1、 Part Practical ReadingPart Practical ReadingPart Further DevelopmentPart Further Development Part Reading MaterialPart Reading Material Part Just for LaughPart Just for LaughPart Practical ReadingPart Practical ReadingPassage APassage AValve TrainValve Train The valve train consists of the valves a
2、nd a The valve train consists of the valves and a mechanism that opens and closes them.The opening and mechanism that opens and closes them.The opening and closing system is called a camshaft.The camshaft has closing system is called a camshaft.The camshaft has lobes on it that move the valves up an
3、d down.Most lobes on it that move the valves up and down.Most modern engines have what are called overhead camshaft.modern engines have what are called overhead camshaft.This means that the camshaft is located above the This means that the camshaft is located above the valves.The cams on the shaft a
4、ctivate the valves valves.The cams on the shaft activate the valves directly or through a very short linkage.Older engines directly or through a very short linkage.Older engines used a camshaft located in the sump near the crankshaft.used a camshaft located in the sump near the crankshaft.Rods linke
5、d the cam below to valve lifters above Rods linked the cam below to valve lifters above the valves.This approach has more moving parts the valves.This approach has more moving parts and also causes more lag between the cams and also causes more lag between the cams activation of the valve and the va
6、lves activation of the valve and the valves subsequent motion.A timing belt or timing chain subsequent motion.A timing belt or timing chain links the crankshaft to the camshaft so that the links the crankshaft to the camshaft so that the valves are in sync with the pistons.The camshaft valves are in
7、 sync with the pistons.The camshaft is geared to turn at one-half the rate of the is geared to turn at one-half the rate of the crankshaft.Many high-performance engines have crankshaft.Many high-performance engines have four valves per cylinder(two for intakefour valves per cylinder(two for intake，t
8、wo two for exhaust)for exhaust)，and this arrangement requires two and this arrangement requires two camshafts per bank of cylinderscamshafts per bank of cylinders，hence the hence the phrase“dual overhead cams”.phrase“dual overhead cams”.（See fig.3-1See fig.3-1）Fig.3-1 Valve Train Lobes Lobes The key
9、 parts of any camshaft are the The key parts of any camshaft are the lobes.As the camshaft spinslobes.As the camshaft spins，the lobes open and the lobes open and close the intake and exhaust valves in time with close the intake and exhaust valves in time with the motion of the piston.It turns out th
10、at there the motion of the piston.It turns out that there is a direct relationship between the shape of the is a direct relationship between the shape of the cam lobes and the way the engine performs in cam lobes and the way the engine performs in different speed ranges.When the intake valve differe
11、nt speed ranges.When the intake valve opens and the piston starts its intake strokeopens and the piston starts its intake stroke，the air-fuel mixture in the intake runner starts the air-fuel mixture in the intake runner starts to accelerate into the cylinder.By the time the to accelerate into the cy
12、linder.By the time the piston reaches the bottom of its intake strokepiston reaches the bottom of its intake stroke，the air-fuel is moving at a pretty high speed.If the air-fuel is moving at a pretty high speed.If we were to slam the intake valve shutwe were to slam the intake valve shut，all of all
13、of that air-fuel would come to a stop and not enter that air-fuel would come to a stop and not enter the cylinder.By leaving the intake valve open a the cylinder.By leaving the intake valve open a little longerlittle longer，the momentum of the fast the momentum of the fastmoving moving air-fuel cont
14、inues to force air-fuel into the air-fuel continues to force air-fuel into the cylinder as the piston starts its compression cylinder as the piston starts its compression stroke.So the faster the engine goesstroke.So the faster the engine goes，the faster the faster the air-fuel movesthe air-fuel mov
15、es，and the longer we want the and the longer we want the intake valve to stay open.We also want the valve to intake valve to stay open.We also want the valve to open wider at higher speed open wider at higher speed this parameter this parameter，called valve liftcalled valve lift，is governed by the c
16、am lobe is governed by the cam lobe profile.Any given camshaft will be perfect only at profile.Any given camshaft will be perfect only at one engine speed.At every other engine speedone engine speed.At every other engine speed，the the engine wont perform to its full potential.A fixed engine wont per
17、form to its full potential.A fixed camshaft iscamshaft is，therefore therefore，always a compromise.This always a compromise.This is why carmakers have developed schemes to vary the is why carmakers have developed schemes to vary the cam profile as the engine speed changes.cam profile as the engine sp
