GM Details New Hybrid Transmission

General Motors Corp. is providing details about the new hybrid-electric vehicle (HEV) drivetrain innovation, whose development program with DaimlerChrysler AG surprised the auto sector when announced last month.

The two companies, which have been criticized for slow adoption of HEV technology for production vehicles, say the Advanced Hybrid System 2 (AHS2) will be on the road in about three years and represents hundreds of millions of dollars in investment. (See related story: GM, DC Reach Hybrid Pact)

GM's initial focus for AHS2 is light trucks and SUVs, although it says the system’s architecture permits quick adoption for a wide variety of vehicles. DC is pursuing front- and rear-wheel-drive vehicle applications.

Electronically controlled and actuated “dual-mode” hybrid transmission more efficient than conventional hybrid-vehicle drivetrains, says GM.

GM Hybrid Powertrain Systems Engineering Eirector Pete Savagian says the system improves fuel economy by as much as 25% in combined city/highway driving, based on the Federal Test Procedure (FTP) used by the Environmental Protection Agency to rate light-vehicle fuel economy.

Savagian says several key system features explain AHS2’s marked efficiency-enhancing capabilities.

The system is based on two electronically-controlled and -actuated planetary gearsets. One gearset covers the ratio range from infinity to 1.7:1 and is the only gearset employed for high-load conditions up to about 60 mph (97 km/h).

At lower loads and at speeds both lower than and higher than the single-gearset top-speed level, a second planetary gearset, with a ratio range from infinity to 0.7:1, is engaged in combination with the single-gear mode.

The two gearsets are illustrated by the accompanying graphic and identified as “input split” and “compound split.” The second gearing is situated prior to the transmission’s final drive in the compound-split mode illustration.

Savagian says the 300-volt electrical system and its small, powerful electric motors – to control the gear ratio changes and provide battery charging via the engine, deceleration (regenerative) charging and short-duration power boost – is less than half the relative size of electrical systems in other single-mode HEV drivelines now on the market.

He says size is important because the electric path is mostly in the 70% efficiency range, whereas mechanical gearing (the planetary units) is up to 97% efficient.

He also says AHS2 not only makes less use of the lower-efficiency electrical path in general, but the electrical system power use is reduced to low levels characteristic of the hydraulic spin loss of conventional automatic transmissions when the planetary gearing ratio is stabilized at cruising conditions. This advantage is maximized by the system’s dual-gearing arrangement.

The AHS2 system further permits, by way of a broad ratio range, the optimization of both engine BMEP (brake mean effective pressure) and friction losses to optimize engine efficiency at all speed/load conditions.

Further optimization is achieved in combination with engine cylinder deactivation – which GM and DC both already use in production engines. The broad gear ratio range also improves launch acceleration, Savagian says.

The ASH2 system’s reduced reliance on electrical components is demonstrated by the fact that at maximum engine output of 300 hp, an additional 60 kW is available from the nickel-metal hydride (NiMH) battery pack for added acceleration boost. In effect, the electrical component of total maximum power is 21%, significantly less than other single-mode HEV systems.

The Toyota Prius electrical/gasoline engine power split is 46%/53%, for example, versus 21%/79% for the AHS2.

Savagian says brief electric power boost is useful as “tip-in” power at the initial phase of acceleration, which helps reduce fuel consumption and emissions during this transient point.

An important advantage of the system is that it fits in the same space as conventional automatic transmissions (not counting the 300-volt battery pack) and is applicable to current-production gasoline and diesel engines, thereby voiding the need for specially developed HEV engines combined with a larger amount of low-efficiency electrical hardware.

Savagian says AHS2 is able to provide all-electric drive at low speed for short distances but he will not quantify this aspect and is unable to give the useful battery capacity (kWh or hp/seconds) planned for the initial target vehicles.

The useful available energy from the Toyota Prius NiMH battery, for instance, is limited because the life of this type of battery depends on maintenance within a narrow state-of-charge range.

The system’s design, says Savagian, explains the virtue of the AHS2’s limited reliance on stored electrical power and a downsized internal combustion engine.

Savagian says collaboration with other auto makers will “enable more rapid (AHS2) technology advancement.” Nonetheless, it will be more than two years before motorists have their chance to evaluate ASH2.