Someday, millions of cars will come off assembly lines as plug-in hybrids. Analysts are mostly agreed on that. But for now, in the whole wide world fewer than 200 PHEVs roam the roads. Plug-in conversions are a cottage industry in North America, offered in kit or turnkey form by perhaps a dozen businesses and organizations. Over the past two years, the budding industry has been aided and abetted by a remarkable and vocal network of plug-in owners, advocates, and fans. They swamp Internet bulletin boards and e-mail their government representatives, arguing for a vast R&D effort aimed at producing cars that get 100 miles per gallon or more, powered as much as possible by grid ­electricity. The key enabler is recent advances in large-­format lithium-ion batteries [see “Lithium Batteries Take to the Road,” IEEE Spectrum, September 2007].

One influential advocacy group is the California Cars Initiative, a Palo Alto–based nonprofit start-up of entre­preneurs, engineers, environmentalists, and consumers. CalCars maintains a popular wiki on all ­topics having to do with plug‑in hybrids, particularly public policy and technology developments (http://www.calcars.org). By stirring demand, the organization hopes to “encourage automakers to produce 100+ MPG ‘no-sacrifices’ high-performance, clean hybrid cars,” to quote its mission statement. Or as Carl Lawrence, lead founder and CEO of converter Hybrids Plus, says, “It’s about creating the perception that this can be done—that if you’re not talking 60 or 80 miles per gallon, then you’re wasting your time.”

It’s quite a big tent of proponents, with some of them worried more about national security than ecology. R. James Woolsey Jr., former director of the CIA, told an IEEE symposium last fall that he views plug-ins as one way to help “destroy oil as a strategic commodity.”

Right now, though, if you want a plug-in, converting an existing conventional hybrid-electric vehicle is the only way to go. It will be three years or more before any of the major automakers sells a car designed from the ground up as a PHEV. Even then, the industry will most likely have to subsidize the cost of the battery packs for years. It’s widely assumed that Toyota has subsidized its hybrid vehicles, which make up almost 80 percent of the hybrids sold worldwide since the first-­generation Prius was introduced in 1997.

The second-generation Prius, introduced in 2003, is the highest-volume hybrid vehicle ever made. Toyota has now built more than half a million of them. Last year, the company sold 181 221 Priuses in the United States alone; the model’s distinctive wedge profile suggests “hybrid car” the same way the distinctive radiator shell of a Rolls-Royce intones “luxury.” The Prius also makes a surprisingly good PHEV, considering that it was never intended to be one. And to explain why, we’ll start by tearing apart a stock Prius.

Toyota’s Hybrid Synergy Drive system seamlessly shuffles power among the combustion engine, two electric motor-generators, and a battery pack. The main 50-kilowatt (67‑­horsepower) motor-generator does just one thing: it drives the front wheels through a reduction gear, which reduces the motor-­generator’s rotational speed to the wheels’ lower speed. The secondary motor-generator serves several masters. It recharges the 1.3-kilowatt-hour nickel-metal-hydride (NiMH) battery pack, and it supplements the power from the main electric motor when more propulsion is needed. The secondary motor-generator also quickly starts the gas engine. To save fuel, the Prius, like most conventional hybrids, shuts off its gasoline engine whenever the car is not in motion—for example, at a stoplight—and then restarts it when the car gets back up to about 25 km/h or whenever the driver steps down hard on the accelerator.

The mechanism by which the system shuffles power, and the heart of Hybrid Synergy Drive, is known as a planetary gear set. In this set, a central gear (the  sun”) connects to the drive shaft of the secondary motor-­generator. Its “planet” gears are in turn surrounded by a ring gear that drives, or is driven by, the main motor-generator—which is also connected to the differential that turns the wheels. The carrier for the planet gears is connected to the engine’s output shaft.

By varying the speed at which the planet gears spin, this arrangement allows the system’s control software to alter the power split between primary and secondary motor-generators. More speed means more power going to the main motor. All three components—the ring gear, the planetary carrier, and the sun gear—work in unison to control the torque output through the ring gear to the wheels.

In a Prius, then, engine power may either turn the wheels or recharge the battery pack. The car’s control software makes those decisions, splitting motive power between engine and electric motors, recharging, and regenerating power on braking.