9G-Tronic

Premiere of the new 9G-TRONIC

Discovering the inside of the 9G-TRONIC

Innovative solutions for maximum comfort and outstanding energy efficiency

• Patented gearing concept with four planetary gear sets and six shift elements
• Nine gears with a wide ratio spread of 9.15
• New torque converter: high efficiency level of 92 percent     

• More comfort: twin-turbine torsion damper with centrifugal pendulum and torque converter lockup clutch with return spring
• Demand controlled oil supply: highly efficient vane cell pump and electric auxiliary pump
• Innovation: main transmission shaft with three deep-drilled holes
• Compact lightweight construction: less installation space and reduced weight

The drive element of a classic automatic transmission is the hydrodynamic torque converter. In the new 9G-TRONIC, the engineers have improved the hydraulic circuit in the torque converter and increased its efficiency to up to 92 percent.

This extraordinarily good figure is important for fuel economy, as the losses imposed by physics when transferring the engine torque to the transmission’s input shaft are kept to a minimum.

In short: the 9G-TRONIC converts a very high proportion of the engine power into tractive power.

In the first-generation 7G-TRONIC dating from 2003, the efficiency of this component was only 85 percent.

The heat generated during operation is reliably dissipated via the transmission oil cooler. Conversely, the 9G-TRONIC requires no additional radiator to warm the cold transmission oil when cold-starting under Arctic conditions.

The second-generation, synthetic Fuel Economy low-friction oil also performs reliably at extremely low temperatures.

 

More comfort: twin turbine torsion damper and centrifugal pendulum

One of the most comfort-enhancing and at the same time fuel-saving features is the torsion damper, which compensates even more effectively for eccentricities and vibrations within the transmission.

A basic physical law operates in this case: the lower the rpm and road speed, and the lower the number of cylinders, the more pronounced these can be.

This results in a conflict of aims between comfort and fuel-efficient operation. It is resolved by the use of a twin turbine torsion damper additionally fitted with a centrifugal pendulum.

Depending on the rpm, this shifts the centre of gravity and also allows comfortable vehicle operation in the most fuel-efficient operating range.

Moreover, the optimised damping enables slip in the torque converter lockup clutch to be reduced considerably, which likewise contributes to fuel economy.

For the first time, a return spring has been integrated into the torque converter lockup clutch of the 9G-TRONIC. The multi-disc lockup clutch was previously only hydraulically controlled.

Use of the return pressure spring allows reliable and comfortable activation even at very low rpm.

 

New gearing concept with nine speeds patented worldwide

An intensive, computer-based system analysis and mock-up made it possible to realise a nine-speed transmission with just four simple planetary gear sets and six shift elements.

Mercedes-Benz has secured a worldwide patent for this specific configuration, which the engineers consider to be the best possible.

A planetary gear set consists of the outer ring gear, the inner centre gear and between them the planetary carrier with the four planetary gears and their bearings.

Four planetary gears are required in the 9G-TRONIC so that the expected torque of up to 1000 newton metres can be reliably transferred in future engine/transmission combinations.

The ring gear, planetary carrier and centre gear in a planetary gear set are connected by carriers and multi-disc clutches, or braked by the multi-disc brakes which are supported by the transmission housing.

This enables the planetary gears to transfer drive torque to the inner teeth of the outer ring gear or to the outer teeth of the inner centre gear.

The result is several gear ratios, and at the same time it is possible to reverse the direction of rotation for e.g. reverse gear.

The gear ratio is the ratio between the number of gear teeth on the driving and transferring gears. Depending which planetary gear sets are connected in series, blocked or separated, multiplying the part ratios produces the overall ratio for the relevant transmission gear.

In the 9G-TRONIC, the individual gears are engaged by three multi-disc clutches and three multi-disc brakes.

The purpose of the multi-disc clutches is to transfer the drive torque between two components as a friction connection.

The ratio configuration of gears one to nine allows a wide ratio spread of 9.15. For the same performance compared to preceding transmissions, the rpm level is considerably lowered – as a decisive factor for the high energy efficiency and NVH comfort of models equipped with the 9G-TRONIC.

Reliable oil supply using the power of two pumps

To ensure the reliable and at the same time highly efficient supply of the durable and shear-resistant 2nd-generation synthetic fuel economy engine oil, the 9G-TRONIC is fitted with two pumps.

The considerably size-reduced, mechanical main pump installed “off-axis” lies next to the main shaft and is driven via a chain.

