Transmissions for the new A Class

Transmissions: Continuously variable driving pleasure

  • Full range: three transmission variants for the new A-Class

  • AUTOTRONIC: debut of the continuously variable automatic transmission at Mercedes-Benz

  • Manual transmission: newly developed six-speed unit

From autumn 2004 Mercedes-Benz is ushering in a new era in the field of transmission technology with the new A-Class.
For the first time in its history the Stuttgart auto brand is offering a continuously variable automatic transmission – a technology which has particular advantages for compact cars and can therefore expect a very promising future.
This new AUTOTRONIC transmission developed by Mercedes is available for all engine variants of the new A-Class as an optional extra. In addition Mercedes-Benz has developed a new six-speed manual transmission which is standard equipment in the A 180 CDI, A 200 CDI and A 200 TURBO. The well-proven five-speed manual transmission is used as standard in the A 150, A 170, A 200 and A 160 CDI.

AUTOTRONIC operates on the "continuously variable transmission" (CVT) principle, in which the ratios are selected with the help of a pulley wheel variator. Accordingly it does not employ the paired gear wheels normally used in other automatics.

The main advantages of the newly developed AUTOTRONIC compared to a conventional automatic transmission are improved ride comfort, more flexibility and more rapid acceleration:

Ride comfort:
The ratios are changed without jolting
The engine speed is lower at constant speed
The most suitable ratio is always selected on downhill gradients
There is a variable engine braking effect on downhill gradients

Acceleration:
The ratios are changed without interrupting tractive power
The engine reaches its rated output more rapidly

Flexibility:
The engine always performs at its best possible operating point

These advantages ensure a considerable improvement in driving pleasure and performance.

Choice of three shift programs

The new AUTOTRONIC is just as easy to operate as a conventional Mercedes automatic transmission. The selector lever positions "P", "R", "N" and "D" correspond to the familiar shift gate. Pressing a button enables the driver to choose between two shift programs: "C" for Comfort and "S" for Sport.

In the Comfort program the transmission keeps the engine speed at the lowest possible level, so that for instance the A-Class accelerates gently and uses less fuel. In the Sport program AUTOTRONIC becomes adaptive: it automatically recognises the individual style of driving and adapts the gearshift strategy accordingly. If the driver has a more sporty driving style, for example, the transmission makes higher performance reserves available by raising the engine speed, while in the case of a more relaxed, comfort-oriented style the engine speed is reduced.

In manual mode, which is activated by briefly nudging the shift lever sideways, AUTOTRONIC divides its complete ratio range into seven virtual stages. The gears are then shifted up or down by briefly nudging the shift lever. A display in the instrument cluster shows the gear currently selected.

Variator for continuously variable ratios

The centrepiece of the AUTOTRONIC transmission is the so-called variator, which basically consists of two pairs of pulley wheels. These are referred to as the primary and secondary disc sets by specialists. Both sets are made up of a fixed disc and a sliding disc, the latter being axially moveable with the help of a hydraulic pump at a pressure of up to 67 bar. Varying the pressures on the two sliding discs both adjusts the contact pressure on the steel thrust belt and changes the radii of the belt on the primary and secondary sides.

In this way the most suitable ratio is selected for any driving situation – immediately, smoothly and always very comfortably, so that the occupants of the A-Class hardly notice the AUTOTRONIC transmission in operation.

The spread between the largest and smallest ratio is 6.41, which means that the ratio range is significantly wider than in a conventional five-speed automatic transmission. During brisk acceleration, for example, AUTOTRONIC moves the sliding discs so that the steel thrust belt has maximum tension at the primary end and moves inwards between the secondary discs to select the highest ratio (13.27). The position of the variator is exactly the opposite for the lowest ratio (2.07), which makes particularly economical driving possible.

The steel thrust belt which moves between the discs has a width of 30 millimetres and consists of 400 individual links. The complete AUTOTRONIC unit has a length of only 345 millimetres and is therefore the world’s most compact continuously variable automatic transmission. It contains only 334 components, the only gear wheels in the compact transmission housing belonging to the reversal unit, which reverses the movement of the discs and thereby enables the vehicle to drive backwards.

A further special feature typical of Mercedes is the torque converter between the engine and the AUTOTRONIC transmission. This helps the A-Class to move off particularly comfortably, yet very briskly. As in all automatic transmissions by Mercedes-Benz, the torque converter is locked up in many operating situations to compensate the slip between the pump and turbine wheels and prevent performance losses. The lockup clutch can be activated with a need-related rate of slip in all ratios, thereby preventing drive train vibrations from being transmitted.

