Use this URL to cite or link to this record in EThOS:
Title: The physical processes in urea SCR : an experimental investigation of DEF injection and decomposition in hot-flows
Author: Gaynor, Paul D.
ISNI:       0000 0004 7971 0151
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2017
Availability of Full Text:
Access from EThOS:
Access from Institution:
With over a decade of development for mobile application, urea selective catalytic reduction is now the dominant technology for NOx abatement within heavy duty diesel after-treatment. Its capacity to reduce high levels of engine out NOx to within legislated limits has given engine manufactures the flexibility to explore control strategies and operation approaches, such as engine down-speeding and modified injection timing, that can offer improved brake specific fuel consumption (BSFC) at the cost of higher combustion emissions. Efforts to extend the capability of current SCR technology and further reduce the reliance on in-cylinder emissions control are ongoing to enable improved efficiency and continued reductions in the CO2 output of next generation heavy duty vehicles. This gives rise to the need to dose and decompose urea in ever increasing quantities, while avoiding the formation of liquid films and solid deposits within the exhaust, commonly found when dosing at elevated rates. This thesis documents the detailed study of urea dosing systems, capturing the processes involved in ammonia generation from DEF injection into hot gas flows. This was performed following the construction of an optically accessible hot-air flow rig, enabling the application of advanced optical diagnostics for observation of DEF injection and decomposition in conditions representative of the exhaust flow of a 7.1 litre diesel engine. Experimental investigation of four production injection systems – each providing a fundamentally different dosing approach – was performed to determine the current state of technology. High speed shadowgraphy identified the break-up features of each spray and allowed comparison of the global spray structures, while phase Doppler interferometry provided a wealth of quantitative data on the droplet distribution within each spray plume. Characterisation not only exposed the extent of variation that exists between DEF sprays, but provided a means of understanding and selecting features of dosing systems desirable for increasing performance levels. Differences in spray structure were found to have a significant impact on droplet entrainment within the flow, in turn affecting the level of spray-wall impingement seen, and demonstrating the importance of detailed characterisation of an injector before any design optimisation is performed. Exploring the energy demands of increasing dosing rates revealed the need for more effective thermal energy transfer into DEF sprays, identifying the uniformity of spray mass distribution and area of impaction surfaces as critical enablers for more effective decomposition of the dose. Further insight into the processes of urea decomposition within after-treatment systems was gained through monitoring of the major decomposition species. The ability to catalytically promote the second decomposition stage, hydrolysis, was confirmed when DEF dosing, while the exothermic energy release from the reaction at the point of DEF decomposition was shown to help to mitigate liquid film formation. When dosing heavily, evidence of urea storage was seen within the system, its detection with FTIR offering a novel indicator of long term deposit growth. An increased understanding of the processes involved in urea SCR gained throughout the duration of this project has fed directly into the validation of simulation and control models, as well as the development of a prototype high efficiency urea SCR dosing system. On-engine testing of the prototype has demonstrated that the system is capable of over 98 % NOx reduction across a wide range of application specific drive cycles.
Supervisor: Not available Sponsor: ETI
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Mechanical Engineering not elsewhere classified ; Urea SCR ; Selective catalytic reduction ; Urea-water sprays ; NOx reduction ; Urea decomposition ; Optical diagnostics ; Hydrolysis catalysts ; Diesel after-treatment ; DEF ; AdBlue