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Old January 12th, 2010, 12:19   #12
T'sTDI
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Join Date: Feb 2008
Location: Maryland
TDI(s): 2001 Jetta
Default The Basic Vegetable Oil Burning Theory

The Basic Vegetable Oil Burning Theory

Reaching this temperature results in no polymerization, just ignition of the oil (fuel) itself. This is the basic theory on why burning vegetable oils is possible. Although these fuel temperatures can’t be achieved through the use of a WVO kit, it can be achieved in the combustion event thanks to diesel compression ratio, boost pressures and cylinder pressures. All of these factors combine to raise the temperature beyond the ignition temperature of vegetable oil.
The only scientific data with thorough analysis of vegetable oil burning properties is the ACREVO study (advanced combustion research for energy from vegetable oils). In this study, pure rapeseed oil (virgin VO) was used, unlike WVO. The objective of the study was to observe the burning characteristics of vegetable oil droplets under high temperature and pressures. The complete study can be found here: http://www.biomatnet.org/secure/Fair/F484.htm

According to this study, “Atomization tests showed that at 150°C the performance of the rapeseed oil are comparable with that of the diesel oil.”
150°C equates to 302°F which is impossible to achieve through heat derived from a WVO kit alone. However, there are two parts to this equation and further reading results in the following information.
The overall combustion performance of the rapeseed oil is very satisfactory in comparison with the diesel fuel while the rapeseed oil produces almost 40 % less soot than diesel fuel. The different volatility of this fuel respect to the diesel fuel is responsible of the different behavior of the sampled gas concentrations in the base of the flames while at the end of the flames, both attain almost the same values. It has been established that an addition of 9 % of ethyl alcohol (95 %) bring a great benefit regarding the pre-heating oil temperature. In fact, the presence of alcohol allows a reduction in the inlet oil temperature from 150 °C to 80 °C. Moreover, the combustion of the emulsion produces less soot and, at the exhaust, the amount is almost one half less than that produced by the combustion of rapeseed oil.”
Amazingly a small amount of ethyl alcohol (ethanol) of 9% by volume of vegetable oil resulted in a required pre- inlet combustion temperature from 150°C to 80°C or 176°F. 176°F represents the typical fuel temperature achieved through heat of the WVO kit. Using a Greasecar kit, a 9% volume amount would equate to 1.17 gallons of ethanol to 11.83 gallons of WVO for a total of 13 gallons.
I DO NOT recommend adding ethanol to your VO mix but understand the chemistry of what is occurring. Ethanol is measured by octane, not cetane. Octane and diesel engines DO NOT MIX and should never be attempted. If you have ever mixed gasoline in your diesel tank, you would know this… Quite frankly, I doubt anyone who “splash mixes” fuels is doing much more than hoping for the best. However, understand the chemistry that caused the tremendous drop in pre-inlet temperature of the fuel before combustion. An inherently thin fuel that is much more volatile than vegetable was to be mixed with a thicker fuel oil. When introduced to the combustion chamber, the ethanol in small particle amounts is the first to ignite which becomes a catalyst for the entire mixture of vegetable oil to ignite more thoroughly. This causes the pre-inlet temperature to be much lower than if we were running 100% rapeseed oil.

Two things of interest here:
Pure virgin vegetable oil is thicker than waste vegetable oil and therefore will require more heat to achieve the adequate viscosity for combustion than waste vegetable oil.

In a two tank setup, small quantities of diesel fuel are sent back to the WVO tank in the purge cycle which is achieving the above scenario (thinner fuel mixed with a thicker fuel).

Diesel fuel is not as volatile as ethanol is; however, you’re achieving the same principle behind introducing other fuels to thin the WVO mixture out. Therefore, there is a less need for extreme amounts of heat to achieve the same viscosity of diesel fuel.
I do not blend fuels with my two tank setup, although I have greatly considered it. My recommendation if you feel you’re not getting your oil up to temperature (even with the FPHE and heated injection lines) would be to splash mix a gallon of kerosene or diesel in your WVO tank. Kerosene is known for its extreme thinning properties and would be an effective way of lowering the viscosity requirements of WVO for combustion.
My experience has shown this is not required as I have always had plenty of power and similar fuel economy compared to diesel fuel. However, the science behind it proves me wrong. Let it be known, there have been diligent people who have operated their WVO kit in the correct fashion and have had plenty of long term success.
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2001 Jetta TDI- 190K
17/22, T4 764's, KermaTUNE, Euroject SMIC, Southbend Stg2 Endurance Clutch, LSD, .681 5th

WVO no More 187k
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