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Discovery Guides
Automotive Transmissions: Efficiently Transferring
Power from Engine to Wheels
By Chao-Hsu Yao
Introduction
Before the steam engine was 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 the steam 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 called atmospheric 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.
Figure 1 Newcomen Steam Engine. Image is from
“Newcomen Steam Engine,” Answers Corporation.
http://www.answers.com/topic/newcomen-steamengine
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 into high pressure steam. This steam pushes the piston, generating a forward and backward motion (see http://static.howstuffworks.com/flash/steam-engine.swf
for an animated picture) [3]. Because the combustion room is located outside the engine, the
steam engine is also called the external combustion engine.
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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 the speed 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 drawback 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 internal 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 wave file
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 the steam 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
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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, the gear ratio between 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 2 shows 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
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.
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Different Kinds of Transmission Systems Used for the Automobile
The most common transmission systems that have been used for the automotive industry are
manual transmission, automatic transmission, semi-automatic transmission, and continuouslyvariable 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.
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
An automatic transmission uses a fluid-coupling torque converter to replace the clutch to avoid
engaging/disengaging clutch during gear change. A completed gear set, called planetary 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.
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Manual Transmission
Manual 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
the clutch works. From Figure 5, the gear selector fork can be moved forward and backward to move the collar to engage it to the tar5 Two-speed manual transmission gearbox. Image is
get gears (blue). The gears (blue) are driven by Figure
from “How Manual Transmissions Work,” HowStuffWorks,
the engine, while the collar (purple) connects Inc. http://auto.howstuffworks.com/transmission.htm
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 a
synchronizer is 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
Figure 6 Clutch. Image is from Research Machines plc
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.
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For a standard 5-speed manual transmission system, 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/clutchfig5.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 the clutch
is 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. 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
the first “adjust-on-the-fly” clutch in 1968 [12].
Regular adjustment will no longer be necessary,
but transmission oil must still be frequently Figure 8 The Side-View of a Standard 5-Speed Manual
changed to enhance the transmission‟s life. Al- Transmission System. Image is from “How Manual
Transmissions Work,” HowStuffWorks, Inc.
most all of the manual transmission vehicles http://auto.howstuffworks.com/sequentialbuilt later than 10 years ago were equipped with gearbox.htm/printable
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.
http://auto.howstuffworks.com/transmissi
on.htm
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a self-adjusting clutch, which is said to last longer
than 120,000 miles without breaking down.
Figure 9 The Top-View of The Standard 5-Speed
Manual Transmission System. Image, from “How
Manual Transmissions Work,” HowStuffWorks, Inc.
http://auto.howstuffworks.com/sequentialgearbox.htm/printable
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 collars aren‟t equipped with
synchronizers. 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,
http://www.youtube.com/watch?v=p--jE1cfhj0,
shows how to do double clutching when driving a
1960 GMC bus.
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.
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Automatic Transmission
Figure 10 Planetary Gear Set. Image is from “The Transmission Bible,”
Carbible.com, http://www.carbibles.com/transmission_bible.html
Figure 11 Compound Planetary Gear Set. Image is from “The Transmission
Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html
Driving a manual transmission vehicle
is not an easy job for a beginner, and
it takes time for a new driver to attain
the skill. Improper driving always
causes the car to stall, and could
damage the transmission system. For
some disabled people who are not
able to use both legs, driving a manual
transmission car is impossible. In
1941, Chrysler introduced the first
automatic transmission system, which
included a fluid coupling between engine and clutch. The gear set is the
same as those in a manual transmission box; however, a vacuum cylinder
or a hydraulic cylinder is used to perform automatic gear shifting. The
clutch selects the gear range only but
isn‟t used when driving. The first
range (or low range) contains the 1st
and 2nd gears, while the second
range (or high range) contains 3rd
and 4th gears. To move the car, the
clutch and brake must be depressed
and a gear range must be selected
(low, high, or reverse.) After the
gear range is selected, the clutch can
be released. To move the car, the
brake is released and the gas is
stepped on. The gear is changed
automatically (between 1 and 2 or
between 3 and 4) during driving [6].
