SCR Ammonia Pre-dosing for Transient NOx and DPF Regeneration
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Solution Overview
Problem
Existing exhaust aftertreatment systems face challenges in achieving simultaneous high NOx conversion and passive regeneration of catalytic particulate filters, particularly during transient engine conditions, due to lag in ammonia storage and dosing response in selective catalytic reduction (SCR) systems.
Innovation Solution
A method for controlling an exhaust gas aftertreatment system that estimates future NOx conversion demand based on predicted vehicle operating conditions, adjusting reducing agent dosing rates between baseline and elevated levels to optimize NOx conversion and ensure sufficient passive regeneration of catalytic particulate filters, using a dual-SCR configuration with separate dosing devices upstream and downstream of the filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the pre-SCR ammonia storage is kept low to enable high NOx conversion during transient conditions, then the NOx removal efficiency is improved, but the passive regeneration capability of the cDPF deteriorates
Solution Approach 1:
The control system performs preliminary action by increasing ammonia dosing to the pre-SCR catalyst before transient conditions occur (based on predicted vehicle operating conditions), thereby pre-storing ammonia in the catalyst. This allows the system to immediately achieve high NOx conversion when transient conditions arise, without having to wait for the natural lag in ammonia storage buildup, thus maintaining both high NOx removal efficiency and sufficient NOx availability for cDPF regeneration during non-transient periods
2Reliability
If ammonia dosing is increased to meet sharp increases in NOx conversion demand, then the NOx removal efficiency is improved, but the response time is reduced due to lag in ammonia storage buildup
Solution Approach 1:
The system applies preliminary action by using a look-ahead approach to predict upcoming transient conditions and pre-increasing ammonia dosing to the pre-SCR catalyst accordingly. This proactive dosing strategy builds up ammonia storage in advance, eliminating the typical lag time that would otherwise occur during sudden NOx conversion demand spikes, thereby achieving both high conversion capacity and rapid response
Solution Approach 2:
The control system incorporates feedback by continuously monitoring actual vehicle operating conditions and comparing them with predicted conditions, then adjusting the ammonia dosing rate dynamically. This feedback mechanism ensures that the ammonia storage in the pre-SCR catalyst is optimized in real-time to match actual NOx conversion demands, preventing both insufficient conversion and excessive ammonia slip
3Object-generated harmful factors
If the pre-SCR limits NOx reduction to supply high levels of NOx to the cDPF for passive regeneration, then the passive regeneration is improved, but the overall NOx removal from the exhaust stream deteriorates
Solution Approach 1:
The system applies segmentation by dividing the NOx conversion function between two separate SCR catalysts positioned at different locations in the exhaust stream. The pre-SCR catalyst (upstream of the cDPF) is controlled to maintain lower conversion levels during normal operation to supply NOx for cDPF regeneration, while the post-SCR catalyst (downstream of the cDPF) handles the majority of NOx removal. This segmentation allows each catalyst to specialize in its respective function, achieving both effective passive regeneration and high overall NOx removal efficiency
Solution Approach 2:
The control strategy applies local quality by implementing different NOx conversion targets for different locations in the exhaust aftertreatment system. The pre-SCR catalyst operates with a lower local conversion target to preserve NOx for cDPF regeneration, while the post-SCR catalyst operates with a higher local conversion target to achieve the bulk of NOx removal. This location-specific quality control enables the system to simultaneously optimize both passive regeneration and overall emission reduction
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 enables the exhaust aftertreatment system to effectively handle sudden NOx conversion demands while maintaining passive regeneration of the catalytic particulate filter, reducing the need for additional heating sources and improving fuel economy by optimizing ammonia storage and dosing rates.
Implementation Method 1
The reductant, with the aid of a catalyst, reduces NOx in the exhaust stream to nitrogen gas (N2) and water
Implementation Method 2
an aqueous urea solution is used as a reductant and this urea solution is decomposed to ammonia and carbon dioxide in the hot exhaust stream
Implementation Method 3
the cDPF catalyst oxidizes NOx to NO2 which in turn oxidizes soot at comparatively low temperatures
Data Source
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AI summary
The present disclosure relates to a method for controlling an exhaust gas aftertreatment system, wherein the exhaust gas aftertreatment system comprises a first selective catalytic reduction (SCR) device, a catalytic particulate filter arrangement arranged downstream of the first SCR device, and a second selective catalytic reduction (SCR) device arranged downstream of the catalytic particulate filter arrangement. The method comprises the steps of: - estimating future exhaust conditions based upon predicted vehicle operating conditions (s403); - estimating a future NOx conversion demand based on the estimated future exhaust conditions (s405); - dosing a reducing agent from the first reducing agent dosing device at a rate based at least on the estimated future NOx conversion demand (s406).