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Prius fix blamed for gas mileage plunge

Ralph Vartabedian, Los Angeles Times on

Published in Automotive News

LOS ANGELES--When Robert Enger took his Toyota Prius into a dealership for a safety recall, he didn't expect that his fuel economy would drop.

Just six months after buying the new 2013 Prius, Enger learned that the company was recalling it to fix the car's hybrid electrical system, which was overheating and frying itself. A technician plugged the car into a diagnostic tool that installed new computer code in two electronic modules. That was supposed to fix the problem.

The repair itself has become controversial amid allegations that the electrical systems are still overheating after the software fix. But Enger noticed something else: His fuel economy dropped by 5 miles per gallon in city driving. Enger, an electrical engineer from Hermosa Beach, checks his mileage every fill-up, dividing the number of miles he drove since the last fill-up by the number of gallons he pumped to top off the tank.

About 800,000 Toyota Priuses in the U.S. were recalled in 2014 to address overheating that damages the car's inverter, a key part of the electrical power system. A lawsuit brought last year by one of Southern California's largest Toyota dealers asserted that the software fix did not solve the overheating problem and could lead to an abrupt loss of power. A related complaint by the dealer is now under review by the National Highway Traffic Safety Administration.

Academic experts contacted by the Los Angeles Times said it is likely the software change reduced the car's fuel efficiency. And a lawsuit this month in federal court makes allegation that Toyota "concealed from consumers that the software reflash decreased the fuel efficiency -- defeating the very purpose of owning these hybrid vehicles."

A statement by Toyota did not directly address questions about whether fuel economy and emissions are affected, but said the company would defend itself against such allegations.

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The company's documents show it modified not only software that controls the inverter's function, but also the software in the vehicle's powertrain control computer that determines how much power is supplied to the transmission by the gasoline engine and by the electric motors, according to the experts, hybrid vehicle engineers at major academic research centers.

The inverter, a device about the size of a large shoe box, boosts the battery's 200 volts to about 500 volts for the electric motors and converts the battery's direct current to alternating current (similar to what comes out of a household outlet). When the brakes are applied, the power flows in the other direction to charge the battery.

The change in the powertrain software and evidence of physical problems in the inverter probably shows that the company's modification reduced the power supplied by the battery and increased reliance on the Prius' four-cylinder gasoline engine, according to the academic experts who have reviewed those filings. If so, the car's fuel economy probably dropped and its emissions increased, they say.

The Prius has an EPA fuel economy rating of 51 miles per gallon in city driving for the 2010 model and 49 mpg for the 2014 model. Enger said his city driving mileage dropped from 49 mpg before the software change to 44 mpg afterward.


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