Spray and combustion characteristics of renewable alcohol blends under low charge density conditions

Yan, Junhao

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https://hdl.handle.net/2142/115347 Copy

Description

Title

Spray and combustion characteristics of renewable alcohol blends under low charge density conditions

Author(s)

Yan, Junhao

Issue Date

2022-03-15

Director of Research (if dissertation) or Advisor (if thesis)

Lee, Chia-Fon

Doctoral Committee Chair(s)

Lee, Chia-Fon

Committee Member(s)

• Lee, Tonghun
• Miljkovic, Nenad
• Wang, Xinlei

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• internal combustion engine
• renewable fuels
• emissions
• optical diagnostics
• direct injection
• flash boiling

Abstract

As climate change and environmental pollution become more severe, high efficiency, low fuel consumption and low emissions designs are required to allow internal combustion engine to continuously serve mankind in the future. Comparing with regular operation conditions, engines operating under low charge density can face extra challenges due to low local oxygen concentration and low local energy density. Studies in this dissertation are aimed to understand the effects of low charge density on engine performance and emissions through single-cylinder engine performance and emission tests, optical diagnostics in constant volume chambers, and soot analysis using materials research techniques, in order to overcome some low charge density related issues by using renewable fuels. Since environment pressure decreases with elevated altitude, diesel engines that operate in high-altitude regions commonly experience low charge density that result in poor mixture preparation and decreased combustion efficiency. Low engine efficiency, high fuel consumption, and high emissions are generally observed. As oxygenated fuels, renewable alcohols can enhance mixing quality and reduce local equivalence ratio, thus can be considered as a potential solution. However, their distinctive fuel properties, such as low cetane number and high latent heat of vaporization, can lead to engine instability, particularly under low charge density conditions. Therefore, it is critical to choose the right renewable alcohols for diesel engines operating under high-altitude conditions. In this study, two higher alcohols, n-butanol and n-hexanol, were chosen and mixed with diesel. The results indicated that hexanol-diesel mixture with a 20% blend ratio could increase thermal efficiency and reduce fuel consumption under low charge density conditions. Both butanol and hexanol mixtures could effectively reduce soot and CO emissions, especially when intake pressure was below 1 atm. However, combustion of alcohol-diesel mixtures produced more NOx emissions than pure diesel combustion under the same conditions. In order to further investigate the effects of alcohol addition on spray, combustion and soot emissions under low charge density conditions, optical diagnostics were performed in a constant volume combustion chamber. It was observed that both butanol and hexanol addition could enhance spray evaporation which led to shorter spray penetration under low charge density conditions. Ignition delay and flame lift-off length, which are two major factors related to engine stability and combustion control, were more sensitive to ambient temperature than pressure for the tested fuels. Therefore, engine instability issues caused by low charge density and alcohols addition could be eased by increasing intake temperature with care. Both butanol and hexanol addition could inhibit soot formation and enhance oxidation. Such effects were more pronounced under low charge density conditions. The physical and chemical properties of soot play significant roles for soot after-treatment. Therefore, it is important to understand the effect of alcohol addition on the physicochemical characteristics of soot generated by combustion under low charge density conditions. It has been found that low charge density conditions could reduce the oxidation reactivity of diesel soot and caused extra burden on the after-treatment device. Under the same condition, adding hexanol could mitigate this trend because it could effectively reduce soot primary particle size. The nanostructure of H20 soot particles were also favorable for oxidation. Moreover, soot generated by H20 combustion under low intake pressure condition had more aliphatic and oxygenated functional groups, which further enhanced soot oxidation. Because of these reasons, soot after treatment devices could be designed to operate at a lower temperature with using H20 as fuel, which could reduce energy consumption and extend the lifetime of the device. For gasoline direct injection engines, low in-cylinder pressure caused by low charge density or early injection can lead to spray flash boiling. Flash boiling can enhance spray atomization without requiring ultra-high injection pressure, which provide a solution to improve mixture quality without additional cost. In this work, the macroscopic and microscopic characteristics of multicomponent flash boiling spray have been investigated. Results indicated that spray flash boiling could effectively enhance liquid dispersion and reduce droplet size. It was also found that the morphology of flash boiling spray was highly related to the superheated degree of the fuel regardless the fuel types. Other than reducing the ambient pressure or increasing the fuel temperature, stronger flash boiling could also be obtained by adding volatile components. However, the fuel properties, particularly the thermal properties, need to be considered to achieve an optimized result. Findings in this work provided an in-depth understanding on spray flash boiling and were invaluable for injection strategy design. Through this work, a) Combustion of butanol-diesel and hexanol-diesel under low charge density conditions were studied in diesel engine for the first time and affirmed as possible solution to overcome some high-altitude related issues; b) Spray and combustion characteristics of butanol-diesel and hexanol-diesel have been visualized in a constant volume combustion chamber and a possible solution to improve engine stability under high-altitude application was proposed; 3) Physical and chemical properties of exhaust soot generated by hexanol-diesel combustion under low charge density conditions were explored for the first time; 4) Macroscopic and microscopic characteristics of flash boiling spray with single component fuel and binary mixtures were investigated using different optical diagnostic methods.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Junhao Yan

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