Transmission control as a sophisticated mathematical process

To control the AUTOTRONIC functions Mercedes-Benz has developed a powerful microprocessor which is integrated into the transmission. It is linked to the electronic network in the new A-Class via a data bus and also processes the signals from various transmission sensors. These include:

  • Speed sensors for the primary and secondary discs, as well as the output shaft
  • Temperature sensor for the transmission oil
  • Pressure sensor for the secondary discs
  • Sensor for the selector lever position
The electronic control unit uses the numerous sensor data to calculate the current vehicle speed, the actual torque at the transmission’s input shaft, the flow rates required for the electro-hydraulic transmission control system and the gearshift strategy, i.e. selection of the most suitable ratio and the action of the torque converter lockup clutch. Other parameters relevant to the shift strategy are also included in the calculation process. The transmission control system is also able to take into account the vehicle load, uphill or downhill gradients, the longitudinal and lateral acceleration of the A-Class and the individual style of driving, for example. In this way AUTOTRONIC selects the most suitable ratio for every operating condition and driver requirement.

Six-speed manual transmission: newly developed for high torque

Like AUTOTRONIC, the six-speed manual transmission included as standard in the A 180 CDI, A 200 CDI and A 200 TURBO is a new development. On the basis of the three-shaft concept the engineers in Stuttgart have successfully realised six speeds in an aluminium housing which is similarly compact to that of the well-proven five-speed A-Class transmission. Moreover, the new manual transmission transfers a much higher engine torque – up to 300 Newton metres in the A 200 CDI.

The three shafts – one drive shaft and two output shafts – are constantly in mesh. Gears 1 to 4 are shifted on the output shaft located outside the oil sump, which means that the six-speed transmission has a high level of shift comfort even at low ambient temperatures. The sophisticated synchromesh system for gears 1 to 4 has the same purpose: a triple cone in first and second gear, and a dual cone for third and fourth gear.

Gears 5 and 6, as well as reverse gear, are located on the lower output shaft. As for third and fourth gears, Mercedes-Benz uses a dual-cone synchromesh here. Gearshifting comfort was also a primary concern in other areas of this transmission development. For example the linkages which control the selector forks on two levels are on roller bearings for particularly smooth operation. A module is bolted to the transmission to accommodate all the gearshift functions. These include the thrust bearings for control cable attachment, the levers for shift path and gear selection and the main shaft with the followers for the selector forks.

The shift lever is linked to the shift module by a control cable for selection of the individual gears. A hydraulic central release bearing ensures smooth, maintenance-free operation of the clutch, whose operating forces remain constant throughout the lifecycle of the A-Class. In the A 180 CDI, A 200 CDI and A 200 TURBO a dual-mass flywheel increases comfort even further.

Mercedes-Benz has developed two variants in order to configure the new six-speed transmission as closely as possible to the different operating conditions of the petrol and diesel engines. These differ in terms of the ratio range and torque class. The key data for the two versions of the six-speed transmission at a glance:

A 180 CDI
A 200 CDI

A 200 TURBO

Maximum torque

300 Nm

280 Nm

Ratio range

5.80

5.30

Ratios
1st gear
2nd gear
3rd gear
4th gear
5th gear
6th gear

Reverse
3.93
2.22
1.39
0.98
0.93
0.81
4.68
4.21
2.54
1.71
1.26
1.18
0.95
5.02

Final drive

3.24/2.70

3.24/2.70

Five-speed manual transmission: well-proven technology with detail improvements

The well-proven five-speed manual transmission remains standard equipment in the A 150, A 170, A 200 and A 160 CDI models of the new A-Class. Mercedes engineers have fine-tuned this transmission to suit the new engines. Modifications include new ratios for first and second gears, as well as new final-drive ratios for the petrol engines and the A 160 CDI.

The precise, smooth operation with a short shift travel has been retained. The compact and extremely lightweight five-speed manual transmission is able to transfer a maximum torque of 185 Newton metres, and is installed in the following variants depending on engine model:

A 150
A 170
A 200 A 160 CDI

Maximum torque

140 Nm
155 Nm
185 Nm 180 Nm

Ratio range

4.44 4.44 5.23
Ratios
1st gear
2nd gear
3rd gear
4th gear
5th gear
Reverse
3.64
2.04

1.33

1.03
0.82
3.29

3.64
2.04
1.33
1.03
0.82
3.29
3.64
2.04
1.26
0.88
0.70
3.29

Final drive

3.88 3.72 3.31

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