Are hybrid cars more environmentally friendly?

Hybrid cars, also known as hybrid electric vehicles (HEVs), are powered by two or more energy sources, one of which is electricity - a much cleaner energy source than petroleum fuels. It is estimated that more than 500,000 alternative-fuel vehicles were in use in the United States in 2002; of these, 50% operate on liquefied petroleum gas (LPG, or propane) and almost 25% use compressed natural gas (CNG).

Types of Hybrid Electric Vehicles

Two types of hybrid electric vehicles (HEVs) HEVs exist: series and parallel. In a series hybrid, the vehicle's power is obtained from one source. For example, an electric motor drives the vehicle from the battery pack, and the internal combustion engine powers a generator that charges the battery. In contrast, power in a parallel hybrid is delivered through both paths, with both the electric motor and the internal combustion engine powering the vehicle. Thus, the electric motor may help power the vehicle both while idling and during acceleration. The internal combustion engine takes over while cruising, powering the drive-train and recharging the electric motor's battery.

Some hybrids can operate in electric-only mode. Automobiles with gasoline-electric hybrid engines first appeared on the consumer market in 1999; unhampered by the alternative-fuel vehicle's (AFV's) limitations, sales of these vehicles increased steadily at the beginning of the 21st century.

Other Non-Conventional Fuel Engines

Internal-combustion engines consume relatively high amounts of petroleum, which contributes heavily to air pollution. Therefore, scientists are studying and developing alternative types of fuels and non-conventional engines.

An alternative-fuel vehicle (AFV) is either (1) a dedicated flexible-fuel vehicle, which means that it has a common fuel-tank designed to run on varying blends of unleaded gasoline with either ethanol or methanol, or (2) a dual-fuel vehicle - that is, one designed to run on a combination of an alternative fuel and a conventional fuel -operating on at least one alternative fuel. An advanced-technology vehicle (ATV) combines a new engine, power train, and drive train system to significantly improve fuel economy.

The ideal alternative-fuel engine would burn fuel much more cleanly than conventional gasoline-powered internal-combustion engines and yet still be able to use the existing fuel infrastructure (i.e., gas stations). Compressed natural gas, propane, hydrogen, and alcohol-based substances (gasohol, ethanol, methanol, and other "neat" alcohols) all have their proponents. However, although these fuels burn somewhat cleaner than gasoline, the use of all of them involves trade-offs.

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