Heavy-Duty SCR NOx Sensor Diagnostics During Normal Operation
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Solution Overview
Problem
The performance of NOx sensors in exhaust after-treatment systems varies, making it difficult to ensure accurate and precise measurements of NOx levels, which is critical for maintaining compliance with emissions regulations and avoiding wasteful DEF injection and ammonia slip.
Innovation Solution
Perform intrusive diagnostics by varying the performance of NOx emissions-controlling components during normal engine operation, using linear regression and ammonia-to-NOx ratio calculations to correct sensor measurement errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If NOx sensors are used to control DEF dosing in SCR systems, then NOx emissions can be reduced to within acceptable ranges, but sensor performance varies making accurate measurements difficult
Solution Approach 1:
The system performs preliminary diagnostic tests on the NOx sensor by temporarily varying SCR system performance and measuring sensor responses under controlled conditions. This preliminary characterization establishes baseline accuracy metrics before normal emissions control operation begins, allowing the system to compensate for sensor inaccuracies throughout subsequent DEF dosing operations.
Solution Approach 2:
The system continuously monitors sensor output during diagnostic maneuvers and compares measured NOx levels against expected values based on SCR system state. This feedback loop enables real-time detection of sensor drift or failure, triggering recalibration or fault protocols that maintain reliable emissions control despite sensor performance variations.
2Measurement precision
If intrusive diagnostics are performed by varying SCR system performance, then sensor measurement accuracy can be corrected, but system operation is temporarily disrupted
Solution Approach 1:
The system implements periodic diagnostic testing at predetermined intervals or under specific operating conditions, temporarily pausing normal emissions control to perform sensor characterization. This periodic interruption is minimized in duration and frequency, balancing the need for accurate sensor data against the requirement for continuous emissions compliance.
Solution Approach 2:
The diagnostic maneuvers apply partial variations to SCR system performance rather than complete shutdowns, using subtle adjustments to catalyst temperature or exhaust flow that maintain baseline emissions control while providing sufficient stimulus for sensor characterization. This partial action approach minimizes productivity impact while obtaining necessary diagnostic data.
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
Ensures accurate NOx sensor performance, reducing DEF wastage and ammonia slip, and maintaining compliance with emissions standards without affecting overall system efficiency.
Implementation Method 1
SCR processes use catalysts to catalyze the NOx reduction and a fluid referred to as DEF (diesel emission fluid), which acts as a NOx reductant over the SCR catalyst
Implementation Method 2
DEF is an aqueous solution that evaporates and decomposes to chemically release ammonia so that the ammonia is available for reaction
Implementation Method 3
a diesel oxidation catalyst (DOC) to oxidize unburned fuel and carbon monoxide
Data Source
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AI summary
A heavy-duty truck has a diesel engine, an exhaust after-treatment system, and an engine control unit. The exhaust after-treatment system includes a close-coupled selective catalytic reduction system and an underbody selective catalytic reduction system, a first NOx sensor upstream of the close-coupled selective catalytic reduction system, a second NOx sensor between the two selective catalytic reduction systems, and a third NOx sensor downstream of the underbody selective catalytic reduction system. The engine control unit may perform methods allowing intrusive diagnostics to be performed on exhaust gas NOx sensors using the selective catalytic reduction systems during normal operation of the heavy-duty truck.