2012 Buick LaCrosse

How about a roomy, relatively inexpensive luxury sedan that gets up to 36 miles to the gallon and can, in an emergency, jump start itself?

At first, it sounds like somebody has been smoking something. But it describes the 2012 Buick La Crosse eAssist four-door.

Known in the car biz as a mild hybrid, eAssist is similar to the system in the Honda Insight. It uses a small electric motor to provide a boost to the gasoline engine.

But the Buick engineers combined their hybrid system with other refinements, including unobtrusive stop-start technology, which resulted in a substantial boost in fuel economy.

The 2012 eAssist has an EPA city/highway mileage rating of 25/36 miles to the gallon compared to the 2011 four-cylinder gasoline model’s 19/30 miles to the gallon.

That’s plenty respectable for a luxurious five-passenger sedan that is 16.5-feet long and weighs more than 3,800 pounds.

Though the development took engineering wizardry, describing the eAssist system is relatively simple. A small electric motor-generator, about the size of a standard alternator, sends its power to the drive shaft via a belt drive.

Working seamlessly with the La Crosse’s six-speed automatic transmission, it delivers an extra 15 horsepower, when needed, to augment the gasoline four-banger’s 182 horsepower. It kicks in on long upgrades, freeway on-ramps and passing on two-lane roads.

If the gasoline engine is in the correct gear within its optimum power range, the electric motor lies in wait. The instant the onboard computer senses that the engine is struggling, it kicks in.

In highway cruising, mileage is enhanced by low engine revolutions and transmission ratios optimized for fuel economy. In addition, the La Crosse e-Assist makes use of an automated streamlining system, identical to that on the high-mileage Chevrolet Cruze Eco, which closes cooling vents in the grille to re-direct air flow around the car.

The eAssist uses a lithium-ion battery pack to power the electric motor. The pack, which carries an eight-year, 100,000-mile warranty, is mounted in the trunk and takes up about two cubic feet of space, reducing the trunk volume to about 11 cubic feet. It is recharged by regenerative deceleration and braking.

That’s where the jump start comes in. Daryl Wilson, the eAssist’s lead development engineer, said he and his team lobbied internally to include the peace-of-mind system, which for now may be unique to the La Crosse.

The eAssist has a conventional 12-volt battery. But it is used only to start the car and operate auxiliary features like the audio system when the engine is not running.

But the lithium-ion battery in the trunk also stores electricity. So if the regular battery weakens, the driver simply selects “jump start” from the vehicle menu and the trunk-mounted hybrid battery shoots a charge into the main battery, which then gets recharged after the engine starts. However, it won’t start a completely depleted battery.

The eAssist substitutes for the former four-cylinder engine, which accounted for about 15% of La Crosse sales. Buick officials expect that to rise to about 25% for the eAssist. La Crosse customers, as before, are mainly expected to choose the 3.6-liter V6 engine, which has received a horsepower boost to 303, with city/highway fuel economy of 17/27 miles to the gallon.

Going green with the eAssist exacts a penalty in the form of leisurely acceleration. The factory estimates a zero to 60 miles an hour acceleration time of 9.3 seconds. That’s two seconds slower than the V6 La Crosse and means that the eAssist could be smoked at stoplights by some economy cars.

On the road, it doesn’t feel sluggish, although you have to carefully estimate your passing distances on two-lane highways. But the electric motor does kick in at opportune times on uphill grades, which reduces but does not eliminate automatic transmission downshifting. Highway cruising is quiet, with little intrusion of mechanical, road or wind noise.

The La Crosse feels bigger than its mid-size classification. Because it was designed to appeal to luxury buyers in China, who often are chauffeur-driven, it has generous back seat room and comfort. The center-rear position, however, is severely compromised by a hard cushion and a large floor hump.

Although the trunk is small because of the battery pack, the rear seatbacks fold to expose an opening about two feet wide by four inches high—enough to accommodate long skinny items.

The La Crosse interior is nicely done, with soft-touch materials and faux wood grain trim tastefully applied. Front seats are supportive and comfortable for long distances. On the test car, the rear-seat entertainment system had screens that folded neatly into the front seatbacks.

The eAssist starts at $30,820. With a load of options that included blind spot warning, navigation, and heated and cooled front seats, the tested La Crosse came in at $40,070.

UPDATE: If you must have speed, Cadillac has extended its 556-horsepower V power train to the CTS station wagon, one of the hottest wagons you can buy. Zero to 60 from the 6.2-liter supercharged V8 is less than four seconds, but figure on a sticker of about 70 grand.