The United States government has set 2050 as the target for net-zero greenhouse gas emissions due to their increasing levels and the subsequent rise in global temperatures. To meet this target, there has been renewed interest in the combustion of high-energy biofuels that could combat these issues. Thus, the Department of Energy started the Co-Optimization of Fuels and Engines program to find bioderived blendstocks that can harmonize with current and future generation engines to increase power and efficiency, all while reducing overall emissions. As part of this program, it is crucial to understand the combustion of these fuels at the temperatures and pressures internal combustion engines operate at. Therefore, the oxidation and pyrolysis of several advanced biofuels--cyclopentanone, prenol, 1-pentene and trans-2 pentene, and methyl propyl ether--have been studied in a shock tube reactor to quantify some of their fundamental combustion properties. Measurements include ignition delay times and time-resolved species concentrations, including that of fuel decomposition and formation of intermediate species such as carbon monoxide and ethylene. These measurements are useful for validating and updating chemical kinetic mechanisms that provide the chemistry input into computational fluid dynamic codes. This study's measured data are compared to the predictions of the most recent literature chemical kinetic mechanisms for each fuel. When appropriate, sensitivity analyses were conducted to highlight reactions sensitive to the conducted measurements, and some reaction rate modifications were made.

The dissertation discusses the details of the four following items: 1) design, assembly, and testing of a shock tube setup as well as a laser diagnostics apparatus for studying ignition characteristics of fuels and associated reaction rates, 2) measurements of methane and propanal infrared spectra at room and high temperatures using a Fourier Transformed Infrared Spectrometer (FTIR) and a shock tube , 3) measurements of ignition delay times and reaction rates during propanal thermal decomposition and ignition, and 4) investigation of ignition characteristics of methane during its combustion in carbon-dioxide diluted bath gas. The main benefit and application of this work is the experimental data which can be used in future studies to constrain reaction mechanism development.

