Investigation of energy transformation and damage effect under severe knock of engines

Abstract

Under severe knock of IC (internal combustion) engines, engine parts like pistons are vulnerable to be damaged rapidly, the mechanism of which remains unsolved. Especially how the chemical energy released from fuel burning is transformed into the force damaging the material of pistons is still unclear. In order to reveal the in-cylinder energy transformation as well as its damage mechanism under severe knock, an assumption of shock wave from severe knock converging in combustion chamber was proposed. Based on the assumption, several quasi three-dimensional numerical simulations had been conducted to investigate into the flow behaviors of pressure wave in chamber. The results calculated from the numerical simulations showed that the shock waves transformed from the energy released by fuel burning could be categorized into Axial Wave and Radial Wave respectively. The interaction between these two types of waves would result in severe energy convergence on certain positions, which was strong enough to damage the piston. The intensity of energy convergence was not only related to the interaction of the two types of shock waves but also to the chamber shape while severe knock happened. A detonation bomb device with the interior configuration similar to that of product spark ignition engines was designed and a series of three-dimensional experiments had been conducted in it. Acetylene which is easy to cause detonation was taken as fuel and burned with pure oxygen inside of the bomb. The experimental results showed that the regions near the center and at the edges did exhibit the greatest amplitude pressure oscillations compared with those at the other regions. And the experimental results were well fit with those results from simulation calculation. Meanwhile, the similar results were achieved when the actual in-cylinder pressure curves obtained from IC engine bench tests under severe knock were analyzed. The damaged pistons were presented to validate the experimental and numerical results. Based on this knowledge, the assumption of piston damaged by knocking wave convergence was validated. Additionally, the method of power vector introduced and combined with strength calculation and energy calculation were conducted to further illustrate that the piston was vulnerable to be damaged by energy transformation under severe knock.