End-Gas Autoignition Fraction and Flame Propagation Rate in Laser-Ignited Primary Reference Fuel Mixtures at Elevated Temperature and Pressure
Engine knock in spark-ignited (SI) engines is initiated by autoignition and detonation in the unburned gases (i.e., end-gas) upstream of the spark-ignited, propagating, turbulent premixed flame. Knock propensity of fuel/air mixtures is quantified by research octane number (RON), motor octane number (MON), or methane number (MN; for gaseous fuels), which are typically measured using single-cylinder, variable compression ratio research engines. In this study, we demonstrate the ability to evaluate knock propensity of SI fuels via observations of end-gas autoignition (EGAI) in unburned gases upstream of laser-ignited, premixed flames at elevated pressures and temperatures in a rapid compression machine. Stoichiometric primary reference fuel (PRF; n-heptane/isooctane) blends of varying reactivity (50 ≤ PRF ≤ 100) were ignited using an Nd:YAG laser at five discrete temperature and pressure conditions, all in excess of 485 K and 16.75 bar. Laser ignition produced outwardly-propagating, laminar premixed flames. High-speed pressure measurements, paired with high-speed Schlieren imaging, clearly indicated the presence of EGAI. The fraction of the total heat release attributed to EGAI (e.g. EGAI fraction) varied inversely with octane number. EGAI fraction and flame propagation rate were both influenced by the magnitude of convection in the unburned gases upstream of the propagating flame caused by variation in piston timing. Experiments were accompanied by three-dimensional computational modeling with detailed chemical kinetics performed using CONVERGETM. The model results reveal low-temperature heat release and hydrogen peroxide formation in the end-gas upstream of the propagating laminar flame. Low-temperature heat release increases the temperature and degree of chain branching in the end-gas and ultimately leads to EGAI.
- Research Organization:
- Colorado State Univ., Fort Collins, CO (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE), Transportation Office. Vehicle Technologies Office
- DOE Contract Number:
- EE0008331
- OSTI ID:
- 1772956
- Report Number(s):
- Paper # 71IC-0208; DE-FOA-0001813
- Resource Relation:
- Conference: 11th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute March 24–27, 2019 Pasadena, California
- Country of Publication:
- United States
- Language:
- English
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