Plantoil/diesel conversion basics
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VO/PO conversions require heated fuel line to ensure that any solidified/thick VO/PO is fully liquid by the time the engine is warm enough for a switch to VO fuel from diesel. These are even more critical in wvo conversions.

Two basic designs of heated VO/PO fuel lines exist in common use today:

HOH - Hose On Hose- Lines that hot coolant flows through are bundled with the line(s) that VO/PO flows through. Often the entire bundle is sheathed in insulation to conserve coolant heat.  HOH lines are usually intended to liquefy any VO which has solidified in the line during the time the engine has been shut down and to slightly warm the VO as it travels toward the engine from the VO/PO tank. HOH is usually easier to bend than HIH and so can be simpler to install and provide a "cleaner" looking installation.


HIH - Hose In Hose- A line through which VO/PO flows is completely encased in a large line that hot coolant flows through. HIH usually is able to add more heat to the VO flowing through the fuel line than HOH and is often sheathed in insulation to conserve coolant heat.

I strongly suggest that if you plan to use a HIH fuel line you avoid PEX and use aluminum as the inner line instead. 
Using PEX as the inner line in a HIH represents a significantly greater danger that the fuel line will be compromised or kink and that as a result either minor (fuel restriction) or major (fuel contamination) problems will result.

  No matter which style (HIH or HOH) you use you will still need a dedicated coolant/fuel heat exchanger (most use FPHES now) to lower the vo viscosity sufficiently.  So a more expensive and complicated HIH line may represent wasted effort and funds.

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