Exhaust gas aftertreatment systems

Abstract

A method are presented for monitoring an amount of ammonia stored in a urea-based SCR catalyst based on a response of a sensor coupled downstream of the catalyst to a periodic desorbtion of a small amount of ammonia. The sensor could be an ammonia sensor, or a NOx sensor whose signal is sensitive to presence of both NOx and ammonia. The method can be performed at engine start to establish initial ammonia storage amount, or, alternatively, to adjust ammonia storage amounts or diagnose degradation when NOx conversion efficiency of the catalyst is below a predetermined value.

Description

FIELD OF INVENTION [0001] The present invention relates to an emission control system for diesel and other lean-burn vehicles, and more specifically, to achieving optimal NOx conversion efficiency while minimizing ammonia slip in a urea-based Selective Catalytic Reduction (SCR) catalyst by maintaining an optimum amount of ammonia stored in the catalyst. BACKGROUND AND SUMMARY OF THE INVENTION [0002] Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas. [0003] In this regard, Selective Catalytic Reduction (SCR) catalysts, i...

AI Technical Summary

Benefits of technology

[0009] An advantage of the present invention is that ammonia slip can be minimized by performing ammonia storage estimates under known operating conditions by releasing only an insignificant amount of ammonia. Another advantage of the present invention is that an accurate estimate of the amount of ammonia stored in the SCR catalyst is obtained. Therefore, improved NOx conversion efficiency can be achieved by maintaining an optimum ammonia storage amounts in the SCR catalyst. Yet another advantage of the present invention is that cost savings can be achieved by eliminating the need for an ammonia sensor downstream of the SCR catalyst. Additionally, since the downstream NOx sensor is cross sensitive to ammonia, minimizing the ammonia slip improves the accuracy of the NOx sensor readings, and thus improves overall system NOx conversion efficiency.

Problems solved by technology

Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas.

Under-injection of urea may result in sub-optimal NOx conversion, while over-injection may cause tailpipe ammonia slip.

However, such systems are inherently inaccurate and require a large amount of system memory.

However, a disadvantage of such system is that ammonia sensor technology is not mature yet.

Further, once slip is detected, only a corrective action of reducing reductant injection can be taken, which does not guarantee expedient elimination of slip.

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