This article was first published in 2003.
While they have been around for decades, Continuously Variable Transmissions (CVTs) are only now starting to make real inroads into the auto transmission market. The breakthrough has been in the development of CVTs that can handle lots of torque. Audi has introduced their Multitronic CVT in a range of cars - it's able to cope with 310Nm - and Japanese auto trans manufacturer JATCO has recently released a completely new design of CVT that is being fitted to rear wheel drive cars developing up to 370Nm. An 8-speed Skyline anyone? It's already available!
So how do these transmissions work?
Audi's Multitronic uses a chain that links two pulleys that can be altered in effective diameters. The system - known as a variator - uses two tapered disc pairs to form the two pulleys. The chain is squeezed between the tapered discs - push the discs harder together and the chain rises higher on the faces, giving the effect of a larger pulley. As the discs are pulled apart, the chain rides lower on the faces - resulting in the forming of a smaller pulley. Both pulleys are adjusted simultaneously in order that the chain remains tensioned correctly.
The Multitronic CVT is the first to use a chain drive - previous CVTs of this design used a rubber V-belt. (DAF, for example, built its first rubber-belt CVT in 1958.) The chain is of unconventional design - unlike a normal chain where the gear teeth of a cog transmit torque through the chain by bearing on the pins between the chain link plates, the Multitronic CVT uses a chain where the pins are extended so that they project from either side of the links. These pins are jammed between the two faces of each pulley - the torque is transmitted from the pulley to the chain by the frictional forces acting on these extended pins. In fact it's a little more complex than this: each pin is actually formed in two shaped parts that roll against one another as they 'turn the corner' around the pulley. Audi claims that "in this way, lost power and wear are minimised despite the high torque and the large angle of the bend... the result is long service life and optimal efficiency".
By varying the relative size of the pulleys, gear ratios all the way from 2.444 to 0.385 can be steplessly achieved. (The diff ratio is 5.297, so explaining the extraordinary 0.385 ratio!) Because the pulleys can be altered in size 'on the run' (there's no pause in torque transmission as the next 'gear' is selected) the transmission has effectively a near infinite number of automatic ratios. The Multitronic system currently being used has 6 driver-selectable manual ratios, however there's no reason that this could be 10 or 20, if desired.
The trans uses an oil/coolant heat exchanger to cool the transmission oil, with an ATF filter provided on the return line from the radiator. The electronic control unit is integrated within the transmission itself, and uses a strong alloy baseplate to provide rigidity and help dissipate heat. Some of the input sensors are built directly into the unit, while wiring connections are largely avoided by the use of plug-in extensions for the connection of other sensors. Three levels of fault indication are provided:
- In the most minor, back-up values are provided - the vehicle will drive near normally
- The next level of fault causes the 'PRND' display on the dashboard to reverse itself (ie red letters on black become black letters on red) - the vehicle can still be driven, however service help should be sought
- The final level of fault indication causes the reversed 'PRND' display to flash. In some circumstances the car will only run until the next time the vehicle stops.
The engine torque is transmitted to the gearbox not by a torque converter but rather by means of a flywheel damper unit or twin-mass flywheel, depending on the engine. Forward and reverse rotation directions are achieved by a planetary geartrain, while two wet multiplate clutches are used to select the direction. These clutches also provide 'creep' (the in-gear characteristic of a conventional auto trans that will cause the car to move slowly when neither throttle or brakes are applied) and hill-holder functions, where just enough pressure is applied to the chain pins to transmit some torque.
By having a ratio literally for all occasions, both performance and fuel economy can be improved. Testing by Audi showed that an A6 accelerated to 100 km/h 1.3 seconds faster than with a 'stepped' conventional automatic transmission - and 0.1 seconds faster than with a manual 5-speed box! Fuel consumption of the Multitronic A6 was 0.9 litres/100 km better than the conventional auto, and 0.2 litres/100 km better than the car equipped with the manual 5-speed. Tests apparently indicate 1mm of wear on the link pins after 300,000km of driving. This wear is automatically compensated for by moving the sides of the pulleys closer together for the development of the same ratio.
JATCO's Toroidal CVT
A completely different design approach has been adopted by JATCO in their JR006E CVT. Rather than using a steel or rubber belt working between two varying size pulleys, rollers bear upon curved discs. The ratio range of the trans is 2.86 - 0.66. Externally the transmission looks similar to a conventional stepped RWD auto trans - but inside everything that's to do with changing gears is different.
The transmission uses a torque converter equipped with a lock-up clutch, while forward and reverse gears are selected by means of wet multiplate clutches working on a planetary geartrain. Ratio changes are caused by two sets of variators, with each unit consisting of an input disc, output disc and two power rollers. The use of four power rollers both improves the torque capacity of the transmission and also allows the use of smaller diameter discs, allowing the assemblies to fit within a 'normal' size transmission case. Synchronisation of the load being taken by each roller is important for durability.
But how do the rollers and the curved discs work to change gear ratios? Each disc has a curved face, grading from being almost flat at the full diameter of the discs through to being almost vertical near to the shaft on which it turns. (In a way the shape of the discs is similar to a smoothed off turbo exhaust wheel.) The input discs are arranged at each end of the assembly, with the two output discs placed back to back, each facing an input disc. Placed in between the input and output discs are the rollers, which transmit the torque between the discs. These rollers are arranged in such a way that they can rotate around their axes, causing their extremities to bear simultaneously on the small diameter of the input discs and the large diameter of the output disc - or vice versa. Or anywhere in between...
The rollers are pressed between the input and output discs by means of a loading cams and dish springs; however, the power rollers don't make actual physical contact with the curved discs - there is always an oil film between them. Unlike a traditional lubricant, this oil is of a 'high traction' formulation allows the transmission of the force by resisting shear. This traction coefficient for oil suitable for use in a toroidal CVT is over four times as high as traditional ATF. The required loading force is determined by the variation in the required torque transmission, in addition to oil temperature and the peripheral speed of the power rollers.
The movement of the power rollers is controlled by oil pressure that is regulated electronically. High speed ratio changes can be accomplished, giving good vehicle response. Gear shift modes include normal automatic, engine braking, and manual where 6 or 8 ratios are provided. A power/normal mode switch is also used which not only adjusts the gear shift ratios to lower values but also increases the sensitivity of the throttle. The pictured Skyline 350 GT uses the toroidal CVT with 8 manual ratios available.
With the ability to provide better fuel economy, better acceleration, a less jerky transmission of power, and the potential for as many 'manual' ratios as desired, Continuously Variable Transmissions are likely to slowly take over all automatic transmission functions.
Thanks to JATCO TransTechnology Ltd and Audi for supplying technical material to allow the assembly of this story.