Essentially, alcohol created from vegetable matter and mixed with gasoline or used undiluted and "straight up." It is 85 percent ethanol and 15 percent gasoline, commercially known as E85. GM and Ford both offer E85 compatible new cars and trucks designed to safely use this fuel (they can also run on regular gas).
An interesting historical fact is that diesel engines were originally designed to run on vegetable oil, not petroleum-based diesel fuel. Biodiesel is made from raw vegetable oil. it requires pre-heaters and other fuel system upgrades, is also less toxic than table salt -- and degrades as fast as sugar.
The idea of eliminating combustion engines entirely has always had tremendous appeal. Electrics cars were offered to the public since mid 1990s, the problems of limited range (typically less than 100 miles per charge), lengthy recharge times (several hours/overnight) and relatively poor performance compared with gas-powered (or diesel) vehicles remain.
There are also environmental concerns, including the storage/recycling of hundreds of pounds of lead-acid battery packs (per car) and the source of the electricity used to charge those battery packs. In the U.S., a large portion of the electrical energy we use is generated by coal-fired utility plants.
They produce millions of tons of carbon dioxide, a known greenhouse gas, each and every year. Until the environmental issues are resolved, it's not likely we'll see mass produced electric cars. Solar-powered vehicles are also in their developmental infancy and unlikely to see production anytime soon.
4- Hydrogen/fuel cells
liquid hydrogen as the "fuel." The electricity produced by the catalytic reaction in fuel cells can then be used to run electric motors which propel the car.
Unlike current electric cars, which have to be plugged in to recharge their batteries, a fuel cell vehicle creates its own electricity.
Hydrogen is the most abundant element and the energy is produced by a fuel cell free of harmful byproducts (water is the primary "emission"). However, practical problems remain: the economical mass production of pure hydrogen and the infrastructure (pipelines, refueling facilities, etc.) necessary to get the hydrogen to end users safely and efficiently.
But several automakers -- including General Motors and Honda -- have prototype fuel cell vehicles under development and we may see a breakthrough sometime during the next five to 10 years.
5- Compressed Natural Gas
Like hybrid gas-electric vehicles, the use of compressed natural gas (CNG) is seen as a workable intermediate step between conventional gas-burning cars and a future form of propulsion which doesn't use gasoline.
Large reserves of clean-burning CNG are availabe at the states and it is relatively easy to modify a conventional car engine to operate on this fuel. In addition, because CNG has long been used in the home, some of the necessary infrastructure to get CNG to end users is already in place.
GM, Ford and Chrysler have been building CNG-capable cars and trucks for several years -- and offering them for sale to both private individuals and municipal fleets. The cost per car is roughly $1,500 to $4,000 more than a gas-only version of the same vehicle.
While development of these future fuels continues, the automakers are also devoting much effort to continuous refinement of the century-old internal combustion engine. Today's gas engines run cleaner and more efficiently than ever before -- with no loss of power or performance.