Exhaust Aftertreatment Reductant Dosing via Dual NOx Sensor Feedback
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Solution Overview
Problem
Current exhaust gas aftertreatment systems face challenges in meeting future stringent NOx emission standards due to the limitations of existing NOx sensors, which cannot accurately measure NOx levels below 0.1 g/kWh, leading to over-dosing of reductant and increased N2O emissions.
Innovation Solution
The system employs a feedback NOx sensor placed downstream of the SCR device and upstream of the ammonia slip catalyst, in combination with a tailpipe NOx sensor, to adjust reductant dosing and achieve the desired ammonia:NOx ratio, ensuring NOx emissions are below detectable levels while minimizing N2O emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If feedback control using tailpipe NOx sensor is used to meet 0.25 g/kWh emission standard, then NOx emissions are reduced to acceptable levels, but the system cannot achieve below 0.1 g/kWh due to sensor measurement accuracy limitations
Solution Approach 1:
The patent divides the aftertreatment system into two functional zones with separate sensing: upstream zone (SCR) for active feedback control and downstream zone (ammonia slip catalyst) for final emission polishing. This segmentation allows the upstream sensor to operate in a measurable range while the downstream catalyst handles ultra-low emissions.
Solution Approach 2:
The ammonia slip catalyst acts as an intermediary component between the SCR catalyst and tailpipe. It converts excess ammonia to nitrogen and water, enabling the system to achieve emissions below the tailpipe sensor's detection threshold while using the upstream sensor for feedback control.
2Manufacturing precision
If reductant dosing is increased to compensate for sensor inaccuracy, then NOx emissions are reduced below detectable levels, but N2O emissions increase due to ammonia slip
Solution Approach 1:
The patent converts the potentially harmful excess ammonia (which would cause N2O emissions) into a beneficial resource by routing it through the ammonia slip catalyst. This catalyst transforms the excess reductant into harmless nitrogen and water, eliminating N2O formation while maintaining ultra-low NOx emissions.
Solution Approach 2:
The system changes the operational parameters of the ammonia slip catalyst to optimize its function. By controlling temperature and residence time in the ammonia slip catalyst zone, excess ammonia is efficiently converted to nitrogen and water rather than N2O, even under varying load conditions.
3Measurement precision
If NOx sensor is placed downstream of ammonia slip catalyst to measure tailpipe NOx, then accurate tailpipe emissions are measured, but the sensor cannot differentiate between NOx and NH3 leading to unreliable feedback control
Solution Approach 1:
The patent segments the sensing function from the emission polishing function. The NOx sensor is placed upstream of the ammonia slip catalyst where it measures both NOx and NH3 for feedback control, while the downstream catalyst zone handles the separation of these components, converting NH3 to N2 and H2O.
Solution Approach 2:
The ammonia slip catalyst serves as an intermediary that processes the exhaust stream between the upstream sensor and tailpipe. It selectively converts excess ammonia while allowing the upstream sensor to utilize its non-selective measurement capability for effective feedback control.
4Manufacturing precision
If consistent over-dosing of reductant is applied to ensure NOx removal, then NOx emissions are reduced below sensor detection limits, but reductant consumption and economic cost increase
Solution Approach 1:
The patent implements feedback control using the upstream NOx sensor to monitor exhaust composition and dynamically adjust reductant dosing. This prevents both under-dosing (incomplete NOx removal) and excessive over-dosing (wasted reductant), optimizing urea consumption while achieving ultra-low emissions.
Solution Approach 2:
The system transitions from static fixed-dose reductant injection to dynamic dosing that adapts to real-time exhaust conditions. The control system continuously adjusts reductant dosage based on upstream sensor readings, load variations, and temperature conditions, minimizing reductant consumption while maintaining emission compliance.
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 allows for precise feedback control of reductant dosing, reducing NOx emissions below detectable limits and minimizing N2O emissions, thereby meeting future emission standards while optimizing reductant consumption.
Implementation Method 1
Selective catalytic reduction (SCR) is an effective technology to reduce tailpipe nitrogen oxides (NOx) emissions. It involves adding a reductant, such as ammonia, to the vehicle exhaust stream. The reductant, with the aid of a catalyst, reduces NOx in the exhaust stream to nitrogen gas (N2) and water.
Implementation Method 2
An ammonia slip catalyst is typically provided in the aftertreatment system to take care of this ammonia slip and prevent ammonia tailpipe emission.
Implementation Method 3
In practical implementations in motor vehicles, an aqueous urea solution is typically used as a reductant and this urea solution is decomposed to ammonia and carbon dioxide in the hot exhaust stream.
Data Source
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AI summary
The present disclosure relates to an exhaust gas aftertreatment system (4), a vehicle (1) comprising such a system and a method for controlling an exhaust gas aftertreatment system. The method comprises the steps: -providing (s503, s603) an initial dosing of reductant from a reductant dosing device (23); -obtaining (s505, s605) a feedback signal from a feedback NOx sensor (33) and a tailpipe NOx signal from a tailpipe NOx sensor (31 ); and -adjusting (s511, s513, s611, s613) the dosing of reductant until the feedback signal exceeds the tailpipe NOx signal by a value within a predetermined positive interval.