Ideally for a four cylinder in-line engine, the engine mounts are designed to isolate the second order engine vibrations from the chassis at idling engine speed. However it has been found that orders other than the second order and its harmonics are also present as sources of vibration excitation. The traditional engine mount design can help in the attenuation of the second and higher orders. The problem of isolation then exists with the lower orders which can pose significant problems such as idle shake.
This paper presents a brief review of different sources of half order excitations. A study of vibration measurements conducted on a four cylinder in-line engine with a predominant 0.5 order excitation is also discussed.
Vibration measurements were made at the engine mounting points on both the engine and the chassis and the 0.5 order excitation was first identified using order tracking analysis. It was found that there is considerable excitation at idling rpm due to the resonant natural frequency of the engine on its mounts coinciding with the 0.5 order of idling rpm. A feasible solution to overcome this problem was found by modifying the stiffnesses of the engine mounts such that the resonant frequency is shifted towards higher rpms and in the speed range when the engine is generally in its acceleration mode. This ensures that a state of resonance dwell is not reached. This solution was tested and successfully implemented.