Global Hybrid Cooperation, formerly Advanced Hybrid System 2 (AHS2), is a set of hybrid vehicle technologies jointly developed by General Motors, Daimler, and Chrysler LLC, with BMW joining in 2005. It uses 2 or 3 planetary gearsets in an automatic transmission: one on the internal combustion engine (ICE) side (input split) paired with a second (output split), forming the compound split, and possibly one third additional planetary gearset to multiply the number of fixed gear ratios (up to 4). General Motors has stopped using the "AHS2" name as of 2006, preferring to call it simply a two-mode hybrid system.
This technology was named as "Technology of the Year" for 2007 by Automobile magazine.[1]
The dual-mode hybrid concept, as described in 1994, was developed to optimize vehicle efficiency by switching between parallel and series hybrid operation, taking advantage of series operation in stop-and-go traffic to minimize engine speed variance, and parallel operation at highway speeds to use efficient mechanical transmission.[2] On September 30, 1993, the United States Department of Energy signed a contract with General Motors to develop and demonstrate hybrid electric powertrains for light duty vehicles.[3] AHS-2 was derived from a dual-mode hybrid system developed earlier by Allison Transmission, then a division of GM, for transit buses. The Allison system first had been announced for buses in 2003.[4]
The GM/DaimlerChrysler partnership was announced on December 13, 2004, with Dieter Zetsche of DaimlerChrysler joining Rick Wagoner of GM on stage with a prototype.[5] Negotiations had begun earlier in October, after engineering managers at both companies discovered they were independently working on similar dual-mode hybrid concepts while participating on the same conference panel.[6] The agreement was not signed until the following August, however. GM was reportedly responsible for development of rear- and four-wheel drive truck and front wheel drive car systems while DaimlerChrysler was focused on a rear wheel drive luxury car application. It was announced on September 7, 2005, that BMW would also join the alliance, likely using then-archrival DaimlerChrysler's rear wheel drive system.[7]
The three companies formed an organization called Global Hybrid Cooperation with engineering and management centered at the GM, DaimlerChrysler and BMW Hybrid Development Center in Troy, Michigan.[7] In 2006, an article published by Autoweek stated the three automakers planned to spend US$1 billion collectively to develop the front- and rear-wheel drive hybrid transmissions.[8]
The two-mode hybrid system was introduced to the light-duty vehicle market as a drivetrain option for the 2008 model year Chevrolet Tahoe and GMC Yukon,[9] with an observed improvement in fuel economy of 27–58% compared to a conventionally-powered equivalent. GM introduced the hybrid option for 2009 model year Chevrolet Silverado,[10] GMC Sierra, and Cadillac Escalade. The largest improvement was in the EPA city rating.[11] The dual-mode hybrid equipment added approximately, but this increase was offset completely by lighter seats, wheels, and 12-volt battery; aluminum engine and body components; and eliminating the starter motor.[12] It was equipped with a 6.0L LFA and LZ1 V-8 engines featuring Active Fuel Management, which was larger than the standard 5.3L LY5 base engine.[12]
The GM products were followed by a hybrid Dodge Durango and related Chrysler Aspen for the 2009 model year in late 2008; the 5.7L HEMI V-8 engine on these was equipped with a stop-start system to improve fuel economy.[13] [14] BMW and Mercedes introduced the X6 ActiveHybrid and ML450 Hybrid, respectively, in 2009,[15] [16] but these were discontinued in 2011.[17]
Earlier, on March 1, 2007, BMW and DaimlerChrysler announced that they were entering a separate partnership to develop a mild hybrid module for rear wheel drive premium cars.[18] [19] They planned to roll out the new system within the next three years on BMW and Mercedes-Benz vehicles. GM did not participate in the new partnership, and did not announce plans to develop a similar hybrid RWD system for cars.[20]
In 2009 Mercedes released the S400 mild hybrid, using a lithium ion battery.[21] It was reported in July 2009 that after the upcoming introductions of two-mode hybrid models of the BMW X6 (marketed globally) and the Mercedes-Benz M class (only in the United States), the two-mode hybrid joint venture would be dissolved. Daimler indicated that it wants to avoid investing in aftersales and service for a vehicle which will only be produced in small quantities, and will instead concentrate on modular hybrid building blocks with scalable lithium-ion batteries, based on the hybrid drive developed for the S-class and 7 Series sedans by the joint venture with BMW and auto supplier Continental AG.[22]
By 2014, the two-mode hybrid drivetrain was no longer offered on any light-duty vehicles.[9]
The dual-mode hybrid drive unit includes two AC motor-generators (MG-A and MG-B, each are three-phase permanent magnet machines with peak output),[23] three planetary gear sets (P1, P2, and P3), four selectively engaging friction clutches (C1, C2, C3, and C4), and two oil pumps.[1]
This hybrid drive unit is coupled to the engine, taking the place of a conventional transmission. The entire drive unit is comparable in size and shape to an Allison 1000 automatic transmission. A 300-volt battery pack is housed elsewhere in the vehicle to store energy for the two motor-generators.