In an automatic transmission such as 7G-TRONIC, the main oil pump previously ringed the transmission shaft and was directly driven.

This meant that the diameter of the transmission shaft prevented the pump from being reduced in size as desired.

For this reason the highly efficient vane cell pump is now placed alongside the main shaft (“off-axis”), and is reduced in size to suit requirements.

The mechanical main pump, which ensures the oil supply to the electrohydraulically controlled automatic transmission when the internal combustion engine is running, is backed up by a separate electric auxiliary pump.

On the one hand this design enables the flow of lubrication and coolant to be controlled actively on demand, and at the same time also means that the 9G-TRONIC can benefit from a start/stop system.

In subsequent hybrid applications, this additional oil delivery also allows so-called “sailing”, i.e. maintaining a constant speed without using the internal combustion engine.

When the engine is off – for example at a red traffic light in start/stop mode – the electric auxiliary pump is actuated, ensuring a defined basic pressure and guaranteeing that all necessary functions are maintained.

When the driver wishes to move off on a green light, oil delivery by the electric pump after engine-starting guarantees immediate and agile acceleration.

In certain operating states with the engine running, the auxiliary pump also assists the main pump, for example at very low engine speeds or in very high temperatures.

In this case the flow of oil is added as needed to ensure smooth gear shifts or when there is a higher cooling requirement.

This innovative oil supply concept using a mechanical main pump and electric auxiliary pump, as well as demand-related control, accounts for around 54 percent of the total fuel-saving potential of the 9G-TRONIC.

The relationship is clear: the less oil that has to be moved within the transmission by more efficient pumps, the higher the overall efficiency.

The fully synthetic Fuel Economy low-friction oil also contributes to this.

 

Maximum precision: main transmission shaft with three deep-drilled holes

The main transmission shaft is another technical highlight of the 9G-TRONIC. Firstly, it is one of the longest shafts in the entire automotive industry at 550.9 millimetres.

Secondly, it performs other functions in addition to its main purpose of power transmission: using a sophisticated internal ducting system, the shaft also performs various lubricating, cooling and control functions.

On the engine side, a large axial hollow-drilled hole measuring a few centimetres supplies the front planetary gear set with oil, which reaches the right places via smaller transverse holes.

The drilled holes on the output side of the main shaft are far more interesting, however. Three parallel holes each measuring 6.1 millimetres are deep-drilled into the transmission shaft with a core diameter of 16 millimetres to a depth of up to 361.5 millimetres.

These three deep-drilled holes have various functions in the 9G-TRONIC: via transverse holes, they ensure a defined oil flow rate to lubricate and cool the planetary gear sets and shift elements.

They also perform an important control function, and transfer the set gearshift pressure to the muilti-disc clutches and brakes.

 

World premiere of a new drilling process

The machining of such a shaft is a production engineering masterpiece. The requirements are particularly exacting when drilling the three deep holes.

To date no other automotive manufacturer or machine tool producer has dared to undertake such a task, with such a ratio between shaft diameter and hole depth.

To illustrate the requirements: Over their entire length of up to 361.5 millimetres, the deep-drilled holes must precisely meet the requirements to just a few thousandths of a millimetre. Machining must follow precisely defined geometrical specifications:

• distance and parallelity of the holes versus each other
• distance and parallelity of the holes versus the outer surface of the shaft
• avoidance of twisting during the drilling process
• radial positioning of the three drilled holes to ensure a free flow to the transverse holes
• correct depth of the individual holes
• residue-free drilling with no microfine swarf remaining in the holes

Reduction in cooling lubricants by 99.9 percent

As if the requirements were not already stringent enough, the developers set themselves other challenges.

The operation was to be carried out with the minimum quantity of cooling lubricants. In addition, complex guidance of the 370-millimetre long drill bits was to be avoided and the processing time was to be considerably reduced.

Implementing these requirements produced impressive results. Conventional cooling lubricants are completely dispensed with when producing the 9G-TRONIC drive shaft.

During the drilling process, a fine oil/air mist is sufficient for lubrication; this reaches the drilling face through a duct in the single-fluted drills.

The generated heat and swarf are conducted away with no residue via a bead in the side of these single-fluted drills.

The savings made possible by this so-called mist cooling technology are enormous. The requirement is reduced by 99.9 percent compared with conventional production processes.

Specifically: whereas around 18,000 litres of cooling lubricant per hour were previously needed, the mist cooling technology requires only 0.3 litres.

