Fig. 5 shows the spectrum map of cylinder liner vibrations in running-in of engine (50% load) and in this figure the acceleration signal in use of adhesive cement is demonstrated. As we can see from the spectrum, peculiar curved stripes appears symmetrically with respect to the position of T. D. C. The piston speed of this engine rises to the peak at the crank angles of θ1 and θ2 as shown in Fig. 6, and the curved stripes of Fig. 5 are constituted by a set of curves proportional to the piston speed. At θ1, for example, fundamental frequency is approximately 1.64 kHz, and many peaks are regularly arranged at the frequencies of integral multiplies of the fundamental frequency.
To examine the geometric changes of curved stripes with engine speed, the relationship between fundamental frequency at θ1 and engine speeds at loads of 4 cases is shown in Fig. 7. As we can see easily, fundamental frequency is directly proportional to engine speed. It can be therfore understood that vertical intervals of stripes expand with the increase of engine speed.
Generation of stripes is considered to be related to the geometric shape of inner surface of cylinder liner. To progress a capability to keep oil-film thick enough, liner surface of low-speed large marine engine is fabricated into corrugated shape (or wave cut) as shown in Fig. 8, and that is the same as measured engine. In this case, from the pitch L of corrugated shape and piston speed V, it follows that the piston ring comes into contact with the swelled part of inner surface at a period of T (= L/V). As shown in Fig. 9, if impact forces are applied at a period of T, responses in the frequency domain appear at regular intervals, which are represented by the frequencies of integral multiplies of fundamental frequency l/T. Fundamental frequencies shown in Fig. 5 nearly correspond to the values of l/T calculated from the piston speed of measured engine, and hence the stripes are considered to be generated by the corrugated shape of liner surface.
In contrast with the above, the spectrum of impulsive vibrations induced by following sources is distributed in the shape of vertical line: closing of exhaust valve, contact of piston rings with non-continuous parts of liner surface, opening and closing of F. O. needle valve. In the case of vibrations induced by combustion and flow of exhaust gas, a spread of the distribution in the direction of time is observed in spectrum map. These spectrum are featured by the continuous distribution in the wide frequency range.
As mentioned above, STFT enables the distinction of vibration sources. From the difference in spectrum distribution, it is possible to distinguish the rubbing vibrations of liner/rings from other vibrations.
In addition, while high responses are conspicuous in frequency range between 20 kHz and 25 kHz at any crank angle in Fig. 5, this is caused by the contact resonance of used accelerometer as described later. And, it must be noted that in general the foregoing stripes are caused also by the rubbing vibration in the adjacent cylinder to measured cylinder, though the phenomenon is not apparent in Fig. 5.
Fig. 5 A spectrum map of cylinder liner vibration by STFT (load:50%)
Fig. 7 The relationship between fundamental frequency and engine speed at crank angle θ1
