Experimental Study On Autoignition And Detonation Of End Gas In A Closed Space Of Macromolecular Fuel

In response to the challenges of energy crisis and environmental pollution,the spark ignition(SI)engine is developing towards downsizing and turbocharging.The load of the SI engine is increased and the knock tends to occur,leading to a highfrequency shock wave and reducing the reliability of the engine.Based on the optical results,it is generally recognized that detonation is caused by the spontaneous combustion of end gas.In the current work,based on macromolecular fuel,the mechanism of end-gas autoignition and detonation is studied in a constant-volume combustion bomb(CVCB-TJU).The influencing factors of end gas autoignition are explored in order to further reveal the mechanism of knocking and reduce knock phenomenon.Firstly,by elevating the oxygen concentration of the mixture,controllable autoignition with detonation is achieved.The mechanism of autoignition and detonation is studied: the shock wave generated by the accelerated flame propagates repeatedly in the combustion chamber,compressing the unburned gas.Therefore,the temperature and pressure of the mixture is increased,leading to autoignition.The autoignition flame and the shock wave are coupled to form a detonation wave.Then,the effects of oxygen concentration and initial pressure on end gas autoignition is investigate.On the one hand,the increase of oxygen concentration improves the reaction activity of the mixture,on the other hand,it increases the flame propagation speed.Therefore,controllable autoignition can occur due to the increase of oxygen concentration.As the initial pressure increases,the required oxygen concentration to achieve controllable autoignition gradually decreases.End gas autoignition is more likely to occur under high initial pressure,and the detonation wave generated is stronger.Secondly,the effect of flame propagation velocity on end gas autoignition is investigated.Under the same conditions,different flame propagation speeds are achieved by using perforated plates with different porosities.It is found that the high flame speed leads to a stronger shock wave,which promotes combustion mode transformation from normal combustion to end wall autoignition and two-point autoignition.Then different combinations of double-perforated plates are used to achieve controllable autoignition under different acceleration conditions.The results show that controllable autoignition can occur under conditions of high oxygen concentration and low flame velocity,or under conditions of low oxygen concentration and high flame velocity.Meanwhile,the combustion modes with the same flame velocity and different oxygen concentration are compared.The results demonstrated that the increase of oxygen concentration is beneficial to the occurrence of end gas autoignition,indicating that the flame propagation velocity is not the only factor affecting autoignition,but the reaction activity of mixture also has an important effect on the generation of autoignition.Finally,the types of inert gas and fuel in the mixture are changed to explore the effect of reaction activity on the autoignition of end gas.When different inert gases and fuels are applied,the normal combustion mode can be changed to end gas autoignition and detonation mode with the increase of oxygen concentration.The ability of inert gas to suppress autoignition increases in the order of argon,nitrogen,and carbon dioxide.And three different combustion modes of autoignition are discovered.When using different fuels,the required oxygen concentration to achieve controllable autoignition increases gradually according to the order of hydrogen,n-heptane,PRF50,isooctane and methane.Besides,the autoignition tendency of the fuel decreases in this order as well.