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Compressed Natural Gas (CNG), because of its low cost, high H/C ratio, and high octane number, has great potential in automotive industry, especially for heavy-duty commercial vehicles. However, relative slow flame speed of natural gas leads to long combustion duration and low thermal efficiency and tends to cause knock combustion at high load, which will aggravate engine thermal load and reliability. Enhancing turbulence intensity in combustion chamber is an effective way to accelerate flame propagation speed and improve combustion performance. In this study, the flow simulations of several piston bowls with different inner-convex forms were carried out using three-dimensional computational fluid dynamics (3D-CFD) software CONVERGE. The numerical results showed the piston bowls with inner-convex could disturb the charge swirl motion and enhance turbulence of different intensity. A hexagram geometry bowl was proved to have the best function in strengthening turbulence intensity.

A new ignition method named Flame Accelerated Ignition (FAI) is proposed in this paper. The FAI system composes of a spark plug and a flame acceleration tunnel with annular obstacles inside. The FAI was experimentally investigated on a rapid compression machine (RCM) with optical accessibility and a single-cylinder heavy duty research engine. In RCM, the flame is significantly accelerated and the combustion process is evidently enhanced by FAI. The ignition delay and the combustion duration are both sharply decreased compared with conventional spark ignition (CSI) case. According to the optical diagnostics, the flame rushes out of the exit of the flame acceleration tunnel at maximum axial speed over 40 m/s, which exceeds 10 times that of CSI flame propagation. In radial direction, the flame curls outwards near the tunnel exit and keeps growing afterwards.