The Study Of The Detonation Wave Formation And Its Destruction Mechanism In The Closed Space Under Severe Knock In Internal Combustion Engines

Knock is a kind of abnormal combustion phenomenon which is always a trouble for the IC(internal combustion)engines.Once conventional knock changes into the severe knock,it will cause the severe destruction on the engine parts like the piston.With the more strict demand of energy saving and emission reduction,IC engines are intensified: in-cylinder thermal conditions become more severe thus the combustion is hard to be controlled at high load,which results in the destructive severe knock.Especially,the engine-downsizing technology sharply increases the in-cylinder thermal conditions,finally leading to a “super knock”.Once the “super knock” accompanies with the detonation phenomenon,it will turn into the destructive severe knock.Besides,other new combustion technologies may also result in the severe knock.Once the knock becomes severe,it will destroy the engine parts in a short time,making the engine breakdown.Even though the knock phenomenon has a long history,the destruction mechanism and the formation mechanism of the severe knock still remain unclear.The purpose of this study is to explore the severe knock formation and destruction mechanism.Furthermore,this study put forward some methods to avoid the severe knock and its destruction to facilitate the further energy saving and emission reduction.Based on the analysis of a large amount of destroyed piston materials,it’s found that the destruction under severe knock is usually caused by the combined effect of the pressure force and the thermal force.According to the above analysis,it’s assumed that the destruction under severe knock is caused by the formation of the detonation wave and its wave convergence.In order to validate this assumption,experiments combined with numerical simulations were conducted.In experiments,a set of detonation bomb system has been developed to simulate the detonation formation and the pressure oscillation under severe knock condition.As one goal of this research,the destruction mechanism under severe knock needs to be revealed.Considering this,a high-energy spark plug was inserted into the center of the bomb to ignite a detonation wave so that the wave oscillation,reflection and focusing behavior can be observed,which can reproduce the process that the detonation wave destroy engine parts.As the other goal,the detonation formation factors need to be found.Towards this goal,the high-energy spark plug was used to ignite a deflagration instead of the detonation wave,of which the leading shock wave behavior can be studied.Such experiment can reproduce the severe knock formation process that the leading shock wave causes the severe end-gas auto-ignition which causes the detonation in the edge region.For both of these two goals,the in-cylinder pressure distribution in every moment should be obtained for analysis.Therefore,four pressure sensors were installed in four different positions of the chamber wall.These four positions are separately the center of the piston,the half of the piston,the edge of the piston and the position where the in-cylinder pressure is usually obtained in bench test-“cylinder head”.According to the synchronous acquisition of the pressure data obtained by these four pressure sensors,the pressure distribution in different moments can be obtained,thus the in-cylinder wave oscillation behavior can be derived,which can be used as a proof to tell how the detonation is formed and where the engine parts are destroyed by the impaction.Further more,the bomb were redesigned with different chamber shapes to explore the effect of the chamber shape on the above results.In numerical simulations,a two dimensional axisymmetric numerical model was developed based on the experiments.Such numerical model adopts Euler equations,a flux-vector scheme called Advection Upstream Splitting Method(AUSM),a thirdorder spatial discretization scheme called MUSCL,dynamically adaptive mesh technology and a simplified chemical reaction mechanism to simulate the propagation of the detonation wave and the oscillations of the shock waves.Thus the experimentenal results can be explained,the mechanism can be revealed.It’s found that the pressure distribution is severely uneven under severe knock condition.Decided by the chamber shape,the wave focusing phenomenon will occur in the chamber,which makes the oscillation amplitude in the focused place several times higher than that of other places.The focused shock wave will form a severe impaction on the focusing point which exceeds the material strength of the engine parts,making the engine parts vulnerable to be destroyed.The focusing point in the cone-roof chamber is always on the piston center and the piston edge positions where the engine parts are usually destroyed under severe knock.On the other hand,the leading shock wave of the deflagration focusing on the edge region can result in a severe auto-ignition in the end gas,causing a detonation wave and forming the sever knock.In a conclusion,the wave convergence can’t only lead to the destruction but also facilitate the detonation formation and result in the severe knock.Therefore the assumption can be proved that the destruction of the engine parts is caused by the behavior of the detonation wave.Furthermore,it’s found that through changing the chamber shape,the wavefocusing phenomenon can be avoided,thus avoiding the severe knock as well as the destruction of the detonation wave.To be more specific,by increasing the edge clearance and adding a pressure relief pit on the center of the piston,the convergence can be eliminated.According to the above modification and the optimization of the chamber shape design,the severe knock as well as its destruction can be finally avoided.Such research can provide a very important and detail theoretical basis for the chamber design to avoid the severe knock as well as its destruction effect.