Problems With Viscosity of Diesel Fuel
Viscosity is a very important issue with diesel fuel. Viscosity is the resistance to deformities of a substance caused by stress. It is also described as how thick something is. The reason a substance, such as diesel has a higher viscosity in lower temperatures and a lower viscosity in higher temperatures is that in higher temperatures, more energy from heat is being added to the substance to break the bonds, or weaken the intermolecular forces. This makes something less viscos because the property of viscosity is due to intermolecular forces. The atoms with stronger intermolecular forces are pulling each other together making it harder to move through the substance macroscopically; this substance is thicker and more viscos.
This causes a problem with diesel fuel. In the winter time, when the temperature gets cold enough, diesel fuel becomes too viscos and will not ignite or pump. It will not ignite because diesel needs to be aerosolized to ignite. Simply, it must be easily turned into a gaseous material which is difficult to do if it is too viscos. Also it is too thick to pump throughout the engine. It is like trying to swim through honey, it would be very difficult.
Preheaters are used in diesel engines to get around the cold weather problem. This is used to heat the fuel during start up to make it less viscos so the car can start. They can only operate for a few seconds while the car is starting up. If they go for any longer they may burn out. This problem is now much easier to get around with computers that monitor things like that.