Even more specifically: rather than the capacity of an entire road tanker, the content of a household drinking glass is sufficient to ensure the high quality of the machining process.

 

Shorter processing times with improved quality

The processing time per shaft has been shortened just as dramatically, while improving production quality.

To minimise cycle times in the production process, the production engineers eliminated the usual guide sleeves when spot-drilling.

The machines drill the deep holes “free-hand”, so to speak, while maintaining absolute dimensional accuracy.

In addition, the single-fluted drills of cemented carbide allow high working speeds. Whereas the figure was around 125 millimetres per minute in the case of 7G-TRONIC PLUS, the new drills allow a speed of over 250 millimetres per minute.

The net result is that it takes just under three minutes to process the main transmission shaft – around 63 percent less than the cycle time during the production of the 7G-TRONIC PLUS.

With the above improvements, Mercedes-Benz is further implementing the “Green Production” project in its production locations.

This is because taken together, these improvements not only increase productivity and process quality, but of course also reduce the environmental burden by enabling savings in terms of cooling lubricants and a reduction in energy requirements thanks to shorter processing times.

 

Compact lightweight construction: reduced weight with the same installation space

With the change in generation from the 7G-TRONIC to the 9G-TRONIC, the development engineers placed further emphasis on the area of “compact lightweight design”.

Despite two additional gears and a maximum transferable torque of up to 1000 newton metres, the new automatic transmission requires just as little installation space as its predecessor, the 7G-TRONIC PLUS, and is also one kilogram lighter.

The two-piece housing design has been retained: the torque converter housing is made of lightweight aluminium alloy.

The transmission housing with a weight-optimised plastic oil pan is made of an even lighter magnesium alloy.

Transmission controlled by electronics

Artificial intelligence for a natural driving experience

• Supreme mechatronics: fully integrated transmission control
• All gearshifting, lubricating and control components completely integrated into the housing
• Extensive sensor system for outstanding gearshift quality
• Optimised electromagnetic compatibility ensures disturbance-free operation
• Three transmissions modes: Economy, Sport and Manual

A special feature of the 9G-TRONIC is the “fully integrated transmission control”.

This design is based on the following principle: all the components for gearshifting, lubricating and control processes are fully integrated into the transmission housing, improving the control quality and reliability of the 9G-TRONIC.

The advantage of this new direct control is more efficient use of the hydraulic power. In this direct control system, the hydraulic gearshifting element is directly linked to the electromagnetic valve.

The hydraulic control slides now only have one third of their original diameter. This means that control of the six shift elements (three multi-disc clutches and three multi-disc brakes) can be much faster and more efficient.

 

9G-TRONIC command centre integrated into the onboard electronics architecture  

In the case of “fully integrated transmission control”, the actuating unit for the electric transmission oil pump, the control unit, all the electro-magnets, as well as the complete sensor system comprising rpm, temperature, pressure and position sensors, are combined together on a single mounting bracket.

The control unit becomes the command centre for the 9G-TRONIC, and is incorporated into the electronics architecture of the vehicle.

Apart from data obtained from the transmission itself, the integrated 9G-TRONIC control system uses information from the drive control (e.g. engine speed, accelerator position), the dynamic control systems (steering angle, linear and lateral dynamics) and the safety systems (interventions by ABS, BAS Plus, Collision Prevention Assist, DISTRONIC), and is able to control all shift processes optimally using these data.

There are also advantages in terms of electromagnetic compatibility (EMC), as mutual influencing by various electronic components is avoided.

Extensive tests in the EMC laboratory showed this to be the case. The software necessary for all control processes was developed in-house, at the Mercedes-Benz Technology Center (MTC). Only the control units, i.e. the hardware, come from a supplier.

In addition to the temperature, pressure and position sensors, three rpm sensors continuously monitor the operating state of the 9G-TRONIC and provide the transmission control system with the following data for effective gear shifts:

• internal transmission rpm (rpm of the main transmission shaft).
• rpm of the turbine (output rpm of the torque converter).
• output speed.

The extensive, networked sensor system with its continuous comparison of all rpm values makes it possible for several gears to be skipped when accelerating or decelerating, should the driving situation call for it.

 

Who does what with whom: the power flow in 1st gear as an example

The following is an example using the power flow in 1st gear. Planetary gear sets 1 / 2 / 3, the multi-disc brake A / B and the multi-disc clutch E are involved here:

• The inner centre gear of the 1st gear set is part of the drive shaft, and therefore permanently connected to it.