18、eed changes.Camshaft arrangements Camshaft arrangements There are several different arrangements of There are several different arrangements of camshafts on engines.The most common type camshafts on engines.The most common type contains the Single OverHead Cam(SOHC)enginecontains the Single OverHead
19、 Cam(SOHC)engine，the Double OverHead Cam(DOHC)enginethe Double OverHead Cam(DOHC)engine，the the pushrod engine.pushrod engine.Single overhead cams Single overhead cams This arrangement denotes an engine with one cam This arrangement denotes an engine with one cam per head.So if it is an in-line 4-cy
20、linder or in-per head.So if it is an in-line 4-cylinder or in-line 6-cylinder engineline 6-cylinder engine，it will have one cam it will have one cam；if it if it is a V-6 or V-8is a V-6 or V-8，it will have two cams(one for each it will have two cams(one for each head).The cam actuates rocker arms tha
21、t press down on head).The cam actuates rocker arms that press down on the valvesthe valves，opening them.Springs return the valves opening them.Springs return the valves to their closed position.These springs have to be to their closed position.These springs have to be very strong because at high eng
22、ine speedvery strong because at high engine speed，the valves the valves are pushed down very quicklyare pushed down very quickly，and it is the springs and it is the springs that keep the valves in contact with the rocker arms.that keep the valves in contact with the rocker arms.If the springs were n
23、ot strong enoughIf the springs were not strong enough，the valves the valves might come away from the rocker arms and snap back.might come away from the rocker arms and snap back.This is an undesirable situation that would result in This is an undesirable situation that would result in extra wear on
24、the cams and rocker arms.See Fig.3-2.extra wear on the cams and rocker arms.See Fig.3-2.Fig.3-2 Overhead Cam Double overhead cam Double overhead cam A double overhead cam engine has two cams A double overhead cam engine has two cams per head.So in-line engines have two camsper head.So in-line engine
25、s have two cams，and and V engines have four.UsuallyV engines have four.Usually，double overhead double overhead cams are used on engines with four or more valves cams are used on engines with four or more valves per cylinder per cylinder a single camshaft simply cannot a single camshaft simply cannot
26、 fit enough cam lobes to actuate all of those fit enough cam lobes to actuate all of those valves.valves.The main reason to use double overhead cams The main reason to use double overhead cams is to allow for more intake and exhaust valves.is to allow for more intake and exhaust valves.More valves m
27、ean that intake and exhaust gases More valves mean that intake and exhaust gases can flow more freely because there are more can flow more freely because there are more openings for them to flow through.This increases openings for them to flow through.This increases the power of the engine.On single
28、 and double the power of the engine.On single and double overhead cam enginesoverhead cam engines，the cams are driven by the the cams are driven by the crankshaftcrankshaft，via either a belt or chain called via either a belt or chain called the timing belt or timing chain.These belts and the timing
29、belt or timing chain.These belts and chains need to be replaced or adjusted at regular chains need to be replaced or adjusted at regular intervals.If a timing belt breaksintervals.If a timing belt breaks，the cam will the cam will stop spinning and the piston could hit the open stop spinning and the
30、piston could hit the open valves.valves.Pushrod engines Pushrod engines Like SOHC and DOHC engines Like SOHC and DOHC engines，the valves in a the valves in a pushrod engine are located in the headpushrod engine are located in the head，above above the cylinder.The key difference is that the the cylin
31、der.The key difference is that the camshaft on a pushrod engine is inside the engine camshaft on a pushrod engine is inside the engine blockblock，rather than in the head.The cam actuates rather than in the head.The cam actuates long rods that go up through the block and into long rods that go up thr
32、ough the block and into the head to move the rockers.These long rods add the head to move the rockers.These long rods add mass to the systemmass to the system，which increases the load on which increases the load on the valve springs.This can limit the speed of the valve springs.This can limit the sp
33、eed of pushrod enginespushrod engines；the overhead camshaftthe overhead camshaft，which which eliminates the pushrod from the systemeliminates the pushrod from the system，is one is one of the engine technologies that made higher of the engine technologies that made higher engine speed possible.(See f
