migrating into the lead-bronze layer 覆镀材料能够对腐蚀提供一定程度的保护，覆镀一薄层的镍就能够防止铅向青铜层的迁移。The backs of the steel shells may have a soft layer which is designed to prevent corrosion. 钢衬壳也可以加一层软材料防止腐蚀。During the 1990s a number of engine builders returned to bi-metal bearings which employ a layer of aluminium-tin on the steel backing shell. 在20实际90年代，一些柴油机厂又从新生产在钢衬壳内部加一层锡铝合金的双金属轴承。Others employ a multiple layer bearing with a tin-antimony rubbing surface over a copper-lead layer on the steel backing shell. 另外一些厂家利用在钢衬壳内部的铜合金层之上又加了一层表面粗糙的锡锑合金生产多层轴瓦。Bearing technology continues to change to meet the demands of higher loads but problems of lubricants contaminated by water and residual fuels must also be overcome. 轴承技术不断发展为满足了更大负荷的需要，但是还需要克服由于水和残油对润滑油所造成的污染问题。 Crankshaft 曲轴
Crankshafts for medium speed engines are solid forgings from a single alloy steel ingot. 中速机的曲轴是用一块合金钢锭实锻而成的。Solid forging avoids stress problems, which would result from the presence of shrink fits, and ensures even stress transmission between journals, webs and pins. 实锻避免了因收缩配合带来的应力问题，避免了轴颈、曲柄臂和曲柄销上的应力集中。There are very rigorous Classification Society rules governing the dimensioning of crankshafts, based on a combination of theory and experience. 基于理论和实践经验的结合，船级社规则对曲轴的尺寸有严格的要求。Current designs make considerable use of computer finite element analysis which allows a wide range of loadings, dimensions and cylinder firing orders to be analysed before any manufacture takes place.现在的设计利用了计算机有限元分析方法，在任何生产之前，都对大负荷范围、不同尺寸和汽缸点火顺序等进行分析。
A crankshaft must be able to transmit the torque developed during operation. 曲轴必须能够传递柴油机工作过程中所产生的扭矩。This torque imposes stresses on the crankshaft journals. 扭矩在曲轴轴颈上产生扭转力。Pins are subject to direct stresses from the connecting rods, which impose bending and shear stresses. 曲柄销承受从连杆传来的力，这个力会在曲柄销上弯曲力和剪切力。Webs bend due to this loading but they also have a tendency to twist. 由于这个负荷曲柄臂会产生弯曲，但是它们也有一个扭转的趋势。Crankshafts will bend under load as each cylinder unit section acts like a simply supported beam, with the main bearings acting as the supports. 曲轴像一个被简单支起来的梁，主轴承就像支撑体，这样曲轴在每一个缸所产生负荷的作用下就会产生弯曲，Loadings vary with time making the analysis of stresses a complex matter, further complicated by the presence of oil holes through journals, webs and pins.负荷随时间的变化使应力的分析变得复杂，轴颈、曲柄臂和曲柄销中钻孔后使应力的分析变得更加复杂
The firing order of an engine is primarily chosen to reduce engine torque fluctuations and minimize vibration. 对于降低扭矩的波动和降低振动，发动机点火顺序的选择很重要。Any final solution must be something of a compromise as other factors have to be considered including exhaust arrangements, bending loads on the crankshaft and torsional vibration. 任何最终方案都是一个折中选择，因为排气布置、曲轴上的弯曲负荷和扭转振动等其他因素也要考虑。Reciprocating masses-the pistons, connecting rods, webs and crankpins induce vertical forces which can result in vibration. 往复运动质量，如活塞、连杆、曲柄和曲柄销都会产生垂直方向上的力，进而导致振动。Reducing the mass of these components through the use of lighter materials and removal of all unnecessary materials will help to minimize vibration but will not eliminate the problem. 通过使用轻金属和去除不必要的材料都会降低这些组件的质量，减轻
振动，但不会消除这个问题。Most medium speed engine crankshafts are machined all over in order to reduce excess weight.为了去除多余的重量，中速机的曲轴需要整体车削。 They are provided with adequate fillets at all changes of section in order to avoid stress raisers which could give rise to Fatigue. 为了防止应力集中造成应力疲劳，在任何横截面积的改变处都加上足够的圆倒角，Changes in section occur between journal and webs, crankpins and webs, and at oil holes. 截面改变一般产生于轴颈和曲柄连接处、曲柄和曲柄销连接处及一些油孔处。In many cases balance weights are fitted to some or all of the webs, these are designed to oppose the vibration induced by the reciprocating masses. 一些情况下，会在部分或全部曲柄上安装配重，这些配重用来抑制由往复质量产生的振动。The solution is only partial as a rotary mass will not only induce vertical forces to oppose the reciprocating masses but it will also induce variable sideways forces, causing transverse vibration. 只有部分配重充当回转质量，这不仅会产生垂直方向上的力平衡往复质量，还会产生变化的横向力，引起横向振动。The size and positioning of balance weights are carefully arranged by the designer and should not be changed.配重的大小和安装位置需要由设计者仔细选择，并且不应改变。 Connecting Rod 连杆
