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Why was diesel fuel safer to store and handle on submarines than gasoline?

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diesel engine and precombustion chamber — diesel engine Diesel engine equipped with a precombustion chamber.

Diesel fuel was considered safer to store and handle on submarines primarily because it is significantly less volatile than gasoline.1

The safety difference is rooted in the physical properties of the fuels, specifically their flash point—the lowest temperature at which a liquid forms enough vapor to ignite when exposed to a flame. Automotive gasoline has a very low flash point of approximately −43 °C (−45 °F), making it highly flammable and prone to releasing explosive vapors at room temperature. In contrast, automotive diesel fuel has a much higher flash point of approximately 38 °C (100 °F), meaning it does not produce sufficient vapor to support combustion under normal storage conditions.2

Because diesel fuel consists of heavier hydrocarbons with higher boiling points than gasoline, it is less prone to evaporation. This reduced volatility made diesel a more reliable and safer choice for the confined, high-stakes environment of wartime submarine operations.31