Mapping of Fuel Anti-Knock Requirements for a Small Remotely Piloted Aircraft Engine 2016-32-0045

Small remotely piloted aircraft (10-25 kg) powered by internal combustion engines typically operate on motor gasoline, which has an anti-knock index (AKI) of >80. To comply with the single-battlefield-fuel initiative in DoD Directive 4140.25, interest has been increasing in converting the 1-10 kW power plants in the aforementioned size class to run on lower AKI fuels such as diesel and JP-8, which have AKIs of ~20. It has been speculated that the higher losses (short-circuiting, incomplete combustion, heat transfer) that cause these engines to have lower efficiencies than their conventional-scale counterparts may also relax the fuel-AKI requirements of the engines. To investigate that idea, the fuel-AKI requirement of a 3W-55i engine was mapped and compared to that of the engine on the manufacturer-recommended 98 octane number (ON) fuel. The knock limit was established to be a peak-pressure rise rate of 5 bar/deg or a maximum amplitude of pressure oscillations of 5 bar for 1% of 400 consecutive cycles, whichever was more conservative. The 3W-55i engine was able to develop full power at all speeds above 6000 rpm (which included the peak power of 5 kW between 6500 and 7000 rpm) running on a 20 ON primary reference fuel (PRF) blend. Below 6000 rpm engine performance was knock-limited on the 20 ON PRF blend, resulting in up to a 30% decrease in power at some operating points. The results validate the speculation that losses in small engines can permit a direct conversion to low-AKI fuel with minimal impact on engine performance.