Fuels with a viscosity higher than that for which the engine was designed will tend to cause operating or reliability issues. Heating high viscosity fuels will reduce viscosity offering improved performance.
Cold weather can cause problems with diesel fuel as it can begin to solidify below -7°C and will block, or wax, the fuel filter. Products that use heat to aid diesel fuel flow in cold weather are available and many are suitable for application with fuels that wax at a higher temperature. Sometimes it may be appropriate to modify these products to get them to work effectively with fuels that have a higher wax point.
Fuel heaters are often added to engines running on biodiesel and are almost always used when fuelling with vegetable oil or cooking oil
Effective application of fuel heaters
Where necessary fuel heaters should be placed to suit the engine, fuel properties and fuel system design. If a fuel is to be operated below the temperature at which it solidifies it will be necessary to heat the entire fuel system. There are a number of areas where elevated fuel temperature can be beneficial.
Fuel filters
Any fuel filter, strainer or gauze will be restrictive to fuel flow and prone to waxing. Heating fuel in or prior to any filter will help to reduce the restriction and the occurrence of waxing.
Excessive restriction at the filter can cause fuel starvation and air ingress. With some types of injection equipment, fuel restrictions will alter the engine speed / injection timing relationship.
Fuel injection pump
Rotary fuel injection pumps are lubricated by the fuel. Rotary pumps which use a radial fuel delivery pump design, such as those made by Lucas / CAV, are prone to damage when fuel with with Pure plant oils. This has been attributed to insufficient rotor lubrication due to the higher viscosity of cold fuel.
Axial rotary pumps, Bosch VE design and licensed copies, tend to be more robust although it is wise to limit engine speed when the fuel is thick (eg. until the engine has warmed up)
Heating the fuel at or prior to the injection pump will reduce its viscosity and decrease the possibility of damage. Due to the difficulty of safely adding fuel heaters to the injection pump generally incoming fuel is heated. A cold injection pump will act to remove heat from the fuel for some time.
Some twin tank systems heat the starting fuel to provide heat to the injection pump. This helps to negate the heat sink effect the large mass of the injection pump will have on the secondary fuel.
Fuel injector
Fuels with a high viscosity and surface tension have been shown to provide degraded sprays. Increasing the temperature of the fuel reduces viscosity and surface tension and improves the injector spray.
Fuel heater design
Engine Coolant Heat
Waste heat from liquid cooled engines can be usefully employed to heat fuel using a suitable liquid to liquid heat exchanger.
Engine Oil Heat
Engine oil can also be used to heat the fuel using a liquid to liquid heat exchanger
Exhaust Heat
The heat from gasses in the engine exhaust system can be used to heat the fuel. Exhaust heat can have large variation and care should be taken to not overheat the fuel.
A robust system is unknown at this time although exhaust heat can simply be used by running fuel lines in close proximity to the exhaust system.
A reported system used metal fuel line wrapped around the exhaust system of a genset. As a generator engine runs at a steady rate using a steady fuel flow heat was regulated by the number of turns around the exhaust. The unit was not dual fuel and there were some problems with cold starting. Using this heat source in a dual fuel system could lead to problems overheating the oil before switching.
Returned Fuel Heat
Fuel returned from the injectors and injection pump will have taken on some heat. This heat can be utilised by sending the returned fuel back into the incoming fuel or through a heat exchanger with the incoming fuel.
Electrical Heat
An electrical heating element can be used to heat the oil to suitable temperatures. In a vehicle application care has to be taken not to have a power demand that would be excessive for a given electrical system. Often the surplus electrical generation capacity of a vehicle engine is not sufficient to provide enough energy to raise PPO fuel temperature to a level where its viscosity and surface tension are reduced to close to that of conventional diesel fuel.
In single tank systems electrical heat can be usefully employed at or just before any fuel filters to stop them waxing with cold fuel upon cold starting.