The automatic system for current
automobiles uses a planetary gear set instead of the traditional manual transmission gear set. The
planetary gear set contains four parts: sun gear, planet gears, planet carrier, and ring gear (see
Figure 10.) Based on this planetary set design, sun gear, planet carrier, and ring gear spin centriProQuest Discovery Guides
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fugally. By locking one of them, the planetary set can generate three different gear ratios, including one reverse gear, without engaging and disengaging the gear set. The shockwave file
http://static.howstuffworks.com/flash/automatic-transmission-planetary.swf shows how this
mechanism works when you click on the left buttons. Normally an automatic transmission system has two planetary gear sets with different sizes of sun gears with their planet gears intermeshed (see Figure 11.) Only one planet carrier is used to connect both sets of planet gears. This
is called a compound planetary gear set. This design can generate four different gear ratios and
one reverse gear. The inputs, outputs, and gear ratios are summarized in Table 1 [7].
Gear
Input
Output
Fixed
Gear Ratio
1st
30-tooth Sun
72-tooth Ring
Planet Carrier
2.4:1
30-tooth Sun
Planet Carrier
36-tooth Ring
2.2:1
Planet Carrier
72-tooth Ring
36-tooth Sun
0.67:1
Total 2nd
1.47:1
2nd
3rd
30- and 3672-tooth Ring
tooth Suns
Over Drive or 4th
Planet Carrier
72-tooth Ring
36-tooth Sun
0.67:1
Reverse
36-tooth Sun
72-tooth Ring
Planet Carrier
-2.0:1
1.0:1
Table 1
In order to lock the gears to perform gear ratio change, a band and clutches are used. The band is
connected to an actuator piston by way of a lever link (see Figure 12). The piston pushes the
lever link to force the band to lock the gear.
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The clutch lock is much more complicated
than the band lock. A set of clutches is
shown in Figure 13. It is constructed with a
clutch housing (drum), clutch plates (pressure plates), and clutch discs (friction
plates).
Figure 12 The structure of the actuator piston, lever link, and band
system. Images are from “The Transmission Bible,” Carbible.com,
http://www.carbibles.com/transmission_bible.html and Martin L.
Culpepper, “2.000 How and Why Machines Work [3-3-3],
http://pcsl.mit.edu/2.000/start.html
From different auto makers, several different types of automatic transmission systems
were designed and developed. Figure 14
shows one of the simple automatic transmission system designs. From this design,
the sun gear (yellow) is connected to a drum
(yellow), which can be locked by a band
(red). The ring gear (blue) is directly connected to the input shaft (blue), which transfers power from the engine. The planet carrier (green) is connected to the output shaft
(green), which transfers power into the
wheels. Based on this design, when in neutral, both band and clutch sets are released.
Turning the ring gear can only drive planet
gears but not the planet carrier, which stays
static if the car is not moving. The planet
gears drive the sun gear to spin freely. In
this situation, the input shaft is not able to
transfer power to the output shaft. When
shifting to 1st gear, the band locks the sun gear by
locking the drum. The ring gear drives the planet carrier to spin. In this situation, the ring gear (input shaft)
spins faster than the planet carrier (output shaft). To
shift to higher gear, the band is released and the clutch
is engaged to force the sun gear and planet carrier
(output shaft) to spin at the same speed. The input
shaft will also spin at the same speed as the output
shaft, which makes the car run faster than in 1st gear.
Using a compound planetary gear set generates more
gear ratios with a special gear ratio, over-drive gear,
Figure 13 Clutch Packs. Image is from Charles
Ofria, “A Short Course on Automatic
Transmission,” The Family Car Web Magazine,
http://www.familycar.com/
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whose gear ratio is small than 1. This will make the gear shift more smooth [8].
How can the band and
clutches perform the
lock function to select
the right gear automatically? Both the
band and clutch piston
are pressurized by the
hydraulic system (see
Figure 15). The part
connecting the band or
clutches to the hydraulic system is called the
shift valve, while the
Figure 14 Planetary Gear Sets with Band and Clutches. Image is from Charles Ofria, “A Short
one connecting the hyCourse on Automatic Transmission,” The Family Car Web Magazine,
draulic system to the
http://www.familycar.com/
output shaft is called
the governor (see Figure 16). The governor is a centrifugal sensor with a spring loaded valve.
