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.
The Control Space for Knock Mitigation in Two-Stroke Engines for 10–25 kg Remotely Piloted Aircraft
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.
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Ausserer, J. K., Polanka, M. D., Litke, P. J., and Baranski, J. A. (June 17, 2019). "The Control Space for Knock Mitigation in Two-Stroke Engines for 10–25 kg Remotely Piloted Aircraft." ASME. J. Eng. Gas Turbines Power. September 2019; 141(9): 091010. https://doi.org/10.1115/1.4043745
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