automotive transmissions_efficiently transferring power from engine to wheels

7/30/2019 Automotive Transmissions_Efficiently Transferring Power From Engine to Wheels

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Automotive Transmissions: Efficiently Transferring Power fromEngine to Wheels(Released January 2008)

byChao-Hsu Yao

Review Key Citations Media Visuals Glossary Editor

Continuously-VariableTransmission (CVT)

Printable Version (PDF)

Tell a FriendThe continuously-

variable transmissionis also an automatic

transmission system,

which changes the

diameters of input

shaft and output shaft

directly, instead of

going through several

gears to performgear

ratiochange. This

design can generate

an infinite number of

possible gear ratios.

Unlike the complicated planetary automatic transmission system, a CVT only has three majorparts; a drive pulley connected to the input shaft, a driven pulley connected to the output

shaft, and a belt. Figure 21 shows the structure of the most common pulley based CVT

systems. If two cones of the pulley fall apart, the diameter of the pulley becomes small; while

they are close, the diameter of the pulley is large. Because the length of the driving belt is

fixed, when the diameter of the drive pulley becomes small, the diameter of the driven pulley

must become large by closing two cones of the pulley together, and vice versa. Based on this

infinite number of gear ratios design, it is said that CVT is the most efficient transmission

system in the world.

Contents

Introduction Different Kinds of Transmission

Systems Used for the Automobileo Manual Transmissiono Automatic Transmissiono Continuously-Variable Transmission (CVT)o Semi-automatic Transmission

Comparison of Transmission Systems The Future Development of

Automotive Transmission Systems
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Another popularCVTis Nissan Extroid

toroidal CVT. It uses two discs instead of

pulleys and two rollers (wheels) instead of

a belt, compared with a pulley based CVT.

Figure 22 shows that the two rollers spin

around the horizontal axis, and also tilt in

or out around the vertical axis. When thetwo rollers tilt to the upper disc in Figure

22, they touch the upper disc with a larger

diameter, while touching the bottom disc

with a smaller diameter. This works

according to the same idea as pulley based

CVT, by generating an infinite number of

gear ratios.

Even though the pulley based CVT is

currently the most common CVT in the

world, the belt is under extremely large

stress between pulleys when they are

spinning. Therefore, in the past the CVTcould only be used for some low torque

engines like lawn mowers or snow blowers. It was also used for some small engine vehicles

under 1,500 cc. With the progress of material technology, Nissan has been able to put a pulley

based CVT into a 3,500 V6 Altima, which dramatically improves the fuel efficiency for a large

engine vehicle.

Figure 22: Nissan Extroid toroidal CVT.

Because CVT is basically designed to perform automatic transmission without manually

engaging/disengaging the clutch, it generates another issue: How to prevent the engine from

stalling when the car is not moving? CVT has several clutch designs to achieve this goal.

Figure 23 is a CVT clutch assembly normally used for a scooter. The CVT clutch for an

automobile is much more complicated but the idea is similar. The CVT clutch has two parts, a

clutch disc with frictional materials attached on the side, and a clutch cover (drum). The cover

encircles the disc, with the frictional materials barely touching the inside wall. The cover is

connected to the drive pulley, while the disc is connected to the engine. When the engine is in

idle, the clutch disc spins slowly, and the frictional pad doesn't contact with the inside wall of

the cover. At high speed, the centrifugal force causes the frictional pad to move away from its

spinning axis to touch the inside wall of the cover, and drives the cover to spin. That's why for

most of CVT vehicles, when the gear is shifted to "D", the car is still not moving when the

engine is in idle, which is different from an automatic transmission with a torque converter.

Figure 21: Pulley Based CVT.
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Figure 23: CVT Clutch Assembly for 152QMI & 157QMJ Engine.

Go ToSemi-automatic Transmission

2008, ProQuest LLC. All rights reserved.

