Conference PaperPDF Available

Lubricating oils for modern automatic transmissions of motor vehicles

Authors:

Abstract

Automatic gearboxes of motor vehicles become more complex mechanical assemblies intended for transmission of torque from the engine to the differential of a motor vehicle. In order to be a core function of the transmission performed, it is necessary to provide a certain quality of working fluid - oil whose role in the multiple transmission. Multiple roles ATF oil is reflected in the lubrication of the planetary gear as main gear train torque, ensuring precision and quality of work of the hydraulic system to switch gears and drive hydraulic torque converter. In addition to these functions, in newer hybrid transmissions, oil must lubricate and certain elements of the drive electric motor / generator, in order to ensure that longer-lasting working life of the transmission. The paper will be processed latest standards prescribed by the manufacturer of gearboxes and necessary physical and chemical characteristics of ATF oils necessary for a reliable and long term operation of modern automatic transmissions.
_____________________________________________________________________________
451
LUBRICATING OILS FOR MODERN AUTOMATIC TRANSMISSIONS
OF MOTOR VEHICLES
Mitar Jocanović
1
, Velibor Karanović
2
, Marko Orošnjak
3
, Nebojša Nikol
4
,
Darko Knežević5
Abstract: Automatic gearboxes of motor vehicles become more complex mechanical
assemblies intended for transmission of torque from the engine to the differential of a
motor vehicle. In order to be a core function of the transmission performed, it is
necessary to provide a certain quality of working fluid - oil whose role in the multiple
transmission. Multiple roles ATF oil is reflected in the lubrication of the planetary gear
as main gear train torque, ensuring precision and quality of work of the hydraulic
system to switch gears and drive hydraulic torque converter. In addition to these
functions, in newer hybrid transmissions, oil must lubricate and certain elements of the
drive electric motor / generator, in order to ensure that longer-lasting working life of the
transmission. The paper will be processed latest standards prescribed by the
manufacturer of gearboxes and necessary physical and chemical characteristics of
ATF oils necessary for a reliable and long term operation of modern automatic
transmissions.
Key words: Automatic Transmission Fluid, oil standards, process lubrication, tribology,
hybrid transmission
1 INTRODUCTION
The world's largest car manufacturing companies have always been trying to
install the most economic and the most efficient engines and transmission systems in
their cars. Recently we have been witnessing the application of combination of internal
combustion engine with hybrid transmission systems, which transforms mechanical
1
PhD, Mitar Jocanović, University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia,
mitarj@uns.ac.rs
2 PhD, Velibor Karanović, University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia,
velja_82@uns.ac.rs
3 MSc, Marko Orošnjak, University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia,
orosnjak@uns.ac.rs PhD,
4 PhD, Nebojša Nikolić, University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia,
nebnik@uns.ac.rs
5 PhD, Darko Knežević, University of Banja Luka, , University of Banja Luka, Faculty of Mechanical
Engineering, BiH, darko.knezevic@unibl.rs
Mitar Jocanović, Velibor Karanović, Marko Orošnjak, Nebojša Nikolić, Darko Knežević
452
work into electrical energy for battery charging, while the vehicle travels, whenever it is
possible.
Design of modern automatic transmissions, containing in its module
motors/generators for the transformation of mechanical energy into electrical energy
and vice versa, differs from manufacturer to manufacturer.
Figure 1 shows a simplified model of transformation of mechanical energy into
electrical energy; depending on the required torque at the wheels of the vehicle.
Figure 1 One of the methods of transformation of mechanical energy into electrical
energy and vice versa in modern hybrid transmissions [1]
Figures 2 and 3 show two entirely different designs of automatic/hybrid
transmissions that are installed in different vehicles (BMW and Toyota). The concept of
transformation is similar, but the design is different.
Figures 2 and 3 Examples of different designs of automatic/hybrid transmissions
(ZTF and Toyota)
What is common in all types of hybrid transmissions is that in order to enable a
Lubricating oils for modern automatic transmissions of motor vehicles
453
quality and long-lasting exploitation of the transmission, it is necessary to provide an
adequate and quality working fluid having required physical and chemical
specifications.
2 AUTOMATIC TRANSMISSION FLUID
Figures 2 and 3 show that design concepts in ZTF and Toyota are completely
different. ZTF design uses a traditional automatic transmission with an incorporated
electric motor at the entrance of the transmission, while Toyota simplifies the design
with a central shaft that is attached to two generators/motors and to an auxiliary shaft
that has a special transmission speed converter, which provides different rotation
speeds of the main shaft. For the operation of the above mentioned transmissions a
special hydraulic system is used that provides proper operation of working elements of
the transmission.
