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Over the years methane and natural gas (NG) as a fuel have become an important source of energy in the power generation and transportation sectors, with the latter most prominently adopting propane used in public transportation shuttles. Notable interest in using liquid methane or NG fueled rocket engines has recently gained traction and are currently in development/production at SpaceX (raptor engine, full-flow staged combustion cycle) and Blue Origin (BE-4, staged combustion cycle). Given the variety of applications, sourcing and refinement of methane/NG may be completely different to ensure a certain purity is met, especially in an advanced rocket engine cycle like the raptor engine, that may be highly sensitive to minor fuel variances. The current study seeks to understand the ignition behavior of methane (with and without CO2 dilution) and NG surrogate mixtures, including CH4/C2H6/C3H8 and CH4/C2H6/C3H8/i-C4H10/n-C4H10, using a high-pressure shock tube facility at reflected shock conditions relevant to advanced rocket engines and gas turbine cycles. This included pressures near 16, 100, and 200 bar throughout a temperature range of 1000-1621 K. Ignition delay time data were compared with predictions of a chemical kinetic mechanism, and along with performed sensitivity and pathway analyses, further aided in the understanding of the observed ignition behavior. The combined effects of lower activation energy, increased concentration of OH and CH radicals, and increased heat of combustion all play some role in the observed promotion of ignition. These combined effects are primarily a function of fuel mixture and concentration, and combustion relevant pressure and temperature conditions (reflected shock conditions T5 and P5), which in addition dictate the propensity of preignition and deflagration-to-detonation transition behavior observed in the shock tube.

Understanding the combustion chemistry of ethanol is critical for continued proliferation and use in future internal combustion engines (ICEs) that will operate in a downsized, turbo-charged, high compression configuration. Detailed chemical kinetic reaction mechanisms already exist for ethanol, which have been validated over a range of operating conditions; however, capturing the conditions that may be seen in future ICEs requires extension of these conditions, namely at elevated pressure. Investigating the kinetics of ethanol existing in a combustion system first involve, for example, understanding a key global metric like ignition delay time (IDT) and measuring major or minor species in a time resolved fashion capturing both formation and decomposition stages. A shock tube facility offers ideal (thermodynamically) operation that can be used to study the high pressure kinetics across a wide range of temperatures, all the while enabling non- intrusive temporal in situ measurements within the given test time. Oxidation of ethanol was carried out behind reflected shock waves at elevated pressures by measuring IDTs and carbon monoxide (CO) time-histories, the latter of which utilized a distributed feedback quantum cascade laser centered at a wavelength in the infrared (IR). With the gathered data, various ignition regimes and sensitive chemistry were investigated for high pressure CO formation. Since CO is an important product of combustion, having an accurate prediction of its formation is necessary to preliminarily understand the efficiency and sustainability of future engine designs. Moving forward, hazardous products like CO among other harmful emissions will have stricter governmental constrains, which further supports studies as these that aid in the continued refinement of such chemical kinetic mechanisms.