Project report on power transmission in automobiles

Project report on power transmission in automobiles, Bài đọc hiểu tiếng anh chuyên ngành ô tô, bao gồm tất cả các phần và hệ thống trên xe ô tô như: gầm (phanh, treo lái, hộp số), động cơ, điện. Project report on power transmission in automobiles, Bài đọc hiểu tiếng anh chuyên ngành ô tô, bao gồm tất cả các phần và hệ thống trên xe ô tô như: gầm (phanh, treo lái, hộp số), động cơ, điện.

CHAPTER 1 INTRODUCTION

Before the steam engine was invented, all of the physicallydemanding jobs like construction, agriculture, shipping, and eventraveling, were done by strong animals or human beingsthemselves The invention of the steam engine prompted theIndustrial Revolution, at which

time human beings started using

automated machines to reduce

human work load and increase

job efficiency In 1705 Thomas

Newcomen invented the first

version of the steam engine,

which is also called atmospheric

engine The Fig in right hand

side (Fig 1) shows Newcomen steam engine From this design,

water (blue) is boiled and vaporized into steam (pink), whichpushes the closed right valve (red) open (green) The steampushes the piston to move up, which causes the pressure insidethe cylinder to decrease Gravity will push the water from theupper tank to open the left valve, and splash the water into thecylinder to cool steam The steam inside the cylinder therefore is

cycle The Newcomen Steam Engine was only used to pumpwater out of mines at that time In 1769, James Watt improved thefunction of the steam engine and made it practical in the realworld, which is why most people still think Watt invented thesteam engine James Watt’s steam engine is designed so thatwater goes into a high temperature boiler, is boiled and vaporized,and turns into high pressure steam This steam pushes the piston,generating a forward and backward motion Because thecombustion room is located outside the engine, the steam engine

is also called the external combustion engine

According to the physics rule of motion, when an object is

in static status it needs a larger force to overcome friction Whenthe object starts moving, the needed driving force becomessmaller and smaller, and the speed becomes faster and faster.Therefore, to move the piston in a steam engine from staticposition, very high pressure must be generated to push the piston.When the piston starts moving, the pressure decreases, because it

is released from the exhaust by the movement of the piston,before it can be compressed into high pressure air At low speed,the engine creates high pressure steam to push the piston, while athigh speed, the steam pressure becomes low That’s why the oldsteam powered locomotives start very slowly, but still can reach avery high speed The steam engine is very efficient at generating

power based on the physics rule of motion; however, it takes awhile before the machine can reach its highest efficiency Anotherdrawback is that the steam engine occupies too much space.Therefore, scientists tried to develop an engine with smaller size,but that can instantly generate the power needed The internalcombustion engine, which has been used for most machineryincluding vehicles, was invented Several kinds of internalcombustion engines have been widely used for vehicles, forexample, in the two-stroke combustion cycle, four-strokecombustion cycle, and rotary engines The first engine to use afour-stroke combustion cycle successfully was built in 1867 by N.

A Otto The design of the internal combustion engine is muchmore complicated than the steam engine, however All internalcombustion engines need to go through the following procedures

to finish the combustion cycle: intake, compression, combustion,and exhaust First, the piston moves downward and at the sametime gasoline is injected into the cylinder through inlet valve.Second, the piston moves upward and compresses the air Third,the compressed air is fired and moves the piston downward again.Finally, the fired air is exhausted through exhaust valve andmoves the piston upward again While fired once every twocycles for a four-stroke cycle internal combustion engine, a two-

The internal combustion design can instantly convert the powergenerated by the explosion of burning fuel into high pressure air

to push the piston Unlike the steam engine, for an internalcombustion engine to move the piston faster and faster, more andmore fuel is needed to generate higher pressure In other words,for an internal combustion engine, high pressure is needed to keepthe piston running at a high speed, while at low speed, only lowpressure is necessary This is just opposite to the function of thesteam engine

Even though it solves the dimension and slow start issues ofthe steam engine, the internal combustion engine generatesanother serious problem When the piston is running at highspeed, the pressure needed is also high, which violates the physicsrule of motion Running an engine at high speed with highpressure is not efficient, and also decreases the engine life Tosolve this problem, the transmission system was invented

To transfer engine power efficiently, the gear ratio between theengine and wheels plays a very important role When we use ascrewdriver, the portion we hold has a larger diameter, while theportion contacting with the screw has smaller diameter Thisdesign makes users use less force to unscrew a screw whileapplying force on a larger diameter portion of the screw driver.Therefore, attaching a smaller gear to the engine side and

connecting it to a larger gear to deliver power to wheels helpsovercome friction when moving a static vehicle