List of Visuals

Figure 21: Pulley Based CVTImage is from "How CVT Works?" HowStuffWorks Inc.

http://auto.howstuffworks.com/cvt2.htm

Figure 22: Nissan Extroid toroidal CVTImage is from "How CVT Works?" HowStuffWorks Inc.

http://auto.howstuffworks.com/cvt2.htm

Figure 23 CVT Clutch Assembly for 152QMI & 157QMJ EngineImage is from "CVT Transmission Parts for Qingqi, Taishan & Geely Scooters,"

ScootUSA.comhttp://www.scootusa.com/CVT-transmission.htm



byChao-Hsu Yao


IntroductionPrintable Version (PDF)

Tell a FriendContents


Systems Used for the Automobile
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>Arts & Humanities

>Natural Sciences

>Social Sciences

>Technology

>List All

Before thesteam enginewas

invented, all of the physically

demanding jobs like construction,

agriculture, shipping, and even

traveling, were done by strong

animals or human beings

themselves. The invention of thesteam engine prompted the

Industrial 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 calledatmospheric engine[2]. Figure 1 shows the

animation of how the Newcomen steam engine works. From this design, water (blue) is boiled

and vaporized into steam (pink), which pushes the closed right valve (red) open (green). The

steam pushes the piston to move up, which causes the pressure inside the cylinder to

decrease. Gravity will push the water from the upper tank to open the left valve, and splash

the water into the cylinder to cool steam. The steam inside the cylinder therefore is

condensed, which turns the cylinder vacuum and sucks back the piston. The descending piston

shuts two valves and finishes one cycle.

The Newcomen Steam Engine was only used

to pump water out of mines at that time. In

1769, James Watt improved the function of

the steam engine and made it practical in the

real world [1], which is why most people still

think Watt invented the steam engine.

James Watt's steam engine is designed so

that water goes into a high temperature

boiler, is boiled and vaporized, and turns intohigh pressure steam. This steam pushes the

piston, generating a forward and backward

motion (seehttp://static.howstuffworks.

com/flash/steam-engine.swffor an animated

picture) [3]. Because the combustion room is

located outside the engine, the steam engine

is also called theexternal combustion engine.

According to the physics rule of motion,

when an object is in static status it needs a

larger force to overcome friction. When the

object starts moving, the needed driving

force becomes smaller and smaller, and thespeed becomes faster and faster. Therefore,

to move the piston in a steam engine from

static position, 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 at high speed, the steam

pressure becomes low. That's why the old steam powered locomotives start very slowly, but

still can reach a very 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. Another drawbackis 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. Theinternal

o Manual Transmissiono Automatic Transmissiono Continuously-Variable

Transmission (CVT)o Semi-automatic Transmission

Comparison of Transmission Systems

The Future Development ofAutomotive Transmission Systems

To view the animations in this articledownload the Adobe Shockwave Player athttp://www.adobe.com/shockwave/download/

Figure 1: Newcomen Steam Engine.
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combustion engine, which has been used for most machinery including vehicles, was invented.

Several kinds of internal combustion engines have been widely used for vehicles, for example,

in the two-stroke combustion cycle, four-stroke combustion cycle, and rotary engines. The first

engine to use a four-stroke combustion cycle successfully was built in 1867 by N. A. Otto [9].

The design of the internal combustion engine is much more complicated than the steam

engine, however. All internal combustion engines need to go through the following procedures

to finish the combustion cycle: intake, compression, combustion, and exhaust. The shock wavefile http://static.howstuffworks.com/flash/engine.swf[11] shows how the four-stroke internal

combustion engine works. First, gasoline comes from 'C' and moves the piston downward.

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 'L' and

moves the piston upward again. While fired once every two cycles for a four-stroke cycle

internal combustion engine, a two-stroke combustion cycle internal engine is fired once per

cycle, which can be seen on the shock wave file http://static.howstuffworks.com/flash/two-

stroke.swf[10]. The internal combustion design can instantly convert the power generated by

the explosion of burning fuel into high pressure air to push the piston. Unlike thesteam

engine, for an internal combustion engine to move the piston faster and faster, more and more

fuel is needed to generate higher pressure. In other words, for an internal combustion engine,

high pressure is needed to keep the piston running at a high speed, while at low speed, only

low pressure is necessary. This is just opposite to the function of the steam engine.

Even though it solves the dimension and slow start issues of the steam engine, the internal

combustion engine generates another serious problem. When the piston is running at high

speed, the pressure needed is also high, which violates the physics rule of motion. Running an

engine at high speed with high pressure is not efficient, and also decreases the engine life. To

solve this problem, the transmission system was invented.

To transfer engine power efficiently, thegear ratiobetween the engine and wheels plays a

very important role. When we use a screwdriver, the portion we hold has a larger diameter,

while the portion contacting with the screw has smaller diameter. This design makes users use

less force to unscrew a screw while applying 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 helps overcome friction when moving a static vehicle. Figure 2shows that the large gear of the wheels needs less force to drive it. However, it also shows

that when the engine gear turns one circle, the wheel gear only turns about one half. The car

won't run as fast as possible.

