Researches On Thermoacoustic Mechanism During Combustion Of I.C. Engines

The essence of thermoacoustic is a phenomenon of reciprocal transformation of thermal energy and acoustic energy in medium. At the present time, both in China and other countries, the thermoacoustic field concentrates its attention on thermoacoustic refrigeration and rocket propellers. Although the researchers in the field of internal combustion engine have recognized that the pressure oscillation play a great role in combustion chamber, especially in knock conditions. However, there is still no definite theory about the role because of complicacy of the acoustic wave in cylinder during combustion. Therefore, it is of great importance to study the thermoacoustic problems during the combustion of an engine.In this paper, a SI engine is chosen as research object. By analyzing the high-speed photographs taken from an optical engine, a model which can be used to describe acoustic characteristics of structure of the combustion chamber of the optical engine is established to calculate the acoustic modes of the chamber and the frequency responses is analyzed in the acoustic software SYSNOISE. Base on the characters of burnt and unburned regions of the chamber, a time domain model is built to calculate the transient responses of pressure wave in the combustion chamber under Single-point stimulation and multi-point stimulation.Subsequently, according to discussion of existing classical solution of wave equation in cylindrical coordinate, it is revealed that one will face awkwarddifficulties when the classical method is adopted to simulate the acoustic wave during combustion of an IC engine. Thus a quasi-dimensional model is applied to couple with an acoustic model. With rational simplifications made to the coupled models, a procedure is compiled and used to calculate the heat release and pressure oscillation. The results indicate that the thermoacoustic characteristics can be explored in this way approximately.For profound comprehension of the acoustic oscillations of pressure wave in combustion, with analysis of arithmetic in fluid field embedded in KIVA, a wave equation is deduced based on the equations set that is included in KIVA for fluid solution, which can be applied on the theromacoustic computation during the combustion of an IC engine. The wave equation shows that, in an IC engine, acoustic wave is stimulated by multiple source-items, such as chemical reactions,viscous dissipation,turbulence generation energy and the rate its dissipation,heat transfer and enthalpy dissipation. To solve this three-dimensional transient wave equation, the calculation should be combined with KIVA simulation. For the wave equation, corresponding difference equation is constituted and the stability conditions while it is coupled with KIVA calculation and mathematics justifications are provided. Aformula is derived for the reflect coefficients for a three layer cylindrical structure of mixed gas- cylinder-water, which is applied in the thermoacoustic computation.Based on the wave equation, subroutines are programmed and real-time quantitative results of the stimulating source items in combustion chamber are obtained during the combustion simulation. Through the results, it is obviously that the chemical reaction is the mainly source to the acoustic wave in combustion. Then various knock conditions of a SI engine are computed. By comparison of cylinder pressure data and the simulation results, a conclusion can be drawn that it is feasible to apply the wave equation on calculation of acoustic field in an IC engine. Oscillating courses is extracted from several specific points at different position, showing that acoustic structure and combustion parameters influence the acoustic wave in combustion explicitly.After the contrast of discrepancies and similarities between IC engine combustion and general thermoacoustic equipments, the wave equation is coupled with KIVA3V to predict the quantificational thermoacoustic effect in IC engines according to thermoacoustic theories. For the SI engine in this paper, high-speed photographs are quoted to denote the development of flame in contrast with the contours from simulation. By Rayleigh integral, we can conclude that before the time of knock, thermoacoustic effect can enhance the instability of the IC engine's combustion system, while it will drive the system to uniformity after knock. When the knock takes place, the steep increase of acoustic pressure may reach the magnitude of mega Pascal and temperature fluctuation will surpass 200 K due to the acoustic wave. In the end, the couple procedure is introduced in acoustic and thermoacoustic computation to figure out its acoustic wave and temperature fluctuation in the chamber.