汽车检测与维修技术毕业设计

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1、无锡科技职业学院毕业设计（论文）专业外文翻译材料 专业外文翻译材料 系 别 南京机电学院 专 业 汽车检测与维修技术 班 级 汽修 1003 学生姓名 岳祥龙 学 号 1101513240 指导教师 张锦龙 2013年4月目 录第一部分 专业外文翻译材料原文1第二部分 专业外文翻译材料译文12参考文献21 i专业外文翻译材料原文Engines Cylinder Block The cylinder block is the largest past of the engine.Its upper section carries the cylinders and pistons.Normall

2、y,the lower section forms the crankcase,and supports the crankshaft.It can be cast in one piece from grey iron.Or it can be alloyed with other metals like nickel or chromium.The iron casting process begins by making up the shapes of what will become water jackets and cylinders as sand cores which ar

3、e fitted into moulds.The stops there parts becoming solid iron during casting.Molten iron is poured into sand moulds that are formed by patterns in the shape of the block. After casting,core sand is removed through holes in the sides and ends,leaving spaces for the cooling and lubricant passages.The

4、re holes are sealed with plugs.The casting is then machined.Cylinders are bored and finished,surfaces smoothed,holes drilled and threads cut.All cylinder blocks are made with ribs,web and fillets to provide rigidity but also keep weight to a minimum. Cylinder Block ConstructionAs more manufacturers

5、try to make vehicles lighter and more fuel efficient,more and more engine blocks are being cast from aluminium.A block made of aluminium alloy is lighter than if it were made of cast iron.So if two engines are generating the same power,the alloy version would have a better weight-to-power ratio than

6、 the cast alloy version.Aluminium alloy blocks are made by various casting processes,including pressure casting.Another method is gravity casting,where the molten metal is poured into molds.Cast iron liners are usually used in the cylinders of aluminium blocks,and sometimes in cast-iron blocks.Some

7、sleeves are cast into the block.Grooves on the outside form a key that stops any movement in the cylinder.They also increase surface area to assist heat transfer from the sleeve to the block. Some blocks dont need liners.They can be made of wear resistant material that makes a hard-wearing surface f

8、or the pistons and piston rings.Or the cylinder bore may have some sort of surface treatment to make it hard-wearing. When the cylinders,block and crankcase are all cast together,it is called a monoblock construction.A horizontally-opposed block has a split crankcase.The two engine blocks are joined

9、 together by the flanges of the crankcase. In air-cooled engines,the cylinders are usually made as separate parts,then bolted to the same crankcase.Each cylinder has cooling fins.Theyre often machined to give uniform thickness and allow free flow of air. Pistons The piston,with its connecting rod an

10、d bearing,transfers the force of the combustion and expansion of the power stroke to the crankshaft.The piston itself,its rings,and the piston pin,also known as the gudgeon pin,are together called the piston assembly.The cutaway shape on this piston allows it to clear the counterweights on this rota

11、ting crankshaft. The connecting rod connects the piston to crankshaft. It is fastened to the piston at its little end,by a piston or gudgeon pin.The big end of the connecting rod has a detachable cap,and carries 2 halves of the big end bearing.The big end is attached to the crankshaft at the crankpi

12、n journal. Cylinder SleevesCylinder sleeves are used in engine blocks to provide a hard-wearing material for pistons and piston rings. The block can be made of one kind of iron thats light and easy to cast,while the sleeve uses another kind that is better able to stand up to wear and tear.There are

13、three main types of sleeves-dry,flanged dry,and wet. The dry sleeve can be cast in or pressed into a new block, or used to recondition badly-worn or damaged cylinders that cant easily be re-bored.Its a pressed fit in its bore in the cylinder blocks.Its wall is about 2mm thick.Its outer surface is in

14、 contact with the block for its full length.Its top finishes flush with the top of the block and can hardly be seen.Once in place,dry sleeves become a permanent part of the cylinder block.A flanged,dry sleeve is like a normal dry sleeve,but a flange at the top fits into a recess in the surface of th

15、e engine block.Its not a tight fit and it can be replaced if its worn. With a wet sleeve,the outer surface is part of the waterjacket around the cylinder.Its called wet because it has coolant against its outer surface.This helps speed up heat transfer between the sleeve and coolant.The sleeve is sea

