Mixture non-uniformity in SCR systems: Modeling and uniformity index requirements for steady- state and transient operation

Selective catalytic reduction (SCR) of NOx is coming into worldwide use for automotive diesel emissions control. To meet the most stringent standards, NOx conversion efficiency must exceed 80% while NH3 emissions or slip must be kept below 10-30 ppm. At such high levels of performance, non-uniformities in ammonia-to-NOx ratio (ANR) at the converter inlet can limit the achievable NOx reduction. Despite its significance, this effect is frequently ignored in 1D catalyst models. The corresponding model error is important to system integration engineers because it affects system sizing, and to control engineers because it affects both steady-state and dynamic SCR converter performance.A probability distribution function (PDF) based method is introduced to include mixture non-uniformity in a 1D, real-time catalyst model. The model is subsequently applied over a broad range of steady engine operating conditions to identify regimes where non-uniformity has greatest impact on NH3 slip limited conversion efficiency. Non-uniformity effects during transient emission cycles (US EPA HD-FTP and SET) are also studied for a 15 liter heavy-duty diesel which targets EPA-2010 standards and uses a copper-zeolite formulation. It is shown that the required ANR uniformity index is a function of not only the engine operating conditions and emissions test cycle, but also catalytic coating characteristics.