To date, Buick and "fuel economy" haven't often been used in the same sentence.

But the 2012 Buick LaCrosse, the brand's largest surviving car, will offer one engine option that's projected to deliver 25 mpg city, 37 mpg highway.

That's not bad for a midsize near-luxury sedan (though 1 mpg lower on city mileage than Buick had hoped).

We've just driven a development prototype of the 2012 LaCrosse fitted with the eAssist system that will be standard on the 2.4-liter four-cylinder engine, the smaller of two engines (the other is a 3.6-liter V-6).

Electric "boost"

It includes not only a start-stop function, saving gasoline when the car isn't moving, it also recaptures electric power via regenerative braking and stores it for use by an electric motor that assists the gasoline engine in short bursts.

2010 Buick LaCrosse

2010 Buick LaCrosse

That's what allows this "mild hybrid" system to improve fuel economy on the highway as well as in the city. The eAssist system was unveiled as an option for the 2012 Buick Regal at the Chicago Auto Show too; both cars will go on sale by the end of the year.

For 2011, the 2.4-liter engine is rated by the EPA at 19 mpg city, 30 mpg highway. So an increase to 25 mpg city, 37 mpg highway would be substantial: more than 30 percent in the city, almost 25 percent on the highway.

The big question is: Does the LaCrosse with eAssist drive like a Buick?

Imperceptible start-stop

In a word, yes. The engine switching itself off and on was completely imperceptible in the prototype we drove, which engineers cautioned wasn't yet a final production vehicle.

Without looking at either of two dashboard displays or the tachometer--which has an "AutoStop" zone replacing the "0" on a standard gauge--we couldn't feel the engine switching off and didn't notice anything as it switched back on.

Acceleration wasn't swift--this is a large sedan powered by a smallish engine--but Buick claims the eAssist model is a few fractions of a second quicker to accelerate from rest to 60 mph, a statistic we couldn't verify in our half-hour drive.

Acceleration dip

The one telltale was a momentary dip in acceleration after the vehicle moved away from a stop. It was the kind of thing that might well be eliminated by the time the vehicle reaches showrooms late this year.

Buick eAssist Hybrid Technology

Buick eAssist Hybrid Technology

And Buick engineers have put a great deal of effort into making the system quiet and smooth. Engine mounts were extensively retuned, since the engine weighs more with the larger starter-alternator and the weight is distributed differently.

They even use quick pulsing of the electric motor--again, imperceptible to the driver--to compensate for engine vibrations under certain conditions. It all seems to work fine.

High-end howling

One advantage of the eAssist mild hybrid system is that it keeps engine speeds lower, by feeding in electric "boost" where the previous 2.4-liter LaCrosse engine would have had to shift down a gear to provide power.

The downside is that when full power is needed, and the transmission does shift down a gear or even two, the engine is correspondingly much louder than it otherwise has been under normal use, where it mostly sits between 1,500 and 2,000 rpm.

Revving up well past 4,000 rpm causes the engine to howl from up there under the hood. It's startling simply because the car has been so hushed and sedate otherwise.

2012 Buick LaCrosse eAssist Live Shots

2012 Buick LaCrosse eAssist Live Shots

How many Buick owners will ever accelerate that hard is another question, but the difference is pronounced. It's not awful, but it's definitely less "Buick-like" when floored than under most other circumstances.

Base price "around $30K"

Buick hasn't yet announced pricing, but said the four-cylinder version would have a base price "around $30,000". That's roughly a 10-percent increase over the base price of $26,995 for the 2011 Lacrosse with the 2.4-liter engine.

Executives said the 2012 eAssist model would have an upgraded stereo system, alloy wheels, and other features to keep it competitive on value.

2012 Buick Lacrosse with eAssist, front fascia with aero shutter

2012 Buick Lacrosse with eAssist, front fascia with aero shutter

They also said Buick will keep the base prices of the 2.4-liter eAssist version the same as that of the more powerful 3.6-liter V-6, which offers better performance but lower gas mileage (its 2011 ratings are 17 mpg city, 27 mpg highway).

A 2011 Buick Lacrosse with the 3.6-liter V-6 has a base price of $28,365.

Mild hybrid, Take Two

What's now called "eAssist" is a completely redesigned version of the GM mild hybrid system known as Belt-Alternator-Starter, or BAS, which had a rocky launch in 2007 through 2009 models of the Saturn Vue and Aura and Chevrolet Malibu.

It uses a 0.5-kilowatt-hour lithium-ion battery pack mounted at the front of the LaCrosse trunk floor. The air-cooled pack includes not only the cells themselves, made in Japan by Hitachi, but also the power electronics that convert electric voltage as needed.

Buick Lacrosse Hybrid eAssist System

Buick Lacrosse Hybrid eAssist System

The electric motor, which also serves as the alternator and starter motor, sits in the customary place on the engine block. A special belt-tensioning system allows it both to generate power and to act as a motor, contributing torque to the engine's crankshaft.

Aerodynamic underbody trays and an active front grille shutter adapted from the Chevrolet Cruze Eco improve aerodynamics, reducing drag and allowing more of the car's coastdown energy to go into regenerative braking.

Marketing

One interesting facet of the marketing for this new system, which promises to pose its own challenges: Buick is not using "the H-word" (hybrid) anywhere on the car. In 2012, in fact, no badges will distinguish either the engines or the trim levels.

In part, said lead eAssist development engineer Daryl Wilson, that's because Buick felt that for some buyers, "hybrid" had come to mean a car that behaved differently than a "normal" car.

2012 Buick Lacrosse with eAssist, underbody aero panels

2012 Buick Lacrosse with eAssist, underbody aero panels

That could mean sacrifices in sound quality, or perhaps ride. Buick deliberately didn't fit ultra-low-rolling-resistance tires to the 2012 LaCrosse with eAssist because it didn't feel they lived up to the car's quiet driving experience.

Buick also wanted to focus on the overall benefits of the LaCrosse as a car first, with better fuel economy being just another feature it could promote--rather than, in effect, pigeonholing one model of the car by its technology.

2012 Buick Lacrosse with eAssist, underbody aero panel

2012 Buick Lacrosse with eAssist, underbody aero panel

The words "hybrid power" do appear on the dashboard displays, and in the owner's manual, Wilson said. "It's a mode of operation," he explained. We'll see what the brochures and magazine ads say later in the year.

Buick provided airfare, lodging, dinner, and one Tim Horton's doughnut to enable High Gear Media to bring you this first-person drive report.

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