Today’s Automatic Transmission Fluids: a Study in Complexity

transmissionarevThe step-type automatic transmission is one of the most sophisticated components in today’s motor vehicles. Using a hydraulic clutch, it transfers torque from the engine through a series of gears, and shifts between them so smoothly that people in the vehicle will barely feel the shifting.

In order to work smoothly, the design of the transmission must take into consideration many aspects of the vehicle – including power, use, speeds and loads. What’s more, vehicle manufacturers must look at the transmission as a vital component in their search for fuel economy to meet new efficiency standards all over the world. The result is a transmission that’s typically optimized to the vehicle and market it serves.

This optimization takes many forms. For example, the frictional properties of the shifting clutches are designed for maximum energy transfer. By contrast, the frictional properties of the torque converter are optimized for minimal noise, vibration and harshness (NVH). The computer controls the speeds and torques for each shift as they’re generated, and may even adjust the revs of the engine as shifting occurs.

The Automatic Transmission Fluid (ATF) is an integral part of the whole system. It must function as a hydraulic oil, a gear oil, a torque conversion fluid, a clutch fluid and a heat removal fluid. It is a complex and truly multifunctional fluid design.

Understanding the critical importance of viscosity.

The viscosity of the ATF is defined so that the hydraulics will provide the required pressure and circulation within the transmission. If the viscosity is too low, it may not pump efficiently and result in low pressure, leading to a variety of problems like …

  • Bad shifts and sometimes even clutch slipping under higher loads
  • Excess heat, resulting in fluid degradation and friction plate wear and glazing
  • Additional wear on pumps, bearings and gears
  • Ineffective sealing, leading to more pressure loss and lubricant starvation

On the other hand, if the viscosity is too high, the energy required to pump it around the system increases. This creates its own set of problems …

  • Critical parts may not receive the correct level of lubrication
  • Heat may not be removed from mating parts quickly enough
  • At higher temperatures, the fluid will be thinner, which may mean less protection from wear or metal fatigue

To combat these issues, an ATF will often contain polymers especially designed to reduce wide viscosity swings with temperature. These polymers are finely tuned to the transmission demands, and tend to be somewhat different from those used in engine oils and other lubricants.

As the transmission fluid lubricates the hydraulics, bearings, gears and other parts of the transmission, there is a tendency for these polymers to shear and become smaller. Shear affects the polymer’s ability to thicken the oil, and the viscosity may drop as a result. Therefore, viscosity loss due to shear under operating conditions is factored into the design of both the fluid and of the transmission.

Frictional properties are even more complex.

Viscosity is a relatively simple parameter to measure, yet it is so critical to get it right with an ATF formulation. Friction is more difficult to measure and to comprehend.

Frictional properties of a system are governed by the hardware – the material that’s being lubricated — and by the fluid which contacts it. The frictional properties of a fluid are dependent on the material which is being lubricated. Conversely, the frictional properties of any transmission material are dependent on the fluid which contacts it. The two are mutually dependent and integrated, and when you change one, the other is affected.

To complicate matters, transmissions have more than one frictional system served by the same fluid. This necessitates the need for friction modifiers — chemicals in the fluid which control frictional properties. A typical ATF may contain multiple friction modifiers. In combination, they impact the frictional properties of several different frictional surfaces on the clutch plates and in the torque converter. As with viscosity, the qualities of the friction modifiers in an ATF can negatively impact the transmission operation.

Viscosity and friction are just two of the attributes of an automatic transmission fluid. But fully understanding the complexities of Automatic Transmission Fluids requires the investigation of others, too. In the future, we will cover and explain such attributes as corrosion protection, foaming prevention, wear protection, cleanliness characteristics and many more.

For those of you charged with protecting and extending the life of complicated components like automatic transmissions, it’s clear that choosing the right ATF is a challenge. As you’ve seen today, it’s always best to follow the recommendation in the vehicle handbook to be certain that the fluid has been designed specifically for the vehicle itself, and takes complexities like viscosity and friction into account.