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General Motors Rear Wheel Drive Eight Speed Automatic Transmission
ISSN: 1946-3995, e-ISSN: 1946-4002
Published April 01, 2014 by SAE International in United States
Citation: Hart, J., "General Motors Rear Wheel Drive Eight Speed Automatic Transmission," SAE Int. J. Passeng. Cars - Mech. Syst. 7(1):289-294, 2014, https://doi.org/10.4271/2014-01-1721.
General Motors shall introduce a new rear wheel drive eight speed automatic transmission, known as the 8L90, in the 2015 Chevrolet Corvette. The rated turbine torque capacity is 1000 Nm.
This transmission replaces the venerable 6L80 six speed automatic. The objectives behind creation of this transmission are improved fuel economy, performance, and NVH. Packaging in the existing vehicle architecture and high mileage dependability are the givens. The architecture is required to offer low cost for a rear drive eight speed transmission while meeting the givens and objectives.
An eight speed powerflow, invented by General Motors, was selected. This powerflow yields a 7.0 overall ratio spread, enabling improved launch capability because of a deeper first gear ratio and better fuel economy due to lower top gear N/V capability, relative to the 6L80. The eight speed ratios are generated using four simple planetary gearsets, two brake clutches, and three rotating clutches. The resultant on-axis transmission architecture utilizes a squashed torque converter, an off-axis pump and four close coupled gearsets. The three rotating clutches have been located forward of the gearsets to minimize the length of oil feeds which provides for enhanced shift response and simplicity of turbine shaft manufacturing.
The transmission architecture features a case with integral bell housing for enhanced powertrain stiffness. A unique pump drive design allows for off-axis packaging very low in the transmission. The pump is a binary vane type which effectively allows for two pumps in the packaging size of one. This design and packaging strategy not only enables low parasitic losses and optimum priming capability but also provides for ideal oil routing to the controls system, with the pump located in the valve body itself. The transmission controller is externally mounted, enabling packaging and powertrain integration flexibilities. The controller makes use of three speed sensors which provide for enhanced shift response and accuracy.
Utilization of aluminum and magnesium components throughout the transmission yields competitive mass. The dedicated compensator feed circuit, used in GM six speed designs, has been supplanted by a lube-fed design in order to simplify oil routing and enhance shift response. Packaging is within that of the GM 6L80 design, allowing for ease of application integration.
The overall result is a robust, compact, and cost effective transmission which offers significant fuel economy and performance benefit, over its six speed counterpart, and shall provide an attractive balance of overall metrics in the automatic transmission market.