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

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.