Experimental Study of Flame Accelerated Ignition on Rapid Compression Machine and Heavy Duty Engine 2017-01-2242
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. As a result, a cylindrical flame column is generated in the main-chamber, inducing the following combustion process completed rapidly. The results indicate the potential to increase thermal efficiency by FAI due to the accelerated flame and enhanced combustion. Experiments were conducted in the engine to validate the improvement potential on efficiency and emission performance. The FAI system with long tunnel failed to operate due to misfire caused by the unscavenged residual gas. A modified FAI system with shortened tunnel was designed and stably operated in the engine. Compared to CSI, the ignition delay is longer in FAI mode, which is different from the RCM result. However, the following heat release is still faster. Lower negative work and comparable combustion phase are achieved due to the accelerated combustion process and the enhanced heat release. The combustion efficiency is improved due to more complete combustion as the CO emissions are remarkably reduced. Moreover, the combustion stability is enhanced. As a result, both emission performance and thermal efficiency are improved.