Date of Graduation
Statler College of Engineering and Mineral Resources
Mechanical and Aerospace Engineering
With growing demands of adopting tighter control on tailpipe NOx regulations for heavy duty trucks, it is an active topic of discussion by regulators in the U.S. to further tighten the existing NOx standards from 0.2 g/bhp-hr to pursue a next phase of reduction as low as 0.02 g/bhp-hr for Class-8 application. High-levels of NOx conversion, in the order of ~98% may be required by SCR system in the future to provide an operating margin to increase fuel efficiency gains. To unleash this potential gain, the SCR technology employed should be capable of achieving high NOx conversion even during low exhaust temperature operating conditions, at the same time allow limited reductant (NH3) slip over transient drive cycle application. The present work will present findings from exploring areas of improvement in SCR system and in general the aspects of diesel engine aftertreatment system affecting SCR performance such as engine exhaust conditions including non-uniformity of flow, temperature and mal-distribution of NH3, urea mixing pipe design, etcetera in contribution towards meeting the projected regulations for NOx tailpipe limits of 0.05 g/bhp-hr in 2025 and 0.02 g/bhp-hr by 2035, for heavy duty vehicles. The central objective of this study is deemed that an optically accessible capability to visualize the SCR system and in specific urea spray injection process in detail would serve as a beneficial tool in investigating the specific conditions responsible for severity in factors contributing to reducing de-NOx performance
The proposed work can be broadly outlined into the following two sections,
- To identify the factors contributing to deterioration of SCR de-NOx performance and thereby explore innovative solutions to abate them
- To develop a research capability for optical access visualization of SCR system, as a tool to aid in the assessment of factors contributing in affecting the NOx reduction performance, from the standpoint of after-treatment system design, thermal strategy and engine operating conditions
Preliminary results indicate there exist a non-uniformity in the exhaust gas temperature at the inlet face of SCR catalyst brick. The resulting effect is likely to cause a non-uniformity in the NH3-NOx distribution. Uniformity of NH3/NOx species concentration, a.k.a., ammonia to NOx ratio (ANR) plays a very critical in affecting SCR conversion performance. The non-uniformity effect was studied over surrogate HD-FTP, and selective steady state points comprising of a test matrix with a combination of 3 levels of space velocities, 3 levels of SCR inlet temperature, to represent light, medium and high: engine load and engine speed activity regimes. Results indicate that at a high load condition, even though the NO2/NOx ratio were close to 1:1 molar ratio, the ANR was very poor suggesting that an alternative pathway in SCR reduction could have taken place and at high temperature, owing to the storage buffer depleting at a fast rate, with a NH3 slip average of 77ppm. The study suggests it is necessary to obtain a more accurate map of temperature mal distribution across the different monolith channels of the SCR under transient conditions to better understand the factors responsible for localized undesirable ANR. Another principal factor affecting the SCR performance is the inlet NO2/NOx ratio. Controlling the NO2/NOx ratio close to 0.5 is conducive for fast-SCR reaction kinetics. Preliminary studies were conducted to understand the effect of this parameter on SCR NOx reduction, NH3 slip and NH3 storage buffer on different engine operating conditions. Further, this work would present results from study the benefit of utilizing low grade heat rejected from the engine, such as coolant thermal energy, in raising the temperature of NOx reductant being dosed. Furthermore, detailed characterization studies of particle size distribution and particle number (PN) concentration from the standpoint of different urea injection strategies would be presented. Results from this study would provide insights into the effect of adopting different dosing strategies, on the compliance of tailpipe PN limits.
The research work is significant because, there is limited availability of systemic experimental work and in-fact presence of very few platforms that has included the modern heavy-duty stock engine set-up without significant design modifications, after-treatment conditions replicating real world challenges and related in-vehicle equipment configuration for the studies involving SCR performance using optical access setup. The work will contribute to building fundamental knowledge in understanding urea-based deposits formation process from advanced diesel engine after-treatment system configuration; the interaction between engine operating conditions, DEF injection rate and after-treatment system design that influence the urea formation & decomposition process under investigation.
Padmanaban, Vishnu, "Investigation of Pathways to Enhance SCR de-NOx Efficiency in Heavy-Duty Diesel Vehicle using an Optical Access Visualization Approach" (2019). Graduate Theses, Dissertations, and Problem Reports. 7370.
Available for download on Monday, December 07, 2020