General Motors has designated the drive unit as the 2ML70;[24] for BMW and DaimlerChrysler vehicles, it is the GM-Allison AHS-2.[25] The technology is known as a "two-mode" hybrid system because the transmission / drive unit can transfer either electrical power, mechanical power, or a blend of both to the wheels, operating both as a series hybrid, using the internal combustion engine solely to generate electrical power, or as a parallel hybrid, using the electrical motor(s) to augment the mechanical power from the engine.[26] The two modes of operation are:
This system amplifies the output of the electric motors similarly to the way in which a conventional transmission amplifies the torque of an internal combustion engine. It also, when required, permits transfer of more of the engine's torque to the wheels, making the transmission more efficient even without the electric motors in use. Although the transmission mechanically has only four conventional gear ratios, the electric motors allow it to function as a continuously variable transmission. This variable ratio functions in addition to the torque multiplication of the planetary gears.
Mode I ("input-split") is intended for the range of speeds commonly encountered during urban driving, while Mode II ("compound-split") is intended for the higher range of speeds encountered during highway travel. Within these modes, there are four possible fixed gear ratios, two in each mode, in which one additional clutch is engaged.
The lowest fixed gear ratio, which is in Mode I, synchronizes both motor-generators, allowing the vehicle to use these as motors to augment mechanical power for stronger acceleration, or as generators for regenerative braking. The second fixed gear ratio, also in Mode I, allows the motor-generators to freewheel which reduces losses at higher speeds and improves overall efficiency.
Similarly, the third fixed gear ratio, which is in Mode II, synchronizes both motor-generators to allow electric motor boost or regenerative braking, and the fourth fixed gear ratio, also in Mode II, allows both motor-generators to freewheel for improved efficiency, relying solely on mechanical power transmission to move the vehicle.
Generally, the vehicle will start from rest in Mode I; as speeds increase, the transmission will transition to the first fixed gear ratio, shift back to Mode I, then transition to the second fixed gear ratio before entering Mode II.
In Mode II, unlike Mode I, there are no defined speed setpoints to enter or leave the third or fourth fixed gear ratios, but the third fixed gear ratio is intended for changing speeds (accelerating to pass or slowing), using inputs from the throttle position, while the fourth fixed gear ratio serves the same purpose as an overdrive ratio, for improved efficiency at high speeds.
When reversing, the vehicle remains in Mode I, using one of the motor-generators for traction.
