Optimum Bore-to-Stroke Ratio for a Flathead Uniflow Two-Stroke SI Engine and its Impact on Different Fuels

This numerical study investigates a spark-ignited, two-stroke engine employing uniflow scavenging, flathead cylinder head design, and an exhaust valve system to identify the optimal bore-to-stroke (B/S) ratio for maximizing brake efficiency at fixed displacement. A single-cylinder prototype engine was constructed, and its experimental data validated a 1D GT-SUITE simulation model. This validated model was then utilized to simulate a full-scale, 1.5-liter displacement, horizontally opposed four-cylinder engine with supercharger-assisted boosting, intended for small aircraft propulsion. The simulations explored a range of B/S ratios from undersquare (0.7) to oversquare (1.5), maintaining a consistent brake power output of 60 kW at 3000 rpm and lambda 0.9. Results showed that increasing the B/S ratio enhanced brake efficiency from 26.0% at B/S=0.7 to 27.0% at B/S=1.5, largely due to reduced frictional losses attributed to shorter stroke and lower piston speeds, decreased heat transfer losses, and a modest reduction in compressor power demand. Frictional power decreased from 12.7 kW at B/S=0.7 to 9.6 kW at B/S=1.5, while heat transfer losses dropped from 43.5 kW to 40.6 kW respectively. Fuel analyses involving gasoline E27, ethanol (E100), and aviation gasoline (AvGas) revealed ethanol (E100) provided the highest brake efficiency yet increased fuel consumption (BSFC). AvGas presented the lowest BSFC, with gasoline E27 performing intermediately. A key finding is the inverse trend in heat transfer losses, where the undersquare configuration exhibited greater losses than those of the oversquare geometry, contrary to conventional expectations. Combined with improved mechanical efficiency due to reduced friction, the oversquare design emerged as the most efficient configuration. These findings challenge traditional heat transfer assumptions in common two and four-stroke engines and highlight the benefits of higher B/S ratios for improving overall performance in flathead uniflow two-stroke engines. The results will serve as the foundation for the design of the full-scale four-cylinder aeronautical engine.