• The planetary carrier of the 1st gear set is connected to the outer ring gear of the 2nd gear set via the 2nd multi-disc clutch.

• The multi-disc brake A brakes the inner centre gear of the 2nd gear set. This increases the torque while reducing the rpm.
• The outer ring gear of the 2nd gear set has a mechanical connection to the inner centre gear of the 3rd gear set.

• The planetary gears of the 3rd gear set rotate in the outer ring gear braked by multi-disc brake B, and transfer the increased torque at reduced rpm to the drive shaft.

• The output shaft rotates with reduced input rpm and a significant torque increase in the engine’s direction of rotation.

• As shown in the diagram:

Power transfer follows the following principle:

• The output rpm is reduced in the lower gears. This leads to lower speeds at the drive wheels while increasing tractive power and drive torque.

• Conversely, the output rpm is much higher in the higher gears. This leads to higher speeds at the drive wheels, accompanied by lower drive torque.

 

Variety of modes: Economy, Sport, Manual and “Short-term M”

The three transmission modes of the 9G-TRONIC allow an individual control strategy depending on the traffic situation or the driver’s personal preferences.

In ECO mode the control is equivalent to a very economical driving style: upshifts are performed sooner, and the handling is gentler overall to support an economical driving style at lower engine speeds.

In SPORT or MANUAL modes the response and shift times are shortened, and there is higher revving in the gears to support a dynamic and sporty driving style.

Like the 7G-TRONIC PLUS before it, the 9G-TRONIC also has the automatic mode “short-term M”, which makes operation even easier and more comfortable.

The driver can now also engage the required gear using the shift paddles in ECO and SPORT mode, without first activating MANUAL mode.

“Short-term M” remains active if there are repeated manual gear shifts or a sporty driving style is maintained with higher linear and lateral acceleration levels.

In contrast to permanently activated MANUAL mode, however, “Short-term M” is deactivated after a certain period without higher power requirements, and the transmission reverts to the original mode.

Operation of the 9G-TRONIC is unchanged compared with the preceding 7G-TRONIC: R, N and D are selected using the DIRECT SELECT lever on the right of the steering column, while the three transmission modes ECO, SPORT and MANUAL are activated with a switch on the centre console.

The well-proven positioning and operation of the steering wheel shift paddles are also unchanged.

The central display in the centre dial instrument reliably informs the driver which gear is engaged, and which transmission mode is active (e.g. “D9 E” = ninth gear, ECO mode).

Developed for in-house drive systems

The best out of 85 billion possibilities

• Innovative, computer-based systems analysis and synthesis to define the 9G-TRONIC gear set concept
• 85 billion transmission structures examined
• Improved configuration thanks to a new powertrain test bench
• Prototype testing over 5 million kilometres

The 9G-TRONIC was designed and implemented using a completely new development process.

For the first time an intensive, computer-based systems analysis and synthesis was used to define the gear set concept.

Here the developers were able to define and implement a single optimum solution from among the total of more than 85 billion possible structures which would theoretically be feasible from the combination of planetary gear sets and shift elements for a nine-speed automatic transmission.

Mercedes-Benz has secured this configuration for its own use and patented it worldwide.

Another goal was to implement the nine gears with a minimal number of planetary gear sets and shift elements.

The digital transmission synthesis described enabled this requirement to be met using only four simple planetary gear sets and six shift elements.

However, this new development approach in the conceptual phase is not an automatic transmission programme culminating in a fully-fledged automatic transmission that only needs to be realised.

Instead, after an initial phase in which the transmission structures are first presorted with precise parameters, the search begins for the best solution for a complete transmission.

This must meet all the functional requirements while fitting into the available installation space.

 

Virtual testing to verify the hardware and software

The developers already carried out extensive virtual test series before practical trials of the new 9G-TRONIC on test benches and in prototypes began.

Using Software in the Loop (SiL) or Hardware in the loop (HiL) processes, each new software function in the entire transmission control system was thoroughly verified.

For this purpose the entire electronics architecture of the 9G-TRONIC with all its control units, sensors, lines, actuators and the complete control software was put together and connected to a digital simulator that represented real operating conditions with a wide range of parameters.

Using the results of these tests, the transmission control was then refined to perfection.

This was followed by extensive and detailed practical trials, which were very demanding in view of the planned applications for the 9G-TRONIC ranging from saloon and sports cars through to off-roaders.