34、ig.3-3)engine speed possible.(See fig.3-3)Fig.3-3 Valve Safety The camshaft in a pushrod engine is often The camshaft in a pushrod engine is often driven by gears or a short chain.Gear-drives are driven by gears or a short chain.Gear-drives are generally less prone to breakage than belt drivesgenera
35、lly less prone to breakage than belt drives，which are often found in overhead cam engines.which are often found in overhead cam engines.Variable valve timing Variable valve timing There are a couple of novel ways by which There are a couple of novel ways by which carmakers vary the valve timing.One
36、system used carmakers vary the valve timing.One system used on some Honda engines is called VTEC(See fig.3-on some Honda engines is called VTEC(See fig.3-4).VTEC(Variable Valve Timing and Lift 4).VTEC(Variable Valve Timing and Lift Electronic Control)is an electronic and Electronic Control)is an ele
37、ctronic and mechanical system in some Honda engines that mechanical system in some Honda engines that allows the engine to have multiple camshafts.allows the engine to have multiple camshafts.VTEC engines have an extra intake cam with its VTEC engines have an extra intake cam with its own rockerown
38、rocker，which follows this cam.The profile which follows this cam.The profile on this cam keeps the intake valve open longer on this cam keeps the intake valve open longer than the other cam profile.At low engine speedsthan the other cam profile.At low engine speeds，this rocker is not connected to an
39、y valves.At this rocker is not connected to any valves.At high engine speedshigh engine speeds，a piston locks the extra a piston locks the extra rocker to the two rockers that control the two rocker to the two rockers that control the two intake valves.intake valves.Fig.3-4 Variable Valve Timing Sev
40、eral engine manufacturers are experimenting Several engine manufacturers are experimenting with systems that would allow infinite variability with systems that would allow infinite variability in valve timing.For examplein valve timing.For example，imagine that each imagine that each valve had a sole
41、noid on it that could open and valve had a solenoid on it that could open and close the valve using computer control rather than close the valve using computer control rather than relying on a camshaft.With this type of systemrelying on a camshaft.With this type of system，you would get maximum engin
42、e performance at every you would get maximum engine performance at every RPM.Something will be looked forward to in the RPM.Something will be looked forward to in the future.future.Phrases and ExpressionsPhrases and Expressionsvalve trainvalve train气阀结构，气门气阀结构，气门overhead camshaft overhead camshaft 顶
43、置凸轮轴顶置凸轮轴valve lifter valve lifter 起阀器起阀器timing belt timing belt 正时皮带正时皮带timing chain timing chain 正时链条正时链条dual overhead cam dual overhead cam 双顶置凸轮轴双顶置凸轮轴intake stroke intake stroke 进气冲程进气冲程intake valve intake valve 进气阀进气阀compression stroke compression stroke 压缩冲程压缩冲程valve lift valve lift 气门升程气门升程r
44、ocker armrocker arm摇臂摇臂double overhead cam enginedouble overhead cam engine双顶置凸轮轴发动机双顶置凸轮轴发动机open valve open valve 开阀开阀pushrod engine pushrod engine 推杆发动机推杆发动机engine block engine block 发动机组，汽缸体发动机组，汽缸体Honda engine Honda engine 本田发动机本田发动机variable valve timing and lift electronic control variable valv
45、e timing and lift electronic control 可变气门正时和升程电子控制可变气门正时和升程电子控制RPMRPM每分钟转数每分钟转数snap back snap back 迅速跳回，很快恢复迅速跳回，很快恢复cylinder head cylinder head 汽缸盖汽缸盖Notes to the PassageNotes to the Passage1.A timing belt or timing chain links the crankshaft 1.A timing belt or timing chain links the crankshaft to
46、the camshaft so that the valves are in sync to the camshaft so that the valves are in sync with the pistons.with the pistons.曲轴与凸轮轴通过正时皮带或正时链条连接起来，以便曲轴与凸轮轴通过正时皮带或正时链条连接起来，以便使气门与活塞同步。使气门与活塞同步。in sync with in sync with与与同步。同步。2.The camshaft is geared to turn at one-half the 2.The camshaft is geared to
47、 turn at one-half the rate of the crankshaft.rate of the crankshaft.凸轮轴转动应与曲轴相配合，其速度是曲轴的一半。凸轮轴转动应与曲轴相配合，其速度是曲轴的一半。be geared to be geared to 使适合；与使适合；与配合。例如：配合。例如：Education should be geared to childrens needs.Education should be geared to childrens needs.教育应适合学生们的需要。教育应适合学生们的需要。3.As the camshaft spin
48、s3.As the camshaft spins，the lobes open and close the lobes open and close the intake and exhaust valves in time with the the intake and exhaust valves in time with the motion of the piston.motion of the piston.当凸轮轴快速旋转时，凸角可随着活塞的运动及时地开当凸轮轴快速旋转时，凸角可随着活塞的运动及时地开启和关闭进、排气门。启和关闭进、排气门。4.It turns out that t
49、here is a direct relationship 4.It turns out that there is a direct relationship between the shape of the cam lobes and the way between the shape of the cam lobes and the way the engine performs in different speed ranges.the engine performs in different speed ranges.这证明了凸轮凸角的形状与发动机在不同速度范围内的这证明了凸轮凸角的
50、形状与发动机在不同速度范围内的工作情况有直接的关系。工作情况有直接的关系。turn out turn out证明（是），结果（是），常与证明（是），结果（是），常与 to to 或或 that that 连用。例如：连用。例如：Things turned out to be exactly as the professor Things turned out to be exactly as the professor had foreseen.had foreseen.事情正如教授所预见的那样。事情正如教授所预见的那样。5.These springs have to be very stro
