Control of Fuel Octane for Knock Mitigation on a Dual-Fuel Spark-Ignition Engine

A two-port fuel-injection (PFI) system is added to a Rotax 914 four-cylinder spark-ignition engine to allow two fuels of different reactivity to be injected simultaneously in order to vary the fuel octane number during engine operation. Engine performance using the dual-fuel PFI system is compared to that using injection of primary-reference-fuel (PRF) blends via a single-PFI system for fuel octane ratings of 50, 70, and 87 octane. The on-the-fly octane control of dual-PFI system is found to control fuel-octane well enough to produce maximum indicated mean effective pressure (IMEPn) results within ± 2% of single-PFI PRF IMEPn results. IMEPn is compared among dual-PFI blends from 20 to 87 octane, neat n-heptane, neat JP-8, and JP-8/isooctane blends. Maximum IMEPn for these fuels is established for the Rotax 914 engine operating from 2500 to 5800 rev/min. IMEPn limitations for dual-PFI blends of JP-8 and isooctane are investigated at typical general-aviation cruise-power requirements to establish the volume proportion of JP-8 that can be used to sustain flight. The octane value of the sample of JP-8 tested is ∼ 20, and enabling JP-8 cruise operation requires a 2.5 - 3.5-bar shift in maximum IMEPn with JP-8. Results for typical cruise operation using JP-8/isooctane blends show that a maximum volume flow proportion of 88% JP-8 at low-load cruise, and 40% at high-load cruise could be used to sustain flight. These results reveal low-load neat JP-8 cruise as a possibility if the octane appetite of the Rotax 914 is reduced.