In recent years, hundreds of consumer complaints about Prius inverter overheating have been filed on the NHTSA website and posted on various online forums. In one of many examples, an owner reported that the car stalled out on a freeway when the dashboard warning lights indicated a hybrid power failure.
The inverter is roughly the size of a box for hiking boots with a thick aluminum case that is packed with high power transistors, capacitors, microprocessors and a liquid cooling system. The inverter is able to handle more power than often runs through a household wiring system. Its power conversion occurs in a series of transistors that turn on and off thousands of times per second.
According to University of Michigan electrical engineering professor Heath Hofmann, a hybrid systems consultant, the auto industry is trying to find a substitute for the transistors, which are prone to high temperatures.
The changes made in the software update could be reducing the amount of power that flows through the inverter, which could affect the Prius' fuel economy and emissions, said University of Maryland professor Michael Pecht, who founded the school's Center for Advanced Life Cycle Engineering, which focuses on electronics reliability.
"They could be reducing the battery use," Pecht said. "It would increase use of the gas engine. Absolutely, gas mileage goes down and emissions go up."
Hofmann also said the vehicle's fuel efficiency and emissions might be affected. But it is also possible that Toyota found a defect in the inverter software that caused the overheating. As an example, he said, the transistors could short-circuit if the software that controls the power switching was faulty.
A Toyota spokeswoman, asked whether the software would affect the vehicles' fuel efficiency or emissions, said, "We don't have any comment on that."
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