The figure 2 shows that the large gear of the wheels needs less force to drive it However, it also shows that when the enginegear turns one circle, the wheel gear only turns about one half The car won’t run as fast as possible

Consider the following situation from Figure 3: the wheel gearhas a smaller size, which needs more force to move it while thecar is static

It won’t even be possible to move the car if the engine power isnot large enough However, when the engine gear turns 1 cycle,the wheel gear may turn 2, which makes the car run faster

Based on the physics rule of motion, after the object startsmoving, the driving force needed becomes smaller Therefore, if

the car can run on the large gear condition (Figure 2) whenstarting, but change to a small gear (Figure 3) when moving, that

is, applying a large force when starting, but a small force whenmoving, this will makes the power transmission much moreefficient

Kinds of Transmission Systems Used For the Automobile:

The most common transmission systems that have been used forthe automotive industry are:

The first transmission invented was the manual transmissionsystem The driver needs to disengage the clutch to disconnect thepower from the engine first, select the target gear, and engage theclutch again to perform the gear change This will challenge anew driver It always takes time for a new driver to get used tothis skill.

Automatic Transmission:

An automatic transmission uses a fluid-coupling torqueconverter to replace the clutch to avoid engaging/disengagingclutch during gear change A completed gear set, called planetarygears, is used to perform gear ratio change instead of selectinggear manually A driver no longer needs to worry about gearselection during driving It makes driving a car much easier,especially for a disabled or new driver However, the indirect gearcontact of the torque converter causes power loss during powertransmission, and the complicated planetary gear structure makesthe transmission heavy and easily broken

Semi-Automatic Transmission:

A semi-automatic transmission tries to combine the

but avoid their disadvantages However, the complicated design

of the semi-automatic transmission is still under development,and the price is not cheap It is only used for some luxury orsports cars currently

Continuously Variable Transmission (C.V.T.):-

The Continuously Variable Transmission (C.V.T.) is atransmission in which the ratio of the rotational speeds of twoshafts, as the input shaft and output shaft of a vehicle or othermachine, can be varied continuously within a given range,providing an infinite number of possible ratios The othermechanical transmissions described above only allow a fewdifferent gear ratios to be selected, but this type of transmissionessentially has an infinite number of ratios available within afinite range It provides even better fuel economy if the engine isconstantly made run at a single speed This transmission iscapable of a better user experience, without the rise and fall inspeed of an engine, and the jerk felt when changing gears

CHAPTER 2 MANUAL TRANSMISSION

SYSTEM

Manual transmissions also referred as stick shift

transmission or just ‘stick', 'straight drive', or standard transmission because you need to use the transmission stick every

time you change the gears To perform the gear shift, thetransmission system must first be disengaged from the engine.After the target gear is selected, the transmission and engine areengaged with each other again to perform the power transmission.Manual transmissions are characterized by gear ratios that areselectable by locking selected gear pairs to the output shaft insidethe transmission

The main components of manual transmission are:

Clutch is a device which is used in the transmission system

of automobile to engage and disengage the engine to thetransmission or gear box It is located between the transmissionand the engine When the clutch is engaged, the power flows fromthe engine to the rear wheels in a rear-wheel-drive transmissionand the vehicle moves When the clutch is disengaged, the power

is not transmitted from the engine to the rear wheels and vehiclestops even if engine is running

It works on the principle of friction When two frictionsurfaces are brought in contact with each other and they areunited due to the friction between them If one is revolved theother will also revolve The friction depends upon the surface areacontact The friction surfaces are so designed that the drivenmember initially slips on driving member when initially pressure

is applied As pressure increases the driven member is broughtgradually to speed the driving member

The three main parts of clutch are:

The driving member consists of a disc or plate called clutchplate It is free to slide length wise on the splines of the clutchshaft It carries friction materials on both of its surfaces when it isgripped between the flywheel and the pressure plate; it rotates theclutch shaft through splines.

