Diesel Beetle
Volkswagen actually commissioned an air-cooled, flat-four, Beetle based diesel from its regular future projects supplier-Porsche-way back in 1951. This was given project number 508 and carried to the level of two test vehicles. It even provided vital strength data for Porsches own 1.3/1.5 crankcase, but it would be 1981 before the public learned of this decidedly different VW engine