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1、Lesson 12Marine medium speed diesel engine IuniversityThe term(术语) medium speed diesel engine is currently(普遍 地) taken to mean an engine with an operating rotational speed of between 300 to 1,200 rev/min.universityThis group of engines are now entirely of the trunk piston(筒 形活塞) type, mainly operati
2、ng on the four stroke cycle anduniversityand majority are now fitted with exhaust gas driven turbochargers(废气涡轮增压器 ) and charge coolers(增压空气 冷却器、中冷器).universityRationalization(合理化) of cylinder layout(布置) and numbers of cylinders has been carried out in the power range 300 to 750 kw by most manufactu
3、rers, universityresulting in the basis configurations(结构形式) of either six or eight cylinders in line, or alternatively(二者择一) six or eight cylinders in a Vee form.universityFor power requirements greater than, say(比方说), 900kw, 8, 12 and 16 cylinders Vee or twin bank(双列式) engines are used. universityC
4、ylinder 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. universityThe cooling water is in direct(直 接) contact with the outer surface of the liner as a result of(作为结果) which it is termed a wet li
5、ner(湿式气缸套).universityA number of (numbers of 许多) cylinder liners are enclosed in( 装入) one cast iron casing to form a cylinder block(气缸体). universityThe cooling water jacket(冷却 水套) is common(共用的) to all the cylinders but there are often dividing walls to ensure that each cylinder receives the right a
6、mount and flow of water. universityDoors are provided on the cylinder casing; through which the water spaces may be cleaned and inspected when overhauling the engine.universityIn an alternative construction each cylinder is enclosed by a separate water jacket. universityOn a bedplate is mounted a ca
7、sing termed the column(机架 ) which forms the crankcase and supports the cylinder block. universityIn many designs the force produced by the reaction(反作用 力) of the cylinder heads to the gas pressure in the cylinders is transmitted fromuniversityfrom the top of the cylinder block directly to the cranks
8、haft main bearings by through bolts( 贯穿螺栓).universitySmaller engines can be made more rigid(刚性的) than large ones and may not have through bolts. universityIn addition, because the castings( 铸件) are not so big and heavy, the column and cylinder block may be made in one piece.universityFor Vee-form en
9、gines the top of the column is shaped to give two sloping surfaces on which the cylinder blocks are placed at the correct relative angle to each other.universityDoors are formed in the columns of the both in-line and Vee engines to give maintenance access(维修通道) to the crankshaft main and connecting
10、rod large end bearings.universityCrankshafts Crankshafts of medium speed engines are almost invariably(总 是) solid forged(整体锻造) from a single piece of steel. universityThe cranks of a multi-throw(多 拐的) shaft are set at angles to each other giving a “firing order(点火顺序)” for the engine. universityAltho
11、ugh the crankshafts of medium diesel engines appear to(看来) be robust (坚固的、结 实的), they rely on the support of the main bearings to develop(进 一步提高) their full strength(整 体强度).universityWhen a crankshaft has to be handled outside the engine, it should be carefully supported in a manner(以.方式) which will
12、 avoid imposing(施加) high bending moments(弯曲力矩) on it. universityIn the engine it is essential to ensure that the bearings carrying it(crankshaft) are in good alignment(良好的对中).universityConnecting Rods With a few exceptions(除个别之 外) medium speed diesel engines have trunk pistons with the result that p
13、istons and connecting rods have to be fitted together before being assembled into the cylinder.universityFor one or two types of engine, the piston and rod can be withdrawn(抽出) downwards from the cylinder into the crankcase and then out through the crankcase door.universityHowever, this design tends
14、 to(有 某种倾向；有的趋势) result in a high engine and therefore the piston and rod is more usually withdrawn upwards passing through the bore of the cylinder.universityIn case of(如果) engines have separate waterjackets, the whole cylinder assembly with the piston and rod inside may be removed as a single unit
15、; universitythe aperture(孔径) in the frame for the waterjacket being much larger than the cylinder bore gives much more room for the connecting rod large end to pass.universityConnecting rod large ends are either of fixed center(固定中心 式的) or “marine” type (可调中 心式、船用式) design. universityIn the latter design, the large eng bearing is separate from the rod. universityA distance piece(调距片), with ground faces, known as a compression plate(压缩片) may be interposed(插入) between the rod and the bearing housing, universityits thickness being chosen to ensure the
