SCR Reducing Agent Metering Control for NOx Conversion
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Solution Overview
Problem
Existing methods for metering ammonia-releasing reducing agents in internal combustion engines face challenges in dynamically adjusting to changing operating conditions and environmental influences, leading to inefficiencies in nitrogen oxides conversion and increased emissions due to sensor inertia and reliance on steady-state operating conditions.
Innovation Solution
A method that uses a control unit to adjust the metering of ammonia-releasing reducing agents based on a model that accounts for various operating points, incorporating multiple operating parameters and integrating sensor data to determine correction values, allowing for real-time adaptation and reducing the need for steady-state conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a pure open-loop control method is used to meter the reducing agent, then the device complexity is reduced, but the manufacturing precision of NOx conversion is insufficient due to inability to compensate for errors and environmental influences
Solution Approach 1:
The patent implements a closed-loop control system that continuously measures actual NOx emissions and ammonia slip using downstream sensors, compares these measurements with target values, and dynamically adjusts the reducing agent metering rate accordingly. This feedback mechanism enables real-time compensation for environmental influences and system variations, significantly improving NOx conversion precision while maintaining reasonable device complexity.
2Manufacturing precision
If a standard closed-loop control circuit with sensors is used, then the manufacturing precision of NOx conversion is improved, but the loss of time occurs due to sensor inertia and system response delay
Solution Approach 1:
The control system performs preliminary adjustments by predicting required reducing agent quantities based on upstream operating parameters (engine load, exhaust flow rate, temperature) before actual emissions are measured. This anticipatory control reduces the time lag caused by sensor inertia and system response delays, allowing the system to maintain high NOx conversion precision during transient operating conditions.
Solution Approach 2:
The patent introduces an intermediary control strategy that combines open-loop prediction based on engine operating parameters with closed-loop correction based on sensor feedback. This hybrid approach acts as a mediator between the fast-responding open-loop system and the slow-responding closed-loop system, optimizing both response time and precision.
3Measurement precision
If the system waits for steady-state operating conditions to determine actual emissions, then the measurement precision is improved, but the productivity is reduced due to inability to adapt during transient conditions
Solution Approach 1:
The control system dynamically adapts its measurement and control strategy based on operating conditions. During steady-state conditions, it uses full closed-loop measurement for high precision. During transient conditions, it switches to predictive control with faster response, accepting slightly reduced measurement precision in exchange for much faster adaptation speed and continuous optimization of NOx conversion.
4Ease of operation
If the metered quantity is determined solely from models and stored data, then the ease of operation is improved, but the reliability is reduced due to inability to compensate for actual emission variations
Solution Approach 1:
The system automatically adjusts metering quantities by combining model-based predictions with real-time feedback from emissions sensors. The control unit continuously compares actual emissions with target emissions and automatically modifies the reducing agent injection rate, maintaining high reliability without requiring manual intervention or complex operator decisions.
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 enables efficient and dynamic adjustment of reducing agent metering, improving nitrogen oxides conversion rates while minimizing emissions of nitrous oxide and unconsumed ammonia, even under non-steady-state conditions, by using class-based correction values and interpolation to maintain accurate emissions control.
Implementation Method 1
They have been used for many years in the power plant industry and recently also in internal combustion engines. A detailed description of these methods can be found, for example, in DE 34 28 232 A1. V2O5-containing mixed oxides such as those in the form of V2O5/WO3/TiO2 can be used as SCR catalysts.
Implementation Method 2
To raise the selectivity and the NOx conversion rates, a reducing agent is present in the oxygen-rich exhaust gas. These approaches have become known under the collective term 'SCR method', where SCR stands for 'selective catalytic reduction'.
Implementation Method 3
Urea decomposes at high temperatures into isocyanic acid and ammonia: (NH2)2CONH3+HNCO (1)
Implementation Method 4
The isocyanic acid is hydrolyzed by water in the exhaust gas to NH3 and CO2: HNCO+H2ONH3+CO2 (2)
Implementation Method 5
One mole of ammonia is required to convert one mole of nitrogen monoxide: 4NO+4NH3+O24N2+6H2O (4)
Implementation Method 6
If, to form NO2, a platinum-containing NO oxidation catalyst is installed upstream of the SCR catalyst: 2NO+O22NO2 (5)
Implementation Method 7
the SCR reaction can be greatly accelerated, and the low-temperature activity can be significantly increased. NO+2NH3+NO22N2+3H2O (6)
Implementation Method 8
Nevertheless, in the presence of NO2, it must also be expected that the emissions of nitrous oxide will also increase according to the following reaction: 2NH3+2NO2+1⁄2O22N2O+3H2O (7)
Data Source
AI summary
A method for use in conjunction with an exhaust gas post-treatment system for metering an ammonia-releasing reducing agent into the exhaust gas stream of an internal combustion engine installed in a motor vehicle and operating with excess air. A control unit meters the quantity of reducing agent as a function of a stored model and, during the operation of the internal combustion engine, determines, by comparison of a value proportional to the nominal emission or the nominal conversion stored in the control unit of the internal combustion engine for the associated operating point of the internal combustion engine and/or of the exhaust gas post-treatment system with a value proportional to the actual emission or the actual conversion metrologically determined by the control unit.