Connecting rods are forged from alloy steel and machined all over. 连杆是由合金钢锻造并进行车削。The basic connecting rod requires a provision for attaching it to the crankpin, at the bottom or large end bearing, and to the piston at the gudgeon pin, the top or small end bearing. 最基本的连杆在底端或大端轴承处，需要一个机构把它连接到曲柄销上，在上端或小端轴承处需要一个机构把它通过活塞销连接到活塞上。The small end generally consists of a bush inserted in a hole bored in the top of the connecting rod. 连杆小端通常在钻孔内安装一个衬套。The bush may be a simple bronze affair but some engines, particularly a high powered unit, have white metal lined, or tri-metal, bushes. 衬套是个简单的青铜件，但是一些柴油机尤其是大功率的机型，会在衬套内部再衬上白合金，或者使用三合金衬套。Lubrication for the top end bearing comes from the crankshaft via the bottom end and a hole drilled in the connecting rod. 给连杆顶端轴承润滑的油，是由曲轴经过连杆底端和连杆上的钻孔供入的。This passageway is also used to conduct cooling oil to the piston. 这个通道也用来为活塞输送冷却油。Because of the high loadings on the small end bearings and the absence of complete rotary motion, an adequate supply of lubricant is essential at all times. 因为小端轴承的负荷重，并且缺少完全的回转运动，所以在任何时候向小端输送足够的润滑油是非常重要的。Stepped small end bearings are used to provide a large bearing surface on which the piston force may act via the gudgeon pin. 用踏板轴承提供大的承载表面，活塞力经活塞销传到小端。
The large end bearing of connecting rod can present certain maintenance problems. 连杆大端轴承也会出现维护保养的问题。The traditional fixed centre connecting rod has the upper part of the large end housing forged as part of the rod, which means that the lower part of the connecting rod is wide in order to accommodate the bearing shell and securing bolts. 传统的固定中心式连杆大端的上半部分是与连杆锻造在一起的，也意味着连杆大端的下半部分是开放式的，可以安装轴瓦和紧固螺栓。In order to remove the piston, with the connecting rod still attached, the bottom part of the connecting rod must be no wider than the cylinder bore through which it will be lifted. 为了能够在连杆不拆除的情况下吊起活塞，连杆底部不能够比缸套内径大，这样活塞可以通过缸套吊出来。Modern engines employ large diameter crank pins in order to provide large area bearings and low bearing loads. 现代柴油机采用大直径的曲柄销来提供大的承载面积，降低轴承单位负荷。A fixed centre connecting rod design would not be suitable. Alternative arrangements are available.固定中心式设计将不是一个合适的备用方案。
Bearing shells for large end bearings are similar to main bearing shells but are provided with large oil holes in a central channel. 连杆大端轴瓦和主轴承的轴瓦相似，但是会在中心油路加大润滑油的供给。These allow oil to flow from holes in the crankpin, through the bearing, to passageways in the large end and then up the hole in the connecting rod. 这样润滑油就会通过曲柄销上的油孔、主轴承、连杆大端上的通路向上流向连杆中的油路。Tags on the bearing halves mate with location points in the housings ensure correct alignment. 轴瓦上的销钉和轴承座上的点相对应，保证正确的定位。As with main bearings the thin shells rely on the accuracy of the bore machined in the large end to preserve a true circular shape. 连杆大端内孔的精确加工，对于薄的轴瓦保持圆环形起决定作用。Correct tightening of the large end bolts is, therefore, essential.因此，大端连接螺栓的正确紧固也是很重要的。
The articulated connecting rod arrangement has a single large end bearing block to which both connecting rods are attached. 主副连杆式连杆（又称关节式连杆）有一个单独的大端轴承体，主副连杆连到轴承体上。The master rod is connected rigidly by means of a palm end, while the slave rod is attached by an articulated arrangement, such as a pin and bush.主连杆通过平的端面刚性连接到轴承体上，副连杆通过一个销和衬套的铰链机构连到轴承体上。 This is necessary because the motions of both pistons, and of their connecting rods and large ends, differ as the crank rotates. 当曲轴回转的时候，由于两个活塞、它们的连杆和大端的 运动是不同的，所以这种设计是有必要的。The articulated large end arrangement has cylinder centre in line across the engine and both pistons may be removed without touching the large end bearing. 铰链式大端的设计（主副连杆式结构），主机各个缸的中心线在同一平面上，两个活塞都可以在不影响大端轴承的情况下拆除。
LESSON 4 Engine Construction II