The faster the governor spins, the more the valve opens. The more the valve opens, the more the
fluid goes through and the higher the pressure applied on the shift valve. Therefore, each band
and clutch can be pushed to lock the
gear based on a specific spin speed detected by the governor from the output
shaft. To make the hydraulic system
work efficiently, a complex maze of
passages was designed to replace a
large number of tubes (see Figure 17).
For modern cars, an electronic controlled (computer controlled) solenoid
pack is used to detect throttle position,
vehicle speed, engine speed, engine
load, brake pedal position, etc., and to
automatically choose the best gear for a Figure 15 Hydraulic system. Image is from HowStuffWorks, Inc.,
http://auto.howstuffworks.com
moving vehicle.
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Figure 16 Governor. Image is from HowStuffWorks, Inc., http://auto.howstuffworks.com
Figure 17 Hydraulic System with Maze Structure. Image is from Charles Ofria, “A Short Course on Automatic
Transmission,” The Family Car Web Magazine, http://www.familycar.com/
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Unlike a manual transmission system, automatic transmission doesn‟t use a clutch to disconnect
power from the engine temporarily when shifting gears. Instead, a device called a torque converter was invented to prevent power from being temporarily disconnected from the engine and also to prevent the vehicle from stalling when the transmission is
in gear. Consider two fans facing each other: when
one of them is turned on and starts spinning, the other
one will also start spinning at a lower speed (see Figure 18). That‟s because the first fan moves the air to
drive the second fan to spin. This is the same idea as
the torque converter of an automatic transmission
system, except that it uses fluid instead of air as the
Figure 18 Image is from Charles Ofria, “A Short
transportation media.
Course on Automatic Transmission,” The Family
Car Web Magazine, http://www.familycar.com/
A torque converter consists of four parts, a pump (impeller), turbine, stator, and transmission fluid. The
pump is connected to the engine and transfers engine power to the transmission fluid. The fluid
causes the turbine, which is connected to the input shaft, to spin. The stator is used to redirect
fluid returning from the turbine before it hits the pump, again to increase the efficiency. In this
design, even when the vehicle is in gear but not moving (the turbine is forced to stop), the pump
can still keep spinning without causing the engine to stall.
When the vehicle speed is slow, the turbine
is always spinning more slowly than the
pump. However, when the vehicle moves at
high speed, the turbine can spin at almost the
same speed as the pump. Therefore, for modern cars, a “lock up” will occur between
pump and turbine when the vehicle is at high
speed in fourth gear, which makes the torque
converter work more efficiently.
Tiptronic transmission is a special type of
Figure 19 The Structure of a Torque Converter. Image is from
automatic transmission with a computer
“What is a Torque Converter?” Pacific Driveline,
controlled automatic shift. The driver can
http://www.pacificdriveline.com/torque.htm
switch the transmission to manual mode,
which lets her shift the gear at her wish sequentially up (+) or down (-) (see Figure 20) without
disengaging the clutch. This works just like a manual transmission; however, it still uses a torque
converter to transfer power from the engine. Unfortunately, this is less efficient than a manual
transmission.
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Figure 20 Tiptronic transmission. Image is from “Test Drive: 2002 Audi A4 1.8T,” Canadian Driver,
http://www.canadiandriver.com/articles/pw/02a4cvt.htm
Continuously-Variable Transmission (CVT)
The continuously-variable transmission is also an automatic transmission system, which changes
the diameters of input shaft and output shaft directly, instead of going through several gears to
perform gear ratio change. This design can generate an infinite number of possible gear ratios.
Unlike the complicated planetary automatic transmission system, a CVT only has three major parts; 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
Figure 21 Pulley Based CVT. Image is from “How
CVT Works?” HowStuffWorks Inc.,
CVT is the most efficient transmission system in the
http://auto.howstuffworks.com/cvt2.htm
world.
Another popular CVT is 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 the two rollers tilt to the upper disc in Figure 22, they touch the upper disc with a larger
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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
CVT could only be used for some low torque engines like lawn mowers or snow blowers. It was
Figure 22 Nissan Extroid toroidal CVT. Image is from
also used for some small engine vehicles under
“How CVT Works?” HowStuffWorks Inc.,
1,500 cc. With the progress of material technolhttp://auto.howstuffworks.com/cvt2.htm
ogy, 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.
