Plantoil/diesel conversion basics
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Posts: 934
Reply with quote  #1 
There are two basic classes of VO conversions.

The simplest is the "single tank" conversion named because it requires only one fuel tank. Single tank conversions are STARTED AND RUN on VO fuel. VO viscosity is lowered by using some of the same components used for this purpose in two tank conversions ..or blending.
For more info  HERE.

Two tank conversions  have two tanks because they are started on diesel fuel (or bio-diesel) and then switched to VO fuel once they have reached operating temperature. This requires a tank for each fuel.

Single tank conversions can be MUCH simper to fabricate and install than two tank conversions but they have some serious drawbacks. These drawbacks are the main reason that nearly every kit vendor only offers two tank conversion kits.

Since many diesel engines will start and RUN on VO some considering converting to VO fuel assume that this indicates a single tank conversion is comparable to a two tank conversion and choose this seemingly simpler and less expensive conversion option. And in some cases single tank conversions ARE the most appropriate choice.

Single tank conversions are probably more appropriate in situations:
1. Where the vehicle is only to be used in warm climates..where the VO fuel will remain fully liquid.

2. When the vehicle will only be used for trips of over 150 miles and very few if ANY short trips.

3. When the vehicle has an older design IDI (Indirect Injection) diesel engine.
4. Where total engine life is not a major concern.

This is mainly because when VO is used to start a diesel engine not at full operating temperature incompletely combusted VO fuel tends to condense on the cool cylinder walls and accumulate on and behind the upper (compression) piston rings. As the engine heats up the head (and pre-combustion chamber in the head) warm up sufficiently to allow significantly more complete VO fuel combustion and the cylinder walls warm enough to allow significantly less condensation of any incompletely combusted VO fuel.

The piston warms to much higher temperatures than any other part of the engine and so any partially combusted VO fuel that manages to seep around and behind the upper (compression) piston rings turns to carbon. Eventually the accumulating buildup of carbon formed causes the upper (compression) rings to seize in the piston grooves and their ability to contain the high pressures above the piston progressively decreases and more and more uncombusted VO fuel leaks past these (compression) rings.

This has two very undesirable main consequences:

1. Upper ring "land" coking.
This is an accumulation of the same carbon deposit as in the ring "grooves".
(Lands are the areas between the grooves on pistons). These carbon deposits eventually build to the point that they begin to cause scoring of the cylinder walls and even faster degradation of the ability of the upper (compressions) rings to contain the high pressures above the piston.

2. Crankcase oil polymerization.
Polymerization is a reaction which VO fuel can undergo which changes it to a plastic like substance. High heat and Oxygen levels accelerate polymerization..and any VO fuel which is present in the crankcase oil is subject to high levels of BOTH heat and O2. This causes the crankcase oil to turn to a thick jellylike consistency or even a solid once the engine has cooled to ambient air temperatures. This in turn can lead to major bearing and cylinder damage when a cool engine is started with crankcase oil that cannot freely circulate and provide adequate lubrication.

It is possible to monitor an engine to determine if the upper (compression) rings are losing their ability to contain the high pressures above the piston due to carbon accretion (coking). By performing a compression and leak down test on an engine before it is converted and then at regular intervals thereafter one can note when this begins to occur by the lower compression values that are noted as the rings begin to seize in their grooves.

One can also test for crankcase oil polymerization.

If you choose to convert to VO fuel by using a single tank configuration I strongly advise you to regularly perform both of these tests.

Once the rings begin to seize in their grooves due to ring coking there does to appear to be any method to reverse it and if the engine is to be salvaged it must undergo piston ring replacement. The cost of this procedure can easily be several times the added cost of converting using a two tank configuration.

For more info on "Coking" go HERE


Posts: 934
Reply with quote  #2 

Originally Posted by Yugzster

I just bought an 84 300TD Benz. I'm in northern CA but it only gets below 30 a few months a year in the mornings. I'm a little confused over going one tank or two. My commute is about 100 miles each way per day. Pros...Cons??? It seems like one tank is the way to go.

A one tank conversion may be simplest and possibly the cheapest.
But as tempting as this make them...simpler and cheaper is not always better.

The simplest one tank conversion would be to blend diesel and prefiltered/dewatered wvo and simply use it as a substitute fuel. Depending on the amount of fats and hydrogenated wvo in your wvo..and how much you are able to remove... you may be able to save 50% of your fuel costs in this manner.

Since piston ring/land/groove coking occurs mainly during the time that the engine is warming up on VO fuel (this includes VO/Diesel Blends) if you only use your vehicle for your long commute  you should not experience serious PRLG coking for several years. When you finally do  (and who knows how many years you may drive your commute before PRLG coking forces you to replace or re-ring the current engine?) you may have saved enough..or several times more than enough ... to do so. 

The most obvious "con" is that you should probably ONLY use this car for your long commute if you "convert" it with a single tank configuration VO/D blend. I suspect that you will be able to use a higher % of VO in the blend in warm months..and may have to use a lower % of VO in the cold ones. So I use 50% VO as an average. Which brings up the other most obvious "con". If you blend you need to be careful to anticipate what the coldest ambient temperature will be while that tankful of VO/D blend is being used. If you don't you will find that the blend will be too thick to pass through the filter....and you may find you cannot start the engine till the ambient temps warm again.

Another "con" is that if you have problems with your VO fuel system in a single tank you have no alternative but to  fix it "right now". A two tank conversion allows you to switch to diesel and put off fixing the problem till you reach a better spot.

Possibly the biggest advantage of a single tank conversion is that it allows you to start using WVO fuel as soon as you are able to collect and prefilter/dewater it. You start saving money on fuel costs immediately.  I have had several individuals that have started using blends ASAP and put aside the savings for gradual improvements to their conversion. Most have eventually opted to completely convert to a two tank after less than a year since it was pretty simple to do so in "short project" increments and they quickly saw that doing so would allow them to lower the amount of petrodiesel they had to use to less than 5%. The more VO you are able to substitute for diesel fuel the more $ you save on fuel costs.

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