When stored in tanks in enclosed spaces, an influx of oxygen from the ambient air is usually relevant. From a chemical point of view, this produces free radicals over time that can react with oxygen from air and change the molecular structure of the fuel (Oxidation). This is referred to as degradation of the fuel. The introduction of heat or metal ions from copper lines into the fuel catalyzes, i.e. accelerates, the aging process. In addition, oxidation can also lead to the formation of acids in the fuel, which in turn can lead to corrosion of fuel-carrying components of the engine system.

In the case of diesel with a biodiesel content (up to 7% biodiesel is commonly added to mineral oil-based diesel), external influences can also cause polymerization of the fuel. Molecular structures and the properties of the fuel would change during polymerization. Above a certain molecular size, the polymers precipitate out of the fuel and form sticky deposits at the bottom of the tank.

A similar picture is seen with microbiological infestation of biodiesel. This is commonly referred to as “diesel plague” but, strictly speaking, is not a typical aging process. Microbes from the environment (and possibly also from the bio-content of the diesel fuel) enter the fuel, multiply and form a biofilm that settles between a water phase and the fuel phase.