Prius plug-in hybrid put to the test

  • 05-Feb-2011 02:22 EST
aei-Prius-ACbutton.jpg

Pressing the A/C button on the key fob turns on heat and/or A/C to “precondition” the cabin while the Prius PHV is plugged into household current.

The plug-in hybrid vehicle (PHV) version of the Toyota Prius, scheduled for introduction early next year, is a tentative step that demonstrates the difficulties ahead until lithium-ion battery technology advances and costs drop. A test fleet of 500 is currently undergoing field-testing in Japan, Europe, and the U.S.

Automotive Engineering International was able to evaluate one fleet car in early January. Although it has a prismatic lithium-ion-nickel battery pack, the PHV uses the same 1.8-L four-cylinder engine, electric motors, and P401 electronic transaxle as does the conventional Prius hybrid.

In electric vehicle (EV) mode, the PHV has a range of approximately 12-13 mi (19-21 km). Our test average was 12.4 mi (20.0 km) with heater off. It dipped only slightly to 11.4 mi (18.3 km) with the heater set to 68°F (20°C) in an ambient of 22°F (-6°C), because we “preconditioned” the cabin for an hour with household electricity. Preconditioning, while the vehicle is being recharged, is enabled by pressing an A/C button on the remote key fob. It operates only the PTC (positive temperature coefficient) heaters at ambient temperatures up to 28.4°F (-2.0°C). At higher ambients, the A/C compressor also runs, and in warm weather the heaters are off.

In highway use, our recharged test vehicle operated in EV mode up to about 60 mph (97 km/h). But even with some EV range remaining, at 65 mph (105 km/h) and above the PHV was observed operating entirely on engine power, like the hybrid. After EV charge depletion, the PHV reverted to series parallel power flow at speeds up to 60 mph

In hybrid operation, the PHV delivered an indicated 50.7 mpg in “ECO” (economy) mode—in our mix of 250 mi (400 km) of suburban, city, and moderate-speed highway operation. That corresponds closely to the U.S. EPA “window sticker” fuel economy numbers (51 mpg city, 48 mpg highway) for the Prius hybrid currently on sale. And therein lies a dilemma for Toyota as well as its competitors. Motorists seeking high fuel economy and range over the approximately 100 mi (160 km) range of EVs currently on the market can see the conventional Prius hybrid as a choice, priced at $22,800 (up to $28,070 for a fully optioned model). Further, Toyota has announced that a lower priced two-door Prius is coming early in 2012.

That price range might turn out to create a ceiling for electrified middle-market competitors, both recently on sale and scheduled to be introduced. Also apparently affected will be the production version of this Prius PHV itself, with its lithium-ion battery pack vs. the nickel-metal-hydride (NiMH) pack in the Prius hybrid. Over the years, Toyota and battery partner Panasonic have produced some 2 million NiMH batteries and lowered their cost by more than 75%. This has enabled Toyota to price the Prius competitively against the simpler Honda hybrid system vehicles and Volkswagen diesels. The current price advantage of NiMH is why Toyota has been saying this battery type will remain in use for its conventional hybrids for years to come.

The pricing issue is exemplified by the PHV’s pure electric vehicle (EV) range of 12-13 mi. Toyota could have chosen any moderate mileage range and installed an appropriate battery pack. However, there’s that cost of lithium-ion batteries as a constraint. So Toyota provides the Prius PHV with an EV range that should be useful for some reasonable number of motorists but presumably will not mandate a vehicle price that would be noncompetitive with its own NiMH model Prius.

This EV range also permits another economy: full charge with 120-V U.S. household current was accomplished by AEI in just 3 h (3-4 h is Toyota’s official estimate). This compares with 16-20 h at 120 V for EV competitors, or an expenditure for household installation of a 240-V charging station to speed recharging for their greater EV range. Although a 240-V station also would halve the charging time of the Prius PHV, the time that would be saved has minimal significance.

The PHV edition is likely to provide a maximum of 3500-4000 mi (5600-6400 km) per year of EV operation, equal to 70-80 U.S. gal (265-303 L) of Prius hybrid gasoline use. Annual net gasoline expense saving over the Prius NiMH hybrid, minus cost of electricity, is perhaps US$110. What additional price premium PHV buyers would pay largely for the convenience of short-trip EV operation is a concern, a Toyota spokesman conceded, noting that the company must proceed cautiously.

The PHV Prius weighs 3350 lb (1520 kg), about 300 lb (136 kg) more than the hybrid. It has a triple battery pack under the rear seat and in place of the spare tire (so an inflator kit is provided). Total capacity of the three packs is 5.2 kW·h, but only two (each 1.75 kW·h) are for EV operation. The two EV batteries discharge in sequence to 80%. After depletion, the PHV switches to hybrid operation, primarily on the third battery.

EV-operation batteries charge directly from the household outlet. The third one charges through the inverter, a protective strategy appropriate for its operating mode (hybrid vs. EV) and therefore state-of-charge vs. the others.

The pack is air-cooled and uses three blower fans controlled by a module that uses 42 sensors to monitor battery cell temperatures.

Toyota has said that based on input from the field test, it might change to a different battery pack and operating strategy for mass production.

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