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research-article

Experimental Investigation of Fuel Anti-Knock-Index Requirements in Three Small Two-Stroke Engines for Remotely Piloted Aircraft

[+] Author and Article Information
Joseph K. Ausserer

USAF Test Pilot School, 220 Wolfe Ave, Edwards AFB, CA 93524
joseph.ausserer@gmail.com

Marc D. Polanka

Air Force Institute of Technology, 1950 Hobson Way, WPAFB, OH 45433
marc.polanka@afit.edu

Paul J. Litke

Air Force Research Laboratory, 1950 7th St, WPAFB, OH 45433
paul.litke.3@us.af.mil

Jacob A. Baranski

Innovative Scientific Solutions, Inc., 7610 McEwen Rd, Dayton, OH 45459
jacob.baranski.ctr@us.af.mil

1Corresponding author.

ASME doi:10.1115/1.4040520 History: Received May 15, 2018; Revised May 22, 2018

Abstract

Small remotely piloted aircraft (10 25 kg) that are powered by internal combustion engines typically operate on gasoline with an anti-knock index (AKI) >80. Interest has increased in converting power plants for these platforms to run on low-AKI fuels such as diesel and Jet-A with AKIs of ~20. It has been speculated that the higher losses that cause these engines to have lower efficiencies than their conventional-scale counterparts may also relax their required fuel AKI. The fuel-AKI requirements of three two-stroke spark ignition engines with 28, 55, and 85 cm3 displacements were mapped, and the performance was compared to that on 98 ON (octane number) fuel. Switching from 98 ON fuel to 20 ON (Jet-A and diesel equivalent AKI) fuel while maintaining optimum combustion phasing led to a 3-5 CAD (crank-angle degree) reduction in burn angle, a 2 3% increase in power, and a 0.5-1% (absolute) increase in fuel-conversion efficiency at non-knock-limited conditions through shortening of the CA0 CA10 burn angle. The efficiency improvement translates to a 6% increase in range or endurance. The results indicate that abnormal combustion is not a significant obstacle to operating small commercial-off-the-shelf engines on low-AKI fuels and that most of the power and efficiency improvements demonstrated in previous heavy-fuel conversion efforts were the result of modifications made to accommodate low-volatility fuels, not the faster burn rate of the low-AKI fuels themselves.

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