2 The gearbox in a motorcar is the technical aid for converting the engine forces into the varying operating conditions. Operating the clutch and shifting gears make up the lion’s share of the physical effort involved in driving a motor vehicle. The purpose in using „self-shifting gearboxes“ is to considerably reduce this physical effort, enhance active safety so that the driver’s responsiveness is directed fully to the traffic situation. The progress in electronics makes it possible to interlink electronic functional details and hydraulic systems and to achieve safe „automatic“ driving with a high level of effi- ciency. That is why automatic gearboxes feature increasingly as part of the equipment available in modern motor vehicles. The operating principle of an automatic gearbox is basically the same in all passenger cars. They vary in design details depending on how they are installed and on the power output of the engine. This self study programme is intended to impart to you the basic design of an automatic gearbox and the operation of certain components. The components presented apply in the vast majority of cases to all automatic gearboxes or are identical to those used in gearbox 01M fitted to the OCTAVIA. SP20-3 Automatic Gearbox - Fundamentals -
Transcript
2
The gearbox in a motorcar is the technical aid for converting the engine forces into the varying operating conditions.
Operating the clutch and shifting gears make up the lion’s share of the physical effort involved in driving a motor vehicle. The purpose in using „self-shifting gearboxes“ is to considerably reduce this physical effort, enhance active safety so that the driver’s responsiveness is directed fully to the traffic situation.
The progress in electronics makes it possible to interlink electronic functional details and hydraulic systems and to achieve safe „automatic“ driving with a high level of effi-ciency. That is why automatic gearboxes feature increasingly as part of the equipment available in modern motor vehicles.
The operating principle of an automatic gearbox is basically the same in all passenger cars. They vary in design details depending on how they are installed and on the power output of the engine.
This self study programme is intended to impart to you the basic design of an automatic gearbox and the operation of certain components. The components presented apply in the vast majority of cases to all automatic gearboxes or are identical to those used in gearbox 01M fitted to the OCTAVIA.
SP20-3
Automatic Gearbox- Fundamentals -
3
You can find specific information on the automatic gearbox 01M fitted to the SKODA OCTAVIA in the Self Study Programme Booklet 21.
Contents
Power Conversion 4
General Design 7
Determining Shift Point 9
Automatic Transmission Fluid 14
Torque Converter 16
Lock-up Clutch 18
Planetary Gear Systems 19
Shift Elements 23
Multi-disc clutch
23
Multi-disc brakes
24
Band brakes
24
Freewheel
26
Gearbox Control 27
System overview of an automatic gearbox
28
Emergency programme/Self-diagnosis
30
Hydraulic System 31
Hydraulic fluid circuit/hydraulic fluid pump
31
Hydraulic shift control unit
32
Diagram of hydraulic system
33
Pressures in the hydraulic system
34
Hydraulic shift elements
36
Test Your Knowledge 38
4
(N)
v (km/h)
IV
III
II
I
ideal tractive force curve
M
•
n
Why convert the power?
9550
Power Conversion
Let’s recall a number of basic rules of automotive engineering.
The power for driving a motor vehicle and the neces-sary ancillaries (e.g. power-assisted steering, air con-ditioning compressor) is produced by the engine.
The power
P
is the mathematical product of torque
M
multiplied by the speed
n
divided by the numerical factor 9550*. The unit of measure is kW.
The power increases with engine speed and torque.
What does the term torque tell us?Torque describes the power transmission through a shaft or gear.It is designated with the formula character
M
and is formed from the force
F
which acts on the circumfer-ence of the rotating part, multiplied by its radius
r
.The term which describes the engine speed is the angular velocity
ω
in 1/s.The unit of measure of torque is Nm = Newton meter. In the case of the gearbox it is the gears which pos-sess a certain lever arm „r“.
Internal combustion engines can, however, only be operated between idling speed (in the case of a car approx. 600 to 700 rpm) and maximum speed (which differs according to engine design, on average 6000 to 7000 rpm).In contrast, maximum torque is achieved only within a narrow engine speed range.It rises to its maximum value and drops off again within the range of rated engine speed.
That is why we need to have a torque converter in a vehicle for adapting this limited engine speed range to the wide range of the tractive force required.This torque converter is the gearbox.In theory, a gearbox with an infinite variety of steps would be required for adapting to tractive force demand.This is not a practical concept.That is why we attempt to approach the ideal pattern of a tractive force curve by using several constant steps = engageable ratios.
SP20-6
SP20-8
Tractive force - vehicle speed diagram
Power-torque diagram of a petrol engine
M = F
•
r
Tractive force curve of gear I to IV
P =
Tra
ctiv
e fo
rce
at d
riven
w
heel
s
* The numerical factor 9550 results from the conversion of all mathematical quantities when the numerical values for n and M are entered in the equation with rpm and Nm, respectively. This produces the result P in kW.
n = 5000 rpm●
n = 1000 rpmo
SP20-45
F
r
Mω
5
The manual gearbox
When it comes to a gearbox, therefore, we can refer to it as a device for converting torques.Speed
n
and torque
M
behave in this case in the reverse ratio, in other words a torque which flows in at the input of the gearbox appears again intensified at the gearbox output.The torque gain is obtained, however, by a loss in rotational speed.The engine power is not altered as a result of the gearbox.
A manually-shifted gearbox is generally designed as a countershaft gearbox.We are familiar with this from all SKODA models.
The power in this case flows from the input shaft through a fixed gear combination to the main shaft and on to the final drive.
The sliding gears on the main shaft run loose and it is only once a gearshift is performed that they are cou-pled to the shaft by means of sliding sleeves.
Manual gearboxes therefore operate on a positive locking basis - in contrast to automatic gearboxes which operate on a non-positive basis.
The torques vary as a function of the transmission ratio „i“.
M
Output
= M
Input
• i
SP20-7
SP18-22
Important:When starting off and shifting gears in a car fitted with a manual gearbox,the power flow from the engine to the gearbox has to be interrupted.As you know, it is not possible to shift gears when the engine is operatingunder load.This requires a special mechanical device - the clutch.When engaged, the clutch transmits the engine torque to the gearbox andthe driven wheels and, when disengaged, it interrupts the power flow.
n
n
n
1
n
2
i = = Speed of driving gear
Speed of driven gear
6
Power Conversion
Today's manual gearboxes do admittedly reflect the state of the art ...........
