Why is diesel difficult to ignite despite its low flash point?
3 Answers
Diesel has a relatively large molecular weight, which makes it less volatile. Only volatile substances can form explosive mixtures. The flash point is the lowest temperature at which a liquid vaporizes to form an ignitable mixture with air under specific test conditions when exposed to an ignition source. Flash ignition occurs when sufficient vapor is produced on the liquid's surface to mix with air and form a combustible gas, resulting in a brief flame upon contact with a fire source, which extinguishes immediately. The flash point of gasoline is approximately 90°C lower than that of diesel. Precautions for fuel tank usage are as follows: 1. Do not wait for the fuel warning light to come on before refueling: Drivers should check the fuel gauge before driving and calculate whether refueling is necessary based on the trip distance and fuel gauge reading. It is advisable to refuel in advance rather than waiting for the warning light to come on. The fuel pump is located inside the tank and operates at high temperatures when continuously working. Being submerged in fuel helps cool it effectively. When the warning light comes on, it indicates the fuel level is below the pump. Repeatedly waiting for the light to come on before refueling will shorten the pump's lifespan, and having too little fuel in the tank may even burn out the pump. 2. Choose to refuel in the morning or evening: Generally, temperatures are lower in the early morning and evening, and gasoline's vaporization point is relatively low, making these times ideal for refueling. Gasoline expands with heat and contracts with cold, and it is sold by volume. At lower temperatures, gasoline's density is relatively higher, increasing the mass per unit volume, resulting in more fuel per refill. 3. Avoid filling the tank completely when driving in the city: For regular city driving, there is no need to fill the tank completely. A full tank increases engine load and fuel consumption. Since refueling is convenient in urban areas, it is unnecessary to fill the tank to the brim.
Last week, I finally understood this while helping a friend repair a diesel generator. The flash point (minimum ignition temperature) of diesel is indeed higher than gasoline, but its autoignition temperature is much lower. The key lies in two conditions required for ignition: the appropriate fuel-air mixture concentration and sufficient temperature. Diesel molecules are large and highly viscous, making it impossible to evaporate enough vapor at room temperature. It's like pouring diesel onto charcoal—the liquid itself is hard to ignite, but when atomized into a fine mist, it suddenly combusts violently. Inside an engine cylinder, high-pressure injectors break diesel into particles dozens of times finer than a strand of hair, and the compression stroke heats the air above 500°C, allowing diesel particles to autoignite instantly. Without a high-pressure injection system or insufficient cylinder pressure, even using a lighter on diesel will only produce black smoke.
My dad has been driving a tanker truck for thirty years, and when he taught me about diesel safety, he explained the principles. Gasoline is highly volatile and can form an explosive mixture at room temperature when exposed to sparks. Diesel is like cooking oil; if you pour it on the ground and try to light it with a lighter, it will only burn slowly with a flame. The key difference lies in the 'flash point' indicator: gasoline has a flash point of -43°C, while diesel has a closed-cup flash point above 55°C. However, diesel's auto-ignition temperature is only 260°C, whereas gasoline requires 427°C to auto-ignite. When a truck's turbocharger reaches 20 atmospheres, the compressed air can exceed 500°C, igniting the diesel. In winter, tractors are hard to start because the cylinder temperature isn't high enough, requiring a glow plug to heat the combustion chamber.
