Effects of Combustion Boundary Conditions of Iso-Octane Air Mixture on Auto-Ignition and Knock

Engine knock is a major technical obstacle to achieving significant improvements in thermal efficiency in modern downsized internal combustion engines. As yet, we lack a thorough understanding of its mechanism. In this study, we investigated abnormal combustion phenomena involving knock and super-knock in downsized internal combustion engines under low speed and high load conditions. Specifically, we conducted experiments on the effects of the combustion boundary conditions of iso-octane air mixtures on auto-ignition and knock. In the experiments, we measured the transient pressure in the combustion chamber of a rapid compression machine and then analyzed high-speed photographic combustion images, based on the auto-ignition theory and our current understanding of the knock mechanism. In addition, we quantitatively studied the mutual influences of the effective energy density, auto-ignition mode, and knock intensity, and explored the operating mechanism involved in the effect of wall temperature on knock intensity. The results show that with increases in the initial temperature, the iso-octane's auto-ignition timing gradually advances and knock intensity is gradually enhanced with super-knock occurring under these experimental conditions. The effect of initial temperature on the auto-ignition timing and knock are similar, with both being positively correlated with the initial pressure. As the tendency of the equivalence ratio moves to 1, the iso-octane knock intensity is significantly increased and super-knock occurs, and is conversely weakened when the equivalence ratio is slightly rich. However, knock intensity is generally more sensitive to the initial pressure and equivalence ratio than the initial temperature. Engine knock is closely associated with the effective energy density of the mixture, i.e., as the effective energy density is increased, a transition occurs from normal combustion to conventional knock and super-knock. In addition, wall temperature has a significant influence on knock intensity, i.e., under the same effective energy density conditions, a higher wall temperature leads to stronger knock intensity and can even induce super-knock. © 2019, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.