In order to provide perfect operation of all components and transmission
assemblies, it is necessary to use corresponding working fluid. In these types of
transmissions, the working fluid, that is called ATF - Automatic Transmission Fluid, has
to meet specific functions regarding:
Lubrication function (planetary gears that constitute the main assembly of
automatic transmission system, all bearings, especially generator/motor
bearings in hybrid transmission);
Hydraulic oil function (within a complex hydraulic system that is used in
automatic/hybrid transmissions provides easy shifting of gears;
With certain design types it provides a smooth operation of the hydraulic
torque converter that operates as a coupling connecting the driving shaft
with the system of planetary gears;
In addition to all of the above mentioned tasks, automatic transmission fluid
must retain a corresponding coefficient of friction to be able to provide
power transmission between the automatic transmission clutch plates
without slipping
In addition to the above listed functions, ATF is designed to provide other important
functions as follows: cooling, power transmission to the gear sides or torque
converters, wear and corrosion protection, seal compatibility, high oxidation stability
and foaming prevention [2].
2.1 Physical and chemical properties of ATF
ATF is designed to provide a long-lasting exploitation of automatic/hybrid
transmissions. In order to make it possible, it is necessary to provide, in addition to the
base fluid, also an additive package providing the required physical and chemical
properties of the fluid.
The most important physical and chemical properties of ATF are:
Low temperature Automatic/hybrid transmissions are complex mechanical
and electronic systems that require corresponding rheological properties of
oils also at the lowest temperatures. Low-temperature properties of oils are
expressed as the highest allowed value of dynamic viscosity at the
temperature of -40 °C measured by Brookfield viscometer. In order to
achieve the best low-temperature properties of oil, it is necessary to use the
best quality API IV and API V group base oils for blending.
Shear stability Mechanical load that automatic transmission fluids are
exposed to, may cause the break of polymer chains, especially those
Mitar Jocanović, Velibor Karanović, Marko Orošnjak, Nebojša Nikolić, Darko Knežević
454
having larger molecular weight, which results in permanent decrease in
viscosity. Such problems are solved with application of shear-stable
viscosity modifiers along with the use of quality base oils;
Oxidation and thermal stability New designs of automatic transmission are
becoming increasingly smaller and lighter in construction, however, such
designs are required to transmit higher torque values, resulting in a higher
thermal load per volume unit of fluid. Additionally, there is a permanent
requirement to have the longest possible service interval of the fluid. For
these reasons, very stable automatic transmission fluids in respect of
oxidation, are required.
ATF must have a high degree of oxidative stability also at temperatures up to
150°C. At such high temperatures, and in the presence of air, the process of increased
oil oxidation is developing together with many other side effects such as: generation of
increased acid level (intensive oxidation has corrosive effects upon liners and other
construction materials as well as on the sealing materials and clutch plates); change in
friction properties of the fluid (generates excessive clutch plates slipping); fluid
viscosity increases (it causes decrease in automatic transmission functionality);
formation of resin and varnish (due to increased oxidation they may cause congestion
in the control hydraulic valves that may lead to transmission failure); formation and
accumulation of sludge (the result of oxidation may cause congestion of the control
hydraulic valves and lubrication pipes).
Antiwear properties automatic/hybrid transmission fluids must have good
antiwear properties that are required to provide durability of assemblies and
parts of automatic transmission, as well as exploitation properties of the
transmission. Normally, this property also has impact on the possibility of
extension of the fluid service interval.
Friction properties it is an important property for those transmission
designs which contain in their assemblies lamellar couplings. The friction
coefficient value of the fluid must be within the allowed range because the
change of gears is performed by means of lamellar couplings and band
brakes coated with friction lining. The friction property of the fluid makes a
basic difference between automatic transmission fluids, i.e. between
specifications of different manufacturers. Further development of automatic
transmission fluids is leading toward development of special additives, the
so-called friction modifiers which provide an optimum friction coefficient with
its proper existence within the determined ranges.
Viscosity grades for ATF fluids are not subject to recommendation (SAE,
ISO VG), because they are an integral part of the respective specification.
Working properties are defined in General Motors and Ford specifications
shown in Tables 1 and 2. Most of other manufacturers (MAN, Mercedes
Benz, Renk, Voith, ZF) accept one these specifications, although some of
them require additional tests to secure compatibility with their automatic
transmissions (such as Toyota Table 3).
