According to a recent report via German news feed, Deutsche Presse-Agentur (dpa), the judges said the letters were not only used in the EU, but were a common phrase in the automotive world. They said since this was a term used by many other companies and individuals, it could not be owned by Audi.
On top of this, the way the European Union law works, no company can trademark a common abbreviation. In this case, TDI simply stands for 'turbo diesel injection' and 'turbo direct injection'. Although no other company uses the TDI term for their engines, or trim levels, Audi and Volkswagen will not be the owner of the letters.
Audi will have to pay relevant court costs for going to the trouble of trying to trademark the name. What do you think? Should companies be allowed to trademark such terms? TDI is normally only associated with Audi and Volkwagen anyway.