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Research Papers

The Control Space for Knock Mitigation in Two-Stroke Engines for 10–25 kg Remotely Piloted Aircraft

[+] Author and Article Information
Joseph K. Ausserer

USAF Test Pilot School,
220 Wolfe Avenue,
Edwards AFB, CA 93524
e-mail: Joseph.Ausserer@gmail.com

Marc D. Polanka

Air Force Institute of Technology,
2950 Hobson Way,
WPAFB, OH 45433
e-mail: Marc.Polanka@afit.edu

Paul J. Litke

Air Force Research Laboratory,
1950 7th Street,
WPAFB, OH 45433
e-mail: Paul.Litke.3@us.af.mil

Jacob A. Baranski

Innovative Scientific Solutions, Inc.,
7610 McEwen Road,
Dayton, OH 45459
e-mail: Jacob.Baranski.ctr@us.af.mil

Manuscript received November 28, 2018; final manuscript received May 8, 2019; published online June 17, 2019. Assoc. Editor: William Northrop. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Eng. Gas Turbines Power 141(9), 091010 (Jun 17, 2019) (13 pages) Paper No: GTP-18-1718; doi: 10.1115/1.4043745 History: Received November 28, 2018; Revised May 08, 2019

Interest is growing in converting commercially available, two-stroke spark-ignition engines from motor gasoline to low-anti-knock-index fuel such as diesel and Jet A, where knock-limited operation is a significant consideration. Previous efforts have examined the knock limits for small two-stroke engines and explored the effect of engine controls such as equivalence ratio, combustion phasing, and cooling on engine operation during knock-free operation on high octane number fuel. This work culminates the research begun in those efforts, investigating the degree of knock-mitigation achievable through varying equivalence ratio, combustion phasing, and engine cooling on three small (28, 55, and 85 cm3 displacement) commercially available two-stroke spark-ignition engines operating on a 20 octane number blend of iso-octane and n-heptane. Combustion phasing had the largest effect; a 10 deg retardation in the CA50 mass-fraction burned angle permitted an increase in throttle that yielded a 9–11% increase in power. Leaning the equivalence ratio from 1.05 to 0.8 resulted in a 10% increase in power; enriching the mixture from 1.05 to 1.35 yielded a 6–7% increase in power but at the cost of a 25% decrease in fuel-conversion efficiency. Varying the flow rate of cooling air over the engines had a minimal effect. The results indicate that the addition of aftermarket variable spark timing and electronic fuel-injection systems offer substantial advantages for converting small, commercially available two-stroke engines to run on low-anti-knock-index fuels.

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Figures

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Fig. 1

Effect of equivalence ratio on knock-limited IMEP and COV of IMEP when operating on 20 ON PRF blend (filled—98 ON baseline; red—knock-limited; orange—knock-observed)

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Fig. 2

Effect of equivalence ratio on delivery ratio and knock when operating on 20 ON PRF blend

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Fig. 3

Effect of equivalence ratio and throttle setting on knock when operating on 20 ON PRF blend

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Fig. 4

Effect of equivalence ratio on fuel-conversion efficiency and knock operating on 20 ON PRF blend

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Fig. 5

Effect of combustion phasing on knock-limited IMEP and COV of IMEP operating on 20 ON PRF blend

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Fig. 6

Effect of combustion phasing on delivery ratio and knock when operating on 20 ON PRF blend

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Fig. 7

Effect of combustion phasing and throttle setting on knock when operating on 20 ON PRF blend

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Fig. 8

Effect of combustion phasing on fuel-conversion efficiency and knock when operating on 20 ON PRF blend

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Fig. 9

Effect of head temperature on knock-limited IMEP and COV of IMEP when operating on 20 ON PRF blend

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Fig. 10

Effect of head temperature on delivery ratio and knock operating on 20 ON PRF blend

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Fig. 11

Effect of head temperature and throttle setting on knock when operating on 20 ON PRF blend

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Fig. 12

Effect of head temperature on fuel conversion efficiency and knock when operating on 20 ON PRF blend

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