MILFORD, MI – The Buick brand faces a daunting task as it prepares to launch the ’12 LaCrosse: convincing U.S. customers that 4-cyl. engines have a place in midsize luxury cars.

Only Audi has succeeded with a long-term strategy in the U.S. that relies on an excellent direct-injection gasoline 4-cyl. in its highest-volume entry-level luxury cars.

Now, Buick is attempting to one-up the field not only by offering 4-cyl. power in the larger midsize LaCrosse but by pairing it with a mild-hybrid system that boosts fuel efficiency 25%.

General Motors’ confidence level in this new eAssist stop/start technology must be high because it will be standard on every LaCrosse that isn’t equipped with the 3.6L direct-injection V-6.

Going on sale this summer, perfectly timed as fuel prices are predicted to surpass $4 a gallon, the LaCrosse with eAssist clobbers the competition with an Environmental Protection Agency rating of 25/37 mpg (9.4-6.3 L/100 km) in city/highway driving. That beats the 4-cyl. Audi A4, Acura TSX and even the diesel 6-cyl. BMW 335d.

How is eAssist on the road? In a word, impressive.

On a short test drive along undulating 2-lane highways and an expressway here, the system performs as billed.

It propels the LaCrosse effortlessly by providing enough electrical boost under hard acceleration and on uphill grades to prevent GM’s next-generation 6-speed Hydra-Matic 6T40 automatic transmission from downshifting too often or too dramatically.

At stop lights, the engine shuts off, then springs back to life when the driver depresses the accelerator pedal, sending energy directly to the front wheels from the 2.4L Ecotec 4-cyl., augmented by power stored in a 65-lb. (29-kg) 115V air-cooled lithium-ion battery pack stashed in the floor behind the rear seats.

The overall result is a smooth, seamless experience that should please the sensibilities of most Buick customers.

At the end of our test drive, the fuel-economy gauge in the LaCrosse reads 26.6 mpg (8.8 L/100 km). But GM engineers say that number includes repeated thrashing by other drivers eager to experience eAssist at wide-open throttle.

A 4-cyl. engine in a midsize luxury car seemingly would make little sense, but eAssist and the LaCrosse feel as if they were made for each other. There is no sense that performance is compromised.

Granted, a customer wanting a sport sedan will be inclined to choose the 280-hp 3.6L V-6 but will have to settle for city/highway fuel economy of 17/27 mpg (13.8-8.7 L/100 km). In these days of volatile fuel prices, that’s a meaningful compromise.

For everyone else, the eAssist version will be more than adequate. By itself, the Ecotec I-4 delivers 182 hp and 172 lb.-ft. (233 Nm) of torque, and eAssist adds 15 hp and 79 lb.-ft. (107 Nm) of thrust, as early as 1,000 rpm.

’12 Buick LaCrosse eAssist
Vehicle type Front-engine, FWD, 5-passenger sedan
Engine 2.4L DOHC DI Ecotec all-aluminum I-4
Power (SAE net) 182 hp @ 6,700 rpm
Torque 172 lb.-ft. (233 Nm) @ 4,900 rpm
Transmission 6-speed auto
Electric Drive 15-kW belt-driven motor-generator
Battery ii5V Lithium-ion
Maximum electric power 15 hp @ 1,000-2,200 rpm
Wheelbase 111.7 ins. (284 cm)
Overall length 197 ins. (500 cm)
Curb weight 3,835 lbs./ 1,739 kg
Base price N/A
Fuel economy 25/37 (9.4-6.3 L/100 km)
Competition Acura TL; Lexus ES350; Lincoln MKZ
Pros Cons
No-compromise driving Success hinges on fuel prices
Highway rating of 37 mpg Test drive yields only 26.6 mpg
Pricing should be attractive Likely premium above $2,000

The eAssist system replaces GM’s less-impressive belt-driven alternator-starter technology that was introduced in 2006 and couldn’t compete with more advanced hybrid-electric vehicles arriving in the U.S.

The new system, although configured like the old one, relies on a 15-kW electric motor-generator that delivers three times the power. The unit is mounted directly to the engine, replacing the alternator.

During acceleration, the system provides up to 15 hp (11 kW) of additional electric power.

When decelerating to a stop, fuel to the engine is cut off, but the motor-generator continues spinning the engine, ensuring a smooth restart that is barely noticeable to the driver.

Regenerative braking further improves fuel efficiency by providing up to 15 kW of electricity to charge the battery.

Not to be overlooked is the capability of the 2.4L Ecotec, combined with eAssist, to confidently motivate a vehicle weighing 3,835 lbs. (1,739 kg). That’s significantly heavier than the three rivals targeted by Buick: the Acura TL, Lincoln MKZ and Lexus ES350; all of them V-6-powered.

Not long ago, a Buick buyer in this segment wanted at least a V-6 and occasionally a V-8. That the Ecotec with eAssist is up to the task in the LaCrosse speaks volumes about the advancing nature of today’s downsized powertrains.

The 2.4L DI I-4 has been available without eAssist in the LaCrosse since June. In that time, about 12% of LaCrosse customers have taken the 4-cyl. instead of the 3.0L or 3.6L V-6s. The 3.0L since has been discontinued.

With only the eAssist Ecotec and the thirstier 3.6L V-6 offered, volumes surely will escalate for the 4-cyl., given its impressive performance and fuel efficiency.

Even the sticker should be appealing. Pricing will be announced closer to launch, but Buick says the LaCrosse with eAssist (being produced in Fairfax, KS,) should start near $30,000. That represents a premium of about $2,200 over an ’11 4-cyl. LaCrosse.

For the ultimate fuel economy in a 5-passenger vehicle, the Toyota Prius remains the gold standard.

But the LaCrosse with eAssist has earned its place in the market by striking the right balance between performance, luxury and fuel economy.

tmurphy@wardsauto.com