Experimental and numerical investigation of the effects of combustion chamber reentrant level on combustion characteristics and thermal efficiency of stoichiometric operation natural gas engine with EGR

In order to accelerate the flame propagation and hence to improve the thermal efficiency in the stoichiometric operation natural gas engine with exhaust gas recirculation, reentrant modification on the Cylindrical combustion chamber has been conducted in this study. The diameter ratio between chamber throat and cylinder bore (hereafter referred to as diameter ratio) has been gradually reduced from 0.64 to 0.46. The effects of the chamber reentrant level on the engine combustion and thermal efficiency were systematically investigated through experiments. Then the mechanisms of these effects were further clarified through both one-dimensional and three-dimensional simulations. The experimental results indicate that the combustion duration could be reduced continuously with higher chamber reentrant level, which is beneficial for the thermal efficiency improvement. However, this benefit will be gradually offset by the simultaneously increased knock tendency and heat transfer loss, which has been further validated through thermodynamic analysis on thermal efficiency with the help of one-dimensional simulation. As a consequence, the chamber with diameter ratio of 0.5, rather than 0.46, shows the best performance in this study, and the thermal efficiency can be improved by approximately 1.5% at full load condition and 3% at medium to high load conditions. In addition, through the experiments, it is also observed that the combustion duration with spark timing set at maximum brake torque point shows a good linear relationship with the diameter ratio. This function is greatly valuable to optimize the Reentrant combustion chamber since it could be obtained via few engine tests and then used for well predicting the effects of chamber reentrant level on thermal efficiency combined with only one-dimensional simulation. Through the three-dimensional simulation results, it is seen that the acceleration of the flame propagation with higher chamber reentrant level could be mainly attributed to the much increased flame surface wrinkle during the initial combustion period, which results from the increased turbulence level and its more obvious effect on the flame surface before top dead center during the combustion process. (C) 2017 Elsevier Ltd. All rights reserved.