The Environment Protection Agency (EPA) Tier 4 emission standard requires locomotive engines to meet emission compliance at any ambient humidity for an ambient temperature of 60 to 105 F and an altitude level of up to 7000 ft. The engine tests were run at a range of intake air humidity and temperature to determine oxides of Nitrogen (NOx) emissions sensitivity to humidity, intake air temp for large bore diesel engines and compared with humidity correction factor recommended by EPA. Further, the change in NOx concentration altered conversion efficiency of the Selective Catalytic Reduction (SCR) catalyst. Hence, the change in Ammonia to NOx ratio and amount of DEF fluid required to achieve target NOx emissions reduction were studied. Considering this effect, the doser and DEF tank sizing strategy was defined based on in-use ambient humidity, temperature and altitude test conditions in order to mitigate risk of running out of DEF fluid during in-use operation of locomotives.

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