TOKYO – The Prius turned 20 this month.

The world’s best-selling hybrid car went on sale Dec. 10, 1997. Toyota sold only 323 of the bubble-shaped cars that year, enough for it to win the “Japan Car of the Year” award, although most models were not delivered until after the New Year.

Since its launch, Toyota has sold 4.3 million units. Cumulative hybrid sales by the automaker, including 36 other Toyota and Lexus models introduced over the past two decades, have grown to 11 million, including 1.5 million this year. Of these, 220,000 are Priuses.

No other hybrid model by any other automaker, including Toyota, is close. No other automaker is close. Honda, ranked second, has sold 2 million hybrids. The rest of the industry combined has delivered an estimated 2 million.

Counting plug-in hybrids, Toyota accounts for nearly two out of three electrified cars ever sold.

The Prius is profitable, according to analysts. No other automaker can make that claim about any of their electrified models.

Hybrids were not new to the industry when Toyota launched the Prius. They date back to the turn of the previous century in Europe. These early vehicles, plus others in the decades that followed, were developed for the purpose of demonstrating that the technology worked.

Fast-forwarding to the 1990s, hybrids became the focus of the Clinton Admin.’s Partnership for New Generation Vehicles program. In 2001, they were included in California’s Zero Emission Vehicle (ZEV) program as a stopgap when the state couldn’t meet its zero-emissions or all-electric-vehicle sales targets.

Ironically, the decision by California to retreat from its mandate effectively killed General Motors’ EV1 electric-car program around the turn of the century. The same decision effectively gave life to the Prius by granting it “advanced technology” partial zero-emission vehicle (AT-PZEV) status.

The EV1 and Prius crossed paths in another part of their separate stories by way of a Michigan startup named Ovonic Battery, at the time a wholly owned subsidiary of Energy Conversion Devices. ECD was created by the legendary American inventor Stanford Ovshinsky.

Batteries are Part II of the Prius story.

In their simplest form, hybrids combine two power sources to propel the car – generally a gasoline engine and an electric motor, which runs off a second battery. The car’s main battery continues to operate independently for starting the engine and powering various electrical systems including lighting, wipers and steering. The second battery is rechargeable.

Toyota’s hybrid differed from previous hybrids because it split the engine’s power, transmitting it separately to the vehicle’s wheels and a generator. The generator converts its portion of engine power into electricity to drive the car’s electric motor or, during braking, to charge the battery.

This energy-regeneration function during braking contributes to fuel efficiency gains ranging as high as 50% over standard cars. Other features contributing to improved mileage are high-efficiency motors, power electronics and the Atkinson-cycle engine.

Toyota exhibited a Prius concept at the 1995 Tokyo auto show. In March 1997 the automaker outlined its hybrid technology at a press conference in Tokyo.

The speculation was that Toyota would adopt the technology for a new car named “Prius” which, in Latin, means first. In fact, the Prius would be the first mass-produced hybrid car.

Toyota had rushed the car’s launch, assembling a team of some 2,000 engineers, to take advantage of the publicity surrounding the Kyoto Conference on Climate Change held during the first week of December 1997. COP3, as the conference was known, set numerical targets on reducing greenhouse gases for the first time.

More than 170 countries signed the accord on Dec. 11, the day after the Prius went on sale.