Good news, ass-havers! The answer is yes. Even better news is that the vast majority of what's likely to be expelled from your butt has potential to run a car, which is likely the best use for anything that comes out of any butt anywhere. So let's see how this ass-power works.
For the purposes of this article, I'll be focusing on two of the most common forms of ass ejecta, the famous two F's we all know and love: flatus and feces. From these two human exhaust products, there's two primary elements that can be harvested to use to run a car: methane (available in both flatus and feces) and hydrogen (available in flatus only).
Let's start with methane, since it's already in use in cars, though more commonly called natural gas. You can convert almost any internal-combustion car to run on natural gas, and there's several CNG (compressed natural gas) cars and trucks ready for sale today, from Chevy and Honda and a number of other auto makers.
Methane can be a component of farts, though not everybody produces methane when farting. If you're seriously considering fart-powering your car, you should get your emissions tested to confirm you're producing methane. Even if you're not, we have a backup plan we'll cover here next.
According to one of my most favorite books of all time, NASA's Bioastronautics Data Book, human flatus can contain between 0-30.3% methane, with a mean amount of 7.2%. This is from direct rectal catheterization, mind you, so you know these results are good. The rest of the gaseous medley we call farts includes nitrogen, CO2, oxygen, and a surprisingly large amount (3.1-34%) of Hydrogen. The volume of fart gas an average human expels in a day varies a great deal, but on average seems to be just over 100 ml per person, per day.
100ml isn't a hell of a lot— in fact, it's about 0.026 of a gallon, so that means you'd need just over a month to make a gallon of outgas. It would take you months and months to fill an 8 gasoline gallon equivalent (GGE) tank like the one in the Honda Civic CNG. Oh, and remember, of that 100 ml fart, only 30% of that (at best) is methane — so to do this right, you'd want to pool a whole community of farts. It's really the only way.
How best to do this? I'd suggest some sort of bladder-filled fart-collector underpants, which would be both sexy and practical. If we had a large group of fart-producing citizens wearing flatus-collection unmentionables and "uploading" their production to a shared facilities, the methane could be extracted from the flatus, compressed, and placed into standard pressurized CNG tanks, ready to drive your Civic to the next In-And-Out, beginning the cycle anew.
Incredible. Essentially free energy as long as everyone's willing to walk around with a rubber tube up their ass. I can't think of a better way for someone to prove their commitment to the environment and energy independence than that.
If hydrogen engines are more your thing, then those same fart-collecting underpants could be employed for that as well. In fact, it'd be even better, since everyone has hydrogen in their farts, and, the mean amount of hydrogen is around 20%, ranging from 3%-34%. Hydrogen internal combustion engines are basically the same as normal ones, just built stronger and to better tolerances. They're more expensive, but they do produce about 20% more power than equivalent gasoline engines, so that's the fun fuel to get out of your ass.
Now, for many more sensitive folks, driving around with a tube up your ass is a deal-breaker. I respect that. Fortunately, there's another less intrusive but even more disgusting method: methane from feces.
This is actually being done all the time from animal manure, in farms all over the world. In fact, some of the CNG produced from those piles of shit may already be finding its way into fleet trucks and taxis and whatnot. But let's say you want to only run your car on your own poop, which, as we all know, does not stink. You absolutely can.
To do so, you'll need what's known, disgustingly, as a methane biodigester. Essentially, you fill up a tank with wastes (which don't have to be all poop — other biological and food wastes work) and you deprive the yuk-sludge from oxygen, forcing anaerobic composting, which will eventually outgas methane. Here's a whole video on how to make one:
Even if all this seems too much work for you, according to the Bioastronautics Data Book, even feces left unattended and just sitting there will produce up to 1 ml of methane per gram of poop. And that seems to continue for up to the two weeks the tests were run.
As with the fart-collection, working collectively will likely be a better plan here, to take advantage of economies of scale and have a shared facility to compress the gas and get it into safe CNG tanks. If "safe" and "compressed" sound like sissy candy-assed ideas to you, there is one other option: giant gas bags.
Beginning around WWI, popular from desperation in WWII, and even up until fairly recently for buses in China, gas bag cars have been developed to run on uncompressed natural gas (or wood gas, etc). Since the gas is not compressed, it takes up a huge volume of space, which is why those bags are so damn big. Still, you don't really need a fuel gauge when you can just look at your car and see how deflated the massive rubberized sack is on your roof.
Gas bag cars aren't really the most practical solution, but if you've got no other choice, it'll work. I imagine you could take something like a rubber raft and use it as a decent gas bag. Just fill it up from the gas hose from your biodigester, bungie it to your roof (I suggest a roof rack), and run a line from it right into your car's intake manifold. Simple, right?
Look at all these great options. No matter what the fuel situation is in the future, we should always be able to enjoy having cars, because there's literally solutions we can pull right out of our asses.