Numerical Study Of Engine Super Knock

With the increasingly stringent emission regulations,the trend of SI engine is to improve engine efficiency and power density by using in cylinder direct injection and high turbocharging technology.However,when the engine is running at low speed and high load,it will encounter ultra-high intensity knock under boosted condition,which is fatal to engine cylinder components.Different from the general knock caused by the spontaneous combustion of the end gas in the cylinder,super-knock is essentially a detonation,and its combustion propagation speed and pressure intensity are much higher than the ordinary knock combustion.It is generally accepted that the super-knock is triggered by pre-ignition,but not all of the pre-ignition evolve into super-knock.In view of the numerous controversies on the super-knock induction mechanism,a simplified one-dimensional model is used to calculate the auto-ignition and evolution process induced by hot spot and lubricating oil in this work.It is expected that the super-knock induction mechanism can be revealed through the calculation and analysis.For the study of hot spot induced super-knock,there are hot spots with different sizes in the cylinder and the thermodynamic conditions(such as temperature and pressure)may differ somehow.The calculations considered all the possible situations of different hot spot characteristics and thermodynamic conditions.The results show that there are different combustion modes in the boosted engine: 1.The auto-ignition reaction waves induced by the hot spots couple with the sound waves,and mutual amplify to compress the reactive unburned mixture in front of the reaction waves,and the reaction fronts accelerate rapidly to form detonation combustions.Detonation combustion can be formed either inside or outside the hot spots.For those developed inside the hot spots,the hot spots characteristic have major impact on the early stage of detonation formations.For those developed outside of the hot spots,the hot spot characteristics and the thermodynamic state of the unburned mixture may equally important to the development of the detonation;2.The auto-ignition reaction waves develop into subsonic flames or explosion due to the elevation of the end-gas temperature and reduction of the ignition delay time,spontaneous ignition of the end-gas resulting in explosion when the propagation time of the reaction wave exceeds the ignition delay time of the unburned gas.A comprehensive analysis of the auto-ignition reaction wave propagation induced by hot spots shows that when the hot spot size is small,the detonation probability is very small.The reason may be that when the hot spot is very small,the energy transmitted to the sound wave by the heat release of the hot spot reaction is not enough to significantly increase the amplitude of the sound wave and compress the unburned gas effectively.The basic condition for the reaction wave to develop into a detonation is effective compression of unburned gas.Small size hot spot is difficult to develop into a detonation.However,if several small-scale hot spots gather to form a large-scale hot spot,it is possible to initiate a detonation.The chemical activity of lubricating oil is high and it is easy to cause early combustion.Whether the spontaneous combustion wave can develop into detonation is still to be studied.In this work,the numerical calculation is used to explore the detonation induced by lubricating oil.Considering that the lubricating oil entering the combustion chamber comes from different parts,the lubricating oil from piston ring gap,cylinder wall and piston crown may enter the combustion chamber and participate in combustion.There are obvious differences in the release frequency and the amount of oil released from different sources.The lubricating oil from the piston ring gap shows cyclic release feature and the average amount of oil is much less than the piston crown oil scattering.The oil accumulated on the piston crown is scattered every few thousands of revolution,The oil is scattered heavily within a few revolutions based on photographic observation.Note that the intermittent oil release frequency is coincident with the frequency of super-knock event suggesting the direct relevance to detonation.The auto-ignition induced by lubricating oil is numerically studied,it is found that the lubricant released from the piston ring can induce auto-ignition reaction wave which only develops into flame propagation;the lubricant released from the piston crown also induce auto-ignition reaction wave which develops into a detonation.Considering the similar intermittent features between super-knock event and piston crown oil scattering events,of all the possible pre-ignition sources(such as hot spot,oil droplet),the oil released from piston crown could have the largest possibility to trigger a super-knock event.