Investigation of the torque capacity of automatic transmission fluids with various oil additives

A slip-controlled lock-up clutch system is very efficient in improving the fuel economy of automatic transmission-equipped vehicles. However, a special automatic transmission fluid (ATF), which combines anti-shudder properties with a high torque capacity, is required for such a system. An ATF with various oil additives was formulated, the torque capacity of which was evaluated. The results show that certain additives, especially sulphur-containing extreme-pressure and antiwear additives, have an important effect on the torque capacity. Analyses by X-ray photoelectron spectroscopy of the steel surfaces tested clearly reveal the presence of the elements carbon, sulphur (S^2-, S⁶⁺), oxygen, nitrogen, phosphorus, boron, and calcium. Among them, sulphur and phosphorus have the more important effects on the torque capacity of the ATF, while the presence of nitrogen is important with regard not only to torque capacity but also to antiwear characteristics.