No abstract

Spark assisted compression ignition (SACI) is a proven method for extending the load range and controlling the combustion phase of homogeneous charge compression ignition (HCCI) while maintaining high thermal efficiency. However, the occurrence of abnormal combustion, such as knock, limits the improvement of efficiency in SACI combustion. In this study, the effects of a coupling strategy, which combines internal/external exhaust gas recirculation (i & e-EGR) and split injection, on knock suppression in SACI mode were investigated in a high-compression-ratio, single-cylinder gasoline engine with a fully variable valve system. During the experiment, the mass of intake air remained constant while e-EGR was added. The results show that the coupling strategy combines the advantages of e-EGR and split injection, providing an effective method for resisting knock and improving engine efficiency. The results also demonstrate that applying e-EGR to SACI combustion significantly decreases the knock intensity by effectively reducing the in-cylinder temperature. In addition, the effect of split injection on knock suppression is related to the initial in-cylinder temperature and fuel stratification. With high initial in-cylinder temperature, the relationship between knock probability and split injection timing is non-monotonic. However, with low initial in-cylinder temperature, the capacity of resisting knock monotonically increases with the delay of secondary injection timing.

The mass erosion of the kinetic energy of projectiles penetrating concrete targets at high speed is an important reason for the reduction in penetration efficiency. The heat generation and heat conduction in the projectile are important parts of the theoretical calculation of mass loss. In this paper, theoretical models are established to calculate the mass erosion and heat conduction of projectile noses, including models of cutting, melting, the heat conduction of flash temperature, and the conversion of plastic work into heat. The friction cutting model is modified considering the heat softening of metal, and a model of non-adiabatic processes for the nose was established based on the heat conduction theory to calculate the surface temperature. The coupling numerical calculation of the erosion and heat conduction of the projectile nose shows that melting erosion is the main factor of mass loss at high-speed penetration, and the mass erosion ratio of melting and cutting is related to the initial velocity. Critical velocity without melting erosion and a constant ratio of melting and cutting erosion exists, and the critical velocities are closely related to the melting temperature. In the process of penetration, the thickness of the heat affected zone (HAZ) gradually increases, and the entire heat conduction zone (EHZ) is about 5~6 times the thickness of the HAZ.