Full electric launch sequence.
The Hybrid drivetrain includes electrically variable transmission, an energy storage system (ESS), the Vortec 6.0L Gen IV engine with active fuel management (AFM), and late intake valve closing (LIVC). All of it works without a single seam so the even the keenest driver is totally oblivious to what’s going on beneath his/her feet. The Tahoe we drove had lots of “journalist” miles on it (read: drive it like you stole it) but was doubtless the smoothest operating transmission we’ve ever experienced. The EPA rates mileage for the bulbous Tahoe at 21 city and 22 highway, but via the grapevine, we’ve heard that people have gotten as much as 27-28mpg on the open road.
Electrically Variable Transmission
The key to GM’s all-new EVT is two 60 kW electric motors, three planetary gearsets and four traditional hydraulic wet clutches. This arrangement allows continuously variable operation, as well as providing four fixed gear ratios (with operation comparable to that of a standard electronically controlled automatic transmission).
GM engineers chose this design because of the characteristics of electric motors. At slow speed, they are very efficient, but become much less so as motor rpm increases. The opposite is true with GM’s EVT, which can activate any of its four hydraulic clutches to allow power to be transferred via the fixed-gear ratios during high-load conditions.
A sophisticated Hybrid Optimizing System (HOS) constantly receives torque-based data from the powertrain and other vehicle systems, and then determines the most efficient means of moving the vehicle — either via electric power, gasoline engine power, or a combination of the two. The EVT is like having two transmissions in one — continuously variable drive for light-load conditions and fixed-ratio drive for high-load situations.
This EVT schematic illustrates the relationship between the system’s two electric motors, three planetary gearsets and four hydraulic clutches (C1-C4). Also shown is the transmission hydraulic pump and input shaft damper, which takes the place of a traditional torque converter.
By locking the first motor (A), the HOS keeps the output shaft operating at the same speed as the input shaft. This is called the “input-split” mode and is basically the same as that used by most current hybrid vehicles. This approach provides either high fuel efficiency or high power, but not both. It is for this reason that GM added a second or “compound-split” mode, which includes a second electric motor as well as other hardware to the combination. The traditional torque converter is no longer necessary.