高等职业技术教育试用教材：汽车专业英语（电子书）.pdf_大学文库

frequently used engine specification. Displacement indicates engine size, and compression ratio cor the total cylinder volume to compression chamber volume The term“ stroke" describe the movement of the pis ton within the cylinder. The oper- ating cycle may require either two or four strokes to complete. Most &l four stroke cycle als known as Otto cycle, after the name of its inventor, Nikolaus Ot to,who first applied the principle Fig 1-2 The four strokes cycle a)intake stroke b)compression stroke c)power stroke d)exhaust stroke in 1876. In the 4-stroke engine, four strokes of the piston in the cylinder are required to complete one full operating cyele. Each stroke is named after the action. It performs intake, compression, power, and exhaust in that order, shown in ig.12 The piston moves downward to the bottom dead center, a vacuum is created in the cylinder. The intake valve opens and air-fuel mixture comes into cylinder. To obtain the maximum filing of the cylinder the intake valve opens about 10 before t.d. c. giving 20 overlap. The inlet valve remain open until some 50 after b.d. c. to take advantage of mixture 2. Compression strok The air-fuel mixture is compressed within the combustion chamber. While the pres se to about IMP, depending on various factors including the compression ratio, throttle opening and engine speed. The spark plug is fired to ignite the air-fuel mixture prior to the piston being at the t d. c.. Note that both valves are closed 3. Power stroke The air-fuel mixture expands, which creates the power to force the piston downward. The exhaust valve opens near the bottom of the stroke 4. Exhaust stroke As the piston starts to move upward, the exhaust valve is opened The piston moving up forces the exhaust gases out of the cylinder. The intake valve usually opens just before the exhaust stroke This 4-stroke cycle is continuously repeated in every cylinder as long as the engine remains running A2-stroke engine also goes through four actions to complete one operating cycle. However, the intake and the compression actions are combined in one stroke, and the power and exhaust actions combined in the other stroke. The term 2-stroke cycle is preferred to the term 2-cycle, which is really not accurate

3 In automobile engines, all pistons are attached to a single crankshaft 'The more ers has, the more power sinkes Produced for each revolution. This Ine ans that an 8-cylinder engine runs mmore smoothly because the power stokes arm closer together in time: and in degree of engine rotation The cylinders of multi-cylinder automotive engines are arranged in one of three ways 1) Inline engines use a single block of cylinder. This is the simplest and most common arrange ment. with all cylinders arrang lly in line. Most 4-cylinder ad G-cylirxler this design. If an engine has more than eight cylinders, it becomes difficult to make a sufficiently rigid frame and crankshaft with an in-line arrangement. Also thte engine becomes quite long and takes up considerable space 2)v-type engines use two equal banks of cylinders, usually inclined 60 degrees or 90 each other. Most V-type engines have 6 or 8 cylinders, although V-4 and V-12 engines have built 3)Horizontally opposed or"pane ake engines have two qual banks of cylinders 180 degrees apart The se space saving engine designs are often air-cooled. This arrangement is used where there is Fach engine has a few main working parts and auxiliary parts, Such as the cylinder block: cylinder head; crankcase: the piston; the connecting rod; the crankshaft; the: fuei pump ele Many parts are also attached by fastening devices to the average bloxk. These items include the wa- ler pump, oil pan, timing gear, the flywheel or clutch housing, the igmition distributor, oil and fuel pump, and the cylinder head. The water pump is a component of the cooling systerm. The crankshaft usu ally rotates this unit by means solving, the: water circulates coolant between the engine water jackets and the radiator The cylinder block is cast in one piece. Usually, this is the largest and the most complicated single piece of metal in the automobile. it is made of gray iron(east iron)or alumunum and contains the cylin ders ard the: water jackets that surround them. Several engines have aluminum cylinder blocks. Alumi mum is a relatively light metal, weighing much less than cast iron. llowever, aluninun is too soft to use as cylinder-wall material. It wears too rapidly. Therefore, aluminum cylinder blocks rmust have cast-iron cylinder liners or be cast frun an aluminum alloy that has silic on particies in it. The cylinder head closes one end of the cylinder and often contains the valves through which air and fuel are admitted and the ex haust The oil pan and the lower portion of the block together are known as the crankcase; they enclose or encase the crankshaft The oil pan itself is also a reservoir, which usually holds 4 to 9 quarts of lubrical- pump of the Iu i from the pumps it to all the moving parts of the engine. After the oil lubricates these parts, it drains off and run back down into the pan. Conisequtrtly, there is a constant circulation of oil between the pan and the ving parts of the The timing gear or chain cover, as its name implies, encloses and pmlects the timing gears or timing chain and sprockets from foreign materials. The cover has a machined area that accomodates the timing

