Error 404--Not Found

From RFC 2068 Hypertext Transfer Protocol -- HTTP/1.1:

10.4.5 404 Not Found

The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.

If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.

Error 404--Not Found

Error 404--Not Found

From RFC 2068 Hypertext Transfer Protocol -- HTTP/1.1:

10.4.5 404 Not Found

The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.

If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.

Tech Briefs
Audi takes CVT from 15th century to 21st century


Audi multitronic CVT showing the variator with link-plate chain.


General view of Audi multitronic CVT.

In 1490, Leonardo da Vinci made a sketch that indicated the potential of the stepless continuously variable transmission (CVT). Leonardo, it seems, took the invention of the automobile itself as mundane and obvious: He just wanted to get down to details. He reckoned two pedals would be better than three and that conventional gears were already passe. However, he has had a long wait. CVTs may be great in theory but the fact is they have a questionable image and have made, until recently, relatively little impact on the automotive scene.

Now, Audi has revealed that it has developed a new CVT it believes overcomes the drawbacks of earlier systems and will, at last, make the principle generally acceptable. It adds that its CVT, which it calls multitronic, when installed in an A6 sedan or wagon (Avant) not only offers markedly better fuel consumption than a regular automatic but gives marginally improved acceleration to 100 km/h (62 mph) compared to a five-speed manual. And Audi feels it can offer the system at only slightly higher cost than its current conventional automatic. AEI went to Germany to find out more.

In Europe, Daf in The Netherlands produced a CVT for a car in 1958. This was developed and improved over the years. Basically it was a simple (some would say crude) rubber band and cone system. The attractions of CVT are (in theory) many and varied, including seamless power delivery, the ability to allow the engine to rev almost immediately to deliver maximum torque, and a wide spread of ratios. However, the Daf CVT had something of an image problem. Because it was simple to use with just a stick shift for selecting forward or reverse and fitted to a low-powered car (the original production Daf had a 0.6-L engine) it was popular with older people. Later, the system was taken up by various manufacturers but at a time when cars were becoming quieter and more refined. The CVT's trait of going to high revs on wide throttle openings with subsequently increased interior noise levels met with customer resistance. It was also decided that the system should have low speed "creep" similar to that of a regular automatic for low-speed maneuvering or when driving in very slow city traffic. But this created a jerky response. Its performance was improved as more advanced electronics were developed, but the electronics actually became a limiting factor. There was also a problem with regard to the maximum torque that could be handled by a CVT even when the rubber belt was replaced by a steel thrust belt.

Audi's research and development engineers watched all this with caution and it has been almost 20 years since its first tentative CVT work started, and only now it feels that it has overcome the system's minus points and enhanced its pluses. To demonstrate it, Audi invited AEI to sample its multitronic system fitted to a 2.8-L A6, over a mixed route of regular roads and autobahn. It is unlike other CVTs experienced by this journalist. Audi states that multitronic finally overcomes all the drawbacks of the stepless principle, and that the multitronic is the first transmission of its kind not to pay the high price of poorer dynamism and economy for the added convenience it brings.

A key element of the Audi design is a variator that Audi explains adopts a new transmission element called a link-plate chain made entirely from steel, said to be almost as flexible as a V-belt (it has been tested "over a number of years"), to handle the high forces and torque levels of the A6's engine, which has a peak torque of 280 Nm (207 lbft). The variator allows a spread of ratios equating to a six-speed system: due to its high maximum torque ratio, the variator facilitates acceleration from rest and renders a hydraulic torque converter unnecessary. Audi has opted instead to use an oil-cooled multi-plate clutch which it said implements a variety of starting strategies which respond to driver preference via sensors linked to the accelerator pedal. The system allows sport or economy mode driving. The multi-plate clutch also provides constant creep behavior. By optimizing the hydraulics, the transmission engineers have ensured that the adjustment processes take place dynamically and without any trailing effects. The "rubber band effect" or "slipping clutch syndrome," which have been a common source of criticism on conventional CVTs, are essentially banished.


How it works: Audi's new multitronic CVT.

