The effect of the energy convergence and energy dissipation on the formation of severe knock

Abstract

In order to increase the energy utilization efficiency and decrease the emission, internal combustion engines have been pushed to their thermodynamic limits. As a result, abnormal combustion phenomenon would occur which results in the severe knock. Severe knock is usually caused by the onset of the detonation which will damage the engine parts in a short period. How to avoid such destructive detonation as well as severe knock is the aim of this research. This research found that the onset of the detonation is highly related with the chamber shape. In order to validate this relationship, two chamber shapes were studied in a series of bomb experiments. Both of them are the cone-roof type, but one has no clearance while the other one has a clearance of 12 mm. High-energy ignition spark ignites a deflagration from the center of the bomb. Four pressure sensors installed in different positions of either chamber were used to monitor the pressure wave behaviors and the onset of detonation. The experimental results show that under the same initial conditions, the detonation always occurs in the non-clearance chamber but never occurs in the chamber with a larger clearance. Furthermore, a series of numerical simulations have been conducted to reveal the mechanism of the detonation formation caused by the chamber shape. It’s found that the non-clearance chamber has a convergent structure on the edge which will focus the pressure wave and its energy on the edge position causing the formation of detonation. Compared with it, the chamber with a larger clearance doesn’t have such convergent structure so that the detonation as well as the severe knock won’t occur. Therefore, according to the research results, if the chamber shape is properly designed to dissipate the energy or avoid the energy convergence, the severe knock may be weaken or even be avoided though the engines are further pushed to obtain a higher efficiency.