Diesel testing--What is the cetane number of diesel? How is it defined?

Combustion properties of diesel fuel for vehicles – Cetane number (ignition property) (I) Working principle of diesel engines and related phenomena The working principle of diesel engines: Both diesel engines and gasoline engines belong to the category of piston-type internal combustion engines. Based on their working processes, diesel engines can be divided into four-stroke and two-stroke types. Taking the common four-stroke engine as an example, the process begins with intake: the piston in the cylinder moves downward from the top of the cylinder, the intake valve opens, and air is drawn into the cylinder through the air filter. When the piston reaches its bottom dead center, the intake valve closes. Some diesel engines are equipped with air boost blowers to increase the amount and pressure of air supplied, thereby improving the efficiency of diesel use ； Compression: The piston moves upward from the bottom dead center, compressing the air that has been drawn in. Since this compression takes place under nearly adiabatic conditions, the temperature and pressure of the air rise sharply; by the end of compression, the temperature can reach 500–700°C and the pressure can reach 3.5–4.5 MPa ； Expansion work: As the piston approaches the top dead center, diesel passes through the coarse and fine filters, the high-pressure fuel pump, and the nozzle before being injected into the cylinder in a mist form. At this point, the temperature of the air inside the cylinder has exceeded the auto-ignition temperature of diesel; as a result, the diesel vaporizes rapidly, mixes with the air, and burns spontaneously. The combustion temperature can reach 1500–1200°C, while the pressure rises to 4.6–12.2 MPa. The high-temperature gas expands rapidly, pushing the piston downward to do work ； Exhaust: After the piston moves downward to reach the bottom dead center, it moves upward; at this point the exhaust valve opens to release the waste gases, completing one working cycle. It then enters the inhalation stroke to continue the next cycle. As can be seen from the above four processes, the working principle of a diesel engine is both similar to and fundamentally different from that of a gasoline engine; therefore, the requirements regarding the properties of the fuel are necessarily different as well. Firstly, diesel engines draw in and compress air, rather than a mixture of air and fuel; as they are not affected by the properties of the fuel, the compression ratio can be increased as much as possible, which significantly improves the efficiency with which fuel is converted into energy. In fact, diesel engines consume 30–70% less fuel per unit of work compared to gasoline engines, making them very economical. Secondly, gasoline engines are ignited by electric sparks; they are known as spark-ignition engines, and they require fuel with a low ignition point and a high auto-ignition point ； A diesel engine generates power through the expansion caused by the spontaneous combustion of fuel injected into the cylinder; therefore, it is also known as a compression-ignition engine, and it requires fuel with a low auto-ignition point. The rough operation of diesel engines: As can be understood from the working principle of diesel engines, during the compression stroke when the piston approaches the top dead center, fuel is injected in a mist form and rapidly vaporizes and oxidizes. The oxidation process intensifies gradually until it becomes so intense that ignition occurs, leading to expansion and the generation of work. The time period from when the injector begins to spray fuel until the fuel catches fire on its own is known as the ignition lag period (or delay period, expressed in terms of crankshaft rotation angle). Diesel has a low auto-ignition point and a short ignition lag period; after the fuel ignites, combustion occurs while fuel is being injected, resulting in stable engine operation and high thermal efficiency. If the auto-ignition temperature of diesel is high, the ignition lag increases; as a result, when self-ignition begins, more diesel accumulates in the cylinder and ignites simultaneously, causing a sharp rise in temperature and pressure. This leads to intense movement of the piston head, producing a metallic knocking sound. This phenomenon is known as detonation in diesel engines. (II) The significance of evaluating the flammability of diesel Diesel’s flammability refers to its ability to ignite spontaneously; in other words, it is related to the balance of combustion in diesel. The flammability of diesel is usually expressed by the cetane number. Generally, diesel with a high cetane number has better self-ignition properties, more uniform combustion, and improved ignition capability; it is less prone to rough combustion, which leads to higher thermal efficiency of the engine and an extended service life. However, a higher cetane number is not always desirable – using diesel with an excessively high cetane number can also result in black smoke and increased fuel consumption. This is because the ignition delay time of such fuel is too short, preventing proper mixing with air before combustion occurs, thus leading to incomplete combustion and the formation of black smoke due to the thermal decomposition of certain hydrocarbons ； A too high cetane number of diesel can also reduce the availability of fuel sources. Therefore, considering both usability and cost-effectiveness, it is reasonable to use diesel with an appropriate cetane number. The ignition performance of diesel is closely related to its chemical composition and distillation profile. Tests have shown that among different hydrocarbons with the same number of carbon atoms, n-alkanes have the highest cetane number, while alicyclic aromatic hydrocarbons without side chains have the lowest cetane number; n-olefins, cycloalkanes, and isoparaffins fall in between ； The higher the degree of isomerization of hydrocarbons and the greater the number of rings, the lower their cetane number ； As the side chain length increases, the cetane number of aromatics and cycloalkanes rises, whereas an increase in side chain branching leads to a significant decrease in the cetane number ； For the same hydrocarbons, the higher the relative molecular mass, the poorer the thermal stability, the lower the auto-ignition temperature, and the higher the cetane number. (II) Cetane number of diesel: The cetane number of diesel is one of the indicators used to assess its ignition properties. It is determined by comparing a diesel sample with a standard fuel in standard engine tests under specified operating conditions; when the two have the same ignition delay time, the cetane number of the standard fuel is considered the cetane number of the sample. Standard fuel is a mixture prepared by combining hexadecane, which has good ignition properties, and heptamethylnonane, which has poor ignition properties, in various volume ratios. (III) Methods to improve ignition performance. Additives that enhance the ignition performance of diesel are known as diesel ignition performance improvers, or diesel cetane number improvers. Both ester compounds and organic peroxides can significantly increase the cetane number. Different cetane number improvers have varying effects on different types of diesel, meaning they are sensitive to the components of the diesel.