Page 137 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Transmissions and driveline CHAPTER 5.1

Firstly, considering the temperature constraints. Many High-temperature capability – What is the highest
transmission units are installed under the bonnet (hood), temperature the transmission can see in service?
alongside, and attached to, the engine. The influence of Remember it is very difficult to specify ‘excursions’
the engine, cooling pack and exhaust system, together to high temperatures, as it is almost impossible to
with any air ducting, can lead to extremely high air tem- predict the duration and frequency over the life of
peratures around the transmission. This air temperature a vehicle
can certainly reach similar temperatures to the required Viscosity grade and load carrying additives – What
temperature of the transmission oil. Unless the trans- grade of oil is required to provide adequate gear
mission has cooling oil piped to a remote cooler or heat and bearing life? In certain cases this is as dependent
exchanger, the unit must rely on convection to remove any on the action of the additives as on the base oil.
excess heat. This is obviously only possible if there is air of Air release and foaming – This tendency of the oil can
a lower temperature than the oil passing over the outside be vital in certain cases. If the oil behaves badly, it can
of the casing. lead to oil being ejected from the breather
Ideally, the transmission oil temperature will warm up system.
quickly on start-up of the vehicle, particularly if the Corrosion inhibition – Components within the trans-
ambient temperatures are near or below freezing. This mission do get exposed to moisture, mostly from
will enable the transmission to operate efficiently for as condensation. Because of this, the oil needs to pre-
much of the time as possible. The transmission oil will vent the surfaces from corroding, particularly while
obviously start near to the ambient air temperature and the transmission is stood for a long duration.
at low temperatures can be extremely viscous. This can Friction modification – The level of friction, which
be a severe problem with automatic and CVT trans- occurs at the rubbing surface of some parts, needs
missions particularly as the lubricant also acts as a hy- to be controlled within certain limits. An example of
draulic control fluid so fluidity is important. this is in automatic gearbox clutch packs, where too
At the other operating extreme we will want to pre- high a friction could cause problems as significant as
vent the oil getting too hot and compromising the dura- too low a value.
bility of the oil itself or the components in the unit. Viscosity modification – The viscosity grade of the
Maximum operating temperatures are often around 140 base oil used may not be quite what is required.

or 150 C, but it should be noted that these temperatures Additives can be used to improve the rate at which
require a modern specification lubricant. Many older viscosity changes with temperature, providing that

formulations will start to degrade much above 110 C. It the oil is thicker at high temperatures to protect the
can sometimes be very difficult to control the lubricant to running parts, while retaining the required fluidity at
a reasonable upper operating limit when the vehicle is low temperatures.
operating in high ambient temperatures and working
hard. For example when towing a trailer with ambients Finally, other aspects of the operating environment

around 50 C in the Gulf States. also need to be considered. The corrosion of the outside
From the above discussion, it can be seen that the of the transmission case is rarely too much of a concern,
control of the lubricant temperature is not a trivial but if ferrous materials are used then they may need to be
problem and many companies are working towards protected by painting. Similarly, immersion in water is
management of the transmission in much the same way unlikely to cause a significant problem, but for off road
as the engine. That is warming the unit up quickly with vehicles, this needs to be considered along with possible
waste heat from the engine but then controlling the damage from rough terrain or sand etc. Tests on the
upper temperature, perhaps to a temperature below that transmission will be used to validate the design and could
of the engine coolant. Simply linking the cooling systems include salt spray, wading, and various types of off road
of the engine and gearbox is not necessarily the answer. driving. The environment within the engine bay may also
Part of this development also entails careful consider- require the outer transmission parts to resist degradation
ation of the lubricant itself. from fuels, antifreeze, brake fluid, etc.
There is not the space here to discuss the choice of
lubricant in any depth. It is perhaps, sufficient to high- 5.1.6.2 Efficiency
light some of the main considerations.
Before continuing with this section, an important point
Cold-temperature fluidity – Does the lubricant have to appreciate is that the efficiency (or losses) of a trans-
to act as a hydraulic fluid? If so, how cold is the mission unit can be considered in three ways:
transmission expected to operate, and how critical is The load (torque) related efficiency, this occurs
immediate pressure or oil flow from the pump? largely as a result of the friction losses at the gear

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