The operating members consists of a foot pedal, linkage,release or throw-out bearing, release levers and springs necessary

to ensure the proper operation of the clutch

Now the driving member in an automobile is flywheelmounted on crank shaft, the driven member is the pressure platemounted on transmission or gear box input shaft Frictionsurfaces or clutch plates is placed between two members

Types Of Friction Materials:

The friction materials of the clutch plate are generally of 3 types:

1 Mill Board Type

Woven types facing materials are made by impregnating acloth with certain binders or by weaving threads of copper orbrass wires covered with long fiber asbestos and cotton Thewoven sheets treated with binding solution are baked and rolled

TABLE: COEFFICIENTS OF RICTION FOR CLUTCH FACING MATERIALS

Sl No Material Coeffieicent Of

Material(µ)

4 Asbestos Base Materials 0.35-0.4

Properties Of Good Clutching:

1 Good Wearing Properties

2 High Resistance to heat

3 High coefficient of friction

4 Good Binders in it

Operation Of Clutch:

When the clutch pedal is pressed through pedal movement,the clutch release bearing presses on the clutch release lever platewhich being connected to clutch release levers, forces these leversforward This causes the pressure plate to compress pressuresprings, thus allowing it to move away from the clutch drivenplate This action releases the pressure on the driven plate andflywheel, the flywheel is now free to turn independently, withoutturning the transmission

When the clutch pedal is released, reverse action takes placei.e the driven plate is again forced against the flywheel by thepressure plate- because of the force exerted by pressure springs.The pressure plate will keep on pressing the facings of drivenplate until friction created becomes equal to the resistance of thevehicle Any further increase in pressure will cause the clutch

plate and the transmission shaft to turn along with flywheel, thusachieving vehicle movement.

Single Clutch Plate:

It is the most common type of clutch plate used in motorvehicles Basically it consists of only one clutch plate, mounted

on the splines of the clutch plate The flywheel is mounted onengine crankshaft and rotates with it The pressure plate is bolted

clutch shaft when the clutch pedal is operated When the clutch isengaged the clutch plate is gripped between the flywheel andpressure plate The friction linings are on both the sides of theclutch plate Due to the friction between the flywheel, clutch plateand the pressure plate the clutch plate revolves the flywheel Asthe clutch plate revolves the clutch shaft also revolves Clutchshaft is connected to the transmission gear box Thus the enginepower is transmitted to the crankshaft and then to the clutch shaft.

When the clutch pedal is pressed, the pressure plate movesback against the force of the springs, and the clutch plate becomesfree between the flywheel and the pressure plate Thus theflywheel remains rotating as long as the engine is running and theclutch shaft speed reduces slowly and finally it stops rotating Assoon as the clutch pedal is pressed, the clutch is said to beengaged, otherwise it remains engaged due to the spring forces

.

The plates are alternately fitted to engine and gear box shaft Theyare firmly pressed by strong coil springs and assembled in adrum Each of the alternate plate slides on the grooves on theflywheel and the other slides on splines on the pressure plate.Thus, each alternate plate has inner and outer splines The multi-plate clutch works in the same way as a single plate clutch byoperating the clutch pedal The multi-plate clutches are used inheavy commercial vehicles, racing cars and motor cycles fortransmitting high torque The multi-plate clutch may be dry orwet When the clutch is operated in an oil bath, it is called a wetclutch When the clutch is operated dry it is called dry clutch Thewet clutch is used in conjunction with or part of the automatictransmission.

Cone Clutch:

Cone clutch consists of friction surfaces in the form of cone.The engine shaft consists of female cone The male cone ismounted on the splined clutch shaft It has friction surfaces on theconical portion The male cone can slide on the clutch shaft Henthe clutch is engaged the friction surfaces of the male cone are incontact with that of the female cone due to force of the spring.When the clutch pedal is pressed, the male cone slides against thespring force and the clutch is disengaged

The only advantage of the cone clutch is that the normalforce acting on the friction surfaces is greater than the axial force,

as compare to the single plate clutch in which the normal forceacting on the friction surfaces is equal to the axial force Thedisadvantage in cone clutch is that if the angle of the cone is madesmaller than 200 the male cone tends to bind in the female coneand it becomes difficult to disengage the clutch Cone clutches aregenerally now only used in low peripheral speed applications

although they were once common in automobiles and othercombustion engine transmissions They are usually now confined

to very specialist transmissions in racing, rallying, or in extremeoff-road vehicles, although they are common in power boats.Small cone clutches are used in synchronizer mechanisms

in manual transmissions

Dog & Spline Clutch:

This type of clutch is used to lock two shafts together or tolock a gear to shaft It consists of a sleeve having two sets ofinternal splines It slides on a splined shaft with smallest diametersplines The bigger diameter splines match with the external dogclutch teeth on driving shaft When the sleeve is made to slide onthe splined shaft, its teeth match with the dog clutch teeth of thedriving shaft Thus the sleeve turns the splined shaft with thedriving shaft The clutch is said to be engaged To disengage theclutch, the sleeve is moved back on the splined shaft to have nocontact with the driving shaft This type of clutch has no tendency

to slip The driven shaft revolves exactly at the same speed of the

driving shaft, as soon as the clutch is engaged This is also known

Centrifugal Clutch:

