Combined Impacts of Engine Speed and Fuel Reactivity on Energy-Assisted Compression-Ignition Operation with Sustainable Aviation Fuels 2023-01-0263

The combined impacts of engine speed and fuel reactivity on energy-assisted compression-ignition (EACI) combustion using a commercial off-the-shelf (COTS) ceramic glow plug for low-load operation werexxz investigated. The COTS glow plug, used as the ignition assistant (IA), was overdriven beyond its conventional operation range. Engine speed was varied from 1200 RPM to 2100 RPM. Three fuel blends consisting of a jet-A fuel with military additives (F24) and a low cetane number alcohol-to-jet (ATJ) sustainable aviation fuel (SAF) were tested with cetane numbers (CN) of 25.9, 35.5, and 48.5. The ranges of engine speed and fuel cetane numbers studied are significantly larger than those in previous studies of EACI or glow-plug assisted combustion, and the simultaneous variation of engine speed and fuel reactivity are unique to this work. For each speed and fuel, a single-injection of fixed mass was used and the start of injection (SOI) was swept for each IA power. A maximum pressure rise rate (MPRR) limit of 20 bar/CAD and a coefficient of variation of gross indicated mean effective pressure (COV of IMEP_g) limit of 5% were used as the bounds for the safe SOI operating range. Results demonstrated that the use of an ignition assistant at a high input power (70 W) can significantly increase the range of SOIs that fall within the prescribed safe operating range. For the higher cetane number fuels (35.5 and 48.5), that burn well without the IA, an advancement in start of combustion (SOC) and combustion phasing were observed with operation of the IA at 70 W. For the low CN fuel (25.9), which misfired without the use of the IA, complete combustion could be achieved with the use of the IA at 70 W. At higher engine speeds, the effectiveness of the IA diminishes. To better understand why the IA effectiveness decreased, a thermocouple was used to measure the IA tip surface temperature in-cylinder. Under motored engine conditions with fixed IA power, the IA surface temperature decreased as engine speed increased. The lower IA surface temperature along with the shorter time available as engine speed increased contributed to the decreased effectiveness at higher engine speeds.