Plantoil/diesel conversion basics
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Some of the best research done on running diesel engines on VO fuel was conduced during the Carter administration at the University of Idaho during the first world wide "fuel crisis".  More indepth in depth VO fuel research was done by the European Advanced Combustion Research for Energy from Vegetable Oils (ACREVO)  group. 

Since then there has been some research on in Asia but most of THAT ressearch appears to be focused on large bore extremely low RPM single cylinder engines and may have limited application to automotive or higher speed diesel engines.

Most other VO conversion research done in the past 2 decades has been done by private individuals and is either proprietary or very subjective. 

The concept of converting diesel engines to use VO as a diesel fuel replacement is based on overcoming the engine damage problems reported by that early U of Idaho research.  Although problems were noted with carbon accretions on exhaust valves and injector tips the "engine killing" damage noted was piston ring coking. ALL of these problems were a result of incomplete combustion of VO fuel in the cumbustion chamber.  Since those U of Idaho researchers used unheated VO fuel in DI engines that damage began to be noticable  within 100 hours. 

The U of Idaho studys suggested that engine longevity was significantly increased by lowering the viscosity of VO prior to injection. VO viscosity can be dramatically lowered by simply increasing its' temperature, mixing it with a solvent (such as diesel fuel), or modifying its molecular structure through transesterfication (biodiesel).  But it was soon discovered (by researchers at NDSU) that piston ring coking still ocurred if viscosity was simply lowered by thinning VO with diesel fuel.
This led to the concept of bringing diesel engines to operating temperature AND THEN switching to VO fuel to reduce the opportunity for partially burnt VO to reach the cylinder walls (and ending cooking to carbon up in the piston grooves).  In the early stages of kit developement it was noted that cold engines started on VO tended to last longest for those who accummulated lots of highway miles between cold starts. Conversely those who drove very few miles per start (on VO fuel) tended to have the shortest useful engine lifes.
Based on this information it was concluded that maximum engine life when using VO fuel requires use of seperate tanks for VO and diesel fuel to allow all cold starts to be accomplished with diesel fuel and for the fuel system to be completely purged of VO fuel prior to engine shutdown. This became the basis for the "two tank" VO conversion concept.
 We have similarly learned that the hotter VO fuel is prior to being injected into the combustion chamber the longer useful engine life becomes. This is especially true for DI engines.  

What is piston ring coking and how is it caused.
Ring coking is created when partially combusted VO fuel is scraped off the cylinder bore walls by the piston rings as the piston moves up and down in the cylinder bore.
The most common reason that VO fuel fails to fully combust before it reaches the cylinder walls is that its viscosity can be 10 to 15 times that of diesel fuel. When injected through fuel nozzels designed for diesel fuel VO forms  droplets that are too large to fully combust and these partially combusted droplets collect on the edge of the piston top and cylinder walls. The viscosity of VO CAN be lowered to That of diesel fuel if sufficiently heated however. This leads to the questiopn of
How hot is HOT ENOUGH?
The Acrevo Study focused heavily on how to lower the viscosity of VO fuel using heat and established that 
it may be recommended, for any attempt of using VO in an engine, to use the rape seed oil at relatively high temperature, of order 100-130 °C.
(212°F to 266°F)
 Further research indicated that this range is slightly higher for some oils and lower for others but that at temperatures above 100°C the differences all but dissapear. 
A close up of the most pertinent area of the above chart is provided below.
The above chart was created using data from sources other than the Acrevo study..and yet appears to mirror that studys conclusions very closely.
While failing to provide sufficient heat to VO fuel prior to injection is the most common reason piston ring coking occurs and diesel engines using VO fuel fail there are other reasons that can contribute to early engine failure as well.
Injectors that are worn, set incorrectly, or damaged can also cause unburned VO fuel to reach the cylinder walls. So it is very important to make certain that you regularly check the condition of your injectors  by having them pop tested. It is equally important to not use VO fuel which might contain more than 200ppm of water since injectors can be quickly damaged by "cavitation erosion". 
*On IDI engines the pre-combustion chamber can become loaded with carbon and begin to dribble liquid VO fuel into the combustion chamber rather than vaporize it.
It is also very important to not switch to vo fuel before the engine has come to full operating temperature. Until that temperature is reached the pistons have not fully expanded and "blowby" is at its greatest. This not only means that compression may not be high enough for optimal combustion but that any partially burned fuel will be forced by the rings and into the crankcase.
This can not only accellerate piston ring coking but may lead to crankcase oil polymerization which can quickly lead to catastropic engine damage due to low oil pressure. (This is covered in the "operational consideratios section" of this tutorial)
This is also the reason you should thoroughly purge the VO fuel from your IP, injection lines, and injectors prior to shutting down an engine.  Because many IPs have a large volume that is filled with VO fuel when a purge is begun..and the diesel fuel introduced to the IP does not directly displace the VO fuel but rather dilutes it more and more as the purge progresses.  
It is important to not only want to allow plenty of time for the purge cycle to complete but also why you want to make certain that you divert the diesel/VO fuel mix that is created during purgeing to the VO tank..and not the diesel tank.
Finally..if you have converted an engine with a small cylinder bore diameter (like VW TDIs) you have to be extra vigilant on all of the above. Small bore engines are even more succeptable to these problems than large bore engines.
And since it goes without saying..I better say it. If you convert a diesel engine that has lots of blowby even when at operting temp don't expect it to last a long time.  About all one can do short of having the engine "re-ringed" is to ONLY use VO fuel at highway speeds once the engine is completely up to operating condition in addition to all the other considerations provided in this tutorial.
*This is the authors opinion based on "post mortem" examinations of VO fuel damaged engines and discussions with engineers that design diesel engines.

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