The centrifugal clutch uses centrifugal forces, instead ofspring force for keeping it in engaged position Also, it does notrequire clutch pedal for operating the clutch The clutch isoperated automatically depending on engine speed The vehiclecan be stopped in gear without stalling the engine Similarly thegear can be started in any gear by pressing the accelerator pedal

A centrifugal clutch works through centrifugal force Theinput of the clutch is connected to the engine crankshaft while theoutput drives gear box shaft, chain, or belt Asengine R.P.M. increases, weighted arms in the clutch swingoutward and force the clutch to engage The most common typeshave friction pads or shoes radially mounted that engage theinside of the rim of housing On the center shaft there are anassorted amount of extension springs, which connect to a clutchshoe When the center shaft spins fast enough, the springs extendcausing the clutch shoes to engage the friction face It can becompared to a drum brake in reverse The weighted arms forcethese disks together and engage the clutch When the enginereaches a certain RPM, the clutch activates, working almost like

a continuously variable transmission As the load increases theR.P.M drops thereby disengaging the clutch and letting the rpmrise again and reengaging the clutch If tuned properly, the clutchwill tend to keep the engine at or near the torque peak of theengine These results in a fair bit of waste heat, but over a broadrange of speeds it is much more useful then a direct drive in manyapplications Weaker spring/heavier shoes will cause the clutch toengage at a lower R.P.M. while a stronger spring/lighter shoeswill cause the clutch to engage at a higher R.P.M. 

Semi-centrifugal

Clutch:-A semi centrifugal clutch is used to transmit power from highpowered engines and racing car engines where clutchdisengagements requires appreciable and tiresome drivers effort.The transmission of power in such clutches is partly by clutchsprings and rest by centrifugal action of an extra weight provided

in system The clutch springs serve to transmit the torque up tonormal speeds, while the centrifugal force assists at speeds higherthan normal

Besides clutch, pressure plate and splines shaft it mainly consistsof:

 Compression spring (3 numbers)

 Weighted levers (3 numbers)

At normal speeds when the power transmission is low the springkeeps the clutch engaged, the weighted levers do not have anypressure on the pressure plate At high speed, when the powertransmission is high the weights fly off and levers exert pressure

on the plate which keeps the clutch firmly engaged Thus instead

of having more stiff springs for keeping the clutch engaged firmly

at high speeds, they are less stiff, so that the driver may not getany strain in operating the clutch

When the speed decreases, the weights fall and the levers do notexert any pressure on the pressure plate Only the spring pressure

Electromagnetic Clutch:

An electromagnetic clutch is a clutch (a mechanism for

transmitting rotation) that is

engaged and disengaged by

an electromagnetic actuator In

this type of clutch, the flywheel

consists of winding The current

is supplied to the winding from

battery or dynamo When the current passes through the winding

it produces an electromagnetic field which attracts the pressureplate, thereby engaging the clutch When supply is cutoff, theclutch is disengaged The gear lever consists of a clutch releaseswitch When then the driver holds the gear lever to change thegear the witch is operated cutting off the current to the windingwhich causes the clutch disengaged At low speeds when thedynamo output is low, the clutch is not firmly engaged Therefore

three springs are also provided on the pressure plate which helpsthe clutch engaged firmly at low speed also Cycling is achieved

by turning the voltage/current to the electromagnet on and off.Slippage normally occurs only during acceleration When theclutch is fully engaged, there is no relative slip, assuming theclutch is sized properly, and thus torque transfer is 100%efficient

The electromagnetic clutch is most suitable for remoteoperation since no linkages are required to control itsengagement It has fast, smooth operation However, becauseenergy dissipates as heat in the electromagnetic actuator everytime the clutch is engaged, there is a risk of overheating.Consequently the maximum operating temperature of the clutch islimited by the temperature rating of the insulation of theelectromagnet This is a major limitation Another disadvantage ishigher initial cost

Gear Box

Principle Of Gearing

Consider a simple 4-gear train It consists of a driving gear A oninput shaft and a driven gear D on the output shaft In between the

two gears there are two intermediate gears B, C Each of thesegears are mounted on separate shaft.