16、led at the top to prevent coolant leaks.This stops coolant entering the combustion chamber,and the bottom of the crankcase.A flange at the top of the sleeve fits into a recess in the block.The lower end has 1 or 2 neoprene sealing rings.With coolant in direct contact with the cylinder sleeve,corrosi

17、on can be a problem.It can even insulate the sleeve from the coolant,which reduces the main advantage of having a wet sleeve at all. The walls on wet sleeves are thicker than on dry sleeves.They dont have the same support from the block as dry sleeves so they depend on their wall thickness to stop d

18、istortion. In diesel engines,vibration caused by combustion can cause cavitation.This damage appears similar to corrosion and it can eventually destroy the cylinder. Grey Iron Grey iron is a form of cast iron.There are many different kinds of cast iron,depending on the particular materials they cont

19、ain.Grey iron is a cast iron that contains carbon in the form of graphite,plus silicon,manganese and phosphorus.The fractured surface of a cast iron with graphite appears grey,hence the name.It is brittle and cannot absorb shocks.It resists heat and corrosion,and can be cast into many different shap

20、es.It is used for many components.Cylinder Head The cylinder head bolts onto the top of the cylinder block where it forms the top of the combustion chamber.In-line engines of light vehicles have just one cylinder head for all the cylinders.Larger in-line engines can have two or more.V-type and horiz

21、ontally- opposed engines have a separate cylinder head for each bank of cylinders.Just as with engine blocks,cylinders heads can be made of cast of cast iron,or aluminium alloy.A head made of aluminium alloy is lighter than if it were made of cast iron. Aluminium also conducts heat away more quickly

22、 than iron.So with an aluminium-alloy head, the heat of combustion can be conducted away into the coolant more quickly . Manufacturing the head is similar to manufacturing the block.A casting mold is made.Sand cores are put in to form any hollow areas. Depending on the engine,these can be for coolan

23、t and lubricant passages,and inlet and exhaust ports. Air-cooled engines have cooling fins cast into the cylinder head. The underside of the head is shaped to form the combustion chamber.Molten metal is poured in,and allowed to cool. The cores are broken out and removed,and the cylinder head cleaned

24、 of any sand. After casting comes machining.Surfaces that must seal are machined flat.Holes are drilled and tapped for attaching blots and studs. In sand-cast heads, the large holes that had contained sand are machined,then fitted with soft metal plugs, called core plugs. Cylinder Head Design Cylind

25、er heads are designed to help improve the swirl or turbulence of the air-fuel mixture,and prevent fuel droplets settling on the surfaces of the combustion chamber or cylinder walls.When air-fuel mixture is compressed between the piston and the flat part of the cylinder head,it produces whats called

26、squish.That means squeezing of the gases to increase their velocity and turbulence. In gasoline engines,the three most popular combustion chamber designs are called hemispherical pent roof,bath-tub and wedge. A hemispherical combustion chamber has the intake valve on one side of the chamber and the

27、exhaust valve on the other.This provides crossflow.Air-fuel mixture enters on one side,and exhaust gases exit on the other.Positioning the valves in this way leaves room for relatively large valves and ports,and that helps the engine breather. Breathing refers to the engines taking in the air or air

28、-fuel mixture. Fuel starts to burn at the plug,then burning travels outward in all directions.This is called flame propagation.With the plug in the middle of the hemisphere,the flame front has less distance to travel than in some other designs,which gives rapid and effective combustion.This design i

29、s common in a lot of passenger vehicles. The bath-tub combustion chamber is oval-shaped,like an inverted bathtub.Valves are mounted vertically and side by side,making them simple to operate.The plug is to one side,and that creates a short flame path.It all helps increase turbulence. The wedge-shaped

30、 combustion chamber tapers away from the plug which is at the thick end of the wedge.The valves are in line and inclined from the vertical.This design usually has a smaller surface area than the others,with less area where fuel droplets can condense.Less fuel is left unburned after combustion ,which

31、 reduces hydrocarbon exhaust emissions.And since the flame is directed toward the small end of the wedge,damage caused by detonation is reduced. Diesel Combustion Chambers Diesel combustion chambers come in 2 main types.Direct and indirect injection.Both are designed to promote turbulence,to help th

