GM’s current hybrid strategy calls for implementing three distinct hybrid propulsion systems targeting different levels of fuel economy and price points.
At the high end is the two-mode hybrid system that GM will apply first in the Tahoe and Yukon full-size SUVs, followed by the Cadillac Escalade and the Silverado/Sierra pickup truck. At the low end is the current extremely light hybrid implementation in the Silverado Sierra. And in the middle is the hybrid powertrain to be first applied in the Saturn VUE Green Line hybrid (earlier post) that will begin sales later this year.
The VUE hybrid powertrain is based on a Belt Alternator Starter system—essentially what underlies a Start-Stop system—albeit one with some modifications that provide a small degree of traction assistance.
|A plot of estimated combined fuel economy vs. suggested price for four hybrid SUVs. The VUE is a price-performer.|
Combined with other vehicle modifications to decrease fuel consumption, however, the VUE Green Line hybrid delivers an estimated 20% improvement in fuel economy (+5 mpg) that brings it very close in consumption to the other hybrid SUVs with more complex hybrid drives that are currently on the market—and at a substantially reduced price.
The design targets for the hybrid system were, according to Steve Tarnowsky, Assistant Chief Engineer for GM Powertrain Hybrids in a talk given at the SAE Hybrid Vehicle Technologies Symposium 2006:
To create a simple, elegant hybrid architecture flexible enough to implement globally on a broad spectrum of both powertrains and vehicles. To that point, GM will deploy the system on the Malibu sedan next year, and will also deploy the system in a car in China (model and timing yet to be publicly announced).
A system that required no compromises in vehicle utility or performance. For the SUV customers, this partly means towing and acceleration—the Saturn VUE has a 1,500-pound towing capacity.
To deliver exceptional customer value. To be priced at less than $23,000, the Saturn VUE hybrid offers an estimated 29 mpg US combined, compared to the Ford Escape hybrid with 33 mpg US combined at a starting price of $27,500. The Toyota Highlander and Lexus Rx 400h both are rated at 30 mpg US combined, and priced at $33,000 and $48,500 respectively.
|The hybrid accessory drive. The dual tensioner assembly that controls the motoring and generating loads is patent pending. Click to enlarge.|
Functionally, the VUE hybrid system offers start-stop and regenerative braking—features expected in a simple Belt Alternator Starter system. GM, however, developed a dual tensioner assembly for the hybrid accessory drive (the motor/generator package) that will transfer a small amount of torque to the drive system for very brief periods of time.
The assembly combines an hydraulic strut tensioner and a friction-damped rotary tensioner on a common pivoting arm to the control the bi-directional loads (motoring and generating).
This assistance takes three forms: electrically motored creep at startup, light power assist during acceleration, and light electric mode during deceleration.
Upon startup, prior to the driver pressing the accelerator, the hybrid system will begin creeping the car forward to assist with a smooth transition to motion. This creep will last a maximum of 2 to three seconds, at which time the engine automatically kicks in even without the driver pushing the pedal.
|The different modes of operation in driving a VUE hybrid. Click to enlarge.|
During acceleration, the motor will contribute a small amount of electric assistance. And under deceleration, fuel cuts off to the engine, and electric assistance briefly kicks in case the driver wants to tip back into motoring mode. The goal with the latter assistance is, like creep during startup, to smooth the transition from no-engine to engine mode.
The assist is small: the motor (provided by Hitachi) is only a 4kW (mechanical) machine (5kW generating power), although it provides some 60 Nm of motoring torque.
GM’s approach with the VUE has been to eliminate any excess cost. The battery pack (from Cobasys) is just 36V. The hybrid control system is implemented within the existing 32-bit Engine Control Module. The instrument cluster only minimally reflects the hybrid drive.
But the VUE also will be the lowest-cost hybrid SUV on the market.
A Belt Alternator Starter system typically improves fuel consumption in city driving by about 8 to 10%. GM’s other modifications to the VUE (weight, aerodynamics, engine, management regime) contribute the remainder of the 20% improvement—and also highlight the importance of the interplay of a wide range of technology and design decisions in increasing fuel economy. (GM would not disclose the relative contributions from the different systems and changes.)
A different hybrid vehicle based on this BAS system—such as the upcoming Malibu or the Chinese sedan—may not deliver an equivalent 20% improvement. But BAS technology, widely applied, could provide a broad-based boost to fuel economy and reduced emissions.
Market researcher K. G. Duleep, Managing Director of Energy & Environmental Analysis, believes that there is growing possibility that regulators will require idle-stop as a means of reducing inner city emissions. And beyond that,
Within 10 years, all vehicles may be required to have stop-start.—K. G. Duleep
The sales performance of the VUE and subsequent GM BAS systems will provide good feedback as to how the market values this type of low-cost, low-impact approach.