BAS (belted alternator starter) is a category of automotive

parallel hybrid Hybrid vehicle drivetrains transmit power to the driving wheels for hybrid vehicles. A hybrid vehicle has multiple forms of motive power, and can come in many configurations. For example, a hybrid may receive its energy by burning gasoline, but swi ...

technology that uses an

electric motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...

to contribute power to the

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

via a

serpentine belt A serpentine belt (or drive belt) is a single, continuous belt used to drive multiple peripheral devices in an automotive engine, such as an alternator, power steering pump, water pump, air conditioning compressor, air pump, etc. The belt may a ...

. By mounting this motor generator unit in the conventional location traditionally used for the standard automotive

alternator An alternator (or synchronous generator) is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field wit ...

, it permits a low-cost method of adding

mild hybrid Mild hybrids (MHEV) (also known as smart hybrids, power-assist hybrids, battery-assisted hybrid vehicles or BAHVs) are generally cars with an internal combustion engine (ICE) equipped with a minimally extended battery and an auxiliary electric co ...

capabilities such as start-stop, power assist, and mild levels of

regenerative braking Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy or potential energy into a form that can be either used immediately or stored until needed. Typically, regenerativ ...

. BAS systems differ from other mild hybrid systems as they are not run off the vehicle's crankshaft.

Toyota

In June 2001,

Toyota is a Japanese Multinational corporation, multinational Automotive industry, automotive manufacturer headquartered in Toyota City, Aichi, Japan. It was founded by Kiichiro Toyoda and incorporated on August 28, 1937. Toyota is the List of manuf ...

introduced a BAS hybrid system under the Toyota Hybrid System-Mild (THS-M) brand name. It operates similarly to other mild hybrids with a

start-stop system A start-stop system (also referred to as idling stop or micro hybrid) is a technology that automatically shuts down and restarts a vehicle's internal combustion engine to reduce idle time, with the aim of lowering fuel consumption and emissio ...

, in that it shuts down the engine as the vehicle comes to a stop and instantly restarts it when the brake pedal is released. THS-M is capable of modest power assist during launch acceleration and similarly modest levels of "blended" regenerative braking during deceleration. A 36-volt

lead–acid battery The lead–acid battery is a type of rechargeable battery first invented in 1859 by French physicist Gaston Planté. It was the first type of rechargeable battery to be invented. Compared to modern rechargeable batteries, lead–acid batteries ha ...

(housed in the vehicle's trunk and operating in a

42-volt electrical system In automobiles, a 42-volt electrical system was an electrical power standard proposed in the late 1990s. It was intended to facilitate increasingly-powerful electrically-driven accessories in automobiles, and lighter wiring harnesses. Electric mo ...

) is used to operate a permanent magnet motor/generator unit mounted to the engine in a similar fashion to a conventional alternator. Then, through a high-tension drive belt, the motor/generator unit is capable of starting or assisting the engine. A conventional 12-volt battery and starter motor are retained and used whenever the engine is cold as during initial start-up. Although unable to operate in pure all-electric EV mode, THS-M improves city and highway

fuel efficiency Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy, chemical potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work, w ...

over non-hybrid versions. According to Toyota, the system provides a 15% increase in fuel economy. The system is designed to be simple and easily adaptable across a wide range of vehicles, with no significant modifications required to the vehicle chassis to accommodate the system. This allows THS-M vehicles to be produced on the same assembly line as non-hybrid versions, producing substantial cost savings and allowing production to be adjusted more easily. The first production model equipped with THS-M was the eleventh generation Toyota Crown Royal Saloon Mild Hybrid, introduced in August 2001, combining the system with a 3.0 L 2JZ-FSE

straight-six engine A straight-six engine (also referred to as an inline-six engine; abbreviated I6 or L6) is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine balanc ...

. It was followed in October 2002 by the Toyota Crown Sedan Super Deluxe Mild Hybrid, which combines the system with a 2.0 L 1G-FE engine.