We notice that:

Gear A drives Gear B

Gear B drives Gear C

Gear C drives Gear D

Therefore, the over all speed ratios are:

Types of Gear Boxes:

The following types of gear box are used in automobiles:

It is the simplest gear box The following figure shows 4-speedgear box in neutral position 4 gears are connected to the layshaft/counter shaft A reverse idler gear is mounted on another

shaft and always remains connected to the reverse gear ofcountershaft.This “H” shift pattern enables the driver to selectfour different gear ratios and a reverse gear

Gears in Neutral:

When the engine is running and clutch is engaged the clutch shaftgear drives the countershaft gear The countershaft rotatesopposite in direction of the clutch shaft In neutral position only

the clutch shaft gear is connected to the countershaft gear Othergears are free and hence the transmission main shaft is notturning The vehicle is stationary.

First or low shaft gear:

By operating the gear shift lever the larger gear on the main shaft

is moved along the shaft to mesh with the first gear of the countershaft The main shaft turns in the same direction as that of theclutch shaft Since the smaller countershaft is engaged with largershaft gear a gear reduction of approximately 4:1 is obtained i.e.the clutch shaft turns 4 times for each revolution of main shaft

Second speed gear:

By operating the gear shift lever the third gear on the main shaft

is moved along the shaft to mesh with the third gear of thecounter shaft The main shaft turns in same direction as clutchshaft A gear reduction of approximately 3:1is obtained

Third speed gear:

By operating the gear shift lever, the second gear of the mainshaft and countershaft are demeshed and then the third gear of themain shaft are forced axially against the clutch shaft gear.External Teeth on the clutch shaft gear mesh with the internalteeth in the third and top gear The main shaft turns in samedirection as clutch shaft A gear reduction of approximately 2:1isobtained i.e the clutch shaft turns 2 times for each revolution ofmain shaft

Fourth speed gear/ Top or High-Speed Gear:

By operating the gear shaft lever the third gears of the main andcountershaft is demeshed and the gears present on the main shaftalong with the shaft is forced axially against the clutch shaft gear.External teeth present on the main shaft engage with the internalteeth present on the main shaft The main shaft turns along withthe clutch shaft and a gear ratio of approximately 1:1 is obtained

shaft This reverses the rotation of the wheels so that the wheelbacks.

Constant Mesh:

In this type of gear box, all gears of the main shaft are in constantmesh with the corresponding gears of the countershaft (Layshaft) Two dog clutches are provided on the main shaft- onebetween the clutch gear and the second gear, and the otherbetween the first gear and reverse gear The main shaft is splined

and all the gears are free on it Dog clutch can slide on the shaftand rotates with it All the gears on the countershaft are rigidlyfixed with it

When the left hand dog clutch is made to slide to the left bymeans of the gear shift lever, it meshes with the clutch gear andthe top speed gear is obtained When the left hand dog clutch

meshes with the second gear, the second speed gear is obtained.Similarly by sliding the right hand dog clutch to the left and right,the first speed gear and reverse gear are obtained respectively Inthis gear box because all the gears are in constant mesh they aresafe from being damaged and an unpleasant grinding sound doesnot occur while engaging and disengaging them.

Syncromesh Gear Box:

In sliding Mesh Gear box the two meshing gears need to berevolve at equal peripheral speeds to achieve a jerk lessengagement and it is true for constant mesh gear box in which theperipheral speeds of sliding dog and the corresponding gear onthe output shaft must be equal The peripheral speed is given by

Where d1 and N1 are pitch circle diameter and r.p.m of gear and

d2 andN2 diameter and r.p.m of attached dog respectively Now

N1 ≠ N2 since d1 ≠ d2 Thus there is a difference in gear and dogwhich necessitates double declutching The driver has todisengage the clutch twice in quick succession therefore it is

referred as double declutching There are two steps involved in

this process:

1 The clutch is disengaged i.e first declutching and the gearsystem is placed in its neutral position Now the clutch isreengaged and acceleration pedal is pressed to adjust theengine speed according to driver’s judgment.

2 The clutch is disengaged(i.e second declutching) again theappropriate gear is engaged and then the clutch is reengaged

It is that gear box in which sliding synchronizing units areprovided in place of sliding dog clutches as in case of constantmesh gear box With the help of synchronizing unit, the speed ofboth the driving and driven shafts is synchronized before they areclutched together through train of gears The arrangement ofpower flow for the various gears remains the same as in constantmesh gear box The synchronizer is made of frictional materials.When the collar tries to mesh with the gear, the synchronizer willtouch the gear first and use friction force to drive the gear to spin

at the same speed as the collar This will ensure that the collar ismeshed into the gear very smoothly without grinding

Synchromesh gear devices work on the principle that two gears to

be engaged are first bought into frictional contact which equalizestheir speed after which they are engaged readily and smoothly.The following types of devices are mostly used in vehicles:

i Pin Type