In the process the prototypes were spared no torture: as when testing a new vehicle generation, the 9G-TRONIC had to prove its worth in all manner of operating conditions in real vehicles.

This included test series such as on and off-road, in trailer operations, long journeys under full load, on mountain passes or in inner-city stop-and-go traffic.

To ensure that the 9G-TRONIC also performed reliably under the toughest climatic conditions ranging from freezing Arctic temperatures to burning tropical heat, a worldwide programme of trials was initiated that reflected the special demands on an automatic transmission and covered all climatic zones.

These driving tests were supplemented with tests in the state-of-the-art climate tunnels in Sindelfingen.

Here the encapsulated dynamometers are able to set and maintain temperatures ranging from -40 to +60 degrees Celsius, and speeds of up to 200 km/h.

These standardised conditions enable much more rapid and precise adjustments to be made.

 

Premiere of the new vehicle powertrain test bench (FATS)

The new vehicle powertrain test bench (German abbreviation “FATS”) is one of the most modern test benches of its kind in the automotive industry, and was first used during the development phase for the 9G-TRONIC.

In contrast to conventional dynamometers, the test vehicles in FATS are on wheels but these do not spin during testing.

Instead special test wheels are used – these have a freewheeling through-drive, and allow the front and rear wheel hubs to be directly coupled to the test unit via shafts.

Each of these four shafts in turn transfers the torque generated by the electric motors to the wheel hubs, based on the relevant test programme.

The advantages of this setup: the entire powertrain and all dynamic control systems respond as they would under real conditions on the road.

Moreover, FATS allows the simulation of all road surface and operating conditions, which is why this test unit is also known as a “road to rig” test bench.

From configurations such as different levels of grip on the left/right sides of the road and different levels of grip between the front and rear axles to the so-called “chess board” (different levels of diagonal grip between the axles), and right up to uphill and downhill gradients or a wide variety of driving manoeuvres – the new test rig is able to examine the powertrain dynamics of a vehicle in many different ways.

This was previously not possible, as conventional dynamometers are unable to simulate these test parameters.

In addition to an automatic programme in which the relevant control elements of the 9G-TRONIC are actuated by robots and defined test routines are carried out, FATS also allows a manual mode.

In this case the test engineers sit inside the vehicle during the test cycle, and operate the controls as they would in normal road traffic.

This enables problems that only occur irregularly or haphazardly in normal test operations to be precisely localised, so that suitable counter-measures can be taken.

 

Blocked or pivoted: test benches for endurance runs and to verify the oil supply

In addition to FATS, other test units were used during the development phase for the 9G-TRONIC which tested the long-term durability and oil circuit of the transmission under all conceivable conditions.

In the so-called block test rig, the powertrain with its engine, 9G-TRONIC and propshaft is assembled and tortured on previously recorded, digitised test routes – for example with high-speed endurance runs on an oval circuit, or on steep mountain gradients with or without a trailer.

Air drag and rolling resistance are simulated by an electric motor connected to the propshaft.

As a reliable oil supply and the necessary oil levels within the transmission under all operating conditions can only be partly simulated in a computer model, the engineers came up with a test rig with “X-ray eyes”.

They produced a transparent transmission housing for the 9G-TRONIC from a high-strength plastic, installed the complete automatic transmission assembly inside it and mounted this “glass 9G-TRONIC” on a special pivoting test rig.

During the subsequent test runs the glass transmission was subjected to different engine speeds, oil temperatures from -20 to +120 degrees C and various tilting angles along the longitudinal and transverse axis to simulate linear and lateral acceleration.

In the process the oil circuit was monitored with the help of HD cameras and optimised where necessary.

Thanks to this intensive preparation with virtual tests and extensive test rig operation, the practical trials with prototypes were able to commence at a very high level of maturity, making the entire test procedure much more effective.

More than 5 million kilometres were covered during these practical trials in a wide range of prototypes.

The extensive test programme ensures the reliability and long-term durability of the 9G-TRONIC as a hallmark of the brand.

Review of automatic transmission development

22 million automatic transmissions from Mercedes-Benz 

• 1961: first automatic transmission from Mercedes-Benz
• The first electronically controlled 7-speed automatic transmission with torque converter, the 7G-TRONIC, came from Mercedes-Benz
• The 9G-TRONIC is already the seventh automatic transmission generation from Mercedes-Benz

In the interests of improving driving comfort, Mercedes-Benz began the development of transmissions to reduce the driver’s workload as an early stage.