General Motors

Generation I BAS

General Motors General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. The company is most known for owning and manufacturing f ...

introduced a BAS hybrid system in the 2007

Saturn Vue The Saturn Vue is a compact SUV that was built and marketed by Saturn, and it was Saturn's best-selling model. It was the first vehicle to use the GM Theta platform when it was introduced in 2001 for the 2002 model year. The Vue was facelifte ...

Green Line. The BAS is in the 2008–2009

Chevrolet Malibu The Chevrolet Malibu is a mid-size car that was manufactured and marketed by Chevrolet from 1964 to 1983 and from 1997 to 2025. The Malibu began as a trim-level of the Chevrolet Chevelle, becoming its own model line in 1978. Originally a rear-w ...

Hybrid. It operates on the same principle as Toyota's THS-M using a 36-volt electrical system (at 42–45 volts). According to the EPA, the 2009 Saturn Vue BAS hybrid garners an improvement of 32% city (19>25mpg) and 24% highway (26>32mpg) making the combined economy improvement 27% (22>28mpg) over the base 4-cylinder FWD version. The system is reasonably simple and inexpensive, making BAS equipped vehicles some of the least expensive hybrids available. Vehicles with the BAS system use a conventional 4T45-E

automatic transmission An automatic transmission (AT) or automatic gearbox is a multi-speed transmission (mechanics), transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions. The 1904 ...

which has been modified to include a more efficient final-drive ratio and includes an electrically driven pump to provide pressure in auto-stop mode. The

air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

compressor continues to be operated through a belt-driven pulley, but for fuel economy improvement it can be disabled in auto-stop mode if the "ECO" A/C mode has been selected by the operator.

Cobasys Cobasys LLC is an American automotive technology corporation. It supplies nickel metal hydride (NiMH) batteries, battery control systems, and packaged solutions for automotive applications, uninterruptible power supplies, telecommunications appl ...

, which supplies the BAS system's battery pack, had to conduct a recall in 2008 due to apparent internal leakage of the battery modules. Citing supplier issues and slow sales, GM has relegated the 2010 model year BAS hybrids to "fleet only" status.

Generation II BAS (eAssist)

At the

LA Auto Show The Los Angeles Auto Show, also known as the LA Auto Show, is an auto show held annually at the Los Angeles Convention Center in Los Angeles, California, United States. It is open to the public for ten days, filling of exhibit space. Since 2006 ...

, on November 15, 2010, General Motors announced that it would be releasing an all-new version of the BAS system available in the 2012

Buick LaCrosse The Buick LaCrosse is a four-door, front-wheel-drive sedan manufactured by Buick since model year 2005, and marketed variously across four generations for the North American and Chinese markets. The first-generation LaCrosse replaced the Centu ...

. While still a Belted Alternator Starter system, the system is named eAssist and includes a larger more powerful Hitachi-supplied 115-volt lithium-ion battery and a motor-generator that delivers of torque. The additional power provided by the more powerful battery and motor provides the ability to contribute more power, and more often able to electrically start and assist the 2.4L engine. The eAssist system also includes a specially modified GM 6T40 6-speed automatic FWD transaxle.

Generation III (BAS3)

A third generation was introduced in the 2016 Chevrolet Silverado/GMC Sierra and can improve fuel efficiency by about 13%. This adds about to the total weight of the truck but provides an additional and . The eAssist system includes a modified GM 8L90

. It uses the same battery cells from the

Chevrolet Malibu Hybrid The Chevrolet Malibu is a mid-size car that was manufactured and marketed by Chevrolet from 1964 to 1983 and from 1997 to 2025. The Malibu began as a trim-level of the Chevrolet Chevelle, becoming its own model line in 1978. Originally a rear-w ...

and the software is a modified version of the

Chevrolet Volt The Chevrolet Volt is an electric vehicle car that was manufactured by General Motors, and also marketed in rebadged variants as the Holden Volt in Australia and New Zealand and the Buick Velite 5 in China, and with a different fascia as the ...

's.

