The hot asphalt of the test track stretches ahead. I stomp on the accelerator of the Fiat Siena and shift swiftly through the gears. The engine roars, the wind whooshes. A flock of quero-quero birds lazing nearby in the scorching Brazilian sun shriek in protest.

You’d never know it by looking at this peppy white sedan—or even by driving it—but it is one special little vehicle. When it comes to fuel, it is the most flexible car on earth. The Siena carries both natural gas and a mixture of gasoline and ethanol. It will burn the natural gas—the cheapest car fuel in Brazil—while cruising, and it will switch on the fly to the liquid fuel mix whenever it needs more power, for example, when passing another car or going up a hill.

And here’s the best part: you can put any mixture of gasoline and ethanol into its tank—from 100 percent gasoline and no ethanol to 100 percent ethanol and no gasoline. The engine automatically adjusts its ignition timing and the quantity of fuel injected into the cylinders on each cycle to get the most power out of whatever mixture you’ve got while keeping emissions under control.

Cars that can use different mixes of gasoline and alcohol have been around for years. And vehicles that let the driver switch between natural gas and gasoline aren’t new, either. But one car that can do both—switching automatically between the fuels and adjusting its engine to suit an arbitrary gasoline-alcohol mix—that’s very new indeed.

”The concept is very powerful, and it certainly is portable,” says William L. Sharfman, an automotive expert and principal of Sharfman & Co., a strategic consulting firm in New York City. ”Once you have the concept, it’s probably applicable to other kinds of fuel.”

As concerns about high oil prices and greenhouse-gas emissions drive legislation that is pushing demand for such alternative fuels as ethanol to record levels in many countries, the allure of such an engine isn’t hard to fathom. Sitting next to me, in the passenger seat of the Siena, automotive engineer Alfredo Silvio Castelli considers the many fuel mixtures taking hold around the world and asks, ”Why choose one if you can have all of them?”

Castelli is the head of the experimental laboratory here at the Brazilian unit of Magneti Marelli, in Hortolândia, an hour and a half drive from São Paulo. Marelli, a ¤4 billion subsidiary of the Fiat Group, is one of the world’s largest manufacturers of automotive systems, supplying fuel-­injection modules, robotized gearboxes, and onboard electronics to automakers and racing teams worldwide. Headquartered in Milan, Marelli has industrial and R&D facilities in 15 other countries, and it was a team of the company’s engineers here in Brazil that created the engine controller that lets the Siena run on multiple fuels.

Fiat recently began selling, in Brazil only, a version of the Siena with Marelli’s system. The car’s 1.4-liter, four-cylinder engine has one set of injectors for the ­liquid fuel and another for the natural gas. The liquids are stored in a 48-L conventional tank, the compressed natural gas in two 6.5â¿¿cubic-meter cylinders in the trunk. Marelli calls the system TetraFuel; it’s a reference to the fact that it can run on pure gasoline, pure ethanol, gasohol (in Brazil, a gasoline mix with 20 percent ethanol), or straight natural gas.

Marelli says that the TetraFuel system lets an average driver cut fuel expenses by 25 percent to 40 percent compared with an ordinary gasoline-powered car. Of course, the savings depend on the relative costs of ethanol and natural gas—both of which, as it happens, have been high in North America and Europe lately. Reductions in carbon dioxide, however, are independent of market forces. According to tests Marelli conducted, a TetraFuel car running on etha­nol emits, on average, 12 percent less CO2 than when it runs on gasoline; with natural gas, the reduction reaches 24 percent.

Still, the question hangs like a mid­afternoon haze over São Paulo: In an automotive sector that seems increasingly convinced that its future is in hybrids, electrics, and even fuel-cell cars, is there room for an alt-fuel vehicle whose technological breakthroughs are all in software and electronics rather than in batteries and ionic membranes?

Without a doubt, the economics are enticing. Consumers could be liberated from dependence on a single fuel and its price oscillations. Ethanol, natural gas, and other alternative fuels could become more attractive despite their much smaller distribution networks, because drivers would know they could always turn to gasoline if they found themselves in a place where they couldn’t get the other options.

Put it all together and multifuel technology starts looking like a bridge from petroleum to other possible technologies and fuels, such as bioethanol from cellulose waste. And it would let automakers keep making internal combustion engines, something they’ve gotten very good at during the past century.

Taking stock of the various electric and partially electric drivetrains that now dominate automotive R&D, Silverio Bonfiglioli, president and CEO of Marelli’s power-train division for North and South America, says, ”Those are alternatives, sure. But not for now. The alternatives available today at low cost and large scale are biofuels: ethanol and biodiesel.”