Diesel Aftertreatment System Analyses with DOC and SCR Models

Santhoji  Katare, Ford Motor Company

Stringent emission regulations are a key bottleneck in the progress towards viable lean burn diesel vehicles that offer significant fuel economy advantages over gasoline vehicles. Major advances have been made in recent years at Ford in the design and development of diesel emission Aftertreatment (AT) systems. Mathematical models of AT components have facilitated this progress. This presentation focuses on a hybrid approach to develop robust models of two key diesel AT components – (1) the Diesel Oxidation Catalyst (DOC) that oxidizes carbon monoxide (CO) and un-burnt hydrocarbons (HCs), and (2) the urea based Selective Catalytic Reduction (SCR) catalyst that hydrolyzes aqueous urea to ammonia (NH3) which in turn reduces the Nitrogen Oxides (NOx). The DOC and SCR models have been developed by integrating information from multiple sources – detailed thermal balances, laboratory performance data, phenomenological description of adsorption and desorption in catalyst pores, and experience based correlations – using optimization and statistical tools. The DOC model can predict cumulative HC and CO tailpipe vehicle emissions as well as DOC NOx outlet composition (NO vs. NO2). The SCR model uses the NO2 information from the DOC model to predict NOx conversion and NH3 slip. System level analyses by combining the DOC and SCR models have resulted in important insights used to improve system performance. The talk will highlight some of these case studies including: (1) Extrapolation of the downstream injection exotherm data for a fresh DOC on a Ford Focus to an aged DOC, (2) Effects of NO2 produced in the DOC, SCR aging conditions and NH3 storage capacity on the NOx reduction performance of the SCR on a Light-duty truck, and (3) Development of urea injection strategies based on the trade-off between NOx reduction and NH3 slip.

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