Self-Study Programme 20 Automatic gearbox fundamentals.pdf

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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 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.
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Contents
Power Conversion
General Design
Determining Shift Point
Automatic Transmission Fluid
Torque Converter
Lock-up Clutch
Planetary Gear Systems
Shift Elements
Multi-disc clutch
Multi-disc brakes
Band brakes
Freewheel
Gearbox Control
System overview of an automatic gearbox
Emergency programme/Self-diagnosis
Hydraulic System
Hydraulic fluid circuit/hydraulic fluid pump
Hydraulic shift control unit
Diagram of hydraulic system
Pressures in the hydraulic system
Hydraulic shift elements
Test Your Knowledge
4
7
9
14
16
18
19
23
23
24
24
26
27
28
30
31
31
32
33
34
36
38
You can find specific information on the
automatic gearbox 01M fitted to the SKODA
OCTAVIA in the Self Study Programme Booklet
21.
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Power Conversion
Why convert the power?
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.
* 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.
P=
M
•
n
9550
SP20-6
Power-torque diagram of a petrol engine
F
M=F
•
r
r
ω
M
SP20-45
Tractive force at driven
wheels
(N)
I
II
III
IV
v (km/h)
Tractive force - vehicle speed diagram
ideal tractive force curve
SP20-8
Tractive force curve of gear I to IV
q
n = 5000 rpm
o
n = 1000 rpm
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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“.
n
n
SP20-7
SP18-22
i=
Speed of driving gear
n
1
=
n
2
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.
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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 determina-
tion of the shift point using fuzzy logic, such arguments no longer hold true.
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