Until recently, the phrase “diesel fuel for cars” could bring a smile to the faces of experienced drivers. This is not surprising, because this oil was used mainly to fuel trucks and large special equipment. But due to the significant progress in refining processes and engine manufacturing it has become possible to use diesel fuel in cars.
The issue which has always worried each driver is the quality of the fuel in the fuel tank. A decisive role has been played by the ways to determine which petroleum product could be regarded as good, and which one – not so good. It is more or less clear with gasoline, but how to determine the quality of diesel fuel?
What is the cetane number?
Among the most important qualities of diesel fuels, it is worth highlighting their ability of ignition and self-ignition. This property is expressed as a numerical parameter, such as a cetane number. By its physical nature, a cetane number is the time between the moment when fuel enters an engine cylinder and its combustion. The higher the cetane number, the shorter the time interval, and the better is diesel fuel. Running on fuel with a high cetane number is characterized by increased engine power, and at higher speed.
What else does the cetane number value mean?
In addition to the ability of ignition and self-ignition, a cetane number also characterizes the degree of environmental friendliness of fuel. The fact is that the ignition ability depends on the hydrocarbon composition of fuel. Here is an example. If paraffinic compounds dominate in oil, it ignites relatively easily. When aromatic hydrocarbons dominate, the ignition occurs with a certain delay.
In the case of using diesel fuel with a cetane number below 40, it increases the risk of rough engine performance, which manifests itself in rapping at idle. This results in deterioration of the engine performance, wearing on bearings and other critical components and parts of the fuel system.
Working conditions put forward certain demands to the cetane number of winter and summer diesel fuel. In the former case this indicator should be higher than in the latter case. Also, it should be understood that cetane number is different for engines operating on ships (20), trucks (40) and cars (55).
Cetane number measurement
For diesel fuels, the following method of cetane number determination is applicable. First of all, the value is numerically equated to the percentage of cetane in its mixture with alpha-methylnaphthalene in fuel equivalent. In further calculations the flammability of cetane is conventionally assumed to be 100%, and that of alpha- methylnaphthalene in fuel equivalent is 0%. Then field trials are conducted, during which the time of fuel self-ignition is measured and compared with the figures for fuel equivalent.
Here is an example. If it was determined that diesel fuel was ignited in a combustion chamber during the time equal to the time of ignition of a 30% cetane and alpha-methylnaphthalene mixture, it has a cetane number equal to 30.
Increasing cetane number
It should be noted that the quality of diesel fuel is determined not only by the value of its cetane number, but also by other factors, like oil contamination with mechanical impurities and the presence of water. Nevertheless, the issue of increasing the cetane number to normalized values does not become less important.
Filtration and separation have almost no effect on the time of ignition and self-ignition of fuel. They allow to clean fuel from water and mechanical impurities, but not more than that.
Special additives are used in practice to improve cetane number. They are substances which are added to oil in a small amount in order to improve certain performance characteristics. They compensate for the missing units of a cetane number of diesel fuel, normalizing the time of its ignition.
During industrial mixing of additives and petroleum products, it is necessary to ensure getting the most stable mixtures that would not break down for a long time.
Wyen using conventional mixing techniques, known today, fuel tends to break do