Blending vegetable oil with solvents

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Vegetable oil may be blended with solvents as a substitute for petroleum-based diesel fuel. A blend thins the oil to near the viscosity of diesel fuel to improve the burn characteristics and reduce the gel-point. The resulting should be a uniform solution without precipitates.

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WVO before solvent is added to it (source: Jeffrey S. Brooks)
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WVO as solvent is added to it (source: Jeffrey S. Brooks)
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WVO just after solvent is added to it (source: Jeffrey S. Brooks)
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WVO five minutes after solvent added to it. The solution has become uniform (source: Jeffrey S. Brooks)

Safety of light solvents in a diesel engine

Blending waste oils with solvents such as gasoline (petrol) or kerosene (paraffin) requires overcoming some myths that persist in the diesel industry. It is commonly believed that they will cause damage to a diesel engine; however, the British owner’s manuals for Mercedes-Benz ref, Mercedes 190D 2.5 Manual ref, BMW Owner's Manual (British) ref and Volkswagen Pickup owner’s manual (equipped with diesel engines from 1978 through 1991) state "UP TO 30% GASOLINE" (or kerosene in the USA) may be added for cold weather starting.

The Haynes Techbook (10330) Diesel Engine Repair Manual also states, "If gasoline is accidentally pumped into the fuel tank, it won't hurt the fuel system or the engine… Gasoline in small amounts – 0 to 30% - isn't noticeable. At higher ratios, the engine will make a knocking noise, which will get louder as the amount of gasoline goes up..."

Research at the University of Wisconsin shows gasoline can be used as a "spark plug" when blended with diesel fuel improves economy and emissions ref

Below 30% in diesel fuel system will not cause mechanical failure, though with higher solvent ratios, hot fuel system components may potentially cause a vapour-lock that prevents the fuel from being pumped.

Cosolvent blending

"Vegetable oils are an attractive renewable source for alternative diesel fuels. However, the relatively high kinematic viscosity of vegetable oils must be reduced to make them more compatible with conventional compression-ignition engines and fuel systems. Cosolvent blending is a low-cost and easy-to-adapt technology that reduces viscosity (and gel point) by diluting the vegetable oil with a low-(molecular weight solvent)." <ref>{{cite journal|last= Dunn|first=R. O.,|title= Low-temperature flow properties of vegetable oil/cosolvent blend diesel fuels.

The common method of blending

A petroleum distillate is blended with a vegetable oil to thin it out to near the viscosity of diesel fuel. The most common petroleum distillates used in blending are gasoline (petrol), kerosene (paraffin) and/or diesel fuel.

Note: kerosene is not a street-legal fuel in some countries, so there might be heavy fines resulting from its use. In the UK no solvent should be used that has not been subject to fuel duty.

Other solvents

Solvents successfully used in making blends for diesel engines are:

  • Acetone
  • Methyl Ethel Ketone (MEK)
  • Pure Gum Turpentine (PGT)
  • Xylene
  • Toluene lacquer thinner
  • Citrus oils
  • Mineral spirits
  • Paint thinners

Note: as above, some of these solvents are not street-legal, depending on your location, so there might be heavy fines resulting in their use. They may also have detrimental effects on hoses and seals.

Theory behind blending

The viscosity of vegetable oils varies from plant oil to plant oil; however, the average is about 12 times as viscous as diesel fuel. Viscosity is also a function of temperature, thus blend ratio is also a function of operating temperature.

Solvent ratios

Since the viscosity of diesel fuel is twice that of gasoline, then twice as much diesel fuel then gasoline would be required for the same thinning effect. The viscosity of kerosene (paraffin) is halfway between that of diesel fuel and gasoline.

Low Temperature Gelling

The gel-point of biodiesel, oils and solvents is the point at which a substance begins to solidify. When these substances are cooled below the gel-point, some of the molecules in these substances begin to aggregate and form crystals, which can plug a fuel filter. There are a number of commercially available additives that will significantly lower the pour point and cold filter plugging point of biodiesel and waste oils. Winter operation is also possible by blending these substances with other fuel oils including #2 low sulfur diesel fuel and #1 diesel / kerosene, and even gasoline.