There has been a considerable improvement in the operation of manual gearboxes in recent years:
– gearshifting simplified by means of synchromesh
– quiet gearshifting as a result of helical gears
– transmission ratios adapted to engine output and tractive force demands optimally
matched between the gears
– designing car gearboxes which for the most part offer 5 gears.
Clutches, too, have also undergone numerous improvements, in particular in respect of reducing the pedal forces required.
How a clutch and gearbox for modern cars are designed and operate is described in Self Study Programme 18.
But ............
If a car is driven a distance of 10,000 km - as test runs have revealed - the clutch pedal is moved about 30,000 to 40,000 times.
And the gears are shifted manually with the gearshift lever just as often.It's therefore no surprise at all that opinions vary considerably when it comes to a manual gearbox.
SHIFTING GEARS IS FUN
- IS THE ONE OPINION
SHIFTING GEARS IS HARD WORK
- IS THE OTHER OPINION
that is why, have the work done for your
automatic gearbox!
Nevertheless, there were a great many prejudices to overcome in the course of developing automatic gearboxes.They were considered as "weak" and "not sporty".
Nowadays, thanks to computer technology in the car with electronic shift programmes and determina-tion of the shift point using fuzzy logic, such arguments no longer hold true.
SP20-13
SP20-14
7
General Design
The differences
What are the differences between a car fitted with a manual gearbox and a car fitted with an automatic gearbox?
with automatic gearboxwith manual gearbox
SP15-19
Torque converter automatically separates the rotating engine from the stationary gearbox when the car is not moving, but also performs addi-tional tasks and can be regarded as a hydraulic gearbox.
Planetary gear
Is an essential requirement for at all making use of automatic con-trol, automatic non-positive torque transmission through clutches and brakes.
More relaxed driving, sensors detect the driving resistances. Electronic gearbox control proc-esses information for selecting a gear which is engaged by hydraulic shift elements.
Automatic gearboxes operate without any inter-ruption to the power flow and therefore acceler-ate continuously.When it comes to acceleration performance, they are the equal of a manual gearbox.
Driving comfort is enhanced, stress is reduced, overall safety is improved.
Mechanical clutch, operated manually
Manual gearbox with countershaft
with positive-locking gearshift mechanism (using gearshift lever, shift fork, sliding sleeve) for transmitting the torque
Driver involved in shifting gears.Eyes and ears sense the driving situation.
The power flow is interrupted during a gearshift.As a rule, the car moves without power for 1 to 2 seconds during a gearshift (depends on driver).
Increased physical demand on the driver, full concentration on driving situations.
The power flow in both cars is the same Engine - Clutch - Gearbox - Differential - Drive Shafts - Running Gear
SP15-18
8
General Design
Automatic gearboxes therefore perform the tasks of
– starting off
– selecting the transmission ratio
– engaging the selected gear automatically.
The only element which is involved in starting off is a hydrodynamic torque converter
Torque converter
For starting off, for increasing torque and minimizing vibrations
Automatic gearbox- Main components -
Planetary gear
For mechanically forming the ratios/gears
Shift elements
hydraulic pressure-operated multi-disc clutches and brakes(assigned to the individual elements of the planetary gear train)
for carrying out the gear change
Freewheels
For optimizing the engagement of load with the shift elements
Gearbox control
(electronic/hydraulic) based on shift programmes
Oil pump
For supplying the shift elements, the torque converter and for lubricating the gearbox
9
Determining Shift Point
When it comes to carrying out the automatic gearshifts, in other words converting the torque in line with the driving situations which exist, what is of interest in addition to the purely mechanical gearbox design (planetary gear), are the following three questions.
1. How does the automatic gearbox control detect adaptive shift curvewhen a gearshift should be made?
2. Who supplies this information to the control unit? sensors
3. How are the gearshifts effected? hydraulically by means ofactuators/solenoid valves
Let's look in this connection at the system functions for an automatic gearbox, as exist also in the SKODA OCTAVIA.
Accelerator pedal position
Vehicle speed
Gearbox speed
Torque converter lock-up valve
Control unit
Main pressure valve
Gear selectionvalve
Selector lever position
ATF temperature
Engine speed
Kickdown switch
Brake pedal operation
Engine torque reduction
Selector lever lock
Starter lockout
Selector lever display
Idle speed increase
Air conditioningSelf-diagnosis
Sensors Actuators
The shift logic is computed through digitally by a microprocessor in the control unit.The electronic gearbox control constantly repeats the detection of the sensor signals, calculates the shift decision and transmits it to the actuators. This cycle is completed in 20 ms.
10
Determining Shift Point
SSP172/116
SSP172/117
3 – 4
4 – 3
3 – 4
3 – 4
4 – 3
4 – 3
Conventional shift characteristic line
Shifting between two gears is carried out by the electronic gearbox control on the basis of a shift characteristic line. This takes into account vehicle speed and accelerator pedal position.A different characteristic line applies to upshifts than to downshifts.A shift characteristic line is stored in the control unit for each gear change as a func-tion of vehicle speed and accelerator pedal position.
This selection of shift points is relatively rigid as gearshifts are always made at the same points in line with the position of accelerator pedal and the speed of the car. The diagram illustrates only the 3rd - 4th gearshift.
Sport characteristic lineECO characteristic line
During the initial period of electronic gear-box controls, therefore, only fixed shift char-acteristics were programmed.
As the electronic gearbox control under-went further development, it was possible to select between two programmes:
- a sporty
- and an economic programme
A separate switch on the selector lever pro-vided the
driver
with the possibility of
selecting
one or the other shift characteris-tics. A subsequent development was to automate this switchover.This was done by sensing the rate at which the accelerator pedal was depressed. Nev-ertheless, as with the previous system, this was also an absolute decision
"ECO" or "SPORT"
Position of accelerator pedal
Veh
icle
spe
ed
Upshift
Downshift
Position of accelerator pedal
Veh
icle
spe
ed
Sport
ECO
11
SP20-11
100 %%
8090
70605040302010
00
1
Adaptive characteristics
Modern electronic gearbox controls - as is also the case for the 01M gearbox in the SKODA OCTAVIA, calculate a shift in the characteristic line from a large variety of information which con-stantly describes the current operating and driving situation.
