Understanding the 2012 Buick LaCrosse with eAssist System
V. V. Haldavnekar
With the 2012 Buick LaCrosse General Motors (GM) introduced the second generation Belt Alternator Starter (BAS) system, officially known as the eAssist, at the 2010 Los Angeles Auto Show. GM claims that eAssist, which means "light electrification" helps the near 2 ton car achieve a 25 percent increase in the fuel economy compared to the fuel economy of the 2.4L, 4 cylinder/6-speed LaCrosse. GM expects the eAssist LaCrosse to deliver a class leading fuel economy of 25 mpg city and 37 mpg highway. The system will be standard on all 4-cylinder equipped LaCrosses.
The eAssist system in the 2012 Buick LaCrosse is coupled with a 180-hp 2.4L Ecotec direct-injection 4-cylinder engine and a 6-speed automatic transmission. An advanced 115V lithium-ion battery and latest-generation 15-kW motor-generator unit help increase fuel economy through regenerative braking that provides up to 15-hp of electric power assistance during acceleration, automatic engine shut-off when the vehicle is stopped, aggressive fuel cut-off during deceleration down to zero vehicle speed, and intelligent charge/discharge of the high-voltage battery.
The eAssist system will become the standard powertrain on the LaCrosse starting next summer, with prices to start around $30,000.
Belt Alternator Starter System
GM introduced the BAS system in a 2007 Saturn Vue Green Line. The technology later proliferated into Saturn Aura Green Line and Chevrolet Malibu Hybrid. The BAS system is also referred to as a mild hybrid.
A mild hybrid vehicle is essentially a conventional vehicle with oversized starter motor that also acts as a generator and an electric turbo. The electric generator/motor/starter not only permits the engine to be turned off whenever the car is coasting, braking, and stopped, but also aids it to restart quickly and cleanly. Accessories can continue to run on electrical power while the engine is off, and as in other hybrid designs, the motor is used for regenerative braking to recapture energy. The larger motor is used to spin up the engine to operating rpm speeds before injecting any fuel. The motor also kicks in anytime when sensors detect heavier load on the gas engine, such as operations during a hill accent or under acceleration. Although unable to operate in pure all-electric EV mode, the BAS system provides improvements to both city and highway fuel efficiency over similar non-hybrid versions.
Given the simplicity of the system, BAS equipped vehicles are relatively inexpensive compared to a full hybrid counterpart. Another perceived benefit of the BAS technology is that it fits in the same space as a conventional engine. No significant modifications are required to the vehicle chassis to accommodate the BAS system, with the battery pack housed in the trunk or spare tire well. This allows the BAS equipped vehicles to be produced on the same assembly line as the non-hybrid versions, resulting in substantial cost savings and allowing flexibility in production.
The downside of early mild hybrids was that the air conditioning suffered in performance when the gasoline engine shut down, but with the eAssist GM assures that the problem is essentially solved using electric air conditioning. Another disadvantage BAS was that the engine starting and stopping operations were easily noticeable. The control system for BAS initially was somewhat crude in comparison to the full hybrid engine startups and for some people it turned to be annoying. However with 2012 LaCrosse eAssist, GM claims the start-stop efforts will be seamless.
GM's Generation I BAS System
The Generation I BAS used a 36 volt electrical system (operating range between 42 and 45 volts), which was powered by a 10kW nickel-metal hydride battery pack. The battery pack consisted of three 12V NiMH battery cassettes in series, with each cassette having two 12V modules in parallel. The electrical system operated a permanent magnet motor/generator unit mounted to the engine in a similar manner as to a conventional alternator. Through a high-tension drive belt, the BAS system was capable of starting or assisting the 2.4L Ecotec engine used in the application. A conventional 12V starter motor was retained and used during cold start operations. The air conditioning compressor was operated through a belt-driven pulley, but for fuel economy improvement it could be disabled in auto-stop mode if the "ECO" A/C mode has been selected by the operator.
A conventional 4T45-E 4-speed automatic transmission was used in the vehicles equipped with the BAS system. The transmission had been modified to include a more efficient final-drive ratio and included an electrically-driven pump to provide pressure in auto-stop mode.
The instrument cluster of the BAS vehicles was modified to inform the driver on operations of the hybrid powertrain:
- AutoStop indicator in the tachometer notifying the driver when the engine is in AutoStop mode,
- ECO (Economy) indicator light notifying the driver when the vehicle is exceeding the EPA fuel economy estimates,
- Charge-Assist gauge that indicates whether the system is charging or providing assist power,
- The Driver Information Center (DIC) is programmed to provide messages specific to the Hybrid operation.
The Gen I BAS system resulted in modest to significant gains in fuel economy. For 2009, the Malibu Hybrid was rated at 26 city and 34 highway – resulting in a 4-mpg increase over the base-level Malibu. The conventional Malibu started at $20,745, while the Chevy Malibu Hybrid started at $24,695. The 2009 FWD Saturn Vue BAS hybrid was rated 25/32/28 mpg for city/highway/combined cycles, respectively with improvements of 32% /24%/ 27% for city/highway/combined cycles, respectively over the base 4cyl FWD version.
