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Inside the fuel-efficient 9-speed 9G-TRONIC from Mercedes-Benz
6 March 2014
|Design of the Mercedes-Benz automatic transmission 9G-TRONIC. Click to enlarge.|
Mercedes-Benz’ new 9-speed 9G-TRONIC, the first rear-wheel drive nine-speed automatic transmission with torque converter, is making its debut in the E 350 BlueTEC as the standard transmission paired with the 185 kW (252 hp) V6 diesel. Announced in 2013 (earlier post), the 9G-TRONIC represents the seventh automatic transmission generation from Mercedes-Benz.
The E 350 BlueTEC offers an average NEDC fuel consumption (sedan model) of 5.3 liters of diesel per 100 km (44.4 mpg US). While an overall reduction in engine speed improves NVH comfort and also cuts down external noise by up to 4 dB(A), 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, Mercedes says.
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 ease of shifting. In manual mode and S mode in particular, the 9G-TRONIC responds significantly more spontaneously and enhances driving pleasure.
The ease of shifting of the new 9G-TRONIC—a focal point during development—comes from a comprehensive package of measures. These include the novel direct control system (more below) 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 practice this translates into being able to drive at 120 km/h (75 mph) in 9th gear with an engine speed of around only 1350 rpm, for example.
The 9G-TRONIC can support a range of drive configurations including: rear-wheel drive; 4MATIC all-wheel drive; hybrid drive; and plug-in hybrid drive. ECO start/stop is standard. Suitable engines for the 9-speed are 4, 6, 8, 12/in-line and V engines.
|Lightweight design and fuel economy measures. Click to enlarge.|
Lightweight. The development engineers also focused on the area of “compact lightweight construction”. Despite two additional gears and a maximum transferable torque of up to 1000 N·m (738 lb-ft), the new automatic transmission requires as little installation space as its 7-speed 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-optimized plastic oil pan is made of an even lighter magnesium alloy.
New gearing concept. Another goal was to implement the nine gears with a minimal number of planetary gear sets and shift elements. Computer-based system analysis and mock-up made it possible to realize this goal with only 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 center 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 1000N·m can be reliably transferred in future engine/transmission combinations.
|Structure of the planetary gear set. Click to enlarge.|
The ring gear, planetary carrier and center 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 center 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 the 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.
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.
Twin turbine torsion damper and centrifugal pendulum. One of the most comfort-enhancing and at the same time fuel-saving features of the 9-speed 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 center of gravity and also allows comfortable vehicle operation in the most fuel-efficient operating range.
Moreover, the optimized 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.
Torque converter. 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%. 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 the first-generation 7G-TRONIC dating from 2003, the efficiency of this component was only 85%.
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.
Oil supply concept accounts for 54% of fuel savings potential. 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% of the total fuel-saving potential of the 9G-TRONIC. 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.
Main transmission shaft with three deep-drilled holes. The main transmission shaft is another technical highlight of the 9G-TRONIC. First, at 550.9 millimeters, it is one of the longest shafts in the entire automotive industry. Second, 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 centimeters 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 millimeters are deep-drilled into the transmission shaft with a core diameter of 16 millimeters to a depth of up to 361.5 millimeters. 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 multi-disc clutches and brakes.
The machining of such a shaft is a masterpiece of production engineering. The requirements are particularly exacting when drilling the three deep holes. To date no other automotive manufacturer or machine tool producer has undertaken such a task, with such a ratio between shaft diameter and hole depth.
Over their entire length of up to 361.5 millimeters, the deep-drilled holes must precisely meet the requirements to just a few thousandths of a millimeter. 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; and
Residue-free drilling with no microfine swarf remaining in the holes.
The machining operation is carried out with the minimum quantity of cooling lubricants. In addition, complex guidance of the 370-millimeter long drill bits was to be avoided and the processing time was to be considerably reduced.
To achieve this, Mercedes-Benz dispensed with 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% compared with conventional production processes. Whereas around 18,000 liters of cooling lubricant per hour were previously needed, the mist-cooling technology requires only 0.3 liters. In other terms: 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.
The processing time per shaft has also been shortened, while improving production quality. To minimize 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 millimeters per minute in the case of 7G-TRONIC PLUS, the new drills allow a speed of over 250 millimeters per minute. The net result is that it takes just under three minutes to process the main transmission shaft—around 63% less than the cycle time during the production of the 7G-TRONIC PLUS.
Fully integrated transmission control. 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.
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 center 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); and 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.
Power transfer follows the following principles:
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.
Three modes. 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 center console. The well-proven positioning and operation of the steering wheel shift paddles are also unchanged. The central display in the center dial instrument reliably informs the driver which gear is engaged, and which transmission mode is active (e.g. “D9 E” = ninth gear, ECO mode).
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