SCR Catalyst NOx Sensor Cross-Sensitivity Decoupling
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Solution Overview
Problem
Existing exhaust aftertreatment systems for internal combustion engines, particularly those with SCR catalysts, face inefficiencies in NOx conversion due to cross-sensitivity issues with NOx sensors, leading to unstable system behavior and inadequate feedback control.
Innovation Solution
Incorporating a NOx sensor downstream of the SCR catalyst to estimate NOx and NH3 levels, decoupling cross-sensitivity, and implementing feedback control mechanisms to adjust reductant dosing based on these estimates, using a controller with modules for NOx and NH3 estimation, correction, and reductant dosing control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a NOx sensor is used for feedback control of reductant dosing, then NOx conversion efficiency can be improved, but cross-sensitivity to NH3 causes unstable system behavior
Solution Approach 1:
The patent segments the SCR catalyst into multiple zones (upstream bed, mid-bed, downstream bed) and places sensors at different locations. A first NOx sensor is positioned upstream of the SCR catalyst, while a second NOx sensor is positioned in the downstream bed. This spatial segmentation allows the system to distinguish between NH3 slip (detected by the second sensor) and unconverted NOx (detected by both sensors), thereby resolving the cross-sensitivity issue and stabilizing feedback control.
Solution Approach 2:
The patent introduces an intermediary computational layer that processes sensor signals through a signal model. The controller uses the outputs from both NOx sensors along with a signal model to estimate actual NOx and NH3 concentrations. This intermediary processing decouples the cross-sensitivity effect, allowing accurate feedback control without direct reliance on the cross-sensitive second NOx sensor reading alone.
2Speed
If feedforward reductant dosing is used, then dosing speed is improved, but inability to account for system variations reduces NOx conversion efficiency
Solution Approach 1:
The patent implements a feedback control mechanism using the second NOx sensor positioned in the downstream bed of the SCR catalyst. The controller continuously monitors the output of this sensor and adjusts the reductant dosing rate accordingly. This feedback loop enables the system to account for variations in catalyst performance, temperature, and other operating conditions, thereby maintaining high NOx conversion efficiency while preserving the speed benefits of electronic control.
Solution Approach 2:
The patent employs dynamic adjustment of reductant dosing based on real-time sensor feedback. The controller modifies the dosing rate dynamically in response to changing system conditions, including catalyst temperature, space velocity, and measured NOx/NH3 levels. This dynamic control strategy allows the system to adapt to varying operating conditions while maintaining optimal performance.
Data Source
AI summary
Systems, methods and apparatus disclosed that include an internal combustion engine and an exhaust system that includes an exhaust aftertreatment system with an SCR catalyst. A NOx sensor downstream of the SCR catalyst is provided along with techniques for estimating an amount of NOx and NH3 at the tailpipe to decouple the impact of cross-sensitivity of the NOx sensor to NOx and NH3. Feedback control of the reductant dosing amount based on these estimates is also provided.