Top Dead Center上止点 intake yalve进气阀 exhaust valve排气阀 oil pan( oil sump)油底壳,油盆 Notes 1. Of all automobile components, an automobile engine is the most complicated assembly with domi nant efTects on the function of an automobile 汽车发动机是切汽车部件中结构最复杂,对汽车功能起支配作用的一个总成。 2. In the 4-stroke engine, four strokes of the piston in the cylinder are required to complete one of full operating cycle 在四冲程发动机中,需要四个行程来完成一个工作循环 3. To obtain the maximum filling of the cylinder the intake valve opens about 10 before t de ing20° overlap 为了使进气充分,进气门在活塞到达上止点之前约10打开,使进、排气门有20的打开重合角 4. The spark plug is fired to ignite the air-fuel mixture prior to the piston being at the t.d. c 在活塞到达上止点前,火花塞发火点燃可燃混合气 5. Aluminum is a relatively light metal, weighing much less than cast iron 铝是比较轻的金属,其重量比铸铁轻多了。 Unit two Connecting Rods and Crankshaft In a reciprocating engine, the power mechanism is called the crankshaft and connecting rod assem- bly. In this assembly, all of the major units such as the engine crankcase and cylinder block, the piston and connecting rd, and the crankshaft and flywheel work in close cooperation to convert thermal energy into mechanical energy used to drive the vehicle. The engine crankcase and block are usually cast into one piece and therefore can be seem as the largest and most intricate piece of metal in automobile. They are usually made of high-grade cast alloy iron to improve wear characteristics of the cylinder. This major unit must be strong and rigid enough to withstand any bending or distortion The piston converts the potential engines of the fuel into the kinetic energy that tuns the crankshaft The piston is a cylindrical shaped hollow part that moves up and down inside the engines cylinder. The piston is composed of piston head, piston ring, piston lands, piston skirt and piston pin hole. see Fig 1-3.)The piston head or"crown is the: top surface against which the explosive force is exerted. It may be flat, concave, and convex or any one of a great variety of shapes to promote turbulence or help control combustion. In some application, a narrow groove is cut into the piston above the top ring to serve as a heat dam"to reduce the amount of heat reaching the top ring. The piston rings carried in the ring groove are of two basic types: compression rings and oil-control ring. Both types are made in a wide variety of designs. The upper ring or rings are to prevent cormpression leakage; the lower ring or rings control the

