The fluid performance of diesel

The fluidity of diesel fuel is generally expressed
by the freezing point (referred to as the freezing point) and viscosity.
The freezing point is mainly used to characterize the low temperature fluidity of diesel and is the most important performance index of diesel, and the viscosity indicates the quality of diesel oil supply performance and atomization performance
.

The freezing point refers to the temperature
at which diesel loses fluidity under certain conditions.
Because the freezing point of diesel is generally just within the range of daily ambient temperature changes, when the conditions of use are not appropriate, diesel will become a paste at low temperatures, resulting in the inability to supply fuel to the engine and affect the normal work
of the diesel engine.
In addition, the storage and transportation of diesel fuel at low temperatures is also related
to the level of the freezing point of diesel.
Therefore, low temperature fluidity is of great significance for diesel
.

There are two types of causes of oil solidification, namely, viscous temperature solidification and structural solidification
.
For diesel with relatively low wax content, the viscosity increases with the decrease of temperature, and when the viscosity increases to a certain extent, the diesel will lose its fluidity due to being too viscous, called viscous temperature solidification, which is somewhat similar to the state of
edible peanut oil at low temperatures in winter 。 Structural solidification refers to diesel with relatively high wax content, with the reduction of temperature, the solubility of wax is reduced, it will crystallize and precipitate from diesel, when the content of crystalline wax in diesel reaches a certain level, it will form an interconnected mesh structure, and other diesel components that are still in a state of flow are wrapped in the mesh structure, so that the fluidity of diesel becomes worse, just like a dozen soccer balls are installed in the net, although each soccer ball still has a certain degree of freedom of activity, but the net can enclose all soccer balls together, It doesn't scroll
around.

Whether it is viscous temperature solidification, or structural solidification, in fact, diesel does not really solidify (solidification generally refers to the liquid crystal material with the reduction of temperature and the formation of a regular arrangement of stable structure), but the fluidity has become very poor, and did not completely lose the fluidity, in fact, touch the solidified diesel with your hand, you will find that the diesel is still soft
.
The reason why diesel cannot be like water, which immediately loses its fluidity at a certain temperature and becomes a solid is because diesel is not a pure compound, but a complex mixture, amorphous with no fixed freezing point
.

Because most of the diesel freezing point is within the range of daily ambient temperature changes, when the engine is added to the diesel with a relatively high freezing point, if the temperature is suddenly encountered, the diesel will become a paste with poor fluidity in the engine tank and the oil pipeline, and the engine cannot be properly supplied, so some diesel engines are difficult to start in the winter morning, which is related
to this.
When the temperature rises, the diesel will become a liquid with better fluidity again
.
For example, in the northern winter countryside, you will see people roasting tractors with corn stalks to remelt
solidified diesel.

Low temperature fluidity is the most important performance of diesel, once diesel loses fluidity, it can not supply the engine normally, diesel engine can not work normally, even if the other performance of diesel is meaningless, especially when working in a low temperature environment is more important
.
For example, in autumn and winter, some diesel vehicles add high-grade diesel when the temperature is relatively high the day before, and the next day suddenly cools down, and the engine cannot start; Or some cars have a high grade of diesel, when the car has been running, the engine runs normally, but when the car stops for a period of time, the engine temperature drops, and then it will be difficult to
start.
Therefore, diesel is to characterize the low temperature flow performance of the index - the freezing point to divide the grade, such as 0# diesel is that the freezing point of this brand of diesel can not be higher than 0 °C
.

At the freezing point, the diesel is already in a solidified state, becoming a paste, losing fluidity and cannot supply the engine normally, and the diesel has been completely unusable
.
In fact, diesel is no longer usable
until the temperature reaches the freezing point.
In order to prevent a very small amount of solid matter mixed into the diesel into the engine and cause damage to the cylinder, the engine oil pipeline is equipped with a strainer, the filter is not selective, when the viscosity of diesel or the wax crystals precipitated in it are large enough, although the diesel at this time also has a certain fluidity, but the filter has begun to hinder the normal passage of diesel, resulting in engine fuel supply difficulties
.
The minimum temperature at which diesel fuel can pass through the filter normally is called the cold filter point, that is, under the specified conditions, the maximum temperature when the flow rate of diesel fuel through the filter (filter) is less than 20 milliliters per minute, which is the ultimate low temperature
that diesel can be used normally.
The cold filter point of the same diesel fuel is often 5 to 10% higher than the freezing point, so when going to the gas station to add diesel, you should generally choose a grade of diesel fuel that is 5 to 10 times lower than the ambient temperature, that is, the ambient temperature cannot be lower than the cold filter point
of the selected diesel.

In addition to using freezing points and cold filter points to indicate fluidity at low temperatures, diesel also uses viscosity to represent fluidity in the conventional state, mainly characterizing the flow, flow pressure drop and atomization state of diesel fuel through pipelines and nozzles
.
During the work of the engine, the diesel fuel is first injected into the engine cylinder through the nozzle through the
high-pressure oil pump, and then combusted and exothermic.
If the viscosity of diesel is not appropriate, the range of diesel injection into the cylinder, the distribution of fog droplets in the cylinder and the particle size are not suitable (you can imagine the case of the spray can spraying water droplets at too much pressure or too hourly), and it cannot be mixed evenly with the air in the cylinder, resulting in incomplete diesel combustion and increased fuel consumption
.
The viscosity of diesel fuel is too large, which can also cause engine fuel supply difficulties
.

At the same time, diesel is also used as a lubricant for diesel engine oil pump and high-pressure oil pump, if the viscosity is not appropriate, the lubrication performance of diesel fuel is not good, and it is impossible to provide reliable lubrication for the oil pump, resulting in excessive wear
of the precision components of the engine.
Therefore, the viscosity of diesel must be appropriate, not too large or too small, and each different grade of diesel has its viscosity range limited
.