Modern Automatic Transmission Fluids (ATF’s) are formulated with the most complex chemistry of all lubricating fluids. During the late 1930’s General Motors developed the first truly automatic transmission that used hydraulic fluid to change gears. It was introduced as the Hydra-Matic transmission in their 1940 Oldsmobile range. Take a trip down memory lane and experience the introduction of the Hydra-Matic auto box by visiting www.youtube.com/watch?v=8vv400bysiM
Today’s automatic transmissions are worlds apart from the original designs with only two forward gears which were used during the roaring forties of the previous century when all cars would run quite well using the same ATF. The first major change came about in the 1950’s when ATF became available in two variants: ATF Types A and F. General Motors specified Type A whilst Type F was developed for Ford ATF’s. These specifications have been revised and improved repeatedly since then to bring about the current General Motors DEXRON and Ford MERCON transmission fluids. In addition, most other manufacturers have also developed their own proprietary ATF specifications.
Automatic transmissions used in present-day vehicles are nothing short of mechanical marvels. Many vehicle manufacturers are using six- and seven-speed automatic transmissions to improve fuel efficiency, performance, and drivability. Various top of the range luxury cars are now available with eight-, nine- and even ten-speed auto boxes. These transmissions are incredibly sophisticated with many of them requiring their own specific fluid formulations, such as the Mercedes-Benz 9G-Tronic transmission on the right.
An ATF has various functions to fulfil. Not only does it have to reduce friction to prevent wear like all other lubricants, it also has to allow a certain level of friction to enable the transmission’s internal clutch materials to engage. Since most manufacturers use proprietary frictional materials, virtually every ATF is manufacturer specific. In some cases, they are transmission-specific. A typical example is the Mercedes-Benz oil specification MB 236.17 that was specifically developed for the Mercedes-Benz 9G-Tronic nine-speed automatic transmission. This oil is not suitable for use in older Mercedes five- and seven-speed auto boxes. ATF’s must also be compatible with all transmission components, they have to transmit power and act as a hydraulic medium, operate at both low and high-temperature extremes, and maintain constant performance for extended periods of time. In addition, they must also control sludge and varnish, resist oxidation and prevent rust and corrosion. To fulfill all these complex tasks, a typically ATF formulation will contain the following additive components:
Pour Point Depressant
Seal Swell Agents
Dyes are also added to ATF’s to distinguish them from other fluids such as engine oil, brake fluid, and antifreeze. Traditionally all ATF’s were dyed red, but nowadays ATF’s are available in other colours, such as blue, green and yellow, depending on what is specified by the transmission manufacturer.
One may well ask whether having an automatic transmission with so many gears is really better and, if so, what the limit is. With more gears in modern automatic transmissions, they can match the engine’s optimum torque and power curve with what is needed to propel a vehicle better under all driving conditions. Simply put, extra gears allow an engine to operate more efficiently and economically, regardless of the type of operation. The downside is that more gear ratios come with some specific disadvantages. These include transmission size and weight, complexity, possible reliability issues and, last but not least, more frictional losses. As a result, you lose the efficiency benefits of more gear ratios. It is, therefore, possible that we may have reached “ultimate” auto boxes where having more and more gears will begin to see diminishing returns. In fact, some manufacturers are now focusing on Continuously Variable Transmissions (CVT’s) that can change seamlessly through an unlimited range of gear ratios.
Continuously Variable Transmissions are not a new concept. For many years motor scooters have been fitted with CVT’s, usually the rubber belt with variable pulley variety, commonly known as twist-and-go transmissions. These transmissions consist of two variable-diameter pulleys, each shaped like a pair of opposing cones, with a rubber belt running between them. One pulley is connected to the engine and the other to the rear wheel. The halves of each pulley are movable. As the pulley halves come closer together, the belt is forced to ride higher on the pulley, effectively making the diameter of the pulley larger. Changing the diameter of the pulleys varies the ratio of the transmission. Making the input pulley smaller and the output pulley larger gives a low ratio for better low-speed performance. As the scooter accelerates, the pulleys vary their diameter to lower the engine speed.
In CVT’s fitted to cars the rubber belt is replaced with a metal belt or chain running between the variable-diameter pulleys. This poses a unique set of different challenges as opposed to traditional ATF’s such as requiring higher shear stability and maintaining the appropriate amount of metal-to-metal friction while having enhanced anti-shudder performance. As in the case of ATF’s, there is not one universal CVT fluid that is suitable for all Continuously Variable Transmissions.
There is, however, a downside to CVT’s as well. CVT’s generally perform well in combination with smaller displacement engines, but engines developing more horsepower and torque exceed the (current) capacity of CVT’s. For this very reason CVT’s are presently not used in larger vehicles and some major manufacturers, including Chrysler and Ford, have in fact dropped CVT’s from their line-up. Other disadvantages associated with CVT’s are driver acceptance (changes in engine speed sounds like a slipping transmission), belt noise and durability (slipping CVT belts in particular).
It is, therefore, safe to assume that conventional automatic transmissions will still be with us for quite some time while other technologies are being refined. The only question is the maximum number of gear ratios that will be engineered into conventional auto boxes. Different automakers commit to transmission technologies for any number of reasons such as cost, durability, branding, experience, and drivability. In fact, because of the different advantages and disadvantages, it is hard to say that any one technology is best.