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The main parameters of diesel include: flammability, volatility, fluidity, stability, and corrosivity. (1) Flammability: The level of flammability of diesel directly affects the operation of diesel engines. Cetane number is an important indicator of a diesel’s ignition and combustion performance in engines. The cetane number of diesel directly affects the combustion process of the fuel in diesel engines. Diesel has a high cetane number and a low auto-ignition point, allowing it to ignite easily in the cylinders of diesel engines, resulting in smooth engine operation. If the cetane number of diesel is too low, it is difficult for the fuel to ignite, resulting in abnormal combustion and a reduction in engine power. However, the cetane number of diesel should not be too high either; if it is too high, the diesel cannot burn completely, resulting in increased fuel consumption. The cetane number of diesel is related to its chemical composition. Normal alkanes have the highest cetane number, followed by naphthenes, while polycyclic aromatic hydrocarbons have the lowest cetane number. Typically, the cetane number of diesel fuel for vehicles should be in the range of 45 to 60. (2) Volatility: For the engine to start and operate properly, diesel must have good volatility. However, the volatility should not be too high either, as an excessive evaporation rate can lead to the accumulation of large amounts of diesel during combustion, causing unstable engine operation. At the same time, high volatility, meaning light fractions, inevitably results in low viscosity, which not only increases wear on the fuel injection pump but also reduces spray quality, thereby worsening the combustion process. In other words, it is not suitable if the volatility of diesel is too high or too low, that is, if the fraction is too light or too heavy. The volatility of diesel is primarily evaluated using the boiling range and closed-cup flash point. ①Range, 50% recovery temperature: The lower this temperature, the more light components are present in the diesel, resulting in better volatility and making it easier to start the engine. The standard specifies that the 50% recovery temperature should not be higher than 300°C. 90% recovery temperature and 95% recovery temperature: The lower these temperatures, the fewer heavy components are present in the diesel, which improves the combustion performance of the diesel and the power output of the diesel engine, reduces fuel consumption, and decreases mechanical wear. The standards specify that the 90% recovery temperature and the 95% recovery temperature shall not exceed 355°C and 365°C respectively. ②Flash point: The flash point of diesel is both a parameter for controlling its evaporation rate and a factor that ensures its stability. It is generally believed that light fuels are safe for storage and transportation if their flash point is above 35°C. The standard specifies that the flash point of diesel fuel grade 0 should be no lower than 55°C. (3) Fluidity: The fluidity of diesel is mainly indicated by viscosity, freezing point, and cold filter plugging point. ①Viscosity is an important performance parameter of diesel; it is closely related to the supply amount of diesel, its atomization properties, combustion characteristics, and lubricity. During operation, high-speed diesel engines have an injection time of only 0.001 to 0.002 seconds per injection. To enable the injected diesel to vaporize and ignite within such a short period, the diameter of the droplets must not exceed 0.025 mm in order to ensure complete combustion. The quality of atomization depends on viscosity; if the viscosity is too high, the droplets are large, resulting in uneven mixing with air and incomplete combustion that leads to carbon buildup. If the viscosity is too low, although atomization is good, the injection angle is large, which also prevents complete mixing with air. Additionally, it reduces the lubrication properties of components such as the fuel injectors, increasing wear. The standard requires that the kinematic viscosity of No. 0 light diesel at 20°C be between 3.0 and 8.0 mm2/s. Only within this range can it ensure good lubricity of the diesel for the engine’s fuel supply system, as well as optimal atomization and supply of the diesel, thereby enabling better combustion performance. ②Freezing point and cold filter plugging point are two key indicators for assessing the low-temperature fluidity of diesel. In China, diesel is classified according to its freezing point, which represents the highest temperature at which the diesel can no longer flow. However, in practical use, wax crystals begin to form before the diesel has completely solidified. Once these crystals reach a certain size, they can clog the filter elements, preventing the diesel from being usable even before it reaches its freezing point. The highest temperature at which diesel cannot pass through the filter under specified conditions is called the cold filter plugging point of diesel. The cold filter plugging point has a good correlation with the performance of diesel; the actual operating temperature ranges for different grades of diesel are determined based on the cold filter plugging point. (4) Stability: The stability of diesel has a similar impact on engines as that of gasoline. Diesel has poor stability; it oxidizes and deteriorates easily, its color darkens, and the amount of precipitates and gums increases, which can clog filters. It also tends to form large amounts of carbon deposits in the combustion chamber. In addition, a film forms in the diesel injection system, causing the piston rings to stick together and increasing wear, all of which have a significant impact on the storage and use of diesel. The stability indicators of diesel are mainly expressed by the 10% residue carbon and total insoluble matter, while the color intensity and its changes can also reflect the quality of diesel stability. (5) Corrosivity: The corrosivity of diesel is roughly the same as that of gasoline, and it is controlled through three indicators: sulfur content, acidity, and copper sheet corrosion. ①Acidity: The acid value, or acidity, represents the total amount of acidic substances in petroleum products. Generally, diesel is expressed in terms of acidity. Diesel with high acidity not only corrodes components but also increases coking and fouling in the fuel injectors and combustion chamber. **The standard specifies that the acidity of diesel should not exceed 7 mgKOH/100 ml. ②Corrosion test: The corrosion test is a qualitative test used to assess the corrosive effect of oils on one or several metals. Its purpose is to determine whether the oil contains thiols, reactive sulfur or free sulfur, as well as acidic substances, alkaline substances and moisture, which can cause corrosion to metals. **The standard specifies that copper sheet corrosion shall not exceed grade 1. ③Sulfur content: The sulfur content refers to the amount of sulfur and its derivatives present in petroleum products. It is an indicator that ensures machinery using these oils is not corroded, protects the health of operators, and prevents environmental pollution. When the sulfur content in fuel is high, reactive sulfur can corrode the equipment used for storing and transporting oil, as well as the fuel supply systems of machinery. Inactive sulfur, upon combustion, forms SO2 and SO3; when these substances come into contact with water, they form sulfurous acid and sulfuric acid, which can corrode machinery. Moreover, the release of SO2 and SO3 into the atmosphere causes pollution. The standards specify that the sulfur content in diesel should not exceed 0.2%, while that in diesel for use in vehicles should not exceed 0.05%. (6) Density: The density of petroleum increases as the contents of carbon, oxygen, and sulfur in its composition increase. Since density decreases as temperature rises, in China the density measured at 20°C is generally used as the standard density; the standard density of diesel is typically between 0.81 and 0.86 grams per milliliter. Apparent density refers to the density at the test temperature (ambient temperature); generally, customers require the density of oils to be measured as apparent density. If the density of diesel is too low, it can cause engine knocking and increase fuel consumption; if the density is too high, the diesel cannot burn completely, leading to carbon buildup in the cylinders and nozzles, which causes cylinder wear and blockages in the fuel supply lines, thereby increasing fuel consumption. (7) Moisture and mechanical impurities Moisture and mechanical impurities are important quality indicators for most petroleum products. During storage and transportation, oils may become contaminated with water and mechanical impurities for various reasons, which is harmful to their use. It can clog the pipes and filters in the fuel supply system, as well as increase wear on the machinery that uses oil. When water is present in diesel fuel, it not only increases corrosion and reduces the efficiency of equipment, but also deteriorates the fuel process. At low temperatures, moisture in the fuel freezes and blocks the engine’s fuel lines, affecting fuel supply. (8) Chroma Chroma is an indicator of the color of diesel; **the standard specifies that the chroma of light diesel should not exceed 3.5. The color of diesel is related to the quality of crude oil, the refining process, and the degree of purification. The color of diesel produced by different refineries can vary significantly.