Gel-point of blends

  • Vegetable oils tend to gel at about 32F (0C)
  • Diesel fuel gels at about -22F (-30C)
  • Kerosene (paraffin) gels at about -60F (-51C)
  • Gasoline (petrol) gels at about -131F (-90C)

However, a blend gels at a point between the gel points of its components, and is a function of the volumetric relationship between those components.

The typical blend ratios and temperature ranges

Blend Upper temp - heated fuel line Lower temp - ambient conditions
50% diesel fuel to 50% vegetable oil 212F (100C) 28F (-2C)
30% kerosene (paraffin) to 70% vegetable oil 212F (100C) 15F (-10C)
20% gasoline (petrol) to 80% vegetable oil 120F (50C) 0F (-18C)

Blends of more than one solvent

Some people prefer blending diesel fuel, vegetable oil and gasoline. Here are some practical ratios:

Gasoline (petrol) Diesel Vegetable oil
5% 40% 55%
10% 30% 60%
15% 20% 65%
20% 10% 70%
25% 0% 75%

Blends with biodiesel

It is also common practice to blend solvents into biodiesel to prevent it from gelling at cold temperatures.

Gasoline (petrol) Biodiesel Lower operation point
5% 95% 28F (-2c)
10% 90% 20F (-7c)
15% 85% 10F (-12c)
20% 80% 0F (-18c)
25% 75% -10F (-23c)

Note: not all biodiesel gels at the same temperature, so biodiesel made from high melting point esters (HMPE) may require higher levels of gasoline (petrol) added to it to prevent gelling.

Blending Method

Blending methods vary; however, the most common method is to blend the solvent with the source vegetable oil prior to settling, de-watering and filtering, because blending solvents with vegetable oils has three basic functions. Blending reduces the viscosity of the source oil, reduces its gel-point, and tends to force water and other contaminants out of solution. Thinned oil will drop its contaminant load much more quickly than the more viscous source oil. While Straight Vegetable Oil users may settle their oil for weeks, blenders find they can settle their blend for as little as a few hours to a few days.

Thinned vegetable oils will also pass through a filter more quickly than the source oil. Where Straight Vegetable Oil users may gravity feed their SVO through a filter taking days, thinned vegetable oils will pass through a filter in minutes.

Blended oils will also remain liquid at lower temperatures, so that filtering can take place at much colder temperatures than the source oil will allow.

A solvent that is readily soluble in vegetable oils, such as petroleum distillates, do not require a great deal of agitation to dissolve into the source vegetable oil, and will remain in solution indefinitely. Most blenders fill a settling drum to about 75% full with the source oil, then pour on top of that oil their solvent, such as gasoline (petrol), then rock the drum a little, or swirl a paddle a few times around in the solution.


Settling prior to filtering is simply allowing the blend to remain undisturbed for anywhere from 3 hours to 3 days.

After an adequate settling period most blenders will drain off the precipitates that settle to the bottom of the settling drum. To facilitate this use a cone-shaped drum (or drum with bell-shaped bottom) with a valve at the bottom for draining off those precipitates.


Precipitates will begin to settle out within minutes after agitation ceases, and consist of primarily particulates and liquids. Particulates are dust, free-carbon and food. The liquid precipitate is mostly water, but there can also be animal fat and lacquers. The water precipitate can contain dissolved components which are common in the commercial kitchen, such as salt, sugar and spices. Animal fat is problematic because of its high melting point. Lacquer is naturally occurring in vegetable oils.

Filters and Filtering

Cartridge and bag filters that are designed for filtering drinking water are fine for filtering blends. However, keep in mind that alcohol may be present in small quantities in gasoline fuel stocks. The alcohol content can cause deterioration of fuel hoses, seals and filter seals in the filter system. Alternatively, filter with bag filters which are not affected by alcohol content.

External links

Jhananda 15:42, 20 April 2011 (BST) Edited and reformatted by Tony 5 March 2013 (BST)