Experimental investigation of octane requirement relaxation in a turbocharged spark-ignition engine

Date of Award


Degree Name

M.S. in Mechanical Engineering


Department of Mechanical and Aerospace Engineering


Advisor: Scott D. Stouffer


Wide adoption of unmanned aerial systems (UAS) powered by spark ignition (SI) engines that require high-octane gasoline has triggered an increase in fuel costs incurred by the U.S. Department of Defense (DoD). Most current United States Air Force (USAF) vehicles are fueled with JP-8, a low-octane kerosene-like fuel that is well suited for turbine engines. A relaxation in octane requirement is required to fuel current SI engines with a low-octane fuel like JP-8 and avoid destructive end-gas knock. In this thesis, a two-phase octane requirement study is conducted using a Rotax 914 four-cylinder turbocharged SI engine. In phase one, net indicated mean effective pressure (IMEPn) is characterized at typical cruise speeds as fuel octane number (ON) is varied on-the-fly using a dual port-fuel-injection (PFI) system. IMEPn is compared among dual-PFI blends from 20 to 87 ON, neat n-heptane, neat JP-8, and JP-8/iso-octane blends. A JP-8/iso-octane demonstration is conducted to show the volume proportion of JP-8 that could be used to sustain flight. Results for typical cruise operation using JP-8/iso-octane 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. Although an impractical configuration, these results reveal that low-load neat JP-8 cruise is a possibility if the octane requirement of the Rotax 914 can be relaxed.The second phase of testing focuses on achieving full-load takeoff performance on 87 ON, since high-load operation is impractical with JP-8. The effects of intake air temperature (IAT), equivalence ratio, ignition timing, and dual-simultaneous ignition on knock are investigated. The combination of delayed combustion phasing with dual-simultaneous-ignition and increased equivalence ratio enables greater maximum IMEPn on 87 ON than the base configuration on 100 ON. To offset the additional fuel used for takeoff, a cruise fuel consumption study is conducted to characterize the reduction in indicated specific fuel consumption (ISFC) with an optimized fuel-lean, dual-simultaneous-ignition, and advanced ignition timing configuration compared to base conditions. The ISFC reduction in the optimized cruise configuration can directly offset the additional fuel used in the optimized 87 ON take-off configuration for flights as short as 4 hours. The 87 ON optimized cruise and take-off configurations can be combined to allow up to 3.5 additional hours of cruise.


Gasoline Anti-knock and anti-knock mixtures, Drone aircraft Fuel consumption, Spark ignition engines Fuel systems, Aerospace engineering; automotive engineering; mechanical engineering; engine knock; low-octane fuel; spark-ignition engine; autoignition; engine control unit; internal combustion engine

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Copyright © 2013, author