Application of Sustainable Aviation Fuels in Compression Ignition Aviation Engines: In-Flight Investigations

Abstract

To achieve a significant reduction in net CO2 emissions in the aviation sector, sustainable aviation fuels (SAFs) are considered a key factor. Current research efforts are therefore focused on SAFs, which exhibit properties that differ from conventional kerosene, particularly in aspects critical to compression-ignition (CI) engines, such as cetane number, evaporation behavior or lubricity. These differences necessitate dedicated investigations to assess their suitability and performance in such engines. However, real operating conditions - such as intake air- and exhaust- pressure levels during flight - cannot be fully replicated on standard engine test benches. For this reason, real flight experiments were conducted to address these limitations. Notably, this work marks the first instance of in-flight testing of SAFs in CI aviation engines, constituting a significant milestone in this research area. In the course of these investigations, ASTM D7566 Annex A2-compliant HEFA (hydroprocessed esters and fatty acids) was tested across a wide range of blend ratios, including pure HEFA. The in-flight tests were performed, using a DA42 aircraft from Diamond Aircraft Industries (DAI), equipped with two AE330 engines from Austro Engine (AE). For safety reasons, one engine was left in its original configuration, while the other one was outfitted with advanced measurement systems, including in-cylinder pressure sensors, a mobile exhaust gas analysis system (portable emission measurement system - PEMS), and various pressure and temperature probes. This setup enabled precise measurement of gaseous emissions, particle number (PN), efficiency, and combustion parameters. The operation of the test engine with HEFA blends, up to and including pure HEFA, was successfully demonstrated. The flight campaign highlighted the emission potential of this aromatics-free fuel. Additional test bench investigations confirmed the findings from the initial flight campaign: while soot mass emission decreased significantly with increased HEFA-share, particle number (PN) remains on a constant level, indicating a shift in the particle size distribution. This emphasizes the importance of understanding the trade-offs and adapting engine calibration when using SAF in CI (aviation) engines.