Optical Investigation of the Combustion Process of Energy-Assisted Compression-Ignition for Low Cetane Number Sustainable Aviation Fuels

Jet fuels lack a specification on the cetane number (CN) and the CN of petroleum-derived jet fuels can vary from 30 to over 50. The push to reduce greenhouse gas (GHG) emissions from aviation has led to the development and approval of a range of sustainable jet fuels with even wider potential variation in CNs, ranging from ∼15 to ∼60. This wide CN variation can lead to misfire and “flame-out” when compression-ignition engines used in small aircraft are operated at altitude. Ground vehicle CI engines are designed to operate on #2 diesel, which has a minimum CN of 40. This work investigates an approach to over-come the challenges associated with operating CI engines on jet fuels with widely varying fuel reactivity. The approach involves depositing thermal energy locally in-cylinder with an ignition assistant (IA) to aid the ignition and combustion of low reactivity fuels, here termed energy-assisted compression-ignition (EACI). Multiple studies were performed in this work to understand various aspects of the EACI combustion process: the ignition and combustion of the fuel jet interacting with an ignition assistant, split-injection injection strategy impacts on the ability to achieve complete stable combustion with acceptable pressure rise rates, and the potential for custom piston bowl shapes to improve the EACI combustion process. Throughout the work, seven fuels were utilized, covering a wide range of CNs, from 2 to 48. The fuels used were a highly-branched alcohol-to-jet fuel (ATJ), F-24 (Jet-A with military additives), two binary blends of ATJ with F-24, toluene, ethanol, and methanol (the last three fuels were only studied with respect to there initial ignition characteristics). In-cylinder pressure measurements, OH chemiluminescence imaging, and schlieren imaging were employed to assess the EACI ignition and combustion process. The results indicate there is an IA temperature threshold of ∼ 1300-1350 K above which the ignition delay for the fuel jet interacting with the IA is independent of fuel CN. However, shot-to-shot variability of the injector and potentially turbulent fluctuations in the fuel jet result in a somewhat stochastic process and cycle-to-cycle variability in ignition for the experimental setup used in this work. Results from split-injection strategies indicate that three different combustion modes can occur during EACI combustion, dependent on fuel reactivity and operating parameters: jet-to-jet propagation, end-gas autoignition, and mixing-controlled combustion. Based on the results, achieving mixing-controlled combustion is desirable as it results in lower pressure rise rate and higher combustion efficiency. The results also showed that custom piston bowl designs could be successfully used to promote mixing-controlled combustion, improving the combustion process. However, small changes in bowl design or operation parameters can result in negative impacts on the combustion process. Taken together, the results provide a first of their kind base of knowledge that can be built on to design efficient EACI combustion engines capable of running on fuels with CNs ranging from 2 to greater than 48.