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Urea/SCR systems have been proven effective at reducing NOx over a wide range of operating conditions on mid/heavy duty diesel vehicles. However, design changes due to reduction in the size of modern compact Urea/SCR systems and lower exhaust temperature have increased the possibility of urea deposit formation. Urea deposits are formed when urea in films and droplets undergoes undesirable secondary reactions and generate by-products such as ammelide, biuret and cyanuric Acid (CYA). Ammelide and CYA are difficult to decompose which lead to the formation of solid deposits on the surface. This phenomenon degrades the performance of the after treatment system by decreasing overall mixing efficiency, lowering de-NOx efficiency and increasing pressure drop. Therefore, mitigating urea deposits is a primary design goal of modern diesel after-treatment systems.

Increasing concerns due to global warming have led to stringent regulation of greenhouse gas (GHG) emissions from diesel engines. Specifically, for GHG phase-2 regulation (2027), more than 4% improvement is needed when compared to phase-1 regulation (2017) in the light heavy-duty (LHD) diesel engine category. At the same time, California Air Resources Board (CARB) and Environmental Protection Agency (EPA) have proposed the new Low NOx standards that require up to 90% reduction in tailpipe (TP) NOx emissions in comparison to the current TP NOx standards that were implemented in 2010. In addition, CARB and EPA have proposed new certification requirements – Low Load Cycle (LLC) and revised heavy-duty in-use testing (HDIUT) based on the moving average window (MAW) method that would require rigorous thermal management. Hence, strategies for simultaneous reduction in GHG and TP NOx emissions are required to meet future regulations.

Stringent emission and GHG regulations drive Diesel engine manufactures to improve hardware and control strategies every model year, lead to increasing calibration development time and cost. Although physics based models has been used in industry for decades, due to accuracy deficiency and high computational time, the usage is still limited to initial design and research phase. In order to fully utilize physics based model’s potential in calibration development with minimal data, improved techniques been shown to overcome those limitations in this paper. Phenomenological combustion and emission models along with air path dynamics are developed in GT-Suite using steady state points over engine operation area. Models are validated using data with limited data corresponding to variation in actuators and Calibration parameters to understand any limitations of using models over wide range of variations.

Various techniques are constantly being devised to accelerate model generation leading to shorter product development cycle. This work proposes and implements a reduced synthetic gas bench (SGB) test protocol for a commercial Pt-Pd diesel oxidation catalyst (DOC) that can be used to develop global reaction kinetics. The kinetics thus developed were implemented in a 1D model to predict DOC emissions accurately over a wide operating window. Hydrocarbons (HCs) in the exhaust were categorized as Propylene (C3H6) representing partially oxidized hydrocarbons and n-Decane (C10H22) representing unburnt fuel. Test protocols were defined using the order of inhibition of the various species present in the exhaust, namely, CO, NOx (NO+NO2) and HC for the specific reaction under consideration. The oxidation reactions for CO and HCs were found to be inhibited competitively by CO and HCs; both the NOx species inhibited these reactions to the same extent.

This work is a part of medium-duty Low NOx technology development project with a focus on evaluating a combination of engine and advanced aftertreatment for 0.02 g/bhp-hr NOx regulation proposed by CARB (California air resource board). In this project, a control oriented chemical kinetics model of SCR (Selective catalytic reduction) was used in the aftertreatment controller that is susceptible to performance degradation due to hydrothermal and chemical aging. This paper focuses on modeling the NOx conversion and NH3 storage characteristics using a controls oriented SCR plant model which is further used for a model-based urea dosing scheme. A set of steady state reactor tests were used to calibrate the SCR performance at degreened, hydrothermal only and hydrothermal + chemical aging conditions and also to determine inhibition factors related to aging. The resultant model is capable of simulating SCR performance deterioration such as a reduction in NOx conversion and NH3 storage.

Typical two-site storage-based SCR plant models in literature consider NH3 stored in the first site to participate in NH3 storage, NOx conversion and second site to only participate in NH3 storage passively. This paper focuses on quantifying the impact of stored NH3 in the second site on the overall NOx conversion for an ultra-low NOx system due to intra site NH3 mass transfer. Accounting for this intra site mass transfer leads to better prediction of SCR out NH3 thus ensuring compliance with NH3 coverage targets and improved dosing characteristics of the controller that is critical to achieving ultra-low NOx standard. The stored NH3 in the second site undergoes mass transfer to the first site during temperature ramps encountered in a transient cycle that leads to increased NOx conversion in conditions where the dosing is switched off. The resultant NH3 coverage fraction prediction is critical in dosing control of SCR.

When used with injecting urea-water solution forming ammonia, Selective Catalytic Reduction (SCR) catalyst is a proven technology for greatly reducing tailpipe emission of nitrogen oxides (NOx) from Diesel engines. However, one major shortcoming of an SCR-based system is forming damaging urea deposits (crystals) in low temperature exhaust operations, especially exacerbated during lower exhaust temperature operations or higher injection rates. Deposits reduce SCR efficiency, damage exhaust components, and induce high concentration ammonia slips. We describe here an Electrically Heated Mixer (EHM™) demonstrated on a Diesel engine markedly inhibiting deposit formation in urea SCR systems, both in low (near 200 °C) and higher exhaust temperature operations and for both low and high urea injection rates in various, realistic engine operations. Engine test runs were conducted in long durations, 10 to 20 hours each, for a total of nearly 100 hours.