In the current engine development, fuel reformulation is considered as one of the potential strategies to improve fuel efficiency, reduce petroleum consumption, and minimize pollutant formation. Oxygenated fuels can be used as neat fuels or additives in spark-ignition and diesel engines to allow for more complete combustion. To understand the influence of oxygenated fuels on engine performance, accurate comprehensive kinetic mechanisms, which can consist of hundreds to thousands of elementary reactions, are needed to describe the chemistry of the combustion events, such as autoignition and pollutant formation. The primary objective of the research presented in this dissertation is to provide reliable experimental kinetic targets, such as ignition delay times, species time histories, and direct reaction rate constant measurements, using shock tube and laser absorption techniques in order to evaluate and refine the existing kinetic mechanisms for two different types of oxygenated fuels (i.e., ketones and methyl esters) and to reexamine the kinetics of the H2 + OH reaction. The topics of this work are mainly divided into three sections: (1) H2 + OH kinetics, (2) ketone combustion chemistry, and (3) methyl ester + OH kinetics. The reaction of OH with molecular hydrogen (H2) H2 + OH → H2O + H (1) is an important chain-propagating reaction in all combustion systems, particularly in hydrogen combustion, and its direct rate constant measurements are discussed in the first part of this dissertation. The rate constant for reaction (1) was measured behind reflected shock waves over the temperature range of 902-1518 K at pressures of 1.15-1.52 atm. OH radicals were produced by rapid thermal decomposition of tert-butyl hydroperoxide (TBHP) at high temperatures, and were monitored using the narrow-linewidth ring dye laser absorption of the well-characterized R1(5) line in the OH A--X (0, 0) band near 306.69 nm. Consequently, this work aims to report the rate constant for reaction (1) with a much lower experimental scatter and overall uncertainty (as compared to the data available in the literature). Ketones are important to a variety of modern combustion processes. They are widely used as fuel tracers in planar laser-induced fluorescence (PLIF) imaging of combustion processes due to their physical similarity to gasoline surrogate components. Additionally, they are often formed as intermediate products during oxidation of large oxygenated fuels, such as alcohols and methyl esters. In the second part of this dissertation, the combustion characteristics of acetone (CH3COCH3), 2-butanone (C2H5COCH3), and 3-pentanone (C2H5COC2H5) are discussed in the context of the reflected shock wave experiments. These experiments were performed using different laser absorption methods to monitor species concentration time histories (i.e., ketones, CH3, CO, C2H4, CH4, OH, and H2O) over the temperature range of 1100-1650 K at pressures near 1.6 atm. These speciation data were then compared with the simulations from the detailed mechanisms of Pichon et al. (2009) and Serinyel et al. (2010). Consequently, the overall rate constants for the thermal decomposition reactions of acetone, 2-butanone, and 3-pentanone CH3COCH3 (+ M) → CH3 + CH3CO (+ M) (2) C2H5COCH3 (+ M) → Products (+ M) (3) C2H5COC2H5 (+ M) → Products (+ M) (4) were inferred by matching the species profiles with the simulations from the detailed mechanisms at pressures near 1.6 atm. In addition, an O-atom balance analysis from the speciation data revealed the absence of a methyl ketene removal pathway in the original models. Furthermore, the overall rate constants for the reactions of OH with a series of ketones CH3COCH3 + OH → CH3COCH2 + H2O (5) C2H5COCH3 + OH → Products (6) C2H5COC2H5 + OH → Products (7) C3H7COCH3 + OH → Products (8) were determined using UV laser absorption of OH over the temperature range of 870-1360 K at pressures of 1-2 atm. These measurements included the first direct high-temperature measurements of the overall rate constants for reactions (6)-(8), and were compared with the theoretical calculations from Zhou et al. (2011) and the estimates using the structure-activity relationship (SAR) (1995). Biodiesel, which consists of fatty acid methyl esters (FAMES), is a promising alternative to fossil fuels. The four simplest methyl esters include methyl formate (CH3OCHO), methyl acetate (CH3OC(O)CH3), methyl propanoate (CH3OC(O)C2H5), and methyl butanoate (CH3OC(O)C3H7), and their combustion chemistry is a building block for the chemistry of large methyl esters. In the third part of this dissertation, the rate constant measurements for the reactions of OH with four small methyl esters are discussed: CH3OCHO + OH → Products (9) CH3OC(O)CH3 + OH → Products (10) CH3OC(O)C2H5 + OH → Products (11) CH3OC(O)C3H7 + OH → Products (12) These reactions were studied behind reflected shock waves using UV laser absorption of OH over 876-1371 K at pressures near 1.5 atm. This study presented the first direct high-temperature rate constant measurements of reactions (9)-(12). These measurements were also compared with the estimated values from different detailed mechanisms and from the structure-activity relationship (SAR).

High-temperature reacting systems are central to many fields including propulsion, power generation, and transportation. Studying such systems requires the use of experimental facilities such as shock tubes to obtain the relevant high temperature conditions, and non-intrusive diagnostic tools for monitoring parameters of interest in the reaction zone, including temperature and species concentration time-histories. Laser absorption spectroscopy offers high-speed, in-situ measurements of the reacting flow field and provides direct measurements of species concentrations and temperature. Applying absorption spectroscopy via different diagnostic strategies enables tailored measurements of these parameters across a variety of combustion systems for chemical kinetic model refinement and enhancing the fundamental understanding of combustion over a broader range of conditions, ultimately aiding in the development of more efficient and lower-emissions fuels and engines.

