The dynamic behavior of an OHC valve train system of a spark ignition engine is investigated to characterize the source and transmission of the valve train (VT) vibration and noise and to improve the VT design for better sound quality. The spectral properties of vibration caused by highly transient dynamics of VT system are characterized for the high frequency ranges over 3 kHz, although the overall sound pressure level due to the VT is negligible [1, 2].
For the analysis of valve train a lumped parameter model with 4 d.o.f.'s is developed and validated with the experimental results from a test rig. Experiments are performed on the test rig to measure the valve acceleration, the surface vibration of cylinder head during the operation, and the transfer functions. Also a measurement of cylinder head vibration in a real vehicle has been performed to correlate with the rig test results. With the results from the simulation and experiment, the vibration generation mechanisms from the valve train are identified. For the spectral analysis of non-stationary signals, Short-Time Fourier transform is applied.
It is revealed that there are two major sources of vibration from the valve train during its operation. The dynamic interaction force between the rotating cam and tappet movement is one of the source mechanisms and the valve seating impact is another. Through the spectral analyses for the source and transmission mechanisms, the ‘cam force’ as well as the ‘seating impact’ is found to be a dominant source of vibration up to the frequency of 6 kHz, and the ‘seating impact’ is solely dominant over the range of 10-20 kHz. The investigation will provide a new design insight on the valve train noise and vibration and useful information for the other engine development purposes such as knock calibration, or identifying other impact-like vibration.