Technology of the Year: GM's Two-Mode Hybrid System

General Motors' Two-Mode hybrid system is the most significant gasoline-electric propulsion advancement since the original Toyota Prius. Chevrolet Tahoes and GMC Yukons equipped with this technology can achieve 20 mpg or more, which is equivalent to that of a four-cylinder mid-size sedan in city driving.

With oil approaching $100 per barrel and a trip to the pumps capable of inflicting a like-size dent in your wallet, big trucks that don't guzzle gas on an ordinary commute seem like manna from the Middle East. To celebrate their arrival, we're saluting GM's Two-Mode as Automobile Magazine's 2008 Technology of the Year.

Like Honda's Integrated Motor Assist and Toyota's Hybrid Synergy Drive, Two-Mode blends the best attributes of internal combustion and battery-powered electric motors. But that's the only thing these three prevailing hybrid systems have in common.

GM's hybrid powertrain consists of a husky 332-hp, 6.0-liter V-8 bolted to what looks like an overgrown automatic transmission. Actually, that box (pictured above) is packed with the following hybrid components:

• two 80-hp AC motors;
• three planetary gear sets;
• four multiplate clutches;
• two hydraulic oil pumps.

A metal container located under the second-row seats contains the 300-volt nickel-metal hydride battery pack. A second box mounted under the hood is loaded with power inverters and control circuits.