Theses and Dissertations

Advisor

Hwang, Joonsik

Committee Member

Shanti, Bhushan

Committee Member

Zhao, Jian

Committee Member

Es-Shali, Omar

Date of Degree

12-12-2025

Original embargo terms

Immediate Worldwide Access

Document Type

Dissertation - Open Access

Major

Engineering (Mechanical Engineering)

Degree Name

Doctor of Philosophy (Ph.D.)

College

James Worth Bagley College of Engineering

Department

Michael W. Hall School of Mechanical Engineering

Abstract

The regulation of diesel and gasoline direct injection (GDI) during cold starts has become subject to stricter numerical limits by both the Environmental Protection Agency (EPA) in the US and the European Commission (EC). For diesel engines, the EPA’s Clean Trucks Plan (Model Year 2027+) sets a certification NOₓ limit of 0.035 g/bhp-hr and an in-use fleet average limit of 0.050 g/bhp-hr, along with a PM limit of 0.005 g/bhp-hr, representing an 82.5% reduction in NOₓ compared to the former 0.2 g/bhp-hr standard. For gasoline engines, US standards regulate cold-start pollutants such as hydrocarbons (HC) ranging from 0.03 to 0.08 g/mi and nitrogen oxides (NOx) from 0.06 to 0.7 g/mi [1]. In contrast, the European Union adopts a stricter approach through Euro standards. The upcoming Euro VII regulation (effective May 29, 2028) harmonizes steady-state, transient, and real-driving emissions (RDE) for heavy-duty diesel engines by setting unified limits of 0.200 g/kWh for both NOₓ and PM. For light-duty gasoline vehicles, Euro 6d specifies cold-start limits for HC and NOx at 30 mg/km [2]. Gasoline vehicles exhibit significantly higher Total Hydrocarbons (THC) and NOx emissions at −7°C compared to 23°C, with THC increasing 6.5-fold and NOx increasing 1.7-fold under cold-start conditions [3]. To meet increasingly stringent emission regulations for diesel combustion systems and cold-start GDI engines, which produce hydrocarbon (HC) and nitrogen oxides (NOx) emissions, extensive experimental, computational fluid dynamics (CFD), and simulation studies have been conducted to analyze the effects of injector operating conditions and fuel characteristics on spray behavior and spray topology. These studies offer a deeper understanding of how injection pressure and fuel temperature affect spray behavior and provide insights into atomization, air–fuel mixing, spray penetration, width, and 3D topology. To complement this analysis, CFD simulations using the Large Eddy Simulation (LES) model and KH-RT breakup model were employed to numerically replicate spray dynamics and validate results. Additionally, a machine-learning model was developed to predict spray characteristics and 3D spray morphology using high-speed imaging, extinction imaging, schlieren photography, and 3D tomographic reconstruction from LVF data captured at 0°, 11.25°, and 22.5°, trained on injection pressure, fuel temperature, and PLV.

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