As early as 1930, the Model 770 “Grand Mercedes” was equipped with a semi-automatic, suction-operated linkage for the overdrive.

This milestone was a first step by Mercedes-Benz on the way to an automatic transmission.

However, the first really automatic transmission, a three-speed torque converter transmission, offered by Mercedes-Benz for the Model 300 “Adenauer” from autumn 1955 was an American design by Borg-Warner.

In 1957 the “Hydrak” hydraulic automatic clutch developed in-house appeared as an optional extra for the 190, 219 and 220 S models.

This system combined a hydraulic start-off clutch and a conventional single-disc dry clutch for clutch engagement/disengagement during gear changes with an overrunning clutch bypassing the hydraulic clutch.

With “Hydrak” the Mercedes-Benz driver no longer needed to operate a clutch, though gear changes continued to be manual.

The first automatic transmission from Mercedes-Benz appeared in 1961

As a completely new development produced in-house, the fully-fledged four-speed automatic transmission available from 1961 for the luxury class saloons, coupés and convertibles proved a trailblazer for the entire industry.

In contrast to Borg-Warner’s automatic transmission, this in-house design had no torque converter but rather a hydraulic clutch, with the advantage of lower power losses.

The downstream four-speed planetary gearbox consisted of two planetary gear sets, three multi-disc clutches and three band brakes.

From 1963 every Mercedes-Benz car model – and even the 230 SL sports car – was optionally available with an automatic transmission.

In 1972 Mercedes-Benz changed over to a newly developed generation of automatic torque converter transmissions, which established themselves as three or four-speed transmissions in all model series by 1973.

From 1979 Mercedes-Benz introduced a newly developed generation of automatic torque converter transmissions which excelled with high efficiency, a wider torque conversion range and reduced fuel consumption.

The version for the most powerful engines was now also equipped with four rather than three gears. In 1989 a variant with five speeds was added for the six-cylinder models.

In 1995 the new five-speed automatic introduced by Mercedes-Benz for the V12 and V8 models was a major milestone in automatic transmission development.

For the first time it combines an electronic control system and a torque converter lock-up clutch.

The result was significantly improved ride comfort and reduced fuel consumption. The transmission’s computer automatically adapted its gearshifting characteristics to the current driving situation and the individual wishes of the driver.

In addition the electronics adjusted the shift points on uphill or downhill gradients, preventing sudden gear changes that could unfavourably affect vehicle handling.

It also excelled by having two transmission modes and being of modern lightweight construction.

In 1998 an improved variant of the five-speed automatic transmissions entered series production for the new S-Class (220 series), featuring a further development in the form of Touchshift.

7G-TRONIC: the first electronically controlled seven-speed automatic torque converter transmission

In 2003 Mercedes-Benz reached new heights with the 7G-TRONIC, the world’s first electronically controlled seven-speed automatic torque converter transmission for passenger cars.

This made Mercedes-Benz models more economical and faster in acceleration. In addition this transmission significantly improved gearshift comfort.

Having seven ratios allowed small increases in engine speed – making it easier to ensure the right ratio for the current driving situation.

At the same time the ratio spread between the lowest and highest gear became larger.

This reduced the average engine speed – a plus point in terms of both fuel consumption and noise comfort.

During rapid downshifts (kick-down) the individual gears were no longer necessarily shifted in sequence.

Instead the control system of the new 7G-TRONIC transmission skipped individual gears if necessary, e.g. shifting from seventh directly to fifth gear and then to third gear.

Depending on the driving situation, this meant that the previous four gearshift operations required to accelerate the car rapidly during kick-down were reduced to two.

At the same time the quality of gear shifts was greatly improved. They became much smoother and hardly perceptible, particularly in the upper gears.

In 2005 Mercedes-Benz dispensed with the usual automatic selector lever in the centre console of the new M-Class, the R-Class and the S-Class, and instead used a stalk on the steering column with which the driver operated the transmission – the DIRECT SELECT gearshift.

Additional steering wheel shift buttons allowed manual preselection of the seven gears.

This enabled the driver to make optimal use of the power from six or eight cylinders in any driving situation.

From 2008 the Mercedes-Benz SL became available with 7G-TRONIC Sport and steering wheel shift paddles.

This featured a double-declutch function activated during manual downshifts. This not only produced a very sporty sound, but was also in the interests of safety and comfort, as the double-declutching synchronised the crankshaft and transmission speeds.

