I recently drove the 2008 Chevrolet Tahoe Hybrid to get a full size, SUV-flavored preview of the 2009 Silverado Two-Mode pickup. The next-gen Silverado Hybrid is expected to go on sale a year from now.
General Motors co-developed Two-Mode Hybrid technology with Daimler|Chrysler and BMW. It's more capable and versatile than the parallel hybrid technology it replaces.
The original Silverado Hybrid was considered mild because it used what was basically a super-sized alternator to power the truck’s electrical systems at full stop while the V8 hibernated to save fuel. The eight cylinder gas engine started back up as soon as the driver’s foot lifted off the brakes, to solely drive the rear wheels. This tag-team approach gained about 1.5 miles-per-gallon in fuel economy over conventional GM full size pickups.
A slightly different version of this system is used in GM's current Saturn VUE. The VUE’s Belt Alternator Starter (BAS) not only stands in for the gas engine at stops but also provides a supplemental boost during acceleration, making it a true parallel hybrid because both the engine and electric motor work at the same time to power the vehicle.
But it's Toyota that has profited most from parallel hybrid technology. Cars like the Prius, Camry, and Lexus LS600h use sophisticated electro-conventional drivetrains mated with continuously variable transmissions (CVT). Depending on driving conditions, either the electric motor, internal combustion engine, or both power the vehicle. Parallel hybrids have a drawback, though. Most are optimized to run in low-speed driving conditions because the electric motor, limited batteries, and CVTs can't stand in for conventional engines at more than several miles-per-hour and can't optimally spread torque across the power band. Since they tend to excel in only one set of conditions, parallel hybrids are also called single-mode hybrids.
Two-mode hybrids throw out the CVT and large dynamo in favor of an advanced fixed-gear automatic transmission that's supplemented with two compact electric motors.
At low speeds (Mode 1), the first electric motor, which also replaces the torque converter, silently drives the truck off the batteries up to 32-mph, and as far as one to two-miles, depending on charge levels.
At high speeds (Mode 2), the second electric motor, which is housed at the back of the transmission, works with the fixed gears to create an electronically variable transmission (EVT). Think of it like a virtual CVT. At the same time, the first electric motor stands in as a digital supercharger to the 6.0-liter 332-horsepower Miller-cycle V8. Miller-cycle V8s leave their intake valves open longer than Otto-cycle engines, to reduce the amount of energy lost compressing the air charge during the compression stroke. Leaving the valves open longer causes torque loss at low RPMs, which the e-motor steps in to supplement during gear changes. The Active Fuel Management (i.e. cylinder shutoff) V8 also uses this electric boost to spend more time running in V4 mode to optimize fuel economy. The large 6.0-liter displacement helps keep the truck in V4 mode longer while providing a steadier torque curve across RPMs for the transmission to manage.