SCR Catalyst Observer for NOx Sensor Ambiguity
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Solution Overview
Problem
Current SCR control systems for reducing NOx emissions in combustion engines face inaccuracies due to ammonia (NH3) slip ambiguity in NOx sensors, leading to destabilization of the control system, as the sensors are cross-sensitive and lack direct adjustment capabilities for the NO2/NO ratio, which complicates distinguishing between NOx and NH3, resulting in potential misinterpretation and incorrect urea injection adjustments.
Innovation Solution
An observer device is implemented to estimate NOx or NH3 levels by regulating gains based on the difference between estimated and measured values, allowing the system to distinguish the polarity of sensor outputs and control the ammonia storage level in the first cell of a cascaded SCR catalyst, thereby overcoming sensor ambiguity and improving accuracy without requiring external NH3 slip detection systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a closed loop controller based on an NOx sensor downstream of the SCR catalyst is used, then the overall NOx conversion efficiency can be controlled, but the sensor signal becomes ambiguous when NH3 slip occurs, leading to destabilization of the control system
Solution Approach 1:
The SCR catalyst is divided into multiple storage cells, with the controller specifically managing the ammonia storage level in the first cell. This segmentation allows the system to control the critical ammonia storage region separately, improving the accuracy of NOx conversion efficiency control while maintaining stability even when NH3 slip occurs, as the controller can detect and respond to ammonia accumulation in the first cell before it affects overall sensor readings
Solution Approach 2:
The patent introduces an intermediary calculation method that uses the known relationship between ammonia storage level and sensor output to determine the operating point on the sensor characteristic. By calculating the expected sensor output based on the model and comparing it with actual sensor readings, the system can distinguish between NOx and NH3 signals, resolving the ambiguity that would otherwise destabilize the control system
2Device complexity
If the SCR catalyst is modeled as one NH3 storage tank, then the control system is simple to implement, but the accuracy of NOx conversion efficiency control is reduced
Solution Approach 1:
The SCR catalyst is divided into multiple storage cells (first cell, second cell, etc.), each representing a distinct region of ammonia storage. The controller specifically manages the ammonia storage level in the first cell, which has the most significant impact on NOx conversion efficiency. This segmentation provides more accurate control while keeping the system manageable by focusing on the critical first cell rather than attempting to control all cells equally
Solution Approach 2:
The controller applies different control strategies to different regions of the catalyst. The first cell, which is most critical for NOx conversion, receives direct control attention through ammonia storage level management. Other cells are managed based on their relative importance, creating a hierarchical control approach that optimizes overall performance without requiring equal complexity for each region
3Adaptability or versatility
If the N02/NO ratio is not directly adjustable, then the DOC/DPF system operates autonomously, but the SCR controller cannot optimize NOx conversion efficiency
Solution Approach 1:
The system implements feedback control by continuously monitoring the sensor output and comparing it with the expected output based on the ammonia storage model. The controller adjusts the urea injection rate based on the difference between actual and expected sensor readings, effectively using the sensor feedback to optimize NOx conversion efficiency even though the N02/NO ratio itself cannot be directly adjusted. This indirect feedback mechanism allows the SCR controller to adapt to changing DOC/DPF conditions
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the accuracy of NOx conversion efficiency and ammonia slip limitation by ensuring the controller knows the sensor polarity, allowing for precise urea injection adjustments and maintaining system stability, even without real-time NH3 measurements, thus improving overall SCR catalyst control.
Implementation Method 1
The urea is transformed into ammonia (NH3), which in turn reduces the NOx to harmless nitrogen (N2) and water (H2O) in the SCR catalyst
Implementation Method 2
The relevant chemical reactions occur after adsorption of the ammonia on the catalyst surface
Implementation Method 3
The relevant chemical reactions occur after adsorption of the ammonia on the catalyst surface
Data Source
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AI summary
The present invention provides for a method and apparatus for controlling an SCR catalytic converter. The invention describes a model-based control method including a physical model of the SCR catalyst (2) with more than one NH3 storage cell and a physical model of a NOx sensor (4). The observer feedback gain forces the estimated sensor outputs from the model to converge to measured ones, therefore, there is not ambiguity in the determination of the operating point.