With gas prices continuing to jump up and down, there's increasing attention on finding viable alternative fuels. You may know all about electric cars or natural gas, but what about the coffee car? Here are some unusual alternative fuels that could find their way into your gas tank -- and a few that shouldn't.

1. Coffee

A British team (the same people behind the BBC show Bang Goes The Theory) outfitted a 1988 Volkswagen Scirocco to run on coffee grounds, arguing that because coffee contains some carbon it would make a viable fuel. Hopped up on java, the car ended up breaking a speed record for organic waste vehicles, but was not the most efficient vehicle available. It cost far more than simply powering with gasoline and the drivers had to stop every 30 to 45 minutes to clear out filters in the engine.

2. Water

A Pakistani scientist made waves earlier this month by claiming to have developed a car that runs only on water, by splitting the hydrogen and oxygen atoms. The revelation could be huge for clean-fuel cars and for Pakistan's energy crisis. The only problem? Scientists say it can't be true. The claim has been widely debunked as untrue, essentially because extracting energy from just water requires another powerful energy source. It's far from the first time somebody's claimed to have built a water-powered car -- similar boasts have been made since at least the 1930s, but none have been borne out.

3. Chocolate

Finally, a car Cathy can get behind. Bacteria can use fuel from a variety of sugar waste, including chocolate, to produce hyrdrogen and generate power. That theory has even been put to the test, when a group of researchers at England's University of Warwick built a racing car that ran on chocolate and was built entirely from recycled sources. The WorldFirst Formula 3 car -- which also used components from vegetables in the body -- used a biodiesel engine that ran on chocolate waste and vegetable oil.

4. Nothing but Air

Compressed air cars work much the same way as a steam engine, but using electric power to push the compressed air through a piston engine. With refueling only involving replacing an air tank, and no emissions, the cars may sound like a great alternative. But researchers say that the energy process isn't efficient and ends up losing enough energy that it's not more effective than an electric vehicle (and powering the electricity with a coal mix ends up producing more emissions than a gas-powered car). Still, a number of automakers are working on air-powered cars, notably Honda and India's Tata Motors.

5. Sawdust

Sawdust is one of the many potential feedstocks for biofuels, because it can be heated into liquid form before it starts to burn. With a few additions or a gasifier attached to an engine, sawdust can effectively power a car. However, the process is still years away and researchers say it would take far more supply than is currently available to make sawdust viable as a fuel source.

6. Straw

Like sawdust, straw is another potential biofuel source that wouldn't compete with existing food stock -- a constant concern for corn-based ethanol. Researchers are also looking at woodchips, corn stalks and grass as other possible sources.

7. Styrofoam

A team at Iowa State University is studying a way to turn plastic waste into fuel by dissolving the polystyrene in Styrofoam and other plastics into biodiesel. At low concentrations, the plastic-blended fuel worked well in engines for electricity generation, although at a certain point it got too thick and caused overheating. The emissions from the engine were also dirtier than normal because of the polymers in the plastic, so researchers are working to refine the process and clean the fuel up.

8. Cooking Grease

The idea of using vegetable oil as a biodiesel has been around for a long time. Even Rudolf Diesel intended for his engine to someday run on vegetable oil so farmers would have a regular supply. Nowadays, vegetable oil would be plentiful from restaurants, which are already discarding used cooking oils, and would burn far cleaner than petroleum. But outside of some self-modified cars, so-called "french fry cars" have not made much headway because the oil can be volatile in extreme temperatures.

9. Diapers

Any manner of garbage could be used to create fuel during the pyrolysis process (similar to incineration, except instead of burning the waste it is instead heated until it breaks down into byproducts), but disposable diapers offer a plentiful and never-ending supply. Quebec company AMEC is already working on using diapers in its power plants because of the plentiful supply and the ability to fine-tune machines to use every part of the diaper.

10. Human Fat

In 2008, Beverly Hills cosmetic surgeon Dr. Craig Alan Bittner claimed that he had figured out an unusual use for the discarded fat from liposuctions -- he was using it to power his SUV and his girlfriend's Lincoln Navigator. While human fat could be tweaked to become biodiesel, most questioned Bittner's claims. WIred pointed out that to use fat, there would need to be a diesel engine and the Navigator did not have a diesel option. The state of California ended up investigating (gas replacement isn't an authorized use for human waste) and Bittner ended up facing lawsuits alleging that unauthorized staff had worked on patients.

11. Ammonia

Anhydrous ammonia, which can be used as a fertilizer, also has the potential to be burned in an internal combustion engine with little or no emissions. However, ammonia's energy density is less than half that of gasoline, so the range would be quite limited, and there are concerns about any emissions it does produce being potentially harmful when inhaled. Still, that hasn't stopped researchers from trying to figure out a way to use it. Even the Defense Department was experimenting with ammonia-fueled vehicles in the 1960s.