Plantoil/diesel conversion basics
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Diesel engines are known for their durabilty.
But feed any diesel "out of spec" fuel and you can kiss that legendary durability goodbye!

WVO is clearly "out of spec" fuel.
For one thing it is far too viscous to atomise as well as diesel fuel once it is injected into the combustion chamber. This is why the main focus of VO conversion is to heat WVO fuel to as high a temperature prior to injection as possible. (This subject will be dealt with seperately)

But even if you have a state of the art conversion capable of heating WVO fuel to 230 degrees F you cannot ignore fuel quality. There are 3 main contaminats in wvo that must be removed if you hope to obtain close to normal engine life when using this alternative fuel.

1. Particulates- Obviously you cannot simply use unfiltered wvo as a fuel. Your onboard fuel filters will quickly clog and starve the engine of fuel. WVO must at the very least be filtered to the same micron rating as the stock fuel filters. 

2. Water- Less obvious than particulate contaminants are liquid contaminants such as water. Since water does not easily remain in suspension in petrodiesel the only provisions in is a "water trap" that catches visible droplets of water before the fuel reaches the Injection Pump. Why is water a concern? Because water droplets exposed to rapid pressure changes (like those in a diesel fuel injection system) can quickly erode metal surfaces in the fuel injection system components. These include the close tolerance internal components of Injector Pumps, the internal components of Fuel Injectors, and the orifices at the tips of some Fuel Injectors. This phenomenon is referred to as cavitation erosion and will at time be mistaken for acid etching.

In fuel systems with IPs the injectors are triggered by the pulse of high pressure fuel the IP provides. If that pulse does not reach a critical "pop" pressure at the correct time the spray of fuel provided by the injector is "late" (in terms of engine event timing) and the combustion of that fuel will not be optimal.  This in turn will create piston ring coking and the early demise of the engine.

In fuel systems which do not rely on the fuel pulse provided by the IP cavitation caused by "wet fuel" will damage the fuel injector internal components causing either mis-timed injection similar to that described above, poor atomization of the fuel in the combustion chamber, or post injection fuel "dribble". These conditions will in turn create piston ring coking and the early demise of the engine. much water is TOO much water?
The ASTM standard for MAXIMUM water content in diesel fuel is 500PPM (Parts Per Million).  But diesel fuel rarely has that much water in it. However diesel fuel usually tests at 200-300ppm when samples are gathered at fuel station pumps.   Common wisdom would lead us to believe that a H2O levels similar to diesel fuel is "good enough". However...experience has shown me otherwise.  I reccomend to clients that 200ppm should be considered a MAXIMUM alowable H2O content in wvo fuel becasue I have noticed that cavitation seems to be more pronounced in wvo fuel than petrodiesel. Why..I am not sure at this point.  My best (educated) guess at this point is that cavitation damage is more pronounced in wvo fuel because even when heated to 230F VO is more viscous than diesel fuel at the point it passes through the injectors.  More research is needed to be certain.

3. Salts and water soluble acids- WVO becomes contaminated by salts as the juices in food being cooked in it leak out into the oil. Some foods (like chicken) have salt in the "crispy coating" as well as in liquid injected into them to enchance thier flavor.  Acids are most commonly created as food particles contained in the used VO decompose in the collection drum or "dumpster" during the time the oil is "disposed of" by the cooks and when it is picked up to be used as fuel feedstock.  Both water soluble acids and salt are dissolved in the water content of wvo and so it is possible to remove these contaminants along with the water if water removal is not done using a method that relys on evaporation.  When water that contains salt or acids is evaporated it eventually becomes so saturated with salt or acid that what water remains cannot evaporate. (As the saturation level increases so does the "boiling point") This remaining salt or acid saturated water is extremely hygroscopic and so will pull water from the air. The result is wvo fuel that may test at 200ppm after processing..but that quickly draws enough water from the air to reach H2O levels in excess of 500ppm or higher shortly after being added to your vehicles fuel tank.

 Neither the Hot Pan Test nor the "Crackel Test" can be considerd sensitive enough to be useful as a test for water wvo fuel anymore. The ONLY method to test for H2O levels of under 500ppm is the Sande Brae method.

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