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

December 27, 2007
0712 01z+GM Two Mode Hybrid+cutaway View
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.
0712 02z+2008 Chevrolet Tahoe Hybrid+system View
Because Two-Mode is a second-generation approach conceived by transmission experts instead of engine specialists, gasoline and electric power sources are more ingeniously combined with fixed and variable gear ratios to achieve higher efficiency over a broader range. Alterations to the vehicle--weight savings from aluminum body panels and wheels, elimination of the spare wheel and tire, and subtle aerodynamic tweaks--also contribute to heightened efficiency.
Since GM partnered with BMW and the former DaimlerChrysler to develop Two-Mode for front-, rear-, and four-wheel-drive applications with gasoline and diesel engines, you'll see it in more cars, pickups, SUVs, and crossover applications. And not a moment too soon.
Here's how it works
  • In Auto Stop mode (engine switched off), motor B propels a lightly loaded vehicle up to 30 mph as long as the driver maintains gentle pressure on the accelerator pedal.
  • In reverse, motor B supplies all of the drive torque.
  • Motor A restarts the engine on cue.
  • The engine and both motors combine forces to accelerate the vehicle and up to 6200 pounds of towed load from rest to cruising speed. Four fixed gear ratios and two variable-ratio modes optimize performance and efficiency.
  • During light-load cruising in top gear, four of the engine's cylinders are shut down to conserve fuel. Electric motor B contributes up to 30 hp of assistance.
  • During braking, one or both electric motors work as a generator to convert unwanted momentum to electricity for battery-pack recharging.
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