Automatic Gearbox - Fundamentals -
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.
SP20-3
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 efficiency. 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.
2
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 Multi-disc clutch Multi-disc brakes Band brakes Freewheel
23 23 24 24 26
Gearbox Control System overview of an automatic gearbox Emergency programme/Self-diagnosis
27 28 30
Hydraulic System Hydraulic fluid circuit/hydraulic fluid pump Hydraulic shift control unit Diagram of hydraulic system Pressures in the hydraulic system Hydraulic shift elements
31 31 32 33 34 36
Test Your Knowledge
38
You can find specific information on the automatic gearbox 01M fitted to the SKODA OCTAVIA in the Self Study Programme Booklet 21.
3
Power Conversion Why convert the power?
P=
Let’s recall a number of basic rules of automotive engineering.
M•n 9550
The power for driving a motor vehicle and the necessary ancillaries (e.g. power-assisted steering, air conditioning 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.
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
Power-torque diagram of a petrol engine
F
r
M=F•r
ω
M SP20-45
Tractive force at driven wheels (N)
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 circumference 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 possess a certain lever arm „r“.
I
II III IV
v (km/h)
SP20-8
Tractive force - vehicle speed diagram ideal tractive force curve
* 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.
4
Tractive force curve of gear I to IV ●
n = 5000 rpm
o n = 1000 rpm
n
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.
n
The manual gearbox
A manually-shifted gearbox is generally designed as a countershaft gearbox. We are familiar with this from all SKODA models.
SP20-7
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 coupled 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“.
i=
Speed of driving gear n1 = n2 Speed of driven gear
M Output = M Input • i
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 operating under load. This requires a special mechanical device - the clutch. When engaged, the clutch transmits the engine torque to the gearbox and the driven wheels and, when disengaged, it interrupts the power flow.
SP18-22
5
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
SP20-13
SHIFTING GEARS IS HARD WORK - IS THE OTHER OPINION that is why, have the work done for your
automatic gearbox!
SP20-14
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 determination of the shift point using fuzzy logic, such arguments no longer hold true.
6
General Design The differences What are the differences between a car fitted with a manual gearbox and a car fitted with an automatic gearbox? The power flow in both cars is the same Engine - Clutch - Gearbox - Differential - Drive Shafts - Running Gear with manual gearbox
with automatic gearbox
SP15-19
SP15-18
Mechanical clutch, operated manually
Torque converter automatically separates the rotating engine from the stationary gearbox when the car is not moving, but also performs additional tasks and can be regarded as a hydraulic gearbox.
Manual gearbox with countershaft
Planetary gear
with positive-locking gearshift mechanism (using gearshift lever, shift fork, sliding sleeve) for transmitting the torque
Is an essential requirement for at all making use of automatic control, automatic non-positive torque transmission through clutches and brakes.
Driver involved in shifting gears. Eyes and ears sense the driving situation.
More relaxed driving, sensors detect the driving resistances. Electronic gearbox control processes information for selecting a gear which is engaged by hydraulic shift elements.
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).
Automatic gearboxes operate without any interruption to the power flow and therefore accelerate continuously. When it comes to acceleration performance, they are the equal of a manual gearbox.
Increased physical demand on the driver, full concentration on driving situations.
Driving comfort is enhanced, stress is reduced, overall safety is improved.
7
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
Automatic gearbox - Main components Torque converter For starting off, for increasing torque and minimizing vibrations
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
8
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 when a gearshift should be made?
adaptive shift curve
2. Who supplies this information to the control unit?
sensors
3. How are the gearshifts effected?
hydraulically by means of actuators/solenoid valves
Let's look in this connection at the system functions for an automatic gearbox, as exist also in the SKODA OCTAVIA.
Sensors
Actuators
Accelerator pedal position
Gear selection valve
Vehicle speed
Torque converter lock-up valve
Gearbox speed
Main pressure valve
Engine speed
ATF temperature
Selector lever lock Control unit Engine torque reduction
Selector lever position
Starter lockout
Brake pedal operation
Idle speed increase Self-diagnosis
Kickdown switch
Air conditioning
Selector lever display
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.
