Whenever trends raise their heads in modern society, there’s always a group of outliers who want to take that new practice to the limit. The rise of biodiesel fuel over the last decade has also given rise to more people who talk about running their vehicles and engines on straight vegetable oil or animal fat. "I just get the used fry grease from the KFC down the street and my car runs great!"
The former (biodiesel fuel) and the latter (running a diesel engine on straight vegetable oil) are linked because biodiesel is made from vegetable or animal fat via a relatively simple series of chemical reactions that can be done by almost anyone with rudimentary chemistry knowledge. Or at least, that’s what they think. It’s one thing to think you can make biodiesel in your garage (as some people try to do), but it’s another to actually do it well and produce high quality biodiesel fuel without impurities. But that’s another show for another time.
Skipping The Chemistry
The people running their engines with straight vegetable oil or animal fat are just trying to skip the chemistry step. The whole purpose of processing these fats and oils in this way (i.e. turning them into biodiesel) is to convert the fat molecules into molecules (called FAME) that will, in a diesel engine, burn and perform the way conventional petroleum diesel does. At least some of these people reason that since biodiesel can come from fry grease or peanut oil or lard, they can just put the straight stuff in their engines without a problem. And some of them will tell you straight up “I’ve been doing this for years and I’ve never had any problems”. In scenarios like that, who are we to tell them they’re wrong? It’s their experience and nobody is standing by their side, observing what’s going on.
But for those of us on the outside looking in, their descriptions of their experiences with running their diesel car on vegetable oil have an implication that “if I can do it, you can, too”. And that resonates with at least some portion of the population. So the important question is, what’s likely to happen if you want to try and run your diesel engine on “straight” vegetable oil or animal oil?
Well, the first thing to say is that you wouldn’t have nearly the problems that you would have if you tried to do that in a gas engine. Vegetable oil/animal fat is so different from gasoline (which is a lighter and much more volatile hydrocarbon than diesel fuel is) that the spark ignition in the gas engine would find it difficult to make the vegetable oil burn. And the fuel lines and fuel pump aren’t designed to handle the difference between, say, peanut oil and gasoline. You would have bad news all around.
The Key Difference With Vegetable Oil
As far as in diesel engines, the biggest difference between vegetable oil (we really mean either vegetable oil or animal fat, but in order to keep us from having to repeat those phrases over and over, we’ll just refer to it all as vegetable oil) and conventional diesel fuel is the viscosity. Vegetable oil is thicker and has a higher viscosity than diesel fuel does. This difference will do some things in your diesel engine.
Since the vegetable oil is thicker than diesel fuel, it’s going to change the way the fuel injectors spray the fuel into the combustion chamber. Fuel injectors are designed to atomize diesel fuel so that it burns in the best way possible. They’re not designed to achieve the same effect with a thicker, more viscous fuel. So what will happen is the injectors will spray the vegetable oil into the cylinder, but the spray won’t be as fine. That means you’ll get a lot more unburned or partially burned fuel, which means lower mileage and higher emissions. You’ll also get significant deposit buildup in all areas of the engine that are associated with combustion – the fuel injectors, combustion chamber, etc. And since it is thicker, you may also expect to wear out your injectors and fuel pump from having to work harder to pump and spray the thicker fuel.
Filtration Can't Solve Everything
Another potential pitfall of using vegetable oil in your diesel vehicle is the fuel filtration issue. Many of the people who espouse this practice will use post-consumer oils – stuff like used fry grease. After all, it’s just going to go to waste and they’re trying to recycle it. But those kind of oils have to be filtered really well, and even if you think you’ve filtered them enough, there’s always the chance of clogging your fuel filters at an accelerated rate, simply because of what’s in the used vegetable oil. And there’s also the chance of substantial engine damage over time because used waste oils can tend to have high acid content. They've been exposed to high heat over extended periods of time in their former life, and parts of the waste oils have broken down and reacted to form, shall we say, undesirable elements. Even if you can filter out all the damaging particulates, you can’t filter out the acid content, and that can have serious consequences for the health of your engine.
The bottom line here is that even the vocal proponents of “SVO” (straight vegetable oil) will tell you that they don’t recommend you just put it in your engine and go. They will say that you can run SVO in your diesel engine IF you’ve made some modifications to it. Many of them recommend kits that will change your injectors and add parallel fuel filters. They also recommend using a dual tank system with one tank housing the SVO and the other tank housing your regular diesel fuel. The SVO would be heated before use, and the vehicle would also start up and shut down on regular diesel fuel – similar to how hybrid cars are designed.
If all of that sounds overly complicated, then the next best alternative recommended by the people who’ve done it before is to just mix your vegetable oil with diesel fuel or kerosene or even 20% gasoline. Of course, that changes the whole game – now you’re not “running your car on straight vegetable oil”. You’re just using a fuel mixture.
The Final Answer
The takeaway here is that no matter what someone on some web page out there says, it is not a good idea to run your diesel engine on straight, unmodified vegetable oil or animal fat, UNLESS you’re willing to make some of the changes we just talked about. Without these modifications, you’re heading down the road towards big engine problems that will make you wish you had reconsidered your decision to do that.