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Evaluation of a cast-joining process of dual metal crankshafts for heavy-duty engines with ductile cast iron and high strength forged steel(구상흑연주철과 고강도 단조강의 주조접합 이종금속을 이용한 중대형 엔진 크랭크샤프트의 평가)
Three unprecedented dual metal crankshafts for heavy-duty diesel engines up to 3,000 hp are proposed and thoroughly investigated by comprehensive multi-body dynamic finite element analysis and supporting experiments. The dual metal crankshafts are basically made of ductile cast iron and high strength forged steel, and the two dissimilar metals are proven to be seamless by a series of tensile tests using bimetallic specimens. The dual metal approach is made in order to achieve both cost effectiveness and shortened manufacturing lead time simultaneously. The new approach is basically realized by casting of ductile cast iron in a mold assembled with pre-machined steel parts. Historical researches and experiments already proved that the monometallic crankshaft of only ductile cast iron is not able to replace a conventional forged steel crankshaft. Therefore, casting-based crankshaft should be reinforced by high strength structures, especially for critical areas such as the fillets of crank pins and journals. Three dual crankshafts are designed to achieve equivalent structural strength not sacrificing manufacturing advantages of casting. The reinforcing members of forged steel are implemented in three ways for the three dual metal models. The first dual metal approach so-called BM1 crankshaft is to put barrel-shaped steel pipes in the center of both crank pins and journals to complement inferior mechanical properties of ductile cast iron, FCD700. The second duel metal approach named BM2 crankshaft uses reinforcing steel members more extensively than the first one does, putting forged steel pipes in the center and outside of crank pins and journals. The second model is more advanced than the first one because the second one effectively strengthens up the most critical areas of a crankshaft, the fillets of crank pins and journals, by the high strength steel. The third model designated as BM3 crankshaft is quite a bold approach to have only crank webs made of ductile cast iron. Including crank pins and journals, the reset of the body of the third model is all made of forged steel, which is meant to maximize its torsional stiffness and minimize its torsional vibration. The conventional crankshaft and the three proposed dual metal ones are evaluated by state of the art multi-body dynamic analysis. The multi-body dynamic approach proposed in the study is very comprehensive because it includes all the key components of a cranktrain such as a crankshaft, pistons, connecting rods, a flywheel, a pulley and even dynamometer. In addition, hydrodynamic journal bearings and allowable radial clearance of journals are effectively implemented by so-called quasi-hydrodynamic approach which is given in this study. Combination of the quasi-hydrodynamic method and external loading by constant velocity on the dynamometer is quick and effective to simulate closely any cranktrains of either in-line or Vee formation. The proposed multi-body dynamic approach with a full deformable crankshaft is supposed to bear much more accurate results than static analysis with partial portion of a crankshaft, for instance, single or half crank throw. Another competitive advantage of the proposed multi-body approach is no need to consider the worst loading case and the most severely loaded part of a crankshaft, which overcomes all the historical struggles to find them. The proposed multi-body dynamic analysis turned out that the third dual metal crankshaft with the steel crank pins and journals connected by the crank webs of ductile cast iron was capable of replacing conventional monometallic steel crankshaft in term of fatigue safety factor which is calculated by scaled normal stress method. Even in extremely harsh virtual operating condition with maximum gas pressure of 210 bar, the third model was expected to operate normally. On the other hand, other two dual metal designs were not up to the minimum fatigue requirement especially in case of the extreme virtual operating case with 210 bar Pmax. Since modern heavy-duty diesel engines generally have maximum gas pressure up to 210 bar, the successful operation in this condition is prerequisite to realize the ultimate objective of this study. Feasibility of the simulation is proved by an experiment of torsional vibration with actual full-scale operational diesel engine using optical rotary encoder. The natural frequencies of the conventional crankshaft were obtained by the experiment, and the results revealed that there was good agreement of the natural frequencies between the simulation and the experimental results. Consequently, one of the proposed dual metal crankshafts is expected to be able to replace conventional monometallic one finally achieving both cost effective and reduced manufacturing lead time in accordance with the corresponding fatigue performance to the conventional one.