NOx Sensor Offset Diagnostic in Idle and After-Run
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Solution Overview
Problem
Current NOx sensor diagnostics face challenges in accurately detecting sensor offsets due to cross-sensitivity with NH3 slip, leading to false failures, especially in transient conditions and when an SCR catalyst is missing, and are often compromised by competing diagnostic priorities, making it difficult to run diagnostics during non-fueling conditions.
Innovation Solution
A system and method for running NOx sensor offset diagnostics during after-run or low idle conditions, where the engine is at a stable state, using enablement criteria to confirm the engine's status and comparing sensor signals to a NOx model or expected values to identify real offsets, thereby reducing noise and interaction with other diagnostics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If NOx sensor diagnostics are run during transient conditions or when SCR catalyst is present, then NH3 slip can cause false failures in downstream NOx sensor offset diagnostic, but running diagnostics during these conditions is necessary for continuous monitoring
Solution Approach 1:
The diagnostic routine is segmented into multiple phases: a first phase runs during transient conditions to detect NH3 slip and set a flag, and a second phase runs during stable conditions to perform the actual offset diagnostic only when the flag indicates no NH3 slip occurred. This segmentation allows continuous monitoring while preventing false failures.
Solution Approach 2:
Before running the NOx sensor offset diagnostic, the system performs a preliminary check during transient conditions to detect NH3 slip and sets a flag. This preliminary action prevents the diagnostic from running under inappropriate conditions, thereby avoiding false failures while maintaining diagnostic coverage during stable conditions.
2Productivity
If multiple diagnostics run during overrun conditions, then diagnostic coverage is improved, but availability to run NOx sensor diagnostics is reduced due to higher operating priority of other diagnostics
Solution Approach 1:
The diagnostic availability is made dynamic by adjusting the run flag based on real-time detection of NH3 slip during transient conditions. The system dynamically enables or disables the NOx sensor offset diagnostic based on current operating conditions, allowing flexible diagnostic coverage without compromising reliability.
3Reliability
If NH3 slip threshold is set very high (>300 ppm) to prevent false failures, then diagnostic reliability improves, but sensitivity to actual NH3 slip decreases
Solution Approach 1:
The system performs preliminary detection of NH3 slip during transient conditions and sets a flag before the main diagnostic runs. This preliminary action allows the system to use a high threshold for preventing false failures while still maintaining sensitivity through the flag-based control mechanism that prevents diagnostic execution when NH3 slip is detected.
Data Source
AI summary
A method for reducing sensor noise in an automobile vehicle NOx sensor offset diagnostic includes: connecting an exhaust system to an engine of an automobile vehicle; sensing a condition of the exhaust system using at least one NOx sensor; identifying when the at least one NOx sensor is at a low noise condition; and running a diagnostic to identify conditions of the at least one NOx sensor. The method further includes selecting one of the low noise condition as the engine in an after-run condition or as the engine in an engine idle condition.