Selective catalytic reduction system

Abstract

A method of treating exhaust gas in an exhaust passage using a selective catalytic reduction system is provided. The system comprises a hydrolysis catalyst in the passage upstream of a SCR catalyst, and a diesel exhaust fluid (DEF) dosing unit for injecting DEF onto the hydrolysis catalyst at a variable DEF dosing rate. The method comprises the steps of predicting an initial DEF dosing rate for converting all nitrogen oxide (NOx) contained in the exhaust gas, and estimating an amount of ammonia stored on the SCR catalyst. The method further comprises the steps of measuring a NOx conversion rate for the system, and adjusting the initial DEF dosing rate based upon the ammonia storage estimate and the measured NOx conversion rate to produce a first adjusted DEF dosing rate. An amount of ammonia-equivalent stored on the hydrolysis catalyst is then estimated, and the first adjusted DEF dosing rate is adjusted based upon the ammonia-equivalent storage estimate to produce a second adjusted DEF dosing rate. DEF is then injected at the second adjusted DEF dosing rate.

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