Toyota is preparing to launch its second hybrid-only car, the Sai sedan.

The Sai, based on the Lexus HS 250h hybrid, will go on sale in Japan on Oct. 20, Toyota told Automotive News. The automaker didn’t disclose when or whether the car would be brought to the United States, where Toyota’s other hybrid-only car, the Prius hatchback, is the best-selling hybrid in the market.

Toyota has talked for several years of expanding the Prius brand name to other vehicles, and analysts speculate that if the Sai is indeed exported to the U.S., it will carry the Prius brand. 

The Sai apparently will have the same platform and powertrain as the HS 250h, which went on sale in Japan in July and is the only hybrid-only Lexus model. The exterior and interior of the two cars will be different, however. (You can see photos here.)

The HS 250h has a 2.4-liter gasoline engine, more powerful than the Prius’ 1.8-liter power plant. It’s less efficient too. The EPA rates the HS 250h’s fuel economy at 35 mpg in combined city-highway driving, compared with 50 mpg for the new 2010 Prius.

-- Martin Zimmerman