Experimental Combustor Research in Sustainable Aviation Fuel at Rolls-Royce: Strategy, Achievements, and Outlook

Sustainable Aviation Fuel (SAF) is a major enabler for the near to mid-term future drive towards net zero aviation. Emphasising this importance, in October 2023, Rolls-Royce successfully completed a programme to demonstrate compatibility of all their in-production engines with 100% sustainable fuel. In addition to the lifecycle CO2 emissions reduction, there is the potential to also reduce non-CO2 effects of aviation by emitting lower levels of particles. In this respect, so-called non-drop-in SAF pathways are of interest. Typically, those paraffinic structured fuels do not contain any aromatic or sulphur content, which is known to reduce particle formation. However, it is important to understand the potential effects of such novel SAF pathways on combustor operability. From a combustor point of view, it is crucial to understand and predict the impact of different fuels on spray, ignition, flame stabilisation, and combustion efficiency. As non-drop-in SAF pathways may show some physical fuel properties outside Jet A/A-1, e.g. a lower density along with a higher heating value per mass and possibly also higher viscosity, they can currently only be used as blend components in line with ASTM D 7566. The paper outlines the Rolls-Royce combustor SAF strategy and status by presenting and discussing key tests and results of emissions and operability tests. The testing strategy follows the fundamental assumption that physical fuel properties can be used to predict the impact of different fuels on combustion. Eventually, new fuels can be characterised and evaluated based on former test evidence looking at their physical fuel parameters only. For operability, several key parameters have been identified like viscosity, boiling curve and shape, and Derived Cetane Number. Potentially, others need to be included as well, like surface tension, specific heat capacity or flame speed. This work will help to define a future non-drop-in SAF fuel specification. Finally, an outlook to future work and main goals is provided.