Note: This is a new tutorial and can probably stand improvement. If you have a suggestion or question just post it below in this discussion and it will be answered. There are no stupid questions and if you don't fully understand something chances are that a dozen others don't either. Please don't be afraid to ask "what do you mean by this?" or "can you explain that further." Questions like those will only make this more useful to everyone.
The majority of VO conversions of diesel engines running today are "two tank" conversions. This refers to the fact they have one tank that contains diesel fuel and another that contains VO fuel. The main aim of most of the components in a VO conversion is to add heat to the VO fuel passing through them in order to lower its viscosity. Since VO can be too viscous/thick to flow through a fuel line from the VO fuel tank the first heat adding component in usually incorporated into the fuel tank.
The tank containing diesel fuel is used to provide fuel to start the engine and fuel it until it is warm enough to safely run on VO fuel. (For more information on this go HERE) By the time the engine temperature is up to "operating temperature" (typically 150°F to 190°F) waste engine heat is available to add heat to the VO fuel using the hot liquid coolant which circulates through the engine as well as the radiator and passenger compartment heating system. VO fuel needs to be heated in order to lower its viscosity so it can easily flow through the fuel supply system and so it is able to atomize into small enough droplets to be efficiently combusted inside the engine.
The tank containing VO is where VO fuel begins being progressively heated to the point it can be used as fuel by the engine. In the early days large coolant "coils" were installed in VO tank that tended to heat all of the VO in them to very high temperatures ..eventually. It was soon discovered though that in cold climates it took much longer for liquid VO to be available at the tank outlet than for the engine to reach operating temperature. More recently it has been revealed that when VO is heated to high temperatures polymerization of the VO occurs. This may cause VO filters to clog prematurely. Most modern VO conversions concentrate heat near the VO tank outlet...or use a heated fuel pickup in the tank itself. The heated outlet or heated VO pickup is the first conversion component that VO passes through as it travels toward the engine. The VO then passes directly into heated fuel lines.
In the heated fuel line VO is heated in order to keep it liquid enough to flow easily as it journeys toward the next VO heating component. Heated VO lines are also necessary throughout the conversion so any VO which is solidified due to cold ambient temperatures (during the time the engine is not running) is liquefied enough to easily flow.
There are two types of heated fuel lines Hose ON Hose (HOH) and Hose IN Hose (HIH).
HOH fuel lines are fabricated by simply bundling hoses which have hot engine coolant passing through them with those containing VO. The heat from the hot engine coolant is transferred through the walls of the coolant and VO hoses and into the VO. Flexible synthetic hose is usually used for HOH lines and the "hose bundle" is usually insulated with a flexible foam sheath to speed initial VO heating and conserve heat.
HIH fuel lines are fabricated by inserting the fuel line inside a larger diameter engine coolant line. Heat transfer is more efficient than exhibited by HOH fuel lines since heat must only travel through one hose wall and hot coolant surrounds the entire fuel line. However there is the danger of coolant leaking into the fuel and causing engine damage which is not possible if HOH lines are employed. HIH heated lines are also significantly more difficult to fabricate and install than HOH lines.
Optimally heated VO lines should be used throughout the conversion since VO may solidify in any unheated VO line and so prevent the flow of VO through the entire VO fuel system. The next most common component that the VO passes though is the coolant/fuel heat exchanger.
As the VO passes through the coolant/fuel heat exchanger its temperature is raised significantly. Since "waste" engine heat is the most abundant heat source available for raising the temperature of VO this is the component where most of the VO heating occur rs.
Two types of coolant/fuel heat exchangers are available, tube in tube (TIT) and Flat Plate heat exchangers (FP or FPHE). Of the two FP heat exchangers are much more compact and efficient. If space is very limited the clear choice is a FPHE. On the other hand FP heat exchangers cannot be fabricated by the DIYer and TIT heat exchangers can be easily fabricated using common plumbing supplies. After passing through the coolant/fuel heat exchanger VO is liquid enough to easily pass through a filter element. Therefore most VO filters are located after a coolant/fuel heat exchanger.
VO filters are necessarily heated filters primarily due to the fact that the VO inside them must be heated to a point where it is fully liquid before it can pass though the filter element. The filter element is the most restrictive point in the entire fuel supply system and so must optimally be able to heat the cold VO it contains to at least 100°F+ before the engine reaches operating temperature. Although fuel filters designed to be used with diesel fuel are also heated they are not well adapted to use with VO since the heating capacity is usually less than 1/4 what is required for VO use and the have an internal thermostat which only allows them to reach a maximum temperature much cooler than is necessary for use with VO. They are marginally adaptable at best. There are however commercially available VO filters, adapters to add more heating capacity for commonly available fuel filters, and DIY options for fabricating VO filters capable of reaching the necessary 100°F+ temps.
Although it is possible to use a single shared filter for BOTH diesel and VO fuel it is not generally considered desirable to do so for both convenience and safety reasons. It is desirable to install a dash mounted fuel pressure/vacuum gauge or indicator to provide a warning that your VO fuel filter is reaching a point where it should be replaced or is malfunctioning. Similarly a dash mounted fuel temperature gauge is also considered a very desirable option by many. The heated VO travels from the VO filter to the Injector Pump (IP) where it is boosted to extremely high pressure and distributed to the Injector Lines and fuel Injectors which inject the high pressure fuel into the engine itself. The last opportunity to heat the VO is as it passes through the Injector lines using Injector Line Heaters (ILH). Not all modern diesel engines have Injector Pumps and Injector Lines.
Injector Line Heaters allow VO to be heated to temperatures where VO is nearly identical in viscosity to diesel fuel. It is not possible to add this heat to the VO prior to it passing through the IP since most IPs will absorb heat from the VO if it is hotter than the operating temp of the IP itself. Most IPs run at a temp of 130-150°F and may experience accelerated wear if constantly operated at temperatures much higher than this. Injector line heaters are essentially 12v electrical resistance heaters which are wrapped onto the individual injector lines and transfer the heat they generate through the steel wall of the injector line and into the high pressure VO passing through them. VO temperatures of as high as 280°F can be achieved in this manner which allows the VO passing through the injectors to be sprayed in a pattern nearly identical to the diesel fuel it is replacing. This allows it to be combusted as fully as possible thereby increasing efficiency and reducing carbon deposits in the engine which may significantly reduce engine life.
In addition to the above VO heating components other common VO conversion components include:
3 and/or 6 port remove fuel valves (motorized, manual, or solenoid operated)
Electric fuel pumps
One way valves
Temperature and pressure/vacuum gauges
and of course
Wire, relays, and various types of electrical switches.