Power transmission: Transaxle and dual clutch

Nov 17, 2014
When it comes to the power transmission of the new GT the Mercedes‑AMG engineers rely on the combination of a front mid-engine and a seven-speed dual clutch transmission in transaxle configuration at the rear axle – this layout has already proven highly successful in the SLS AMG. The weight distribution with its ratio of 47 percent (front) to 53 percent (rear) benefits from this configuration. The AMG SPEEDSHIFT DCT 7-speed sports transmission has been further improved in terms of efficiency, gearshift speed and precision for use in the new sports car. In 'sailing' mode the engine is decoupled from the powertrain, allowing an effective reduction in fuel consumption.
Thanks to the larger gear ratio spread, the pronounced torque plateau of the V8 turbo engine can be used to optimum effect. The powertrain's overall design is ideal for an uncompromising sports car. At the same time the power transfer meets the exacting requirements in terms of everyday practicality, noise level, ride comfort and fuel consumption.
To transfer the driver's shift commands even more directly, communication between the engine and transmission has been fundamentally revised. This means that the previous limits of shift performance and the response time have been substantially increased. The driver can individually tailor the GT to their wishes with the aid of AMG DYNAMIC SELECT. Various modes are available: "C" (Comfort), "S" (Sport), "S+" (Sport Plus) and – completely new – "I" (Individual). The manual "RACE" drive program reserved exclusively for the GT S optimally adjusts the shift strategy of the dual clutch transmission to the needs of racetrack usage – shortest shift speeds and highly emotional engine sound are included. By pressing the "M" button, the driver can activate the manual transmission mode in any drive mode.
The driver selects the different transmission modes using the AMG DYNAMIC SELECT controller on the left of the centre console (see also section AMG DYNAMIC SELECT). In addition to the transmission's gearshift strategy, the transmission modes influence the engine characteristics including the exhaust system, suspension setup, steering, 3-stage ESP®, the ECO start/stop function incl. sailing function and the extending rear aerofoil.
In "Comfort" mode the focus is on low engine speeds in the highest possible gears. Given a corresponding driving style, the transmission performs very early upshifts to the next-highest gears – benefiting not only fuel consumption, but also noise levels. Thanks to its powerful, readily available torque even at low engine speeds, the AMG V8 biturbo unit is absolutely ideal for this style of driving. Comfort also means comfortable gearshifts and a "soft" accelerator response setup for smooth power delivery. The ECO start/stop function is only available in "C" mode.
The integrated "sailing function" for a further reduction in fuel consumption is a completely new feature in this transmission mode: when the driver releases the accelerator in a speed range from 60 to 160 km/h, the clutch of the DCT transmission is disengaged. This reduces the engine speed to idling level, and the driving resistance is reduced by the compression and frictional forces of the engine on the overrun.
The engine and transmission are considerably more agile in "Sport" mode. Accelerator movements are more directly translated into drive power, and to this end the downshifts are faster. The gears are revved higher and the gearshifts are faster than in "Comfort" mode. In "Sport +" mode the gearshifts are even faster than in "Sport". The same applies to manual transmission mode "M", which the driver activates using a separate button.
Manual transmission mode "M": No automatic gearshifts
In manual transmission mode "M" the driver benefits from the high torque of the V8 biturbo engine. There is no automatic downshift under full load and on kick-down; the transmission remains steadfastly in the selected gear. Moreover, the 7-speed sports transmission does not perform an automatic upshift in manual mode when the rev limit is reached - ideal for a highly dynamic driving style on the racetrack.
In "M" mode the instrument cluster shows the current gear. Shortly before reaching the red section on the speedometer, the driver is also given a visual prompt to change to the next-highest gear by an upshift recommendation showing white/yellow/red LEDs. This means that a particularly sporty driver can use the superior performance potential to its fullest extent. When approaching the lower rev limit, for example when braking the vehicle, there is an automatic downshift to the next-lowest gear also in "M" mode.
The engine control unit also partially suppresses cylinders in the sporty transmission modes under full load: precisely interrupting ignition and injection for brief periods leads to even faster gearshifts than before. This is accompanied by a particular engine note during every gearshift. Using kickdown, the transmission also allows fast, spontaneous multiple downshifts e.g. from seventh to fourth gear, or from fifth to second gear.
The driver experiences the automatic double-declutch function in "Sport", "Sport +", RACE (only GT S) and in the manual "M" mode. Each manual or automatic downshift is accompanied by precisely metered double-declutching, of increasing intensity from "Sport" to "Sport +" and up to RACE (only GT S). This not only heightens the emotional experience for the driver: the double-clutch function reduces load cycle reactions, which has a positive effect when braking for bends and increases handling safety in wet or slippery conditions.
Momentary "M" mode and RACE START for even more dynamism
With "momentary M mode", in modes C, S or S+ the driver is able to activate manual transmission mode by nudging the "up" or "down" shift paddle once, without removing a hand from the wheel. In momentary M mode, when in any of the automatic transmission modes the driver experiences the dynamism and performance obtainable from manual gearshifts, with the addition of automatic upshifts and downshifts. This allows the driver to enjoy the vehicle's impressive performance to the full.
Momentary M mode combines the best of two worlds: whichever transmission mode is active at the time, the combination of automatic gearshifts and manual shifts using the shift paddles offers an impressively broad scope of variability. The system deactivates momentary M mode and reverts to the mode last activated ("C", "S" or "S+") fully automatically, based on the current driving situation.