| Schematic | Clutch engagement | MG operation | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| C1 | C2 | C3 | C4 | MG-A | MG-B | ||||
| Mode I: First Power Split Mode (Input-Split) | rowspan=3 | Gen | Mot | Throughout Mode I, clutch C1 remains engaged. This locks the ring gear of planetary gearset P3, which in turn forces the rotational speed of the second motor-generator (MG-B, connected to the sun gear of P3) to be proportional to the output driveshaft (planetary gears of P3). The first planetary gearset pair acts as a four-driveshaft power-split transmission, and the last planetary gearset reduces the rotational speed. MG-A acts as a generator, while MG-B acts as a motor. | |||||
| within Mode I | First Fixed Gear ratio | Sync'd | In general, when maximum power is required at low speeds, C4 is engaged in addition to C1, entering the first fixed gear ratio. By engaging shunt clutch C4, the first two planetary gearsets (P1, P2) are maintained in a synchronous 1:1 fixed gear ratio, and the last planetary gearset (P3) reduces the rotational speed. With C4 engaged, both motor-generators are mutually synchronous, and both can be operated either as motors for maximum acceleration, or as generators for regenerative braking. It's the midpoint of the first continuously variable range, when both motor-generators rotational speed join. | ||||||
| Second Fixed Gear ratio | Off / freewheel | At higher speeds within Mode I, C2 is engaged alongside C1 to enter the second fixed gear ratio. All three planetary gearsets (P1, P2, P3) are active, collectively forming an intermediate fixed gear ratio, and both motor-generators can be turned off, with the vehicle motivated solely by mechanical power transmitted from the engine. The rotational speed of both motor-generators are very asymmetric (1:9), excluding effective usage of the first one (MG-A). It's the boundary between both continuously variable ranges. The two-modes switching occurs at this point, when the third planetary gearset ring gear reach zero rotational speed, and all the planetary gearset carriers gears reach the same rotational speed together. | |||||||
| Mode II: Second Power Split Mode (Compound-Split) | rowspan=3 | Mot | Gen | The vehicle enters Mode II from the second fixed gear in Mode I by disengaging clutch C1 while keeping C2 engaged. Again, the first planetary gearset pair acts as a four-driveshaft power-split transmission. Neither of the two motor-generators is directly coupled with input or output. At the midpoint of the second continuously variable range, both motor-generators match rotational speeds. MG-A briefly operates as a generator in Mode II when the vehicle is traveling at speeds in the lower range of Mode II, transitioning to operation as a motor at higher speeds. At the same transition point, MG-B switches from a motor to a generator. | |||||
| within Mode II | Third Fixed Gear ratio | Sync'd | The first planetary gearset pair (P1 & P2) are synchronized in a 1:1 fixed gear ratio by engaging C4, which also synchronizes both motor-generators; these can be operated simultaneously as motors, or as generators. | ||||||
| Fourth Fixed Gear ratio | Off / freewheel | The clutch C3 is engaged, which blocks the sun gear of planetary gearsets P2 and P3, and also takes the second motor-generator (MG-B) off-line. | |||||||
Operation of the Allison Transmission (AHS-2), or Two-Mode Hybrid, from the Global Hybrid Cooperation. This transmission is mounted on the BMW X6 ActiveHybrid and the Mercedes-Benz ML450 BlueHybrid.[28]
Per the patent, the planetary gearsets P1 and P2 are compounded, with MG-A driving (or being driven by) the sun gear of P1, which is coupled to the ring gear of P2, and the carriers for the planet gears in P1 and P2 are coupled. Similarly, MG-B drives (or is driven by) the sun gear of P2, which is coupled to the sun gear of P3 via a hollow shaft.
Toyota's Hybrid Synergy Drive may appear similar in that it also combines the power from an ICE and a pair of electric motor–generators; however in its current form, Toyota uses only one planetary gearset providing only single mode functionality (i.e. input split only) using a series/parallel architecture.[29]
Honda's Integrated Motor Assist uses a traditional ICE and transmission where the flywheel is replaced with an electric motor: it is a simple parallel architecture,[29] requiring the addition of a mechanical continuously variable transmission (CVT), i.e. not electrically variable.
The two-mode hybrid drive system manufactured by Allison Transmission was first used in New Flyer transit buses which entered revenue service in 2004;[30] in testing at the National Renewable Energy Laboratory, measured fuel economy improved by, representing an improvement of – % compared to a conventional diesel-powered bus.[31]
By 2008, Allison had delivered 1,000 hybrid powertrains.[32] Many transit operators since then have received buses with the Allison hybrid system, now branded eGen Flex,[33] including:
The longitudinal system for light trucks from General Motors will be manufactured at Baltimore Transmission by GM's PowerTrain division. The nickel-metal hydride batteries will be manufactured by Panasonic EV of Japan.
The system was introduced for the 2008 model year in the full-sized Chevrolet Tahoe and GMC Yukon SUVs as a specific 2-Mode Hybrid model. Rear wheel drive and four wheel drive light duty trucks using the 2-mode hybrid system include: