Internal-combustion engines consume relatively high amounts of petroleum，andcontribute heavily to air pollution; therefore, other types of fuels and nonconventionalengines are being studied and developed. An alternative-fuel vehicle (AFV) is a dedicatedflexible-fuel vehicle (one with a common fuel tank designed to run on varying blends ofunleaded gasoline with either ethanol or methanol) or a dual-fuel vehicle (one designed torun on a combination of an alternative fuel and a conventional fuel) operating on at leastone alternative fuel. An advanced-technology vehicle（ATV）combines a new engine，power train, and drive train system to significantly improve fuel economy. It is estimatedthat more than a half million alternative-fuel vehicles were in use in the United States in2002；50%of these operate on liquefied petroleum gas（LPG, or propane) and almost25%use compressed natural gas (CNG).
The ideal alternative-fuel engine would burn fuel much more cleanly than conventionalgasoline-powered internal-combustion engines and yet still be able to use the existing fuelinfrastructure（i. e. gas stations）．Compressed natural gas，propane, hydrogen, andalcohol-based substances (gasohol，ethanol，methanol，and other "neat" alcohols) all havetheir proponents. However, although these fuels burn somewhat cleaner than gasoline，the use of all of them involves trade-offs. For example, because they take up more spaceper mile driven，these alternatives require larger fuel capacities or shorter distancesbetween refueling stops. In addition, conventional automobiles may require extensivemodifications to use alternative fuels; for example, to use gasohol containing more than17%ethanol，the spark plugs，engine timing，and seals of an automobile must bemodified；since 1998，however, many U. S. automobiles have been manufactured withequipment that enables them to run on E85，a mixture of 85%ethanol and 15%gasoline.Fuels derived from plant materials，such as ethanol，are a popular concept because they donot deplete the world's oil reserves；in various locations，"biodiesel" test cars have run onfuel similar to sunflower-seed oil. Similarly, dual-fuel（gasoline-diesel and gasoline-propane) and water-fuel-emulsion cars are being tested.
Alternative propulsion systems are also being studied. Steam engines，which wereonce more common than gasoline engines，are being experimented with now because theygive off fewer noxious emissions；they are，however, less efficient than internal-combustion engines. Battery-powered electric engines，previously used mainly for localdelivery vehicles，can now be used in automobiles capable of highway speeds，but they arerestricted to relatively short trips because of limitations on the storage batteries that powerthe motors.
Some engineers worry that widespread adoption of electric cars might actuallygenerate more air pollution, because additional electric power plants would be needed torecharge their batteries. Therefore, design and research work has also intensified on solarbatteries，but they are generally not yet powerful enough to power such vehicles. The
most promising technology for electric engines is the fuel cell, but fuel cells currently aretoo expensive for practical applications.
Hybrid vehicles，or hybrid electric vehicles（HEVs)，are powered by two or moreenergy sources， one of which is electricity，to produce a high-miles-per-gallon, low-emission drive. There are two types of HEVs’, series and parallel. In a series hybrid, allof the vehicle power is provided from one source. For example, an electric motor drivesthe vehicle from the battery pack and the internal-combustion engine powers a generatorthat charges the battery. In a parallel hybrid, power is delivered through both paths，boththe electric motor and the internal-combustion engine powering the vehicle. Thus，theelectric motor may help power the vehicle while idling and during acceleration. Theinternal-combustion engine takes over while cruising, powering the drive train andrecharging the electric motor's battery. Some hybrids can operate in electric-only mode.Automobiles with gasoline-electric hybrid engines first appeared on the consumer marketin 1999；unhampered by the AFV's limitations, sales of these vehicles increased steadily atthe beginning of the 21st cent.