by Jim Kerr
Multitronic is Audi’s name for their new continuously variable transmission. The continuously variable transmission (CVT) concept, based on the long-established principle of the chain drive transmission has been improved by Audi for use in high performance premium segment vehicles. Audi recently presented this technology to the “Best New Technology” panel of the Automobile Journalists Association of Canada (AJAC) at their annual Car of the Year test-fest. One technology will be awarded this year’s AJAC Best New Technology award in the new year. Will the Audi Multitronic be the winner?
According to Audi, this transmission sets new standards in driving performance and fuel economy, as well as in handling dynamics and comfort. It can be used in combination with a 3.0 liter V6 putting out 220 horsepower and 221 lbs-ft of torque. Performance of the Multitronic is excellent. In 0-100 kph tests, the Multitronic is 1.3 seconds faster than a regular automatic transmission and .1 second faster than the manual gearbox. Economy is 2.2 mpg better than an automatic equipped vehicle and .5 mpg better than one with a standard gearbox. The numbers are impressive, but how does it work?
If you have ever been on a snowmobile, then you have experienced a CVT. Apply the throttle and the engine revs up. Keep the throttle applied and the vehicle speed keeps increasing, while the engine rpm stays fairly constant. The engine is operated at optimum rpm for best performance while the transmission does all the work. Snowmobiles use rubber belts. Audi uses a chain.
The key component of the Multitronic is the Variator. It allows reduction ratios to be adjusted continuously as the vehicle is being driven. The Variator has two tapered disc pairs – a set of primary drive pulleys and a set of secondary driven pulleys. A special chain runs between the two pulleys to transmit power to the drive wheels. The Variator’s pulleys are adjusted in and out by a computer controlled hydraulic valve body to change the position of the chain on the pulleys and thus the gear ratio.
The chain is not a conventional multi-plate with free pivot pin design. Instead, this CVT design uses adjacent rows of plates linked together with cradle type pressure pieces (oval shaped pins). The cradle type pressure pieces are “jammed” between the taper pulleys of the Variator as the pulleys are pressed together. Torque is transmitted only by the frictional force between the ends of the cradle pieces and the contact faces of the pulleys.
As the cradle pieces are pushed together, the chain locks to form a solid curve around the pulley. The locked chain acts as one piece to transmit torque. As the pulley rotates, the chain cradle type pieces have the pressure removed when they leave the pulley and begin to move freely. This special chain acts like a solid piece of metal as it contacts the pulleys but like a flexible link as soon as the pressure is released. You might expect a lot of wear on the chain as the cradle type pieces move on the pulleys. Tests by Audi show less than 1mm wear on the chain in 300 thousand kilometres of driving. When the chain does wear, the computer can compensate by moving the pulleys closer together.
To change vehicle direction, two engagement “wet” clutches are used. One for forward and one for reverse. A planetary gearset is used in conjunction with the clutches to change direction. These clutches are applied when the shifter is placed in drive or reverse.
No torque converter is used in the Multitronic. This reduces transmission weight and size. Instead, the chain is allowed to slip on the pulleys when the vehicle is stopped, and it is gripped progressively to provide a smooth take-off. Shaft speed sensors allow the computer to monitor operation and the computer can engage “hill holder” function to provide just enough pressure on the chain to prevent the vehicle from rolling backwards. In operation, it drives much like a vehicle with a torque converter.
Although the Multitronic is a continuously variable transmission, it can be shifted into manual gear ranges if desired. When used in Tiptronic mode, the driver can shift the transmission with buttons on the steering wheel. The computer operates the pulley ratios in six distinct steps.
Currently Audi is only using the Multitronic in front wheel drive applications but are looking at using it in their Quattro applications as well. It’s impressive technology. We’ll have to watch and see if it wins this year’s Best New Technology award.