Other names

*Belt-driven starter generator (BSG) *Belt-driven integrated starter generator or belt mounted integrated starter generator (BISG or B-ISG) *Belt-driven starter alternator or belted starter alternator (BSA) *Dynastart

Vehicles

*2001–2003

Toyota Crown The is an automobile which has been produced by Toyota in Japan since 1955. It is primarily a line of mid-size luxury cars that is marketed as an upmarket offering in the Toyota lineup. In North America, the first through fourth generation ...

Royal Saloon Mild Hybrid *2002–2008 Toyota Crown Sedan Super Deluxe Mild Hybrid *2007–2009 Saturn Aura Green Line *2007–2009

Green Line *2008–2010

Hybrid *2012–2014, 2018-2019

with eAssist *2012–2014

Buick Regal The Buick Regal is a line of mid-size cars marketed by Buick since 1973. Serving as the premium mid-size/intermediate car of the Buick product range for nearly its entire production, the Regal initially served as the divisional counterpart of t ...

with eAssist *2013-14

with eAssist *2014

Chevrolet Impala The Chevrolet Impala () is a full-size car that was built by Chevrolet for model years 1958 to 1985, 1994 to 1996, and 2000 to 2020. The Impala was Chevrolet's popular flagship passenger car and was among the better-selling American-made auto ...

with eAssist *2016 Chevrolet Silverado/GMC Sierra with eAssist, only in California *2017

Audi A8 The Audi A8 is a full-size luxury vehicle, luxury sedan (car), sedan manufactured and marketed by the German automaker Audi since 1994. Succeeding the Audi V8, and now in its fourth generation, the A8 has been offered with either front-wheel dri ...

*2017

Audi A7 The Audi A7 is an executive five-door liftback produced by Audi since 2010. The coupé variant of the Audi A6 saloon/estate, the Audi A7 features a sloping roofline with a steeply raked rear window and integrated boot lid (forming the Sportback ...

*2019-2020

Jeep Wrangler The Jeep Wrangler is a series of compact and mid-size four-wheel drive off-road SUVs manufactured by Jeep since 1986, and currently in its fourth generation. The Wrangler JL, the most recent generation, was unveiled in late 2017 and is produced ...

with eTorque *2019 Ram 1500 with eTorque *2022

Jeep Wagoneer The Jeep Wagoneer and Grand Wagoneer is a sport utility vehicle (SUV) nameplate of Jeep vehicles, with several models marketed for the 1963 through 1993 model years and again since the 2022 model year. Various versions of the Wagoneer/Grand Wag ...

with eTorque *2022

Jeep Grand Cherokee The Jeep Grand Cherokee is a range of mid-sized sport utility vehicles produced by American manufacturer Jeep. At its introduction, while most SUVs were still manufactured with body-on-frame construction, the Grand Cherokee has used a unibody ...

with eTorque *2021

FIAT 500 The Fiat 500 (, ) is an Economy car, economy / city car that was manufactured and marketed by Fiat Automobiles from 1957 until 1975. It was sold as a two-door semi-convertible or saloon car and as a three-door panel van or estate car. Launched ...

Hybrid and

FIAT Panda The Fiat Panda is a city car manufactured and marketed by Fiat Automobiles, Fiat since 1980, currently in its third generation. The first generation Panda, introduced in 1980 as the Mk1, was a Car body configurations#Two-box design, two-box, thr ...

Hybrid, with BSG (Belt-integrated Starter Generator) *2023-2024 Audi Q5 45 TFSI (12V MHEV)

References

Bibliography

* * * * {{cite web, url=http://www.gm.com/company/gmability/adv_tech/100_news/hybridvue_10906.html , title=Saturn Vue Green Line Uses New, Less Complex Hybrid System , work=

press release , accessdate=January 9, 2006 , archiveurl=https://web.archive.org/web/20060315174408/http://www.gm.com/company/gmability/adv_tech/100_news/hybridvue_10906.html , archivedate=March 15, 2006 , url-status=dead Engine technology Hybrid powertrain Saturn Corporation General Motors hybrid technologies