An X-type rotary engine (XRE) that utilizes a High Efficiency Hybrid Cycle, combining high compression ratio with constant-volume combustion, could be considered an attractive option for both improved combustion efficiency and decreased emissions. However, research on optimizing its combustion in XREs is still limited. The optimization of ignition strategy can be beneficial for enhancing combustion performance and especially for reducing the unburned region due to the particular recess chamber. In this study, a three-dimensional CFD model of the XRE was developed and verified. A numerical study was conducted to analyze how ignition number, ignition location, and asynchronous ignition on combustion characteristics and energy losses of the XRE. Results showed that the twin-spark scenarios had a significantly higher flame propagation speed than the single-spark scenario due to the formation of larger flame fronts. The twin-spark plugs arranged along the rotor rotating direction (Case A1) had maximum peak pressure and indicated thermal efficiency, which increased by 19.6% and 9.8%, respectively, over the single-spark scenario. However, Case A1 produced higher NOx emissions and had the highest heat transfer losses, which boosted by 14% over the single-spark scenario. For this twin-spark arrangement, advancing the ignition of the leading-spark plug (L-plug) significantly reduced ignition delay and combustion duration, as well as boosted the pressure rise rate, peak pressure, and combustion efficiency at the price of a smaller increment of NOx and CO emissions. Notably, advancing the L-plug timing drastically decreased exhaust losses, but had only a minor impact on heat transfer losses. Thereby, it is recommended to set the L-plug timing in advance for practical engineering applications.
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