NOx Sensor Signal Frequency Analysis for SCR Control
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Solution Overview
Problem
Existing exhaust gas after-treatment systems for diesel engines with active SCR face ambiguity in NOx sensor readings due to cross-sensitivity to ammonia, leading to unreliable NOx measurement and potential excess ammonia emissions, especially in transient conditions.
Innovation Solution
An analysis method that isolates a specific frequency band from the NOx sensor signal downstream of the active SCR, determining if the signal indicates NOx or NH3 based on threshold values, without affecting the urea metering control, and adjusts engine parameters to increase harmonic components for accurate measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If only one NOx sensor is implemented downstream of the SCR to eliminate the NH3 sensor, then device complexity is reduced, but measurement precision deteriorates due to cross-sensitivity ambiguity
Solution Approach 1:
The patent segments the NOx sensor signal into different frequency components, separating the continuous component (indicative of NH3) from harmonic components (indicative of NOx). This segmentation allows the single NOx sensor to provide both NH3 and NOx measurement capabilities, resolving the ambiguity caused by cross-sensitivity while maintaining reduced device complexity.
Solution Approach 2:
The patent introduces a frequency domain dimension to the signal analysis. By transforming the time-domain sensor signal into the frequency domain and analyzing different frequency bands, the system can distinguish between NH3 and NOx measurements using a single sensor, effectively adding an analytical dimension that resolves the measurement precision issue.
2Device complexity
If ammonia emissions are modelled to compensate for NOx sensor inefficiency, then device complexity is reduced, but reliability deteriorates in unpredictable transient conditions
Solution Approach 1:
The patent replaces the chemical/ammonia-based compensation model with a signal processing approach. Instead of relying on ammonia emission models that fail in transient conditions, the system uses frequency domain analysis of the NOx sensor signal itself, which provides reliable measurements regardless of transient operating conditions.
3Measurement precision
If frequency band isolation is applied to the NOx sensor signal to distinguish NH3 from NOx, then measurement precision is improved, but device complexity increases due to additional signal processing
Solution Approach 1:
The patent makes the existing NOx sensor serve multiple functions by processing its own signal in the frequency domain. The sensor signal contains information about both NH3 and NOx, and by analyzing different frequency components of this self-generated signal, the system achieves precise distinction without requiring additional sensors or complex external processing systems.
Data Source
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AI summary
An analysis method of a signal generated by a NOx sensor of an active SCR catalyst, the method comprising a first step of analysing the signal generated by the NOx sensor, isolating a predetermined first frequency band and calculating a relative intensity value and a second consequent step of comparing said value with a first predetermined threshold (Th1); said signal is deemed indicative of a measurement of NOx when said value exceeds said first predetermined threshold.