Effects of high ignition energy on lean combustion characteristics of natural gas using an optical engine with a high compression ratio

Abstract

Natural gas engines often suffer from slow-burning rate and large cyclic variations, which significantly affects engine performance and thermal efficiency. In this study, using an optical spark-ignition engine with a high compression ratio, the role of high ignition energy in lean combustion characteristics of natural gas was investigated. Synchronization measurement of in-cylinder pressure and visualization images were performed to analyze the correlations of engine performance and combustion characteristics. The results show that elevating ignition energy can improve combustion instability and thermal efficiency, manifesting more concentrated heat release and greater influences on the combustion duration from 10% to 50% in burned mass fraction than that of 50%–90% range. The combustion visualizations show that ignition energy mainly affects initial flame kernel formation and early flame development, and improved flame speed and combustion instability can be observed even when excess air coefficient reaches 1.4 at sufficiently high ignition energy. Furthermore, an empirical criterion based on burned mass fraction and combustion visualizations was adopted to quantify early flame development. The shortage of ignition delay time and the acceleration of initial flame propagation are observed with the increases of ignition energy, which leads to lower cyclic variations and a faster burning rate at the early combustion stage.