Absorption spectroscopy is an important branch of spectroscopy that quantitatively measures the level of attenuation on electromagnetic radiation by a test sample. It offers the promise of in-situ, non-intrusive, fast, and sensitive diagnostics for application to transient harsh environments, such as exoplanets, flames, combustion systems, and hypersonic flows. In the endeavor to expand upon existing spectroscopic knowledge of infrared absorption and offer optical sensing solutions to the practical challenges in these complex environments, better experimental strategies of measurement and calibration for the associated high-temperature gas-phase atoms and molecules are warranted. This dissertation describes the development of two experimental approaches for the studies of potassium line shapes and broadband molecular absorption using state-of-the-art lasers at previously unexplored temperature conditions that are made possible by a shock tube. I first present a new approach to seed and produce alkali metal vapor in a shock tube and the resulting measurements of the high-temperature potassium vapor in a controlled laboratory environment. To overcome the experimental challenges associated with the extreme reactivity of potassium, the new method employs shock waves to break apart potassium chloride (KCl) salt precursors and produce atomic potassium in the shock-heated buffer gas. This potassium seeding approach was demonstrated to be effective between 1100 -- 1900 K and is readily deployable for other absorbing species of alkali metals. To overcome the hurdle of the relatively short test time of a shock tube, high-speed tunable diode laser absorption spectroscopy (TDLAS) was deployed. The lasers interrogated the potassium D1 and D2 transitions near 0.77 μm and yielded well-resolved absorption line shapes every 40 μs. The measured spectra were modeled as Voigt profiles. Line shape parameters are presented with temperature-dependent power-law relations for the potassium resonance doublets with argon, nitrogen, helium, and hydrogen as the collisional partners. Secondly, a novel methodology is presented of rapid-tuning broad-scan laser absorption spectroscopy that measures broadband mid-infrared absorption cross sections of gaseous molecules at elevated temperatures. The new method deploys rapid-tuning, broad-scan external-cavity quantum-cascade lasers (EC-QCLs) in a shock tube and can provide quantitative absorption information at a rate over 30,000 cm-1/s at spectral intervals between 0.35 -- 0.6 cm-1. Within the shock tube test time of a few milliseconds, the lasers can sweep across over 100 cm-1 to cover the entire branches, or even entire bands, of the absorption spectra for these species. In total, this method was used to measure the cross section profiles of ethylene, propene, 1-butene, i-butene, cis-2-butene, trans-2-butene, 1,3-butadiene, methanol, ethanol, formaldehyde, acetaldehyde, and acetone. The measurements focus on their strongest mid-infrared absorption bands between 5.4 -- 6.1 μm and 8.4 -- 11.7 μm for various temperatures and pressures up to 1600 K and 5 atm, respectively. The resulting spectra are distributed in a plain text format and archived as the Stanford ShockGas-IR database through a permanent URL https://purl.stanford.edu/cy149sv5686.

This dissertation details the development and application of mid-infrared laser absorption spectroscopy sensing methods towards advancing low-carbon reciprocating engines for high-efficiency and low-emission power generation in a decarbonized energy sector. The scope of this work includes advancement in methods for fundamental spectroscopic studies, integration of advanced sensors into production reciprocating engines for characterization of combustion of low-carbon fuel blends, and computational methods advancement for high-speed real-time signal processing. A high-temperature, high-pressure optical gas cell is designed to enable controlled studies of molecular absorption spectra at high temperatures (>1200 K) and high pressures (>200 atm) to validate spectroscopic parameters at the elevated conditions in combustion engines. A novel optical approach provides access to the mid-wave infrared wherein lies the fundamental rovibrational absorption bands of combustion species critical to characterization of combustion process and emissions formation. Laser absorption sensors are developed and utilized for experimental measurements in the exhaust of a production Honda single-cylinder spark-ignition engine through design of an in-line exhaust sensor module to gain optical access to exhaust gases close-coupled to the exhaust valve. High-temperature opto-mechanical design and laser fiber-coupling assist in achieving robust measurements of cycle-resolved temperature and species (CO and NO) concentration at a rate of 10 kHz. The exhaust sensor is demonstrated by capturing cycle-to-cycle and intra-cycle emissions dynamics and characterizing emissions response to low-carbon fuel blends incorporating natural gas, hydrogen, and ammonia. To enable real-time measurement output at 10 kHz, computational time of the sensor data processing is reduced to sub-ms scales through the use of machine learning algorithms on an embedded processing platform. Compact neural network and ridge regression models are developed to calculate species concentration and temperature directly from transmitted laser signals, removing the need for computationally-intensive nonlinear fitting methods. The machine learning algorithms are deployed to a field-programmable gate array (FPGA) for further acceleration. Hardware-in-the-loop demonstration yields computational time and latency below 100 μs to expand use of the 10 kHz exhaust sensor for real-time sensing applications. Complementary to the sensor development work, a time-resolved chemical-kinetic model is constructed within Cantera to evaluate reciprocating engine performance and emissions during fueling with low- and non-carbon blends. The simulation model provides insights into strategies for optimization of low-carbon combustion and serves as a foundation for sensor interpretation and future work in engine optimization. Discussion of ongoing work includes the design and development of an electro-hydraulic camless valvetrain for future integration into a reciprocating engine architecture to enhance adaptability for fuel-flexible operation.