Diesel fuel:
Diesel fuel is derived from crude oil. Diesel is somewhat heavier than petrol/gasoline and contains more combustion heat. In contrast to petrol/gasoline, diesel is highly prone to ignition, because it must ignite spontaneously as quickly as possible. There are 2 types of diesel fuel: summer diesel and winter diesel. More about this under Cetane number in the next paragraph.
Cetane number:
Diesel fuel must be able to self-ignite easily. The time that elapses between injecting the fuel and the start of combustion must be as short as possible. The tendency of the fuel to self-ignite is expressed by the cetane number. The higher the cetane number, the more easily the fuel ignites. If the diesel engine has to run at high revs, the fuel must have a higher cetane number (56 for indirectly injected engines, 70 for directly injected engines). At high engine speeds, the time available for the fuel to ignite is short.
Color:
The natural color of diesel is light yellow. Dyes have been added for various reasons.
Cloud point:
As the outside temperature drops, the fluidity of diesel fuel decreases. The cloud point is the temperature at which paraffin crystals in the fuel begin to separate out. The petroleum industry supplies summer and winter fuel. With summer fuel, solidification phenomena caused by paraffin separation can occur at -8 degrees. Winter fuel causes no problems down to -15 degrees. Only by heating the clogged parts can the engine be started again.
Viscosity:
For a diesel engine to operate properly it is very important that the fuel has the correct viscosity. With too low a viscosity, the fuel has poor lubricating properties and leaks in the injection pump occur more easily. With too high a viscosity, the injection system is subjected to extra load. Both too low and too high a viscosity affect the droplet size of the injected fuel and therefore also the course of the combustion.