TS-88
SHAFTING ACTIVE TORSIONAL VIBRATION CONTROL FOR LARGE MARINE DIESEL ENGINES
BY PILOT INJECTION AND DELAYED COMBUSTION
V. A. Soloiu* and J. Jenzer**
ABSTIULCT
In this study is investigated the possibility of removal of the barred speed range by reducing only one harmonic component of the excitation, that one responsible for resonance within the barred speed range. The active torsional vibrations control was based on a pilot injection and delayed combustion while keeping mean indicated pressure (m.i.p.) at the required value for a normal operation of the marine engine on the propeller law. The Sulzer two stroke 6RTA62 marine Diesel engine coupled to a fixed pitch 4-blade propeller through an intermediate and propeller shaft were analyzed. The results of the investigations of the combustion law within a large range of injection timings, showed that the refined fuel injection characteristic decreased the amplitude of the dangerous harmonic component of the excitation and the shafting stress within the barred speed range as low as three times. It was proven that no barred speed range would be necessary for the fully satisfactory operation of the engine.
Key Words: active vibration control, operation and propulsion system, propellers and shafting, combustion
1. INTRODUCTION
The need of fast ships with more powerful and efficient engines at lower speeds, coupled directly to the propeller, gives an impulse to develop long stroke, high m.i.p., low speed two stroke Diesel engines. This trend challenges the researchers to find optimum solutions regarding mechanical, thermodynamic and emissions aspects. An important direction of research is the shafting and structures vibration. As the engine torque increased constantly due to higher bore, stroke and m.i.p., the excitation and the supplementary stresses due to the vibrations showed the same trend. For a given mechanical configuration the best way to reduce the supplementary shaft stress is to reduce at acceptable limit the excitation, or the amplitude of the defined harmonic component responsible for the dangerous resonance. This study would like to investigate the possibility of reducing only one harmonic component of the excitation that one responsible for resonance within a barred speed range, by a pilot injection and delayed combustion while keeping m.i.p. at the required value for a normal operation of the engine on the propeller law.
2. THE SHAFTING CONFIGURATION
The two stroke Sulzer 6RTA62 marine Diesel engine, with 620 x 2150 mm, bore x stroke, Maximum Continuos Rating (MCR): 12200 kW at 109 rpm, coupled to a fixed pitch 4 blade propeller through an intermediate and propeller shaft were analysed. The equivalent torsional configuration is presented in Fig. 1. This system has already a large disc at the free end of the engine crankshaft as resulted from the optimisation from the point of view of the torsional vibrations in a previous study [17]. The first mode of vibration has the first natural frequency at 290 rpm and the second mode, at 1076 rpm (Fig. 2). The critical speeds showed resonance during the engine acceleration beginning with the I/12th order harmonic down to I/3rd order harmonic for the first mode of vibration and from II/12th down to the II/8th order harmonic for the second mode of vibration. Special remark for the II/9th and II/8th harmonics in the second mode of vibration that will excite forced torsional on the left resonant flank especially at engine MCR and during the sea trials. The calculations have been confirmed by on board measurements. For this particular configuration the critical speed of the 6th order harmonic in the first mode of vibration is at 48 rpm., [1]. During the acceleration the engine is passing through this critical speed and high amplitude torsional vibrations occur. This makes the additional torsional stress τ6 in the engine crankshaft to be at the limit according to the International Association of the Classification Societies/New Sulzer Diesel (IACS/NSD) and to be exceeded in the intermediate shaft according to Nippon Classification Society (NK) (Fig. 3). This last value brought the necessity of a barred speed range from 44-53 rpm and additional countermeasures to reduce the stresses below the NK Transient Limit (NK-FT) limit. Several classic ways to bring the additional torsional stress within the Classification Societies limits where analysed during the study but being not in direct connection with the proposed topic, they will be listed here only: increased intermediate shaft diameter, modified propeller mass moment of inertia small damper at crankshaft's free end, greater strength of the intermediate shaft material and so on. The aim of this paper is to propose a novel way to solve this problem.