Figure 2

Consider the following situation from Figure 3: the wheel gear has a smaller size, which needs

more force to move it while the car is static. It won't even be possible to move the car if the

engine power is not large enough. However, when the engine gear turns 1 cycle, the wheel

gear may turn 2, which makes the car run faster.

Figure 3
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Based on the physics rule of motion, after the object starts moving, the driving force needed

becomes smaller. Therefore, if the car can run on the large gear condition (Figure 2) when

starting, but change to a small gear (Figure 3) when moving, that is, applying a large force

when starting, but a small force when moving, this will makes the power transmission much

more efficient.

Go ToDifferent Kinds of Transmission Systems Used for the Automobile


List of Visuals

Figure 1: Newcomen Steam EngineImage is from "Newcomen Steam Engine," Answers Corporation

http://www.answers.com/topic/newcomen-steam-engine

Figure 2Developed in-house at ProQuest

Figure 3Developed in-house at ProQuest


>Arts & Humanities

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>List All


byChao-Hsu Yao


Different Kinds of TransmissionSystemsUsed for the Automobile

Printable Version (PDF)

Tell a Friend

The most common transmission

systems that have been used for

the automotive industry are

manual transmission,automatic

transmission,semi-automatictransmission, andcontinuously-

variable transmission (CVT).

The first transmission invented was

the manual transmission system.

The driver needs to disengage the

clutch(see Figure 4) to disconnect

the power from the engine first,

select the target gear, and engage

the clutch again to perform the

gear change. This will challenge a new driver. It always takes time for a new driver to get used

to this skill.

Contents


Systems Used for the Automobileo Manual Transmissiono Automatic Transmissiono Continuously-Variable



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Figure 4: The transmission system delivers the engine power to wheels.

An automatic transmission uses a fluid-couplingtorque converterto replace the clutch to avoid

engaging/disengaging clutch during gear change. A completed gear set, calledplanetary gears,

is used to perform gear ratio change instead of selecting gear manually. With the invention of

the automatic transmission, a driver no longer needs to worry about gear selection during

driving. It makes driving a car much easier, especially for a disabled or new driver. However,

the indirect gear contact of the torque converter causes power loss during power transmission,

and the complicated planetary gear structure makes the transmission heavy and easily broken.

A semi-automatic transmission tries to combine the advantages of the manual and automatic

transmission systems, 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 or sports cars currently.

CVT has been used for low-powered machinery like scooters for a long time due to its highly

efficient gear change. However, it is a challenge to install it on high power machinery because

of the strength of the driving belt. With the progress of materials technology, engineers have

been successfully installed it on automobiles, making the power transmission efficient.

Go ToManual Transmission


List of Visuals

Figure 4: The transmission system delivers the engine power to wheels Image is from "How Manual Transmissions Work," HowStuffWorks, Inc.

http://auto.howstuffworks.com/transmission.htm
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>Arts & Humanities

>Natural Sciences

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>List All


byChao-Hsu Yao


Manual TransmissionPrintable Version (PDF)

Tell a FriendManual transmission is also

referred to as stick shift

transmission because you need to

use the transmission stick every

time you change the gears. To

perform the gear shift, the

transmission system must first be

disengaged from the engine. After

the target gear is selected, the

transmission and engine are

engaged with each other again to

perform the power transmission.

Figure 5 shows the components of

the manual transmission gearbox,

and Figure 6 shows how theclutch

works. From Figure 5, the gear selector fork can be moved forward and backward to move the

collar to engage it to the target gears (blue). The gears (blue) are driven by the engine, while

the collar (purple) connects to the wheels. To engage the collar to the gear, the power from

the engine must be temporarily removed so the gear will lose the driving force. Therefore, the

gear and the collar can be engaged without causing grinding due to different spin speeds.However, the collar and gear still have different spinning speeds due to the gear ratio change

when shifting. For the modern car, a mechanism called asynchronizeris added to the collar to

synchronize the spinning speed between collar and gears to make the gears mesh smoothly.

The synchronizer is made of frictional materials. When the collar tries to mesh with the gear,

the synchronizer will touch 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 is meshed into the gear very

smoothly without grinding (see Figure 7). For some cars without synchronizers, a driving skill

called "double clutching" must be performed during gear shifting to avoid gear grinding.

