Lubricants

Automotive Drivetrain Lubricants with Fuel-Saving Performance

Automotive Drivetrain Lubricants with Fuel-Saving Performance

When it comes to technology for improving automobile fuel efficiency, much of the focus is on engine performance and on making vehicles more lightweight and aerodynamic. What's often overlooked is that improving the efficiency of the drivetrain can also produce significant gains in fuel economy. With the introduction of new designs and new technologies, transmissions have been getting more efficient by the year. However, these new transmissions require special lubricants to match in order to perform to their full potential. This has created a growing need for fuel-efficient drivetrain lubricants.
A transmission is made up of many rotating parts, and simply filling it with a lighter oil (reducing viscous drag) will make the transmission more efficient and thus improve the fuel efficiency of the vehicle. Viscous drag can be reduced by using a lower viscosity lubricant. The problem, however, is that the viscosity may become too low at high temperatures, which can be damaging to the transmission. That's why low-viscosity lubricants designed to improve fuel efficiency are being formulated with new technologies.

Automatic transmissions (AT), continuously variable transmissions (CVT), and transmission fluids

An AT consists of several units, including a torque converter, a multiplate wet clutch (for gear changes), a planetary gear unit, and a control unit (hydraulic controller) (Fig. 1). The torque converter transmits torque from the engine to the transmission and provides torque multiplication. The multiplate wet clutch, which is engaged and disengaged by hydraulic control, is used to hold different members of the planetary gearset motionless as needed to provide the various gear ratios. The automatic transmission fluid (ATF) not only performs a lubricating and cooling function inside the AT, it also acts as a hydraulic fluid for operating the wet clutch and must maintain adequate friction so that the clutch does not slip.

Schematic of an AT and role of the ATF

The units that make up a CVT include a torque converter, a metal belt and two pulleys (primary and secondary), and a control unit (hydraulic controller). Torque from the engine goes through the torque converter to the primary pulley, and is then transmitted to the secondary pulley via the metal belt. The gear ratio in a CVT can be varied continuously, using hydraulic pressure to change the width of the gap between the pulley halves. The CVT fluid not only performs a lubricating and cooling function inside the CVT, but also acts as a hydraulic fluid for operating the pulleys and must maintain adequate friction so that the metal belt and the clutch in the torque converter do not slip.

Schematic of a CVT and role of the CVTF

Fuel-Saving WBASE and Friction Control Technology

WBASE

To create a lubricant with low viscosity that still performs reliably, we need to focus on improving the viscosity index (VI), which is an indicator of how much viscosity will be affected by temperature. The higher the viscosity index, the smaller the changes in viscosity will be relative to changes in temperature. By raising the viscosity index, we can avoid having the oil be too thick in the operating temperature range, while still ensuring it has sufficient viscosity in the high temperature range. This is aimed at reducing churning loss in the transmission.
JXTG developed WBASE, a synthetic base oil with a low pressure-viscosity coefficient and a viscosity index higher than anything else on the market today. WBASE allows us to formulate oils with both lower viscosities and higher viscosity indices.

1.15% higher viscosity index than conventional synthetic base oils

With WBASE, the lubricant will not get too thin at high temperatures (e.g. during high speed driving), nor too thick at low temperatures (e.g. during cold starts). This greatly reduces churning drag and energy losses in the transmission and improves power transmission performance.

2.Around 10% lower pressure-viscosity coefficient than regular base oils

A lower pressure-viscosity coefficient means that viscosity is less affected by changes in pressure. This means the viscosity of the lubricant will increase less under the high pressures inside the transmission, thus significantly reducing energy losses (viscous drag) and improving power transmission performance.

Friction Control Technology

Friction Control (FC) is an additive technology developed by JXTG to control friction for better power transmission performance.
Lubricants are designed to form an oil film on metal parts to prevent metal-to-metal contact and reduce friction to promote smooth operation. With engine oils, lowering the viscosity reduces fiction to improve fuel efficiency. ATFs and CVTFs, on the other hand, must have characteristics that are in opposition. In other words, we want to reduce friction to reduce energy loss and improve fuel efficiency, but if the friction is not high enough for the clutch or the metal belt and pulleys, the power from the engine will not be transmitted efficiently to the wheels. By ensuring high friction where it's needed and reducing friction where it's not, FC technology provides an optimal balance of friction characteristics to improve performance in automatic transmissions.

Esso and Mobil are trademarks of Exxon Mobil Corporation, used under license.