Ethanol

GM markets many of its cars as flexible fuel vehicles, meaning they can run on a fuel mixture up to 85 percent ethanol and just 15 percent gasoline, referred to as E85. Ethanol is an alcohol commonly produced from corn in the United States and sugar cane in other parts of the world.

Burning ethanol as a fuel reduces carbon dioxide emissions by 21 percent over gasoline and releases no particulate matter. A downside to ethanol is that, because the fuel is made from food crops, heavy demand for ethanol drives up food prices. Because global food prices are rising, the trade-off is problematic. Current research into ethanol made of grass or even trash seems poised to help ease this controversy.

In the United States, Ford, GM, Chrysler, Mercedes-Benz, Mazda, and Nissan sell cars that can run on ethanol mixes up to E85. The National Ethanol Vehicle Coalition hosts a map on its website showing the locations of E85 stations around the country.

Hybrids

Hybrid cars have proven a popular green alternative. These cars have a battery pack that gets charged from excess engine energy and braking, and an electric powertrain alongside a traditional gas engine. When the car needs extra power, the electric powertrain draws on the batteries to help turn the wheels. Hybrid cars generally get better mileage, produce lower amounts of the regulated emissions than their gasoline counterparts, and are especially economical in traffic.

There are two main varieties of hybrids on the roads today, full hybrids and mild hybrids. A third type, plug-in hybrids, have generated interest, but are not in mass production.

Full Hybrids
The most successful example of a car that uses a full hybrid powertrain is the Toyota Prius. Full hybrids use one to three electric motors that drive the car under full electric power. Typically, full hybrids run under electric power at low speeds, up to 25 mph. The electric motors also contribute power when the car accelerates, assisting the gas engine. A particular virtue of full hybrids in urban areas is that the engine shuts down when the car is stopped or creeping slowly in traffic, reducing all emissions to zero. The engine comes back on when the battery reserves run low or the driver requires more speed or acceleration.

There is a wide array of full hybrid cars available today, from sedans to SUVs. Because of the extra equipment, especially the batteries, hybrids tend to cost more than their gas-only equivalents. On the plus side, many states let hybrids use car pool lanes no matter how many occupants are in the car.

Mild Hybrids
Honda pioneered the mild hybrid, and there are more recent examples from GM. Mild hybrid systems are less robust than full hybrid systems, differentiated by the fact that they don’t run in an electric-only mode. They usually integrate one small electric motor with the gasoline powertrain. As in a full hybrid, regenerative braking charges the batteries. Mild hybrid systems can also shut down the engine when the car is stopped at a light or in traffic, but won’t drive the car under electric power in slow traffic. Emissions will only drop to zero when the engine is stopped. Mild hybrids cost less than full hybrids, but don’t deliver nearly the fuel savings and emissions reduction.

Plug-in hybrids
Tinkerers have found that recharging a full hybrid’s batteries by plugging it in can yield 100 mpg. To convert a full hybrid into a plug-in, the car has to be modified, usually by changing the batteries from nickel metal hydride, used in current production full hybrids, to lithium ion, a battery type that better handles deep discharging.

As there are currently no production plug-in hybrids, the only option is to buy a hybrid and pay for the conversion, which ranges from $6,000 to $24,000. Major automakers, including Toyota and GM, have expressed interest in developing plug-in hybrids, but no prototypes have yet been shown.

Natural Gas Vehicles

Natural gas is commonly used in the United States to heat homes and water, and fuel ovens and stoves. It can also fuel internal combustion engines, usually as compressed natural gas. Of the regulated emissions, CNG cars produce substantially less carbon monoxide and nitrogen oxide than similar gasoline cars.

Natural gas is currently cheaper than gasoline, and mostly produced domestically, but finding a filling station can be difficult. Natural gas filling stations appropriate for vehicles can be found on the Department of Energy’s Alternative Fuel Locator website. Few car companies make a production model for the United States market, so most are conversions. To get a CNG-powered car, buyers should look at the Honda Civic GX.

Electric Cars

Of the various green car choices, electric cars are the greenest. They produce no emissions, neither smog producing gases nor greenhouse gases, and qualify as Zero Emissions Vehicles. The major drawback of current electric cars is that they don’t offer the performance of gas-powered cars, most notably in range. Current production and research electric cars can travel from 40 miles to 100 miles. Proponents argue that the average commute is less than 20 miles each way, making electric cars work well as everyday vehicles.

No major automaker currently sells a production electric car, although Subaru and Mitsubishi have announced research projects into small, urban-oriented cars. Some notable small manufacturers, such as Tesla, Phoenix, and ZENN offer electric vehicles.

Range Extender Vehicles

GM made big news when it showed off the Chevrolet Volt concept car at the 2007 Detroit auto show. The Volt is an electric car with an onboard electricity generator designed to improve its driving range, otherwise known as a range extender vehicle. Ford has also developed an REV platform called Hy-Series, and has shown it off in concepts at auto shows. REVs have a full electric powertrain, with one or more electric motors driving the wheels and drawing power from a battery.

An onboard generator, such as a small gas engine or a hydrogen fuel cell, generates electricity to recharge the battery as needed. REVs are also designed with plug-in capability to minimize the use of the onboard generator, thereby reducing emissions. Public interest in the Volt led GM to promise a production model sometime about 2010.

Hydrogen Cars

Because standard electric cars have such short range, carmakers have researched using hydrogen fuel cells to generate extra electricity. Fuel cells draw electricity from the reaction when hydrogen combines with oxygen to form water, making water the only emission from a fuel cell vehicle. Almost every major automaker has a fuel cell research program, which has led to good gasoline-equivalent performance from the prototypes, with ranges in excess of 300 miles.

GM has placed 100 Chevrolet Equinox fuel cell cars with consumers as part of its Project Driveway test, while Honda is leasing its Clarity fuel cell vehicle to a select set of consumers. Both of these programs are confined to specific geographic areas, and the automakers choose the participants.

Vegetable Oil

Believe it or not, some people have converted their cars to run on straight vegetable oil, because it can often be obtained for free, and produces very, very low emissions. Unfortunately, because it has a tendency to gel, VO-fueled cars need a separate tank for the oil, and a preheater to keep it in liquid form. While vegetable oil can often be obtained for free from restaurants that use it for frying foods, it must be filtered to remove any solids before it is put in the tank. Believe me, you don’t ever want a french fry in your tank.