Knock Resistance Evaluation of Methane-Based Two-Component Gases by Combustion Visualization and Analysis using RCEM

Gaseous fuel originated from natural gas (NG) has been affected by industrial fields thanks to its low emission feature. The excellent knock resistance of methane, a major component of NG, is another advantage in engine applications, but the composition of NG varies depending on the production region. Methane number (MN) has been widely used to evaluate the knock resistance of certain NG. However, the selection of a reliable knock-resisting index has not been settled because of several definitions of MN, and a new index called the propane knock index was recently proposed. Moreover, the proper index could change with types of gas engines. In this study, a rapid compression-and-expansion machine (RCEM) was prepared to reproduce in-cylinder conditions and combustion processes of a pre-chamber type medium-speed gas engine, and the knocking-like combustion was intentionally generated by setting compression pressure, ignition timing, and fuel density in the mixture to the proper level. The combustion process including torch flame ejection into the main chamber was visualized, and the heat release and the knock intensity were analyzed based on the in-cylinder pressure. Two-component mixtures were tested with methane as a primary component and C2 ~ C4 alkanes as a secondary one. 12 test fuels were blended at three different MNs based on a Wärtsilä MN calculator. The results showed that the second component still affected the heat release and the knock intensity under the same MN conditions. The various indexes for knock resistance of NG were also examined with the experiments in detail.