The Research On The Formation Mechanism Of Overpressure In The Combustion Chamber Of IC Engines Caused By The Knocking

Knock is the common abnormal combustion phenomenon in IC engines, whichhas a long history. Such abnormal combustion phenomenon is so destructive that itwill destroy some parts of IC engines within the knock cycles. Through improving theoctane number of fuel and retarding the ignition time, knock is avoided momentarily,which is no longer researched a lot. But in recent years, new combustion technologyand engine downsizing are widely used. As the application of GDI and turbocharging,a new abnormal combustion phenomenon came out, which is called super knock.When super knock occurs, the maximum cylinder pressure amplitude can reach to25MPa or even higher, and the oscillation frequency can reach to10KHz, more seriousthan that of the normal knock. Such serious pressure oscillation frequently occurringwill damage the piston, spark plug and other parts easily. Because super knock occursbefore the spark ignition, the common method can’t inhibit it any more. So knock isresearched again all over the world. For a long time, the cognition of the knock islimited to the macroscopic phenomenon, and the method to avoid it is alsofrom macroscopic perspective. However, the mechanism of knock and its destructioneffect is not clear, which prevents the knock from being eliminated thoroughly. Onaccount of this, it has a significance to find out the essence of the destroyingphenomenon and reveal its destroy mechanism, which will help to avoid the damageeffect of the severe knock on the engine parts and improve the adaptability of enginesto the downsizing and other new combustion technologies. Therefore, ourresearch work is made on the following aspects to clarify the destroy mechanism ofthe knock.First, our research introduces the nonlinear shock wave theory. In the severeknock, the pressure oscillation amplitude has the same order of magnitude with thestatic pressure in cylinder, which can’t be explained by the traditional smallperturbation acoustic wave equation. Such severe pressure oscillation is in the scopeof nonlinearity. So our research use the method of DDT to illustrate the severe knock.The severe knock is begin with the deflagration and the deflagration front willaccelerate, which will make the deflagration coupled with the pressure wave. Thisprocess will form the severe nonlinear shock wave. Such shock wave will reflect inthe cylinder many times which cause the cylinder pressure fluctuating dramatically.Second, our research build the mathematic model to illustrate the process thatshock wave propagates in the closed space. Though the large pressure gradient, shock wave can be formed. And we use shock-capturing method to capture the process thatshock wave propagates reflects and converges in the closed space.At last, the research is conducted to reveal the effect of shock waves on thecombustion chamber based on the Fluent. Two kinds of representative combustionchamber are researched: the cone roof one and the ω type one. The overpressuredistribution of top piston surface is found. Numerical simulation showed that thedetonation wave could converge in special zone because of the shape of combustionchamber, which made the overpressure there much higher than that in the other zones.The numerical results were compared with the damaged pistons, it’s found that thedetonation wave converged from calculation of numerical simulation could be thereason to cause the piston local region damaged under super knock. The resultsobtained in the study prove that convergence of shock wave did exist and it alsoreveal the destroy mechanism of knock.