Factors for Decay of Diesel-Engine-Vibration Energy

Abstract

This study focused on the factors for the decay of diesel-engine-vibration energy experimentally and numerically. We considered four factors of the vibration decay processes: internal damping of each engine part, friction between the contact surfaces of assembled parts, oil viscous friction on joint parts, and asperity friction on joint parts. First, we conducted hammering tests on each engine part to investigate the mode frequencies and modal damping ratios. A single part damps itself by internal damping. Second, we conducted hammering tests on the two parts assembled to investigate the influence of slip friction between the contact surfaces of two parts. We confirmed that if the mode shape of one part differs from that of the other part and its phase is in opposition to the other one, friction occurs on the contact surfaces and thus, the modal damping ratio of assembled parts becomes much bigger than that of each part. Third, we conducted engine firing experiments to investigate the effect of oil viscosity on the combustion-induced vibration. We confirmed that the decay rate with high-viscosity oil becomes smaller than that with low-viscosity oil at some frequencies. To detect the cause of this phenomenon, we performed simulation for different oil viscosities. The simulation results show that there is an asperity contact on the main bearing with low-viscosity oil while there is no asperity contact with high-viscosity oil and the asperity contact increases the decay rate in a wide frequency. Finally, we discussed the damping ratio for simulation. We found that containing the slip friction between the contact surfaces of two parts in Rayleigh damping can improve simulation compared with the Rayleigh damping ratio considering only internal damping.