(1) Closing of exhaust valve and flow of exhaust gas
In the signal measured at exhaust valve, an excessive vibration due to the flow of exhaust gas is observed at the crank angle of approximately 45 degrees (before B. D. C. ), and this vibration is detected also in other locations. At the crank angle of approximately 90 degress (after B. D. C. ), an impulsive large vibration occurs due to the closing of exhaust valve, that is an impact force on the valve seat. This impulsive vibration is distinguished particularly in the signals measured at cylinder cover and cylinder liner. Naturally, the amplitude of these vibrations is large at the upper position of cylinder unit.
(2) Combustion and opening/closing of F. O. needle valve
The effect of combustion appears as a comparatively large vibration at the upper position of cylinder. It can be seen from careful observation of the waveform measured at cylinder cover that the vibration in combustion process includes the transient vibrations [3] corresponding to the motion of F. O. needle valve, and this transient vibrations are observed also at cylinder liner. These are induced by the impact force which occurs with opening and closing of F. O. needle valve, and are therefore essentially the same phenomenon as mentioned in the case of closing of exhaust valve. It should be noted that the vibrations observed in the period of F. O. injection include above transient vibrations as well as the vibrations due to combustion.
(3) Contact of piston rings with non-continuous parts of cylinder liner surface
There are non-continuous parts such as (upper and lower) cylinder oil grooves and scavenging ports on inner surface of cylinder liner. When piston rings run across the non-cotinuous parts, impulsive vibrations are generated by the contact with the parts. In Fig. 4, it is noticeable that two vibrations occur symmetrically before and after T. D. C. at the crank angle corresponding to contact of top-ring with oil grooves. When piston rings run across scavenging ports, in some case relevant vibrations occur and in other case don't; in Fig. 4, the vibrations are not apparent. This is supposed to be the phenomenon caused by the positional relationship between scavenging ports and free ends of piston rings.
(4) friction between cylinder liner and piston rings
The vibrations due to the friction between cylinder liner and piston rings appear especially at cylinder liner in the vicinity of crank angles which maximize piston speed; the angles, shown as 01 and 02 in Fig. 4, equal 66. 7 degrees (before and after T.D.C.) in measured engine. Amplitude of the vibrations changes slowly compared with other instantaneous vibrations. In comparison between 01 and 02, amplitude at 01 is generally larger than that at 02 though the difference is not apparent in Fig. 4. The friction of liner/rings depends upon their lubricating condition in addition to piston speed. Consequently, if piston is moving, rubbing vibrations may appear even in combustion process, which decrease the thickness of oil-film. However, it is difficult in the time domain to distinguish the rubbing vibrations from those induced by combustion.
3.2 Features of spectrum map
When the waveform of measured signals changes transiently in one frame of the analysis, it is a general way to extract the features of the frequency domain using time-frequency analysis. In this study STFT (Short Time Fourier Transform), which is a kind of time-frequency analysis, was employed with following conditions:
・the width of a data frame for FFT is 1024 points (equivalent to approximately 10 msec);
・a quantity of shift of data frame for 2D map is half of frame width;
・hanning window is employed for data window.
And, vibration level (dB re = 10-5 m/sec2) is represented by gradation pattern from white (70 dB) to black (130 dB).
