There's a choice to make for running diesels on biofuels:
make biodiesel and just use it, no need to modify the engine, or
buy a ready-made conversion system for your vehicle or build your own so you can run it on straight vegetable oil (SVO) -- no need to process the fuel, just put it in and go.
Does the SVO option work? Yes -- IF you go about it the right way. It's not quite that simple a choice. For one thing, if you want to use waste vegetable oil, which is often free, you're going to have to process it anyway, though less so than to make biodiesel. And it still might not be a very good fuel.
But read on -- you CAN run your diesel motor safely on straight vegetable oil, just put it in and go. There are pitfalls and provisos, but we'll help you to steer your way through them.
One of the great advantages of biodiesel is that it will work in any diesel motor -- see Biodiesel and your vehicle. (More on the choice between biodiesel and SVO.)
The same claim is often made for straight vegetable oil fuel systems, such as this: "Ready-to-install kit that will allow you to run any diesel on waste vegetable oil."
Is it true? Yes, but for how long?
Diesel motors last a long time, half a million miles or more is not unusual, and there are very few thorough, long-term studies of the effects of using straight vegetable oil in diesel motors. What is clear, however, is that "any diesel" is an exaggeration.
Some vegetable oils are better than others.
Some diesels are more suitable than others.
Some injection pumps work better than others.
Some computerized fuel systems don't like vegetable oil at all.
There are doubts about using vegetable oil in DI (Direct Injection) diesels.
There are also doubts about using waste vegetable oil.
(See The TDI-SVO controversy)
The main problem is that vegetable oil is much more viscous (thicker) than conventional diesel fuel (DERV, petro-diesel). It must be heated (thinned) so that it can be properly atomised by the fuel injectors. If it's not properly atomised, it won't burn properly, forming deposits on the injectors and in the cylinder head, leading to poor performance, higher emissions, and reduced engine life.
This can also happen with unsuitable oils, for instance those with a high iodine value, such as linseed oil (see Iodine Values), which can form tough epoxy deposits, not good for engines. It's argued that a good SVO system will prevent this, but solid proof is lacking.
"In the high temperatures commonly found in internal combustion engines, the process is accelerated and the engine can quickly become gummed-up with the polymerised oil." -- From "Waste Vegetable Oil As A Diesel Replacement Fuel":
Waxes can clog up the fuel system, especially in cold weather. Waste oils can contain acids that cause corrosion in the injector pump, and impurities that can cause coking and further corrosion.
Cutaway view of an injector pump -- complex, expensive
"In autumn 2001 an injection pump was damaged for the first time ... so that an exchange was necessary. The vehicle had previously been driven without problems for two years. An examination of the defective sections found substantial surface erosion of the hardened steel high pressure parts, which are not acid-proof." The problem was traced to a supply of soy oil which was not the usual food-grade oil and had a high acid-content. BioCar (German page):
Here's some more:
"Rapeseed oil ... can only be used as a diesel fuel extender, with inclusion rates of up to 25%." -- From "Results of engine and vehicle testing of semi-refined rapeseed oil":
And yet more:
"Deacidified rapeseed oil can be used as fuel for a diesel engine. Degummed oil and crude rapeseed oil were found to be unsuitable for use as fuel due to the high level of incombustible materials in oil." -- From "Operation of a Diesel Engine Using Unrefined Rapeseed Oil as Fuel":
http://ss.jircas.affrc.go.jp/engpage/ja ... gashi.html
Nonetheless, experience is showing that SVO systems are a practical proposition with a lot of advantages to offer, not least of all that using SVO can be cheaper than turning it into biodiesel and uses less energy than making biodiesel does.
It's vital to pay close attention to the quality and condition of the oil -- much more critical with an SVO system than if you're going to convert the oil into biodiesel. Study the resources below carefully, as well as the information on oil and fuel qualities and properties on our Oil yields and characteristics page.
That done, get a good system matched to the right kind of engine with the right kind of injection pump, as well as to your climate, and you'll be just fine, like thousands of others