Investigation of rapid flame front controlled knock combustion and its suppression in natural gas dual-fuel marine engine

Abstract

In this study, a new-type knock mechanism for large-bore marine engines, which differs from the existing end-gas auto-ignition knock theory and detonation wave theory, is fully revealed, using a high time resolution dynamic pressure difference method. The results indicate that the disparate enhancement effects of the flame front on the high-pressure region and the low-pressure region result in an increasing pressure difference, eventually forming the knock. The distinctions between this knock and the conventional end-gas auto-ignition knock were examined. Subsequently, the different functions of swirl in the two knock mechanisms were investigated. The results illustrate that, increasing swirl in end-gas auto-ignition knock can accelerate the termination of flame surface propagation and reduce the end-gas temperature by shortening the end-gas heat accumulation, thereby decreasing the knock intensity. However, reducing swirl in the new-type knock can diminish the combustion intensity of the flame front, thereby reducing its enhancement of the pressure wave. Consequently, a reduction to one-fourth of the initial swirl level can reduce knock intensity by 91%. Particularly, a novel knock suppression method of positioning the jet flame in the opposite direction of the swirl is proposed, which can avoid the local rapid combustion to effectively eliminate knock while maintaining power.