9
Determining Shift Point
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 function of vehicle speed and accelerator pedal position.
3–4 4–3 Vehicle speed
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.
Upshift Downshift
Position of accelerator pedal
Sport characteristic line ECO characteristic line
SSP172/116
During the initial period of electronic gearbox controls, therefore, only fixed shift characteristics were programmed. As the electronic gearbox control underwent further development, it was possible to select between two programmes: - a sporty - and an economic programme
3–4 3–4 4–3
Vehicle speed
A separate switch on the selector lever provided the driver with the possibility of selecting one or the other shift characteristics. A subsequent development was to automate this switchover. This was done by sensing the rate at which the accelerator pedal was depressed. Nevertheless, as with the previous system, this was also an absolute decision "ECO" or "SPORT"
Sport ECO
4–3
Position of accelerator pedal SSP172/117
10
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 constantly 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
1
%
90 80 70 60 50 40 30 20 10 0 0 Sporty factor
ECO
100 % Sporty factor
Sport
Accelerator pedal speed SP20-11
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.
11
Determining Shift Point What does fuzzy logic mean? 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 cameras 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". SP20-46
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 computer 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.
Switching states hot
80 °C
1
1
Yes
Switch closed or
cold
0
0 0 °C Temperature
No
Switch open
SSP172/107
12
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".
80 °C
hot 1 almost hot too warm
warm
lukewarm cool almost cold cold 0
10
20
30
40
50
60
70
80 °C SP20-10
The upper limit of "hot" and the lower limit of "cold", as well as all the intermediate levels, are assigned to precise temperatures.
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 intermediate values.
cold
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.
lukewarm 19 °C hot
The fuzzy logic recognizes "lukewarm".
SP20-9
13
Automatic Transmission Fluid Automatic Transmission Fluid = ATF (Automatik Transmission Fluid) 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 lock-up clutch) – Lubricate parts (all rotating gearbox parts) – Dissipate heat – Remove abrasion. 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.
SP20-4
Note: Use only the automatic transmission fluid approved by the vehicle manufacturer. Other fluids or additives result in changes to the properties and have a detrimental effect on the operation and life of the gearbox. In particular, water elements in the automatic transmission fluid can cause operational problems. The ATF is kept clean by passing it through a filter as it flows out of the oil pan. A strong permanent magnet in the oil pan traps any metallic abrasion.
14
Additive
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 worldwide.
SP20-5
ATF level/ATF temperature 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 and ATF temperature have a major influence on proper operation of an automatic gearbox.
ATF level
Too high
Too low
Clutches/brakes close too slowly ATF temperature too high
ATF foams Gearshifts performed with time lag
Gearshifts take too long
ATF flows out of breather Operational problems in gearbox
Service Inspect ATF level and adjust, if necessary
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.
Pay particular attention to the inspection temperature of the ATF if adjusting the ATF to the correct level. The inspection temperature should be measured 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 temperature increase by changing the oil pressure in order to ensure uniformly smooth gearshifts. 15
Torque Converter The hydrodynamic torque converter The hydrodynamic torque converter is, in fact, an additional hydrodynamic transmission to the automatic 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 engineering. 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.
SP20-15
Impeller Pump wheel
Turbine wheel
Components
Propulsion effect
16
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.
Torque increase In the torque conversion phase, the torque converter converts the reduction of rotational speed into an increase in torque. Pump wheel
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.
Turbine wheel
Freewheel
Impeller
SP21-31
The torque converter slip is the operationally necessary 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 gearbox 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.
Transmitting energy by means of ATF flow
Starting-off Torque conversion phase 1
The turbine wheel is stationary. Pump wheel is rotating. ATF flow sharply deflected. High slip. Gearing down. Maximum boost in torque.
Torque conversion phase 2
Turbine speed increases. ATF flow is "stretched". Slip is reduced, transmission ratio decreases. Torque boost decreases.
