In the race to adopt automatic transmissions with more than six forward ratios, for improved vehicle fuel economy, GM is tapping a proven source to get it in the game with its first production 8-speed.
Aisin AW will supply its TL-80SN planetary automatic for the 2014 Cadillac CTS, to be mated with the new LF3 twin-turbocharged 3.6L V6 and its naturally aspirated cousin. The Aisin unit, equipped with torque converter and paddle-shift levers for greater driver engagement, is also expected to go into other GM vehicles, according to GM and supplier sources.
In the twin-turbo CTS application, the 8-speed is expected to deliver a 1.5% fuel economy improvement vs. the GM 6L50 6-speed automatic in the outgoing 2013 CTS V6 model, according to Rich Bartlett, Assistant Chief Engineer on the LF3 engine program. (See V6 article: http://www.sae.org/mags/sve/11941/.)
The TL-80SN’s wide 6.71 ratio spread (compared with 6.04 on the 6-speed) enables the V6 to pull a taller (numerically lower) 2.85:1 final drive ratio, compared with 3.27:1 on the 6-speed car. Its 0.69-ratio eighth gear helps maintain a low (1800-rpm) engine speed at 70 mph (113 km/h). That’s approximately 200 rpm lower than the 6-speed model, Bartlett said, contributing to lower cabin noise as well as greater vehicle efficiency.
“This transmission enables us to reduce the ‘N-over-V’ [engine speed to vehicle speed] by 11%,” Bartlett said. He said the engine development team’s use of Gamma Technologies’ GT-Power analysis tool helped them to simulate torque output and create a transient response map with which they then could “play through different drive scenarios” during powertrain integration.
The CTS development team and GM Powertrain engineers worked with their AW colleagues to tailor the 8-speed to the higher-output turbo V6. They also had to tailor its performance and response to the lower vehicle mass of the new CTS, which is based on GM’s lightweight Alpha RWD architecture pioneered on the 2013 Cadillac ATS. Upgrades to the transmission, which AW had been producing prior to the 2014 GM applications, include additional heat-treatments on gear sets and other internal components, and additional clutch plates.
The team also designed in four driver-selectable performance modes—Tour, Sport, Track, and Winter— that enable the powertrain to hold certain gears during specific driving situations. The transmission’s electronic control strategy allows it to skip one or more gears, depending on load and other inputs, when up-shifting or down-shifting. In manual-shift mode, torque converter lockup is used in second through eighth gear to balance performance and efficiency, while preserving maximum torque multiplication in first gear for best launch performance.
The TL-80SN will be manufactured by Aisin in Japan. It is expected to serve as GM’s 8-speed until the automaker’s in-house 9- and 10-speed transmission programs enter production later this decade.