There are at least 3 ways to run a diesel engine on biofuel utilizing veggie oils, animal fats or both. All three are utilized with both fresh and used oils.

1. Use the oil simply 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 fuel;

3. Convert it to biodiesel.

The very first 2 approaches sound easiest, but, as so frequently in life, it's not quite that easy.

1. Mixing it

Grease is a lot more viscous (thicker) than either petro-diesel or biodiesel. The purpose of blending it or mixing it with other fuels is to reduce the viscosity to make it thinner so that it streams more freely through the fuel system into the combustion chamber.

If you're blending veg-oil with petroleum diesel or kerosene (like # 1 diesel) you're still utilizing fossilfuel-- cleaner than many, but still not clean enough, many would say. Still, for each gallon of

grease you utilize, that's one gallon of fossil-fuel conserved, which much less climate-changing carbon in the atmosphere.

People utilize numerous blends, varying from 10% veggie oil and 90% petro-diesel to 90% grease and 10% petro-diesel. Some people simply use it that method, launch and go, without pre-heating it (that makes veg-oil much thinner), or even utilize pure grease without pre-heating it, which would make it much thinner.

You may get away with it with an older Mercedes 5-cylinder IDI diesel, which is a very tough and tolerant motor-- it will not like it but you probably won't eliminate it. Otherwise, it's not wise.

To do it properly you'll need 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 requirement for the blends.

Blends with various solvents and/or with unleaded fuel are "speculative at best", little or absolutely nothing is learnt about their results on the combustion qualities of the fuel or their long-term results on the engine.

Higher viscosity is not the only problem with utilizing veggie oil as fuel. Veg-oil has different chemical properties and combustion characteristics from the petroleum diesel fuel for which diesel engines and their fuel systems are designed.

Diesel engines are modern devices with very precise fuel requirements, particularly the more contemporary, cleaner-burning diesels (see The TDI-SVO controversy).

They are difficult however they'll just take so much abuse. There's no guarantee of it, but utilizing a blend of up to 20% veg-oil of good quality is stated to be safe enough for older diesels, particularly in summer.

Otherwise utilizing veg-oil fuel requires either an expert SVO service or biodiesel. Mixes and blends are usually a bad compromise. But blends do have a benefit in cold weather condition.

As with biodiesel, some kerosene or winterised petro-diesel fuel combined with straight vegetable oil decreases the temperature at which it begins to gel. (See Using biodiesel in winter) More about fuel mixing and blends.