2018-01-0391 Published 0 3 Apr 2018 © 2018 General Motors LLC.General Motors Hydra-Matic 9T50 Automatic Transaxle Thomas Martin and James Hendrickson General Motors LLC Citation: Martin, T., and Hendrickson, J., “General Motors Hydra-Matic 9T50 Automatic Transaxle,” SAE Technical Paper 2018-01-0391, 2018, doi:10.4271/2018-01-0391. Abstract General Motors Global Propulsion Systems’ first nine- speed automatic transmission makes its debut in the 2017 Chevrolet Malibu, advancing a legacy of multi - speed transmissions designed to optimize efficiency, perfor - mance and refinement. The Hydra-Matic 9T50 nine-speed is paired with a Ecotech 2.0L Turbo engine in the Malibu, contributing to an EPA estimated 33 mpg highway, a three-percent increase over the 2016 Malibu with an eight-speed automatic paired to the same engine. The 9T50 has a wider 7.6:1 overall ratio, which is the ratio between the first gear ratio and the top gear ratio, - compared to the six-speed’s 6.0:1 ratio. The 9T50 is fitted with a “deep” 4.69 first gear ratio for excellent off-the-line acceleration and a “tall” 0.62 top gear ratio for low-rpm highway cruising. That balance optimizes acceleration and fuel economy while reducing engine noise during cruising. With nine available forward ratios, the step size between the gears is reduced providing smooth, precise upshifts and excellent refinement. Engineering features like the Selectable One Way Clutch (SOWC), step pinion gear set, and control valves integrated into the pump support allows the 9T50 to package in the same vehicle architectures as the 6T50 six speed automatic. Vehicle integration and packaging costs have been significantly reduced so interchangeability in existing vehicles will be quick and use less capital. Introduction In the 2008 model year General Motors introduced a family of on-axis small front wheel drive (FWD) six speed auto - matic transmissions to replace its traditional four speed automatic wrap around transmissions. Moving to the on-axis transmission marked a revolutionary change in the packaging and layout of FWD under hood architectures. The six speed family has succeeded in delivering improved fuel economy, performance, mass and N&V at a lower cost with world class quality and reliability levels. Ten years later, General Motors builds on its small front wheel drive six-speed heritage with the introduction of the 9T50 front wheel drive nine-speed automatic transmission. The 9T50 retains the basic shape and packaging of the six-speed transmission and fits in existing under hood architec - tures. The 9T50 delivers up to 2 percent improvement in fuel economy, with improved N&V at similar mass and packaging versus the Hydra-Matic 6T50. Powerflow The General Motors patented 9T50 powerflow is derived from GM’s FWD six-speed. The 6T50 has three simple planetary gear sets, three friction brake clutches, a single one-way brake clutch, and two rotating clutches. The 9T50 retains two of the simple planetary gear sets and replaces one simple gear set with a stepped planetary arrangement. The 9T50 also retains both rotating clutches and adds an additional rotating clutch for fourth gear, retains two of the three brake clutches and adds an additional brake clutch for second and ninth gear. The one-way brake clutch is replaced by an industry leading selectable one-way clutch (SOWC). A lever diagram is shown below in Figure 1 . Carry-over elements from the 6T50 are sketched in black and 9T50 specific elements are sketched in red. A velocity diagram for second gear operation is overlaid to illustrate how nodal speeds are effected by the application of the CB29 and CB123456 brake clutches. Sun 2 is referred to as a “hot node” due to its fixed connection to the input shaft. This arrangement allows first through fourth gear states without the application of an input clutch. The C4 clutch is not attached to input or to ground. When applied C4 provides synchronous overdrive gear set operation. The