## 1 Introduction  
High boost and fuel direct injection possess the potential of enhancing power density and fuel economy in downsized spark ignition (SI) engines running under high load conditions. However, high loads also induce more harsh thermodynamic conditions in the cylinder, which promote the occurrence of the typical abnormal combustion phenomena such as the conventional knock even super-knock [1; 2]. [PERSON] et al. reported that engine knock is related to auto-ignition of end gas [3]. In previous research, engine knock occurred owning to detonation [4]. In the recent decade, the development of high boost and direct injection in gasoline engines under low engine speed and high load operating conditions, nevertheless, has been threatened by the occurrence of a new abnormal combustion, namely super-knock or deto-knock [5; 6]. The peak pressure in the cylinder can exceed 30 MPa and the pressure oscillation amplitude can exceed 20 MPa, which is extremely destructive and it can cause spark plug breakage, valve blowout, or piston ablation [5; 7]. The super-knock mechanism is remarkably different from that of the conventional knock, showing that it occurs owing to a different combustion mode in the cylinder. In the literature [8; 9; 10], a number of researches on end-gas autoignition because it is relevant to engine knock or super-knock. [PERSON] et al. [8] studied the influencing factors of the starting point of the knock from a single-cylinder gasoline engine. During the experiment, it is found that the auto-ignition of the end-mixture already appeared in several cycles before the knock occurred. [PERSON] et al. [9] investigated the effects of different fuel injection strategies on the fuel-air blending in the cylinder and the corresponding combustion processes in a gasoline engine. AVL FIRE simulations are conducted to study the turbulent flow in cylinder and to evaluate the different mixture formation on flame propagation. [PERSON] et al. [10] used precise LES (large eddy simulation) codes and modified ECFM-LES (extended coherent flame model-LES) and TKI (tabulated kinetics of ignition) models to predict in-cylinder pressure oscillation, frequency of knock occurrence and the corresponding crank angle at the different ignition timing strategies. It was found through three-dimensional analysis that the knock occurred at a random position, but mainly on the side of the exhaust valve. In addition, some issues represent that the abnormal combustion such as super-knock is closely relevant to the auto-ignition of the mixture before spark ignition, also known as pre-ignition, and it possesses sporadic and transient characteristics [5; 11; 12; 13]. However, the understanding of auto-ignition is not clear enough, and the mechanism for the occurrence of super-knock is not yet distinct. [PERSON] et al. [14] adopted a visualized rapid compression machines (RCM) to study the relationship between the pre-ignition and super-knock, and found that the early start of the hotspot may cause a conventional knock instead of a super-knock. The most important difference between a super-knock and conventional knock is the amplitude of the pressure rise at the start of the pressure oscillation. In addition, a conventional knock continuously occurs because the spark-triggered flame propagation compresses the end unburned gas to auto-ignition. Nevertheless, the super-knock is an occasional event and the combustion starts prior to spark timing. If the hotspot starts later, most of the combustible mixture has already been burned and the pressure rises less, then the knock or super-knock will not be triggered. [PERSON] et al. [15] used visible methods to directly observe the pre-ignition caused by solid particles. There was a large amount of floating particles in the cylinder after pre-ignition and these floating particles caused the next premature combustion.