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
Figure 23 CVT Clutch Assembly for 152QMI & 157QMJ
two parts, a clutch disc with frictional materials at- Engine. Image is from “CVT Transmission Parts for
Taishan & Geely Scooters,” ScootUSA.com,
tached on the side, and a clutch cover (drum). The Qingqi,
http://www.scootusa.com/CVT-transmission.htm
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.
Semi-automatic Transmission
A semi-automatic transmission is a very advanced system, which still uses a clutch to perform
the gear shift instead of a torque converter. Unlike the manual transmission, the computer does
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all of the clutch disengaging, gear shifting, and clutch engaging. This not only makes the gear
shifting faster than manual transmission, but also prevents the vehicle from stalling when the car
is stationary. Like a tiptronic transmission, a semi-automatic transmission can also be switched
to manual mode to perform gear shifting at the drivers‟ wish. The two most common semi-automatic transmissions are direct shift transmission (aka dual-clutch transmission) and electrohydraulic manual transmission (aka sequential transmission).
The direct shift transmission was designed and developed by the Audi and
Volkswagen auto groups. Figure 24
shows the structure of the direct shift
transmission. Like a conventional manual
transmission system, it uses a collar, synchronizer, and gear set to perform gear
shift. The clutch set is like that inside the
automatic planetary gear transmission
system, which controls the gear ratio
change. Unlike the conventional manual
transmission system, there are two different gear/collar sets, with each connected
to two different input/output shafts. One
set contains odd (1st, 3rd, 5th…) gears,
while the other contains even (2nd, 4th,
6th…) gears. It is just like two conventional manual transmission gear boxes in
Figure 24 6-Speed Direct-Shift Gear Box. Image is from “How Dualone. To automatically shift from 1st gear
clutch Transmissions Work?” HowStuffWorks Inc.,
http://auto.howstuffworks.com/dual-clutch-transmission.htm
to 2nd gear, first the computer detects
that the spinning speed of the input shaft is too high, and engages the 2nd gear‟s collar to the 2nd
gear. The clutch then disengages from 1st gear‟s input shaft, and engages the 2nd gear‟s input
shaft. Controlled by computer, the gear shift becomes extremely fast compared with a conventional manual transmission. Using direct contact of the clutch instead of fluid coupling also improves power transmission efficiency. Another advanced technology used for direct shift transmission allows it to perform “double clutching” by shifting the gear to neutral first, adjusting the
spinning speed of the input shaft, and then shifting to the next gear. This makes gear shifting
very smooth.
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Another famous semi-automatic transmission is an electrohydraulic manual
transmission (aka sequential transmission). The gear set is almost the same
as the conventional transmission system, except that the shifting of the selector is not an „H‟ pattern. Instead, all
selector forks are connected to a drum
(see Figure 25). The drum has several
grooves, and each has a ball sliding in
it. Each fork hooks up to a ball and can
be moved forward and backward when
the drum is turning (see Figure 26).
Based on the pattern of the grooves on
Figure 25 Electrohydraulic Manual Transmission. Image is from “The
the drum, by turning the drum, each
Transmission Bible,” Carbible.com,
fork can move forward and backward
http://www.carbibles.com/transmission_bible.html
in turn, which makes gear selection sequential. Therefore, it is impossible for an electrohydraulic manual transmission to perform a
gear shift from 1st to 3rd or 4th to 2nd. The shifting must be sequential, like
1st2nd3rd4th, or 4th3rd2nd1st.
Comparison of Transmission Systems
The traditional automatic transmission system
is much more complicated than the manual
transmission system. It occupies more space,
so normally a small car can easily be designed
up to 5-speed manually, but only up to 3- or 4speed automatically. The automatic transmission system is also heavier than the manual
transmission system, which wastes more
gasoline. The fluid torque converter also loses
some power transferred from the engine due to
26 Collar and Fork Move When Drum is Turning.
indirect gear contact. The worst part of the Figure
Image is from “How Sequential Gearboxes Work?”
automatic transmission is that it is easily bro- HowStuffWorks Inc.,
http://auto.howstuffworks.com/sequential-gearbox.htm
ken due to its complicated design, and when
there is a problem it is very costly to fix.
