Stop/start system on Buick's LaCrosse boosts mpg 25 percent

General Motors Co. is installing a relatively inexpensive start/stop system to give the 2012 Buick LaCrosse a 25 percent boost in fuel economy.

The technology, now dubbed eAssist, is modular and likely will spread across a range of GM vehicles.

"From a business standpoint," said Steve Poulos, eAssist global chief engineer, "there is an opportunity to bring this kind of light electrification and regenerative braking to the vast majority of our vehicles."

Poulos said eAssist is similar in principle to the belt-alternator starter start/stop system first used on the 2007 Saturn Vue Green Line.

"We looked at other economy-enhancing systems and realized that there were significant benefits to developing a more powerful next-generation BAS system in terms of performance and cost," he told Automotive News.

The 2012 LaCrosse with eAssist is expected to deliver 25 mpg in city driving and 37 mpg on the highway. Those numbers are up from 19 city and 30 highway for the 2011 LaCrosse--with the same engine.

Consumers will be given a clear choice: fuel economy or power. For an identical sticker price, they will be able to choose their 2012 LaCrosse with the eAssist 2.4-liter four-cylinder or the 3.6-liter V-6. The V-6 engine option costs $1,370 on the 2011 LaCrosse.

The pricing strategy is reminiscent of that for the 2011 Lincoln MKX Hybrid. Ford Motor Co. is charging the same for the hybrid and nonhybrid variants.

The eAssist system is more complex than the start/stop technology used in the 2011 Porsche Panamera and Cayenne but less complex than full hybrid systems such as those on the Toyota Prius or Ford Fusion Hybrid. Buick's eAssist is closer in overall function to the technology used in the BMW ActiveHybird 7 and the Honda Insight

Don't say 'hybrid'
GM will not refer to eAssist as a hybrid or mild hybrid, in part to avoid direct comparison to even higher-mileage hybrids.

The eAssist's defining component is an alternator/starter motor (the BAS) that is connected by a drive belt to the engine's crankshaft. The BAS is a three-phase AC induction motor that is capable of multiple functions: generating electricity like an alternator, recharging the mild hybrid's lithium ion battery array using engine torque or regenerative braking, starting the engine, and driving the vehicle in limited conditions.

GM didn't pursue the same technology as BMW and Honda, Poulos said, because "there are challenges with adding an integrated motor into transmissions. Particularly in a front-wheel-drive application, packaging becomes an issue because it adds length."

He also said GM gained valuable experience with the original BAS system and thought that further development of the concept led to significant economy gains from comparatively simple and cost-effective components.

The system's lithium ion batteries are from Hitachi. GM declined to name the BAS supplier.

The battery pack generates only 0.5 kilowatt-hours, much less than a Prius' 1.3-kilowatt pack or the Chevrolet Volt's massive 16-kilowatt array.

"Our duty cycle is unlike a full hybrid," Poulos said. "Hybrid batteries provide power for longer durations. Our system is designed to charge and discharge in bursts so we don't need more capacity."

Added horsepower
The eAssist system enables the start/stop function and adds up to 15 hp and 79 pound-feet of torque to help accelerate the vehicle. Unlike the first-generation system, eAssist alone also can propel the vehicle while cruising at steady speeds--say, along a flat highway.

In start/stop, eAssist delivers torque to restart the engine by spinning the crankshaft up to operating speed before the engine's ignition fires. The BAS then supplies torque assist as needed when the driver accelerates. The system also allows regenerative braking, converting energy from braking into electricity.

Buick's eAssist operates at 115 volts, about three times the voltage used in the 2007 Saturn Vue Green Line. This enables eAssist to produce 11 kilowatts of power compared with the Green Line's 2 kilowatts.

The Saturn also operated with a four-speed transaxle. The Buick uses a twin-clutch six-speed automatic.

The electronic controllers and battery array are housed in a single 65-pound unit behind the rear seat. The single module is one reason the system can easily be considered for other nameplates.

The packaging leaves the 2012 LaCrosse with 10.9 cubic feet of trunk volume, down from 13.3 in the 2011.

(Source: Automotive News)