This individually adapted, non-rigid shift characteristic is used in the control unit for the shift decision. We talk in this connection of an adaptive shift characteristic.
The
driving resistance-based shift programme
recognizes driving resistances such as climbing or descending a hill, towing a trailer and driving into the wind.The control unit calculates the driving resistance on the basis of the speed of the car, the position of the throttle valve, the engine speed and the acceleration of the car, and uses this as a basis for specifying the shift points.
The calculation of the gearshift point based on the driver's style and driving situation is conducted using the fuzzy logic principle
Sporty factor
Sporty factor
Sport
Accelerator pedal speed
With the speed at which the driver operates the accelerator pedal, he produces a sporty factor which is determined by the fuzzy logic.This sporty factor is used to determine a floating gearshift point between a shift point design oriented more to good fuel economy or more toward performance.It is thus possible to have any number of shift points between the "ECO" and the "SPORT" shift characteristic.It is thus possible to react much more responsively to the individual driver's wishes.
ECO
12
Determining Shift Point
SSP172/107
0
11
0
80 °C
0 °C
SP20-46
Fuzzy logic is something which we encounter today in a large number of items of equipment which are part of our daily life.Washing machines, vacuum cleaners, video cam-eras or electric razors nowadays are controlled by fuzzy logic.
The word fuzzy in this connection means more or less a "specifically applied out-of-focus".
What we do when we make use of fuzzy logic is to eliminate the classical hard shift states for a rigid classification does not permit any tolerance in assigning quantities.
Classical division
The example below is intended to illustrate to you the classical rigid allocation of quantities in a compu-ter without fuzzy logic:If a computer has to distinguish between hot and cold, it is then necessary to provide it with a fixed limit (in this example 80°C).
The computer is able to distinguish between hot and cold on the basis of the switching states.This rigid classification does not allow the computer any tolerance in allocating quantities.
What does fuzzy logic mean?
hot
cold
Switching states
Switch closed
or
Switch openTemperature
Yes
No
13
19 °C
SP20-9
SP20-10
It is often necessary to take decisions which come somewhere between these rigid statements of "hot" and "cold".
Fuzzy logic makes allowance for an intentional out-of-focus (fuzziness) which does not operate with two values but with result quantities.
What we then have is an infinite variety of intermediate values such as "almost cold", "cool", "lukewarm" or "too warm".
The size of the areas produced by the intersections - blue area relative to red area - enables the fuzzy logic to recognize how these different temperature levels are assigned to the previously precisely specified inter-mediate values.
Consequently, at 19°C, 88% of the entire area is assigned to blue = cold and 12% of the entire area is assigned to red = hot.
The fuzzy logic recognizes "lukewarm".
cold
lukewarm
hot
10 706050403020 80 °C
80 °Chot 1
almost hot
too warm
warm
lukewarm
coolalmost cold
cold 0
The upper limit of "hot" and the lower limit of "cold", as well as all the intermediate levels, are assigned to precise temperatures.
The fluid in the automatic transmission has to satisfy varying demands as it circulates.
It has to
– Transmit forces (in torque converter)– Perform shift movements (in the hydraulic shift elements)– Produce friction (in the multi-disc clutches and brakes, in the
The automatic transmission fluid has to perform these tasks within a temperature range from -30°C up to 150°C (temperature measuring points in the oil pan of the gearbox).
Temperatures of up to 250°C to 400°C may occur for short times during a gearshift at the multi-disc clutches and brakes.
That is why the mineral base oil for automatic gearboxes is provided with a number of additives to enable and to perform all these tasks no matter the conditions which exist. In particular, the viscosity index is improved in order to ensure uniform viscosity over the entire temperature range.Standards which have been compiled for this purpose by General Motors (ATF Dexron) and Ford (ATF Mercon) are recognized world-wide.
SP20-4
SP20-5
Note:Use only the automatic transmission fluid ap-proved by the vehicle manufacturer.Other fluids or additives result in changes to theproperties and have a detrimental effect on theoperation and life of the gearbox.In particular, water elements in the automatictransmission fluid can cause operational prob-lems.The ATF is kept clean by passing it through a fil-ter as it flows out of the oil pan.A strong permanent magnet in the oil pan traps any metallic abrasion.
Ad
ditiv
e
15
ATF level and ATF temperature have a major influence on proper operation of an automatic gearbox.
ATF level/ATF temperature
Pay particular attention to the inspection tem-perature of the ATF if adjusting the ATF to the correct level.
The inspection temperature should be meas-ured with the diagnostic tester and set to the specified temperature.
When inspecting the ATF level, proceed as stated in the current Workshop Manual for the relevant gearbox.
If the quantity of ATF is correct, the electronic gearbox control automatically compensates for a change in viscosity as a result of a tem-perature increase by changing the oil pres-sure in order to ensure uniformly smooth gearshifts.
Even if the temperature is exceeded by a small amount this can result in changes to the ATF level. The expansion of the ATF takes place not in the oil passages but occurs in the oil pan.In particular, the heating up of the ATF in the torque converter forces it into the oil pan.
An excessive ATF level results in foaming and in ATF flowing out of the overflow.
Important!Incorrect filling of an automatic gearbox can result in operational problems and damage to the gearbox.
Too high Too low
ATF tempera-ture too high
Gearshifts take too long
Clutches/brakes close too slowly
Gearshifts performed with time lag
Operational problems in gearbox
ATF foams
ATF flows out of breather
ServiceInspect ATF level and adjust, if necessary
That is why automatic gearboxes feature a temperature sensor which measures the ATF temperature, and also an ATF cooler. The block diagram below illustrates the interrelationships.
ATF level
16
Torque Converter
SP20-15
The hydrodynamic torque converter is, in fact, an additional hydrodynamic transmission to the auto-matic gearbox.
It forms the initial element of the automatic gearbox.
The principle of the torque converter was first used in 1905 by Hermann Föttinger in marine engineer-ing.That is why the hydrodynamic torque converter is often referred to as the Föttinger converter.
The principle of the torque converter:A pump draws in a fluid - in our case the special automatic transmission fluid - accelerates it and passes it to a turbine.The hydrodynamic energy is thus converted into a mechanical rotating movement.
The torque converter consists of three main parts:
– Pump wheel (this is also the housing of the torque converter)
– Turbine wheel (which powers the turbine shaft and thus the gearbox)
– Impeller (connected by a freewheel to the gearbox housing, it is able to rotate only in the same direction as the pump wheel and turbine wheel)
It is filled with a special automatic transmission fluid and is pressurized.
