Browse Publications Technical Papers 2018-32-0046
2018-10-30

A Study of the Control Space for Knock Mitigation in 28, 55, and 85 cm3 Displacement, Two-Stroke, Spark-Ignition Engines 2018-32-0046

Interest is growing in operating commercially available, two-stroke spark-ignition engines that are designed for motor gasoline on low-anti-knock-index fuel such as diesel, Jet A, and Jet A-1. Such fuels have an octane rating of ~ 20, comparted to >80 for motor gasoline; therefore, knock-limited operation is a significant consideration. In this work commercially available 28, 55, and 85 cm3 displacement, two-stroke spark-ignition engines were operated on a 20 octane-number blend of iso-octane and n-heptane. The engines were converted from air-carburetion to throttle-body fuel injection to improve repeatability and controllability. Knock was controlled by throttling each engine to maintain a knock limit of <5 bar maximum amplitude of pressure oscillations or 5 bar peak-pressure rise rate (whichever was more conservative) for 1% of 400 consecutive cycles. Equivalence ratio, combustion phasing, and engine cooling were then varied to investigate which variables could be employed to relax the knock limit, allow opening of the throttle, and increase power . Combustion phasing had the largest impact, with a 10° retardation in the CA50 mass-fraction burned angle, which translated to 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 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 impact. 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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