There are at least 3 ways to run a diesel engine on biofuel using vegetable oils, animal fats or both. All three are used with both fresh and used oils.
1. Use the oil just as it is-- generally called SVO fuel (straight grease);
2. Mix it with kerosene (paraffin) or petroleum diesel fuel, or with biodiesel, or blend it with a solvent, or with gas;
3. Convert it to biodiesel.
The very first 2 approaches sound simplest, but, as so often in life, it's not rather that simple.
1. Mixing it
Vegetable oil is far more viscous (thicker) than either petro-diesel or biodiesel. The function of blending it or blending it with other fuels is to lower the viscosity to make it thinner so that it flows more freely through the fuel system into the combustion chamber.
If you're mixing veg-oil with petroleum diesel or kerosene (exact same as # 1 diesel) you're still utilizing fossilfuel-- cleaner than the majority of, however still not tidy enough, lots of would say. Still, for each gallon of
grease you use, that's one gallon of fossil-fuel saved, which much less climate-changing carbon in the atmosphere.
People utilize different mixes, varying from 10% grease and 90% petro-diesel to 90% grease and 10% petro-diesel. Some people just use it that way, begin up and go, without pre-heating it (which makes veg-oil much thinner), and even utilize pure grease without pre-heating it, which would make it much thinner.
You might get away with it with an older Mercedes 5-cylinder IDI diesel, which is a really difficult and tolerant motor-- it will not like it however you most likely won't kill it. Otherwise, it's not smart.
To do it properly you'll require what amounts to an SVO system with fuel pre-heating anyway, ideally using pure petro-diesel or biodiesel for starts and stops. (See next.) In which case there's no need for the blends.
Blends with different solvents and/or with unleaded gasoline are "experimental at best", little or absolutely nothing is understood about their impacts on the combustion characteristics of the fuel or their long-term effects on the engine.
Higher viscosity is not the only problem with utilizing grease as fuel. Veg-oil has different chemical residential or commercial properties and combustion qualities from the petroleum diesel fuel for which diesel engines and their fuel systems are designed.
Diesel engines are modern devices with really precise fuel requirements, particularly the more modern-day, cleaner-burning diesels (see The TDI-SVO controversy).
They are difficult however they'll only take so much abuse. There's no warranty of it, but utilizing a mix of up to 20% veg-oil of excellent quality is said to be safe enough for older diesels, especially in summer.
Otherwise utilizing veg-oil fuel needs either a professional SVO option or biodiesel. Mixes and blends are usually a bad compromise. But blends do have a benefit in winter.
As with biodiesel, some kerosene or winterised petro-diesel fuel blended with straight grease decreases the temperature level at which it begins to gel. (See Using biodiesel in winter) More about fuel mixing and blends.
