Separating NOx and NH3 in Lean NOx Trap Exhaust Signals
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Solution Overview
Problem
Current vehicle exhaust systems face errors in calculating NOx and NH3 levels due to the NOx sensor's inability to distinguish between the two, leading to overloading of the SCR with NH3 and subsequent NH3 slip at the tailpipe, which is not adequately addressed by existing methods.
Innovation Solution
A method to determine NOx and NH3 levels exiting a Lean NOx Trap by measuring sensor output, using a model to estimate NOx entering the LNT, and identifying time points based on regeneration phases and exhaust flow thresholds to separate NOx and NH3 contributions in the sensor signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the raw NOx sensor signal is used directly for calculating NOx entering the SCR, then the calculation is simple, but it leads to errors in urea dosing and SCR control due to NH3 interference
Solution Approach 1:
The patent segments the sensor signal into distinct NOx and NH3 components by identifying specific time windows during LNT regeneration. The method divides the regeneration process into phases where NH3 production occurs separately from NOx release, allowing independent calculation of each component's contribution to the sensor signal.
Solution Approach 2:
The patent performs preliminary identification of NH3 production time points during LNT regeneration before finalizing the NOx calculation. By detecting when NH3 is produced and establishing this as a reference time point, the method prepares the data structure needed for accurate separation of NH3 and NOx contributions in subsequent processing steps.
2Measurement precision
If a model of NH3 produced during LNT regeneration is used to compensate NH3 under-estimation, then NH3 compensation is improved, but it does not account for catalyst aging and still results in SCR overdosing
Solution Approach 1:
The patent uses feedback from the NOx sensor signal combined with knowledge of LNT regeneration timing and exhaust flow conditions to continuously refine the separation of NH3 and NOx components. The method adjusts the calculated NH3 and NOx values based on real-time sensor data and accumulated exhaust flow measurements, creating a self-correcting system that adapts to changing catalyst conditions.
Solution Approach 2:
The patent changes the approach from using a fixed NH3 production model to dynamically calculating NH3 based on variable parameters including sensor signal levels, exhaust flow rate, and time since regeneration start. This allows the system to adapt to catalyst aging by using actual measured data rather than relying on predetermined model parameters that become inaccurate over time.
3Ease of operation
If the NOx sensor signal is used without correction during LNT regeneration, then the system operation is simple, but it causes SCR overloading with NH3 and NH3 slip at the tailpipe
Solution Approach 1:
The patent extracts the NH3 component from the combined NOx sensor signal by identifying specific time periods during LNT regeneration when NH3 is produced. By separating the NH3 contribution from the total sensor signal, the method enables accurate calculation of actual NOx levels entering the SCR, preventing both SCR overloading and tailpipe NH3 slip.
Data Source
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AI summary
A method of determining the level of NOx and/or NH3 levels exiting a Lean NOx Trap (LNT) in a vehicle exhaust system, said exhaust system including a sensor located downstream of the LNT and being responsive to both NOx and NH3 and adapted to give an output signal indicative of the level of both NH3 and NOx, comprising the steps of: measuring the sensor output, and a) determining the NOx output to be the level indicated by the sensor, and/or the NH3 output to be zero during a first period; b) determining a first time point subsequent to a LNT regeneration cycle, where the NOx output is determined to be zero and/or the NH3 output determined to be the level indicated by the sensor c) determining a second time point, whereafter the NOx output is determined to be the level indicated by the sensor, and/or the NH3 output is determined to be zero.