Cold behaviour, filterability

At low temperatures, the precipitation of paraffin crystals can cause clogging of the fuel filter, and thus result in interruption of the fuel supply. The start of paraffin precipitation can be as early as 0C in most cases. Consequently, winter diesel fuels must be specially selected, or treated, in order to guarantee problem free operation in cold weather.

Normally, flow improvers are added at the refinery. Although these do not prevent precipitation of the paraffins, they limit the crystal growth to a very high degree. The crystals that do form are so small that they can still pass through the filter pores. As a result, filterability can be extended down to lower temperatures. In accordance with DIN 51 601, filterability should be guaranteed down to at least -15C.

The resistance to cold can be improved even further by additives which prevent precipitation of paraffin crystals. The win ter diesel fuels which are widely available today guarantee cold resistance down to at least -22 C.

Additional measures are the addition of petroleum to the diesel fuel. Admixing of regular grade petrol can also delay crystal precipitation; however, the ignition quality is impaired and the flash point considerably reduced (petrol has very low cetane numbers).

Today, if correct fuels are used as stipulated by Australian standards, such additives are no longer necessary.