The Analysis Of Knock During Engines Downsizing Which Based On The Deviation Of Wave Equation

Dynamic is one of the main important indicators of a car engine. With the development of people’s awareness of environmental protection and regulations which related to limit the carbon dioxide emission will be more stringent, energy conservation and pollution reduction has had become the direction of development of automobile industry. In order to reach this target, the engine downsizing technology that engine volume would be reduced by booster under guaranteed performance, which has become the future development of automobile.However, the direct consequence of gasoline engine supercharged is knock, which has become one of the biggest obstacles of limited gasoline engine downsizing. The causes of knock are very complex, yet there are unified conclusions. The mainstream views are: the "end gas ignition theory" and "flame speed theory". These two views all believe that a special shock wave would be produced as the rapid release of energy.For the gasoline engine, the pressure fluctuation is a typical form of the knock. There is a closely relationship between the phenomenon of pressure fluctuations during combusting progress and the acoustic characteristics of engine combustion chamber. But the problem is most research of the combustion pressure oscillations didn’t focus on the formula for intuitive representation, which could not explain the relationship between combustion chamber pressure fluctuations and strong non-steady-state of instantaneous variables during combustion progress. In this paper, the pressure wave equation was derived and the boundary conditions: the expression of the reflection coefficient of the wave equation was explored. Then we confirmed the range of reflection coefficient of the three-layer structure and discussed the influence on calculating of pressure fluctuations combination with different conditions. Then, the numerical calculation of the pressure fluctuations during combustion process was achieved. The calculation results and analyzing was compared with the results of original wave equation and experimental data which proved that the equation of this paper can be used to simulate the phenomenon of pressure fluctuations more accurately. Then, some key parameters on the mean pressure and oscillating pressure were discusses during gasoline downsizing, such as inlet pressure, inlet temperature, compression ratio, ignition advance angle and speed. The effects of parameters on two parts of largest explosion pressure of the pressures were also discussed. The fluctuation results of simulation in some of the parameters changed are more rapidly responded and the amplitude changes are more obvious. There are multiple resonance peaks during the pressure oscillation process, which will help us to explore the causes and controlling of combustion noise method. Finally, the two most critical parameters: the inlet pressure and compression ratio were selected to discuss with EGR rate on engine performance. It was founded that if it is separately used, the technology of boost on engine performance improvement is more appropriate. Then, three-parameter will be discussed concurrently in order to analyze its function of power enhancement and inhibit the knock during gasoline engine downsized. For this simulation model, the relationship of compression ratio, pressure and EGR rate matching were studied. The gasoline downsized combined with suitable EGR technology can not only serve to enhance engine performance but also the effect of inhibiting the pressure fluctuations. .