The BAS equipped vehicles received mild reviews from the publications citing the deficiencies stated above. Cobasys supplied the BAS system's nickel-metal hydride battery pack for Gen I system. The company had to conduct a recall in 2008 apparently due to leakage of the battery modules.
GM Gen II BAS System (eAssist)
The eAssist is mated to a 2.4L Ecotec direct injection four-cylinder engine and a next-generation six-speed automatic transmission. The engine is rated for 180 horsepower. A 115V lithium-ion battery and a latest-generation 15-kW motor-generator replace the original 36V NiMH battery pack and the older 10 kW motor. Following are the significant features of the eAssist system:
- Providing up to 15 kW of electricity to charge the battery from regenerative braking,
- Providing up to 11 kW (15 hp) of electric power assistance during acceleration,
- Shutting off engine automatically when the vehicle is stopped,
- Cutting off power down to zero during deceleration, enabled by the torque smoothing provided by the motor-generator unit,
- Charging and discharging of the high-voltage battery using intelligent systems.
The eAssist system"s 115V air-cooled lithium-ion battery bolsters the 2.4L Ecotec four-cylinder engine with approximately 11 kW (15 horsepower) of electric power assist during heavy acceleration and 15 kW of regenerative braking power. Those numbers are significantly better than the 2 kW of power assist and 5 kW of regenerative power on the previous BAS system. The greater power capability enables greater energy capture during regenerative braking for improved fuel economy.
While the eAssist system shares the same basic belt-alternator-starter configuration of previous BAS designs, it delivers more than three times the power and is more refined and capable than the Gen I BAS system. Its ability to integrate regenerative braking with the latest lithium-ion battery technology creates a system that delivers fuel-efficiency gains.
The eAssist system's electric motor-generator is mounted to the engine in place of the alternator to provide both motor assist and electric-generating functions through a new engine belt-drive system. The motor-generator is a high-performance, compact induction motor that is liquid-cooled for increased performance and efficiency.
The system also enables the Ecotec engine to shut down fuel delivery in certain deceleration conditions, which saves additional fuel. While in fuel shut-off mode, the motor-generator unit continues spinning along with the engine to provide immediate and smooth take-off power when the driver presses on the accelerator. Furthermore, as the vehicle comes to a stop, the motor-generator unit spins the engine, bringing it to a smooth stop and also properly positions the vehicle for a smooth restart.
GM's global chief engineer of the eAssist system, Steve Poulos says that the battery system is designed to provide power assistance to the internal combustion engine, rather than storing energy for all-electric propulsion. Mr. Poulus further states that the eAssist is an extension of the conventional internal combustion engine, but not its replacement.
The eAssist power pack contains the 115V lithium-ion battery pack, the integrated power inverter and 12V power supply. It is located in a compartment between the rear seat and trunk, and is compact and lightweight. GM states that the system weighs only about 65 pounds. Due to its compactness, rear access to the trunk is allowed via the split-folding rear seat, an attribute which GM claims is not shared with other hybrid competitors. The trunk space for the eAssist equipped LaCrosse is down to 10.9 cubic feet compared to 13 cubic feet for the conventional 4-cylinder model. An electric fan cools the power pack, drawing air from a vent located in the package tray, behind the rear seat. The fans are designed to be quiet from the cabin.
To further boost the fuel economy, Buick LaCrosse with eAssist technology also features improved underbody aerodynamics and tires optimized for performance and fuel economy, as well as active control of front-end airflow that improves aerodynamics and engine warm-up. Electronically controlled shutters in the lower grille close at higher speeds to push more air over the vehicle, thus increasing the aerodynamic efficiency to enhance fuel economy.
LaCrosse with eAssist includes an ECO gage 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. The operation is achieved by greatly reducing the tendency of the vehicle to roll backward with the engine in shut-down mode.
A Special Mention – New GM 6-Speed Transmission
GM will introduce the next-generation Hydra-Matic 6T40 in the 2012 LaCrosse eAssist. GM says the new transmission incorporates features specifically designed to enhance powertrain efficiency.
Significant internal transmission changes to clutch controls and hardware are incorporated in the new transmission 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 drivability. The system's capability of providing some electric assistance at cruising speeds allows the driver to accelerate lightly or ascend mild grades without the transmission downshifting.
An auxiliary, electric-driven transmission oil pump has been added to the 6T40, which keeps the transmission primed and the fluid flowing when the engine shuts down at a stop. This auxiliary system keeps the transmission ready to perform when the driver accelerates, providing a seamless delivery and smooth driving experience.
Summing it up
GM has been one of the main proponents of BAS system and with eAssist has shown renewed commitment to the system. GM replaces the 4-cylinder Buick LaCrosse with the eAssist system as standard for the 2012 model. The price increase over the conventional 4-cylinder CX model is expected to be approximately $3,000, thus making the base MSRP of the LaCrosse in the range of $30,000. The price bump also aids in creating a gap between the Regal and the LaCrosse that are currently priced within few hundred dollars of each other. While the real world advantages of the system can only be judged by a road test, the 2012 Buick LaCrosse eAssist looks stronger on paper than any GM Gen I BAS vehicle had.
The infomercial from GM, portraying the eAssist technology can be found in the following video.