Improved Efficiency Through New Designs

GM’s innovative triple clutch concept can provide up to 12 forward ratios. It is aimed at pickups and RWD applications

The imperative of driving down emissions and improving efficiency to comply with increasingly stringent legislation – such as the forthcoming WLTP test cycle regulations – is impacting every aspect of vehicle design and engineering.
This means more efficient engines, lighter vehicles and, increasingly, more sophisticated transmissions. These, as countless key speakers and presenters made clear at December’s CTI Symposium on Transmission Design in Berlin, are a key element in the equation as they can be engineered so the engine runs at its most optimally efficient speed.

These regulations have led to a proliferation of ratios in both traditional torque-convertor type transmissions and the increasingly popular dual clutch gearboxes. As some degree of vehicle electrification is now a virtual necessity for any larger capacity gasoline or diesel engine to satisfy future legislation, there is the perfect opportunity for transmission designers to integrate electric motors into the driveline – giving the potential to provide either a power boost for downsized engines or to deliver short-range electric driving. Again, both approaches were in evidence at the Berlin Symposium last month.

“Future engines will have more torque at low(er) engine speeds,” predicted Darrell L. Robinette, an assistant professor at Michigan Technological University. “It’s part of our job when designing transmissions to maximize that efficiency but, also, to ensure a pleasurable driving experience.”

Robinette presented a GM paper on a concept 10-speed step gear automatic triple clutch transmission with the potential of running an electric motor in parallel. Designed principally for American light trucks powered by downsized turbocharged V6 or naturally aspirated V8 engines, the new concept could also be applied to rear-wheel drive sedans, he said.

Robinette’s research revealed that gasoline engines returned the best fuel consumption with nine or 10 ratios, while for diesel it was 12 speeds. The triple clutch transmission also features “Sport,” “Eco” and “Comfort” settings.

The concept enables clutch-to-clutch skip shifting from seventh to 11th gear, plus a 12-speed option with two dual transition shifts. This yields a potential five percent reduction in fuel consumption on the WLTP drive cycle compared to the currently best-available six-speed planetary torque convertor automatic, say the concept’s engineers.

Ford has calculated that 10 speeds and a ratio span of 7.4 provide the optimum overall performance. The RWD unit is co-produced with GM.

Ford: favors ten speeds

Analysis by Ford engineers, explained Jim Centlivre, the company’s automatic transmission and driveline engineering manager, led them to conclude that 10 ratios with a span of 7.4 was “optimum” to deliver a three to four percent improvement over a six-speed automatic. “Our analysis showed benefits up to eight speeds but diminishing returns beyond 10,” he added.

Ford’s new 10-speed automatic, the 10R80, is a joint development with GM and features an internal e-pump for stop-start compatibility. Beyond third gear, the steps between ratios are less than 20 percent to give smoother shifting patterns, though these can be made more aggressive if the driver opts for the transmission’s Sport mode. In addition, the box itself is only 25 mm longer than Ford’s current six-speed auto, yet weighs one kilo less, claimed Centlivre.

Honda’s nine-speed super sports DCT, showing innovative separation of oil systems in clutch and gear housings

American-based OEMs are not the only ones pursuing transmissions with double-digit ratio counts. Although not present at CTI, Toyota has announced two new rear-drive automatics: an eight-speed Direct Shift 8-AT and the 10-speed Direct Shift 10-AT. The latter features what are described as “new techniques” to the gear surfaces to achieve lower friction coefficients when the gears engage, and by optimizing the clutch friction material configuration, torque losses during rotation are reduced by almost 50 percent.

Honda and JLR go the DCT route

Meanwhile the Jaguar Land Rover eight-speed DCT, now known as Transcend rather than Project Polar, is being developed to incorporate 48-volt hybridization. First-level prototypes will be undergoing evaluation from spring 2017, according to a spokesman from JLR partner Drive System Design.

Honda, for its part, has developed a nine-speed DCT with what it claims is a unique first gear structure to help reduce the transmission’s overall length to 633 mm. The improved packaging and minimal rear overhang are key criteria for the company’s recently launched high-performance NSX hybrid sports car. The new 9DCT features a hydrostatically actuated dual wet clutch, with the odd-numbered gears on the main shaft and even-numbered on the secondary shaft.

An interesting aspect of the transmission is the choice of ratios, as Ryuhei Kataoka, assistant chief engineer at Honda research and design, explained: “First gear was kept very short for maximum acceleration, whilst second through to eighth are more closely stacked to maintain the engine at its peak performance from 6,000 rpm upwards. The 191 mph (308 km/h) Vmax is reached in eighth gear, ninth being reserved for relaxed cruising.”

One of the most novel aspects of the new transmission is the inclusion of separate lubrication systems for the gears and the clutch assemblies. These two fluids are kept apart by a unique seal developed by Honda. The design allows the clutches to run in ATF and the hypoid and transmission gears to use a high viscosity oil.

A three-way cooling system featuring both water and oil heat exchangers for the ATF is situated directly above the transmission, whereas a third air cooler, for the gear oil, is located behind the front spoiler.

Lower costs and compact packaging for emerging markets

In complete contrast, Belgium-based Punch Powertrain has developed a low-cost seven-speed wet clutch DCT for emerging markets such as India. Using only one layshaft, the transmission measures just 367 mm in length. Yet, says business development manager Alex Serrarens, what is even more important for packaging is the fact that the unit is just 305 mm long to the vehicle’s side member. “Because it uses just one layshaft, it’s very slender at the rear and easy to install under the side member,” he added.

Serrarens explained that Punch replaced the conventional dual clutch pack with a “power shift module,” eliminating two of the layshafts. The power shift module is a combination of a planetary gear, brake and a normal clutch, in effect making it a dual-ratio dual clutch pack. The two ratios provided by the module multiplied by the further four ratios inside the gearbox give a total of eight potential ratios.

Compared to a five-speed manual transmission tested on the NEDC, there is a three percent improvement in efficiency. However, Serrarens predicted a further three percent gain following the development of a hydraulic control system.

The clutches are independent of each other, with their own cooling circuitry to ensure thermal stability. Additional cooling is provided through the housing during launch phases. Unusually, the gearbox has a very low first gear for initial launch, though in very slow traffic it starts off in second.

Initial versions of the design have a rated torque capacity of between 170 and 230 Nm, but Serrarens went on to suggest that an even smaller 120 Nm five-speed version could be possible. The new transmission’s price point was “substantially” lower than a CVT, he observed.

Punch Powertrain’s prototype low-cost seven-speed DCT is notably compact to suit tight packaging spaces