Photos: Close-up of the 2008 GMC Yukon Hybrid

GM's two-mode hybrid system is the biggest news in the hybrid world since the introduction of Toyota's wildly successful Hybrid Synergy Drive. The two-mode system, which will debut later this year in the GMC Yukon and the Chevy Tahoe, makes use of advanced drive train technology to improve the mileage and the environmental image of two of Detroit's most popular full-size SUVs.

Kevin Massy
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Like GMs "mild" hybrids (the Saturn Vue Green Line and the Aura Green Line), the Yukon Hybrid features an Auto Stop mode, which indicates that the gasoline engine is not running. Being a full hybrid, the Yukon can run in Auto Stop mode under electric power.
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Up to a speed of 32mph, the Yukon Hybrid can run solely under electric power via its 60-watt motor, which is fed by the battery pack mounted under the second-row seats. To achieve this speed, however, drivers need to apply gentle throttle inputs.
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The Yukon Hybrid can run on hybrid power--gasoline engine plus electric motor--in two modes. In low-speed city driving, the car can split engine demand between both propulsion sources. At higher speeds, the electric motor assists the gasoline engine, which makes use of a number of electronic controls, such as Active Fuel Management (which closes off four of the engine's eight valves when demand is low); cam phasing; and late-intake valve closure.
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Like many other hybrid cars, the Yukon Hybrid features regenerative braking, which channels brake friction via the alternator to charge the battery. Additionally, the car makes use of regenerative deceleration, which does a similar thing by converting friction from the wheels into battery power when the driver releases the gas pedal.
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In addition to its high-tech drive train technology, the Yukon Hybrid features some unique aerodynamic modifications to reduce wind resistance and improve fuel economy. External design tweaks include a spoked front grille, an aluminum hood, and sealed-in headlight assemblies. The aerodynamic mods give the Yukon Hybrid a drag coefficient of 3.4 compared to the standard Yukon's 0.365.
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Paradoxically, the Yukon Hybrid has a larger engine than the gasoline-only model. The model features a six-liter small-block V-8 compared with the (non-Denali) gasoline-only Yukon's 5.3-liter engine. Nevertheless, GM maintains that the Yukon Hybrid achieves a 25 percent efficiency overall compared with its gasoline-only counterpart.
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All electrical systems in the Yukon Hybrid are powered by the 300-volt battery. The car's air-conditioning system runs directly from the battery, while the rack-mounted power steering is stepped down to 24 volts and other cabin electronics run off a 12-volt circuit.
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According to GM, the 2008 Yukon Hybrid will achieve a 40 percent improvement in city fuel economy over its gasoline-only counterpart. The hybrid will be less than 100 pounds heavier than the standard Yukon, and will come with a lower price tag than the top-of-the range gasoline-only Yukon.
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GM's two-mode hybrid system, which was developed in partnership with BMW and DaimlerChrysler, will later be expanded to the Cadillac Escalade and to the Silverado and Sierra pickups.
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Specifically designed 12-spoke alloy wheels, special front and rear spoilers, and unique running boards all help to make the Yukon Hybrid lighter and more aerodynamic.
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Inside, the Yukon Hybrid features some specific cabin instruments that enable the driver to monitor the status of the hybrid system. As well as two specific gauges in the instrument cluster, the Yukon hybrid has a Priuslike flow chart on its in-dash LCD screen.
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The Yukon's hybrid system--including its lithium-ion battery pack and two electric motors--adds 400 pounds to the curb weight of the standard Yukon. However, due to a number of weight-saving devices--including the aluminum hood and lighter seat materials--the Yukon hybrid ends up being less than 100 pounds heavier.
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GM's two-mode Yukon Hybrid makes use of some sophisticated drive train technology. In place of the regular Yukon's automatic gearbox, the hybrid gets four fixed gear ratios and two electronically variable transmission (ECVT) modes that make use of planetary gear sets --thus the "two-mode" tag. The first ECVT mode operates with the vehicle in electric-only drive mode and through the second fixed gear ratio. The second ECVT mode kicks in after the second fixed-gear ratio. Thirteen processors located throughout the vehicle constantly monitor driving conditions, terrain, and driver inputs to make a decision on the optimum drive train configuration.

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