A single-cylinder engine was found to have abnormal sound at idle

and the issue was addressed by a study on the identification and control of the source of the noise. Through a comprehensive application of the subjective assessment of sound quality

noise spectral analysis and three-dimensional sound distribution analysis

the noise was identified as high-frequency valve slamming sound caused by the abnormal dynamics of valve train at idle. Six segments of 5-order polynomial were used for the valve lift

from which the cam profile was obtained based on kinematic relations. Then the cam profile was optimized based on multi-body dynamics by using a direct search method. The optimization results show that the spring oscillation amplitudes of the intake and exhaust valves

which have serious influence on dynamics of a valve train

are decreased by 89% and 91% respectively. Finally

a new camshaft with the optimized profile was manufactured and the engine with the new camshaft was tested on an acoustic engine test bench. The test result shows that the abnormal sound is eliminated and the engine noise is effectively improved by about 4 dB(A) reduction.