Estimation of automotive urea-based selective catalytic reduction systems during low temperature operations

Urea-based selective catalytic reduction (SCR) systems have become indispensable for automotive Diesel engines to meet the tightened NO_x emission standards. The modeling, estimation, and control of urea- SCR system are more challenging during low-temperature operations than during high-temperature operations, mainly due to the poor urea-to-ammonia conversion process in cold exhaust gas condition. As a result, the actual ammonia concentration input to the SCR system for NO_x reduction and total ammonia slip including the unconverted urea solution and isocyanic acid (HNCO) may be unknown since the urea-to-ammonia conversion process can be delayed downstream to the catalysts or even be incomplete. To help addressing these issues, Lyapunov-based observers are proposed in this study to estimate the ammonia coverage ratio, actual ammonia concentration input, and the total ammonia slip, for a urea-SCR system during low-temperature operations. Simulation results from an experimentally-validated full-vehicle simulator verify that the proposed observers possess the capability of estimating the important states and input in urea-SCR systems with high accuracy under the FTP-75 cycle.