There are at least three ways to run a diesel engine on biofuel using veggie oils, animal fats or both. All 3 are utilized with both fresh and secondhand oils.
1. Use the oil just as it is-- normally called SVO fuel (straight vegetable oil);
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 first two approaches sound simplest, however, as so frequently in life, it's not quite that basic.
1. Mixing it
Vegetable oil is a lot 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 easily 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 using fossilfuel-- cleaner than most, but still not clean enough, many would state. Still, for every gallon of
grease you use, that's one gallon of fossil-fuel saved, which much less climate-changing carbon in the atmosphere.
People use different blends, ranging from 10% grease and 90% petro-diesel to 90% veggie oil and 10% petro-diesel. Some people simply use it that method, launch and go, without pre-heating it (which makes veg-oil much thinner), and even use 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 an extremely hard and tolerant motor-- it will not like it however you probably won't eliminate it. Otherwise, it's not sensible.
To do it effectively 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 requirement for the mixes.
Blends with numerous solvents and/or with unleaded gasoline are "experimental at best", little or absolutely nothing is understood about their results on the combustion qualities of the fuel or their long-lasting effects on the engine.
Higher viscosity is not the only issue with using grease as fuel. Veg-oil has different chemical residential or commercial properties and combustion attributes from the petroleum diesel fuel for which diesel motor and their fuel systems are designed.
Diesel motor are state-of-the-art machines with really exact fuel requirements, particularly the more contemporary, cleaner-burning diesels (see The TDI-SVO debate).
They're difficult however they'll only take a lot abuse. There's no guarantee of it, however using a mix of up to 20% veg-oil of good quality is stated to be safe enough for older diesels, particularly in summertime.
Otherwise using veg-oil fuel requires either a professional SVO option or biodiesel. Mixes and blends are normally a poor compromise. But mixes do have an advantage in cold weather.
As with biodiesel, some kerosene or winterised petro-diesel fuel combined with straight grease decreases the temperature level at which it starts to gel. (See Using biodiesel in winter) More about fuel blending and blends.