The variator uses a novel dual-piston system and oil flow is separated into high pressure and cooling circuits. Pump output of the hydraulic system is said to be lower than that of a conventional transmission, which aids in efficiency and road performance. The rubber band effect is avoided by electronically controlled speed tracking, producing, says Audi, dynamic driving properties in conjunction with a reassuringly familiar pattern of sound — in other words, the engine does not rev with manic insistence as the car's speed "catches up." Multitronic also has a "manual" mode with six fixed transmission stages, working in a similar way to the manual one-touch sequential element of a conventional automatic transmission. Audi claims that the multitronic A6 accelerates from 0-100 km/h (0-62 mph) 1.3 s quicker than a geared automatic transmission and is 0.1 s quicker over the same speed than an equivalent model with "optimum" use of a five speed manual gearbox. Gasoline consumption measured to EU standards shows a 0.9-L (0.25-gal) savings over 100 km (62 mi) less than an automatic and 0.2 L (0.04 gal) less than a manual. Audi claims it to be the first automatic transmission to achieve lower fuel consumption and better performance than an otherwise identical model with a manual five-speed gearbox. Audi opted to use magnesium for the gearbox housing and claims the multitronic as the first automatic transmission to use the material. "This factor alone brought about a weight reduction of approximately 7 kg (15.4 lb)," says Reinhard Gesenhaus, Audi's Manager of Transmission Engineering, who discussed some of the other key elements of the multitronic system. "We used an oil cooled drive-off clutch to replace the torque converter usually found in automatic transmissions. An input side step-down gear matches the torque to the variator and provides a suitable overall ratio. The variator with link-type chain provides a continuously variable ratio according to torque and engine speed. Output to the front wheels is via an integral front axle differential. The hydraulic control unit with integrated pump and local electronic system is installed at the rear of the transmission."

He adds that CVTs currently on the market use the van Doorne company's steel thrust belt as a transmission element. "We decided to go for a link-type chain that we developed jointly with the LUK company. It consists of 1025 individual links and 75 pairs of pins serving as cradle links, all made of high-strength steel. In each case, either 13 or 14 links are lined up next to each other. This gives the chain a width of 37 mm (1.45 in); a length of 715 mm (28.2 in), and a weight of approximately 1.7 kg (3.74 lb). This link-type chain achieves a higher degree of efficiency, reduces the radii of the tracks, and has a higher torque capacity from a comparable installation space."

Gesenhaus cites the use of a torque sensor as of particular significance in the multitronic. Torque transmitted through the gearbox is registered continuously by the sensor, which, during normal operation, sets the pressure value proportional to the torque, which in turn clamps the chain. "In the event of a sudden alteration to the torque value, the two split shells rotate," Gesenhaus says. "The ramps and the bearings located between them cause an axial displacement of the movable right hand split shell, which acts directly on the clamping zone. The pump effect generated in this manner causes the chain's clamping load to be increased immediately and prevents the chain from slipping even during peak loads."

Also regarded as being of great significance is what Audi calls its dual-piston principle. Says Gesenhaus, "The variator's travel disks primarily fulfill two functions. One is the transmission of the clamping load to the chain, the other is the variation in gear ratio. We have separated these two mechanical functions in the multitronic and developed the so-called dual-piston principle. While both clamping cylinders with the larger surface areas ensure that the chain is sufficiently clamped, the smaller, adjusting cylinder can be called on to provide additional force if the transmission has to be altered. The advantage of this system lies in the fact that during adjusting procedures, the pump only has to compensate for the volumetric difference in the two adjustment chambers. In the clamping chambers, the volume of oil is simply moved from one clamping chamber to the other. The new development plays a significant part in enabling the Audi multitronic to achieve highly dynamic gear ratio changes. And to do so with a smaller, low-loss pump."

As fitted to the A6, multitronic is part of a longitudinally positioned powertrain. The company is developing the system for application to transverse engine layouts and for four-wheel-drive quattro installations. It believes the system could cope with torque levels of up to 350 Nm (258 lbft). Audi has not confirmed that multitronic will be offered on its new small A2 MPV (multi purpose vehicle).

Stuart Birch

Error 404--Not Found

Error 404--Not Found

From RFC 2068 Hypertext Transfer Protocol -- HTTP/1.1:

10.4.5 404 Not Found

The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.

If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.