Clutch phase
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. 17
Lock-up Clutch Torque converter lock-up clutch - a mechanical clutch
Why is the torque converter locked up? Once the clutch phase is reached, in other words the torque ratio is 1:1, the torque converter 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 highperformance engines which operate at high speeds.
Lock-up clutch
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.
Turbine wheel
SP21-34
Torque flow
The lock-up clutch is integrated in the housing of the torque converter. It is provided with a ringshaped friction lining and is connected to the turbine wheel. It is pressed by means of oil pressure 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 torsion dampers for reducing engine torsional oscillations. It is the control unit of the automatic gearbox which determines when the lock-up clutch closes or opens.
Operation
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 automatic gearbox by 2 to 8 % by means of a torque converter lock-up clutch. Self Study Programme 21 contains further information on the hydraulic control of a torque converter lock-up clutch.
18
Planetary Gear Systems The gear change - manual gearbox
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 gear change - automatic gearbox
The automatic control unit cannot derive from the traffic situation when it would be the right moment to interrupt the power flow.
What alternatives are there?
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 automatic gearboxes.
1
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
2
3
4 SP20-31
– a central gear the 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.
19
Planetary Gear Systems The sun wheel -1- rotates in the inside on a central shaft. The planet gears -2- mesh with the teeth of the sun wheel.
1
2 3
4 SP20-2
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 movement 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 direction of rotation is reversed. If two parts are held fixed, the planet gear blocks and the ratio is 1:1.
SP20-16
- Hollow gear fixed - Sun wheel driving = large down gearing ratio
20
SP20-17
- Sun wheel fixed - Hollow gear driving = low down gearing ratio
SP20-18
- Planet carrier fixed - Sun wheel driving = reversal of direction of rotation
It is possible to form additional transmission ratio versions from a combination of driving and braking (holding fixed) parts
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
Hollow gear
The parts of the planetary gear set therefore have to be braked or driven from outside.
Planet gears with planet carrier Sun wheel
Rotation superposed on that of hollow gear, superposition of speed (escalator effect)
Turbine shaft
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.
SP20-20
Block diagram of input and output of a planet gear set
During braking, the brakes are supported against the gearbox housing (refer also to the section on shift elements).
21
Planetary Gear Systems 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. Wilson gearbox
➙
Simpson gearbox
➙
Ravigneaux gearbox
➙
The different combinations and technical standard designs are named after their inventors.
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 permanently 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 connected to each other. The forward gears are each driven through the hollow gears. This design was often used in the age of threespeed automatic gearboxes.
consists of 2 planet gear sets with a common planet carrier. This design is similar to that used in the 01M automatic 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 suitable for use in front-wheel-drive vehicles.
SP20-19
22
Shift Elements Multi-disc clutch Each gear features at least one shift element which creates the power flow by means of friction.
Externally-toothed disc, positively connected to outer part
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 externallytoothed discs which are both connected to rotating 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.
Internally-toothed disc, positively connected to inner part
Outer part
Externally-toothed disc
SP20-22
Internallytoothed disc
Ball valve
Inner part
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 highstrength 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.
Plunger
SP20-21
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.
SP20-25
Disc carrier (clutch bell) for accommodating externally-toothed discs
23
Shift Elements Multi-disc brakes 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 connected 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 compresses 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 automatic gearbox of the SKODA OCTAVIA.
Externally-toothed disc, supported at gearbox
SP20-24
Internally-toothed disc, positively meshed with rotating part
Gearbox housing
Externally-toothed disc, fixed
Internallytoothed disc
Band brakes
Rotating part
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.
Plunger
SP20-23
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.
SP20-26
24
Overlap 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.
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.
p t
p
= =
Pressure Time
=
Pressure pattern of disengaging shift element at zero overlap
=
Pressure pattern of engaging shift element at zero overlap
=
Negative overlap
=
Positive overlap
Pp P0 Pn
t
P0 =
Point of zero overlap
Pn =
Point of negative overlap
Pp =
Point of positive overlap
SP20-27
In addition to zero overlap, there are also negative and positive overlaps which are applied specifically for certain operating states. Negative overlap
Positive overlap
Engaging shift element accepts too late.