32、e compressed air and injected fuel mix well.Engines using direct injection have cylinder heads with a flat face.The combustion chamber is formed in the top of the piston.Sometimes, the rim of the piston provides squish,forcing the air to the center of the combustion chamber.This causes turbulence as

33、 fuel is injected into the cylinder. In indirect injection,the piston is fairly flat,or has a shallow cavity.The main combustion chamber is between the cylinder head and the top of the piston,but a smaller,separate chamber is in the head.Fuel is injected into this smaller chamber.It can have various

34、 designs.A swirl chamber is spherical,and connected to the main chamber by an angled passage .Both the injector and glow plug are screwed into the head.The glow plug preheats the air inside to help start the engine.During compression ,the spherical shape makes the air swirl in the chamber.This helps

35、 make a better mixture of the air and fuel,which improves combustion .This combustion chamber is divided into a main combustion chamber and an air cell, joined by a throat. The injector is in the throat. When injection commences,combustion pressure forces the air to flow from the air cell where it m

36、ixes with fuel from the injector.The rush of air from the air cell produces a rotary motion of gas in the main chamber which helps make combustion more efficient.This pre-combustion chamber is screwed into the cylinder head.The injector is mounted in the upper end. Injection occurs near the top of i

37、nto the compression stroke .Only part of the fuel is burned in the pre-combustion because of the limited amount of air there.The high rise in pressure forces burning fuel into the main chamber.This happens very rapidly,which helps make more efficient combustion. Intake and Exhaust PassagesThe size o

38、f passages in the head can affect engine output.Smaller intake and exhaust passages allow more torque at low engine speeds.This is because smaller passages improve mixing of air and fuel fat low speeds ,which causes more efficient combustion .At high speeds however,these smaller passages restrict ai

39、rflow .To reduce the effect of this ,this engine has two inlet valves.One opens at low speed and other operates at higher engine speeds .Larger passages produce greater power at high engine speeds. Each intake and exhaust passage can be formed separately in the head.Intake passages for adjacent pass

40、ages may have a common,thin wall between them .This is called siamesed.Exhaust ports in the same head can also be siamesed. When all intake and exhaust ports are on one side,it is called a counter flow head.They can be cast separately or siamesed.When all of the intake ports are on one side and exha

41、ust ports are on the other ,it is called a cross-flow head.This allows for straighter passageways and higher efficiency. GasketsGaskets form a seal by being compressed between stationary parts where liquid or gas could pass.Most gaskets are made to be used only once.They can be made of soft material

42、s such as cork,rubber,paper,asbestos.They can also be made of soft alloys and metals such as brass,copper, aluminium or soft steel sheet metal. Choosing which material and design to use depends on the substance to be sealed,the pressures and temperature involved.The materials and mating surfaces to

43、be sealed.Head gaskets seal and contain the pressures of combustion within the engine,between the cylinder head and block.They also seal oil passages between block and head.And control the flow of coolant between the block and the head. Some gaskets provide or adjust clearances. Some joints between

44、surfaces on modern engines are being sealed with special sealants which eliminate the use of gaskets in some applications. Gaskets around a rotating part would quickly wear out and leak. To seal these parts,oil seals are needed .Many different kinds have been developed,including oil slinger rings. T

45、he most widely used is the lip type dynamic oil seal. It has a shaped dynamic rubber lip thats held in contact with the shaft to be sealed by a circular coil spring called a garter spring. A similar sealing principle is used to seal the valve stem to prevent oil entering the engine combustion chambe

46、r. Rotating or sliding shafts can also be sealed by using O rings, but generally they are not as durable in most applications as the lip-type seal.As a general rule ,oil seals must be replaced when a component is overhauled. Gaskets And Oil SealsModern head gaskets have to be constructed to resist h

47、igh temperatures and engine detonation. Some modern high temperature head gaskets are called anisotropicin nature.This means that the gasket is designed to conduct heat laterally to transfer heat from the engine to the coolant faster.They are normally constructed with a steel core.Special facing mat

48、erials are added to both sides of the gasket core to provide a comprehensive seal under varying torque conditions. With the advent of environmental factors and a reduction in the use of asbestos,replacement materials have been developed.Some of these modern special materials that are now used for th

