NOx Sensor Filtering for SCR Exhaust Systems
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Solution Overview
Problem
In exhaust gas systems with SCR catalytic converters, distinguishing between NOx and NH3 measurements is challenging due to the sensitivity of NOx sensors, leading to difficulties in accurately monitoring NOx conversion and NH3 slip, especially when insufficient or excessive NH3 is dosed, and temperature conditions are not suitable.
Innovation Solution
A filtering method for NOx sensors that determines NOx concentration upstream and downstream of the catalytic converter, models NOx conversion and NH3 slip behavior, calculates errors, and assigns sensor data based on error ratios to differentiate between NOx and NH3 measurements, allowing for accurate data processing without additional hardware, using kinetic models and linear-time algorithms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a NOx sensor is used to monitor exhaust gas composition downstream of an SCR catalytic converter, then information about NOx conversion efficiency can be obtained, but the sensor cannot reliably distinguish between NOx and NH3 measurements
Solution Approach 1:
The system uses feedback from multiple sensors (NOx sensor upstream and downstream, NH3 sensor downstream) to continuously adjust and refine the models predicting NOx conversion and NH3 slip behavior. The measured values are compared with modeled values, and the model parameters are adapted based on the errors, creating a closed-loop system that improves measurement reliability over time.
Solution Approach 2:
Mathematical models serve as intermediaries between the physical measurement process and the interpretation of sensor data. The NOx conversion model and NH3 slip model act as mediators that translate raw sensor signals into meaningful information about catalytic converter performance, allowing indirect inference of NH3 concentrations through the NOx sensor readings.
2Reliability
If additional NH3 sensors are installed to accurately monitor NH3 slip, then measurement reliability improves, but system complexity and cost increase
Solution Approach 1:
The NOx sensor downstream of the catalytic converter is made multi-functional by using it for both NOx measurement and NH3 detection. Through the filtering method that compares modeled versus measured NOx values, the single sensor performs dual functions, eliminating the need for separate NH3 sensing hardware while maintaining monitoring capability.
Solution Approach 2:
Instead of directly measuring NH3 concentrations with dedicated sensors, the system creates a virtual copy or model of NH3 slip behavior through mathematical modeling. The NH3 slip model reproduces the expected NH3 behavior based on operating conditions, and deviations between model predictions and actual NOx sensor readings indicate NH3 slip events.
3Measurement precision
If the NOx conversion model and NH3 slip model are continuously adapted, then measurement accuracy improves, but computational complexity increases
Solution Approach 1:
The model adaptation focuses on adjusting only the necessary parameters that have the greatest impact on prediction accuracy, rather than recalculating entire models. The filtering method applies partial adjustments to model parameters based on observed errors, performing just enough computational action to maintain accuracy without excessive processing overhead.
Data Source
AI summary
Embodiments for determining NH3 slip past a catalyst are provided. In one example, a filtering method for a NOx sensor of an exhaust gas system having an SCR catalytic converter comprises determining NOx concentration upstream of the catalytic converter, measuring NOx concentration downstream of the catalytic converter, modeling NOx conversion, modeling NH3 slip behavior, calculating a NOx error of the NOx modeling, calculating an NH3 error of the NH3 modeling, assigning data of the NOx sensor as NH3 measured values when a ratio of the NOx error to the NH3 error is greater than an upper threshold value, and assigning data of the NOx sensor as NOx measured values when the ratio of the NOx error to the NH3 error is smaller than a lower threshold value. In this way, NOx slip and NH3 slip may be determined from a common sensor.