3.3 Changes of spectrum with time
In order to examine the changes of spectrum with time, FFT analysis with the averaging of eight revolutions was carried out using one frame data ( l024 points) at the crank angle of θ1 Fig 10 and Fig.11 show the spectrum of cylinder liner vibrations measured at 50% load in running-in and after 20 hours' running respectively, and in both figures examples in use of adhesive cement are demonstrated. For the manner of representation of data, the horizontal axis (frequency axis) with a logarithmic scale is used to compare with the resonance characteristics curves of accelerometers.
In Fig.10, many peaks corresponding to stripes shown in Fig.5 are prominent. However, if the fluctuations are neglected, the response curve Is similar to the frequency characteristics of accelerometers in use of adhesive cement shown in Fig.3, and is of the shape on which the contact resonance of approximately 25 kHZ has a strong effect. On the other hand, in Fig.11, the significant decrease of responses can be seen at the higher frequency range than 3 kHz, though the existence of contact resonance is confirmed.
Since the principal difference between both figures is running hours of engine, this decrease is supposedly caused by the change of rubbing of liner/rings. It is said that the abnormality such as scuffing of cylinder liner brings the increase of responses at higher frequencies [4]. In this measurement, to the contrary, the decrease of the responses with time was verified taking advantage of running-in of the engine, in which liner/rings are comparatively in severe condition of rubbing.
Thus, changes of spectrum in lower load, 250% and 50"lo, were notable, but the changes in higher load than those were not remarkable. This is considered to be related to the way of loading; that is, the engine was initially operated at step-up loads, 25%, 50%, 75% and 90%, and also 20 hours later at the same step-up loads. It is therefore assumed that the rubbing of liner/rings at the first load of 25% and 50% was in severe condition, but subsequently slight rubbing lasted.
Fig. 8 Corrugated surface of cylinder liner
Fig. 9 Generation mechanizm of stripes in spectrum map
Fig. 10 A spectrum of cylinder liner vibrations at θ1 (load: 50%, running-in)
Fig. 11 A spectrum of cylinder liner vibrations at θ1 (load: 50%, 20 hours later)