PISTONS AND PISTON RINGS 活塞、活塞环
Pistons for medium speed engines must be capable of withstanding the gas loads experienced during peak firing periods without any appreciable leakage. 中速柴油机的活塞必须能够承受最高燃烧时所产生的气体负荷，并且没有明显的漏泄。As a result the sealing arrangements provided by the piston rings must be effective, even though lubrication at the top of the cylinder may be marginal. 即使是汽缸的顶部润滑油的量较少，活塞环也必须达到密封效果，Unlike crosshead engines most medium speed engines rely on splash lubrication from the crankcase, which must be controlled by scraper rings located on the piston crown or on the skirt. 不像十字头式柴油机，许多中速柴油机采用飞溅式润滑，润滑油量通过位于活塞头或活塞裙上的刮油环来控制。A skirt is necessary on two stroke cycle engines in order to seal the ports when the piston is at the top of its stroke.二冲程柴油机必须设有活塞裙，当活塞位于行程顶部的时候用来关闭气口。 In a four stroke engine, the skirt also guides the piston into the cylinder, countering the side thrusts caused by the angularity of the connecting rod.在四冲程柴油机中，活塞裙在汽缸中起导向作用，并且承受由于连杆倾斜所产生的侧推力。
A piston assembly consists essentially of the piston crown, the skirt, a gudgeon pin and a number of piston rings. 活塞的主要组件有活塞头、活塞裙、活塞销和一些活塞环。The crown is subject to the highest combustion pressure and thermal load. 活塞头承受燃烧所产生的高温
高压，It may also suffer from impingement by the combustion flame if the atomiser spray is defective. 如果雾化器效果差的话，还需要经受火焰的冲击。Piston crowns are of the dished type to provide a combustion chamber between the piston crown and the cylinder head. 活塞头是中凹形的，与汽缸盖形成燃烧室。To give the desired combustion, the shape of the bowl formed in the crown depends on many factors, including the choice of atomiser spray. 良好的燃烧效果取决于很多因素，包括活塞头的形状和喷油器的选择。The flat rim of the piston surrounding the dished combustion chamber generally contains cut-out sections to prevent the piston from hitting the inlet and exhaust valves. 活塞中凹形顶部的圆环面上通常车有凹坑用来防止进排气阀撞击到活塞。The piston crown is generally made from high quality, deformation resistant forged steel and contains internal cooling surfaces and the provision for a number of piston rings. 活塞头通常采用质量好且耐变形的锻钢制作，其内部有冷却空间，外部有安装活塞环的环槽。The crown may taper slightly in order to allow for thermal expansion while in service. 活塞头可以有轻微得锥度，以适应其工作时的热膨胀。During operation the upper part of the crown becomes hotter than the lower part so it will expand more.在工作时，活塞头上部会比下部热，所以也膨胀的多。 Tapering allows a narrow clearance between piston and liner to be maintained.这个锥度就使得活塞和缸套之间保持一个小的间隙。
A piston skirt guides the piston in the cylinder, minimising the tipping of the piston which causes high contact pressures at upper and lower parts of the piston/skirt assembly and disturbs gas sealing of the rings. 活塞裙引导活塞在汽缸中运行，减轻活塞的倾斜。倾斜会引起活塞上部和下部组件产生高接触压力以及破坏活塞环的密封。The piston skirt is subject to lower thermal and mechanical loads than the crown and can be made from materials of lower strength. 活塞裙所受的热负荷及机械负荷都比活塞头低，可以用低强度的材料制作。It is subject to greater rubbing against the liner, however, and should have a lower coefficient of friction. 活塞裙与汽缸套之间摩擦严重，最好选用摩擦系数低的材料，Nodular cast-iron is often used for piston skirts due to its low coefficient of friction and ability to be readily cast into a fairly complex shape.球墨铸铁的摩擦系数较低，并且能够很容易救助造成复杂形状，所以经常采用球墨铸铁制作活塞裙。 Aluminium is also used for piston skirts. Its low density is an advantage in terms of inertia forces, although it has a higher coefficient of thermal expansion than nodular cast iron. 铝也经常用来制作活塞裙，虽然它比球墨铸铁的热膨胀系数大，但是由于它的密度低利于减小惯性力。At low loads an aluminium skirt will give increased cylinder clearance compared with one of nodular iron, so the piston axis will differ slightly from the cylinder axis. 与球墨铸铁的活塞裙相比，铝制活塞裙在低负荷时的汽缸间隙较大，所以活塞的轴线会稍微偏离汽缸中心线。At reduced loads an aluminium skirt will therefore not provide as stable a platform for the piston rings as a nodular iron skirt. 负荷降低时，铝制活塞裙不会像球墨铸铁活塞裙那样为活塞环提供一个稳定的平台。The skirt does not need to be perfectly cylindrical, as long as it is shaped to assist the formation of an effective lubricant film between piston and liner. 活塞裙不必要加工成一个理想的圆柱体，只要他能够在活塞和汽缸套之间形成一个有效地润滑油膜就可以。Some skirts taper slightly from bottom to top while others are of oval section, with the minor axis parallel to the axis of the gudgeon pin.一些活塞裙会从底部向上有一个锥度，一些会有椭圆横截面，椭圆的短轴平行也活塞销的轴线。