Lubricating oils for modern automatic transmissions of motor vehicles
455
2.2 ATF Specification
Table 1. General Motors [3] ATF Specification
Type A
The first automatic transmission specification released in 1949
Type A Suffix A
(TASA)
Models 1957. An outdated specification, but sill in use. Suffix A means
improved oxidation properties.
DEXRON B
Models 1967
DEXRON II
Models 1973. The most used specification. Includes special requirements
for low static coefficient of friction, high oxidation stability and good
corrosion protection in the wet chamber.
DEXRON II D
Models 1981
DEXRON II E
Models 1991. Includes, in addition to laboratory tests of physical and
chemical properties, some special requirements for low static coefficient of
friction, high oxidation stability, flash point and fire point, antifoaming agents,
good corrosion protection in wet chamber, seal compatibility requirements.
DEXRON III F
Models 1994. The Specification contains upgraded characteristics in relation
to the previous, and primarily higher flash points and fire points and lover
flash and fire tendency. A successor of DEXRON II D and DEXRON Il E.
DEXRON III G
Successor of DEXRON III (F) automatic transmission fluids. According to
Specification, a fluid similar to DEXRON Il E, however with upgraded
antioxidation and antiwear properties. Launched in 1997.
DEXRON III H
DEXRON III H launched in June 2003, to replace DEXRON III G fluids.
They contain base oils of very high oxidation stability (group 2 and 3). Fluids
from this group have excellent friction and antiwear properties, better flash
and fire control and longer service interval.
DEXRON VI
Specification released in 2005 to replace DEXRON III H fluids. The
Specification provides a higher fluid slipping stability, a good oxidation
stability and good antifoaming properties. Fluids that satisfy this
Specification can be used in extended service interval and provide
significant saving of energy.
Table 2. Ford [4, 5] ATF Specification
SQM-2C9007A (M2
C33-G)
This type of fluid has completely different friction properties for all other
ATF types for which reason it must not be blended with other fluid types.
Models since 1967.
SQM-2C9010A (M2
C138-CJ)
Introduced in 1973.
ESPM-2C166-H (M2
C166-H)
Introduced in 1981.
MERCON
Introduced in 1987. The Specification specifically points out the
requirements related to wear, antifoaming, anticorrosion, seal
compatibility and flash point.
MERCON
Introduced in 1992. The revision of the previous MERCON. It requires
more strict control of volatility, colour and all other properties from the
previous Specification.
MERCON V
Introduced in 1996. The Specification requires more strict wear, friction
and oxidation tests.
MERCON SP
Introduced in 2004. It has a better oxidation stability, antiwear and
friction properties in relation to previous fluid types.
Mitar Jocanović, Velibor Karanović, Marko Orošnjak, Nebojša Nikolić, Darko Knežević
456
Tabela 3. Toyote [5, 6] ATF Specification
Toyota Types T,
T-II and T-III
Toyota Type T-IV
Toyota WS
3 CONCLUSION
On the basis of the above presented ATF Specifications and required physical
and chemical properties, we can conclude the following:
Each type of transmission generally requires a specific type of working fluid,
especially if it is automatic transmission, or even more, modern concept of
automatic/hybrid type;
Fluids designed for lubrication of modern automatic and automatic/hybrid
transmissions have specific values of viscosity, friction coefficient and
additive package. ATF types are designed specifically for operation in
automatic and automatic/hybrid transmissions.
ATF types are not interchangeable. It is not allowed to blend different types,
even in cases when they have the same formulation but different base;
The car and transmission manufacturer’s recommendation must be strictly
observed regarding the use of ATF of required quality and compatibility;
It is allowed to use ATF types of other manufacturers provided that they
have specifications designed for the respective type of transmission.
REFERENCES
[1] Toyota Prius; http://www.zerauto.nl/2014/01/06/toyota-investeert-in-zeldzame-
aardmetalen/;
[2] Leslie R. Rudnick, Synthetics, Mineral Oils, and Bio-Based Lubricants, 2006.
[3] General motors oil specification,
http://www.oilspecifications.org/general_motors_gm.php;
[4] James Dickey, The History of ATF, 1939-2006,
http://www.lifeautomotive.com/pdfs/history_of_atf.pdf
[5] Ralph Kalal, SELECTING THE PROPER AUTOMATIC TRANSMISISON FLUID,
2011, https://www.cartechbooks.com/techtips/autotransfluid;
[6] Transmission Fluid Guide NO. TB 150122.1, Technical Service Bulletin
Transmission Fluid Guide, 2015, http://martinlubricants.com/technical-service-
bulletin-transmission-fluid-guide/;
ResearchGate has not been able to resolve any citations for this publication.