Lesson 12　MARINE MEDIUM SPEED DIESEL ENGINE Ⅰ 轮机英语课件.ppt

Lesson 12 第12课 MARINE MEDIUM SPEED DIESEL ENGINE Ⅰ The term medium speed diesel engines is currently taken to mean an engine with an operating rotational speed of between 300 to 1200 r/min. This group of engines is now entirely of the trunk piston type, mainly operating on the four-stroke cycle and the majority is now fitted with exhaust gas driven turbochargers and charge coolers. *Rationalization(合理配置) of cylinder layout(布置) and numbers of cylinders has been carried out in the power range of 300 to 750 kW by most manufacturers, resulting in the basic configuration(结构,外形) of either six or eight cylinders in line, or alternatively(或者) six or eight cylinders in a Vee form. For power requirements greater than, say, 900kW, 8, 12 and 16 cylinders Vee or twin bank engines are used. Cylinder and crankcase The cylinders of medium speed engines are water cooled. The bore of each cylinder is formed in a liner which can be replaced when worn out. The cooling water is in direct contact with the outer surface of the liner as a result of which it is termed a ‘wet’ liner. A number of cylinder liners are enclosed in one cast iron casing to form a cylinder block. The cooling water jacket is common to all the cylinders but there are often dividing walls to ensure that each cylinder receives the right amount and flow of water. Doors are provided on the cylinders casing, through which the water spaces may be cleaned and inspected when overhauling the engine. In an alternative construction each cylinder is enclosed by a separate water jacket. ＊On a bedplate is mounted a casing termed the column(机架) which forms the crankcase and supports the cylinder block. In many designs the force produced by the reaction　of the cylinder heads to the gas pressure in the cylinders is transmitted from the top of the cylinder block directly to the crankshaft main bearing by through bolts(long-tie bolt贯穿螺栓). Smaller engines can be made more rigid than large ones and may not have through bolts. In a