Selective NOx catalytic reduction system including an ammonia sensor

Abstract

An improved SCR system for controlling NOx levels in internal combustion engine exhaust, comprising a least one ammonia sensor disposed at an intermediate longitudinal location in an SCR catalyst and in communication with a System Control Module (SCM). The ammonia measurement permits calculation of ammonia storage on catalyst sites via a stored SCM algorithm. Locating the ammonia sensor midway in the catalyst allows for optimum control of NOx reduction and permits the portion of the catalyst downstream of the sensor to be treated as a slip catalyst, thus minimizing or eliminating the need for a second slip catalyst and housing, and reducing the size, volume, complexity, and cost of an SCR system. In-brick ammonia sensor permits the system to manage engine exhaust to a desired NOx conversion level and ammonia slip target value, thus minimizing the rate of consumption of ammonia while meeting required limits for NOx emissions.

Description

TECHNICAL FIELD[0001]The present invention relates to systems for controlling the level of nitrogen oxides (NOx) in internal combustion engine exhaust; more particularly, to such systems for catalytically reducing NOx to N2 by reaction with ammonia; and most particularly, to an improved system having an ammonia sensor disposed within the catalyst for feedback control to manage exhaust levels of NOx and ammonia.BACKGROUND OF THE INVENTION[0002]Reducing and controlling engine emissions of oxides of nitrogen are important considerations in modern internal combustion engines, both spark-ignited and compression-ignited. NOx emissions are an element of smog production, and emissions limits mandated by state governments and / or the federal government are likely to become even more stringent in the future.[0003]One known approach to reducing NOx emissions is to reduce NOx formation by reducing combustion temperatures, such as by recirculation of exhaust gas into the engine firing chambers to...

AI Technical Summary

Benefits of technology

[0016]Briefly described, an improved SCR system in accordance with the invention comprises a least one ammonia sensor disposed at an intermediate longitudinal location in an NOx-reducing SCR catalyst and in communication with an Engine Control Module (ECM). Locating the ammonia sensor within the catalyst allows for optimal NOx reduction and permits the downstream NOx catalyst to be treated as an effective slip catalyst, thus minimizing or eliminating the need for a second slip catalyst and housing, and reducing the size, volume, complexity, and cost of the SCR system. Continuous measurement of ammonia concentration in exhaust at an intermediate location in the SCR catalyst permits calculation of ammonia storage amounts on active catalyst sites throughout the catalyst brick, and thus permits the engine exhaust to be managed to a desired NOx conversion level and ammonia slip target value. Further, placing the ammonia sensor closer to the point of urea introduction reduces hysteresis in the SCR system, allowing faster response to NOx load changes.

Problems solved by technology

Even under the best of control, however, untreated engine exhaust typically contains an unacceptable level of NOx.

In practice, this has proved to be very difficult to achieve.

First, excess ammonia in the tailpipe exhaust can be due to incomplete reaction of the SCR as shown in Eqs. 1-3.

Second, in the SCR catalytic reaction mechanism, NOx reacts with ammonia stored on the catalyst. Ammonia storage capacity is highly dependent on temperature of the catalyst, with capacity at low temperatures being significantly greater than at higher temperatures. Because of this effect, even when dosing is greatly reduced or even stopped completely during hot exhaust transients, unreacted ammonia can be desorbed from the SCR catalyst and pass into the tailpipe exhaust.

Lastly, dosing at low temperatures can lead to solid or liquid urea deposits in the exhaust system which, upon subsequent heating, can lead to additional ammonia release unaccounted for in the dosing control.

First, significant inadequacies in the prior art require a slip catalyst element and volume of slip catalyst to offset lack of optimal dosing control.

Second, due to hysteresis the system cannot be responsive to abrupt changes in NOx load or catalyst temperature, which occur frequently in actual engine usage.

Third, when the sensor is placed after the slip catalyst, ammonia sensed by the ammonia sensor is by definition lost to atmosphere, as the system has no means for absorbing or oxidizing slipped ammonia after the sensor.

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