Filling the gas tank is expensive, leading some to search for alternative fuels. Some go with standard biofuel or electricity, while others opt for the bizarre. Here are just a few strange things people put in their car’s gas tank. Some are effective under the right circumstances, while others are not.

Gas tank follies No. 1 – Coffee grounds

In the U.K., an enterprising group of tinkerers modified a 1988 Volkswagen Scirocco to run on coffee grounds. The prime argument is that coffee had the carbon to make it a viable alternative fuel. The coffee-powered VW did break a speed record for organic waste-powered vehicles, but it cost more to rig the engine than to simply bite the bullet and stick with gasoline. Then there were those ugly filters that needed cleaning.

Gas tank follies No. 2 – Water

Not as extreme as some substances on this list, a Pakistani scientist claimed to have made an engine that would split hydrogen and oxygen atoms and power a car. Considering the energy that would be needed to make that happen, however, the existence of such a working consumer engine has been debunked.

Gas tank follies No. 3 – Chocolate

Bacteria can use fuel from sugar waste to produce hydrogen and generate power. Enter sweet chocolate. Researchers at England’s University of Warwick built a race car from recycled vegetable sources that ran on chocolate. Called the WorldFirst Formula 3, the car’s biodiesel engine ran on chocolate waste and vegetable oil.

Gas tank follies No. 4 – Air

A compressed air car works in a way similar to a steam engine, using electric power to move compressed air through a piston engine. While no emissions are involved, this kind of an engine isn’t a favorite with engineers, as too much energy is lost. Thus, it’s less effective than your standard EV. Yet Honda and India’s Tata Motors are still working on this one.

Gas tank follies No. 5 – Sawdust

This biofuel can be heated into liquid form before it burns. Attach a gasifier to an engine and sawdust can work, at least in theory. Researchers suggest that you’d need a lot of sawdust to actually power a vehicle over long distances.

Gas tank follies No. 6 – Straw

Similar to sawdust, straw also has some potential as a biofuel. Corn-based ethanol has proven to be a more popular fuel source, however.

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Gas tank follies No. 7 – Styrofoam

Iowa State University researchers have looked into turning plastic waste into fuel. The process involves dissolving the polystyrene in Styrofoam into biodiesel fuel. At low concentration, plastic fuel blends will generate electricity, but if the concentration is too thick, overheating occurs. The polymers in the plastic make the process a dirty one, too.

Gas tank follies No. 8 – Diapers

Deriving energy from burning garbage via the pyrolysis process, where items are heated until breakdown into individual byproducts, works just fine with diapers, says Quebec company AMEC. The company is working on an automotive power plant that can effectively break down every part of a diaper.

Gas tank follies No. 9 – Human fat

In 2008, plastic surgeon Dr. Craig Alan Bittner of Beverly Hills, Calif., claimed he’d figured out how to use fat from liposuctions to power his SUV and Lincoln Navigator. Experts called this bogus, however, as the Navigator does not come with a diesel engine option that would be necessary for conversion to biofuel. As California doesn’t allow human waste to be used as an alternative fuel for a vehicle, authorities investigated and found that unauthorized staff had worked on patients. So Bittner’s credibility came into question.

Gas tank follies No. 10 – Ammonia

Anhydrous ammonia is sometimes used as fertilizer, so why not? Burned in an internal combustion engine, there would be little or no emissions. However, as ammonia is less than half as dense as gasoline, vehicle range would be very limited. Plus, there are health concerns over the small amount of emissions that would be produced. That didn’t stop the U.S. Defense Department from experimenting with ammonia-fueled vehicles during the 1960s, but it will likely prevent it from behind used as a consumer vehicle fuel source any time soon.

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A car that runs on coffee grounds?

MCDE Editorial