## 1 Introduction
They proposed a possible mechanism of pre-ignition caused by sediments: That is, the floating particulates formed after the sediment fall off will burn in the normal combustion cycle; after that, although the flame on the particle surface is extinguished during the gas exchange process, the chemical reaction on the surface continues, which can allow the particles to maintain a certain temperature. Then, during the compression stroke, for the reason of the temperature and pressure of the ambient mixture rising, the chemical reaction on the particle surface accelerates and ignites the surrounding mixture, resulting in a pre-ignition. [PERSON] et al. [16; 17] used endoscopic optics to observe the spatial position of the pre-ignition point in the cylinder at the moment of super-knock. It was found that the pre-ignition point was widely distributed along the plane of the cylinder gasket and that super-knock is not induced by structural hot spots such as sparking plugs. In their experiments, it was found that the use of three kinds of lubricating oil sold in the market induced the abnormal combustion such as pre-ignition, indicating that the pre-ignition and super-knock caused by lubricating oil were not incidental. [PERSON] et al. [18] cited the hotspot self-ignition theory and concluded that detonation may occur within a hot spot in the event of super-knock. The unburned mixture can react chemically in a short time and in a small space with rapid exothermic after shock compression. The released energy in turn supports the propagation of the precursor shock. Due to the strong coupling of this precursor shock and chemical reaction, detonation combustion can reach extremely high pressure peaks, and a super-knock is more likely to be the result of the mutually reinforcing 'developmental detonation' generated by multiple hotspots. From the literatures mentioned above, it can be seen that pre-ignition and super-knock are utilized to represent the engine knock occurred at low speed and high load operation conditions of the engines. Pre-ignition indicates the combustion of the fuel-air mixture triggered by hot-spots in cylinder other than spark ignition prior to the spark timing. Researchers usually distinguished the distinction between pre-ignition and super-knock by experimental investigation and numerical simulation. A numerous experiments to characterize the super-knock combustion process have been carried out including optical engine and rapid compression machine [4; 5; 9; 12; 13; 17]. In addition, computation fluid dynamics (CFD) has been widely used to study the abnormal combustion in the gasoline engines [10; 11; 19; 20; 21]. [PERSON] et al. [20] adopted KIVA-3V code and Chemkin to conduct engine simulation and chemical kinetics investigations of the effects on ignition delay time and flame speed. [PERSON] et al. [21] used STAR-CD to investigate the interaction between in-cylinder flow conditions and flame propagation, and tumble and swirl flows were produced in cylinder by inserting various baffle plates in the middle of the intake port. Nevertheless, there are few literatures concentrating on the formation mechanism of a super-knock under the practical engine operating conditions. The existing studies focus on optical engines, visualization constant-volume devices and numerical simulation carried out to investigate the end-gas auto-ignition under approximate engine operating conditions. There are still gaps between the above mentioned studies and the end-gas autoignition of the gasoline engines in service under practical engine conditions. Simultaneously, the end-gas autoignition process by means of the multi-dimensional simulations can fully represent the knock phenomena compared to 1D simulations. 3D simulation takes into account the essential factors such as turbulence, wall heat transfer and boundary layer, which play significant roles in an engine knock.