The pump wheel (at the same time the housing) is driven by the vehicle engine with a direct speed.As a result of the centrifugal force the ATF is forced out between the blades of the pump wheel.It is deflected toward the turbine wheel at the inner wall of the housing.This hydrodynamic energy is absorbed by the blades of the turbine wheel and it rotates.The hydrodynamic energy is converted into a mechanical rotating movement.The ATF flows back in the vicinity of the shaft of the torque converter through the vanes of the impeller positioned relative to the pump wheel.The internal ATF circuit in the torque converter is closed.
The hydrodynamic torque converter
Propulsion effect
Components
Turbine wheel
Impeller
Pumpwheel
17
In the torque conversion phase, the torque con-verter converts the reduction of rotational speed into an increase in torque.
At the moment the vehicle starts off, only the pump wheel rotates initially. The turbine is stationary. The difference in speed - known as slip - is 100 %.Slip is reduced to the extent that the ATF passes the hydrodynamic energy to the turbine wheel. Pump speed and turbine speed move closer together.
The torque converter slip is the operationally nec-essary criterion for converting the torque.At a high slip, the torque boost is at its maximum, in other words if there is a large difference in speed between pump wheel and turbine wheel, the ATF flow is deflected by the impeller.The impeller thus has the effect of boosting torque in the torque conversion phase.It is supported by means of a freewheel at the gear-box housing during this operation.When slip is low, in other words when pump wheel and turbine wheel are operating at practically the same speed, the impeller no longer has the effect of boosting the torque.In this case, it then rotates in the same direction as the pump wheel and turbine wheel thanks to the freewheel. It thus causes scarcely any losses in efficiency.
Torque increase
The turbine wheel is stationary.Pump wheel is rotating.ATF flow sharply deflected. High slip.Gearing down.Maximum boost in torque.
Turbine speed increases.ATF flow is "stretched". Slip is reduced, transmission ratio decreases.Torque boost decreases.
Turbine speed practically pump speed.Low slip, impeller also rotates.Torque ratio shrinks to 1:1.Operates only now as clutch.
Hence: The torque converter operates in the slip range as a hydraulic gearbox with a variable ratio.
Starting-offTorque conversion phase 1
Torque conversion phase 2
Clutch phase
Pump wheel
Turbine wheel
Freewheel
Impeller
Transmitting energy by means of ATF flow
SP21-31
18
Lock-up Clutch
SP21-34
Why is the torque converter locked up?
Once the clutch phase is reached, in other words the torque ratio is 1:1, the torque con-verter operates with relatively high losses.The efficiency as a rule is around 85 %, and may even be as much as 97 % in the case of high-performance engines which operate at high speeds.
Two to three percent of slip are always required, however, for transmitting the power otherwise the ATF flow would come to a stop.
Losses in transmitting power always, however, have an impact on economic operation of the vehicle.That is why modern automatic gearboxes are equipped with a lock-up clutch. This locks up the torque converter, if necessary, if the slip level is low and takes it out of operation.
The lock-up clutch is integrated in the housing of the torque converter. It is provided with a ring-shaped friction lining and is connected to the tur-bine wheel. It is pressed by means of oil pres-sure against the torque converter housing which also serves as the torque input.This ensures a rigid, slip-free transmission of power.
In the same way as a normal dry friction clutch, the torque converter lock-up clutch features tor-sion dampers for reducing engine torsional oscil-lations.It is the control unit of the automatic gearbox which determines when the lock-up clutch closes or opens.
Depending on the characteristics of the vehicle and gearbox, it is possible in practice to improve the fuel economy of a car fitted with an auto-matic gearbox by 2 to 8 % by means of a torque converter lock-up clutch.
Self Study Programme 21 contains further infor-mation on the hydraulic control of a torque con-verter lock-up clutch.
Operation
Torque flow
Torque converter lock-up clutch - a mechanical clutch
Lock-up clutch
Turbine wheel
19
Planetary Gear Systems
SP20-31
A gear change in the manual gearbox proceeds as follows, as most of you will be aware:
– Disengage shift sleeve, power flow interrupted
– Gear is brought to the same speed,– Then, the selected shift sleeve is engaged
and the power flow is restored
There is no possibility in the case of an automatic gear change for any interruption to the power flow, which is what we wish from an automatic gearbox.
The automatic control unit cannot derive from the traffic situation when it would be the right moment to interrupt the power flow.
A planetary gear system is composed of two to four planet gear sets.These are permanently connected to each other or by means of clutches.The operating principle can be explained by taking one planet gear set.
A planet gear set consists of
– a central gearthe sun wheel - 1 -
– several planet gears (three to six) - 2 -– the planet carrier - 3 -– an external internally-toothed
hollow gear - 4 -
All the pairs of gears are constantly meshed.It is not necessary to have shift sleeves. The gear speeds do not have to be synchronised.
When it comes to an automatic gearbox, it is only gearboxes which can also be shifted without any interruption of the power flow, which are suitable.This is the case for planetary gear systems. That is why they form the design basis of almost all auto-matic gearboxes.
What alternatives are there?
The gear change - automatic gearbox
The gear change - manual gearbox
1
2
3
4
20
1
2
3
4
Planetary Gear Systems
SP20-16 SP20-17 SP20-18
The sun wheel -1- rotates in the inside on a cen-tral shaft.The planet gears -2- mesh with the teeth of the sun wheel.
The planet gears are able to rotate both about their own axis as well as on an orbit around the sun wheel.
The planet gears are housed together with their shafts in the planet carrier -3-.The planet carrier permits the rotational move-ment of the planet gears around the sun wheel and, logically, also thus around the central shaft.
The inner teeth of the hollow gear -4- mesh with the planet gears and surround the entire planet gear set.The central shaft also forms the rotational point for the hollow gear.
The hollow gear, planet carrier and sun wheel are each connected to a shaft.
It is possible to achieve both large as well as smaller high and low up and down gearing with a planet gear set if one of the gear elements is held fixed and the two others perform the task of input and output.When the planet carrier is held fixed, the direc-tion of rotation is reversed.If two parts are held fixed, the planet gear blocks and the ratio is 1:1.