49、e side layers of head gaskets are designed to withstand temperatures up to 2100F or 1150 .Such materials are also designed to allow the cylinder head and block,some of which have considerable distortion rates,to move slightly on the head gasket as they expand during engine warm-up.This feature is vi

50、tal for preventing head gasket failure.Some head gaskets also incorporate stainless steel fire rings to help to contain heat and pressure within the cylinder.In addition,many head gaskets also have an added silicone based outer coating on both sides of the side material layers to provide additional

51、cold sealing ability during start-up and warm-up.Head gaskets also seal oil passages,and control the flow of coolant between the cylinder block and head and are fitted with beads or rings to prevent leakage and corrosion.Some joints between surfaces on modern engines are being sealed with special se

52、alants which eliminate the use of gaskets in some applications.Pure rubber,or conventional cork rubber is unable to deal with the stresses and pressures in modern engines.Modern gasket manufacturers are producing improved material combinations such as nitrile and cork blends to deal withhigh tech en

53、gine demands.Some materials are designed to swell in application and increase sealing ability.For instance when oil inside a valve cover penetrates the edge of the gasket material,it is designed to swell by approximately30%.This swelling effect increases the stealing pressure between the head and va

54、lve cover sealing surfaces and helps to seal potential leaks. Some gasket materials are designed to have high tensile strength.They are designed to resist breakage during dismantling or installation processes. Various materials are used in modern oil seals,some being impregnated with special coating

55、 materials that are designed to increase their sealing ability on worn shafts. Turbulence Turbulence refers to the swirling motion of a liquid or a gas .It helps to maximize the mixing of air and fuel,which helps make sure the combustion process occurs efficiently.Without turbulence ,the air-fuel mi

56、xture can form local areas of high pressure and temperature that can cause detonation during combustion .A high level of turbulence can prevent this.Camshafts and DrivesThe position of the camshaft depends on the design of the engine.It can be in the engine block close to the crankshaft,this is a ca

57、lled a pushrod or overhead valve system.Or there can be one or two camshafts mounted in the cylinder head.But in both designs it dose much the same job,driving the valves and the distributor,and sometimes the fuel pump,and the oil pump. The camshaft is made of hardenable iron alloy or steel,and it c

58、an be cast or machined. The cam lobes are ground to the proper shape and position in relation to one another.Accuracy is crucial.If the cam is not exactly the required shape,or if it becomes worn,there can be impacts,fast valve wear,or noisy operation. The bearing surfaces on the camshaft are ground

59、 smooth,and the distributor drive gear is machined into the shaft. The cam lobes are then flame or induction hardened. The camshaft has a cam for each valve.In some case,there is an additional cam known as an eccentric,to operate the fuel pump.A gear on the camshaft drives the ignition distributor,a

60、nd,often,an oil pump.Overhead CamshaftIn modern engines,the pushrod system is being replaced by the simpler overhead camshaft arrangement. The overhead camshaft is located in the cylinder head.there can be 1 or 2 camshafts.Lets look at a single overhead camshaft arrangement.Single overhead camshafts

61、 can use rocker arms.The cam can lift one end of the rocker arm,or it can press down on the rocker arm. On double overhead camshaft systems,the most common arrangement is to use a bucket tappet or lifter.It operates in a guide that protects the valve against side thrusts which it would receive if th

62、e cam operated directly against the valve.The adjustment of valve clearance is usually done by changing accurately machined spacers.Spacers are available in a range of thicknesses,and theyre exchanged to obtain the correct clearance.Some overhead cam engines use a hydraulic lash adjuster to reduce l

63、ash in the valve train.They have zero clearance at the valve stem so theres no need for tappet adjustment.It can be put in the valve end of the rocker arm.Like the hydraulic valve lifter,it has a body with plunger hold against the valve stem by a spring. Oil supplied to the adjuster keeps the plunge

64、r in contact with the valve and eliminates lash. Lash adjusters can be put in the cylinder head at the end of the rocker arm.The lash adjusters are stationary and have a pivot for the end of the rocker arm.The plunger in the adjuster holds the rocker up against the cam.In the lash adjuster inside the bucket tappet,the plungers hydraulic action holds the bucket body against the cam on the camshaft and also against the tip of the valve stem so that there is zero cl