A study on the influence of burning rate on engine knock from empirical data and simulation

Abstract

There is contradictory literature about whether faster combustion may increase or decrease the likelihood of knock in spark ignition (SI) engines. Faster combustion allows less time for end-gas autoignition to occur, but also increases the end-gas pressure and temperature, which may reduce the time required for autoignition. By using the duration from ignition to 70% mass fraction burnt (MFB0−70%) as an explanatory variable, the hypothesis that “knocking cycles are the cycles with shorter MFB duration when they are compared with normal cycles” is proposed. In the experimental work, MFB0−70% duration of normal cycles is calculated by the conventional method. For knocking cycles, which have non-uniform pressure, this conventional method cannot be used. Instead, it has been demonstrated that the MFB0−70% duration can be estimated by a sine wave estimation (SWE) method with negligible errors. This MFB0−70% duration is then used to represent the burning rate of knocking cycles. The proposed hypothesis is verified by the relationships between MFB0−70% duration and the maximum rate of change of pressure. In the simulations, a multi-step adiabatic constant-volume zero-dimensional (MACZ) model is developed using Cantera software. In the model, the most recent detailed mechanism for gasoline surrogate, developed at the Lawrence Livermore National Laboratory (LLNL), is used. The MACZ model simulates the chemical kinetics of unburned air–fuel mixture from the start of compression stroke to the end of expansion stroke based on the recorded in-cylinder pressure. The simulation results match the experimental results fairly well. The simulation results suggest that increasing burning rate will promote knock, which is in agreement with the experimental results presented here. The contributions of burning rate to the knock are categorised by two factors described as “pre-dominant” steps and “post-dominant” steps. It is found that both factors have significant influences on the knock mechanism, even though the chemical reactions occurring in the pre-dominant steps are of low exothermicity.