The advantage: gear shifts became even smoother, and typical load cycle reactions were noticeably reduced. In 2009 the AMG SPEEDSHIFT DCT 7-speed sports transmission appeared for the SLS AMG.

This excelled with fast gear changes and practically no interruptions in tractive power. Four different transmission modes were available to the driver: the fuel-efficient “C” (Controlled Efficiency), “S” (Sport), “S+” (Sport plus) and “M” (Manual).

An automatic double-declutching function was active in Sport, Sport plus and Manual mode, and all modes could be conveniently operated via a rotary control in the AMG DRIVE UNIT.

The RACE START function allowed optimal traction – as did the mechanical differential lock integrated into the compact transmission housing.

 

2010: the sixth generation of automatic transmissions – 7G-TRONIC PLUS

In 2010 Mercedes-Benz presented a further development of its seven-speed automatic transmission with the new-generation CL-Class (216 series): 7G-TRONIC PLUS.

This was even more comfortable, faster and more economical than the first-generation 7G-TRONIC.

An engine connection with further reduced torque converter slip and significantly lower internal power losses optimised the efficiency.

The various innovations ensured that in ECO mode, there was a reduction in engine speeds that greatly contributed to fuel savings.

In 2012 the new SL was likewise launched with the 7G-TRONIC PLUS. New feature: the Roadster was the first Mercedes-Benz to be equipped with the newly developed DIRECT SELECT gearshift in the centre console, a feature reminiscent of a high-quality joystick.

In this way the premium brand established a completely new generation of centre gear shifts for vehicles with shift-by-wire transmission technology.

The latest development stage of the 7G-TRONIC PLUS entered use in 2013, in the E 300 BlueTEC HYBRID.

Here Mercedes-Benz combines the 2.2-litre four-cylinder diesel engine developing 150 kW (204 hp) with a powerful hybrid module.

The 20 kW electric motor is integrated into the transmission’s torque converter housing.

The electric motor assists the diesel engine when accelerating (boosting), and is used to recuperate braking energy in generator operation.

At the same time it is suitable for purely electric driving.

 

First AUTOTRONIC CVT transmission in the A-Class

In 2004 Mercedes-Benz introduced the AUTOTRONIC system developed in-house as an optional extra for the A-Class (169 series).

AUTOTRONIC operates on the principle of a “continuously variable transmission” (CVT), in which the ratios are varied continuously using a pulley wheel variator.

Accordingly, it does not employ the paired gear wheels normally used in automatic transmissions.

The main advantages of AUTOTRONIC compared with a conventional manual transmission are improved ride comfort, extra flexibility and quicker acceleration.

 

The 7G-DCT dual clutch transmission: comfortable and efficient 

In 2011 a further Mercedes-Benz automatic transmission celebrated its debut in the B-Class: the 7G-DCT dual clutch transmission.

Extremely compact, highly adaptable to engine speeds thanks to seven gears and start/stop capable by virtue of an electric oil pump, this transmission performs gear shifts with no interruption in tractive power and combines the comfort of an automatic with the efficiency of a manual transmission.

7G-DCT is an automated three-shaft manual transmission consisting of two partial transmissions, each with its own clutch.

Both clutch actuation and gear changing are fully automatic without any interruption in tractive power, It allows both exceptionally comfortable and dynamic driving.

Thanks to its seven gears, it offers an exceptionally large spread of up to 7.99. This means that a very short transmission ratio is available when moving off on an uphill slope with a high payload, for example, while during cruising the engine speed can be lowered considerably.

For the first time the efficiency of this transmission matches that of a manual transmission.

The new 4MATIC for the compact models also combined with 7G-DCT. The special feature of this technology is that the power flow to the rear drive train is provided by a compact power take-off unit (PTU) which is fully integrated into the main transmission and supplied with lubricant from the latter’s oil circuit.

This configuration gives rise to substantial weight advantages over competitor systems that branch off power by means of an add-on component with its own oil circuit.

The models A 45 AMG, CLA 45 AMG and GLA 45 AMG feature a particularly sporty variant of this drive technology.

The AMG SPEEDSHIFT DCT 7-speed sports transmission is specifically configured for the world’s most powerful four-cylinder engine, and thrills with its additional functions of double-declutching and RACE START.

The first nine-speed automatic transmission with torque converter, the 9G-TRONIC for premium vehicles, is now celebrating  its debut: equipped with the innovative power transmission, the E 350 BlueTEC is set to become one of the most fuel-efficient six-cylinder diesel models in its class.