- Hollow gear fixed- Sun wheel driving =
large down gearing ratio
- Sun wheel fixed- Hollow gear driving =
low down gearing ratio
- Planet carrier fixed- Sun wheel driving =
reversal of direction of rotation
SP20-2
21
Hollow gear Sun wheel Planet carrier Ratio
Fixed Output Input High, fast
Output Fixed Input Low, fast
Input Output Fixed Fast, direction of rotation reversed
Fixed Fixed Output No planet gear set blocked
Input low Input normal Rotation superposed on that of hollow gear, superposition of
speed (escalator effect)
It is possible to form additional transmission ratio versions from a combination of driving and braking (holding fixed) parts
SP20-20
Block diagram of input and output of a planet gear set
The parts of the planetary gear set therefore have to be braked or driven from outside.
If this is to operate properly, all the shafts of the parts in question have to be led to the outside and connected to countershafts.
This is solved in design terms by means of intermeshed hollow shafts.
These are shaped on the outside like a bell (clutch bells) and are positively connected to the similarly shaped countershafts, depending on their actuation.
The clutch bells in turn support in this case the clutches and brakes.
During braking, the brakes are supported against the gearbox housing (refer also to the section on shift elements).
Hollow gear
Planet gears with planet carrier
Turbine shaftSun wheel
22
Planetary Gear Systems
SP20-19
consists of 3 planet gear sets.The first hollow gear, the second planet carrier and the third hollow gear are permanently connected to each other.In addition, second and third sun wheel are perma-nently connected to each other.The forward gears are driven through this double sun wheel.
consists of 2 planet gear sets with a common sun wheel.The planet carrier of the one set, the hollow gear of the other and the input shaft are permanently con-nected to each other.The forward gears are each driven through the hol-low gears.This design was often used in the age of three-speed automatic gearboxes.
consists of 2 planet gear sets with a common planet carrier.This design is similar to that used in the 01M auto-matic gearbox of the SKODA OCTAVIA.
The planet carrier features two sets of planet gears:
- short planet gears with a large diameter which mesh with a small sun wheel
- long planet gears with a small diameter which mesh with a large sun wheel and the short planet gears.
The Ravigneaux gearbox features only one hollow gear which surrounds the short planet gears.
Power output is always through the hollow gear. The design of the Ravigneaux gearbox makes it possible to provide 4 forward gears and one reverse gear.
Because of its compact design it is particularly suit-able for use in front-wheel-drive vehicles.
Ravigneaux gearbox
Simpson gearbox
Wilson gearbox ➙
➙
➙
The different combinations and technical standard designs are named after their inventors.
Several planet gear sets are positioned one after the other for an automatic gearbox in a vehicle. It is then possible to combine the required gearbox steps from this combination.
23
Shift Elements
SP20-22
Each gear features at least one shift element which creates the power flow by means of fric-tion.
Multi-disc clutches are used in order to create the power flow from the turbine shaft to the planet gear set.
They possess internally-toothed and externally-toothed discs which are both connected to rotat-ing parts. They are interlaced in the form of chambers. In the non-operated state, there is a gap between them and they are filled with oil so that they are also able to rotate freely.The set of discs is compressed by a hydraulic plunger which rotates together with its oil filling which acts from the rear on the plunger.The oil is therefore supplied through a hollow shaft. The pressure on the multi-disc clutch is released by means of springs when the clutch is disengaged (compression springs or also disc springs).Ball valves (some in the plunger, the others in the disc carrier) ensure that the pressure is able to be reduced rapidly in the non-operated state and the oil is able to flow off.
The disc carriers at the inner part as well as the outer part support the discs by means of lugs, which produces a positive connection.
The externally-toothed discs are made of steel.The internally-toothed discs are made of high-strength plastic.At the same time, they perform the function of the friction lining.The supporting framework is made of cellulose.The temperature resistance is achieved by admixing aramide fibres, a high-strength plastic.To influence the friction coefficient, minerals are added for bonding phenol resin.
The number of discs differs considerably depending on the gearbox version.The play between the discs is of importance for operation of the automatic gearshift and is fixed as a result of the design.It is set separately during installation.
We also find the principle of multi-disc clutch in the 01M automatic gearbox of the SKODA OCTAVIA.
Externally-toothed disc, positively connected to outer part
Multi-disc clutch
SP20-21
SP20-25
Internally-toothed disc, positively connected to inner part
Outer part Externally-toothed disc
Inner part
Ball valve
Plunger
Internally-toothed disc
Disc carrier (clutch bell) for accommodating externally-toothed discs
24
Shift Elements
SP20-24
The multi-disc brakes are used for holding a part of the planetary gear set fixed. They are similar to the multi-disc clutches and likewise feature internally-toothed and externally-toothed discs. The internally-toothed discs are likewise con-nected to the rotating part by means of lugs whereas the externally-toothed discs are held in position, supported at the gearbox housing.
During actuation, a hydraulic plunger com-presses the set of discs.The hydraulic plunger does not move in contrast to the multi-disc clutch.In the case of the multi-disc brake as well, the play between the clutches is of importance for proper operation of the shift mechanism and is set separately.
This type of brake is also used in the 01M auto-matic gearbox of the SKODA OCTAVIA.
Externally-toothed disc, supported at gearbox
Multi-disc brakes
SP20-23
SP20-26
Internally-toothed disc, positively meshed with rotating part
Gearbox housing
Externally-toothed disc, fixed
Rotating part
Plunger
Internally-toothed disc
The band brake offers a further design possibility of holding the elements of a planet set fixed.
The outer shape of the shaft is designed in a similar way to a brake drum.A steel brake band, as the braking element, is closely wound around this brake drum, which rotates freely in the non-operated state.The brake band is supported at one end against the gearbox housing.At the other end, the piston force is active during hydraulic actuation and brakes the drum until it comes to a stop.
A disadvantage of the band brake is that large radio forces act on the gearbox housing.
This principle is used, for example, in gearbox 001 of the Arosa.
Band brakes
25
During an electro-hydraulic gear change one shift element is opened, another is closed.This process occurs within fractions of a second.During this operation, the torque transmitted by the opening shift element drops. The torque transmitted by the closing element increases. The new gear meshes at the moment where the torque at the engaging shift element is greater than at the disengaging shift element.
Overlap
SP20-27
Negative overlap
Engaging shift element accepts too late.
