Researches On Thermo-acoustic Mechanism Of High-frequency Combustion Pressure Oscillations In An IC Engine

High-frequency oscillations of in-cylinder pressure are an important factor of combustion noise in the engine. The high or low combustion noise can be judged by a frequency spectrum of an in-cylinder pressure curve. Therefore, it is very important to accurately obtain an in-cylinder pressure curve for analysis of combustion noise. A spectrum analysis of an in-cylinder pressure curve from experiments is a traditional approach to assess combustion noise. An in-cylinder pressure curve from experiments often shows pressure fluctuations. However, the pressure curve, obtained by using various engine simulation programs, is a smooth curve. Analyzing a smooth pressure curve obtained by simulation will bring about strong distortions if one estimates combustion noise before experiments.During the combustion process of in-cylinder air-fuel mixture, the volume of the chamber changes constantly with time. The distribution of combustion sources in the chamber is also changes. The in-cylinder combustion process in the engine is a complicated physical and chemical process process. The model of in-cylinder pressure field about thermo-acoustic coupling is built on the basis of combustion and acoustic theory. The in-cylinder pressure oscillations are analyzed by this model.The two simulations methods about pressure wave propagation are proposed for solving pressure field changes caused by in-cylinder combustion through combining combustion theory and acoustic one. The first method is for simulating pressure wave propagation about air-fuel mixture combustion. The second method is for simulating pressure wave propagation about reflected waves at a cylinder wall.For simulating pressure wave about combustion, the simulation grid cells in the combustion chamber whose heat release changes as the crank angle changes were served as combustion noise sources in KIVA. These sources produced acoustic waves. This distance of acoustic wave propagation in the cylinder is positive correlation with time. The regions around noise sources were divided into the cells inside acoustic wave, and outside acoustic wave respectively according to the hysteresis effect of acoustic theory. The pressure of the grid cells will change within the acoustic waves.For simulating pressure wave about reflection, a local fluid-structure interaction model is built by linking air-fuel mixture with cylinder wall by SYSNOISE. Acoustic waves will happen to reflect when acoustic waves propagate to a cylinder wall. Reflected waves will have an impact on the in-cylinder pressure field again. The propagation process of reflected acoustic waves is also based on the hysteresis effect of acoustic theory. The local coupled model better simulated hysteretic propagation of reflected waves.Coding new programs in KIVA call the WIN32API function, the interface program of FORTRAN Language, to start SYSNOISE, which achieve communications between the two softwares. Furthermore, the in-cylinder pressure field could be calculated in every time step.At comparing the simulation and experiment, the acoustic hysteresis effect of incident wave propagation and reflected wave propagation could well explain the thermo-acoustic mechanisms of the in-cylinder pressure oscillations in engines. The main noise sources which caused pressure oscillations can be identified by analyzing the sound contribution of the combustion sources and that of excitation loads. The characteristic of in-cylinder pressure oscillations is quantitatively analyzed. This will provide references and theoretical support for reasonable predictions of combustion noise.