There are several different types of alternative fuel vehicles available to drivers, certainly more than the number of petroleum-based cars. One of the most well-known alternative fuels is ethanol, which can power cars using various blends of gasoline. Ethanol fuel is made from corn, and manufacturers can produce it domestically. Biodiesel, on the other hand, is made from animal fats and vegetable oils and usually produces fewer pollutants than regular petroleum-based diesel. Clean natural gas (CNG) and propane are both naturally available fuels that produce fewer greenhouse gases, and hydrogen fuel cells create zero emissions and have potential as a renewable source. Electric vehicles that get their power from the chemicals inside a battery are also considered alternative fuel vehicles.
The large number of potential alternative fuel vehicles available makes it difficult to come up with any one particular standard along which to compare them, mostly because each fuel gives off emissions in different ways.
One standard that's difficult for many alternative fuel vehicles to meet, regardless of engine type or fuel source, is safety. As manufacturers of new technologies try to shave off as many miles per gallon as possible, engineers take into account more than just the kind of engine or motor an alternative fuel vehicle will use -- they also consider weight. Heavy, bulky cars and trucks suffer when it comes to emissions standards because they burn more fuel just to carry the extra weight. Lighter, more aerodynamic cars require less fuel for propulsion and therefore will burn less fuel to travel the same distance.
Some alternative fuel vehicles, which tend to be compact in design, might have difficulty rating well on crash and safety standards, since small- and mini-sized cars often rate poorly in crash tests. The technology for some designs, like hydrogen fuel cells, is also complicated and requires extensive testing to make sure people are kept safe from dangers like electric shock and compressed hydrogen hazards.
On top of safety, some alternative fuel vehicles are having trouble with renewable fuel standards, once thought to be the driving point behind their environmental importance. While previous studies have found that ethanol derived from corn produces fewer direct emissions than gasoline, researchers from the University of Minnesota published a study stating that corn ethanol is actually more harmful to the environment than gasoline [source: Hill].
For more information about alternative fuels and other related topics, follow the links on the next page.