As we first reported more than a year ago, a beefier version of GM’s mild hybrid system, the kind that was on the previous Chevrolet Malibu Hybrid, is coming next year. Set to debut on the 2012 Buick LaCrosse, GM’s new “eAssist” mild hybrid system is said to improve fuel economy by as much as 25 percent.

In the case of the LaCrosse that means improvements as much as 6 mpg in the city and 7 mpg on the highway compared with the 2011 four-cylinder model. Final estimates are a claimed 25 mpg in the city and 37 mpg on the highway--impressive for a luxury sedan with seating for five adults.

eAssist essentially pairs a small electric motor with the LaCrosse’s 182 horsepower 2.4-liter four-cylinder engine to boost acceleration during high load situations and help reduce fuel consumption. The electric motor also acts as the car’s alternator-generator, topping up the batteries when needed and running ancillary features like the air conditioner. The system also features a regenerative braking system to help charge the car’s air-cooled 115 volt lithium battery pack.

Unlike a full hybrid system, eAssist cannot power a vehicle on electric power alone. Not surprisingly, the electric motor only adds about 15 horsepower during high load situations and the system can generate up to 20 horsepower from its regenerative brakes. The previous generation of GM’s mild hybrid technology only added an extra 3 horsepower of power assist and 7 horsepower of regenerative power. eAssist can, however, shut down the vehicle’s engine when stationary or when coasting to a stop and almost instantly restart it once the accelerator pedal is depressed.

The car also comes with an ECO gauge on the instrument panel that continuously responds to driving behavior, enabling the driver to drive with maximum efficiency. It also features a hill-assist system that captures brake pressure to help the driver more comfortably accelerate from a stop on a moderate or steep grade. It does this by reducing the tendency of the vehicle to roll backward with the engine in shut-down mode. 

As for the new six-speed automatic, the redesigned box features significant changes to clutch controls and hardware to provide reduced spin losses while improving shift response and time. The added electric power provided by the eAssist system allows for higher gearing to improve steady state efficiency without impacting acceleration performance or driveability. The system’s capability of providing some electric assistance at cruising speeds also allows the driver to accelerate lightly or ascend mild grades without the transmission downshifting.

In an interesting move, GM will offer eAssist as a standard feature on the four-cylinder LaCrosse starting next summer. Pricing for the car is expected to come in at around $30,000.

Look out for a reveal of the 2012 Buick LaCrosse with eAssist technology at this week’s 2010 Los Angeles Auto Show.