As much as it has with the Quattro brand, Audi (and its parent company Volkswagen) have more recently turned the letters TDI into an emblem of what it stands for as an automaker. Think of those three letters and you could picture no other marque, that's how closely intertwined the two have become. But an attempt to enshrine that much in law has apparently failed.
According to Autobild, Audi went to the European Union courts in Luxembourg – on behalf of its parent company as well in its own right – to secure dominion over the TDI badge, but in a ruling this week, the judge ruled that the acronym is not theirs alone. Apparently the judge felt that the letters apply to a technology that is common with other automakers – namely turbocharged diesels with direct injection – and therefore rejected Audi's claim of ownership.
The company apparently owns the name in individual countries, so any attempt by a rival automaker to use the name would undoubtedly prove difficult. But as far as the European Union is concerned, it's not illegal.