NOx Sensor Self-Diagnostic Delay for Ammonia Decay
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Solution Overview
Problem
NOx sensors in vehicle exhaust systems face challenges in accurately diagnosing degradation due to ammonia interference and fluctuating environmental conditions after engine key-off, leading to incorrect identification of healthy and faulty sensors.
Innovation Solution
Implementing a method that delays self-diagnostic tests until ammonia levels decay and environmental conditions stabilize, using a controller to determine a sleep duration based on exhaust system temperature and performing tests only when conditions are within thresholds to reduce variance and increase accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If self-diagnostic tests are performed immediately after engine key-off, then testing frequency is increased, but measurement precision deteriorates due to ammonia interference and environmental fluctuations
Solution Approach 1:
The system performs preliminary cooling of the exhaust system after engine key-off before initiating self-diagnostic tests. This preliminary action allows ammonia to decay and environmental conditions to stabilize, ensuring measurement precision is achieved when tests are finally performed, while still maintaining regular testing intervals through scheduled post-cooling test cycles.
2Measurement precision
If self-diagnostic tests are delayed until ammonia decays, then measurement precision improves, but loss of time increases
Solution Approach 1:
The system dynamically determines the cooling duration based on real-time monitoring of exhaust temperature and ammonia levels. The self-diagnostic tests are scheduled to begin when temperature and ammonia conditions meet predefined criteria, optimizing the balance between allowing sufficient decay time for precision and minimizing unnecessary delay. This dynamic approach ensures tests are performed at the optimal moment rather than using fixed delayed intervals.
3Measurement precision
If qualification conditions are set to exclude high NOx readings, then measurement precision improves, but productivity decreases due to sparse valid data
Solution Approach 1:
The system performs preliminary cooling of the exhaust system before self-diagnostic tests, which pre-reduces ammonia levels and stabilizes environmental conditions. This preliminary action ensures that when tests are performed, the NOx readings are more likely to fall within the qualification conditions range, increasing the quantity of valid test data while maintaining precision through the controlled test environment.
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 reliability and accuracy of NOx sensor self-diagnostic tests by minimizing ammonia interference and environmental fluctuations, thereby improving the detection of degraded sensors and maintaining the efficiency of the NOx emission control system.
Implementation Method 1
Under sufficiently high exhaust temperatures, urea in the exhaust system may be converted to ammonia
Implementation Method 2
A reductant, such as urea, may be injected into the exhaust system upstream of the SCR catalyst, and together, the reductant and the SCR catalyst may chemically reduce NOx molecules to nitrogen and water
Data Source
AI summary
Methods and systems are provided for detecting NOx sensor degradation based on results from a NOx sensor self-diagnostic (SD) test performed after a key-off event. In one example, a method may comprise waiting a duration to perform a SD test of a NOx sensor after a key-off event until engine operating conditions stabilize and reach a set of qualifying conditions. One or more SD tests may be performed after waiting the duration, but outputs generated under conditions where one or more of a temperature at the sensor is greater than a threshold, and an oxygen concentration is outside a threshold range, may be excluded when determining whether or not the NOx sensor is degraded.