What is vegetable oil fuel polymerization?
When you open the fuel filler cap on your vegetable oil powered vehicle, do
you smell something like paint fumes? Is there a coating of thick, gummy,
red-brown substance on the metal filler-cap or in the neck of the fuel tank? This is polymerized vegetable oil.
This “stuff” is (Quoting Joe Beatty page 4) “…long
chain, cross-linked polymer…with a molecular weight in the millions.”
What’s happening is that the vegetable oil is reacting to oxygen and the
metal in the fuel tank, creating a plastic-like film. This substance largely
adheres to the tank wall until it becomes too thick and heavy. It then
falls into the fuel, causing problems in the fuel delivery system. This chemical reaction is accelerated by
certain metals (like copper), heat, and light. The more of these things
to which the fuel is exposed, the faster the polymerization takes place.
It also seems that in the short-term, recycled vegetable oils (WVO) are more
likely to polymerize than are virgin oils.
Why do we care about this?
At a minimum, polymerization decreases fuel filter life. The four-orders-of-magnitude-increase
in molecular weight means that the fuel has a more difficult time passing
through the filter media. At a maximum, combined with filter-failure, the
polymerized fuel can cause damage to sensitive fuel injection equipment and
even engine damage in extreme cases.
The most in-depth discussion of polymerization I have seen was written by
Joe Beatty, titled Vegetable Oil as Fuel.
It is available on many vegetable oil-related websites including:
http://www.plantdrive.com/Vegetable_Oil_as_Fuel.pdf
What is the solution?
Ideally, vegetable oil should be used out of plastic tanks rather than metal
tanks. Copper heat-exchangers should be avoided, and oil should not be
allowed to sit around in open containers in the sunlight. Use fuel
systems that heat the fuel only to the extent necessary. Don’t store vegetable oil for long periods of
time.
What is PrairieFire doing about this problem?
PrairieFire is engaging this problem on several fronts.
We are informing those with single-tank conversions on vehicles with steel
tanks that polymerization is a problem.
These vehicles include primarily Volkswagens in the 1980’s and Mercedes
Benz through the 1990’s. Interestingly,
some of these vehicles seem to develop problems very quickly and dramatically,
and others not at all.
We are pursuing coating technologies for the internal sealing of steel
tanks. To this end we applied a
hardening plastic-like coating to the inside of a Mercedes fuel tank. This vehicle will serve as a source of
data. Our fuel-dispensing-equipment
expert has recently mentioned a “glassy like coating.” We are hoping this can be a solution to our storage
and dispensing concerns.
We are searching for aftermarket tank solutions. Perhaps there is a company that already
manufactures plastic replacement tanks for these applications. If there is not a manufacturer of these
items, perhaps we can convince someone to begin manufacture. In the case of VW the tanks are under the
vehicle, so the safety of a plastic tank is not a concern. In the Mercedes sedans, the fuel tank is
inside the vehicle behind the rear seat, isolated from the passenger
compartment by a steel plate. This should
also not cause a safety concern.
Finally, it is possible that there is a chemical solution to this
problem. Again, this is thanks to Joe
Beatty. He has extensively researched
the use of metal inhibitors and antioxidants.
These chemicals used in combination seem very effective in reducing
polymerization in vegetable oil fuels.
The last time I spoke to Joe, he was hoping to be able to sell this
chemistry as a fuel-additive. I have yet
to see this product on the market.