(In other words the pressure reduction of the first shift element is too early in the case of a power upshift/braking downshiftorthe pressure increase of the engaging shift ele-ment is too late during a power downshift/over-run upshift.When the engine is operating under load, engine speed rises as a result of the separation.In overrun engine speed drops off).
This process is known as overlap.In the case of the so-called zero overlap the engaging shift element accepts as much torque as the disengaging element releases. The result is that the torque is retained.The overlap control is performed solely by means of hydraulic shift elements, actuated by the electronic shift control unit.The full working pressure is supplied to the engaging shift element.
Positive overlap
Engaging shift element accepts too early.
(In other words the pressure reduction at the dis-engaging shift element is too late in the case of a power upshift/braking downshiftorthe pressure increase of the engaging shift ele-ment is too soon during a power downshift/over-run upshift. The result is a brief blocking of the gearbox and thus a drop in torque.This can be advantageous if the engine has to be reduced from a high to a lower speed).
In addition to zero overlap, there are also negative and positive overlaps which are applied specifically for certain operating states.
t
p
Pp
Pn
P0
p = Pressuret = Time
= Pressure pattern of disengaging shift element at zero overlap
= Pressure pattern of engaging shift element at zero overlap
= Negative overlap
= Positive overlap
P0 = Point of zero overlap
Pn = Point of negative overlap
Pp = Point of positive overlap
26
Shift Elements
The overlap control can be simplified by provid-ing the assistance of freewheels.The freewheel transmits a torque only in one direction.It rotates freely in the opposite direction.It is used in order to simplify the technical design of a shift mechanism without any interruption to tractive force.It permits exact shift transition without any partic-ular requirements regarding the control of the engaging shift element.
Roller freewheel
Rollers are positioned in the gaps between the inner and outer ring.These move into the narrowing gaps in the lock-ing direction.Inner and outer ring are connected as a result. The springs press the rollers into the gap in order to achieve reliable locking.A roller freewheel is used, for example, in the 01M automatic gearbox of the SKODA OCTA-VIA.
Clamping body freewheel
This is a more involved design than the roller freewheel but enables higher torques to be transmitted within the same size of mechanism.Dumbbell-shaped clamping bodies are posi-tioned between inner and outer ring within a spring cage. They constantly make contact as a result of the spring force. In the freewheeling direction the clamping bodies are tilted and do not impede the freewheel.They move upright in the locking direction.A clamping body freewheel is used, for example, in the automatic gearbox 001 of the Arosa.
Freewheel
SP20-28
SP20-29
When the vehicle is operating in overrun, the power flow is reversed.The freewheel would open as a result and not permit any engine braking action (in a same way as the freewheel of a bike).That is why brakes or clutches are operated in parallel to the freewheel.
27
Gearbox Control
Driver decides when, to where, how quickly, sporty or economicThe "transmitters" are the accelerator pedal and the selector lever.
Operating states are responsible for producing the control pressures and the shift travel.
Electronics driving resistances influence, whether uphill/downhill, towing a trailer, driving into the wind, under power or in overrun.Sensors pass the information to the control unit.
Hydraulics are responsible for producing the control pressures and the shift travel.
This was not yet the case in early automatic gearboxes.The logic of gear selection was performed hydraulically.The operating states were detected by hydraulic, pneumatic and electrical components, converted into pressures and the gear selection activated.
In the course of the development of electronics in vehicle engineering most of these components have been replaced by corresponding electronic ones.The hydraulic gearbox control has been transformed into the electronic gearbox control.
The shift elements are actuated by the electronics.The electronic gearbox control has become the central element of the control logic and execution.The shift points are formed from a large mass of information which describe the current operating and driving situation (see also Determining shift points).
ExceptionsThe main positions of the selector lever - P - R - N - D - continue to be passed in addition mechanically by the selector lever to the selector slide valve in the hydraulic shift control unit, as before.This ensures that the automatic gearbox can continue to operate even if the electronic control unit fails.
Operating states
Automatic gearbox
Electronics Hydraulics
To simplify matters, we can say that four components are involved in the control logic and execution of a modern automatic gearbox
Driver
28
The control unit is always located separately in the vehicle, not at the gearbox.The installation point differs according to the vehicle model (e.g. in plenum chamber, in engine compart-ment, in footwell).
Gearbox Control
System overview of an automatic gearbox
The control unit determines the shift logic with permanent computer operations.It uses these as a basis for actuating the control elements of the electronic gearbox control, the most important of these being the solenoid valves which are located in the hydraulic shift control unit of the gearbox.
The advantages of the electronic gearbox control compared to conventional hydraulic systems:
– Additional signals can be processed without any major additional effort.
– The hydraulic elements can be controlled more precisely.
– The effects of wear and tear can be compensated for by adaptive pressure control.
– The shift characteristics can be designed flexibly.
– The electronics offer enhanced protection against operating errors.
– Faults which occur can be bypassed to a certain extent to ensure that the vehicle continues to operate.
– Faults which occur are stored in the fault memory for the Service sector.
The functions of the sensors and actuators of an automatic gearbox control are described in detail in Self Study Programme 21, Automatic Gearbox 01M.
The electronic gearbox control is not a system which operates in isolation. It communicates with other electronic systems in the vehicle in order to minimise the number of sensors, optimise smooth gear-shifts and enhance road safety.
A large number of signals are used in common by the engine electronics and gearbox electron-ics, for example engine speed, load signal, accelerator pedal position.In order to minimise shift pressures during oper-ation of the shift elements (e.g. multi-disc clutches, multi-disc brakes), the moment of a gearshift is advised to the engine control unit.That is why the control unit of the automatic gearbox is linked by means of a direct line to the engine control unit.During the gearshift, ignition timing is retarded, as a result of which engine torque decreases for a short time.
Certain systems of the electronic gearbox con-trol conduct information transfer with various running gear systems.If a control cycle of a stability control system is activated (e.g. electronic traction control or elec-tronic differential lock), the electronic gearbox control does not carry out a gearshift.In the event of a control cycle which is activated when starting off (anti-slip control) the electronic gearbox control makes use of second gear in order to minimise the torque.The lateral acceleration during tight cornering is detected by a sensor and transmitted to the electronic gearbox control. Gearshifts are sup-pressed during this time.