Engaging shift element accepts too early.
(In other words the pressure reduction of the first shift element is too early in the case of a power upshift/braking downshift or the pressure increase of the engaging shift element is too late during a power downshift/overrun upshift. When the engine is operating under load, engine speed rises as a result of the separation. In overrun engine speed drops off).
(In other words the pressure reduction at the disengaging shift element is too late in the case of a power upshift/braking downshift or the pressure increase of the engaging shift element is too soon during a power downshift/overrun 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).
25
Shift Elements Freewheel The overlap control can be simplified by providing 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 particular requirements regarding the control of the engaging shift element.
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.
Roller freewheel Rollers are positioned in the gaps between the inner and outer ring. These move into the narrowing gaps in the locking 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 OCTAVIA.
Clamping body freewheel SP20-28
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 positioned 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.
SP20-29
26
Gearbox Control To simplify matters, we can say that four components are involved in the control logic and execution of a modern automatic gearbox
Operating states
Driver
Electronics
Hydraulics
Automatic gearbox Driver
decides when, to where, how quickly, sporty or economic The "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). Exceptions The 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.
27
Gearbox Control System overview of an automatic gearbox 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 compartment, in footwell). Engine load Accelerator pedal position Gearbox speed
Hydraulic shift control unit - solenoid valves
Vehicle speed Selector lever lock Engine speed Starter lockout Reversing light Position detection Idle speed increase Engine torque reduction
Brake pedal operation Diagnostic connection
Kickdown
AC cut-off ATF temperature SP20-30
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.
28
Communication with other vehicle systems 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 gearshifts and enhance road safety.
Engine electronics
Running gear electronics
Air conditioning
A large number of signals are used in common by the engine electronics and gearbox electronics, for example engine speed, load signal, accelerator pedal position. In order to minimise shift pressures during operation 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 control 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 electronic 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 suppressed during this time.
If full engine torque is required during fast acceleration, the magnetic coupling of the AC compressor is switched off. The information for this is passed by the electronic gearbox control to the AC control unit once the kickdown switch is operated.
29
Gearbox Control Emergency programme/Self-diagnosis 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 temperature sensor. If the sensor fails, an empirical value of "warm gearbox 70°C" can be used. The signal supplied by the engine coolant temperature sensor can also be used as a substitute.
1 4 7 C
2 5
8 O
3 6
9 Q
HELP
V.A.G. 1552
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 disappears 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, however. Emergency running An emergency running mode is activated if essential signals 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 forward gear D depending on the position of the selector lever. The torque converter lock-up clutch is switched off.
30
SP17-29
Note: When carrying out service work on an automatic gearbox, therefore, always read the fault memory first of all before carrying out any further operations.
Hydraulic System Hydraulic fluid circuit/hydraulic fluid pump 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 section 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 gearboxes 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).
SP21-19
Oil circuit (block diagram)
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. SP21-18
The ATF circuit which is similar on all automatic gearboxes, is also explained there.
Oil pump (ATF pump)
31
Hydraulic System Hydraulic shift control unit The hydraulic shift control unit is the control centre for the ATF pressure.
Hydraulic shift control unit
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 elements. 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 accordance 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.
SP20-32
Solenoid valves Printed conductor along which the signals flow into the solenoid valves
In addition to its electrical connections to the electronic 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 separate oil passage plate.
SP20-33
Oil passages in gearbox housing
32
Diagram of hydraulic system The hydraulic diagram is a simplified detail from the hydraulic plan of an automatic gearbox. Shift elements
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.
The diagram shows the valves in the off position SP20-34
Oil pump
Induction Working pressure for shift elements
Pressure control valve
Working pressure for torque converter lock-up clutch Shift valve pressure
Control solenoid valve
Shift solenoid valve
Control valve pressure Modulating pressure Shift pressure
Shift valve
OO
Shift pressure stabilised
Zero outflow
Lubrication pressure
Restrictor
Control pressure for torque converter lock-up clutch
33
Hydraulic System Pressures in the 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 elements. 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.
