Closed-Loop SCR Control for NOx Slippage Reduction
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Solution Overview
Problem
Engine after-treatment systems face challenges in accurately and timely detecting and predicting changes in engine operating conditions and catalyst performance, leading to NOx and reductant slippage due to variable parameters and catalyst aging, which affects the effectiveness of selective catalytic reduction (SCR) systems.
Innovation Solution
A closed-loop control system that adjusts the reductant dosing command based on real-time feedback and feed-forward models, incorporating high and low frequency adaptation values to account for rapid and gradual changes, using a reference generation module, feed-forward module, and feedback module to optimize SCR conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional open-loop SCR control is used, then the system structure is simple, but the system cannot accurately detect and respond to rapid changes in engine operating conditions, leading to NOx slippage
Solution Approach 1:
The patent implements a closed-loop control system that uses a NOx sensor to measure actual NOx concentrations in the exhaust stream and feeds this information back to the controller. The controller compares measured values with target values and adjusts reductant dosing accordingly, enabling the system to detect and respond to changes in engine operating conditions and catalyst performance, thereby maintaining reliable SCR conversion efficiency
Solution Approach 2:
The control system is divided into distinct functional modules: a feed-forward module that calculates initial dosing commands based on engine operating parameters, a feed-back module that measures actual NOx concentrations, and an adaption module that reconciles discrepancies between predicted and actual values. This segmentation allows each module to specialize in specific tasks while working together to achieve accurate NOx control
2Adaptability or versatility
If the system uses a single time-scale control strategy, then the control logic is simple, but it cannot simultaneously address both rapid transient changes and gradual catalyst aging
Solution Approach 1:
The adaption module is divided into two distinct subsystems: a high-frequency adaption subsystem that processes rapid transient changes using dynamic models and responds within seconds, and a low-frequency adaption subsystem that addresses gradual catalyst aging through slow drift compensation over hours or days. Each subsystem operates on its own time scale with appropriate filtering and integration, allowing the system to simultaneously handle both rapid and gradual variations without excessive complexity
Solution Approach 2:
The control system employs periodic updates at different time scales: high-frequency adaption values are calculated and applied rapidly to track transient changes, while low-frequency adaption values are updated more slowly to capture gradual catalyst degradation. This multi-rate periodic action allows the system to adapt to changes occurring at different speeds without requiring continuous complex processing
3Reliability
If reductant dosing is increased to prevent NOx slippage, then NOx conversion improves, but reductant slippage increases
Solution Approach 1:
The closed-loop control system continuously measures actual NOx concentrations downstream of the SCR catalyst and feeds this information back to the controller. By comparing measured NOx levels with target values, the system can precisely adjust reductant dosing to achieve the minimum required for effective conversion, avoiding excessive dosing that would cause reductant slippage while maintaining reliable NOx removal
Solution Approach 2:
The system uses the NOx sensor measurements and control algorithms to automatically self-regulate reductant dosing without requiring external intervention. The adaption module continuously learns from the difference between predicted and actual NOx concentrations, automatically adjusting dosing commands to optimize conversion efficiency while minimizing reductant consumption and preventing both NOx and reductant slippage
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
The system effectively reduces NOx slippage and reductant slippage by accurately determining and adjusting the reductant dosing command, maintaining optimal SCR conversion efficiency and preventing reductant over-supply, thereby improving the overall treatment of NOx in exhaust gases.
Implementation Method 1
Selective catalytic reduction (SCR) systems typically are configured to provide one or more catalyst elements that, with the aid of a reductant, covert nitrogen oxides (NOx) in exhaust gases into nitrogen (N2) and water
Data Source
AI summary
A system and method for using feed-back information, such as, for example, system-out NOx levels, to provide for closed-loop selective catalytic reduction control. A reference generation module may adjust an ideal SCR conversion value based on a quantity of reductant slippage from the engine system so as to provide an adjusted ideal SCR conversion efficiency value. The adjusted ideal SCR conversion efficiency value may then be used to generate a reductant to NOx ratio that corresponds to an ideal reductant dosing command. The ideal reductant dosing command may be further adjusted to account for low frequency variations, such as, low frequency variations in the SCR system, to generate an adjusted ideal reductant dosing command. The adjusted ideal reductant dosing command may be further adjusted to reflect high frequency variations in the SCR system, before a final reductant dosing command is generated for a reductant injector.