The smaller steps between the gears, compared to the eight-speed, enable smooth, almost imperceptible upshifts, for excellent refinement. No matter the engine torque or vehicle speed, the 9T50 is always in the perfect gear.

—Dan Nicholson, vice president, Global Propulsion Systems

The new transmission also supports stop/start technology that can help save fuel by allowing the engine to shut down in certain stop-and-go driving conditions.

GM will rapidly deploy the new Hydra-Matic nine-speed automatic transmission as an option on 10 vehicle models by the end of 2017.

Selectable one-way clutch. Developed for the tight packaging requirements of transverse propulsion systems, the new 9T50 nine-speed automatic has an on-axis design, where all the gears are in line with the crankshaft, and GM’s first application of a selectable one-way clutch. Both features help reduce the package size, making it roughly the same size as a six-speed transmission.

The selectable one-way clutch, which can hold torque or freewheel—depending on the desired operating mode—contributes to a smaller overall size by eliminating the need for an additional clutch pack.

The selectable one-way clutch is one of the biggest enablers of the 9T50’s balance of efficiency and refinement. It also symbolizes GM’s acumen when it comes to designing advanced, compact multispeed transmissions.

—Steve Saia, chief engineer

For precise and smooth shifts, a specific strategy is used in the selection of the gearsets, clutches and torque converter. Five planetary gearsets are used with four stationary clutches and three rotating clutches, saving space compared to freewheeling designs. Gears change from second- to ninth-gear ratios with precise clutch-to-clutch shifting, where the clutch is engaged in one gear at exactly the same time it is released from another.

The first-to-second upshift is a freewheeling shift. The second-gear clutch engages while the first-gear one-way clutch spins freely, contributing to greater refinement at lower vehicle speeds.

Additionally, this planetary-type automatic transmission employs new torque converter technology to help it deliver smooth take-off performance and additional refinement. The converter features a hyper-elliptical oval cross-section that reduces the thickness of the torque converter, keeping the overall width of the engine and transmission as narrow as possible for packaging advantages.

The single-plate lock-up clutch uses GM’s electronically controlled capacity clutch (ECCC) technology to dampen engine vibrations and ensure smooth operation.

Shift controls and active oil management. Shift controls include automatic grade braking that selects a lower gear to help maintain desired vehicle speed when coasting or braking on a downgrade. This takes advantage of engine braking to help prevent unwanted acceleration and can help reduce the need to brake during a descent. The control module receives input that monitors brake pedal usage, vehicle acceleration, throttle position, grade and whether a trailer is connected to the vehicle.

The 9T50’s hydraulic system is controlled electronically to deliver the pressure and volume required for various operating temperatures for optimal efficiency. The valve body side cover acts as an internal expansion tank, holding the excess oil until it is required and then returning it to the sump when needed.

The 9T50 is filled with DEXRON-VI fluid that does not require changing under normal driving conditions. It was developed for a more consistent viscosity profile, consistent shift performance in extreme conditions and less degradation over time.

Stop/start performance. A unique accumulator within the new nine-speed transmission enables virtually seamless stop/start transitions for greater refinement. The accumulator stores energy for almost imperceptible engine restarts, while GM-designed transmission software calibrated for those restarts further contributes to smooth takeoffs.

Transmission control module. The GM-developed transmission control logic resides in a 32-bit transmission control module (TCM). This module commands all shifting events to provide smooth and precise ratio changes. The TCM also monitors transmission performance and compensates for normal wear in components such as clutch plates so that performance remains consistent. Located outside the transmission to reduce overall packaging size, the module reduces manufacturing complexity and requires only vehicle-specific calibration to be added to the core program. The TCM also enables manual shift control and grade logic.