Development of Model Based Closed Loop Control Strategy of SCR System for Heavy-Duty Diesel Engines

摘要

Urea selective catalytic reduction (SCR) is a key technology for heavy-duty diesel engines to meet the increasingly stringent nitric oxides (NO_x) emission limits of regulations. The urea water solution injection control is critical for urea SCR systems to achieve high NO_x conversion efficiency while keeping the ammonia (NH_3) slip at a required level. In general, an open loop control strategy is sufficient for SCR systems to satisfy Euro IV and Euro V NO_x emission limits. However, for Euro VI emission regulation, advanced control strategy is essential for SCR systems due to its more tightened NO_x emission limit and more severe test procedure compared to Euro IV and Euro V. This work proposed an approach to achieve model based closed loop control for SCR systems to meet the Euro VI NO_x emission limits. A chemical kinetic model of the SCR catalyst was established and validated to estimate the ammonia storage in the SCR catalyst. Based on the model, a PI controller was designed to control the ammonia storage in the SCR catalyst at the level close to the saturated condition to maximize the NO_x conversion efficiency without exceeding the NH_3 slip limit. Both simulation results and engine bench test results demonstrated good effects of the control strategy. With the effects of the control strategy, the ammonia storage in the SCR catalyst can be well controlled, the NO_x emission in the world harmonized transient cycle (WHTC) after the SCR system of a heavy-duty diesel engine with raw NO_x emission at 8.13 g/kW·h can be reduced to about 0.3 g/kW·h, and the averaged NO_x conversion efficiency was higher than 96%, while the averaged NH_3 slip in the cycle kept below 10 ppm, which has met the NO_x emission requirements of the Euro VI emission regulation.