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The Scaling of Loss Pathways and Heat Transfer in Small Scale Internal Combustion Engines

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Technical Report

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Air Force Institute of Technology Wright-Patterson A ir Force Base United States

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Prior literature indicates fuel conversion efficiency and normalized power deteriorate increasingly rapidly with decreasing displacement, but does not fully reveal the driving losses. The literature also suggested that increasing losses relax the required fuel anti-knock index AKI, but offered conflicting conclusions on the performance impact. This comprehensive experimental study of three, 28 cm3 to 85 cm3 displacement, commercial-off-the-shelf COTS, two-stroke ICEs identified short-circuiting as having the most deleterious impact on COTS engine performance in this size range. Heat transfer losses were comparable to larger engines for displaced volumes greater than 10 cm3. An engine friction model was developed that uses the surface area to volume ratio, speed, and throttle to predict friction losses a heat transfer model was also validated. The impact of reducing fuel AKI on performance was systematically investigated. The results showed a dependence on engine size the fuel AKI requirement decreased 20 octane number between 85 cm3 and 28 cm3 displacement. Switching from 98 ON manufacturer recommended to 20 ON JP-8, diesel equivalent fuel increased power 2 -3 and fuel conversion efficiency by 0.5 -1 at non knock-limited conditions.

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