Fuel 101: The truth about what powers cars

Given all the talk about electric cars, clean diesel, ethanol and more, it can be confusing to know how fuels work and their advantages and disadvantages. Here's a guide.

Gasoline

Atomically, the gas we pour into our tanks is a mix of hundreds of hydrocarbons, derived from crude oil. It's distilled in refineries to create motor and jet fuels, diesel, lubricants and other products. In fact, only 19 to 20 gallons of each 42-gallon barrel of oil becomes gasoline.

Gasoline is ignited by a spark in an engine cylinder. As it expands, it pushes down pistons to make your engine run. The burned gas, ejected through the engine's tailpipe, contains carbon monoxide, nitrogen oxides and hydrocarbons, all of which are noxious air pollutants.

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Each gallon of gas represents about 124,000 British thermal units, which is roughly equal to the amount of electrical energy the average U.S. family consumes in a day. Only an average of 15 percent of that energy is actually used to move your car -- engine and drivetrain inefficiencies eat up the rest.

Gas at the pump is rated by its octane grade, which denotes a gas mixture's ability to prevent engine knocking. It works out like this:

* Regular has octane ratings of 85 to 87, the lowest anti-knocking ability

* Midgrade is rated 88 to 90

* Premium is 90 and above and mostly needed for high-compression engines

Ethanol

Praised and vilified in equal measure, ethanol (and, to a lesser extent, methanol) is a biofuel additive that stretches supplies of gasoline. It's an alcohol derived from corn primarily as well as sugar cane and other plant materials, and it was first popularized by an oil-starved Nazi Germany. Today, ethanol comprises only about 3 percent of the total gasoline stores in the United States.

Generally speaking, regular unleaded gas in the U.S. contains no more than 10 percent ethanol (designated at pumps as E10). Beyond that ratio, engine modifications are required to keep exhaust pollution at acceptable levels. Specially modified engines in flexible-fuel vehicles can burn E85, which is 85 percent ethanol. Engines that run E85 are specially manufactured to deal with the high alcohol content in the fuel.

At best, ethanol can produce less pollution than gasoline, and at worst, it can be dirtier than gasoline. Much depends on the entire life cycle of ethanol production, going all the way back to farm tractors putting seeds in the ground. A lot of oil can be burned to create a gallon of ethanol that will extend fuel supplies and potentially burn cleaner than gas.

Of course, ethanol is a renewable energy source, which wins over many environmentalists as well as people who don't want to import oil. The downside is that redirecting edible biomass -- particularly corn -- toward fuel production can inadvertently jack up the cost of food.

Diesel

Like gas, diesel is distilled from crude oil and comprised of hundreds of hydrocarbons, including the carcinogen benzene. But diesel differs from gas in significant ways.

Diesel doesn't need a spark to explode in an engine's cylinders the way gas does. Compressing diesel and air alone is enough to ignite the mixture and power the engine. With more potential energy (139,000 Btu per gallon), diesel pushes a vehicle 20 percent to 40 percent farther than gas. Diesel creates more greenhouse gases than gasoline, but its greater efficiency means it produces 10 percent to 20 percent less of the pollutants per gallon than gas.

Its exhaust differs from gasoline's, too. There are more particles in diesel's exhaust, which can be seen as soot on tailpipes and adjacent bumpers. Fine and ultra-fine particulates have been linked to increases in asthma and lung cancer. Diesel fuel also has comparatively high sulfur content. Sulfur combines with trace metal alloys to act as a lubricant in engines, but it creates acid rain when burned.

Governments in North America and Europe have been restricting the amount of sulfur in diesel for about 20 years. Ceilings in the United States fell from 5,000 parts per million to 500 ppm (low-sulfur diesel) in 1993. All retail pumps in the country are required to dispense ultra-low-sulfur diesel, spewing just 15 ppm, by Dec. 1. Some engine modifications are needed in order to run on ultra-low-sulfur diesel.

As a pollutant, sulfur does double duty. Some antipollution devices capable of curbing other noxious fumes and soot can't be used with low-sulfur diesel engines because the dirtier fuel gums up the works. Ultra-low-sulfur diesel is, in a sense, clean enough to be cleaned.

In another interesting twist, U.S. buyers of diesel European vehicles -- which come standard with ultra-low-sulfur diesel-compliant engines -- have had to pay for engine modifications so that the cars will run on the low-sulfur diesel common here. In a sense, they pay to make their cars more polluting.

Efforts to reduce sulfur in diesel have added to pump prices, but so has increasing demand in the United States, Europe, India and China. In fact, according to the Department of Energy, diesel in the United States has topped regular gas prices for a few years because of its popularity and environmental regulations.

Biodiesel

Vegetable oils, animal fats and waste grease can be processed into a fuel called biodiesel that can be mixed with or even replace traditional diesel fuel in engines.

The big advantages of biodiesel are that it's renewable, nontoxic, biodegradable, produced domestically and less polluting than diesel.

Among its disadvantages are its price, which is currently higher than diesel, lower fuel economy and poor performance in cold conditions. It also emits more polluting nitrogen oxides than diesel. And, of course, it's difficult to find. There's a cottage industry in selling kits to "brew" biodiesel at home.

Hydrogen

Produced by separating hydrogen atoms from other atoms, hydrogen can be burned in internal combustion engines or used in fuel cells to power electric motors. Burned directly, the only pollutants it creates are nitrogen oxides, and it produces no pollution when used in fuel cells.

Of course, separating hydrogen from whatever it's combined with often results in pollution. The process involves steam, pressure, heat, electricity, sunlight or some combination of the five. Raw materials include coal, natural gas, biomass and water.

Hydrogen packs a smaller energy wallop than gas, which means more hydrogen is needed to push a car the same distance as gas. While strides are being made in terms of compact storage in vehicles, there's no hydrogen distribution network.

Electric

Electricity is the flow of electrons from one atom to another, and that flow is created mechanically using a generator or chemically as happens in batteries and living animals.

As a vehicle fuel, electricity performs the same function as a fossil fuel without combustion of any kind. Instead, it powers an electric motor, which transfers power to the transmission and in turn puts the vehicle's wheels in motion.

Electricity has been used to power vehicles since 1832, and there are two categories of electric vehicles today. One is battery-powered electric vehicles that recharge when plugged into the power grid, and the other is hybrids that have gas engines and electric motors.

The all-electric vehicle uses stacks of batteries to power an electric motor. The only polluting output of this vehicle comes from the utility company that sells the electricity. The big downside of EVs is the overall cost of ownership such as replacing the batteries, which run about $1,000 per kilowatt-hour. In this case, a kilowatt-hour is the length of time a fuel cell takes to fully discharge one kilowatt at a given voltage.

Hybrids are as efficient as internal-combustion-engine vehicles for several reasons:

* Electric motors produce a lot of torque at slow speeds, where gas engines typically suffer.

* The gas engine is small -- three or four cylinders -- and sized to deliver enough power for the average demand.

* Electric motors kick in during peak-energy needs, like driving uphill.

* Most models capture and store the energy released while braking.