehicle speed and/or low engine load conditions. 7. The method of claim 1 wherein the first temperature threshold is a temperature below which NOx will be substantially adsorbed by the emission-control device and above which NOx will be substantially desorbed by the emission-control device. 8. The method of claim 1 wherein the second temperature threshold is a temperature at which a NOx reducing device is sufficiently heated to become catalytically active. 9. The method of claim 8 wherein the NOx reducing device is a selective catalytic reducing device. 10. The method of claim 1 further comprising preloading a reductant prior to the purging. 11. The method of claim 1 wherein the turbines are variable geometry turbines. 12. The method of claim 1 wherein the turbines have wastegates. 13. The method of claim 1 wherein following a first engine start with a first amount of NOx in the emission control device, the first duration has a first length; and following a second engine start, different from the first start, with a second amount of NOx, in the emission control device, higher than the first amount, the first duration has a second length shorter than the first length. 14. A method for an exhaust system having first and second parallel turbines, comprising: when exhaust temperature is below a threshold: directing exhaust through the second turbine and an emission-control device, but not the first turbine; andadjusting the second turbine to control intake boost;after exhaust temperature rises above the threshold: directing exhaust through the first turbine, but not the second turbine and the emission-control device; andadjusting the first turbine to control intake boost. 15. A method for an exhaust system having a first turbine and a second turbine, comprising: during a first duration when exhaust temperature is below a first temperature threshold: directing exhaust gas through the second turbine and an emission-control device;adjusting the second turbine to control intake boost;during a second duration following the first duration where exhaust gas does not flow through the emission-control device: directing exhaust gas through the first turbine; andadjusting the first turbine to control intake boost; andwhen transitioning from the first to the second duration, operating for a third duration in which the first turbine spins up in order to continue providing the boost previously supplied by the second turbine. 16. The method of claim 15 wherein turbo lag associated with the first turbine spin up during the transition is off-set by gradually decreasing expansion across the second turbine in correspondence with the gradual increase in expansion across the first turbine. 17. The method of claim 15 wherein the emission-control device includes a zeolite. 18. The method of claim 15 wherein the emission-control device is a selective catalytic reducing device. 19. The method of claim 15 wherein the threshold is a temperature below which NOx will be substantially adsorbed by the emission-control device and above which NOx will be substantially desorbed by the emission-control device.