Audi's Multitronic® Technology

October 2001

SMOOTHNESS PERFECTED

Most of us have flown in an aeroplane at some stage or another, and few of us have not been awed by the exhilarating thrill of unfettered acceleration as the pilot opens the throttle and the aircraft hurtles down the runway. Similar is the sensation when one floors the accelerator of one of Audis new Multitronic® equipped cars. 

The revs shoot up and stay there as the vehicle gathers momentum, ultimately reaching its maximum speed without ever faltering or giving you the slightest sensory clue that it has changed gears in the process. It is entirely jolt free.

But change gears it does, seamlessly and probably many thousands of times as the Multitronic® box ensures that the optimum ratio is selected for the conditions each moment. 

The methodology is called CVT, or Continuously Variable Transmission, and while the basic concept is not entirely new (in fact its forebears could be found in pre-1950 Japanese motor scooters) early executions had drawbacks in the form of both actual and perceived friction losses. Also, little success was achieved in efforts to mate the technology to modern high output engines. Audi's Multitronic®, fully exploits all the advantages of CVT yet avoids its shortcomings by achieving optimum gearshift strategies. For the first time in an automatic changer, the Multitronic's®  gain in convenience is not at the expense of performance, dynamism or fuel economy. In fact, quite the opposite is true and most of the performance data for an A4 with Multitronic® are actually superior to an otherwise identical vehicle with 5-speed manual gearbox. And the more powerful the engine, the bigger the gain in fuel economy for the Multitronic® version. Not forgetting a further advantage that quite literally weighs heavily in its favour: the Multitronic® weighs around 15 kilograms less than a comparable 5-speed automatic transmission. Merely the use of magnesium for its casing saves a healthy eight kilograms.
  

So how does this revolutionary drive concept work? 
  
On all previous transmission types, whether manual or geared automatic transmission, specific transmission stages are physically present inside the transmission casing, in the form of five, six or more pairs of gear pairs or planetary gear trains. On the continuously variable transmission (or CVT) on the other hand, there is just one pair of sturdy gears with a flexible transmission belt, which permits an infinite number of transmission ratios. 

This central component is called the variator: It is a transmission element resembling a V-belt, which links two axially adjustable sets of pulley halves. By adjusting the spacing between them in line with the circumference of the tapered pulley halves, the transmission ratio of this belt drive arrangement can be varied across a wide range. 

 Instead of the mesh belt that is otherwise commonly used on CVTs, Audi has opted for a link-plate chain made entirely from steel, which is nevertheless almost as flexible as a V-belt. It is sufficiently strong to be able to transmit much higher torque and forces than thrust belts. In tests extending over a number of years, this type of chain has proven extremely reliable and guarantees lifelong durability. 

The structure of the link-plate chain is similar to that of other chains, except that it has several layers side by side, making it much stronger. The link plates are connected by pins, the face ends of which press against the tapered surfaces in the variator. The chain's tension is transmitted to the variator pulleys at these points of contact. 

One fundamental advantage of the variator in the Multitronic®  is the high spread of ratios. This means the relationship between the highest and lowest transmission ratios, which in this case is 1:2.1 to 1:12.7. With the spread in this instance more than 1:6, it is very close to the ideal value for a transmission. This on the one hand permits dynamic, sports-style acceleration thanks to the use of the highest possible ratio, and on the other hand fully exploits the engine's potential for economical operation by using the lowest ratio. 

Instead of using the hydraulic torque converter that is fitted on other CVTs to handle the tasks of pulling away - inevitably incurring some degree of loss in the slip phase - Audi has opted for an electronically controlled, oil-cooled multi-plate clutch. 

In addition to its improved efficiency, this type of clutch permits a wide range of starting characteristics. In other words, the multi-plate clutch can be controlled in such a way that every conceivable form of pulling away is possible, from an ultra-gentle edging along on a slippery surface to sports-style acceleration at full throttle. 

The electronics detect from the way the accelerator pedal is being operated whether the driver prefers an energy-conscious or performance-oriented style of driving, and either adapt the engine speed very gently, or push it into the peak engine torque range as rapidly as possible. The electronically-controlled clutch can even implement a warming-up programme to bring the catalytic converter up to temperature more rapidly, with temperature-dependent speed increase upon clutch engagement. 

The electronic control centre is located in the immediate vicinity of the hydraulics. This mechatronic unit combining electronics, sensors and actuators is responsible for opening and closing a whole battery of hydraulic valves via the shortest possible routes. It acts on the basis of stored data which can be called up according to internal parameters, such as operating temperature, and external factors, such as movement of the accelerator pedal. The software stored here takes charge of control processes, some of which are entirely new in nature, in performing complex transmission functions that have never before been available in this form. 

A further special feature is the ability of the electronics to emulate the functions of a manual gearbox. Audi has opted for six forward speeds, which can be selected by the driver with a single touch of the selector lever in the second gearshifting plane, or activated at the push of a button on the steering wheel (optional extra). 

These six speeds are stored as fixed gearshift programmes. Depending on which speed the driver selects, the controller is supplied with the ratio as a setpoint, which it establishes at the variator and maintains. Even these manually prompted gearshifts take place with pleasing smoothness and without jerking, all the while satisfying the desire for sports performance thanks to their continuous adaptation. 

Audi is offering the Multitronic®®, a continuously variable automatic transmission, for the first time in this vehicle category on the new A4 2.0, 1.8T, 2.4 and 3.0 and the A6 2.4. The revolutionary Multitronic® is available for all versions of the front-wheel-drive models.