Three GM SUVs with Two-Mode Hybrid Engines
By Bill Siuru
A Cadillac Escalade SUV that gets up to 20 mpg in city driving? General Motors says that its 2009 luxury SUV will deliver more than a 50 percent improvement in fuel economy in the city when equipped with the company’s new two-mode hybrid system. This is the same percentage increase as the 2008 GMC Yukon Hybrid and Green Car Journal’s 2008 Green Car of the Year, the Chevrolet Tahoe Hybrid, although one mpg less estimated city fuel economy than these latter two. While the Escalade’s fuel economy is not up there with a Prius, this is worlds better than the 12 mpg EPA city rating for a non-hybrid 2008 Escalade and represents an intelligent application of technology to improve environmental performance.
GM’s 2008 Chevrolet Tahoe and GMC Yukon Hybrids will be appearing in dealerships any time now, while the Escalade variant will be available next summer. Chevrolet Silverado and GMC Sierra two-mode hybrid pickups will also show up in 2008. The two-mode hybrid is the product of a joint development program between GM, DaimlerChrysler, and BMW that began in 2004. The technology is based on the very successful Allison hybrid transmission used in large buses now operating in many U.S. cities.
The two-mode designation comes from this parallel hybrid’s two operating modes, each optimized for city or highway driving. Under light loads and low speeds, the two-mode system can operate on its electric motors and battery pack alone, solely on internal combustion engine power, or a combination of the two. At higher speeds and heavier loads, it switches to the second mode and runs on the gasoline engine with the electric motors available if needed for added power to climb steep grades, tow a trailer, or for passing.
The two-mode system uses a pair of 80 horsepower electric motors integrated within a new electrically-controlled, continuously variable transmission (ECVT). Energy is supplied by a nickel-metal-hydride (NiMH) energy storage system. The 300 volt, 1.8 kilowatt-hour Cobasys battery pack is located under the second row seats. In the first mode, the ECVT functions with continuously variable ratios as the power flow is blended between the engine and electric motors. In the second mode, the transmission’s four clutches transition from variable ratio mode to one of four fixed gear ratios.
All three GM hybrid SUVs use a Gen IV 6.0-liter V-8 engine with Active Fuel Management (AFM), a technology that also plays a big part in reducing fuel consumption. AFM enables the V-8 engine to seamlessly shut off half of its cylinders when less power is needed, such as during highway cruising. The engine also features late intake valve closing technology.
This full hybrid system enables vehicles to start up and travel silently at up to 25 mph on battery power alone for short distances. When the battery’s charge level drops, the engine starts up seamlessly. Air conditioning and power steering are electrically powered. An electro-hydraulic brake system utilizes a high pressure hydraulic accumulator to boost braking rather than a conventional vacuum brake booster. Regenerative braking recoups energy to keep the battery pack charged.
The system’s second mode is used primarily at highway speeds. In addition to electric assist, the second mode provides 8-cylinder engine power when needed. The second mode integrates sophisticated electronic controls, such as AFM, cam phasing, and late-intake valve closure for even more efficient engine operation.
A controller identifies the proper operating mode. Input from the controller determines the necessary torque for specific driving conditions and sends a command to the engine and electric motors. In response, the engine and motors transfer torque to a series of gears in the transmission, which multiplies torque in ways similar to a conventional automatic transmission.
Unlike a conventional continuously variable transmission, there are no mechanical belts or bands. Shifts between the two modes are synchronous without engine speed changes. That means ultra-smooth acceleration. The two-mode system is compact and places its electric motors within the approximate space of a conventional automatic transmission, an important efficiency advantage compared to today’s typical single-mode systems with much larger electric motors.
GM says that its hybrid Tahoe and Yukon get 30 percent better fuel economy on the combined EPA city/highway cycles compared to the smaller 5.3-liter V-8 that’s standard in the Tahoe and Yukon. All three of the automaker’s two-mode hybrid SUVs allow a 6,000 pound towing capacity, eight passenger seating, and the option of both rear- and all-wheel drive. The two-mode hybrid system promises to have a major impact on GM vehicles large and small since the system can be scaled up or down as needed. In fact, because the two-mode hybrid was a joint development program, it will certainly have wide-ranging fuel efficiency impacts in BMW, Chrysler, and Daimler products as well.
For example, Chrysler Aspen and Dodge Durango hybrid SUVs using the same two-mode technology will appear in early 2008. Chrysler is using its 5.7-liter Hemi V-8 with a Multi-Displacement System (MDS) that also shuts down four of the eight cylinders in this engine under light load conditions. Chrysler is projecting a 25 percent improvement in fuel economy over the non-hybrid 5.7-liter Hemi V-8. BMW will likely use the two-mode hybrid in its new 2009 X6 and Mercedes-Benz in its M-Class SUV. Because the two-mode architecture can be adapted to wide-ranging gasoline or diesel engines, it can truly be used in many different models to satisfy the needs of diverse markets worldwide.
|
