Why does the oil have a low flash point?

Because the lighter the oil, the greater its volatility, so its flash point is lower~~~:lol

A low flash point of the oil indicates a high content of small-molecule hydrocarbons in it. Small-molecule hydrocarbons have low boiling points, which results in a high partial pressure of flammable gases at the liquid surface; this makes it easy for the oil to ignite at low temperatures, hence the low flash point.

The flash point is the lowest temperature at which a flammable liquid or solid can release enough vapor to form a flammable mixture with air at the surface of the liquid or solid within its container. The flash point of flammable liquids changes with their concentration. 　　The flash point, also known as the ignition point, is the temperature at which a mixture of vapor and air above the surface of a flammable liquid begins to emit sparks when in contact with fire. The flash points of various oils can be determined using standard instruments. The lowest temperature at which the vapor resulting from the evaporation of a liquid, mixed with air, can catch fire is determined using the closed-cup method. The flash point temperature is lower than the ignition point temperature. From a fire safety perspective, the flash point of a liquid is the lowest temperature at which it can cause a fire. The lower the flash point, the greater the risk of fire. 　　The ignition point, also known as the burning point, is the temperature at which a mixture of vapor and air above the surface of a flammable liquid, when in contact with fire, allows the flame to continue burning for at least 5 seconds. It can be continued to be measured in the same standard instrument after determining the flash point. The flash point and ignition point of flammable liquids indicate the likelihood of an explosion or fire, and are of great importance for the safety of their transportation, storage, and use. 　　The flash point refers to the lowest temperature at which a petroleum product, under specified conditions, will ignite instantly when its vapor comes into contact with a flame upon heating. The lighter the oil, the lower its flash point. The hazard level of oils is determined by their flash point. The flash point can be used to determine the severity of an oil’s composition and to assess the risk of fire involving that oil. The flash point determined using a closed-cup flash point tester is called the closed-cup flash point, and it is generally used to determine light petroleum products. The higher the flash point, the safer it is. The flash point is a parameter that indicates the evaporation tendency and safety properties of petroleum products. The hazard level of oils is determined based on their flash point; those with a flash point below 45°C are classified as flammable substances ； Substances with a temperature above 45°C are flammable. During storage and use, it is prohibited to heat the oil to its flash point; the maximum heating temperature should generally be 20–30°C below the flash point. During the use of oil products, the flash point also holds significant importance. For example, if the flash point of the engine oil in use drops significantly, it indicates that the engine oil has been diluted by fuel; the engine should be inspected and the oil replaced.

The flash point is the lowest temperature at which, under specified test conditions, when a petroleum product is heated, the mixture of vapor and air emitted comes into contact with a flame and causes an instantaneous ignition, and it is expressed in °C. Depending on the measurement method and instruments used, there are two types of measurement methods: open-cup and closed-cup. The former is used to measure heavy oil products, while the latter is used to measure light oil products. The flash point is often used to determine the hazard level of oils and fuels; for example, those with a flash point above 45°C are classified as flammable substances, while those with a flash point below 45°C are considered highly flammable. The flash point of gasoline corresponds to the upper limit temperature for explosion, while the temperature of oils such as kerosene and diesel corresponds to the lower limit concentration. The flash point is related to the volatility of the oil, and it is highly correlated with the temperature of the 10% distillate point of the oil.

The flash point of oils is related to their distillation composition, chemical composition, and pressure. The lower the boiling point range of a petroleum product, the lower its flash point. The greater the vaporization tendency of a fuel, the lower its flash point. The flash point of oils increases as pressure rises. The lighter the oil, the easier it is to ignite; therefore, this is reflected in a low flash point.

The general rule is: the higher the vapor pressure of the oil and the lighter its distillate composition, the lower its flash point. Conversely, oils with a heavier distillate composition have a higher flash point ； 2) The flash point can be used to determine the fire hazard of oils. Because the flash point is the lowest temperature at which a fire hazard arises. The lower the flash point, the easier it is for the fuel to burn, and the greater the fire hazard. Therefore, flammable liquids are also classified according to their flash point. Liquids with a flash point below 45°C are called flammable liquids, while those with a flash point above 45°C are called combustible liquids. The various fire safety measures for its transportation, storage, and use can be determined based on its flash point ； 3) For certain lubricants, simultaneously measuring the closed-cup flash point can serve as an indicator of the presence of low-boiling-point contaminants in the oil, useful for production inspection. Generally, the open flash point is 20-30°C higher than the closed flash point, as some of the oil vapor evaporates during the measurement of the open flash point. However, if the two results differ significantly, it indicates that the oil contains light fractions, or that cracking occurred during distillation, or that the solvent used in purification during dewaxing was not completely removed.

Oil is light, has a low boiling point, and vaporizes in large quantities; an increased concentration of oil vapor in the air makes ignition more likely.