Continuously variable transmissions hold potential for significant fuel savings, however up until now, their use has been limited to relatively small vehicles with engines generally 3.0 liters or less.
"We started out three years ago looking for a way to improve our truck and SUV fuel economy, and were totally unprepared for how spectacular the results would be from a driver's perspective,” . What we've ended up with is a tranmission which offers refinement, smoothness, and responsiveness unmatched anywhere in the truck or SUV industry." –said Vance Zanardelli, manager, transmission and engine systems for Ford.
The most common CVT technology uses a belt-and-pulley system to transfer torque through a range of drive ratios. These systems are typically used in front-wheel-drive applications, with relatively small engine sizes, due to an inherent limitation in the design.
Ford’s research system uses a traction drive CVT design that offers several advantages. In this architecture, called a "variator," two discs are arranged on either side of a tilting or articulating roller - one disc for torque input, the other for output. The roller angle between the discs determines the final drive ratio, and there is no belt.
While torque in the belt-and-pulley system is transferred via metal-to-metal friction between the components, the traction drive CVT establishes a fluid film between the polished surfaces of the roller and discs. This boundary layer - about one micron thick - develops enough shear strength to transfer torque between the components, without actual metal-to-metal contact. These qualities allow it to be upsized for larger uses, as in full-size trucks or sport utilities.
Ford engineers estimate it could handle engines up to 7.3 liters in displacement and are projecting a potential, immediate 10% fuel-economy improvement in the test Expedition, with only calibration changes in the powertrain and without sacrificing either engine power or emissions performance.
