D2 is a refinery abbreviation for Gasoil

D2 diesel fuel is the broad category for the second distillate of crude oil. Ultra-low sulfur grades of D2, such as GOST 305-82, reduce sulfur content to a maximum of 0.02 percent and reduce sulfur pollution emissions.

This type of fuel can be used without additives or reformers. It is the second distillate from the crude, and can be used without reformers and additives. So, the first engines used D2 as fuel -D2 is primarily used as fuel in cars.