Mid-IR laser-based diagnostics for hydrocarbon fuel vapor sensing and decomposition species measurements [electronic resource]
Keywords: Mechanical Engineering

The development and optimization of energy conversion systems, such as internal combustion (IC) engines, gas turbines, and pulse detonation engines, relies on two important factors. The first is the accurate measurement of critical engineering variables at the beginning of the engine cycle. Specifically for IC engines, time-resolved incylinder fuel delivery and gas temperature information is needed as an important optimization parameter for advanced IC engine design and development. The second factor is the accurate predictive modeling of chemical reaction progress during hightemperature fuel decomposition. Energy conversion systems can be optimally controlled based on the understanding and modeling of the chemical processes that control their performance. The first goal of this dissertation is to describe the design and implementation of a mid-infrared absorption sensor for crank-angle-resolved in-cylinder measurements of gasoline concentration and gas temperature for spark-ignition internal-combustion (IC) engines. Mid-IR laser light was tuned to transitions in the strong absorption bands associated with C-H stretching vibration near 3.4 [micrometers], and time-resolved fuel concentration and gas temperature were determined simultaneously from the absorption at two different wavelengths. Validation experiments were conducted for a singlecomponent hydrocarbon fuel (2,2,4-trimethyl-pentane, commonly known as iso-octane) and a gasoline blend in a heated static cell (300≤T≤600 K) and behind planar shock waves (600≤T≤1100 K). A novel, mid-IR, scanned-wavelength laser absorption diagnostic was also developed for time-resolved, interference-free, absorption measurement of methane concentration. A differential absorption (peak minus valley) scheme was used that takes advantage of the structural differences of the absorption spectrum of methane and other hydrocarbons. A peak and valley wavelength pair was selected to maximize the differential cross-section (peak minus valley) of methane for maximum signal-to-noise ratio, and to minimize that of the interfering absorbers. Methane cross-sections at the peak and valley wavelengths were measured over a range of temperatures, 1000 to 2000 K, and pressures, 1.3 to 5.4 atm. Cross-sections of the interfering absorbers were assumed constant over the small wavelength interval between the methane peak and valley features. The differential absorption scheme efficiently rejected the absorption interference and successfully recovered the vapor-phase methane concentration. The second goal of this dissertation is to present and analyze fuel decomposition species concentration time-histories that were measured during the high temperature pyrolysis of several fuels including major gasoline n-alkane components as well as dimethyl ether (DME), a bio-fuel. CH4 and C2H4 concentration time-histories were measured behind reflected shock waves during the pyrolysis of two n-alkanes: n-butane and n-heptane. Experiments were conducted at temperatures of 1200-1600 K and at pressures near 1.5 atm, with fuel concentrations of 1% in Ar. CH4 was measured using the methane diagnostic described above. C2H4 was measured using a fixed-wavelength absorption scheme at 10.532 [micrometers] with a CO2 laser. The measured CH4 and C2H4 time-histories in n-butane pyrolysis were compared to simulations based on the comprehensive n-alkane mechanisms and the chemical model was improved based on the measurements. High-temperature dimethyl ether (DME) pyrolysis was studied behind reflected shock waves by measuring time-histories of CO, CH4 and C2H4 in mixtures of 0.5%, 1%, and 2% DME in argon respectively. Experiments were conducted at temperatures of 1300-1600K and pressures near 1.5 atm. A direct absorption strategy with a fixed wavelength (2193.359 cm-1) using a quantum cascade laser (QCL) was used to measure CO concentration time-histories. C2H4 was measured at 10.532 [micrometers] and 10.675 [micrometers] with a CO2 laser using a two species-two wavelength scheme to reject fuel absorption. The measured CH4, C2H4 and CO time-histories during DME pyrolysis were compared to simulations based on detailed chemical mechanisms, leading to improvements in these mechanisms. These measurement strategies can be used to quantify the needed fuel and temperature loading in IC engines, and the kinetics database obtained provides quantitative species time-histories that can be used to test, validate and refine the fuel decomposition sub-mechanism of gasoline surrogate mechanisms.