Contents


Systems Used for the Automobileo Manual Transmissiono Automatic Transmissiono Continuously-Variable



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Figure 5: Two-speed manual transmission gearbox.

Figure 6: Clutch.
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For a standard 5-speedmanual transmissionsystem,

three different gear selector forks are used. Figure 8

and Figure 9 show the side-view and top-view of the

respective positions of three forks. This "H" shift

pattern enables the driver to select fivc different gear

ratios and a reverse gear.

The clutch, used to temporarily disconnect power from

the engine, is shown in Figure 6. When the thrust pad

is pushed, it lifts the pressure pad so the driven pad

(clutch plate) is disengaged from the flywheel. The

flywheel, which is connected to the gears of the

transmission system, will stop spinning. On the other

hand, when the thrust pad is released, the spring

pushes the driven pad back to engage the flywheel and

drives it spinning. The whole procedure is shown on

the following Shockwave file:

http://static.howstuffworks

.com/flash/clutch-fig5.swf.

The driven pad is made of frictional materials like

brake pads, which use friction to drive the flywheel

spinning. Like the brake system, the pad will become

thinner and thinner, and eventually it will need to be

replaced. The life of the driven pad really depends on

the driver's skill and how carefully theclutchis

maintained. Like brake pads, if signs of wear occur, a

proper adjustment is needed. Regular adjustment is

the best way to keep the clutch and brakes in good

shape.

Figure 8: The Side-View of a Standard 5-Speed Manual Transmission System.

With the progress of technology, a new mechanism was invented called a self-adjusting clutch,

which uses hydraulic pressure, instead of the traditional school linkage, to push the thrust pad

[4]. This idea came from the hydraulic brake, which was first invented in 1927 by Magura

located in Bad Urach, Germany and used for BMW motorcycles. The company also invented thefirst "adjust-on-the-fly" clutch in 1968 [12]. Regular adjustment will no longer be necessary,

but transmission oil must still be frequently changed to enhance the transmission's life. Almost

Figure 7: The synchronizer drives the gearto spin at the same speed as the collar to

avoid grinding when meshing.
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all of the manual transmission vehicles built later than 10 years ago were equipped with a self-

adjusting clutch, which is said to last longer than 120,000 miles without breaking down.

Basically, the gear is shifted by depressing the

clutch and releasing the gas, shifting the gear,

and releasing the clutch and stepping on the gas.

For some very old cars, the transmission collarsaren't equipped withsynchronizers. For some

heavy-duty trucks, the synchronizers are removed

in order to carry more heavy products. For some

rally race cars, to reduce gear shift time, the

synchronizers are removed so the driver can

quickly move the stick into the target gear. To

avoid gear grinding, a technique called "double

clutching" is always used. To perform double

clutching, instead of shifting to the target gear

directly, it is necessary to shift to neutral first,

release the clutch, step on the gas when down-

shifting or release the gas when up-shifting so the

gear can reach the same spin speed as the collar,depress the clutch again and release the gas, shift

to the target gear, and finally release the clutch

and step on the gas [5]. This link of a video clip

from youtube , shows how to do double clutching

when driving a 1960 GMC bus:

Double Clutching

Most modern manual transmission cars have been

equipped with synchronizers so double clutching is

no longer necessary. However, you still can use this technique when driving to enhance the

synchronizers and clutch pad's lives.

Go ToAutomatic Transmission


List of Visuals

Figure 5: Two-speed manual transmission gearboxImage is from "How Manual Transmissions Work," HowStuffWorks, Inc.


Figure 6: Clutch

Image is from Research Machines plc Figure 7: The synchronizer drives the gear to spin at the same speed as the collar

to avoid grinding when meshing

Image is from "How Manual Transmissions Work," HowStuffWorks, Inc


Figure 8: The Side-View of a Standard 5-Speed Manual Transmission System Image is from "How Manual Transmissions Work," HowStuffWorks, Inc.

http://auto.howstuffworks.com/sequential-gearbox.htm/printable

Figure 9: The Top-View of The Standard 5-Speed Manual Transmission SystemImage is from "How Manual Transmissions Work," HowStuffWorks, Inc.

http://auto.howstuffworks.com/sequential-gearbox.htm/printable

Double ClutchingZimtok. Youtube.http://www.youtube.com/ watch?v=p--jE1cfhj0

Figure 9: The Top-View of the Standard 5-

Speed Manual Transmission System.
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automotive transmissions_efficiently transferring power from engine to wheels

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