amount of oil being ckepwsited on the cylinder wali. The lower groove or grooves often have holesor slots in the bottom of the grooves to pemi ts oil drainage frum behind the rings. The piston lands are parts of piston between the ring grooves. ' Ihe lands provide a seating surface for the sides of piston rings main section of a piston is known as the skirts. It forms a bearing area in contact with the cylin The piston pinhole in the piston also serves as a bearing for the piston pin, which is used to connecT the conneeting rol. In addition, bx cause pistons operate under exceedingly difficult mechanical and thernal c onditions, piston must be strong enough to stand the force of the expansion, yet light enough to avoid excessive inertia forces when their direction of travel is reversed twice each revolution. Piston nust Ix able t withstand the heat from the buming air-fuel ixture, plus the heat gene rated by friction The connex ting rod is attached to the crankshaft at one end( big end )and to the piston at the other end( small CGas end).In operation, the connecting rod is subjected to both gas pressure and inertia loads, and therefore it must be ad quately strong and rigid and light in weight as well. The con- necting rod showT in Fig. 1-3 is in fonn of a bar with ring- shaped heads at its end. So they are generally fabricated from high quality steel are composed of connecting rod small end connecting md shank, connecting rod big end, connecting md cap, and connecting rod bearing half shells. Shank of the connecting rod is provided with an J-cross section to give the rmd maximum rigidity with the minimum of weight. The big end of the rod is split so that it can be connected to the Fig 1-3 A piston and a connecting-rod crankshaft. To avoid misplacing the rod caps during assem- 1.2-compression rings 3-oil rings 4"piston piston pin 6piston-pin ring bly, the connecting ruls and their mating caps are marked on -connecting rod ushing 8--connexting rod one side with serial numbers, starting with the first rod from -connecting rod llt 10-tonnecting rod nut 11. 12--crank bearing balf shells the radiator, to identify their location in the engine 13-ccnneeting rd eap Some connecting rods have an oil spurt hole in the yoke or at the cap-mating surface to provide cyl inder wall lubrication. The small end of the connecting rod is attached to the piston by a piston pin. In some cases the small end of the rod is clamped to the pin or has a bushing in it to allow the pin and rod oscillation. In other designs the pin is bolted to the rod. Connecting rod are usually drilled to provide h brication to the piston pin and also to spray oil into the bottom of the piston for piston cooling on some de The crankshaft, regarded as the backbone of the engine, serves to change the reciprocating mo- tion of the piston into rotary motion and handles the entire power output. The periodic gas and inertia forces taken by the crankshaft may cause it to suffer wear and bending and tensional strains. The crank- shaft therefore must be adequately strong and wear-resistant. So the crankshaft is either forged from a high ality steel or cast in a high-strong iron. The crankshafts shown in Fig. I-4 is actually mede up of vari

ous parts such as main bearing journals, rod journal, crank am, buring, counter balane ed weight and flywheel end. The crankshaft revolves in bearings located in the engine crankcase but the number of bearings used usually depends on the number of cylinders in the engine. and the design of the engine Mechanically, a crankshaft without spec ial balanced weight would have severe vibration when revolving In order to reduce or eliminate surh vibration, it must be provided with counter balane ed weights that (x- tend radically from the crankshaft centerline in the opposite direction of the crank anmr. Im that way, the fores acting on the crankshaft are balanced and vibration is reduced The md joumals are bored hollow in order to reduce the erankshaft inertia. Drilled diagonally through the: crank arms ure oil holes to supply 04 Fig 1-4 The crankshaft raik haf frmi erd 2-frond main joumal 3 oil peuwsupe helr 4-erumk pin 5-emank wHl il passage plug 8-isil pasage g-eruu k lrft eMlar 10--flywheel 11-Aywheel gear ring 12--flywhed lock plate 13--clutch staft tearin 14--rm ar main brug half shell 15--oil EVe 16--cTankshaft thrust 17-renunl main beiring half shell 18--bearing hall shell 1y--trcnt main bearing half whell The flywheel is a relatively heavy metal wheel, which is firmly attached to the crankshaft. Its func tion is to help the engine to nn smoothly by absorbing some of the energy during the power stroke and re leasing it during the: other strokes Because uf its rotation the flywheel acquires kinetic energy when the flywheel speeds up, it stores additional kinetic energy, and when it slows down it gives back that energy. The anlount of energy that a flywheel will store for a given change in speed depends on its inertia, which, in lum, depends on its mags and its elective diameter. The heavier the flywheel or the larger its diameter the smaller will be the speed changes. For an engine of a given horsepower, single cylinder engines require large flywheels to keep the momentary sptd variations. While multi-cylinder requ flywheels can do it. In practice the automobile engine is usually multi-cylinder engine. Flywheels however, because of its inertia, an cessively heavy flywheel will cause the engine to accelerate and decelerate slowly. For this reason, heav duty or truck engines have large and he avy flywheels, while racing engines or high performance engines ave light flywheel