The acoustic emission technology was used to on-line monitor friction and lubrication state. The wavelet multi-resolution analysis was adopted to inhibit high amplitude signals of oscillatory acoustic emission caused by valve switch and combustion

and the threshold de-noising method was used to extract the acoustic emission signals caused by viscous friction. Then the wavelet envelope analysis was applied to calculate the average amplitude of the acoustic emission signals in the middle of a stroke as criteria for evaluating the lubrication state of a system of piston rings and cylinder liner. The results show that the acoustic emission signals in the middle of a stroke can effectively characterize the friction and lubrication condition between a system of piston rings and cylinder liner

and can distinguish the effect of different types of oil product on the lubrication state of a system of piston rings and cylinder liner through the standard acoustic emission signals. It is also shown that the amplitudes of acoustic emission signals increase slightly with load and significantly with engine speed. The acoustic emission monitoring technique is fully demonstrated to be effective for online monitoring of the friction and lubrication state of a system of piston rings and cylinder liner.