Folks have been trying to run diesel engines on VO since shortly after Rudolf Diesel developed the compression ignition engine. (Someone else had actually already invented it) Contrary to the urban legend the "Diesel" engine was never designed to run on VO..but it was adapted to run on many fuels...including coal dust. In fact some very large diesel engines (those in ships) are run on a fuel that is very close to tar in consistency, smell, and appearance. To do that the thick tar like "bunker fuel" is heated to lower its viscosity much like VO conversions do. Those engines have bores that are several feet to several YARDS across and combustion "events" that are 20 to 50 times slower than the diesel engines in cars and trucks. Which is why THOSE engines can run on such a tar like fuel for hundreds of thousands of hours before requiring overhauls. Essentially the engines were adapted to the thick hard to combust fuel. We don;t have that option.
Instead when trying to adapt car and truck engines to reliably run on VO fuel we modify the fuel delivery systems so VO fuel passing through is heated as much as possible prior to injection into the combustion chambers of our engines.
Research has shown that in order to the viscosity where it combusts as well as diesel fuel VO must be heated to over 300 degrees F. Current commercially available conversion kits are only able to raise the temperature of VO to pre-injection temps of between 100F and 150F. These use high efficiency Flat Plate Heat Exchangers (FPHEs) that transfer heat from engine coolant to VO fuel shortly before he fuel reaches the IP. Optimally the fuel temp exiting the FPHE is near coolant temp (160F to 180F) but generally the fuel temp is 10-20 degrees cooler (140F to 170F) . After passing through the IP the fuel temp of conversions using FPHEs averages 150F and from 20 to 40 degrees of heat is lost as the fuel passes though bare steel injection lines. Fuel temp in common rail engines tends to remain within 10 degrees of what it was after passing through the FPHE.
There is not much that can be done using currently available technology to further warm the fuel in common rail diesel engines. However for those engines with injector lines two components can be added to significantly increase the final pre-injection temperature of the VO fuel.
1. The most obvious is to insulate the steel injection lines to decrease the amount of heat lose to the air blowing by those lines. Light weight insulation material must be used to minimize the possibility of harmonic vibration developing (which can create stress fractures in the lines) .
2. 12volt injector line heating elements may also be installed to actually ADD heat to the VO fuel as it passes through the injector lines. Currently there are two commercial Injector Line Heaters available.
Some of the first ILHs to become available were simply lengths of stainless steel wire that were wrapped around injector lines after a layer of insulation was first applied. the wire and insulation were then held in place using high temp silicone tape. In tests these added from 10 to 20 degrees of heat to the VO fuel as it passed through the injector lines. The efficiency of these heaters were limited by the heavy insulation applied between the resistance wire and the injector lines, the low power the resistance wire were able to draw, and the heavy wire itself, which had a tendency to lose more heat to the air passing over than was transferred to the injector lines and the fuel passing through them. These ILHs are still sold by various vendors but due to their poor performance I cannot recommend using them.
A significantly more effective ILH was developed by Line Heater Specialists. These ILHs use a very thin and heat transmitting sticky layer between the resistance wires and the injector lines. Instead of a single large diameter stainless steel wire 4 very thin wires made of high resistance alloy are used. Not only do these ILHs produce more total heat they are designed to deliver that heat to the injector lines. When installed as directed the fuel temperature gains from 50 to 80 degrees as it passes from the the IP to the fuel injectors via the injector lines. An additional 15 to 20 degrees can be added to the fuel passing the VO fuel by insulating Line Heater Specialists heaters with split silicone tube and poly foam sleeves. This allows VO fuel temperatures to be raise to a final pre-injection temperature of from 220F to 240F.