엔진의 가변실린더 운전이 엔진진동 및 축계 비틀림진동에 미치는 영향

Abstract

Shipping companies are adopting a mega container ship to transport more cargoes to hub port at once and transporting them to each feeder port using 1,000 to 4,000TEU container feeder vessels via the hub-spoke network to increase the operation efficiency. In addition, they are applying slow steaming for the voyage of the mega container ship to reduce fuel costs and emissions of greenhouse gas(GHG) at the same time. Although the efficiency and the performance of the engine are designed and built to be optimal at normal continuous rating(NCR), it is operated at lower engine revolution speed to save the engine fuel. In the case of new vessels for reducing the fuel consumption of the engine, the engine which has higher power at lower maximum continuous rating speed can be adopted with a larger propeller to increase the propulsion efficiency. However, the acceleration problem of the vessel occurs due to the reduction of the engine power or it may cause problems such as heat accidents of the aft-stern tube bearing due to the larger propeller. As a countermeasure against these problems, the concept of "The device for crankshaft angle change and the reciprocating engine including it" was devised, and the technology of variable cylinder operation was introduced. Variable cylinder operation is a technique that makes the optimal efficiency and performance in lower engine revolution speed by cutting off the fuel supply to at least one of the operating cylinders. It can reduce the fuel consumption of the engine by changing the number of the operating cylinders and avoid the power imbalance by adjusting the crank angle for operation cylinders at the same time. For application of the variable cylinder operation, the detail technical review for the methods to adjust the crank angle should be needed and also the excitation’s changes should be confirmed for the effect on the hull, engine structure and the shafting vibration. For the detail adjustment technic for the crank angle such as hydraulic, thermal or key fitting, it is plan to be reviewed later. It has been firstly dealt with vibration point of view in this paper to review the effect of the variable cylinder operation on the hull, engine structure and the shafting torsional vibration. The inertia forces are occurred from reciprocating and rotating masses of piston-crank mechanism during crankshaft rotating. It is called external force of reciprocating engine. The moments that are occurred by the external forces are called external moment. Generally, the crank angles are regularly arranged according to firing order so the inertia forces of moving parts become equilibrium in the crankshaft so the external forces are not occurred. However, during variable cylinder operation, the external forces are occurred because the cranks are regularly arranged for the operating cylinder but other cranks for the deactivated cylinders are not so the external forces are able to be remain and it is able to make the external moments bigger. In this paper, the external forces and moments has been analyzed during normal and variable cylinder operation for 6 ~ 12 cylinders 2 stroke marine diesel engines, and the effect of the external forces have been reviewed and evaluated for the effect on the main bearing by comparing with the maximum cylinder pressure. Also, the external moments have been analyzed for the same objective engines and evaluated by power related unbalance(PRU) to review the effect of the hull vibration from the external moment, and the alternatives have been reviewed by changing the deactivation cylinder to find the condition which the counterweight for normal operation is able to be also applied for variable cylinder operation, and finally, the optimal conditions have been found. While the force from the cylinder pressure is tranfered from piston to crankshaft, the reaction force which is push the cylinder side at the cross head is occurred. The force is called “Guide force” and the moment occurred by the guide force is called “guide force moment”. There are two kinds of guide force moments which are H-moment and X-moment. These are excitations that influence on the engine’s H mode and X mode each, and the engine structure is able to excessively vibrate by them. In general, the engine vibration which is occurred by the guide force moment is controlled by applying the top bracing by increasing the critical speed above the engine operation range, and sometimes the semi-active controlled top bracing which is able to automatically control the top bracing operation “Active” and “Inactive” by setting the engine speed. In this paper, the guide force moments for 6 - 12 cylinder engines have been analyzed during variable cylinder operation and the effects of the guide force moment during variable cylinder operation on the engine structure vibration have been evaluated by estimating the engine H and X mode vibration for 6 and 12 cylinders engine for each. For the results, it is found that the H-mode vibration for 6 cylinder engine is lower then normal cylinder operation because of less maximum engine operation speed and lower major order even if the guide force moment are much higher. For the results of 12 cylinder engine, significantly heavier vibration have been predicted because of higher X-moment compared to normal cylinder operation. The semi-active controlled hydraulic top bracing has been introduced as an alternative and the effect of the top bracing has been confirmed. From the cylinder pressure, the tangential force is acting on crankshaft as the excitation of torsional vibration. It can cause the shafting failure when the excessive torsional vibration is happened. As the general alternatives, the shafting diameters are increased or decreased and the tuning wheel is applied to control the natural frequency of the shafting. When the torsional vibration is highly occurred or 2 node vibration is a problem for the shafting, torsional vibration damper can be applied on fore-end of crankshaft. In this paper, the shafting’s safety against torsional vibration has been reviewed for variable cylinder operation condition and the same countermeasures are also studied for variable cylinder operation and the normal cylinder operation. As the results of the review, the heavier tuning wheel is needed for the shafting applied 6 cylinder engine. For 7 to 9 cylinder engines’ shafting, they are able to apply the same countermeasures for the normal cylinder operation without any changes, and the tensile strength of the intermediate shafts should be increased to use the whole engine operation range for the 10 cylinder engine’s shafting. Finally, it has been found that variable cylinder operation is impossible for the 11 and 12 cylinder engines because of the excessive torsional vibration stresses in the crankshaft. The variable cylinder operation has worse effect for the hull, engine structure and the torsional vibration compared with normal cylinder operation, however, it has been confirmed that it is possibly controlled by the countermeasures generally applied excepting the cases of 11 and 12 cylinder engines that have excessive torsional stresses in the crankshaft due to the 2 node torsional vibration. The smoother engine operation is expected with lower fuel consumption, controlled vibration and noise by applying variable cylinder operation when the cylinder is not operated by malfunction. It is still remained as a problem that the method which can properly change the crank angle by various technics as like hydraulic, shrink or key fitting for the change of the crank angle required for the variable cylinder engine should be found. Although it was not dealt with in this study, it is expected that a proper method for the crank angle change for the variable cylinder engine can be applied with new technical development if the variable cylinder engine is increasingly demanded as a main engine of a ship in the future.