Optical engine investigation of two-injection strategies for energy-assisted compression-ignition combustion of low cetane number sustainable aviation fuels

A two-injection strategy was investigated for energy-assisted compression-ignition combustion using hot surface ignition of low-cetane number sustainable aviation fuels in an optically-accessible engine. In-cylinder pressure measurements and OH chemiluminescence imaging were employed to assess the combustion process. Three experimental studies were performed to understand and optimize the EACI combustion process: a single injection design-of-experiments (DoE), a two-injection injection DoE, and a modified two-injection dwell sweep. The fuel used for this investigation was a cetane number 35.5 binary blend of an alcohol-to-jet (ATJ) fuel with a cetane number of similar to 17 and F-24 (Jet-A with military additives) with a cetane number of 48.5. Results demonstrate a two-injection strategy can enable a complete and stable EACI combustion without excessive pressure rise rates for the right set of ignition assistant temperature and injection parameters. Parameters that assist in establishing this are higher ignition assistant temperatures (>1400 K), longer injection dwells (>1.5 ms), and lower injection pressures (near 600 bar). These parameters increase the extent of heat release from the first injection and influence the location of hot combusted gases. The first injection heat release is essential to enable rapid ignition of a majority of second injection fuel jets. This limits the amount of end-gas autoignition, reducing pressure rise rates and avoiding pressure oscillations. The ability of the first injection to positively influence the ignition process of the second injection is encouraging and provides guidance on the approach to operate with even lower cetane number renewable fuels.