“World firsts” are rife in the automotive industry and BMW M has come up with another, claiming the accolade for its new seven-speed double-clutch gearbox (M DCT) with Drivelogic control, conceived and designed for a “high-speed power unit.” That is the 4.0-L V8 in the M3 range.
Making its debut in the V8 M3 coupe, convertible, and sedan, the M DCT—developed with Getrag—provides, in BMW’s words, shifts without interruption of power and traction, gearshift management with M-specific configuration, and seven gears for dynamic acceleration. The three previous SMG (single-clutch) versions of the BMW M robotized gearbox incurred a short interruption of power (about 80 ms) to shift gears, but the new double-clutch system can do that “without any interruption,” according to BMW.
[Hot on BMW's heels is the recently-announced 2009 Porsche 911, which also features a seven-speed DCT.]
The auto industry’s move toward double-clutch transmission solutions has taken rather longer than originally envisaged by some observers, partly for reasons of cost and subsequent achievement of economies of scale. BMW was also aware of the challenge of applying a DCT to very-high-performance cars. So it came up with the M double clutch with Drivelogic for engine speed applications of up to 9000 rpm.
The technology, which is basically similar to that of established twin-clutch systems, is controlled by steering wheel paddles and a newly designed shift lever.
The M double-clutch transmission uses two oil-cooled, wet multi-disc clutches with carbon coatings, with one or other closed at all times when the transmission is in gear. Under acceleration, the clutches are engaged alternately with one clutch closing as the other opens, to avoid any interruption of power delivery to the wheels. The system incorporates a dual-mass flywheel.
The transmission fluid is integrated within the coolant cycle of the engine, which helps achieve normal operating temperature more quickly, reducing frictional losses and fuel consumption. The transmission fluid also has its own oil/air cooler.
BMW is being cautious about releasing fine details of the system, but M3 Project Manager Rolf Scheibner said that what was special about the gearbox was its application: “How to set up all the details on the hardware and software side. The software covers a very wide range across the different modes in sequential and cruising—automatic—mode. The engineers have set up 11 modes. A normal system of this kind would have just two.
“It is a question of philosophy and experience, including that gained through the earlier single-clutch SMG gearbox. For example, hydraulic pressure is higher than that of a regular twin-clutch system; at the moment we can’t say what that is. Cooling management is also very important. Our M cars must be suitable for high dynamic use, including on the racetrack.
“We have two cooling systems: a water system and an additional oil cooling system so we can ensure that the double-clutch technology is able to achieve very high temperatures.”
The system can be used as an auto (D mode) or manual (S mode) and spans variable shift characteristics from comfort to track. Drivelogic provides 11 electronically controlled driving programs in M-specific setup; five shift programs in auto mode and six in manual, including Launch Control for maximum acceleration from stationary. Programming is via a push of the Drivelogic button on the center console. There is a memory system for driver preference (M Drive), controlled via an M Drive steering-wheel button. The driving programs differ in terms of road speed at which gears are shifted as well as engine rpm. Drivelogic D1 sees the car moving away from rest on a level surface in second gear, while D5 “shifts up almost like a race car,” according to BMW.
Engine speed required for optimum acceleration when shifting up is indicated by eight LED units on the instrument panel. The driver can configure operation of the shift lights via the M drive button. In manual mode Drivelogic provides a Launch Control function for the fastest possible acceleration from standstill. This is in driving program S6. DSC (dynamic stability control) can be deactivated by the driver.
The M3 achieves maximum output at 8300 rpm with an 8400 rpm limit. Maximum torque is 400 N·m (295 lb·ft). Fuel consumption and emissions benefit from the M DCT Drivelogic when compared to six-speed manual and torque converter alternatives.
The open-top M3 is also the first M car to feature a retractable hardtop. Fully automatic, it can be lowered in 22 s. The car joins the M3 coupe and sedan, already on the market, and is similar mechanically, using the same 4.0-L V8 309-kW (414-hp) engine, but it is now available with the new seven-speed twin-clutch transmission. A touring (wagon) version is not scheduled to join the M3 trio.
The folding hardtop adds about 230 kg (507 lb) to the mass of the M3 convertible compared to that of the coupe (which has a carbon-fiber roof). Thanks to weight savings in other areas—notably lightweight body panels, the engine, and suspension system—the new M3 convertible has a better power-to-weight ratio than its predecessor, and in terms of torsional stiffness drivers get a sportier, more agile-handling car compared to a fabric-roofed rival, the company says. With the roof in place, torsional rigidity has improved by about 30% compared to the previous E46 BMW M3 convertible.
When the roof is stowed, mass increase on the rear axle is 25.5 kg (56 lb), which is said to have little impact on fore and aft weight distribution.
The new car is the first open-top M model to be offered with optional active suspension control: EDC (electronic damper control) and DSC+ (dynamic stability control+). The convertible has a bespoke chassis and suspension setup that takes into account the different weight distribution of the car and the variable positioning of the new roof structure, according to BMW.
The car embraces BMW’s EfficientDynamics program and incorporates brake energy regeneration.