SP20-37
Working pressure control valve (a pressure control valve)
Shift element
Shift valve pressure Control valve pressure
Shift valve Shift solenoid valve
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.
Pressure control valve
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 example).
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 lockup clutch.
34
Pressure control valve Control solenoid valve
SP20-38
Pressure control valve
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 pressure. 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
Shift pressure Lubrication pressure The shift pressure is 6 to 12 bar. It is used during the gear change at the shift element to be operated. 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.
SP20-40
Pressure for lock-up clutch The pressure is set by means of a control solenoid 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
35
Hydraulic System Hydraulic shift elements Solenoid valves are used in the electronically controlled automatic gearbox as hydraulic shift elements (shift solenoid valve, control solenoid valve). In addition, shift valves which operate only hydraulically, are also used.
Shift 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.
Shift solenoid valve
Shift valve pressure
P
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.
Valve plunger A Armature
The valve plunger opens the passage from P to A for the shift valve pressure and closes off the outlet O.
Coil O
Shift solenoid valves are actuated by means of a digital shift signal. The shift valve pressure acts as a control pressure on the shift valve.
SP20-41
Shift valve
Off position
The shift valve is a valve which operates purely hydraulically. Its purpose is to distribute the pressure to the shift elements. As a rule, it has only two shift positions which are operated by one or two control pressures.
A
X
O
P
In the off position, the working connection A is linked to outlet O, the shift elements are thus pressureless.
L
A
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. Outlet L acts only as a compensation port.
X
O
P
L SP20-36
Shift position
36
The majority of shift valves are designed as slide valves and are therefore often also called slide valves or shift slide valves.
Example of operation of shift solenoid valve and shift valve - block diagram
Off position The shift solenoid valve is not actuated. No control pressure (shift valve pressure) exists at the shift valve. The zero outlet is open.
A X
Off position
O
P
A X Working position O
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.
P Working pressure
Shift valve pressure SP20-35
Working position The shift solenoid valve is actuated by the electronic 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 connection for the working pressure is opened. The working pressure now acts fully on the shift element (clutch or brake, depending on the control logic). Control solenoid valves set a stepless oil pressure.
Control solenoid valve 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 outlet O. Consequently, the oil pressure drops at A, all the more the greater the actuation current, which thus produces a stepless control.
Control valve pressure
P
A
Low amperage = high pressure High amperage = low pressure
O Valve plunger
Armature Coil
Control solenoid valves are always used in combination with a restrictor and are supplied with the control valve pressure. They do not control the oil pressure of a shift element directly, but supply the control pressure which acts through A on a downstream pressure control valve (e.g. modulating pressure).
SP20-42
37
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. B. C.
?
The hydraulics perform the task of synchronising the wheel speeds. It is possible to shift gears without any interruption to the power flow. 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. B. C.
It has 3 planet gear sets. It has 2 planet gear sets with a common sun wheel. 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. B. C. D.
38
Transmitting forces Performing the synchromesh work Storing heat Performing gearshifts
7.
Which statement is correct? A. B.
8.
Which valve passes the working pressure of ....... bar to the shift elements? A. B. C.
9.
In an automatic gearbox there are only electro-hydraulic functions. In an automatic gearbox too, important selector lever positions are transmitted mechanically to the hydraulic shift control unit.
The control solenoid valve The shift solenoid valve The shift valve
?
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?
When the turbine wheel is stationary At maximum torque boost When pump speed and turbine speed are practically the same.
Answers: 1. Torque converter; 2. B;C; 3. Planetary gear 4. Planetary gear, C; 5. Road speed, accelerator pedal position; 6. ATF;B,C; 7. B; 8. approx. 25, C; 9. Page 33; 10. Diagnostic capability, to interrogate the fault memory; 11. C.
A. B. C.
39
