A new strain of gas/electric hybrid vehicles called plug-in hybrid electric vehicles, or PHEVs, represent the latest wave of fuel-efficiency-boosting technology being explored by carmakers and third-party firms. They promise to boost gas mileage to 100 mpg and beyond by running only on rechargeable electric batteries for limited distances. As the name suggests, they can be plugged in to recharge.

Unlike gas/electric hybrids on the market now, which can't be plugged in, PHEVs require more batteries in order to run on electric power alone. So part of the process of converting an existing hybrid, such as the Toyota Prius, into a PHEV is to add more batteries, particularly newfangled ones like lithium-ion batteries, which are lighter and hold a charge longer than lead-acid or nickel metal hydride batteries.

No PHEVs are currently sold by major auto manufacturers, but Toyota recently pledged to pursue the technology and independent companies are working to convert existing hybrids into plug-ins. A small Southern California company called EnergyCS hopes to offer kits to convert 2004 and newer Toyota Priuses to PHEVs by the end of the year for a target cost of $12,000 per conversion, including installation.

But execs at big car manufacturers, such as Toyota and Ford, have said that PHEVs are still a long way off. And with some evidence that consumer interest in hybrid vehicles is beginning to wane (see Hybrid Downturn? section), PHEVs could face even greater challenges than simply developing reliable technology at a reasonable cost.

Best of Both Worlds

Proponents argue that PHEVs offer the best of both worlds: an alternative fuel source with drastically reduced emissions without the worry of running out of juice. Many viable plug-in electric vehicles have failed over the past decade because of one thing: They can't get very far on a full charge.

And neither can PHEVs when running on electricity alone. But a PHEV has the advantage of behaving just like a gas/electric hybrid when the batteries have been exhausted, and this gives them unlimited range -- meaning that once the batteries run down, they can keep running on gasoline.

Plug-in Priuses currently being tested routinely get more than 100 mpg when driven in the city, thanks largely to their ability to operate only on electric power at speeds under 34 mph. By comparison, the soon-to-be-discontinued Honda Insight is the most efficient gas/electric hybrid currently available and returns an EPA estimated 60 mpg city/66 mpg highway.

Existing hybrid models generally run on gas and electricity at the same time -- except the Prius, Ford Escape Hybrid and Mercury Mariner Hybrid, which already can run on electric power alone, but only for very short distances and at low speeds.

An extension cord plugged into a simple 110-volt household outlet can effectively charge the batteries in PHEV systems currently being explored by EnergeryCS, a small engineering firm based outside of Los Angeles in Monrovia, Calif.

This is an important facet to PHEVs' success. "It's existing infrastructure, so you don't have to build new charging stations to make this technology available to people," said Peter Nortman, president of EnergyCS. "These outlets are in your garage … everyone from a 10-year-old to a 95-year-old can handle it."

Being able to run on electric power alone for extended distances and be plugged in to recharge the batteries are the key differences between PHEVs and conventional gas/electric hybrids, which cannot be plugged in. They instead rely on clever techniques to recharge the batteries, such as capturing energy dissipated during coasting or braking.

Although PHEVs appeal to environmentally conscious consumers, they wouldn't be as appealing to buyers of luxury vehicles, says Art Spinella, president of CNW Marketing Research in Bandon, Ore. "Luxury hybrid buyers are looking for performance, they're not looking for fuel economy," he said.

Lexus promotes its hybrid GS 450h sport sedan, which is equipped with a gas/electric V8 powerplant, as offering performance comparable to that of a conventional V12. Toyota, which operates luxury division Lexus, wouldn't give specifics on PHEV plans, including whether it will explore Lexus PHEVs.

A major benefit of both current hybrids and future PHEVs is that their widespread use promises to not only reduce consumption of non-renewable fossil fuels but also curtail atmospheric pollution and greenhouse gases, which many scientists say cause global warming. A 2004 climate-change study by the California Air Resources Board found that gas/electric hybrid vehicles produce 62 percent fewer greenhouse gases than conventional gasoline-powered vehicles.

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