Mitsui-MAN B&W 12K90MC MK6 Engine
The Mitsui-MAN B&W 12K90MC MK6 engine, which has the largest output in the world at present, was completed in April, 1995 at Tamano Works, Mitsui Engineering & Shipbuilding Co., Ltd. The engine particulars are: cylinder bore: 900 mm, stroke length : 2,550mm, mean effective pressure : 18 bar, number of cylinders: 12, engine output: 74,640BHP and speed : 94rpm. This engine was installed in the largest container ship in the world and has been operating well since it was put in service in January, 1996. The main structures follow those of the K90MC MK5, which has had good results in service. The engine output in has been increased by 6% with the mean effective pressure of 18 bar, which is higher by 6% than that of MK5.
In order to increase the mean effective pressure, while keeping the thermal load of the engine and increasing reliability of the engine, the following technology has been adopted.
Three Fuel Valves
The number of fuel valves was increased to 3 from the conventional 2 in order to obtain a uniform temperature distribution on the combustion chamber parts and as well as to improve combustion. The temperature levels measured at 100% load during the shop test were 550℃ at the exhaust valve bottom, 220℃ at the top ring position of the cylinder liner at TDC, 460℃ at the throat of piston crown, respectively.
Direct Air Intake System
The ship, in which the engine is installed, has a direct air intake system through a duct from the outside the ship. Such system may help in avoiding an increase of exhaust gas temperature during a voyage in tropical weather sea because the engine can intake relatively low temperature air in comparison to a conventional air intake inside the ship. In other words, a direct air intake system may have a similar effect to that of a high efficiency turbocharger. On the other hand, during a voyage in cold weather seas, the scavenging air pressure increases because of the intake of cold outside air, and then the maximum pressure increases. In order to avoid a maximum pressure increase, it is equipped with an automatically operated exhaust by-pass system.
The following systems are applied as measures to increase the reliability in addition to the above mentioned measures for thermal load of engine.
Three Stage Air Cooler
The air cooler consists of 3 cassette type heat exchange units, where fresh water is used as coolant for the upper 2 units (heat exchange at high temperature) and sea water for the third unit (heat exchange at low temperature). Such a system makes it possible to avoid salt separation inside the surface of tubes at the outlet of coolant because fresh water is used as coolant for heat exchange at high temperature. Thus, the number of sea water pipe lines can be reduced, then it leads to easier maintenance work.
Monitoring Systems
During operation of engine, the crankshaft axial vibration at the fore end and the torsion vibration are being monitored all the time in order to confirm the dampers are working well. Furthermore, the cylinder conditions are also being monitored by means of 4 thermocouples installed circumferentially at 5mm depth from the sliding surface of each cylinder liner.
