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单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,*,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,*,Lesson 11 Manual Transmission,Lesson 11 Manual Transmission,1,Cars need transmissions because of the physics of the,gasoline engine,.First,any engine has a redline-a maximum rpm value above which the engine cannot go without exploding.Second,engines have narrow rpm ranges where horsepower and torque are at their maximum.For example,an engine might produce its maximum horsepower at 5,500 rpm.The transmission allows the,gear ratio,between the engine and the drive wheels to change as the car speeds up and slows down.,Cars need transmissions becaus,2,You shift gears so the engine can stay below the redline and near the rpm band of its best performance.Ideally,the transmission would be so flexible in its ratios that the engine could always run at its single,best-performance rpm value,1.That is the idea behind the,continuously variable transmission,(CVT).,You shift gears so the engine,3,A typical layout of transmission system,A typical layout of transmissi,4,A CVT has a nearly infinite range of gear ratios.In the past,CVTs could not compete with four-speed and five-speed transmissions in terms of cost,size and reliability,so you didnt see them in production automobiles.These days,improvements in design have made CVTs more common.The,Toyota Prius,is a,hybrid car,that uses a CVT.,A CVT has a nearly infinite ra,5,The transmission is connected to the engine through the,clutch,.The input shaft of the transmission therefore turns at the same rpm as the engine.,Mercedes-Benz C-class sport coupe,six-speed manual transmission,graphic illustration.,The transmission is connected,6,A five-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm value at the output shaft.Here are some typical gear ratios:,Gear,Ratio,RPM at Transmission Output Shaft with Engine at 3,000 rpm,1st,2.315:1,1,295,2nd,1.568:1,1,913,3rd,1.195:1,2,510,4th,1.000:1,3,000,5th,0.915:1,3,278,A five-speed transmission appl,7,Simple Transmission,To understand the basic idea behind a standard transmission,the diagram below shows a very simple two-speed transmission in neutral:,Simple Transmission,8,neutral position of a two-speed transmission,neutral position of a two-spee,9,Lets look at each of the parts in this diagram to understand how they fit together:,The green shaft comes from the engine through the,clutch,.The green shaft and green gear are connected as a single unit.,Lets look at each of the part,10,The red shaft and gears are called the layshaft.These are also connected as a single piece,so all of the,gears,on the layshaft and the layshaft itself spin as one unit.The green shaft and the red shaft are directly connected through their meshed gears so that if the green shaft is spinning,so is the red shaft.In this way,the layshaft receives its power directly from the,engine,whenever the clutch is engaged.,The red shaft and gears are ca,11,The yellow shaft is a splined shaft that connects directly to the drive shaft through the,differential,to the drive wheels of the car.If the wheels are spinning,the yellow shaft is spinning.,The blue gears ride on bearings,so they spin on the yellow shaft.If the engine is off but the car is coasting,the yellow shaft can turn inside the blue gears while the blue gears and the layshaft are motionless.,The yellow shaft is a splined,12,The purpose of the collar is to connect one of the two blue gears to the yellow drive shaft.The collar is connected,through the splines,directly to the yellow shaft and spins with the yellow shaft.However,the collar can slide left or right along the yellow shaft to engage either of the blue gears.Teeth on the collar,called dog teeth,fit into holes on the sides of the blue gears to engage them.,The purpose of the collar is t,13,Now,lets see what happens when you shift into first gear.,First Gear,The picture below shows how,when shifted into first gear,the collar engages the blue gear on the right:,Now,lets see what happens wh,14,First gear position of a two-speed transmission,First gear position of a two-s,15,In this picture,the green shaft from the engine turns the layshaft,which turns the blue gear on the right.This gear transmits its energy through the collar to drive the yellow drive shaft.Meanwhile,the blue gear on the left is turning,but it is freewheeling on its bearing so it has no effect on the yellow shaft.,In this picture,the green sha,16,When the collar is between the two gears(as shown in the first figure),the transmission is in neutral.Both of the blue gears freewheel on the yellow shaft at the different rates controlled by their ratios to the layshaft.,From this discussion,you can answer several questions:,When the collar is between the,17,When you make a mistake while shifting and hear a horrible grinding sound,you are not hearing the sound of gear teeth mis-meshing.As you can see in these diagrams,all gear teeth are all fully meshed at all times.The grindin
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