In most engines the top three rings are classed as compression rings, although the upper ring takes most of the gas loading. 大多数柴油机中最上面的三道活塞环是压缩环（气环）Ring materials should be compatible with the liner material in order to avoid scuffing as the rings rub over the liner surface. 活塞环的材料应该与汽缸套的材料相适应，避免活塞环擦过汽缸套表
面时造成擦伤。Rings must retain their initial spring at operational temperatures and pressures, resist thermal cracking and maintain their mechanical strength. 活塞环必须在工作压力和温度下保持初始的弹性，不出现热裂纹并保持机械强度。Ideally they should have some self lubricating properties.理想的情况下，它们应该有一定的自润滑特性。 Cast iron is an ideal material but for modern conditions alloying elements such as manganese, chromium and molybdenum are employed to improve mechanical strength, heat resistance and self-lubrication. 铸铁是一种理想的材料，现在也把一些合金元素如锰、铬和钼加入到铸铁当中用来提高机械强度、耐热性和自润滑能力。 Cylinder Liners 汽缸套
Cylinder liners are centrifugally cast from nodular or close grained cast iron to obtain good mechanical strength and wear resistance. 汽缸套是利用球墨铸铁或者细晶粒铸铁离心铸造，已获得良好的机械强度和耐磨性。A rigid liner bore which is free of distortion is ideal and, with the provision of good lubrication, provides optimum running conditions for the piston and piston rings. 理想情况下，如果有良好的润滑，活塞和活塞环工作于最佳条件，一个刚性汽缸套是不会变形的， Liners fit into the cylinder block, but for many highly rated engines the cylinder block does not always house the water jacket which is instead provided by means of a separate unit mounted above the block. 汽缸套安装在汽缸体上，但是一些大功率的柴油机汽缸体不总是都带有水冷却腔，而是在汽缸体上部安装一个独立的冷却单元。Such an arrangement provides cooling just where it is required, at the upper part of the cylinder liner, and also avoids the risk of cooling water leaking into the crankcase or of oil from the crankcase gaining entry to the cooling water system. 这样的布置恰好冷却了汽缸套的上部，避免了冷却水泄漏进曲轴箱和曲轴箱内部的油进入冷却系统的危险。The liner is located within this jacket unit and sealing rings are provided to prevent water leakage.汽缸套位于水套的内部，用密封圈防止水漏泄， The jacket unit sits on the cylinder block and is held in place by the cylinder head.水套安装在汽缸体上，用汽缸盖定位。 Although such jackets may not be shrink fits with the liner they should be neat fits and add strength to the upper part of the liner under working conditions.尽管水套盒汽缸套之间不是红套安装，他们也应该配合良好，当缸套处于工作条件下，水套应该对汽缸套上部施加一定的力。 Individual jackets ensure low cylinder liner distortion and prevent any one cylinder causing distortion of neighbouring cylinders.单独的水套设计保证汽缸套变形低，防止缸套受其它缸影响而变形。 The liners must be a neat fit in the cylinder block to prevent distortion. 汽缸套必须要良好地安装在汽缸套上防止变形。For two stroke engines it is essential that the correct angular position is obtained, so exhaust and air inlet ports are aligned with their respective passages. 二冲程柴油机汽缸套要有正确的安装角度，要使进排气口正好与它们各自的通道成一线。Sealing rings are still provided between the liner and the cylinder block, even though there may be no water jacket in the block, to prevent oil leaking into the cylinder block section. 即使汽缸体上没有水套，在汽缸套和汽缸体之间也要有密封圈，防止油漏泄到汽缸体内。Where the cylinder block provides a water jacket for the cylinder liner, sealing rings are required at the lower part of the liner. 这种情况下，汽缸体就作为了汽缸套的水套，密封圈安装在汽缸套的下部。Some sealing arrangement is also needed in the liner above the jacket. 在衬套里也需要其它一些密封，It is usual to provide a telltale between the two lower sealing rings, so that water leaking past the upper seal ring or oil leaking past the lower ring may be readily detected.它在两道较低密封环 之间形成了一个指示器，所以如果水从上面的密封环漏泄或者油从下面的密封环漏泄都可以容易地检测出来。An alternative to having a jacket around the upper part of the liner is to have a solid upper section liner of the desired thickness for