Fundamental Study Of Flame-shock Interactions And Their Influence On Combustion Modes In Confined Chamber

Knock is a prominent constraint on the efficiency of downsized spark ignition engines.It is generally agreed that knock is caused by autoignition at the end of the combustion chamber,with the high frequency and large amplitude pressure oscillation and make irreversible damage to the mechanical structure.The formation of autoignition and the intensity of in-cylinder pressure oscillation have a strong correlation with the flame-pressure wave interaction.When knocking occurs,the instantaneous high temperature and high pressure due to the extremely rapid energy release can be formed in combustion chamber.The visualization of pressure wave in different intensity and its interaction with flame are difficult to reproduce because of the harsh demands to optical experiments.On the other hand,it is difficult to capture the pressure wave in the confined space because there forms a complex flow caused by the intake,exhaust valve and the piston movement.However,the flame-shock interactions and their influence on combustion modes in confined chamber,and the evolution from autoignition to knock have not been fully understood in current rapid compression machine and optical engine.The present work investigates the interaction of the turbulent flame and shock wave,the end gas autoignition as well as the relationship between combustion modes and cylinder pressure oscillation in a newly designed constant volume combustion bomb using optical methods and theoretical analysis.The different propagation speeds of the turbulent flames and shock waves can be obtained by controlling the initial pressures and the hole size of the perforated plate.The present work attempts to reveal the influence mechanism of pressure wave on autoignition,the interaction of flame-pressure wave and provide a reference for knock suppression theoretically.The process of laminar flame converting into turbulent flame by perforated plates and its influence on the combustion at the end region of the chamber were investigated in a newly designed constant volume combustion bomb by using different perforated plates.The flame morphology and propagation speed were compared and analyzed with and without perforated plate using gasoline fuel.It is found that the laminar flame propagation velocity shows the trend of initial increase and consequence decrease in the confined space.The turbulent flame propagation velocity shows the significantly different trend and demonstrates the “M” shape evolution including the flame acceleration,unstable and deceleration process in the closed chamber.The accelerating flame with perforated plate can cause a severe a burning at the end of the chamber and the peak of the pressure would be higher.In order to reproduce the interaction of flame-shock wave in internal combustion engine.The images of laminar flame converting into turbulent flame were obtained by schlieren method using hydrogen fuel.It is found that there is a clear shock wave formed ahead of the flame front.The formation mechanism of shock wave is analyzed in the view of gas dynamics: the flame propagation in combustion chamber is an acceleration process after orifice plate and thus generates several disturbance waves ahead of the accelerated flame front.Due to the compression from acceleration effects,the disturbance wave formed later propagates faster than that formed previously.With the propagation of flame front,the disturbance waves in the back gradually catch up with the front and coalesce to it.Finally,the shock wave is formed due to the effect of coalescence.In addition,the influence of initial pressure and hole size on flame acceleration were studied.It is found that the laminar flame propagation velocity decreased with the increase of the initial pressure before the perforated plate.The hole size has a significant effect on the turbulent flame propagation,with the increase in hole size,the average speed after flame through the perforated plate gradually reduced.Then the author studied the development of the flame at the end of the combustion chamber by changing initial pressure,hole size,porosity and the mounting position of the perforated plate in the view of flame morphology,flame propagation speed as well as the in-cylinder pressure fluctuation intensity.It is found that the initial pressure has a significant effect on the cylinder pressure.As the initial pressure increases,the peak and the amplitude of the pressure increase.The influence of hole size on the shock wave is significant.When the hole sizes are medium,the shock wave generated at the end of the combustion chamber is clearest.At this time,the amplitude of the pressure is also the largest.Porosity has a significant effect on the flame propagation velocity.As the porosity decreases,the flame propagation velocity increases at the end of the combustion chamber.The peak and amplitude of cylinder pressure increase with the decrease of the porosity.When the perforated plate mounted in the middle of the combustion chamber,the amplitude of the pressure is the largest.In present work,the influence of the flame-pressure wave(including acoustic wave and shock wave)interaction on the combustion mode at end of the chamber were studied.Five combustion modes were obtained clearly under the experimental conditions,such as,(1)normal combustion without pressure wave,(2)weak oscillating combustion with acoustic waves,(3)oscillating combustion induced by shock wave,(4)flame acceleration induced by flame-shock interaction,(5)end gas autoignition induced by strong shock wave.It is found that the transition of furious combustion mode is the reason for high-frequency and large-amplitude pressure,such as,mode 4 and mode 5.Three different autoignition modes,including end gas autoignition induced by flame acceleration,autoignition simulated by heating rod and spark plugs,were compared in present work.It is found that there is no significant oscillation when the autoignition simulated by heating rod and spark plugs.The results show that the rapid energy release induced by autoignition at the end region of the chamber is the basic reason for large oscillation of cylinder pressure.In summary,the constant volume combustion bomb with perforated plate was designed.The influence of perforated plate on combustion at the end region of the chamber,the process of flame acceleration,in influence of initial condition on flame,shock propagation and pressure oscillation,and the influence of flame-shock interaction on combustion modes were studied in detail.Thus attempts to reveal the mechanism of knock for downsized spark ignition engines in the view of flame-shock interaction.