If full engine torque is required during fast accel-eration, the magnetic coupling of the AC com-pressor is switched off.The information for this is passed by the elec-tronic gearbox control to the AC control unit once the kickdown switch is operated.
30
Gearbox Control
The electronic gearbox control features strategies in the event of signal failures = emergency programme.
In the event of an input signal not being received, e.g. as a result of a cable break, the system attempts to switch to a substitute signal in order to maintain safe operation of the vehicle.
Example:The ATF temperature is detected by means of a tempera-ture sensor.If the sensor fails, an empirical value of "warm gearbox 70°C" can be used.The signal supplied by the engine coolant temperature sen-sor can also be used as a substitute.
The description of the sensors and actuators in Self Study Programme 21 Automatic Gearbox 01M also contains the relevant substitute signals.
The gearbox control with diagnostic capability stores any faults which occur in the emergency programme in the fault memory.This fault memory can be read at the diagnostic interface using a fault reader.It is thus possible to draw conclusions regarding the cause of the fault in the Service sector.
A sporadic fault occurs only for a short time and then dis-appears again.Various strategies are used depending on the type of fault:
– Control remains in emergency mode even if fault does not occur again,
– Control returns to normal mode if fault no longer occurs during several start operations.
The information remains stored in the fault memory, how-ever.
Emergency running
An emergency running mode is activated if essential sig-nals are not received or if the electronic gearbox control itself fails.In this case, a purely hydraulic mode is activated. The selector lever remains coupled mechanically to the selector slide valve in order to enable the vehicle to be driven in the emergency running mode.The automatic gearbox is in the N, R position or in a for-ward gear D depending on the position of the selector lever.The torque converter lock-up clutch is switched off.
1552V.A.G.HELPQOC
987
654
321
Note:When carrying out service work on an automatic gearbox, therefore, al-ways read the fault memo-ry first of all before carrying out any further operations.
SP17-29
Emergency programme/Self-diagnosis
31
Hydraulic System
SP21-18
The torque converter, electronics and planetary gear are ideally supplemented in the automatic gearbox by the hydraulic system.
After all, the fluid in the automatic gearbox is the working medium.
That is why particular importance is also attached to the fluid in the automatic gearbox for, in the absence of fluid, all of the functions would be lost (for the importance of the fluid refer also to the sec-tion on automatic transmission fluid).
The hydraulic fluid is pressurized by a separate oil pump and flows through the oil circuit.
The ATF pump used on almost all automatic gear-boxes is a crescent moon pump.It is driven by the vehicle engine at engine speed.Crescent moon pumps are rugged and reliable in operation and produce the working pressure required (up to approx. 25 bar).
They ensure the oil supply of:
– the shift elements– the gearbox control– the hydrodynamic torque converter– all the lubrication points of the gearbox.
The ATF is cooled in a small, separate circuit by the engine coolant.
The pressure control and pressure distribution are performed in the hydraulic shift control unit (usually positioned below the gearbox).
A crescent moon pump is also fitted, for example, to automatic gearbox 01M of the SKODA OCTAVIA which is described in Self Study Programme 21.
The ATF circuit which is similar on all automatic gearboxes, is also explained there.
Hydraulic fluid circuit/hydraulic fluid pump
Oil circuit (block diagram)
Oil pump (ATF pump)
SP21-19
32
Hydraulic System
The hydraulic shift control unit is the control cen-tre for the ATF pressure.
The ATF pressure is controlled in this unit in line with the control signals supplied by the electronic gearbox control, and distributed to the shift ele-ments.
As a rule, the shift control unit consists of several valve housings.A valve housing is the common valve body for all the valves which it contains (shift valves, control solenoid valves, pressure control valves).In addition, it contains the oil passages in accord-ance with the hydraulic diagram.Oil passages in the valve housing are designed to be free of intersections.Any intersections required are created by holes drilled in an intermediate block.This makes it possible to form oil paths in various valve housings placed one above the other.
The valves (solenoid valves) actuated electrically by the electronic control unit, are placed onto the valve housing from the outside.They are therefore easily accessible for service work and can be simply replaced.
In addition to its electrical connections to the elec-tronic control unit, the hydraulic shift control unit is also linked mechanically to the selector lever by means of a hand slide valve.
The hydraulic shift control unit is usually installed below the gearbox.In this case, the gearbox housing then contains part of the oil passages.
The oil passages can also be designed as a sepa-rate oil passage plate.
Hydraulic shift control unit
SP20-32
SP20-33
Oil passages in gearbox housing
Hydraulic shift control unit
Solenoid valves
Printed conductor along which the signals flow into the solenoid valves
33
SP20-34
The hydraulic diagram is a simplified detail from the hydraulic plan of an automatic gearbox.
We can use this diagram to explain the complicated hydraulic control labyrinth.Two shift elements are shown. Depending on the design of a gearbox, this may be six to eight friction elements (clutches and brakes) in a modern four-speed gearbox.
Diagram of hydraulic system
Shift elements
Oil pump
Pressure control valve
Control solenoid valve
Shift solenoid valve
Shift valve
Zero outflow
The diagram shows the valves in the off position
Working pressure for shift elements
Working pressure for torque converter lock-up clutch
Control valve pressure
Modulating pressure
Shift pressure
Lubrication pressure
Shift valve pressure
Control pressure for torque converter lock-up clutchRestrictorO O
Induction
Shift pressure stabilised
34
Hydraulic System
The oil in the hydraulic system has to be present at different pressure levels. Pressure control valves and control solenoid valves are used to produce the pressure stages required.
Working pressure
The working pressure is 25 bar, and is thus the highest pressure in the hydraulic system.It is produced by the oil pump and also exists directly downstream of the latter.It is stabilised by the working pressure control valve by means of a controlled zero outflow.The pressure is controlled by control pulses of the electronic gearbox control in line with the gear engaged.Depending on the gear to be engaged the working pressure is distributed to one or more shift ele-ments.This distribution of pressure is performed by a shift valve.The working pressure exists at a relevant shift element when the gear is being engaged.
Pressures in the hydraulic system
SP20-37
SP20-38
Shift valve pressureControl valve pressure
The shift valve pressure is set to 3 - 8 bar by means of a pressure control valve.It supplies the electrically controlled shift solenoid valves.
Important!Shift solenoid valves use the shift valve pressure to control downstream shift valves, which in turn control the shift elements (refer also to shift exam-ple).