Thanks to the high efficiency level of the patented transmission design, NEDC fuel consumption in the E-Class is reduced to 5.3 litres of diesel per 100 km.

As is typical of the brand, the transmission offers outstanding comfort and barely perceptible gear changes.

In the E 350 BlueTEC, which is available in Saloon and Estate models and comes equipped with the new 9G-TRONIC as standard, the 185 kW (252 hp) V6 diesel engine has an average NEDC fuel consumption of 5.3 litres of diesel per 100 kilometres (Estate: 5.5 l/100 km), corresponding to CO₂ emissions of 138 g/km (Estate: 144 g/km).

As a result, the new models with 9G-TRONIC undercut their predecessors in terms of both consumption and CO₂ emissions.

The higher number of gears and the broader gear ratio spread help to increase drive comfort and allow a significant enhancement to be achieved in terms of converting engine power into traction.

 

Perfect interaction: E 350 BlueTEC with V6 diesel engine and 9G-TRONIC

On the one hand, the overall reduction in engine speed improves NVH comfort and therefore the pleasant sense of well-being on board the E-Class, and on the other also cuts down external noise by up to 4 dB(A), thus reducing the strain on the environment.

The reduced fuel consumption of the E 350 BlueTEC with 9G-TRONIC has primarily been achieved as a result of the high efficiency level of the transmission.

As part of this, the broad ratio spread of 9.15 for gears one to nine allows a clearly perceptible reduction in engine speed and is a decisive factor behind the high level of energy efficiency and ride comfort.

Shortened shift and reaction times ensure optimum spontaneity combined with outstanding ease of shifting.

In manual mode and S mode in particular, the 9G-TRONIC responds significantly more spontaneously and enhances driving pleasure.

The particular ease of shifting of the new 9G-TRONIC – a focal point during development and a hallmark feature of a Mercedes-Benz automatic transmission – comes courtesy of a comprehensive package of measures.

These include the novel direct control system which enables short, barely perceptible gear changes.

The combination of twin-turbine torsional damper and centrifugal pendulum technology in the torque converter ensures outstanding drive comfort.

Together with the extended gear ratio spread, higher speeds can now be driven at lower engine speeds for even greater comfort.

In reality this translates into being able to drive at 120 km/h in 9th gear with an engine speed of around only 1350 rpm, for example.

 

Advanced technology for superior power transmission

In the case of the 9-speed automatic transmission, the development engineers also focussed on the area of “compact lightweight construction”.

Despite two additional gears and a maximum transferable torque of up to 1000 newton metres, the new automatic transmission requires as little installation space as its predecessor and is also lighter in weight.

The two-piece housing design has been retained: the torque converter housing is made of lightweight aluminium, while the transmission housing with weight-optimised plastic oil pan is made of an even lighter magnesium alloy.

Another goal was to implement the nine gears with a minimal number of planetary gear sets and shift elements.

Intensive, computer-based system analysis and mock-up made it possible to realise this goal with just four simple planetary gear sets and six shift elements.

Three speed sensors monitor operation and provide the transmission control system with corresponding data for effective shifting.

Here it is possible for several gears to be jumped when accelerating or decelerating, should the driving conditions call for it.

To ensure the reliable and at the same time energy efficient supply of the durable and shear-resistant second-generation synthetic fuel economy engine oil, the new automatic transmission is equipped with two pumps.

The considerably smaller mechanical main pump, featuring an off-axis design, is located next to the main shaft and is chain-driven and fed by a separate electric auxiliary pump.

This design allows the flow of lubricating and cooling oil to be controlled actively on demand, and also means that 9G-TRONIC can benefit from a start/stop system.

Thanks to the world’s first direct control system in a planetary automatic transmission with nine gears, it has also been possible to significantly increase hydraulic efficiency within the transmission.

 

Around 22 million automatic transmissions from Mercedes-Benz since 1961

Mercedes-Benz has been developing and producing high-quality automatic transmissions featuring outstanding ease of shifting, optimum energy efficiency and dependable durability for more than 50 years.

To date, a total of around 22 million automatic passenger car transmissions have left the manufacturing plant in Stuttgart-Hedelfingen, close to the corporate headquarters in Stuttgart-Untertürkheim.

The 9G-TRONIC is suitable for rear-wheel and all-wheel drives as well as hybrid and plug-in hybrid drive systems, and will be used in almost all model series and engines in the coming years.