Editing Diesel engine (section)

===Gelling===
DIN 51601 diesel fuel was prone to ''waxing'' or ''gelling'' in cold weather; both are terms for the solidification of diesel oil into a partially crystalline state. The crystals build up in the fuel system (especially in fuel filters), eventually starving the engine of fuel and causing it to stop running.<ref name="vB_2017_1018" /> Low-output electric heaters in [[fuel tank]]s and around fuel lines were used to solve this problem. Also, most engines have a ''spill return'' system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. Once the engine has warmed, returning warm fuel prevents waxing in the tank. Before direct injection diesel engines, some manufacturers, such as BMW, recommended mixing up to 30% petrol in with the diesel by fuelling diesel cars with petrol to prevent the fuel from gelling when the temperatures dropped below −15&nbsp;°C.<ref name="BMW_1985" />