Alternative propulsion

Alternative propulsion is a term used frequently for powertrain concepts differing from the standard internal combustion engine concept used in petroleum fueled vehicles.

Definition
The term alternative propulsion or "alternate methods of propulsion" includes both
 * alternative fuels used in standard or modified internal combustion engines (e.g. combustion hydrogen).
 * alternative propulsion systems, this is, not based on internal combustion, such as those based on electricity (for example, electric or hybrid vehicles), compressed air, or fuel cells (e.g. hydrogen fuel cells). Under certain conditions they can be more efficient than petroleum propulsion. However, while some are hoping for spectacular breakthroughs such as bubble fusion or cold fusion (currently discredited), at the moment no technology yields significantly higher fuel efficiency than traditional methods.

Today's cars can be classified in three main groups:


 * Pampetro (also known as non-clean or non-green) vehicles, this is, that only uses petro(leum).
 * Hybrid vehicles, that use petroleum and another source, generally electricity.
 * Petrofree cars, that do not use petroleum, such as battery electric cars, hydrogen vehicles, compressed air.

Motivation and petro-free
The motivation for the research in alternative propulsion in transport is primarily to achieve more sustainable methods of transportation than those relying on fossil fuels.

Petrofree is a term to indicate that petro(leum) is not used or sold (i.e. petrofree fuel station, petrofree vehicle). It's a synonym of post carbon or zero-carbon.

Application fields
Most work in alternative propulsion concepts is focused on replacing traditional internal combustion engines in automobiles. Several contenders, such as Michelin's Challenge Bibendum and the North American Solar Challenge, are designed to provide motivations for effective concepts utilizing alternative propulsion. Hybrid vehicles, which combine an internal combustion engine with an alternative system, are perhaps the most visible example of alternative propulsion systems in use today.

Alternative propulsion is not limited to automobiles, however; it can also apply to types of spacecraft propulsion beyond solid- or liquid-fueled rockets. Propulsion methods such as the ion thruster and solar sail are examples of alternative propulsion options for spacecraft.

Criteria for successful application of alternative propulsion
In order to supplant traditional propulsion systems, alternative propulsion systems must be able to equal or exceed the performance and convenience of traditional systems in several areas:
 * operating radius (range)
 * acceleration and top speed
 * stability of the stored energy (degradation over time, losses or leakages).
 * refueling or recharge procedure.
 * environmental effects (minimizing odor, noise, vibrations, radiation, emission of noxious substances, etc.).