The RACE START function provides maximum dynamism: The driver is able to activate it when stationary, by using the left foot to operate the brake pedal with "ESP SPORT Handling Mode" switched on. When the RACE START programme is preset on the AMG DYNAMIC SELECT controller, a confirmation prompt appears in the instrument cluster. The driver must now confirm the RACE START function by pulling the "UP" shift paddle once, floor the accelerator and release the brakes. The optimum start-off engine speed is set and the GT accelerates. Sensitive control by the dual clutch transmission responds to the current traction of the tyres, ensuring that wheel slip with RACE START remains within the maximum friction coefficient of the tyres. The driver does not need to shift the gears manually, as the AMG transmission changes them automatically with extremely short shift times.
Torque tube with carbon-fibre drive shaft between the engine and transmission
The AMG SPEEDSHIFT DCT 7-speed sport transmission forms a fixed unit with the V8 engine – with the torque tube serving as the connecting element. The engine and the transmission over the rear axle are rigidly connected to each other, and are mutually supporting. This brings decisive advantages in terms of vehicle dynamics and ride comfort, as this sophisticated solution makes it possible to configure the entire powertrain with far less play. The 1.71-metre long torque tube consists of a one-piece, sand-cast housing and weighs less than 27 kilograms.
Inside the tube, a drive shaft rotates at the engine speed. As in racing cars, the shaft is made of carbon fibre. This high-tech material has a major advantage: despite its high strength, the carbon-fibre shaft tips the scales at only 3.9 kilograms. This means that the drive shaft, which has to transfer a full 650 newton metres of torque from the engine to the dual clutch transmission, is much lighter than an equivalent shaft made from steel. A centrifugal pendulum integrated into the two-mass flywheel eliminates oscillations and vibrations to improve ride and noise comfort – especially in the fuel-efficient lower engine speed range.
Electronic rear-axle locking differential as standard on the GT S
The GT comes as standard with a mechanical rear-axle locking differential which ensures outstanding traction and superb driving safety in all driving conditions. The GT S is fitted as standard with an electronically controlled rear-axle locking differential, which is likewise integrated into the compact transmission housing. The electronic function provides even more sensitive, faster control than the mechanical solution, thus pushing the limits of handling dynamics even further.
Not only is the traction of the drive wheels improved further: cornering speeds at the physical limits also increase, and handling stability during slaloms or lane-changes at high speed is optimised. The system operates with a variable locking effect in acceleration and overrun mode, and is perfectly tuned to the various driving conditions and road surface friction coefficients. The engineers developed an efficient cooling system for the AMG SPEEDSHIFT DCT 7-speed sport transmission. The heat exchangers located in the car's rear end are integrated into the highly effective low-temperature water circuit of the V8 engine.
AMG DYNAMIC PLUS package available as an option for the GT S
The Mercedes-AMG GT comes as standard with powertrain damping. A specially tuned damper reduces undesirable rolling motion of the entire powertrain including the transaxle transmission, thus substantially improving comfort and agility.
The AMG DYNAMIC PLUS package further augments dynamism and agility. It is available exclusively for the GT S and includes dynamic engine and transmission mounts. Using these dynamic mounts means that a separate powertrain damper can be omitted. The engine and transmission mounts assume an important function in the case of a transaxle design: soft mounts improve comfort since they provide more effective decoupling of noise and vibration. Handling and agility benefit, however, from an overall stiffer mount set-up.
Mercedes-AMG resolves these conflicting objectives using dynamic mounts, which adjust their stiffness continuously and instantly to the respective driving conditions and handling. This function is performed by special control electronics – the so-called AMG Chassis Controller. The ACC detects driving situations on the basis of CAN data, is connected to the electronic rear-axle locking differential and thus ensures optimum, balanced driving dynamics. AMG has gone a step further when implementing this technology: the engine and transmission mounts are even actuated independently of each other. The advantage: this innovative solution provides an additional substantial improvement in driving precision and lateral dynamics. Instead of conventional rubber mounts between the engine/transmission and the vehicle body, the GT S has mounts containing a magnetic fluid. Depending on the driving situation, a magnetic field is generated in this fluid containing iron particles by an electrical coil. This changes its viscosity.
The driver feels more connected to the vehicle when driving dynamically, as there is more feedback and the steering response is more direct. All in all the driver benefits from more precise vehicle control: minimised mass movements of the major assemblies mean less corrective action to be taken by the driver when steering into bends.
A special engine application in AMG DYNAMIC SELECT "RACE" mode makes the GT S even more dynamic. Both the maximum output and maximum torque are available over a wider engine speed range. The package also includes tauter suspension tuning thanks to raised spring and damping characteristics, more negative camber at the front axle as well as an adapted sports speed-sensitive steering system. Thus the driver benefits from better performance with optimised handling. The performance steering wheel in black DINAMICA microfibre as well as yellow highlights in the instrument cluster round off the AMG DYNAMIC PLUS package.
Mercedes-AMG GT, AMG SPEEDSHIFT DCT 7-speed sports transmission in transaxle configuration at the rear axle
Mercedes-AMG GT, AMG sports suspension
Mercedes-AMG GT, Torque Tube
Mercedes-AMG GT, AMG SPEEDSHIFT DCT 7-speed sports transmission in transaxle configuration at the rear axle
Mercedes-AMG GT, Electronic rear axle differential lock