This has to start with the cetane number of diesel.
The cetane number is the self-ignitability index of diesel when it is burned in a diesel engine. The higher the cetane number, the better the combustion performance of diesel, and the less likely it is to cause abnormal combustion such as knocking. It differs from the diesel label: the diesel label only indicates the anti-coagulation properties of the diesel, and the cetane number is a true indicator of the combustion performance.
The cetane number is related to the nature of the diesel component. In general, alkanes have the largest cetane number, the smallest aromatic hydrocarbons, and the naphthenes and olefins are somewhere in between. N-hexadecane (CN is 100), 1-methylnaphthalene (CN is 0) diesel standard fuel is from n-hexadecane and 2,2,4,4,6,8,8-heptamethyl The decane (CN specification 15) is mixed in a certain ratio. The test oil is compared with the standard fuel on a standard engine. If the test oil has the same degree of knocking as a standard fuel, then it is the same as the standard fuel. The cetane number, the cetane number of the standard fuel, can be calculated from the formula: cetane number = n-hexadecane volume fraction + 0.15 x heptamethyl decane volume percent. Generally, the cetane number of coking diesel is generally around 50, the cetane number of Ciba is generally around 45, and the cetane number of Chai is generally around 35.
The same petroleum, the diesel fuel addition process is different, the diesel fuel has different cetane numbers: straight-run diesel with high alkane content and high cetane number; catalytic diesel isoparaffins and olefins are higher than straight-run diesel. Therefore, its cetane number is lower than that of straight-run diesel; the content of olefins and aromatics in coking diesel is higher than that of catalytic diesel, so its cetane number is obviously lower. The diesel fuel subjected to catalytic hydrorefining is converted into a naphthene by conversion of an olefin into an alkane, so that the cetane number is improved and the performance is improved.
In general, the light diesel fuel used in cars should have a cetane number of no less than 45. If the cetane number of diesel is lower than the working conditions, combustion delay and incomplete combustion may occur, and even knocking may occur, reducing engine power and increasing diesel consumption. However, if the cetane number is too high, it will also cause incomplete combustion and smoke, and increase diesel consumption. The high-speed diesel fuel has a cetane number of about 40-56. Adding a small amount of additives (such as amyl nitrate) can also increase the cetane number of diesel.