Toyota demos Prius plug-in hybrids

  • 28-May-2010 03:39 EDT

Toyota's new Prius Plug-In Hybrid is one of 600 prototypes that will be road tested by average drivers as part of the company's global demonstration program of the technology.

Toyota highlighted another noteworthy step toward its planned 2012 introduction of plug-in hybrid vehicle technology when it recently let members of the domestic auto press test-drive the new 2010 Prius Plug-in Hybrid prototype in San Diego. The cars were the first of 150 that the auto maker is bringing to the U.S. as part of a global demonstration program that will involve road testing of 600 Prius plug-ins over the next two years, said Jaycie Chitwood, Manager of the Advanced Technology group at Toyota. Regional evaluation programs are slated for Boulder, CO, California, Washington, D.C., New York, Oregon, and Pennsylvania.

The demo effort is part of Toyota’s gradual and very deliberate build-up to plug-in hybrid technology. Given current battery development progress and costs, company planners see plug-in hybrid vehicles as the most realistic way to use grid electrical power for vehicle propulsion. But much remains that is unknown, so Toyota’s taking a conservative approach, Chitwood said.

By placing the prototype plug-ins, each fitted with a remote data-collection device from Qualcomm, into the hands of selected government and educational institutions, “we hope to find out what works and what doesn’t, what drivers like and don’t like,” she explained. Toyota planners expect the cars to gather data about real-world use of plug-in vehicles, while they gain insights into establishing warranty standards. They will also help promote the installation of charging stations.

One reason for the measured progress toward commercial plug-in hybrid technology is lingering uncertainty about the battery technology. “We’re taking a cautious approach to battery development,” Chitwood noted. The overall performance in real-world applications of the current, first-generation, large-format lithium-ion batteries is still relatively unproven.

Even basic issues such as optimal battery size remain unclear, so in the latest plug-in prototype Toyota “focused on a smaller battery that is recharged many times as opposed to a larger battery that’s recharged once in the evening,” she said. “The thinking is that plug-ins are good for capturing a lot of short trips in urban areas, but not so much for longer excursions that require better energy capacity.” Short-range applications require higher energy density to meet the greater demands of extended operations that entail large swings in the battery’s charge and discharge state.

According to Toyota management, the smaller the battery in a plug-in hybrid the better, both in terms of its carbon footprint as well as cost. They cite research conducted by Carnegie Mellon University investigators in 2009 that has shown that plug-in hybrid vehicles with smaller batteries, which are charged frequently—every 20 mi (32 km) or less—with average U.S. electricity, produce fewer greenhouse gas emissions than conventional hybrid vehicles. And as battery size grows, so does its cost.

Bill Reinart, Toyota’ National Manager of Advanced Technology Vehicles, wasn’t even sure that lithium-ion is the way to go. “We’re not totally convinced that nickel/metal-hydride batteries are done in hybrids.” He also worried about the lack of an installed electric-vehicle (EV) infrastructure and uncertainties regarding the ways in which plug-in cars will interface with the grid.

“There are many questions that need answering,” Reinart said, and then listed several, including: How will a car communicate with the grid? Who will ensure that different grids can talk to each other so billing can proceed smoothly? When does the owner charge the car? Is there daytime charging? Do you always want to charge the battery overnight? Who is going to pay for the charging infrastructure, including power transmission distribution to the house? And who will tell the homeowner that the house wiring is not up to handling a charger installation? The plug-in hybrid demo program is an effort to address these issues, he said.

The new Prius Plug-in Hybrid itself differs from the standard third-generation hybrid version only in its lithium-ion battery pack and support equipment, a recharging port and power cable assembly, some explanatory dashboard displays (such as an energy monitor to indicate the state of charge), a “blue mica metallic” paint job, body striping, and various distinctive emblems. Unlike its cousin, it has no spare tire. The engine is the same 98-kW (131-hp), 1.8-L gasoline engine and so is the 60-kW permanent-magnet synchronous motor, said Mary Nickerson, Toyota's cross-vehicle marketing manager.

The prototype plug-in model and its new lithium-ion battery pack means that plug-in Prius drivers can now go a bit faster, get somewhat better mileage over short distances, and travel as far as 13 mi (21 km) on EV power alone at a top speed of 62 mph (100 km/h) before the engine engages. (The last requires that you feather the accelerator after startup to avoid engaging the engine.) And of course, in 3 h they can renew the battery charge at a standard 110-V electrical outlet, or in 1.5 h with a 220-V connection (under development), hopefully at off-peak utility rates.

Nickerson said that figuring the actual fuel economy for the new plug-in was complex and depends on several factors related to the motorist’s individual drive cycle, such as trip distance, availability of recharging stations, driver aggressiveness, terrain, and hot or cold weather conditions (the last of which affect battery operations).

In early November 2009, Panasonic Electric Vehicle Energy, a joint-venture battery production company, started production of the first of more than 500 lithium batteries on a dedicated assembly line at its Teiho production facility in Japan. They went into the car’s three-module battery pack, which sits under the rear seat. It consists of a larger hybrid vehicle (HV) battery in the middle that is flanked by two electric vehicle (EV) sub-batteries that operate in tandem, Nickerson said. “The EV sub-batteries charge directly from an outside source, but to protect the HV battery, it’s charged with current that runs through the inverter."

The 5.2-kW·h pack, which is operated by smart digital controls, develops 345.6 V. Each unit contains 96 lithium-ion cells. The battery cooling system includes three blowers, ducting, and 42 temperature sensors. The 2010 Toyota Prius PHV has a mass about 300 lb (136 kg) more than a standard Prius, which places its curb mass around 3350 lb (1520 kg).

When the Prius plug-in first starts up, it operates in all-electric mode, wherein it draws electrical power directly from the first EV sub-battery. When the charge of the first sub-battery is depleted, the second EV sub-battery engages and feeds juice to the motor. When that sub-battery runs low, the propulsion system defaults to conventional hybrid-electric mode, in which the engine runs and the main HV battery the electrical duties.

Nickerson admitted that the lithium-ion power pack in the current Prius plug-in hybrid may not be set up the same way in the 2012 production model. “We want to really understand the conditions under which motorists will operate the plug-in Prius hybrid before we design and build a production model.”

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