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EuroAuto: Gearboxes for Supercars

#HP #Volkswagen #Ricardo


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Although “supercars” carry price tags that brings race cars to mind—a single horsepower can cost as much as $1,200—unlike a racing car, they have to deliver daily driveability to someone who has spent, in the case of the Bugatti Veyron, around $1.2 million on the vehicle. In this case, the onus is less on the engine than on the transmission, the unsung hero or villain of many a supercar. In the case of the Bugatti, it was Ricardo (www.ricardo.com) which had the responsibility of developing “the best transmission in the world”. The Bugatti Veyron has set out to replicate the Royale, a model introduced by the original company in 1927 that was intended to outshine all other luxury cars of the world. Virtually the same track and wheelbase as a London omnibus of the day, it was powered by an eight-cylinder engine of nearly 13 liters that developed 300 hp at a leisurely 1,700 rpm. While it was a model fit for kings, today’s Bugatti is a car built for the elite. It’s a car that has a top speed in excess of 250 mph, one that can accelerate from 0-100 mph in less than 3 seconds and to 185 mph in under 18 seconds. From the outset, the engine’s power output has always been set at 1,001 hp with a maximum torque of 1,258 Nm (927 lb-ft), yet it must have the levels of luxury and refinement characteristic of a fine motor car. As the senior program manager at Ricardo in the UK, Mike Everitt has been involved with the project from the start, over five years ago. “We faced new challenges in every area. Despite the engine’s high power and torque, the car still had to meet high refinement and high durability levels.”

On the 1927 Royale, just a three-speed gearbox—a fly-gear for starting from rest, a direct gear and a geared-up top—located on the rear axle was deemed necessary. The Veyron, with its 6,500-rpm input speed, 11,100-rpm output speed to the rear axle and 7,900-rpm output speed to front axle, needs more. A number of options were considered, but after a process of elimination it was decided that a seven-speed dual clutch transmission, or “DSG” in Volkswagen terminology, was the way to go. “DSG has all the advantages of both a good manual and a good automatic,” says Everitt. “Its manual characteristics include a direct link from the throttle pedal to the wheels, while the automatic transmission benefits included high levels of refinement and advanced shift features, a shift speed optimized in all situations and better clutch protection.” 

However, there were some design and development challenges. Not only was this a unique solution for this type of application, but the design was undertaken simultaneously with the entire car and engine design evolution. What’s more, neither the engine torque nor the rev range were confirmed at the early program stage. There was a question of tires, as well. Another potential problem was the decision made by the new Bugatti team once it had taken over the project from Volkswagen in 2001 that the vehicle had to lose a great deal of weight, with 60 kg needing to be shed from the driveline alone. “In effect,” says Everitt, “this meant a complete redesign of the gearbox and the front and back axles.” An extremely compact design and light weight construction and fully drive-by-wire, the transmission features a wet multi-plate double clutch (supplied by BorgWarner). It has a dry sump lubrication, triple-cone synchromesh on all gears, four internal oil pumps driven at engine speed, separate high-pressure hydraulic gear actuators for each gear pair, a high-pressure external electric pump for gear selection, and a fully active rear axle differential but no center differential. The drive to the front axle, which has a Haldex coupling—the first use of Haldex driving the front wheels in a four-wheel drive configuration—is via the prop shaft direct on the transmission output shaft.

Using advanced of SIL (software in the loop) and HIL (hardware in the loop) simulation to minimize reliance on in-car work during early program stages, Everitt and his team had the first transmission on the test rig within seven months of being commissioned. By February 2001, 11 months after being commissioned on the project, the gearbox was being installed into a vehicle. By the end of the program 57 prototype transmissions had been manufactured for test, development and validation with a constant upgrade program to all vehicles during this phase. The result is a car that has been highly praised for the quality of its transmission amongst other things.

Another would-be supercar manufacturer is hoping to replicate this success by teaming up with a different producer of specialist transmissions. Xtrac, which is more generally known as the supplier of racing car gearboxes, is supplying a new transmission it has developed for high-performance road cars to Brammo Motorsports. This Oregon-based company is in the midst of developing the Rogue GT, a V12-powered supercar delivering 850 hp and 1,000 Nm (737 lb-ft) of torque and the turbocharged Falconer V12 that will deliver up to 1,600 hp. “It’s all very well having a gearbox that can accept such high horsepower and torque figures, but if the end user cannot drive down to the road to the supermarket on a cold and frosty morning, then it defeats the purpose of the car,” says Adrian Moore, Xtrac’s technical director. “This is often the problem with gearboxes in supercars.”

The answer, as far as Xtrac is concerned, lies in the relatively simple H-pattern manual gear change, the advantage of which is a minimal number of components and consequent low of weight in the pump assembly. Porous media filters, all baffles, magnets, breathers, filler and level plugs are of a Formula One/aerospace quality. However, the real secret to ensuring driveability is the thermostatically controlled pressure relief valve that enables the gearbox to achieve an operating temperature quickly resulting in an improved gear change and high efficiency. “It’s not a unique solution,” says Moore, “but there are not that many gearboxes out there that have it, but it’s necessary on a supercar as the gearbox needs to warm up very quickly to make it driveable.”

All gears and internal components are manufactured using steels developed exclusively for Xtrac to its own specification and follow a metallurgy program drawing on the company’s application of design, testing and race experience. In addition, most of the rotating parts are shot-peened to enhance durability and reliability. “In terms of the gearbox’s internals, we carried out significant amounts of analysis in terms of gear design and gear rating,” says Moore. “We also did some simulation work on the mixture of single, double and triple synchronizers, which have been supplied by a third party, and then undertook some rig testing with a mule gearbox.” While a longitudinal layout was originally considered, impending rear-impact regulations steered the project group to designing and developing a transverse layout. The benefit is a short rear overhang while still enabling the car to comply with the legislation. The drive is transmitted via an engine-mounted push-type clutch with a twin-cable gear change actuation system.

“The design philosophy behind this product is one of attention to detail,” says senior project engineer Mel Dickinson. “Our objective was to optimize packaging, weight and efficiency without resorting to expensive and time-consuming exotic materials and processes, also to use analysis techniques to ensure performance targets were met. For maximum reliability proven Xtrac components have been specified throughout.”

Brammo Motorsport has the North American marketing rights but Xtrac will market the transmission in Europe and all other regions of the world. Production in the UK has already commenced with the first batch of transaxles being readied for delivery this spring.