GM's hybrids: beyond the urban environment.
The hybrid system is produced by GM Allison Transmission (Indianapolis). According to Tom Stephens, group vice president, GM Powertrain, the architecture of this system is a precursor to that which will be fitted into the Chevrolet Tahoe and GMC Yukon in late 2007. Remarks Stephens: "This bus uses the most efficient parallel hybrid architecture available in the world today, and has served as a starting point for GM's co-development of a two-mode full hybrid system for application in trucks, cars and sport utility vehicles with a variety of drive configurations, such as front-, rear- and all-wheel drive."
The buses at Yosemite are powered by diesel engines built by Cummins. But whereas there had been 8.9-liter engines deployed in the old bus fleet, the hybrid system allows the engines in the new buses to be 5.9 liters--while offering, Stephens says, 50% faster acceleration. While faster acceleration per se may not seem to be particularly key in a setting like Yosemite, that characteristic of the parallel hybrid system is important to the buses in services like King County, Washington (as in Seattle) and the 21 other cities around the country where the buses are deployed (there are 346 buses in service; so far, these buses have had more than 6.5 million revenue service miles). After all, oftentimes urban buses are merely stopping and starting, from one stop to another, an ideal situation for hybrid systems. Yes, Yosemite will get a fuel efficiency advantage, which is estimated to be from 20 to 55%. Yes, the air in the park will be cleaner, as the buses reduce NOx by about 60% and particulates, hydrocarbons, and carbon monoxide by about 90%. But one of the things that the park rangers at Yosemite are most excited about is the fact that the noise levels will be reduced by 70%, which is certainly key in an environment where it is nicer to hear the finches and warblers than engines.
Jeff Everett, service engineer, Allison, says a key benefit provided by the hybrid system for buses is that the maintenance requirements are greatly reduced as compared with conventional powertrain systems, which increases uptime. Once again, this is something that is particularly advantageous when used in urban transportation applications; it is still something that is important to the concessionaire at Yosemite, North Delaware, which is responsible for running the fleet.
If the maintenance, uptime, and fuel savings are taken into account over the life time of a bus--and transit buses tend to be in service for 10 or more years--then the premium for the hybrid system, which is estimated to be on the order of $150,000 to $200,000, is more than made up for.
Looking ahead to light-duty truck and passenger car applications, Stephens says that focus is on reducing the cost of the components, such as the electric motors. He says that what they're looking toward is a case where the intellectual property will be developed by GM, and then the manufacturing will be outsourced to other suppliers--after all, the manufacture of electric motor and battery packs are not the sorts of things that GM is particularly familiar with. He is not, however, looking for simple production sources, as he expects the suppliers to be conducting R & D. He says, for example, that he'd like the electric motor supplier to show him what the next couple of generations of motors would likely be.
Although Toyota (and to a lesser extent, Honda) is getting a lot of popular run for its hybrid consumer vehicle offerings, Stephens says that he's convinced that supplying buses (i.e., going after the big users of fuel rather than addressing compact cars and crossovers, which tend to be comparatively fuel efficient, anyway) is the better approach. He also says that they've been waiting for the release of the new SUVs on the GMT 900 platform for the hybrid deployment for smaller vehicles because had they put it in the GMT 800 models, they'd not have a sufficiently long life. They'll be waiting for about a year into the GMT 900 production to bring out the hybrid versions so as to not cannibalize conventional internal combustion engine sales.
In the words of Beth Lowery, GM vice president, Environment and Energy, they're pursuing "market-based technologies"-as in technologies that not only make environmental sense, but business sense, as well.--GSV
RELATED ARTICLE: ABOUT THE EP40 HYBRID SYSTEM
The hybrid transmission used in the buses consists of two 100-kW motors and a 600-volt nickel-metal-hydride battery pack; it works in conjunction with the 5.9-liter diesel engines used for the buses. In operation, there is either an input split mode or a compound split mode, with the former being deployed when the vehicle is launched from a stop or operating at low speeds; the compound split mode is engaged when the vehicle is at higher speeds. A controller determines which mode is to be deployed. What's different about this setup from single-mode hybrids is the fact that the two-mode system avails itself more of the mechanical benefits derived from the use of gears like a conventional transmission to multiply torque, thereby requiring smaller electrical motors than would otherwise be required. Even though the speeds are continuously variable, unlike a continuously variable transmission (CVT), there are no mechanical belts or bands used, with shifts between the two modes being synchronous.
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|Title Annotation:||NOTABLE; General Motors Corp.|
|Comment:||GM's hybrids: beyond the urban environment.(NOTABLE)(General Motors Corp.)|
|Publication:||Automotive Design & Production|
|Date:||Jun 1, 2005|
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