The control valve pressure is likewise set by means of a pressure control valve and is 3 to 8 bar.It supplies a control pressure through a control solenoid valve to a downstream pressure control valve, for example for the torque converter lock-up clutch.
Working pressure control valve (a pressure control valve)
Pressure control valve
Pressure control valveControl solenoid
valve
Pressure control valve
Shift solenoid valve
Shift element
Shift valve
35
Modulating pressure
The modulating pressure is proportional to the engine torque, it reflects the engine load.The modulation valve (a control solenoid valve) is actuated by the electronic gearbox control on the basis of the information supplied by the engine electronics, and produces the modulation pres-sure.This is 0 to 7 bar.The modulation pressure flows to the working pressure control valve and thus influences the level of the working pressure. SP20-39
SP20-40
Shift pressureLubrication pressure
The shift pressure is 6 to 12 bar. It is used during the gear change at the shift element to be oper-ated. The shift pressure is set by the electronic gearbox control through a control solenoid valve and a pressure control valve.After the gearshift is completed, it is substituted at the shift element by the working pressure.
The lubrication pressure is 3 to 6 bar. It supplies the torque converter. The hydraulic fluid flows through the torque converter, the ATF cooler and through all the lubrication points of the automatic gearbox.
Pressure for lock-up clutch
The pressure is set by means of a control sole-noid valve and a pressure control valve and is controlled by the electronic gearbox control.The pressure is set in line with the torque to be transmitted.
SP20-44
36
Hydraulic System
Hydraulic shift elements
Shift solenoid valveShift solenoid valves pass on the oil pressure to a shift valve or reduce the oil pressure. In other words, they switch on or off and cause the shift elements to switch over, for example to initiate the shift procedure.
Solenoid valves are used in the electronically control-led automatic gearbox as hydraulic shift elements (shift solenoid valve, control solenoid valve).In addition, shift valves which operate only hydrauli-cally, are also used.
They are closed in the off position by spring force. The armature is connected to the valve plunger. When actuated by the electronic control unit, the armature is pulled against the spring force.
The valve plunger opens the passage from P to A for the shift valve pressure and closes off the out-let O.
Shift solenoid valves are actuated by means of a digital shift signal.The shift valve pressure acts as a control pres-sure on the shift valve.
The shift valve is a valve which operates purely hydraulically. Its purpose is to distribute the pres-sure to the shift elements.
As a rule, it has only two shift positions which are operated by one or two control pressures.
In the off position, the working connection A is linked to outlet O, the shift elements are thus pressureless.
In the working position, the control pressure is effective at connection X, pressure P is switched through to connection A, outlet O is shut off. Out-let L acts only as a compensation port.
The majority of shift valves are designed as slide valves and are therefore often also called slide valves or shift slide valves.
SP20-36
A
X
O P L
A
X
O P L
Shift valve
Off position
Shift position
O
P
A
SP20-41
Shift valve pressure
Armature
Coil
Valve plunger
37
O
P A
SP20-42
Control solenoid valves set a stepless oil pres-sure.
They are shut-off valves toward a zero pressure, pretensioned by spring force.When actuated, the armature is pulled against the spring force and the valve plunger opens the out-let O.Consequently, the oil pressure drops at A, all the more the greater the actuation current, which thus produces a stepless control.
Low amperage = high pressureHigh amperage = low pressure
Control solenoid valves are always used in combi-nation with a restrictor and are supplied with the control valve pressure.They do not control the oil pressure of a shift ele-ment directly, but supply the control pressure which acts through A on a downstream pressure control valve (e.g. modulating pressure).
Control solenoid valve
Armature
Coil
Control valve pressure
Valve plunger
The example of the operation is intended to clearly show us that the working pressure is not supplied to a shift element through the solenoid valve.
Off positionThe shift solenoid valve is not actuated. No con-trol pressure (shift valve pressure) exists at the shift valve.The zero outlet is open.
Working positionThe shift solenoid valve is actuated by the elec-tronic control unit of the automatic gearbox, it is operated electrically.The solenoid valve attracts a valve plunger and opens the passage for the flow of the shift valve pressure to the shift valve.The piston (slide) in the shift valve is now moved hydraulically.Consequently, the zero outlet is shut off, the con-nection for the working pressure is opened.The working pressure now acts fully on the shift element (clutch or brake, depending on the con-trol logic).
Example of operation of shift solenoid valve and shift valve - block diagram
SP20-35
AX
O P
AX
O P
Off position
Working position
Shift valve pressure
Working pressure
38
?Test Your Knowledge
Which answers are correct?Sometimes only one,but perhaps also more than one - or all of them!
Please complete these points ......................
1. In a manually shifted gearbox it is the mechanical clutch which transmits the engine torque to the gearbox.In an automatic gearbox this function is performed by the.........................................
2. Design features of an automatic gearbox are:
A. The hydraulics perform the task of synchronising the wheel speeds.B. It is possible to shift gears without any interruption to the power flow.C. All gear pairs are constantly meshed.
3. The mechanical basis of almost all automatic gearboxes are............................
4. A special design of ...................................... is the Ravigneaux gearbox
A. It has 3 planet gear sets.B. It has 2 planet gear sets with a common sun wheel.C. It has 2 planet gear sets with a common planet carrier.
5. The shift points are determined by the electronic gearbox control in a conventional way using two parameters.These are .......................... and .....................
6. The oil in an automatic gearbox is often characterized by the abbreviation ......... In addition to lubrication, it has to perform a number of other important tasks. Which answers are not correct?
A. Transmitting forcesB. Performing the synchromesh workC. Storing heatD. Performing gearshifts
?
39
?7. Which statement is correct?
A. In an automatic gearbox there are only electro-hydraulic functions.B. In an automatic gearbox too, important selector lever positions are transmitted mechanically to
the hydraulic shift control unit.
8. Which valve passes the working pressure of ....... bar to the shift elements?
A. The control solenoid valveB. The shift solenoid valveC. The shift valve
9. Complete the missing lines and valves in the hydraulic diagram.
10. The electronic gearbox control has a ................................. If, for example, a customer complains of poor performance, the first job to do is therefore ........ ......................... .........
11. When does a torque converter operate only as a clutch?
A. When the turbine wheel is stationaryB. At maximum torque boostC. When pump speed and turbine speed are practically the same.
