A Study on the Influence of the Strength of Ejected Jet on Combustion in a Natural Gas Lean Burn Engine with a Sub-chamber with Direct Injector Inside

抄録

In order to realize lean burn with a natural gas engine, this study considers a ultra-lean rapid two-layer distribution combustion concept using a combustion chamber with a sub-chamber equipped with a direct-injection injector and an ignition plug inside. Through actual machine experiments so far, this concept has confirmed that it has an effect on thermal efficiency enhancement and reducing NOx. In this paper, focusing on the jet from the sub chamber which enables this combustion concept, we tried to investigate the effect of sub-chamber specifications on combustion and obtain guidelines for setting sub-chamber specifications. The heat release rate of both the engine experiment and the unit combustion experiment which simulated the engine experiment was analyzed. From the unit combustion experiment results conducted with various sub-chamber specifications, the rising of heat release increases as the burnt gas jet speed increases. However, it was found that the combustion was delayed and the heat release rate decreased as the speed increased to the level at which ignition delay occurred. From the combustion visualized image, when the jet strength is too strong, the jet penetration grows rapidly in the early stage of combustion, but the development once stagnates soon after that. Such an instability phenomenon is based on the fact that the flame cannot develop because the thermal diffusion by the turbulent mixing is larger than the heat release amount of the flame surface. This analysis makes it possible to grasp the jet intensity limit for stably growing the flame, and shows that the optimum jet strength can be used as an indicator to determine the sub chamber specifications.