Date of Award

9-21-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Benjamin Akih-Kumgeh

Second Advisor

Ashok Sangani

Subject Categories

Engineering | Mechanical Engineering

Abstract

The thesis investigates the problem of knocking combustion in Spark-Ignition engine using Computational Fluid Dynamics (CFD). It seeks to understand the mechanism of engine knock and the role of chemical kinetic models in its prediction. Specifically, it interrogates the claim that low-temperature chemical kinetics is indispensable for knock prediction, compared to high-temperature chemical kinetic models generally viewed as incapable of predicting knock. This work is motivated by the projected continuous relevance of SI engines and the limiting behavior of knock in these engines. Higher efficiencies can be obtained if the compression ratios of engines can be increased while eliminating the undesirable knock. Uncontrolled knock also leads to additional emissions. The computational analysis carried out to address the research question are based on models of turbulent flows, flame propagation, chemical kinetic reactions, and a reasonable engine geometry, whose combination require careful attention. The target engine geometries are those of aspirational compression ratios. The combination of physical models is first validated using engine measurements from the literature. Accompanying the study of the effect of chemical kinetic models is an examination of other parameter effects, such as engine speed, fuel composition, and compression ratio. The work establishes that both high-temperature and low-temperature chemical kinetic models can predict engine knock reasonably well. This unexpected conclusion is explained by the significant role played by heat release from the normal spark-initiated combustion. This heat release establishes high enough temperature that dominates the knocking process near to, and almost overlapping with the flame front. There is relative competition between the flame propagation speed and the rate of chemical reactions that induces knock. This calls for greater care and justification in choosing flame propagation model constants. This work further shows that through direct chemistry integrated simulation of the well-resolved flame front, more realistic estimates can be made of the flame front. This provides reasonable references for the designation of model constants for flame front-tracking approaches to the spark-initiated combustion phase. This work advances our understanding of engine knock mechanism and enable rapid knock simulation through its demonstration of the power of the less costly high-temperature chemistry in engine knock prediction.

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Open Access

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