Dual DEF Dosing for SCR Catalyst Light-Off and Fuel Economy
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Solution Overview
Problem
Current diesel exhaust aftertreatment systems face challenges in achieving better fuel economy while meeting regulated tailpipe NOx emission levels and N2O formation limits, due to the thermal sink effect of Diesel Oxidation Catalyst (DOC)/Diesel Particulate Filter (DPF) combo, which delays SCR system light off and requires frequent active regeneration, leading to fuel penalties and increased complexity.
Innovation Solution
The system incorporates a close-coupled SCR catalyst upstream of the DOC/DPF and a main SCR catalyst downstream, with a dual DEF dosing system using two injectors, where the DEF controller calculates the fraction of total DEF to be injected based on the main SCR catalyst temperature and NOx mass flow, utilizing pre-determined maps to optimize NOx reduction and passive soot oxidation across different temperature zones.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the SCR catalyst is placed downstream of the DOC/DPF combo, then the system can utilize a single SCR catalyst with simpler architecture, but the thermal sink effect of DOC/DPF delays SCR light-off and requires frequent active regeneration, leading to fuel penalties
Solution Approach 1:
The patent divides the SCR dosing function into two separate injection locations: one upstream of the DOC/DPF and one downstream. This segmentation allows the upstream injector to provide early NOx reduction before the thermal sink, while the downstream injector ensures complete NOx control after the DOC/DPF, thereby maintaining fuel economy without requiring frequent active regeneration
Solution Approach 2:
The upstream DEF injector performs preliminary NOx reduction before the exhaust gas enters the DOC/DPF thermal sink. By dosing DEF upstream, the system begins the SCR reaction earlier, reducing the burden on the downstream SCR catalyst and decreasing the frequency of active regeneration events, thus improving fuel economy
2Use of energy by moving object
If the SCR catalyst is placed upstream of the DOC/DPF, then the SCR system achieves light-off faster and reduces fuel penalty, but the NOx/soot ratio entering the DPF is significantly lowered, resulting in little to no passive soot oxidation
Solution Approach 1:
The patent segments the DOC/DPF system into two functional zones with separate DEF dosing: the upstream injector targets the SCR reaction before the DPF to maintain fuel economy, while the downstream injector targets the SCR reaction after the DPF to ensure complete NOx control. This segmentation allows each zone to optimize its function independently
Solution Approach 2:
The patent applies local quality by dosing DEF at two different locations with different purposes: upstream dosing creates a locally optimized environment for early SCR reaction and fuel economy, while downstream dosing creates a locally optimized environment for complete NOx conversion and tailpipe emission control, allowing each location to have the specific quality needed for its function
3Ease of manufacture
If vanadium-based SCR formulation is used downstream of DOC/DPF, then the system benefits from lower cost and lower N2O production, but vanadium releases V2O5 at temperatures above 550°C during active regeneration or malfunction events
Solution Approach 1:
The patent segments the SCR catalyst function across two locations, allowing the use of vanadium-based formulation in the downstream SCR catalyst where it benefits from lower cost and lower N2O production. The upstream SCR catalyst can use alternative formulations that are more resistant to thermal degradation. This segmentation isolates the vanadium formulation from the high-temperature regeneration events that occur upstream of the DPF
Solution Approach 2:
The upstream DOC/DPF combo acts as an intermediary that protects the downstream vanadium-based SCR catalyst from direct exposure to the highest temperature events. By placing the thermal barrier (DOC/DPF) between the regeneration events and the vanadium catalyst, the system enables use of cost-effective vanadium formulation while minimizing V2O5 release risks
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 configuration enhances fuel economy by optimizing NOx conversion and reducing N2O formation, meeting stringent emission standards while minimizing fuel penalties and thermal aging, thereby improving the overall efficiency and cost-effectiveness of the aftertreatment system.
Implementation Method 1
Selective catalytic reduction is a known technology for reducing NOx in engine exhaust gas... the chemistry of SCR aftertreatment of engine exhaust gas using DEF is well-documented. When DEF is introduced into engine exhaust gas of sufficiently high temperature, the water evaporates and the urea decomposes to create ammonia. Ongoing reactions in the presence of the catalyst reduce NOx into nitrogen, and water and produce N2O.
Implementation Method 2
When DEF is introduced into engine exhaust gas of sufficiently high temperature, the water evaporates and the urea decomposes to create ammonia.
Implementation Method 3
When DEF is introduced into engine exhaust gas of sufficiently high temperature, the water evaporates and the urea decomposes to create ammonia.
Implementation Method 4
The DOC/DPF are placed upstream so that the high NOx/soot ratio will facilitate passive regeneration of soot.
Data Source
AI summary
Diesel Exhaust Fluid is metered into an engine exhaust gas aftertreatment system having a close coupled SCR catalyst and a main SCR catalyst. The DEF is injected into two injector locations, one upstream of the close coupled SCR catalyst and another upstream of the main SCR catalyst, the quantity of DEF in each injector being based on primarily the temperature at the main SCR catalyst and the mass flow of the NOx through the aftertreatment system. This method of injection enables a relatively better fuel economy outcome while meeting the regulated tailpipe NOx emission levels and the N2O formation limits.