2. X-MODE VIBRATION CONTROL BY UNEVEN FIRING ANGLES
K90MC-C engine type with 12 cylinder has been mainly used for large container vessels (4400, 5050 TEU) and engine builder Hyundai Heavy Industries had good experience to reduce X-mode vibration by exchanging cylinder firing angle as case A in Table 1 and fitting tuning torsional vibration damper.(4〜5)
For diesel power plant as shown Table 2, the firing angles of case A and B was not relevant in a view of torsional vibration aspect. So, cylinder firing angle was modified as Case B and due to reduce torsional vibration it was unevenly arranged as Case C and torsional stresses at crankshaft are shown as Fig. 2. This method, however, increased 6th X-moment shown as Table 3 and so final cylinder firing angle was decided for Case D considering X-mode and torsional vibration.
Maximum uneven angle between each cylinder for Case D was 13.2 Deg. relatively high comparing to that of even arrangement, which had possibility to increase torsional vibration during one cylinder misfiring condition. Also shown as Fig. 3, the additional stress at crankshaft was high at 77 rpm, 5th order critical speed around which should be set barred speed range to electrically by-passed during engine starting and stop.
During commissioning test in site, the vibration measurement was carried out to find out X-mode resonance with engine speed sweeping and no load condition. However, the resonance of 5th and 6th order X-mode vibration could not be found by the torsional and H-mode vibration effect. And vibration results at generator synchronized speed with 100% load was normal as shown in Table 4.
Table 4 Measured transverse vibration levels of engine top with 100% load(Unit: mm/s)
