So, I'm still learning after a year of ownership. This is from GM Media Online, and has only a little marketing tripe at the beginning. Interesting about the Atkinson-cycle, I didn't realize GM had used this:
Powertrain details
Drawing on its vast experience in developing hybrid bus propulsion systems, GM designed the EVT to provide the best combination of city and highway fuel economy. Used in concert with the Vortec 6.0L V-8 engine with Active Fuel Management, this all-purpose system is well-suited to provide excellent economy and full-size SUV passenger-carrying, cargo-hauling and trailer-towing capabilities.
Furthermore, the EVT is designed to bolt directly to the standard four-wheel-drive transfer case found on the gasoline-only models for true four-wheel-drive capability. The following major components comprise the Yukon Hybrid’s unique powertrain system:

• Electrically Variable Transmission (EVT)
• Energy Storage System (ESS)
• Vortec 6.0L V-8 with Active Fuel Management (AFM) and late intake valve closing (LIVC)

Electrically variable transmission (EVT): The key to GM’s all-new EVT is its unique assemblage of 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).
A sophisticated Hybrid Optimizing System constantly receives torque-based data from the powertrain and other vehicle systems, and then determines the most efficient means of propelling 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.
In Mode One (M1), the EVT provides infinitely variable drive up to 1.70:1, while Mode Two (M2) provides ratios from 1.70:1 to 0.50:1. These two modes include all instances of electric-only (Auto Stop) and electric-gasoline hybrid operation. Depending on driving conditions and vehicle load, a fixed first-gear (G1) ratio of 3.69:1 is available (such as for pulling a large trailer) instead of the variable M1 ratios.
Similarly, a second-gear (G2) fixed ratio of 1.70:1 may be selected by the HOS, if needed, before transitioning to the variable M2 ratios. As road speed increases, and depending on driving and vehicle load conditions, variable-ratio M2 operation can be switched instantly to a third-gear (G3) fixed ratio of 1.00:1, and finally to a fourth-gear (G4) fixed ratio of 0.73:1, which is used mainly for steady-state highway cruising.
Unlike less advanced hybrid systems, the GM system occasionally shuts down the electric motors, allowing the EVT to function as a conventional automatic transmission. Typically, this fixed-ratio operation occurs at highway speeds or when hauling heavy loads, and can actually be more efficient than operating in electric-gasoline hybrid mode under the same conditions. This is because under high-load situations, when the fixed gears are in use, the electric motors can be used to generate electricity — or, if needed, they can be called on to supply additional torque for improved performance.
Energy storage system (ESS): Providing power to the EVT’s two electric motors is a 300-volt nickel-metal hydride Energy Storage System (ESS). This battery pack is located under the second-row seat, where it takes up virtually no additional space and does not interfere with second- or third-row ingress/egress — the fold-and-tumble capability of the second-row seat is maintained, and when the seat is flipped forward, a flat load floor is provided from the liftgate forward to the front seats.
The primary function of the ESS is to provide power (300 volts) to the EVT via the Traction Power Inverter Module (TPIM) and to store captured energy produced during regenerative braking. The ESS can also be charged, when necessary, by the gasoline engine via one of the two electric motors when operated in generator mode.
In addition to supplying power to the EVT, the ESS also provides power to the air conditioning compressor and the Accessory Power Module (APM), which converts the high-voltage supply to 42 volts for the electric power steering system, and 12 volts for the vehicle battery and other 12-volt electrical accessories.
Battery pack durability and reliability is maintained via optimized charge and discharge cycles, as well as a dedicated cooling system that draws air from the passenger compartment. As part of the vehicle’s emission control system, the ESS is warranted for eight years/100,000 miles.
Vortec 6.0L V-8 engine: The Vortec 6.0L V-8 gasoline engine is used in the Yukon Hybrid because it has more favorable torque characteristics than the smaller-displacement 5.3L engine. This is especially important because GM engineers adapted the 6.0L V-8 to operate with late intake valve closing (Atkinson-cycle combustion process) for reduced pumping losses and better overall fuel economy.
Using flat-top pistons, cylinder heads borrowed from GM’s 5.3L high-output V-8 and a 10.8:1 compression ratio, the Vortec 6.0L V-8 produces 332 horsepower (248 kW) at 5100 rpm and 367 lb.-ft. of torque (498 Nm) at 4100 rpm, yet requires only regular unleaded fuel.
In addition, the Vortec 6.0L engine features variable valve timing to control late intake closing, and Active Fuel Management, which allows four of the eight cylinders to be shut off during periods of light load. Gasoline-only V-8 Yukon models with Active Fuel Management benefit from the economy of V-4 operation, but because of the lack of electric boost, they cannot remain in V-4 mode as long as the Vortec 6.0L in the hybrid models.

 

great info Thanks again fjungman.