However, an automatic transmission system is easy for a new driver to learn to use quickly, because engaging/disengaging the clutch by foot is not necessary, and the car will never stall during gear shifting. The CVT uses solid coupling instead of fluid coupling, which not only solves
the power loss problem but also prevents the clutch from engaging/disengaging to perform the
gear ratio change. However, the CVT is also heavy, and the strength of the driving belt is the
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critical issue which determines the CVT‟s performance. The infinite number of gear ratios makes
gear change smooth, and the result is better fuel efficiency. However, some drivers who enjoy
quick acceleration may not like the CVT‟s slow and smooth acceleration.
Two different semi-automatic transmission systems are the most advanced transmission systems
used in race cars and luxury cars. They use computers to perform gear shift faster than conventional manual transmission, and also with better fuel economy. However, the direct shift transmission has too many machine parts, and also a complex clutch set, which makes the gear box
heavy, and easily broken. The high repair cost is also an issue. The electrohydraulic manual
transmission has a simpler gear structure with a rotating drum to perform gear ratio change. The
drum can be rotated manually, or by computer control. This makes the gear shift extremely fast,
and due to its simple structure, the gear box is not heavy, and only occupies a small space. That
is why it‟s also used for manual transmission on motorcycles. However, if you need to make a
sudden stop at high speed, it‟s impossible to shift from 6th gear directly to 1st gear. You will
need to go through all gears from 5th to 2nd. Without a clutch design, the gear shift is not
smooth. That‟s why it is only used for some high speed race cars like Toyota MR2 and Ferrari,
which only consider speed but not comfort.
The Future Development of Automotive Transmission Systems
It is likely that, with the progress of new material technology, the CVT will gradually replace the
conventional automatic transmission due to its high fuel efficiency and smooth gear shift. The
technology of semi-automatic transmission systems will also be improved to perform smooth
gear shift and extend the cars‟ lifetime, without losing fast acceleration and fuel efficiency. The
torque converter with fluid coupling may be improved, or may no longer be used for cars in the
future due to its low-efficiency power transfer. The gear shift of the manual transmission will be
controlled by computer instead of engaging/disengaging the clutch and moving the gear selector
by hand. For some modern cars, using push-buttons behind the steering wheel, instead of a conventional shift lever or stick, also saves gear shift time when shifting manually. An adaptive
transmission control (ATC) has also been invented by using a computer to recognize and memorize different drivers‟ styles, and determining the best shifting timing for different drivers. As
mentioned in the Introduction, a transmission system is needed for a vehicle due to the internal
combustion engine‟s property of running at high pressure at high speed but low pressure at low
speed. If someday an engine with different properties is invented, the transmission system may
no longer be necessary, but can still get the vehicle to reach its maximum speed in a couple of
seconds.
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References
1. “Steam Engine Invented,” Antique Farming.
http://www.antiquefarming.com/steamengine.html
2. “How Steam Engines Work?” HowStuffWorks, Inc.
http://www.howstuffworks.com/steam.htm
3. “Steam Engine Operation, “ HowStuffWorks, Inc.
http://static.howstuffworks.com/flash/steam-engine.swf
4. “Self-Adjusting Clutch Mechanism,” PatentStorm LLC.
http://www.patentstorm.us/patents/5456345-fulltext.html
5. “Double Clutch,” Wikipedia. http://en.wikipedia.org/wiki/Double_clutch
6. “Semi-Automatic Transmission,” Wikipedia Foundation, Inc.
http://en.wikipedia.org/wiki/Semi-automatic_transmission
7. “How Automatic Transmission Works?” HowStuffWorks, Inc.
http://auto.howstuffworks.com/automatic-transmission3.htm
8. Charles Ofria, “A Short Course on Automatic Transmission,” The Family Car Web.
Magazine, http://www.familycar.com/
9. “Internal-Combustion Engine,” Answers Corporation
http://www.answers.com/internal%20combustion%20engine
10. “How Two-Stroke Engines Work?” HowStuffWorks, Inc.
http://science.howstuffworks.com/two-stroke2.htm
11. “How Car Engines Work?” HowStuffWorks, Inc.
http://auto.howstuffworks.com/engine1.htm
12. “Magura Motorcycle Components,” Sand Wizards Racing.
http://www.sandwizards.com/Magura_review.htm
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