Note: This is a “classic” Interesting Thing of the Day article from over 10 years ago. It has not been edited recently, so it may contain broken links, outdated information, or other infelicities. We plan to eventually update or retire most classic articles, as time permits.

While out for a walk in my neighborhood, I noticed an otherwise ordinary-looking pickup truck with a big sign on the back that said “This vehicle powered by 100% used vegetable oil.” That’s nice, I thought, very environmentally conscious and all—as San Franciscans are known to be. I wondered briefly about the technological issues involved in getting a truck to run on vegetable oil, information that surely would be available a few clicks away on the Web. But I also wondered about maintaining a fuel supply. If you’re on a trip and the fuel gauge starts getting low, a gas station would presumably do you no good. Do you start looking for a doughnut shop or a fast-food joint where you can score some used oil? Is there enough to go around? And will it really end up being less expensive than conventional diesel fuel?

The first claim I discovered sounded too good to be true: diesel engines can, without modification, run on vegetable oil—just like that. Now, I’ll be the first to admit I know precious little about engines, but this revelation puzzled me. If true, then why even bother with petroleum-based fuel in the first place? As it turns out, that claim is only approximately true—some diesel engines can run on some kinds of vegetable oil under some conditions without problems. (This trick doesn’t work with gasoline engines, because the sparks produced cannot ignite vegetable oil.) Still, the fact that this can happen at all seemed pretty amazing to me. It shouldn’t have: had I read about diesel engines more carefully when I was researching fire pistons, I would have learned that the first diesel engines ran on peanut oil, and that Rudolf Diesel’s original idea was that this would be a perfect solution for areas with limited access to petroleum. Today, however, nearly all diesel engines are designed to work with petroleum-based fuel, so running such engines on vegetable oil is not entirely straightforward.

The Plot Thins

The first problem is one of viscosity: vegetable oil is a lot thicker than conventional diesel fuel. This means that it has a harder time atomizing, which in turn reduces its ability to burn. One way to reduce the viscosity is to mix it with ordinary diesel fuel, and most of the diesel engines used in cars and light trucks will run quite happily on anywhere from 10% to 50% vegetable oil—poured right into the gas tank along with the diesel from the pump. Another way to thin the vegetable oil is to heat it; heating is the primary function of conversion kits you can buy to run your vehicle on vegetable oil.

But using pure vegetable oil is still problematic because the fuel sitting in your engine (and your fuel line) will be cold when you initially start your car. (This is especially true, of course, in colder climates and during the winter months.) One way of solving this problem is to use two fuel tanks—one for regular diesel and one for vegetable oil. You start your engine with regular fuel, and as it warms up, the heat is used to thin the vegetable oil. After a few minutes or so, you use a dashboard control to switch over to the vegetable oil tank. Dual-tank systems also mean that a vehicle can, if necessary, refuel at a regular gas station. Another approach is to use a special additive (which contains a small amount of petroleum); this enables the vegetable oil to flow more easily, even when cold.

In most cases, you can outfit your existing vehicle with a second tank, heater system, switch, and other necessary apparatus for well under US$1,000; if you go with the additive-based approach instead, you eliminate your up-front costs in exchange for somewhat more expensive fuel. Either way, you’ll save an enormous amount of money on diesel fuel—assuming that you run your vehicle on used vegetable oil. Where I live, at least, the least expensive vegetable oil you can purchase in an ordinary supermarket runs about $5 per gallon—with usual prices being closer to $10. But some proponents of vegetable oil fuel claim that you’ll never have to buy fuel again, as long as you are willing and able to obtain used cooking oil from a restaurant or food processing facility. Many restaurants are apparently happy to give away their used oil, because otherwise they’d have to pay to have it disposed of properly. (Even if you do have to pay for it, the cost would be a mere fraction of what you’d pay for diesel fuel.)

If you’re using waste oil, you’ll need to filter it first—which potentially involves setting up some equipment in your garage and spending a bit of time doing the processing. The inconvenience of obtaining and filtering used vegetable oil is one of the biggest reasons this process hasn’t caught on more widely. But with gas prices rising as they are, this extra step looks like an increasingly minor hassle. As for supply, according to one estimate I read, the United States produces about 3 billion gallons (roughly 11 billion liters) per year of waste vegetable oil, which should keep quite a few cars on the road.

The Gas Is Greener

Apart from cost, using vegetable oil as a fuel provides several other advantages. According to some sources, at least, it’s better for your engine, providing additional lubrication and reducing engine deposits. It’s also less likely to cause a fire or explosion in the case of an accident. Vegetable oil supposedly gives you equivalent fuel efficiency to petroleum-based diesel fuel, which is to say, substantially more than gasoline. It also results in lower emissions. Because the carbon dioxide produced by burning vegetable oil is less than the amount absorbed by the plants from which the oil is obtained, vehicles running on vegetable oil produce no net increase in atmospheric carbon dioxide. And, finally, vegetable oil fuel is indefinitely renewable.

Beyond straight vegetable oil is biodiesel, a fuel that is generally interchangeable with petroleum-based diesel—even in unmodified vehicles—but is made by specially refining vegetable oil and other organic materials. Commercial biodiesel is much more expensive than regular diesel, and though you can, in theory, make it yourself, that’s an expensive and potentially hazardous undertaking. And besides, it’s just not as cool as being able to say your car actually runs on used vegetable oil. I hope some day to see fast-food chains begin to offer low-cost fill-ups with their meals. Drive through to pick up your burgers and fries, and then pull up to the pump for some used oil. Imagine: there may be a benefit to all that deep-fried food after all. —Joe Kissell

More Information

Thanks to reader Dan Bowden for suggesting today’s topic!

This article was featured in Carnival of the Green #31.

For more information about running diesel vehicles on vegetable oil, consult these resources:

Articles and other relevant sites about vegetable oil-powered vehicles:

Diesel Secret Energy makes an additive that claims to enable unmodified diesel cars to run on (almost) pure vegetable oil.

To learn more about biodiesel, read Biodiesel—The Alternate, Vegetable-Based Fuel for Diesel Engines at Global Stewards or Biodiesel in the Wikipedia.